deermaster
11-15-2008, 04:42 PM
how many pounds of thrust in a trolling motor is equal to one horsepower of gas motor? thanks for any help.
View Full Version : how many lbs of thrust is equal to 1 hp?
marshmat
11-15-2008, 06:49 PM
The power absorbed by a hull is its resistance (wave drag, skin friction, etc.) multiplied by the speed at which it is travelling.
The actual power produced by the engine is substantially larger, as much of the engine power is lost to inefficiencies at various points in the system. For example, a gearbox might take 3% off the engine's brake horsepower, and a typical propeller would convert 50-60% of the horsepower that reaches it into useable thrust. (Really good, well-matched props can sometimes be 70%+ efficient.)
When you look at a trolling motor rated for something like "55 lb thrust", the figure given is often the maximum thrust that motor will give. Let's say for the sake of argument that you'll be going at 2 knots, or 1 m/s with the trolling motor at full power. If you're getting the full 55 lbf of thrust at this speed, then your hull has 55 lbf total resistance at this speed. 55 lbf = 0.245 kN, so (0.245 kN)*(1 m/s) = 0.245 kNm/s = 0.245 kW. That's about 1/3 hp.
Now, once again for the sake of argument (these figures might not be representative of your actual setup) let's say your propeller efficiency on that trolling motor is 50%. Then it's actually producing 0.49 kW or 2/3 hp. Electric motors aren't perfect, let's say our example motor is 80% efficient at full power, so we're up to 0.62 kW of electric power that's actually being sent into the thing. At 24 volts that's about 26 amps; at 12 volts, more like 52 amps.
To compare against a gasoline engine is a little harder. An outboard's SHP rating is usually at full power (very near WOT) and usually measured at the propshaft. To get the same 55 lbf of thrust as above, again with a 50% propeller efficiency, we get the exact same 0.49 kW or 2/3 hp. Shaft power has the exact same meaning regardless of what is turning the shaft.
The question becomes, then, what gas engine will produce 2/3 hp at the prop shaft. You'd probably find that a 2 hp outboard at about one-half to two-thirds throttle would be roughly equivalent.
Now, all of what I've just said rests on a big assumption- that we actually know what thrust the trolling motor is making at a given speed. Short of putting strain gauges on its mounting bracket, we generally don't- and can't- know this value accurately.
But the key message in all that math I just posted is this: You cannot make a direct comparison between thrust (force) and power, unless you also know the speed of the vessel when it is absorbing that thrust or that power.
The actual power produced by the engine is substantially larger, as much of the engine power is lost to inefficiencies at various points in the system. For example, a gearbox might take 3% off the engine's brake horsepower, and a typical propeller would convert 50-60% of the horsepower that reaches it into useable thrust. (Really good, well-matched props can sometimes be 70%+ efficient.)
When you look at a trolling motor rated for something like "55 lb thrust", the figure given is often the maximum thrust that motor will give. Let's say for the sake of argument that you'll be going at 2 knots, or 1 m/s with the trolling motor at full power. If you're getting the full 55 lbf of thrust at this speed, then your hull has 55 lbf total resistance at this speed. 55 lbf = 0.245 kN, so (0.245 kN)*(1 m/s) = 0.245 kNm/s = 0.245 kW. That's about 1/3 hp.
Now, once again for the sake of argument (these figures might not be representative of your actual setup) let's say your propeller efficiency on that trolling motor is 50%. Then it's actually producing 0.49 kW or 2/3 hp. Electric motors aren't perfect, let's say our example motor is 80% efficient at full power, so we're up to 0.62 kW of electric power that's actually being sent into the thing. At 24 volts that's about 26 amps; at 12 volts, more like 52 amps.
To compare against a gasoline engine is a little harder. An outboard's SHP rating is usually at full power (very near WOT) and usually measured at the propshaft. To get the same 55 lbf of thrust as above, again with a 50% propeller efficiency, we get the exact same 0.49 kW or 2/3 hp. Shaft power has the exact same meaning regardless of what is turning the shaft.
The question becomes, then, what gas engine will produce 2/3 hp at the prop shaft. You'd probably find that a 2 hp outboard at about one-half to two-thirds throttle would be roughly equivalent.
Now, all of what I've just said rests on a big assumption- that we actually know what thrust the trolling motor is making at a given speed. Short of putting strain gauges on its mounting bracket, we generally don't- and can't- know this value accurately.
But the key message in all that math I just posted is this: You cannot make a direct comparison between thrust (force) and power, unless you also know the speed of the vessel when it is absorbing that thrust or that power.
PAR
11-16-2008, 12:53 AM
Depending on which math formulas and efficiency assumsions you want to accept, a single HP is some where between 75 and 110 pounds of thrust.
For an average trolling motor, figure towards the high end of this scale. If using a better prop, then comes with the typical trolling motor, then the some place in the middle.
The 105 pounds of thrust, 24 volt motors are about a single HP.
For an average trolling motor, figure towards the high end of this scale. If using a better prop, then comes with the typical trolling motor, then the some place in the middle.
The 105 pounds of thrust, 24 volt motors are about a single HP.
FAST FRED
11-16-2008, 08:16 AM
I know formulas vary , but I always use 20 lbs for most boats , 25 for really good prop setups and 30 as the dream of "perfect".
Perhaps Rick will bridge the huge difference between 25 and 100 lbs per hp.
FF
Perhaps Rick will bridge the huge difference between 25 and 100 lbs per hp.
FF
Guest625101138
11-16-2008, 04:47 PM
I know formulas vary , but I always use 20 lbs for most boats , 25 for really good prop setups and 30 as the dream of "perfect".
Perhaps Rick will bridge the huge difference between 25 and 100 lbs per hp.
FF
Fred
HP and thrust are indirectly related. As Matt points out:
Power = Force X Velocity
If you were solely interested in bollard pull without actually moving then you would go for the biggest possible prop and gear it down. So the water velocity needed to generate the thrust is small and the area that it acts over is large. This is what you see with tugs. The power required to achieve a certain bollard pull for a zero loss system is:
Power = (4/pi/9/rho)^0.5 x Thrust^(3/2)/D in Watts
Thrust in Newtons
rho = 1025 for salt water
D prop diameter in metres
Realistically the power will be roughly twice this level to make up for drag of the blades through the water.
If you actually want to move then the prop size will reduce and gearing ratio will be lower so it spins faster. The force will be lower but you now have the blade advancing through the water plus the added slip to generate the thrust.
The smallest commercial combustion engine outboard I know of is 2HP. This is more powerful than any of the Minn Kota range of trolling motors as far I know. However the trolling outboards tend to have larger diameter props than the little combustion outboards so one of the larger trolling motors will produce similar thrust to a 2HP outboard. It is possible to get larger diameter props for many of the small combustion outboards for low speed, high thrust applications.
The original question posed does not make much sense unless there is an application in mind. The thrust rating of trolling motors is nonsense anyhow unless you want to have tug-of-wars. If you actually want to move then it is POWER that matters.
The question would be better framed if the intended purpose was given.
Rick W
thrust will be roughly the same.A mid range troll
Perhaps Rick will bridge the huge difference between 25 and 100 lbs per hp.
FF
Fred
HP and thrust are indirectly related. As Matt points out:
Power = Force X Velocity
If you were solely interested in bollard pull without actually moving then you would go for the biggest possible prop and gear it down. So the water velocity needed to generate the thrust is small and the area that it acts over is large. This is what you see with tugs. The power required to achieve a certain bollard pull for a zero loss system is:
Power = (4/pi/9/rho)^0.5 x Thrust^(3/2)/D in Watts
Thrust in Newtons
rho = 1025 for salt water
D prop diameter in metres
Realistically the power will be roughly twice this level to make up for drag of the blades through the water.
If you actually want to move then the prop size will reduce and gearing ratio will be lower so it spins faster. The force will be lower but you now have the blade advancing through the water plus the added slip to generate the thrust.
The smallest commercial combustion engine outboard I know of is 2HP. This is more powerful than any of the Minn Kota range of trolling motors as far I know. However the trolling outboards tend to have larger diameter props than the little combustion outboards so one of the larger trolling motors will produce similar thrust to a 2HP outboard. It is possible to get larger diameter props for many of the small combustion outboards for low speed, high thrust applications.
The original question posed does not make much sense unless there is an application in mind. The thrust rating of trolling motors is nonsense anyhow unless you want to have tug-of-wars. If you actually want to move then it is POWER that matters.
The question would be better framed if the intended purpose was given.
Rick W
thrust will be roughly the same.A mid range troll
PAR
11-16-2008, 09:53 PM
740 watts is a single HP. It's not a simple nor direct conversion (HP to pounds of thrust).
Lets take Fred's example and say 25 pounds = a single HP. At 1 pound of thrust per amp (not my assumsion, but Minnkota's, which I feel is less then half right) a 24 pounds (24 amps) of thrust motor times 12 volts, equals 288 watts. Clearly this isn't right, so a more effective measure might be .35 pounds per amp. Of course this doesn't count slippage, speed (your not performing work unless you're moving) nor any other variables.
Thrust is a static measurement in pounds. HP is a measurement equal to 550 foot pounds of work per second.
Most trolling motors use a prop designed to provide quick acceleration at the sacrifice top speed. Most are "geared" to reach about 4.25 knots (around 5 MPH). You can trade off acceleration for top speed, with a different prop.
Since the conversion isn't direct, it would be much more helpful if we had more understanding of the application.
Lets take Fred's example and say 25 pounds = a single HP. At 1 pound of thrust per amp (not my assumsion, but Minnkota's, which I feel is less then half right) a 24 pounds (24 amps) of thrust motor times 12 volts, equals 288 watts. Clearly this isn't right, so a more effective measure might be .35 pounds per amp. Of course this doesn't count slippage, speed (your not performing work unless you're moving) nor any other variables.
Thrust is a static measurement in pounds. HP is a measurement equal to 550 foot pounds of work per second.
Most trolling motors use a prop designed to provide quick acceleration at the sacrifice top speed. Most are "geared" to reach about 4.25 knots (around 5 MPH). You can trade off acceleration for top speed, with a different prop.
Since the conversion isn't direct, it would be much more helpful if we had more understanding of the application.
CDK
11-17-2008, 04:05 AM
If you manufacture outboard engines with outputs below 1 hp and advertise them as such, sales figures will be much lower than if you advertise them as having, say, 80 lbs thrust.
Of course the thrust figure is meaningless, but for the public 80 lbs is a considerable weight to lift, so an engine with that much thrust is regarded as powerful. Horse-powers we relate to the figures we know from combustion engines, there anything under 4 hp is almost nothing.
Only jet engine output is measured in lbs thrust, but for totally different reasons.
Of course the thrust figure is meaningless, but for the public 80 lbs is a considerable weight to lift, so an engine with that much thrust is regarded as powerful. Horse-powers we relate to the figures we know from combustion engines, there anything under 4 hp is almost nothing.
Only jet engine output is measured in lbs thrust, but for totally different reasons.
jehardiman
11-17-2008, 12:48 PM
740 watts is a single HP. It's not a simple nor direct conversion (HP to pounds of thrust).
PAR is correct here. See the following threads where I tried to explain the problem.
http://www.boatdesign.net/forums/powerboats/finding-thrust-15967.html
http://www.boatdesign.net/forums/propulsion/hp-watt-thrust-6572.html
PAR is correct here. See the following threads where I tried to explain the problem.
http://www.boatdesign.net/forums/powerboats/finding-thrust-15967.html
http://www.boatdesign.net/forums/propulsion/hp-watt-thrust-6572.html
bruceb
11-17-2008, 03:57 PM
Back in the 80s, both evinrude/johnson and chrysler (maybe others) rated their longshaft "sailboat" engines at about 150lbs thrust for a 6hp and 250lbs for a 10hp. From experience, it did seem close to real world performance. Bruce
Joakim
11-18-2008, 06:43 AM
Power = V * F. Typical system has roughly constant maximum thurst (F) with all speeds. Thus it is enough to calculate the thrust at max speed.
F = Pmax /Vmax * efficiency. For normal, not so good, applications a good first guess for efficiency might be 50%.
Now to stupid non-SI units...
Thus 1 HP (735 W) would acchieve a thrust of 370 N/V[m/s], which would be 160 lbs/V[kn]. Thus for a propeller designed for 3 kn speed (typical electric system) the thrust is ~50 lbs/HP, but for a 6 hp outboard designed for 6 kn it would be less than 30 lbs/HP. If we take a 20 outboard propped for 30 kn speed, we have only 5 lbs/HP.
For a 12 V system 1 HP equals to 60 A, which is close to normal 1 lbs/A of thurst.
Joakim
F = Pmax /Vmax * efficiency. For normal, not so good, applications a good first guess for efficiency might be 50%.
Now to stupid non-SI units...
Thus 1 HP (735 W) would acchieve a thrust of 370 N/V[m/s], which would be 160 lbs/V[kn]. Thus for a propeller designed for 3 kn speed (typical electric system) the thrust is ~50 lbs/HP, but for a 6 hp outboard designed for 6 kn it would be less than 30 lbs/HP. If we take a 20 outboard propped for 30 kn speed, we have only 5 lbs/HP.
For a 12 V system 1 HP equals to 60 A, which is close to normal 1 lbs/A of thurst.
Joakim
daiquiri
11-18-2008, 07:04 AM
This thread illustrates well the nonsense of rating an engine in lbs of thrust instead of giving the power data.
It is similar to a question one guy, a sheet metal worker, has asked me once: "how many square meters are there in 1 cubic meter?"
It is similar to a question one guy, a sheet metal worker, has asked me once: "how many square meters are there in 1 cubic meter?"
bruceb
11-18-2008, 09:07 AM
I think the engine builders were avoiding the issue, but at least you can use lbs of thrust to compare similar engines. As for the different measurement systems, the USA has been trying to convert for 40 years- we just haven't quite got it yet.
marshmat
11-23-2008, 09:10 PM
Hey Bruce, we Canucks are only slightly better.... you still see odd hybrid units like "milligrams per pound" here, and the grocery stores often make the $/lb price bigger than the $/kg price when they have sales on so that food looks cheaper than it really is.
And then there's the infamous 4x8 foot sheet of 6-millimetre plywood. (Some of our mills have converted to metric in one dimension and stuck with feet in the other two.)
But back to the original topic.... to be honest, I don't think the folks who sell trolling motors really care what their actual power rating is. They just want to know which motor will push what size of bass boat, and how soon the battery will die.
And then there's the infamous 4x8 foot sheet of 6-millimetre plywood. (Some of our mills have converted to metric in one dimension and stuck with feet in the other two.)
But back to the original topic.... to be honest, I don't think the folks who sell trolling motors really care what their actual power rating is. They just want to know which motor will push what size of bass boat, and how soon the battery will die.
bruceb
11-24-2008, 10:03 AM
I think it depends if the fish you are trying to catch are measured in inches or kilowatts. Here in the south, we are mostly concerned if the beer will run out before the battery dies.
PAR
11-24-2008, 08:44 PM
We've been using metric tires on cars for a number decades now, except the rim diameters are still inches.
bruceb
11-24-2008, 09:10 PM
Crossing the Atlantic changes the size of a gallon, (I guess the colonies got the small ones) and then you can fix your car with a whitworth wrench. Well maybe.
sailfish25
11-25-2008, 01:01 AM
All this math is hurting my brain!
bruceb
11-26-2008, 10:04 AM
My brain hurts too, but I guess it needed the exercise- I have learned something. Thanks everyone. Bruce - 2,4,6,8, I'm too old to metricate;)
Manie B
11-27-2008, 12:37 AM
thanks i learned a lot from this thread as well :D
mikelinmon
03-21-2009, 02:14 PM
When we (Roger Macgregor and I, Mike Inmon) were testing the righting moment or self righting of the 26M ( you can see the movie on the macgregorsailboats.com) we had a scale and blocks, etc to measure the force needed to hold the Mac on its side; he said "wonder how much thrust the 50hp Merc bigfoot has"? We hooked it up to the dock and to a scale
using one block so we would still be on scale. Result, exactly 750 lbs on the dial at wide-open. 25lbs per hp by measure includes all the prop loses and one ball bearing block thrown in. Cool huh! All that math was just about the same as the totally unrelated testing by totally uninvolved folks. I could have done that math, yes I could have!
using one block so we would still be on scale. Result, exactly 750 lbs on the dial at wide-open. 25lbs per hp by measure includes all the prop loses and one ball bearing block thrown in. Cool huh! All that math was just about the same as the totally unrelated testing by totally uninvolved folks. I could have done that math, yes I could have!
bruceb
03-21-2009, 06:08 PM
Very cool:cool: and! we now know about what it takes to plane a 26' sailboat:) in real numbers. Great! Thanks Mike.
jehardiman
03-21-2009, 08:39 PM
When we (Roger Macgregor and I, Mike Inmon) were testing the righting moment or self righting of the 26M ( you can see the movie on the macgregorsailboats.com) we had a scale and blocks, etc to measure the force needed to hold the Mac on its side; he said "wonder how much thrust the 50hp Merc bigfoot has"? We hooked it up to the dock and to a scale
using one block so we would still be on scale. Result, exactly 750 lbs on the dial at wide-open. 25lbs per hp by measure includes all the prop loses and one ball bearing block thrown in. Cool huh! All that math was just about the same as the totally unrelated testing by totally uninvolved folks. I could have done that math, yes I could have!
My money says you were not getting full power and the number is actually closer to ~30 lbs/hp ;) ....but who really wants to pay for that test.
Very cool:cool: and! we now know about what it takes to plane a 26' sailboat:) in real numbers. Great! Thanks Mike.
Ahhhh...no. there are two problems with trying to corrolate bollard thrust of an OB to eph at planning speed. First, I'll bet bollard was reduced by tip and face caviation. And second, thrust falls off with J so there is actually less thrust a planing speed than at bollard assuming constant sigma (cavitation). Without Kt and Kq curves, it would be a pretty broad guess and only if you knew max rpm and pitch of the wheel.
using one block so we would still be on scale. Result, exactly 750 lbs on the dial at wide-open. 25lbs per hp by measure includes all the prop loses and one ball bearing block thrown in. Cool huh! All that math was just about the same as the totally unrelated testing by totally uninvolved folks. I could have done that math, yes I could have!
My money says you were not getting full power and the number is actually closer to ~30 lbs/hp ;) ....but who really wants to pay for that test.
Very cool:cool: and! we now know about what it takes to plane a 26' sailboat:) in real numbers. Great! Thanks Mike.
Ahhhh...no. there are two problems with trying to corrolate bollard thrust of an OB to eph at planning speed. First, I'll bet bollard was reduced by tip and face caviation. And second, thrust falls off with J so there is actually less thrust a planing speed than at bollard assuming constant sigma (cavitation). Without Kt and Kq curves, it would be a pretty broad guess and only if you knew max rpm and pitch of the wheel.
bruceb
03-21-2009, 11:08 PM
Of course you are correct- but, I think it is still nice to have some measured numbers. I have run a M-26 with about the same set up, and it is performance that I can relate to. Bruce
powerabout
04-11-2009, 02:47 AM
I wonder if you scaled down tug /AHTS vessels hp and bollard pull you get a meaniful number at the bottom end of the scale?
re the low numbers, I;ll bet if you give me a ski handle tied to a say a 15 hp I could hold it on the dock...
Think about say an outboard of say 100hp, put both feet in a single ski and no way will it pull you up so the skier is holding all that force less the fact it will be pulling you through the water at about 4-5 kts...what did I just prove?
re the low numbers, I;ll bet if you give me a ski handle tied to a say a 15 hp I could hold it on the dock...
Think about say an outboard of say 100hp, put both feet in a single ski and no way will it pull you up so the skier is holding all that force less the fact it will be pulling you through the water at about 4-5 kts...what did I just prove?
Guest625101138
04-11-2009, 04:48 AM
I wonder if you scaled down tug /AHTS vessels hp and bollard pull you get a meaniful number at the bottom end of the scale?
re the low numbers, I;ll bet if you give me a ski handle tied to a say a 15 hp I could hold it on the dock...
Think about say an outboard of say 100hp, put both feet in a single ski and no way will it pull you up so the skier is holding all that force less the fact it will be pulling you through the water at about 4-5 kts...what did I just prove?
If you do the calculations on these tugs:
http://www.hitzler-werft.de/tugs.html
They consistently achieve 170N/kW. This is about twice as good as the 50HP outboard per unit of power given earlier. Not unexpected given that a tug is designed to give high thrust at low speed. This is quite the opposite objective for most outboards.
The absolute thrust limit is:
F = rho^(1/3) * pi^(1/3) * (1.5 * Power * Prop Diameter) ^ (2/3)
So each engine in the "Santantonio Primo" has an absolute limit of 737kN assuming prop is 4m diameter while the actual is 320kN. So slightly less than half of the absolute value.
The 50HP (37kW) has an absolute limit of 9.6kN with a 0.3m (12") prop. The 750lbf (3.3kN) measured is hence about one third of the absolute limit. This outboard will not be propped to get the best bollard pull.
The bollard pull gets down to prop design. The 15HP outboard with a low pitch, large diameter prop would not have much trouble pulling you off a wharf. Absolute limit with a 14" prop is 5.8kN (1300lbf). So with a well designed prop you might get 600lbf. I doubt that you could hold that on the pier.
Likewise the 100HP outboard correctly propped for skiing would pull your arms out of their sockets if the rope was up to the task.
If you have been able to hold a 100HP outboard from planing then the motor is not propped for skiing and will not be getting up into its power band. It will be developing much less than 100HP. If the prop diameter is 16" (0.4m) then the absolute limit is 18.7kN (4197lbf). So with the best possible prop and the motor in its power band the bollard pull would be up around 1 tonne.
Rick W
re the low numbers, I;ll bet if you give me a ski handle tied to a say a 15 hp I could hold it on the dock...
Think about say an outboard of say 100hp, put both feet in a single ski and no way will it pull you up so the skier is holding all that force less the fact it will be pulling you through the water at about 4-5 kts...what did I just prove?
If you do the calculations on these tugs:
http://www.hitzler-werft.de/tugs.html
They consistently achieve 170N/kW. This is about twice as good as the 50HP outboard per unit of power given earlier. Not unexpected given that a tug is designed to give high thrust at low speed. This is quite the opposite objective for most outboards.
The absolute thrust limit is:
F = rho^(1/3) * pi^(1/3) * (1.5 * Power * Prop Diameter) ^ (2/3)
So each engine in the "Santantonio Primo" has an absolute limit of 737kN assuming prop is 4m diameter while the actual is 320kN. So slightly less than half of the absolute value.
The 50HP (37kW) has an absolute limit of 9.6kN with a 0.3m (12") prop. The 750lbf (3.3kN) measured is hence about one third of the absolute limit. This outboard will not be propped to get the best bollard pull.
The bollard pull gets down to prop design. The 15HP outboard with a low pitch, large diameter prop would not have much trouble pulling you off a wharf. Absolute limit with a 14" prop is 5.8kN (1300lbf). So with a well designed prop you might get 600lbf. I doubt that you could hold that on the pier.
Likewise the 100HP outboard correctly propped for skiing would pull your arms out of their sockets if the rope was up to the task.
If you have been able to hold a 100HP outboard from planing then the motor is not propped for skiing and will not be getting up into its power band. It will be developing much less than 100HP. If the prop diameter is 16" (0.4m) then the absolute limit is 18.7kN (4197lbf). So with the best possible prop and the motor in its power band the bollard pull would be up around 1 tonne.
Rick W
powerabout
04-11-2009, 05:22 AM
Rick
yes thanks
the outboard cannot get to max torque thats why you can hold them back.
A 100hp will only have 14 1/2 max diameter and a 2:1 box if your lucky.
But saying that a good slalom skiier can pull back any V6 even on the plane
Try pulling 3 to 4 skiiers out with a deep water start and they can certainly hold a v6 back regardless of the prop so when you divide the theory up it would say it would pull your arms off. I still have my arms although long..
I think we are talking theoretical maximum versus what you get as a dock buster.
As you might know, Moomba was the first international waterski tournement to use outboards and the first 200+ foot jump was recorded there.
The secret was the Mercs had 15" props so they came in at about 7000 and the skier could pull them back to under 6k so they pulled against peak torque and couldnt slow the boat very much at all after that. (The boat being split timed twice in the course to ensure speeds in an allowable range)
The speeds kept were better than any inboard hence the skier had better boat speed when he hit the ramp.
We even tried a 2:1 box ( from a 150) on the 200 ( for more torque) and it only lasted a few runs.
Merc told us the pinion is too small a diameter (for the power) and would break..they were right
The same engine on the same boat would have a 21" or more prop to keep it in its correct operating range for a consumer
all that was 20 years ago..
as for a 4 stroke outboards...gutless
regards
powerabout
PS I was a torny skiier but not good enough to represent oz or even Vic
yes thanks
the outboard cannot get to max torque thats why you can hold them back.
A 100hp will only have 14 1/2 max diameter and a 2:1 box if your lucky.
But saying that a good slalom skiier can pull back any V6 even on the plane
Try pulling 3 to 4 skiiers out with a deep water start and they can certainly hold a v6 back regardless of the prop so when you divide the theory up it would say it would pull your arms off. I still have my arms although long..
I think we are talking theoretical maximum versus what you get as a dock buster.
As you might know, Moomba was the first international waterski tournement to use outboards and the first 200+ foot jump was recorded there.
The secret was the Mercs had 15" props so they came in at about 7000 and the skier could pull them back to under 6k so they pulled against peak torque and couldnt slow the boat very much at all after that. (The boat being split timed twice in the course to ensure speeds in an allowable range)
The speeds kept were better than any inboard hence the skier had better boat speed when he hit the ramp.
We even tried a 2:1 box ( from a 150) on the 200 ( for more torque) and it only lasted a few runs.
Merc told us the pinion is too small a diameter (for the power) and would break..they were right
The same engine on the same boat would have a 21" or more prop to keep it in its correct operating range for a consumer
all that was 20 years ago..
as for a 4 stroke outboards...gutless
regards
powerabout
PS I was a torny skiier but not good enough to represent oz or even Vic
Joakim
04-13-2009, 05:21 PM
A 100 hp engine propped for skiing (= for about 30-35 kn maximum speed) shoud have about 2000 N (=200 kg) of thrust. That is quite much to work against and you are not going to be able to hold for more than a couple or tens of seconds (measure how long you can even carry your own weight, I quess about 2 minutes). A 15 hp motor propped for 7 kn speed will have about the same thrust!
Joakim
Joakim
powerabout
04-13-2009, 07:47 PM
When you say 200kg do you mean at max thrust i.e. right on peak torque or do you mean at the rpm it will attain while trying to pull a skier out?
What I can say is just a simple change in the gearbox ratio makes an enourmous difference to pulling skiers out.
Thats why a boat with a 135 or 150 V6 can pull out as many skiers as the same boat with a 200 or 225.
Inboards crap all over outboards in ski boats when it comes to pulling out skiers as they usually get to peak torque, ( just over 2k) even with a 1:1 shaft and a 12 or 13" diameter prop, when pulling them out
Outboard just cant get to peak torque as it will be well over 4000 rpm.
I think someone needs to go water skiing....
BTW you are now able to get torque curves from the BRP site as they are comparing them to 4 stroke Yamahas.
Powerabout
What I can say is just a simple change in the gearbox ratio makes an enourmous difference to pulling skiers out.
Thats why a boat with a 135 or 150 V6 can pull out as many skiers as the same boat with a 200 or 225.
Inboards crap all over outboards in ski boats when it comes to pulling out skiers as they usually get to peak torque, ( just over 2k) even with a 1:1 shaft and a 12 or 13" diameter prop, when pulling them out
Outboard just cant get to peak torque as it will be well over 4000 rpm.
I think someone needs to go water skiing....
BTW you are now able to get torque curves from the BRP site as they are comparing them to 4 stroke Yamahas.
Powerabout
powerabout
04-13-2009, 08:07 PM
"A 15 hp motor propped for 7 kn speed will have about the same thrust!
Jaokim"
Is that regardless of the gear ratio and the prop diameter?
Jaokim"
Is that regardless of the gear ratio and the prop diameter?
Joakim
04-14-2009, 03:06 AM
I have done plenty of water skiing. No problems getting out of water with one ski (only one leg in the bindings though) with a 40 hp outboard and have done it even with a 20 hp outboard and actually also with a 10 hp outboard, but back then I was 10 years old.
Even a 40 hp outboard propped for skiing and on a light boat is a tough "opponent". If you don't know how to get out of water, you are not going to be able to hold the rope for more than a half a minute or so unless you are considerably stronger than average.
The approximate 200 kg thrust is valid from start to top speed, IF the propeller is well sized for that 30-35 kn top speed. Of course it is not absolutely constant, but in order to calculate the thrust curve you need to know much more details. On a heavier boat most of this thrust is "wasted" to the drag of the boat while starting to get to plane.
The problem for a bigger motor in the same boat is that the bigger motor is typically propped for a bigger speed. Thus it is driving on "a bigger gear" and will have less thrust for the same torque and less rpm, thus not able to get to good torque range. It also weighs more, thus the drag of the boat increases considerably, especially at lower speed. Most evident this fact is on some very fast boats, which have problems getting into plane even without a skier despite huge power/weight ratio. If you would prop it for the same top speed, it would be a much better puller.
Of course the thrust of a 15 hp motor propped for 7 kn speed is not regardless of gear ratio and prop diameter. A typical displacement vessel with a 15 hp motor will have a thrust of about 200 kg. If you put a typical 15 hp outboard on that displacement boat, it will have much worse performance, since a 15 hp outboard is designed for 15-20 kn speeds.
The key to all this is the top speed which is used to select the gearing/pitch/diameter combination. Power = Thrust * Speed. When you increase the (top)speed, you reduce thrust with the same power.
Joakim
Even a 40 hp outboard propped for skiing and on a light boat is a tough "opponent". If you don't know how to get out of water, you are not going to be able to hold the rope for more than a half a minute or so unless you are considerably stronger than average.
The approximate 200 kg thrust is valid from start to top speed, IF the propeller is well sized for that 30-35 kn top speed. Of course it is not absolutely constant, but in order to calculate the thrust curve you need to know much more details. On a heavier boat most of this thrust is "wasted" to the drag of the boat while starting to get to plane.
The problem for a bigger motor in the same boat is that the bigger motor is typically propped for a bigger speed. Thus it is driving on "a bigger gear" and will have less thrust for the same torque and less rpm, thus not able to get to good torque range. It also weighs more, thus the drag of the boat increases considerably, especially at lower speed. Most evident this fact is on some very fast boats, which have problems getting into plane even without a skier despite huge power/weight ratio. If you would prop it for the same top speed, it would be a much better puller.
Of course the thrust of a 15 hp motor propped for 7 kn speed is not regardless of gear ratio and prop diameter. A typical displacement vessel with a 15 hp motor will have a thrust of about 200 kg. If you put a typical 15 hp outboard on that displacement boat, it will have much worse performance, since a 15 hp outboard is designed for 15-20 kn speeds.
The key to all this is the top speed which is used to select the gearing/pitch/diameter combination. Power = Thrust * Speed. When you increase the (top)speed, you reduce thrust with the same power.
Joakim
Guest625101138
04-14-2009, 03:46 AM
I....... and have done it even with a 20 hp outboard and actually also with a 10 hp outboard, but back then I was 10 years old.
........
Joakim
Joakim
I can go better than this. How about a 6yo with 1HP-----
Just a matter of having the right prop and good power transmission.
Rick W
........
Joakim
Joakim
I can go better than this. How about a 6yo with 1HP-----
Just a matter of having the right prop and good power transmission.
Rick W
Joakim
04-14-2009, 04:11 AM
Joakim
I can go better than this. How about a 6yo with 1HP-----
I have seen this picture before. Doesn't look very fast and fun and seems to be with two skis, but still a great achievement.
At ten years of age we skied a lot with 10 hp on one ski (9.8 hp Mercury with the default prop). It reached over 20 kn top speed with the skier and that was just fine for that age and skill.
Joakim
I can go better than this. How about a 6yo with 1HP-----
I have seen this picture before. Doesn't look very fast and fun and seems to be with two skis, but still a great achievement.
At ten years of age we skied a lot with 10 hp on one ski (9.8 hp Mercury with the default prop). It reached over 20 kn top speed with the skier and that was just fine for that age and skill.
Joakim
Guest625101138
04-14-2009, 04:24 AM
I have seen this picture before. Doesn't look very fast and fun and seems to be with two skis, but still a great achievement.
At ten years of age we skied a lot with 10 hp on one ski (9.8 hp Mercury with the default prop). It reached over 20 kn top speed with the skier and that was just fine for that age and skill.
Joakim
Brings back memories. A girl I knew in my mid teens had a 14ft run-about with a 25HP outboard (her father was wealthy and she had fun toys). It could get me out of the water on two skis. I would drop one and, while on single ski, I could turn the boat by pulling the stern around on the whip.
On the other hand I have had a 115HP Evinrude outboard bog down dragging a large man feet first through the water when trying a barefoot start off the beach. If I did not get enough speed by the time he hit the water he would stay submerged and just bog the boat down. He was a good skier but had lost condition and weighed around 90kg. The motor was no where near its power band.
Rick W
At ten years of age we skied a lot with 10 hp on one ski (9.8 hp Mercury with the default prop). It reached over 20 kn top speed with the skier and that was just fine for that age and skill.
Joakim
Brings back memories. A girl I knew in my mid teens had a 14ft run-about with a 25HP outboard (her father was wealthy and she had fun toys). It could get me out of the water on two skis. I would drop one and, while on single ski, I could turn the boat by pulling the stern around on the whip.
On the other hand I have had a 115HP Evinrude outboard bog down dragging a large man feet first through the water when trying a barefoot start off the beach. If I did not get enough speed by the time he hit the water he would stay submerged and just bog the boat down. He was a good skier but had lost condition and weighed around 90kg. The motor was no where near its power band.
Rick W
powerabout
04-15-2009, 10:33 AM
Brings back memories. A girl I knew in my mid teens had a 14ft run-about with a 25HP outboard (her father was wealthy and she had fun toys). It could get me out of the water on two skis. I would drop one and, while on single ski, I could turn the boat by pulling the stern around on the whip.
On the other hand I have had a 115HP Evinrude outboard bog down dragging a large man feet first through the water when trying a barefoot start off the beach. If I did not get enough speed by the time he hit the water he would stay submerged and just bog the boat down. He was a good skier but had lost condition and weighed around 90kg. The motor was no where near its power band.
Rick W
HA so the outboard proped for skiing produced a low enough thrust that the guy could hang onto it and keep it at what..5kts?
( or his nose filled up with water)
I thought I was trying to say that but it seems people think a 15 hp can be used as a tug boat?
I wasnt trying to say what could get you skiing, I was stating what you could hold back which means they produce way less than 200kg thrust at that rev/speed
On the other hand I have had a 115HP Evinrude outboard bog down dragging a large man feet first through the water when trying a barefoot start off the beach. If I did not get enough speed by the time he hit the water he would stay submerged and just bog the boat down. He was a good skier but had lost condition and weighed around 90kg. The motor was no where near its power band.
Rick W
HA so the outboard proped for skiing produced a low enough thrust that the guy could hang onto it and keep it at what..5kts?
( or his nose filled up with water)
I thought I was trying to say that but it seems people think a 15 hp can be used as a tug boat?
I wasnt trying to say what could get you skiing, I was stating what you could hold back which means they produce way less than 200kg thrust at that rev/speed
Guest625101138
04-15-2009, 05:26 PM
HA so the outboard proped for skiing produced a low enough thrust that the guy could hang onto it and keep it at what..5kts?
( or his nose filled up with water)
I thought I was trying to say that but it seems people think a 15 hp can be used as a tug boat?
I wasnt trying to say what could get you skiing, I was stating what you could hold back which means they produce way less than 200kg thrust at that rev/speed
The 115HP was not propped for skiing. The 17ft boat was set up for fishing primarily. It was quite heavy by today's standards and a deep "V". The trick was to get the right ballance between ripping his wet suit off as he was dragged down the beach or just not being fast enough when he hit the water. If the speed was too slow the motor just bogged down well below its power band. The same point I made back at post #25.
It all depends on the prop. I could make a single person pedal boat with a 1m diameter prop that you would find hard to hold against from a pier. Thrust in excess of 200lbf.
Rick W
( or his nose filled up with water)
I thought I was trying to say that but it seems people think a 15 hp can be used as a tug boat?
I wasnt trying to say what could get you skiing, I was stating what you could hold back which means they produce way less than 200kg thrust at that rev/speed
The 115HP was not propped for skiing. The 17ft boat was set up for fishing primarily. It was quite heavy by today's standards and a deep "V". The trick was to get the right ballance between ripping his wet suit off as he was dragged down the beach or just not being fast enough when he hit the water. If the speed was too slow the motor just bogged down well below its power band. The same point I made back at post #25.
It all depends on the prop. I could make a single person pedal boat with a 1m diameter prop that you would find hard to hold against from a pier. Thrust in excess of 200lbf.
Rick W
powerabout
04-15-2009, 07:12 PM
Ok
got it.
Regards
Powerabout
got it.
Regards
Powerabout
penguin78
01-31-2010, 04:46 AM
Hi,
I don't like formulas and overly complicated answers.
Horse power and thrust are directly related.
However 'thrust' is the more useful value because it is the force pushing the boat forward.
Horse power is usually the total force (over time) in the whole system.
Some is being lost in the gear box, inefficient prop, vibration and heat.
And a fraction of that total horse power ends up as thrust.
If the thrust figures are the force exerted on the boat when it is stationary at optimum revs..
Then:
I found the thrust figures for a range of 2.5 Horse power 2 strokes and the figures range between 70 and 100 pounds.
That means if you buy a 86 pound thrust electric, then it's giving you as much push as a 2.5 horse power 2 stroke (at low speeds at least).
There, don't need formulas for that.
I don't like formulas and overly complicated answers.
Horse power and thrust are directly related.
However 'thrust' is the more useful value because it is the force pushing the boat forward.
Horse power is usually the total force (over time) in the whole system.
Some is being lost in the gear box, inefficient prop, vibration and heat.
And a fraction of that total horse power ends up as thrust.
If the thrust figures are the force exerted on the boat when it is stationary at optimum revs..
Then:
I found the thrust figures for a range of 2.5 Horse power 2 strokes and the figures range between 70 and 100 pounds.
That means if you buy a 86 pound thrust electric, then it's giving you as much push as a 2.5 horse power 2 stroke (at low speeds at least).
There, don't need formulas for that.
powerabout
01-31-2010, 06:13 AM
so a 2hp could pull a water skier?
I'd like to see a graph of how this drops as the engine moves through the water.
It obviously drops very fast
I'd like to see a graph of how this drops as the engine moves through the water.
It obviously drops very fast
penguin78
01-31-2010, 07:15 AM
Yes,
A 2 Hp could pull a water skier.
Very slowly.
A 2 Hp could pull a water skier.
Very slowly.
Joakim
01-31-2010, 10:52 AM
Horse power and thrust are directly related.
However 'thrust' is the more useful value because it is the force pushing the boat forward.
Yes power and thrust are related, but still you can not get thrust from power without knowing many things.
Bollard pull thrust, which is the one you refer to, is not very useful, since it only tells the thrust at zero speed. Power is much more useful for selecting an engine.
Horse power is usually the total force.
Power is not force! Power is force * speed. Try how much can you pull with your car using 1st gear and then repeat that with 5th gear. You probably get about 50% of your cars weight at 1st gear and not much at 5th gear. It is just the same with an outboard. The ones capable of higher speeds will have less bollard pull at the same power.
I found the thrust figures for a range of 2.5 Horse power 2 strokes and the figures range between 70 and 100 pounds.
That means if you buy a 86 pound thrust electric, then it's giving you as much push as a 2.5 horse power 2 stroke (at low speeds at least).
Yes a typical 2.5 hp outboard will have a bollard pull of 30-40 kg (70-90 lbs), but buying an electrical motor of equal bollard pull will not give you the same performance.
Look at this test: http://www.torqeedo.com/uploads/media/YachtingMonthly_03.09_english_UK.pdf
Note that electrical motors rated at 55 lbs only measured at 13-17 kg (29-37 lbs). Thus buying an electrical motor rated at 86 lbs, will likely give more like 60 lbs.
Torqeedo 801 (800 W = 1.1 hp) gave about the same bollard pull (31 kg) as the 2.5 hp motors, but it only had a top speed of 4 kn vs. 4.5-5.4 kn of 2.5 hp motors. Actually even Torqeedo 2.0 (2 kW = 2.7 hp) only reached 4.9 kn, but it delivered clearly more bollard pull (54 kg vs. 45 kg) than the 3.5 hp motor, which had a top speed of 8.8 kn.
You need the same power for the same speed regardless of engine type. Forget the bollard pull values unless you are only interested in acceleration at very low speeds.
However 'thrust' is the more useful value because it is the force pushing the boat forward.
Yes power and thrust are related, but still you can not get thrust from power without knowing many things.
Bollard pull thrust, which is the one you refer to, is not very useful, since it only tells the thrust at zero speed. Power is much more useful for selecting an engine.
Horse power is usually the total force.
Power is not force! Power is force * speed. Try how much can you pull with your car using 1st gear and then repeat that with 5th gear. You probably get about 50% of your cars weight at 1st gear and not much at 5th gear. It is just the same with an outboard. The ones capable of higher speeds will have less bollard pull at the same power.
I found the thrust figures for a range of 2.5 Horse power 2 strokes and the figures range between 70 and 100 pounds.
That means if you buy a 86 pound thrust electric, then it's giving you as much push as a 2.5 horse power 2 stroke (at low speeds at least).
Yes a typical 2.5 hp outboard will have a bollard pull of 30-40 kg (70-90 lbs), but buying an electrical motor of equal bollard pull will not give you the same performance.
Look at this test: http://www.torqeedo.com/uploads/media/YachtingMonthly_03.09_english_UK.pdf
Note that electrical motors rated at 55 lbs only measured at 13-17 kg (29-37 lbs). Thus buying an electrical motor rated at 86 lbs, will likely give more like 60 lbs.
Torqeedo 801 (800 W = 1.1 hp) gave about the same bollard pull (31 kg) as the 2.5 hp motors, but it only had a top speed of 4 kn vs. 4.5-5.4 kn of 2.5 hp motors. Actually even Torqeedo 2.0 (2 kW = 2.7 hp) only reached 4.9 kn, but it delivered clearly more bollard pull (54 kg vs. 45 kg) than the 3.5 hp motor, which had a top speed of 8.8 kn.
You need the same power for the same speed regardless of engine type. Forget the bollard pull values unless you are only interested in acceleration at very low speeds.
Muerte
12-07-2010, 11:56 PM
While I have greatly appreciated the physics discussed in this overall thread, I find a lot of humor in the responses. While many aspects of what has been said is very accurate, I think the heart of the original question was barely responded to. I am old, and my undergrad and graduate degrees as an M.E. are quite dated, but luckily, physics is near-static at this point...so overall, the numbers are a moot point.
Guessing at what the original writer asked, as I wondered at it for a great time myself - having played in many rivers for decades...I always wondered, 'if I were to build a boat powertrain and wished to set it up to run on trolling motors, what numbers are given, if any, to compare gasoline HP engines to electric trollers? The answer is not directly given in physics, advertising, or the responses until you get down to one reply that references an old OEM statement that a 6-HP motor provides 160-lbs of thrust, and a 10-HP motor offers near 250-lbs.
For those struggling with newtons, force, and the quantum effects of cheesecake and fish snot on the reciprocal contributions of the Coefficient of drag on the waxed hull of the craft...please...stop.
The original question asks comparative thrust. Ok, envision, you have a 300-lb aluminum flat-bottomed skiff. It is tied to the pier, and you are in it trying to depart perpendicular to the anchor-cleat you are still lashed off on. First, you do this with your gas motor, and you have a linear scale inline with your tie-rope, so that you can take a force reading in pounds. Now, depart at WOT. What does it read with a 10, 20, and 30-HP motor? Good, you have your empirical data. Now, remove your gas motor and do this again with a (rated) 50, 70, 100 and 120-pound thrust trolling motor. Did the actual yield measured equal the rating? Ok. Now, compare. Is the 10-HP gas motor exerting a 50, 70, 120-lb force on the scale? While this measure is technically a measure of potential power, this is exactly what the user needs to know...because I know my 20-hp motor can push my boat upstream at 4-kts with 1000-lbs payload, I need to know if I change to electrics, what do I need to perfectly replace the previous thrust and maintain the 4-kts...(this is regardless to effects of voltage drops relative to Peukert's curve, which is compensatible via perpetual solar charging).
In other words...there are a lot of responses here, but you haven't answered the actual question...a few close guesses, but no 'exact' answers or offers of emperical testing data. Again, the closest I have seen offered is the data of 160-lbs from a 6-HP motor, and 250-lbs from a 10-hp motor. I actually doubt these values for a greater reason...because this implies that a 10-HP motor could yield enough force to pull someone up out of the water as a skiier...and I just don't envision that happening...as it normally requires a 70-hp or greater motor to effectively ski behind...else, you would beat the skiier to death pulling them out of the water.
In the end, the only value that truly matters is the force of the thrust, or push, or pull (pick your vector). If I take a ski-rope and tie it to a scale tied to a pier pylon, and I go WOT, what is the greatest force reading I can exert on the scale meter? Ok...now what can a 70-lb trolling motor really exert on this same empirical test? Ok, what can a 10, 20, 50, 100-hp motor exert? Yes, prop shape has a lot to do with this...just like hull shape and drag have a lot to do with speed...but in the end, when you are trying to just compare motors...you really just need to know in the general configuration as you buy the motor, what is true force yield in the water, where it counts...
Guessing at what the original writer asked, as I wondered at it for a great time myself - having played in many rivers for decades...I always wondered, 'if I were to build a boat powertrain and wished to set it up to run on trolling motors, what numbers are given, if any, to compare gasoline HP engines to electric trollers? The answer is not directly given in physics, advertising, or the responses until you get down to one reply that references an old OEM statement that a 6-HP motor provides 160-lbs of thrust, and a 10-HP motor offers near 250-lbs.
For those struggling with newtons, force, and the quantum effects of cheesecake and fish snot on the reciprocal contributions of the Coefficient of drag on the waxed hull of the craft...please...stop.
The original question asks comparative thrust. Ok, envision, you have a 300-lb aluminum flat-bottomed skiff. It is tied to the pier, and you are in it trying to depart perpendicular to the anchor-cleat you are still lashed off on. First, you do this with your gas motor, and you have a linear scale inline with your tie-rope, so that you can take a force reading in pounds. Now, depart at WOT. What does it read with a 10, 20, and 30-HP motor? Good, you have your empirical data. Now, remove your gas motor and do this again with a (rated) 50, 70, 100 and 120-pound thrust trolling motor. Did the actual yield measured equal the rating? Ok. Now, compare. Is the 10-HP gas motor exerting a 50, 70, 120-lb force on the scale? While this measure is technically a measure of potential power, this is exactly what the user needs to know...because I know my 20-hp motor can push my boat upstream at 4-kts with 1000-lbs payload, I need to know if I change to electrics, what do I need to perfectly replace the previous thrust and maintain the 4-kts...(this is regardless to effects of voltage drops relative to Peukert's curve, which is compensatible via perpetual solar charging).
In other words...there are a lot of responses here, but you haven't answered the actual question...a few close guesses, but no 'exact' answers or offers of emperical testing data. Again, the closest I have seen offered is the data of 160-lbs from a 6-HP motor, and 250-lbs from a 10-hp motor. I actually doubt these values for a greater reason...because this implies that a 10-HP motor could yield enough force to pull someone up out of the water as a skiier...and I just don't envision that happening...as it normally requires a 70-hp or greater motor to effectively ski behind...else, you would beat the skiier to death pulling them out of the water.
In the end, the only value that truly matters is the force of the thrust, or push, or pull (pick your vector). If I take a ski-rope and tie it to a scale tied to a pier pylon, and I go WOT, what is the greatest force reading I can exert on the scale meter? Ok...now what can a 70-lb trolling motor really exert on this same empirical test? Ok, what can a 10, 20, 50, 100-hp motor exert? Yes, prop shape has a lot to do with this...just like hull shape and drag have a lot to do with speed...but in the end, when you are trying to just compare motors...you really just need to know in the general configuration as you buy the motor, what is true force yield in the water, where it counts...
Submarine Tom
12-08-2010, 01:03 AM
It seems to me you've overlooked accelerating the boat the motor is attached to...
-Tom
-Tom
Joakim
12-08-2010, 04:46 PM
I know my 20-hp motor can push my boat upstream at 4-kts with 1000-lbs payload, I need to know if I change to electrics, what do I need to perfectly replace the previous thrust and maintain the 4-kts..
Again, the closest I have seen offered is the data of 160-lbs from a 6-HP motor, and 250-lbs from a 10-hp motor. I actually doubt these values for a greater reason...because this implies that a 10-HP motor could yield enough force to pull someone up out of the water as a skiier...and I just don't envision that happening...as it normally requires a 70-hp or greater motor to effectively ski behind...else, you would beat the skiier to death pulling them out of the water.
You would need a 20-hp electrical motor to replace your 4-kts upstream performance. Equal power is needed to produce the same "pull" at the same speed regardless of the source of power.
I have been pulled out of the water with a 20-hp motor on one ski many times. I weigh 82 kg (180 lbs). I have also done it with a 10-hp motor, but then I was a child and only 50 kg (110 lbs). Probably could still do it with two skis and a 10-hp motor, but haven't tried.
My sailboat has a 10-hp motor as well. It can pull about 1000 N (450 lbs), but you can not ski with it, since its top speed is only 6.5 kts.
Again, the closest I have seen offered is the data of 160-lbs from a 6-HP motor, and 250-lbs from a 10-hp motor. I actually doubt these values for a greater reason...because this implies that a 10-HP motor could yield enough force to pull someone up out of the water as a skiier...and I just don't envision that happening...as it normally requires a 70-hp or greater motor to effectively ski behind...else, you would beat the skiier to death pulling them out of the water.
You would need a 20-hp electrical motor to replace your 4-kts upstream performance. Equal power is needed to produce the same "pull" at the same speed regardless of the source of power.
I have been pulled out of the water with a 20-hp motor on one ski many times. I weigh 82 kg (180 lbs). I have also done it with a 10-hp motor, but then I was a child and only 50 kg (110 lbs). Probably could still do it with two skis and a 10-hp motor, but haven't tried.
My sailboat has a 10-hp motor as well. It can pull about 1000 N (450 lbs), but you can not ski with it, since its top speed is only 6.5 kts.
bruceb
12-08-2010, 07:26 PM
I supplied those HP-lbs numbers, and I think they were correct, but have to be used in context. The manufactures put them in their sales guidelines and I suspect they had some test results to base them on. The models were intended for aux power for sailboats, and were propped for the job. Top speed was not important, but good thrust at 0-6kts was. The 6hp engines would push a 5000 lbs sailboat at 5-6 kts in calm conditions and usually manage about 3-4 kts in wind and waves. A 10hp would do about the same up to 7500lbs, and anything larger got inboards, often not as powerful, but the prop stayed in the water much better. I am sure one of the new hi-thrust electric trolling motors would replace my current 5hp engine for manuvering around the dock, but would be useless for a trip down the lake, unless I carried a lot larger battery. There are several electric launches on our lake, around 16'-18' that certainly carry several people easily and they seem to be using about 3hp electric motors. B
Muerte
12-10-2010, 07:24 PM
Well, Joakim and Submarine Tom are kinda missing my point...which was an echo, hollow that it was, of what the first poster said.
Let's restate this kinda, with a much larger ship test, known as a BOLLARD PULL. This is done for tug boats. Since HORSEPOWER isn't really HORSEPOWER in the water, you have to measure true pull in a tug boat, so you do a BOLLARD PULL to determine if your little tuggy is as strong as your older tuggy...
<<<at this point, I ask all in contention to the idea of PULL versus HORSEPOWER, please wikipedia the term BOLLARD PULL>>>
Now, welcome back...
Since I can make a propeller that is 85% efficient, and I can make one 72% efficient, and I can form it to do many different things (i.e.: high rotational speed, low noise, high thrust, minimal cavitation pull-side, minimal cavitation push-side, etc.) two 6-HP motors with differing props can thrust totally different. Saavy? Ok, so since trolling motors are meant to effect lowest noise and disturbance...while outboards, well, outboards are for power-thrust...and hence, you cannot easily compare gas outboards to electric trolling motors based upon the conventional transition of 1 Horsepower is equal to 745.7 watts (or 550 ft-lbs/sec)...because you are talking about the work-head, not the work done. Vast difference. The actual work done, for purposes of barges, tugs, and even pontoon boats is more equated to the measurement given in a BOLLARD PULL test...and hence, you now are fighting the same battle that automotive manufacturers have dealt with for years.
Which is faster, a 500-horsepower Corvette, or a 500-horsepower Mustang? If you say they are equal because of power, I say enter your wager for the Mustang, I shall take the Corvette, and I will also take your money. They are NOT created equal...for there are finite details in the distribution of power to real-world work, that measures the actual winner. To such a point, you can reciprocally ask, 'ok then, what is the measure of the application and the full equation, to where I can calculate the actual horsepower the Mustang will require to be speed-time-performance equivalent to the Corvette. By the way, this can actually be done...ask any race car crew chief.
Now, back to real world...applying a BOLLARD PULL test to my Pontoon Boat, which has a 20-HP outboard motor, what total 'thrust pounds of trolling motor' do I need, to match the 'PULL' exerted by the outboard? If the actual 'PULL/THRUST' is the published value given on the trolling motors per advertising...then we come back to the question...how much 'PULL', in Newtons, KN, Pounds-force, etc. units, by the conventional 20-HP outboard?
I've still not heard the answer...but the answer will be given in LBS-FORCE (my preferred anti-SI units) against the BOLLARD PULL test standard...or you have NOT answered the question.
By the way...I have NOT done this, for I do not have a sufficient scale to do so...but I would estimate my 20-HP motor would exert around 250-300 pounds-force on the linear scale while it is tied-off to a pier cleat. I have not tried this with a trolling motor, to compare it's pull...but the pull from either, whether mounted to a canoe, a flat-bottom boat, a V-hull, or a pontoon boat...really should be within 3-5%, and therefore negligible to whatever hull-form it is attached to. Hence, it is my supposition that the lbs-force yield from a BOLLARD PULL test is the purest form of testing for actual motor comparisons...
Now...all that long-windedness aside...doesn't this seem correct? Does anyone have any data on such testing? That would be most-valuable, indeed.
Let's restate this kinda, with a much larger ship test, known as a BOLLARD PULL. This is done for tug boats. Since HORSEPOWER isn't really HORSEPOWER in the water, you have to measure true pull in a tug boat, so you do a BOLLARD PULL to determine if your little tuggy is as strong as your older tuggy...
<<<at this point, I ask all in contention to the idea of PULL versus HORSEPOWER, please wikipedia the term BOLLARD PULL>>>
Now, welcome back...
Since I can make a propeller that is 85% efficient, and I can make one 72% efficient, and I can form it to do many different things (i.e.: high rotational speed, low noise, high thrust, minimal cavitation pull-side, minimal cavitation push-side, etc.) two 6-HP motors with differing props can thrust totally different. Saavy? Ok, so since trolling motors are meant to effect lowest noise and disturbance...while outboards, well, outboards are for power-thrust...and hence, you cannot easily compare gas outboards to electric trolling motors based upon the conventional transition of 1 Horsepower is equal to 745.7 watts (or 550 ft-lbs/sec)...because you are talking about the work-head, not the work done. Vast difference. The actual work done, for purposes of barges, tugs, and even pontoon boats is more equated to the measurement given in a BOLLARD PULL test...and hence, you now are fighting the same battle that automotive manufacturers have dealt with for years.
Which is faster, a 500-horsepower Corvette, or a 500-horsepower Mustang? If you say they are equal because of power, I say enter your wager for the Mustang, I shall take the Corvette, and I will also take your money. They are NOT created equal...for there are finite details in the distribution of power to real-world work, that measures the actual winner. To such a point, you can reciprocally ask, 'ok then, what is the measure of the application and the full equation, to where I can calculate the actual horsepower the Mustang will require to be speed-time-performance equivalent to the Corvette. By the way, this can actually be done...ask any race car crew chief.
Now, back to real world...applying a BOLLARD PULL test to my Pontoon Boat, which has a 20-HP outboard motor, what total 'thrust pounds of trolling motor' do I need, to match the 'PULL' exerted by the outboard? If the actual 'PULL/THRUST' is the published value given on the trolling motors per advertising...then we come back to the question...how much 'PULL', in Newtons, KN, Pounds-force, etc. units, by the conventional 20-HP outboard?
I've still not heard the answer...but the answer will be given in LBS-FORCE (my preferred anti-SI units) against the BOLLARD PULL test standard...or you have NOT answered the question.
By the way...I have NOT done this, for I do not have a sufficient scale to do so...but I would estimate my 20-HP motor would exert around 250-300 pounds-force on the linear scale while it is tied-off to a pier cleat. I have not tried this with a trolling motor, to compare it's pull...but the pull from either, whether mounted to a canoe, a flat-bottom boat, a V-hull, or a pontoon boat...really should be within 3-5%, and therefore negligible to whatever hull-form it is attached to. Hence, it is my supposition that the lbs-force yield from a BOLLARD PULL test is the purest form of testing for actual motor comparisons...
Now...all that long-windedness aside...doesn't this seem correct? Does anyone have any data on such testing? That would be most-valuable, indeed.
Muerte
12-10-2010, 07:52 PM
Well, I just managed to stumble across some 'mosquito' motors and their related BOLLARD PULL test values...though they are reading far above what I would have thought they could do...for it seems that the 3-HP range are getting close to 75-lbs of thrust...then again, the thrust-force-curve may be quite curved instead of linear...Again, I don't know, but am very interested to find out.
I did 'MS Excel' the following values, and came up with an equation of y=15.7x + 35.3, where x is horsepower and y is gross thrust.
Extrapolated to a 20-HP motor, that would be 349.3-lbs thrust...and as my first 'rough guess' of my 20-hp motor making 250-300 lbs seems about right...then it appears feasible that there is some form of equation for thrust, though there may be some loss factor I am not seeing as the motors get larger.
Overall, it looks like for my application, a couple of high-thrust trolling motors and a rechargeable battery bank would work quite well as a replacement. Again, this is what I was looking for...and I believe what the original poster was looking for. If the above formula is correct, then his answer would be 1-HP is equal to 51-lbs thrust, but this is not a direct amount. It is by using the linear slope of the actual Bollard Tests given below...(again, sure would be nice to have 20-30 samples of real-world data to verify the formula above is correct)
http://ezinearticles.com/?Choosing-an-Outboard-Motor-For-Your-Dinghy&id=2008077
"To judge one engine against the another several tests were completed. A Bollard pull test showed that the Mercury 3.5hp and Tohatsu 3.5hp were the most powerful at 90lbs of thrust (These two engines along with the Mariner are virtually identical). The least effective was the Honda 2.3hp at 66lbs of thrust. In between were the Suzuki 2.5hp at 83lbs of thrust, the Yamaha 2.5hp at 78lbs of thrust and the Parsun 2.6hp at 70 lbs of thrust."
I did 'MS Excel' the following values, and came up with an equation of y=15.7x + 35.3, where x is horsepower and y is gross thrust.
Extrapolated to a 20-HP motor, that would be 349.3-lbs thrust...and as my first 'rough guess' of my 20-hp motor making 250-300 lbs seems about right...then it appears feasible that there is some form of equation for thrust, though there may be some loss factor I am not seeing as the motors get larger.
Overall, it looks like for my application, a couple of high-thrust trolling motors and a rechargeable battery bank would work quite well as a replacement. Again, this is what I was looking for...and I believe what the original poster was looking for. If the above formula is correct, then his answer would be 1-HP is equal to 51-lbs thrust, but this is not a direct amount. It is by using the linear slope of the actual Bollard Tests given below...(again, sure would be nice to have 20-30 samples of real-world data to verify the formula above is correct)
http://ezinearticles.com/?Choosing-an-Outboard-Motor-For-Your-Dinghy&id=2008077
"To judge one engine against the another several tests were completed. A Bollard pull test showed that the Mercury 3.5hp and Tohatsu 3.5hp were the most powerful at 90lbs of thrust (These two engines along with the Mariner are virtually identical). The least effective was the Honda 2.3hp at 66lbs of thrust. In between were the Suzuki 2.5hp at 83lbs of thrust, the Yamaha 2.5hp at 78lbs of thrust and the Parsun 2.6hp at 70 lbs of thrust."
Submarine Tom
12-10-2010, 10:09 PM
I don't believe I have missed your point. What I have missed is "The" question you refer to.
So, it must be the original one. Thrust vs. Hp depends largely on your propeller efficiency and RPM. There is no direct correlation between Hp and thrust.
I should probably point out that as a Master Limited Captain with a towing certificate I am well familiar with the term BOLLARD PULL.
-Tom
So, it must be the original one. Thrust vs. Hp depends largely on your propeller efficiency and RPM. There is no direct correlation between Hp and thrust.
I should probably point out that as a Master Limited Captain with a towing certificate I am well familiar with the term BOLLARD PULL.
-Tom
Joakim
12-11-2010, 04:35 AM
Muerte, I do know very well what bollard pull is and what it is used for. The original question did not state that bollard pull was the main issue, he just wanted to compare gas and trolling motors. For most purposes this comparison should be made using power, not bollard pull.
Please read this: http://www.torqeedo.com/uploads/media/YachtingMonthly_03.09_english_UK.pdf
You can find the measured bollard pull and top speeds for several outboards and trolling motors. Note that Torqeedo 2.0 had a very good bollard pull of 54 kg (120 lbf), but still the top speed was only 4.9 kts. Compare this to say Suzuki 2.5, which is about equal in power. It has only bollard pull of 35 kg (80 lbf), but still it has a top speed of 5.9 kts. So which one of these has more thrust at 4.9 kts? Even Torqeedo 801 had almost as much bollard pull, 31 kg (70 lbf), but only 4.0 kts top speed. How much thrust do you think it has at 4 kts?
So you are running 4 kts upstream. What is your speed through water? Which of the above would provide the biggest thrust at this situation?
Please read this: http://www.torqeedo.com/uploads/media/YachtingMonthly_03.09_english_UK.pdf
You can find the measured bollard pull and top speeds for several outboards and trolling motors. Note that Torqeedo 2.0 had a very good bollard pull of 54 kg (120 lbf), but still the top speed was only 4.9 kts. Compare this to say Suzuki 2.5, which is about equal in power. It has only bollard pull of 35 kg (80 lbf), but still it has a top speed of 5.9 kts. So which one of these has more thrust at 4.9 kts? Even Torqeedo 801 had almost as much bollard pull, 31 kg (70 lbf), but only 4.0 kts top speed. How much thrust do you think it has at 4 kts?
So you are running 4 kts upstream. What is your speed through water? Which of the above would provide the biggest thrust at this situation?
powerabout
12-12-2010, 07:43 AM
the fastest one
Muerte
12-12-2010, 03:42 PM
You gentlemen miss the point, entirely. If I have a boat, and wish to change the powerplant from one motor to another, different one, then that change is SOLELY effected by the BOLLARD PULL test. Not speed, not power, and not propeller efficiency. (This is in regards to my pontoon vessel, and NOT speed-craft, which function under a few different rules, including issues related to prop designs, cavitation, hull cavitations, etc.) Just a slow-moving riverboat...saavy?
Now, if my boat, ship, etc., does 4-kts with its current engine, then it will yield a specific BOLLARD PULL test value in POUNDS-FORCE (Newtons, etc. - pick your UOM). Now, if I put another motor on this SAME VESSEL, I have not changed the hull design, the slipstream, the coefficient of drag, NOTHING. Therefore, logic says, that if I put a new motor onto a flatbottom boat, a ski-boat, etc., and BOLLARD PULL it against a tied cleat, that the new, unknown motor, which I tested in STATIC BOLLARD, will perform within 2-3% identical once it is mounted to my boat...right? I mean, is their a fail to my logic on this? If so, please enlighten me...why would speed even be considered in this event...I just need to know what a new motor pulls/pushes against the transom mount of ANY vessel, so as to replicate 100% of my current motor's mechanical effect to my transom...anything else considered is superfluous to the engineering design or boat effects.
Ok...so if I have a BOLLARD PULL table of actual empirical yields of motors, say from 2-50 horsepower, with common OEM props on each (say 72% efficient), then I should be able to ideally convert outboard (gas motor) thrust to electric motor thrust, if the actual push or thrust of such electric motors is being reported truthfully as a bollard value.
Now, that said, can you unequivocally state that a 55-lb thrust trolling motor is actually yielding a 55-lb BOLLARD PULL value, or is that a base-mounted value for the work-head, or exactly where is that 55-lbs being read from? Is that torque yield from the rotational event itself? Where is "55-lbs" being reported from? My first knee-jerk response to this is that the "55-lbs" is coming from a static head-mount in a lab somewhere, and is tested against ideal conditions and power input closer approaching 13.2 Volts, not 12.2 as is the nominal in the 'field'.
Ok, so let's assume (you know what they say about assuming, right?) that my current 20-HP motor is making Y=15.7x + 35 in thrust, and that converts at .9x to a BOLLARD PULL. Therefore, the motor is making 349-lbs of thrust, and for-which, it is yielding 314-lbs of BOLLARD PULL event. Ok, lovely. Now, I want to either EXACTLY MATCH this force (as work = force x distance, remember?) or add more...I don't want to lose work-potential, when I CHANGE MY GAS MOTOR OVER TO ELECTRIC MOTORS! This is the point that I was trying to get to...what force-potential do I need in electric motors to 100% replace my gas outboard! The original question asked more-simply, "how much force-thrust is equal to 1-HP...to which at the work head, it appears to be generally answered as Y(thrust)=15.7x(x=HP) + 35. AND...it further appears, that conversion, at least in my-case (not sure where it is coming from, unless it has to do with less-than-perfect angles of approach relative to my outboard's trim) that to take pure thrust and convert to a BOLLARD PULL value comes to (.9Y), or 90% of the calculated thrust value.
Ok...so if I have 349-lbs thrust, and 314-lbs of Bollard Pull, then I need trolling motors which can do the same thing...so I can either fit up three quantity 115-lb thrust trolling motors (115 x 3 = 345-lbs thrust x .9 = 310.5-lbs Bollard Value). <This is a snarl-factor...I hate mated linkages>
Now, taking that into power demand against three 115-lb thrust trolling motors, I am looking at (after consideration of Peukert's Curve for storage batteries) around 4-hours load-time against a 6-battery charge bank, or 6-7 hours of cruise time against the same battery bank, with 12-hours charge required at 20-amps for such battery bank...and now, I have resolved how to replace a 20-HP outboard with 3-DC trolling motors and 6-deep cycles, and remove $30 a day of fuel requirements from my boat use.
You see, there is ingenuity and method-to-madness from all of this...the problem is, there are too many intents of seeking "power", "thrust", "pull", etc.; when it all really comes down to one simple fact, and that is whether I have a team of horses pulling 349 constant pounds of force on my vessel, or an outboard or three trolling motors are doing it...doesn't matter. This is the true purpose of a BOLLARD TEST...to compare apples to apples over FORCE POTENTIAL-TO-KINETIC. Believe it or not, Evinrude did just this thing against Yamaha with the eTec motor, by connecting two boats stern-to-stern, and 'PULLING' against each other...while fitted with the OEM recommended propeller...
Yamaha got swamped, Evinrude drug it into the blimey deep, and Yamaha cried foul...that a "BOLLARD PULL" wasn't a 'fair test' to do...that it mean nothing.
Well...what can you say, it is all about the media, hype, and lies. In the end, any engineer will tell you, same boat, same hull, then bollard 350-lbs beats bollard 320-lbs, every single time...and the 350-lbs is stronger AND faster, when you are testing them on the same boat...and is why if you use BOLLARD of a motor to compare motor-to-motor-to-motor, you can find what you need to upgrade, or to change over to other systems...
Sorry...didn't mean to cause an uproar...just tough being retired and no engineering group to sound off to...kinda had to talk myself into the right answer on this one, and now I have found what I required.
Looks like to replace my 20-hp outboard, I will need 3-quantity 115-lb thrust trolling motors, tied with mated linkages...connected to a 6-cell deep cycle bank and 20-amps of solar cell charging overhead on the fixed canopy, with inline charging regulator that can handle up to 600-watts . <wow...gonna look like the African Queen, when I am done!> LOL
I hope SOMEONE other than myself found this 'enlightening'...
Now, if my boat, ship, etc., does 4-kts with its current engine, then it will yield a specific BOLLARD PULL test value in POUNDS-FORCE (Newtons, etc. - pick your UOM). Now, if I put another motor on this SAME VESSEL, I have not changed the hull design, the slipstream, the coefficient of drag, NOTHING. Therefore, logic says, that if I put a new motor onto a flatbottom boat, a ski-boat, etc., and BOLLARD PULL it against a tied cleat, that the new, unknown motor, which I tested in STATIC BOLLARD, will perform within 2-3% identical once it is mounted to my boat...right? I mean, is their a fail to my logic on this? If so, please enlighten me...why would speed even be considered in this event...I just need to know what a new motor pulls/pushes against the transom mount of ANY vessel, so as to replicate 100% of my current motor's mechanical effect to my transom...anything else considered is superfluous to the engineering design or boat effects.
Ok...so if I have a BOLLARD PULL table of actual empirical yields of motors, say from 2-50 horsepower, with common OEM props on each (say 72% efficient), then I should be able to ideally convert outboard (gas motor) thrust to electric motor thrust, if the actual push or thrust of such electric motors is being reported truthfully as a bollard value.
Now, that said, can you unequivocally state that a 55-lb thrust trolling motor is actually yielding a 55-lb BOLLARD PULL value, or is that a base-mounted value for the work-head, or exactly where is that 55-lbs being read from? Is that torque yield from the rotational event itself? Where is "55-lbs" being reported from? My first knee-jerk response to this is that the "55-lbs" is coming from a static head-mount in a lab somewhere, and is tested against ideal conditions and power input closer approaching 13.2 Volts, not 12.2 as is the nominal in the 'field'.
Ok, so let's assume (you know what they say about assuming, right?) that my current 20-HP motor is making Y=15.7x + 35 in thrust, and that converts at .9x to a BOLLARD PULL. Therefore, the motor is making 349-lbs of thrust, and for-which, it is yielding 314-lbs of BOLLARD PULL event. Ok, lovely. Now, I want to either EXACTLY MATCH this force (as work = force x distance, remember?) or add more...I don't want to lose work-potential, when I CHANGE MY GAS MOTOR OVER TO ELECTRIC MOTORS! This is the point that I was trying to get to...what force-potential do I need in electric motors to 100% replace my gas outboard! The original question asked more-simply, "how much force-thrust is equal to 1-HP...to which at the work head, it appears to be generally answered as Y(thrust)=15.7x(x=HP) + 35. AND...it further appears, that conversion, at least in my-case (not sure where it is coming from, unless it has to do with less-than-perfect angles of approach relative to my outboard's trim) that to take pure thrust and convert to a BOLLARD PULL value comes to (.9Y), or 90% of the calculated thrust value.
Ok...so if I have 349-lbs thrust, and 314-lbs of Bollard Pull, then I need trolling motors which can do the same thing...so I can either fit up three quantity 115-lb thrust trolling motors (115 x 3 = 345-lbs thrust x .9 = 310.5-lbs Bollard Value). <This is a snarl-factor...I hate mated linkages>
Now, taking that into power demand against three 115-lb thrust trolling motors, I am looking at (after consideration of Peukert's Curve for storage batteries) around 4-hours load-time against a 6-battery charge bank, or 6-7 hours of cruise time against the same battery bank, with 12-hours charge required at 20-amps for such battery bank...and now, I have resolved how to replace a 20-HP outboard with 3-DC trolling motors and 6-deep cycles, and remove $30 a day of fuel requirements from my boat use.
You see, there is ingenuity and method-to-madness from all of this...the problem is, there are too many intents of seeking "power", "thrust", "pull", etc.; when it all really comes down to one simple fact, and that is whether I have a team of horses pulling 349 constant pounds of force on my vessel, or an outboard or three trolling motors are doing it...doesn't matter. This is the true purpose of a BOLLARD TEST...to compare apples to apples over FORCE POTENTIAL-TO-KINETIC. Believe it or not, Evinrude did just this thing against Yamaha with the eTec motor, by connecting two boats stern-to-stern, and 'PULLING' against each other...while fitted with the OEM recommended propeller...
Yamaha got swamped, Evinrude drug it into the blimey deep, and Yamaha cried foul...that a "BOLLARD PULL" wasn't a 'fair test' to do...that it mean nothing.
Well...what can you say, it is all about the media, hype, and lies. In the end, any engineer will tell you, same boat, same hull, then bollard 350-lbs beats bollard 320-lbs, every single time...and the 350-lbs is stronger AND faster, when you are testing them on the same boat...and is why if you use BOLLARD of a motor to compare motor-to-motor-to-motor, you can find what you need to upgrade, or to change over to other systems...
Sorry...didn't mean to cause an uproar...just tough being retired and no engineering group to sound off to...kinda had to talk myself into the right answer on this one, and now I have found what I required.
Looks like to replace my 20-hp outboard, I will need 3-quantity 115-lb thrust trolling motors, tied with mated linkages...connected to a 6-cell deep cycle bank and 20-amps of solar cell charging overhead on the fixed canopy, with inline charging regulator that can handle up to 600-watts . <wow...gonna look like the African Queen, when I am done!> LOL
I hope SOMEONE other than myself found this 'enlightening'...
DCockey
12-12-2010, 10:43 PM
...
Now, if my boat, ship, etc., does 4-kts with its current engine, then it will yield a specific BOLLARD PULL test value in POUNDS-FORCE (Newtons, etc. - pick your UOM). Now, if I put another motor on this SAME VESSEL, I have not changed the hull design, the slipstream, the coefficient of drag, NOTHING. Therefore, logic says, that if I put a new motor onto a flatbottom boat, a ski-boat, etc., and BOLLARD PULL it against a tied cleat, that the new, unknown motor, which I tested in STATIC BOLLARD, will perform within 2-3% identical once it is mounted to my boat...right? I mean, is their a fail to my logic on this? If so, please enlighten me...why would speed even be considered in this event...I just need to know what a new motor pulls/pushes against the transom mount of ANY vessel, so as to replicate 100% of my current motor's mechanical effect to my transom...anything else considered is superfluous to the engineering design or boat effects.
.....
Different engine/motor, gear ratio and propeller combinations will have different curves of thrust vs boat speed. In general if two combinations happen to have the same thrust at zero speed (bollard pull) then the thrust at non-zero speeds will be different. That's why speed matters.
At sufficiently slow boat speeds the curves will be close to each other if the bollard pull is the same at zero boat speed. How slow is sufficiently slow depends on the particular combinations involved and how close is close enough.
So if your are only interested in slow enough speeds your logic is okay, otherwise it's wrong.
If you're convinced you are right then go ahead and replace the outboard with the electric motors. As long as you're satisfied with going slowly then it may work out.
Now, if my boat, ship, etc., does 4-kts with its current engine, then it will yield a specific BOLLARD PULL test value in POUNDS-FORCE (Newtons, etc. - pick your UOM). Now, if I put another motor on this SAME VESSEL, I have not changed the hull design, the slipstream, the coefficient of drag, NOTHING. Therefore, logic says, that if I put a new motor onto a flatbottom boat, a ski-boat, etc., and BOLLARD PULL it against a tied cleat, that the new, unknown motor, which I tested in STATIC BOLLARD, will perform within 2-3% identical once it is mounted to my boat...right? I mean, is their a fail to my logic on this? If so, please enlighten me...why would speed even be considered in this event...I just need to know what a new motor pulls/pushes against the transom mount of ANY vessel, so as to replicate 100% of my current motor's mechanical effect to my transom...anything else considered is superfluous to the engineering design or boat effects.
.....
Different engine/motor, gear ratio and propeller combinations will have different curves of thrust vs boat speed. In general if two combinations happen to have the same thrust at zero speed (bollard pull) then the thrust at non-zero speeds will be different. That's why speed matters.
At sufficiently slow boat speeds the curves will be close to each other if the bollard pull is the same at zero boat speed. How slow is sufficiently slow depends on the particular combinations involved and how close is close enough.
So if your are only interested in slow enough speeds your logic is okay, otherwise it's wrong.
If you're convinced you are right then go ahead and replace the outboard with the electric motors. As long as you're satisfied with going slowly then it may work out.
Submarine Tom
12-12-2010, 10:49 PM
Well, it sounds to me like you've answered your question.
Personally I think you are mistaken but I don't believe you are really interested in what I think, and that's okay, it may be me that is mistaken.
All the best to you in your efforts and, please, let us know how it turns out.
-Tom
Personally I think you are mistaken but I don't believe you are really interested in what I think, and that's okay, it may be me that is mistaken.
All the best to you in your efforts and, please, let us know how it turns out.
-Tom
DCockey
12-12-2010, 10:52 PM
The Yachting Montly comparison test from March 2009 on the link which Joakim posted provides some interesting data. His points are correct.
I extracted the bollard pull and top speed for each motor.
Gasoline Bollard Pull Top Speed
Honda BF 2.3 13 4.5
Suzuki 2.5 35 5.9
Tohatsu S3.5 45 8.8
Mercury 17 5.4
Parsun 27 4.4
Electric Bollard Pull Top Speed
Minni Kota Riptide 55 17 2.5
Flover 55TGS 13 2.5
Torqeedo Cruise 2.0 54 4.9
Torqeedo Travel 801 31 4
At the same speed the thrust of the engine on the boat will be the same. If the speed is higher the thrust will be higher. It's clear the relationship between bollard pull and thrust at speed is different for the electric outboards compared to the gasoline outboards.
I extracted the bollard pull and top speed for each motor.
Gasoline Bollard Pull Top Speed
Honda BF 2.3 13 4.5
Suzuki 2.5 35 5.9
Tohatsu S3.5 45 8.8
Mercury 17 5.4
Parsun 27 4.4
Electric Bollard Pull Top Speed
Minni Kota Riptide 55 17 2.5
Flover 55TGS 13 2.5
Torqeedo Cruise 2.0 54 4.9
Torqeedo Travel 801 31 4
At the same speed the thrust of the engine on the boat will be the same. If the speed is higher the thrust will be higher. It's clear the relationship between bollard pull and thrust at speed is different for the electric outboards compared to the gasoline outboards.
Submarine Tom
12-12-2010, 11:00 PM
Ah, well, if it's in print in Yachting Monthly then it just has to be true!
-Tom
-Tom
DCockey
12-12-2010, 11:05 PM
Plot of speed vs Bollard Pull from the Yachting Monthly comparison test.
DCockey
12-12-2010, 11:10 PM
Well...what can you say, it is all about the media, hype, and lies. In the end, any engineer will tell you, same boat, same hull, then bollard 350-lbs beats bollard 320-lbs, every single time...and the 350-lbs is stronger AND faster, when you are testing them on the same boat...and is why if you use BOLLARD of a motor to compare motor-to-motor-to-motor, you can find what you need to upgrade, or to change over to other systems...
I'm an engineer and I won't tell you that. 350 lbs of bollard pull will beat 320 lbs of bollard pull in a tug-of-war, but it doesn't indicate which one will push the boat faster.
See the chart posted I posted above which is a test of a variety of engines/motors on a single boat.
I'm an engineer and I won't tell you that. 350 lbs of bollard pull will beat 320 lbs of bollard pull in a tug-of-war, but it doesn't indicate which one will push the boat faster.
See the chart posted I posted above which is a test of a variety of engines/motors on a single boat.
Mr Efficiency
12-13-2010, 12:03 AM
In any event, running a petrol outboard at its max will kill any economy, so assuming the electric can operate to the max without a similar penalty, you have to compare them with that in mind. The figures quoted about 1 hp = 80 lbs or whatever are clearly ridiculous, That would mean even allowing 50% conversion of HP to prop thrust giving about 8000 lbs of thrust with a 200 hp outboard when gunned hard........don't think it would be 25% of that, with change.
Joakim
12-13-2010, 02:31 AM
Ah, well, if it's in print in Yachting Monthly then it just has to be true!
It is of course true one shouldn't trust everything printed on a magazine, but it shouldn't really be hard to measure bollard pull and top speed. All you need is a force scale and a GPS. Do you think this is too complicated for Yachting Monthly crew?
It is of course true one shouldn't trust everything printed on a magazine, but it shouldn't really be hard to measure bollard pull and top speed. All you need is a force scale and a GPS. Do you think this is too complicated for Yachting Monthly crew?
Joakim
12-13-2010, 02:48 AM
Now, if my boat, ship, etc., does 4-kts with its current engine, then it will yield a specific BOLLARD PULL test value in POUNDS-FORCE (Newtons, etc. - pick your UOM). Now, if I put another motor on this SAME VESSEL, I have not changed the hull design, the slipstream, the coefficient of drag, NOTHING. Therefore, logic says, that if I put a new motor onto a flatbottom boat, a ski-boat, etc., and BOLLARD PULL it against a tied cleat, that the new, unknown motor, which I tested in STATIC BOLLARD, will perform within 2-3% identical once it is mounted to my boat...right?
No! That is not the way it works. Trolling and most other electrical motors are especially good at bollard pull, that is ZERO speed NOT 4 kts. Probably their propellers are designed for that and also an electrical motor can deliver its max torque at low (even zero) rpm. If you read the Yachting Monthly or any other test which measures bollard pull and top speed on any vessel, you would know this.
Your logic probably works quite well for bigger gas outboards, that are designed and propped for much higher speeds (15+ kts). Those will likely have about the same thrust at 0 and 4 kts. But this is not true for trolling motors, which loose their thrust rapidly with increasing speed.
Remember also that there are special high-thrust models/propellers for gas outboards. These will be much better at 4 kts than the standard models/propellers designed for higher speeds.
No! That is not the way it works. Trolling and most other electrical motors are especially good at bollard pull, that is ZERO speed NOT 4 kts. Probably their propellers are designed for that and also an electrical motor can deliver its max torque at low (even zero) rpm. If you read the Yachting Monthly or any other test which measures bollard pull and top speed on any vessel, you would know this.
Your logic probably works quite well for bigger gas outboards, that are designed and propped for much higher speeds (15+ kts). Those will likely have about the same thrust at 0 and 4 kts. But this is not true for trolling motors, which loose their thrust rapidly with increasing speed.
Remember also that there are special high-thrust models/propellers for gas outboards. These will be much better at 4 kts than the standard models/propellers designed for higher speeds.
Mr Efficiency
12-13-2010, 03:10 AM
Remember also that there are special high-thrust models/propellers for gas outboards. These will be much better at 4 kts than the standard models/propellers designed for higher speeds.
There are props for the low speed thrust application, but the diameter is pretty limited and well short of ideal for the purpose. Gearcase ratios, too, are restrictive.
There are props for the low speed thrust application, but the diameter is pretty limited and well short of ideal for the purpose. Gearcase ratios, too, are restrictive.
Joakim
12-13-2010, 05:58 AM
There are props for the low speed thrust application, but the diameter is pretty limited and well short of ideal for the purpose. Gearcase ratios, too, are restrictive.
Certainly they are not as good as well designed inboards, but still they can be quite much better than a standard OB. E.g. Yamaha has two 9.9hp models, with likely the same upper part. The standard one (F9.9) has a 1:2.08 gear ratio and 8 1/2" diameter standard propeller. The high thrust (T9.9) one has a 1:2.92 gear ratio and 11 3/4" diameter propeller.
After a quick calculation with a propeller optimization software I would guess that the high thrust one would have thrust of about 1000 N (220 lbf) at 4 kts, which is about the same as my Yanmar 1GM10 diesel inboard.
The standard one is much harder to estimate, since it suffers from severe cavitation far before the engine is limiting the thrust. The cavitation begins already at about 300 N (70 lbf) thrust at 4 kts. This requires only 2 hp from the engine. The high thrust propeller did not show any cavitation at 1000 N!
Then the standard one with a lower pitch propeller (8 1/2 instead of 9 1/4) performs clearly better. It can deliver about 750 N (170 lbf) of thrust without severe cavitation at 4 kts.
Torqeedo 2.0 in the YM test has a 12x10 propeller, thus almost the same as Yamaha T9.9. According to the propeller software it should deliver about 450 N (100 lbf) of thrust at 4 kts at 2 kW power (no cavitation). At 2 kn it can deliver the measured 530 N (120 lbf) at 2 kW power. In a Finnish boat magazine it even delivered 62 kg (610 N, 140 lbf) bollard pull, but then the current at 24 V was 112 A, thus 2.7 kW. At full speed (5.0 kts) it took 88 A (2.1 kW).
Then a 20 hp outboard. Yamaha F20 with high thrust propeller (9 3/4 x 8) should provide about 1600 N (360 lbf) at 4 kts. With the standard propeller (9 1/2 x 12) severe cavitation would start already at 500 N (110 lbf), but I guess it could still provide more than 1000 N (220 lbf) of thrust.
Certainly they are not as good as well designed inboards, but still they can be quite much better than a standard OB. E.g. Yamaha has two 9.9hp models, with likely the same upper part. The standard one (F9.9) has a 1:2.08 gear ratio and 8 1/2" diameter standard propeller. The high thrust (T9.9) one has a 1:2.92 gear ratio and 11 3/4" diameter propeller.
After a quick calculation with a propeller optimization software I would guess that the high thrust one would have thrust of about 1000 N (220 lbf) at 4 kts, which is about the same as my Yanmar 1GM10 diesel inboard.
The standard one is much harder to estimate, since it suffers from severe cavitation far before the engine is limiting the thrust. The cavitation begins already at about 300 N (70 lbf) thrust at 4 kts. This requires only 2 hp from the engine. The high thrust propeller did not show any cavitation at 1000 N!
Then the standard one with a lower pitch propeller (8 1/2 instead of 9 1/4) performs clearly better. It can deliver about 750 N (170 lbf) of thrust without severe cavitation at 4 kts.
Torqeedo 2.0 in the YM test has a 12x10 propeller, thus almost the same as Yamaha T9.9. According to the propeller software it should deliver about 450 N (100 lbf) of thrust at 4 kts at 2 kW power (no cavitation). At 2 kn it can deliver the measured 530 N (120 lbf) at 2 kW power. In a Finnish boat magazine it even delivered 62 kg (610 N, 140 lbf) bollard pull, but then the current at 24 V was 112 A, thus 2.7 kW. At full speed (5.0 kts) it took 88 A (2.1 kW).
Then a 20 hp outboard. Yamaha F20 with high thrust propeller (9 3/4 x 8) should provide about 1600 N (360 lbf) at 4 kts. With the standard propeller (9 1/2 x 12) severe cavitation would start already at 500 N (110 lbf), but I guess it could still provide more than 1000 N (220 lbf) of thrust.
powerabout
12-13-2010, 08:11 AM
I dont think the 4kts comparison is a fair one as you are now relying on the torque curve of the engine assuming it is revving higher than it was at zero speed bollard pull test.
Although I guess if you are buying an engine for a yacht you might be interested in that figure although I think I would want to know what thrust at 5 to 6kts
Although I guess if you are buying an engine for a yacht you might be interested in that figure although I think I would want to know what thrust at 5 to 6kts
Muerte
12-14-2010, 09:27 PM
I'm an engineer and I won't tell you that. 350 lbs of bollard pull will beat 320 lbs of bollard pull in a tug-of-war, but it doesn't indicate which one will push the boat faster.
@DCockey, I am old and retired, but I really was an engineer for many years, too. My M.S. in Mechanical is from Oklahoma State, my B.S. is from Maine...and while my penchant was structural design (yes, I worked for a major manufacturer - OEM - of fiberglass-hull saltwaters), I did work enough with 'prop' guys to have a little insight into the 'black-arts' of prop design and related hassles.
First off, guys, please re-read the 15-or-so replies since I last posted...there are 10-'experts' countering every single thing another says...understand, laypeople see this and only think one thing...there are no experts here...and they don't have a clue. Some of the stuff I have seen in direct response to what I have asked was wrong...but I understand how you got there...ok...but it conflicting with another without explanation is confusing.
Ok, THRUST AND POWER...let's consider it this way: On a 'slow-mover' (tug, barge, pontoon, etc.) let's accept a few of the 'slow motion group laws' work regardless of vessel...in other words, we aren't talking about speedboats on this one...so we will then stay in one class of propeller profile...
Ok, Now, let's take a trip to the Erie Canal, circa 1820...got mules pulling barge-skiffs up the erie...at about 1-kt. Ok, now if you put that practice to play in our BOLLARD discussion, you would see that whether you have a mule connected to a linear scale, connected to your bow plane, or you have a linear scale, connected to your transom, connected to a concrete mooring cleat to your aft, if you exert 350-lbs on the scale, then you are exerting the same amount of force in either direction.
Ok, now if you could keep measuring that motor-thrust in a dynamic BOLLARD at various speeds, you would find that as you departed static BOLLARD and reached 1-kt, 5-kts, 10-kts...that motor at stable-throttle, is still pushing on that boat's transom with the same forces +/- 7%, as when it was tied to a cleat at the dock. Ok, then that means as you approach velocity, you are approaching equilibrium where the friction and hydraulic forces are countering. This is nothing but Newton's 3rd Law of Motion applied. Ok, so for my application, a pontoon boat, a 20-hp outboard hits equilibrium at 4-kts, at acceptable throttle.
Ok, if my 20-hp is producing around 310-335 lbs. of BOLLARD pull, then answer this: If I hook up a team of horses on the side of the river, and have them pull me forward (with a scale in-line of the tow line) while keeping a steady forward load of 330-lbs of force on the line...(discount angular vectors of the rope...no need for that in this proposition), then are there any here who will bet that the boat will go upstream at 4-kts, the SAME VELOCITY as if the outboard was running? Again, since the hull didn't change, and the water profile did not change, and the same pull force has been applied, albeit in a different format, would logic not conclude that +/-5% (allowing for subtle hydraulic changes that would happen around the propeller that will not happen to the horses), the craft is going to go up/down the river in the same way, at the same speed, whether it is by being tethered to horses on the bank, or by a mercury 20 on the stern?
Again...yes...this DOES break down at velocity...when you start hydroplaning a skin atop water, when your prop tips start approaching mach .8, when you have prop cavitation occurring...yes...agreed...but this is not a 1000-hp Scarrab, but merely a 20-hp pontoon boat...
Guys...I tell you...when the margin of error is less than a half-knot for the example given...
Well, sometimes you just have to remember the case-example.
@DCockey, I am old and retired, but I really was an engineer for many years, too. My M.S. in Mechanical is from Oklahoma State, my B.S. is from Maine...and while my penchant was structural design (yes, I worked for a major manufacturer - OEM - of fiberglass-hull saltwaters), I did work enough with 'prop' guys to have a little insight into the 'black-arts' of prop design and related hassles.
First off, guys, please re-read the 15-or-so replies since I last posted...there are 10-'experts' countering every single thing another says...understand, laypeople see this and only think one thing...there are no experts here...and they don't have a clue. Some of the stuff I have seen in direct response to what I have asked was wrong...but I understand how you got there...ok...but it conflicting with another without explanation is confusing.
Ok, THRUST AND POWER...let's consider it this way: On a 'slow-mover' (tug, barge, pontoon, etc.) let's accept a few of the 'slow motion group laws' work regardless of vessel...in other words, we aren't talking about speedboats on this one...so we will then stay in one class of propeller profile...
Ok, Now, let's take a trip to the Erie Canal, circa 1820...got mules pulling barge-skiffs up the erie...at about 1-kt. Ok, now if you put that practice to play in our BOLLARD discussion, you would see that whether you have a mule connected to a linear scale, connected to your bow plane, or you have a linear scale, connected to your transom, connected to a concrete mooring cleat to your aft, if you exert 350-lbs on the scale, then you are exerting the same amount of force in either direction.
Ok, now if you could keep measuring that motor-thrust in a dynamic BOLLARD at various speeds, you would find that as you departed static BOLLARD and reached 1-kt, 5-kts, 10-kts...that motor at stable-throttle, is still pushing on that boat's transom with the same forces +/- 7%, as when it was tied to a cleat at the dock. Ok, then that means as you approach velocity, you are approaching equilibrium where the friction and hydraulic forces are countering. This is nothing but Newton's 3rd Law of Motion applied. Ok, so for my application, a pontoon boat, a 20-hp outboard hits equilibrium at 4-kts, at acceptable throttle.
Ok, if my 20-hp is producing around 310-335 lbs. of BOLLARD pull, then answer this: If I hook up a team of horses on the side of the river, and have them pull me forward (with a scale in-line of the tow line) while keeping a steady forward load of 330-lbs of force on the line...(discount angular vectors of the rope...no need for that in this proposition), then are there any here who will bet that the boat will go upstream at 4-kts, the SAME VELOCITY as if the outboard was running? Again, since the hull didn't change, and the water profile did not change, and the same pull force has been applied, albeit in a different format, would logic not conclude that +/-5% (allowing for subtle hydraulic changes that would happen around the propeller that will not happen to the horses), the craft is going to go up/down the river in the same way, at the same speed, whether it is by being tethered to horses on the bank, or by a mercury 20 on the stern?
Again...yes...this DOES break down at velocity...when you start hydroplaning a skin atop water, when your prop tips start approaching mach .8, when you have prop cavitation occurring...yes...agreed...but this is not a 1000-hp Scarrab, but merely a 20-hp pontoon boat...
Guys...I tell you...when the margin of error is less than a half-knot for the example given...
Well, sometimes you just have to remember the case-example.
Muerte
12-14-2010, 09:39 PM
By the way...there is also the old addage, "if you profess I am wrong, then after you condemn my words, be so kind as to offer up the solution"
For all those advanced of power and propulsion insight...I give you this scenario, and ask for your solution:
I have a 28-ft pontoon craft, all aluminum, loaded with gear, equipment and persons, total mass approaches 1800-lbs. It is pushed by a new-ish Mercury 20-HP 4-cycle outboard. Top speed on calm waters is approximately 4-kts, at 4/5-throttle (what I consider WOT for this motor).
I want to change to electric motors, probably some form of trolling motor(s) due to their ready-availability to market, and require either the specific motor(s) or understanding of how to fit up with them so as to 100% replace my outboard. Ignore added mass of new batteries required to run it...battery mass will replace fuel tank.
Now...all those unequivocally stating I am wrong, please provide the mated answer to this question - WITH SHOWING YOUR WORK of how you came to this conclusion. if you can't solve the question, then don't ridicule my resolution of Y=15.7x + 35, as the formula to transfer horsepower to electric-motor-thrust...I have already provided my answer...but if there actually is a commenting ridiculer here who has more data, greater insight, and a penchant to actually resolve this (without fitting 20-different motors to my vessel...any yard-ape can do that) then please, provide your insight. My already-calculated solution is: 3-quantity hi-thrust 115-lb trollers mounted with mated linkage...which is a $3000 chunk of change...that will require 100-days of hard use to offset the acquisition...at replacing $30 of petro a day....
I am most-interested in anyone's actual working solution on this...
For all those advanced of power and propulsion insight...I give you this scenario, and ask for your solution:
I have a 28-ft pontoon craft, all aluminum, loaded with gear, equipment and persons, total mass approaches 1800-lbs. It is pushed by a new-ish Mercury 20-HP 4-cycle outboard. Top speed on calm waters is approximately 4-kts, at 4/5-throttle (what I consider WOT for this motor).
I want to change to electric motors, probably some form of trolling motor(s) due to their ready-availability to market, and require either the specific motor(s) or understanding of how to fit up with them so as to 100% replace my outboard. Ignore added mass of new batteries required to run it...battery mass will replace fuel tank.
Now...all those unequivocally stating I am wrong, please provide the mated answer to this question - WITH SHOWING YOUR WORK of how you came to this conclusion. if you can't solve the question, then don't ridicule my resolution of Y=15.7x + 35, as the formula to transfer horsepower to electric-motor-thrust...I have already provided my answer...but if there actually is a commenting ridiculer here who has more data, greater insight, and a penchant to actually resolve this (without fitting 20-different motors to my vessel...any yard-ape can do that) then please, provide your insight. My already-calculated solution is: 3-quantity hi-thrust 115-lb trollers mounted with mated linkage...which is a $3000 chunk of change...that will require 100-days of hard use to offset the acquisition...at replacing $30 of petro a day....
I am most-interested in anyone's actual working solution on this...
kerosene
12-14-2010, 11:44 PM
I don't know what ridiculing you are talking about.
Joakim's last answer was very accurate and good.
I think where the disconnect with you and the "others" is that when they say that speed makes an impact you assume that speed needs to be 30knots or something (speed boat per your words) to make a difference. When in fact 0 knots and 4 knots is already very different situation.
in short you can have a 10 hp engine-prop-combo that provides less thrust at 0 knots than another 2hp engine-prop pair. Yet the 10 hp motor could easily have much much more thrust at 4 knots.
The propeller is effectively a transmission. You cannot ask how many hp certain amount of Nm on the wheel of a car equals to. 100hp from engine car can have 10Nm at the wheel or 400nm at the wheel (assuming that you could have whatever gear ratios you choose). You could make a pedal powered vehicle that would win a big 4x4 in tug of war - might take a day to pedal an inch but you can gear small power to enormous force. Prop is not that different.
there is no 'slow motion group laws' as you put it. different speed is different speed and as the thrust formula is:
power (watts) = thrust (newtons) x speed (metres/second)
you can see that 1 knot and 2 knots is quite a different situation.
edit: typos
Joakim's last answer was very accurate and good.
I think where the disconnect with you and the "others" is that when they say that speed makes an impact you assume that speed needs to be 30knots or something (speed boat per your words) to make a difference. When in fact 0 knots and 4 knots is already very different situation.
in short you can have a 10 hp engine-prop-combo that provides less thrust at 0 knots than another 2hp engine-prop pair. Yet the 10 hp motor could easily have much much more thrust at 4 knots.
The propeller is effectively a transmission. You cannot ask how many hp certain amount of Nm on the wheel of a car equals to. 100hp from engine car can have 10Nm at the wheel or 400nm at the wheel (assuming that you could have whatever gear ratios you choose). You could make a pedal powered vehicle that would win a big 4x4 in tug of war - might take a day to pedal an inch but you can gear small power to enormous force. Prop is not that different.
there is no 'slow motion group laws' as you put it. different speed is different speed and as the thrust formula is:
power (watts) = thrust (newtons) x speed (metres/second)
you can see that 1 knot and 2 knots is quite a different situation.
edit: typos
Submarine Tom
12-15-2010, 02:22 AM
You certainly seem to already have all your answers.
What do predict for a completion date?
All the best to you in your efforts and, please, let us know how it turns out and be sure to get lots of pictures.
-Tom
What do predict for a completion date?
All the best to you in your efforts and, please, let us know how it turns out and be sure to get lots of pictures.
-Tom
Joakim
12-15-2010, 07:57 AM
I have a 28-ft pontoon craft, all aluminum, loaded with gear, equipment and persons, total mass approaches 1800-lbs. It is pushed by a new-ish Mercury 20-HP 4-cycle outboard. Top speed on calm waters is approximately 4-kts, at 4/5-throttle (what I consider WOT for this motor).
Ignore added mass of new batteries required to run it...battery mass will replace fuel tank.
Why are you only getting 4 kts? 1800 lbs is not heavy and 28 ft vessel should easily reach 6 kts with much less than 20 hp. Here are some examples of heavier, but shorter, pontoon boats going 14.5 kts with a similar motor:
http://www.yamaha-motor.com/assets/products/otb/bulletins/bulletin_4stroke_midthrustjetport_al_sa53be.tmp.pdf
http://www.yamaha-motor.com/assets/products/otb/bulletins/bulletin_4stroke_highthrust_d-t_sa9f3c.tmp.pdf
This is about the same size, but much heavier. Note that it reaches 3.8 kts with just 0.7 GPH, which is equal to about 7 hp (probably less from a 150 hp engine). With a 20 hp OB, this should go about 10 kts.
http://www.yamaha-motor.com/assets/products/otb/bulletins/bulletin_4stroke_hpv6_bss_ava_29excalibur_f150tlr_2010-08-30_pnt.pdf
So what kind of boat do you actually have? And what propeller do you have on your Mercury? Why would your boat have a resistance of over 300 lbf (1300 N) already at 4 kts? Even a 65 ft 35 tn sailboat wouldn't have that much drag at that speed (at 5.3 kts it would).
Battery mass is not even close to gasoline fuel mass. You need about 100 times more batteries in weight than gasoline to do the same job. Thus your 6 gallon tank needs to be replaced with about 2 tn (4400 lbs) of lead-acid batteries.
Ignore added mass of new batteries required to run it...battery mass will replace fuel tank.
Why are you only getting 4 kts? 1800 lbs is not heavy and 28 ft vessel should easily reach 6 kts with much less than 20 hp. Here are some examples of heavier, but shorter, pontoon boats going 14.5 kts with a similar motor:
http://www.yamaha-motor.com/assets/products/otb/bulletins/bulletin_4stroke_midthrustjetport_al_sa53be.tmp.pdf
http://www.yamaha-motor.com/assets/products/otb/bulletins/bulletin_4stroke_highthrust_d-t_sa9f3c.tmp.pdf
This is about the same size, but much heavier. Note that it reaches 3.8 kts with just 0.7 GPH, which is equal to about 7 hp (probably less from a 150 hp engine). With a 20 hp OB, this should go about 10 kts.
http://www.yamaha-motor.com/assets/products/otb/bulletins/bulletin_4stroke_hpv6_bss_ava_29excalibur_f150tlr_2010-08-30_pnt.pdf
So what kind of boat do you actually have? And what propeller do you have on your Mercury? Why would your boat have a resistance of over 300 lbf (1300 N) already at 4 kts? Even a 65 ft 35 tn sailboat wouldn't have that much drag at that speed (at 5.3 kts it would).
Battery mass is not even close to gasoline fuel mass. You need about 100 times more batteries in weight than gasoline to do the same job. Thus your 6 gallon tank needs to be replaced with about 2 tn (4400 lbs) of lead-acid batteries.
Muerte
12-15-2010, 02:59 PM
Why are you only getting 4 kts? 1800 lbs is not heavy and 28 ft vessel should easily reach 6 kts with much less than 20 hp. Here are some examples of heavier, but shorter, pontoon boats going 14.5 kts with a similar motor:
http://www.yamaha-motor.com/assets/products/otb/bulletins/bulletin_4stroke_midthrustjetport_al_sa53be.tmp.pdf
http://www.yamaha-motor.com/assets/products/otb/bulletins/bulletin_4stroke_highthrust_d-t_sa9f3c.tmp.pdf
My actual craft goes a bit faster than 4.0 kts, but again, no one is able to get out of quoting textbooks to give a valid answer, does it really matter?
So what kind of boat do you actually have? And what propeller do you have on your Mercury? Why would your boat have a resistance of over 300 lbf (1300 N) already at 4 kts? Even a 65 ft 35 tn sailboat wouldn't have that much drag at that speed (at 5.3 kts it would).
I have a 1972 Harrison, 28' with 8' beam, and 4 cyc 20-hp Merc.
Ok, I am not sure where anyone is getting their 'pontoon speed data' from...I've checked other pontoon sources, and so far have found a 50hp Johnson on a 22' vessel going 18 @ WOT, a 20' Harris Sunliner with 40hp Johnson running 17 @ WOT, and a 22' Sweetwater with an amazing 150-hp yamaha going 34.7 @ WOT. Now, by comparison, I have a 28' triple pont, with a total loaded mass of closer to 2800-lbs loaded, pushed by a 20-HP Merc...and in reality is closer to 6-6.5kts than 4.0.
It seems that so many are hung up on the word 'speed' and 'prop' that no one can yield an answer for a simple question...let me try it again...I may have used big words the first time:
Boat...take gas motor off...put electric motors on...with minimal loss of power (ideally, I would like a 100% match of thrust here), what electric motors do I need to put on to MATCH the 20-HP gas motor I am taking off. I have given you 20-HP, gas motor, BOLLARD PULL of 320-lbs. This value seems to be also represented in other publications, for a 20-HP OEM Mercury, with OEM general general use aluminum propeller. Now...
Can you answer this?
As I already have a hypothesis that the answer is 3-quantity 115-lb thrust trolling motors...is this right? Is this wrong? If wrong, then what is the right answer, and how did you come to such answer?
To Heckle as Submarine Tom does, does not impress. I am asking a valid question that seems to draw contempt. If my question, and following statement above is incorrect, all I ask, legitimately, is the corrected answer with justification of how you came to it, be provided.
Battery mass is not even close to gasoline fuel mass. You need about 100 times more batteries in weight than gasoline to do the same job. Thus your 6 gallon tank needs to be replaced with about 2 tn (4400 lbs) of lead-acid batteries.
Wow...I see you don't know a lot about energy. Ok, lead-acid batteries, standard deep cycle batteries (I prefer AGM-type) are around 72-lbs per, so six of them will weigh about 432-lbs. Six good AGM batteries would power a single 115-lb thrust trolling motor for around 20-25 hours (as one would work for 3-4 hours)...now, divide 24-hours by 3 (the number of motors I THINK I will need) and you end up with 8-hours until the battery-bank is dead. Supplant that with under-way recharging, and you might extend that by 2-3 hours...so it is close to say you should be able to get 8-10 hours of use from such a battery bank. That isn't bad. At the same time, my vessel has a 60-gallon tank...which filled with petro at 6.5-lbs per gallon, means that I would carry 390-lbs of fuel at top-off. So, the only major difference is that as the gas is depleted from my tanks, I run higher in the water, hence, less drag...and I would not use the full tank in a 10-hour period...more like a third of it...
BUT...at the end of the day, I DO have to top my tank off again, where-as, with electrics on a battery-bank, I just plug up the bank overnight...or let it sit in the sun and charge for a day, and there is no more expense.
WOW...I really figured there were people here on this board trying to design new craft, and overcome concept limits...but clearly, there are 90% nay-sayers trying to explain theory and how it 'can't work' while the other 10% are left scratching their heads...
What a bunch...indeed. Hey, Submarine Tom, if you want the answer, I recommend you try it yourself...assuming you understand solid state circuitry as well as physics. Good luck, I am sure you will share your testing information with the class, once completed...after all, such a conversion has clearly been done and the data is published for all others to see and clearly understand...
Wait...no, I am mistaken...it hasn't been...but there are a lot that sure seem to have the answer already, and are clearly hording it for themselves...yet I haven't seen any thrust or pull data on any of this. Clearly it is highly classified...
Wow...are you guys going to have a lot of trouble when you can't find gasoline for your boats, or can't afford it when you do...after all, $200 barrel oil is right around the corner...again...
http://www.yamaha-motor.com/assets/products/otb/bulletins/bulletin_4stroke_midthrustjetport_al_sa53be.tmp.pdf
http://www.yamaha-motor.com/assets/products/otb/bulletins/bulletin_4stroke_highthrust_d-t_sa9f3c.tmp.pdf
My actual craft goes a bit faster than 4.0 kts, but again, no one is able to get out of quoting textbooks to give a valid answer, does it really matter?
So what kind of boat do you actually have? And what propeller do you have on your Mercury? Why would your boat have a resistance of over 300 lbf (1300 N) already at 4 kts? Even a 65 ft 35 tn sailboat wouldn't have that much drag at that speed (at 5.3 kts it would).
I have a 1972 Harrison, 28' with 8' beam, and 4 cyc 20-hp Merc.
Ok, I am not sure where anyone is getting their 'pontoon speed data' from...I've checked other pontoon sources, and so far have found a 50hp Johnson on a 22' vessel going 18 @ WOT, a 20' Harris Sunliner with 40hp Johnson running 17 @ WOT, and a 22' Sweetwater with an amazing 150-hp yamaha going 34.7 @ WOT. Now, by comparison, I have a 28' triple pont, with a total loaded mass of closer to 2800-lbs loaded, pushed by a 20-HP Merc...and in reality is closer to 6-6.5kts than 4.0.
It seems that so many are hung up on the word 'speed' and 'prop' that no one can yield an answer for a simple question...let me try it again...I may have used big words the first time:
Boat...take gas motor off...put electric motors on...with minimal loss of power (ideally, I would like a 100% match of thrust here), what electric motors do I need to put on to MATCH the 20-HP gas motor I am taking off. I have given you 20-HP, gas motor, BOLLARD PULL of 320-lbs. This value seems to be also represented in other publications, for a 20-HP OEM Mercury, with OEM general general use aluminum propeller. Now...
Can you answer this?
As I already have a hypothesis that the answer is 3-quantity 115-lb thrust trolling motors...is this right? Is this wrong? If wrong, then what is the right answer, and how did you come to such answer?
To Heckle as Submarine Tom does, does not impress. I am asking a valid question that seems to draw contempt. If my question, and following statement above is incorrect, all I ask, legitimately, is the corrected answer with justification of how you came to it, be provided.
Battery mass is not even close to gasoline fuel mass. You need about 100 times more batteries in weight than gasoline to do the same job. Thus your 6 gallon tank needs to be replaced with about 2 tn (4400 lbs) of lead-acid batteries.
Wow...I see you don't know a lot about energy. Ok, lead-acid batteries, standard deep cycle batteries (I prefer AGM-type) are around 72-lbs per, so six of them will weigh about 432-lbs. Six good AGM batteries would power a single 115-lb thrust trolling motor for around 20-25 hours (as one would work for 3-4 hours)...now, divide 24-hours by 3 (the number of motors I THINK I will need) and you end up with 8-hours until the battery-bank is dead. Supplant that with under-way recharging, and you might extend that by 2-3 hours...so it is close to say you should be able to get 8-10 hours of use from such a battery bank. That isn't bad. At the same time, my vessel has a 60-gallon tank...which filled with petro at 6.5-lbs per gallon, means that I would carry 390-lbs of fuel at top-off. So, the only major difference is that as the gas is depleted from my tanks, I run higher in the water, hence, less drag...and I would not use the full tank in a 10-hour period...more like a third of it...
BUT...at the end of the day, I DO have to top my tank off again, where-as, with electrics on a battery-bank, I just plug up the bank overnight...or let it sit in the sun and charge for a day, and there is no more expense.
WOW...I really figured there were people here on this board trying to design new craft, and overcome concept limits...but clearly, there are 90% nay-sayers trying to explain theory and how it 'can't work' while the other 10% are left scratching their heads...
What a bunch...indeed. Hey, Submarine Tom, if you want the answer, I recommend you try it yourself...assuming you understand solid state circuitry as well as physics. Good luck, I am sure you will share your testing information with the class, once completed...after all, such a conversion has clearly been done and the data is published for all others to see and clearly understand...
Wait...no, I am mistaken...it hasn't been...but there are a lot that sure seem to have the answer already, and are clearly hording it for themselves...yet I haven't seen any thrust or pull data on any of this. Clearly it is highly classified...
Wow...are you guys going to have a lot of trouble when you can't find gasoline for your boats, or can't afford it when you do...after all, $200 barrel oil is right around the corner...again...
Muerte
12-15-2010, 03:18 PM
Why are you only getting 4 kts? 1800 lbs is not heavy and 28 ft vessel should easily reach 6 kts with much less than 20 hp. Here are some examples of heavier, but shorter, pontoon boats going 14.5 kts with a similar motor:
http://www.yamaha-motor.com/assets/products/otb/bulletins/bulletin_4stroke_midthrustjetport_al_sa53be.tmp.pdf
http://www.yamaha-motor.com/assets/products/otb/bulletins/bulletin_4stroke_highthrust_d-t_sa9f3c.tmp.pdf
WOW...I just checked your offered PDFs there...
Those are 18 and 19' boats, with 300-lbs of people on them...Now, add 10-more-ft of boat another pontoon/fuel cell, and 1200-lbs of gear and equipment...oh, yeah, and another 4-inches of draft because of it...also, they are only carrying 24-gallon cells, while mine is 60-gallon...so add about 200-lbs more fuel, too.
Again...great idea, and I see the flyer, but unless you have crystal clear flat waters, you are NOT going to get that speed from a pontoon...sorry, but that is NOT going to happen.
I am thinking that this is a 'barge driver' trying to explain such vessels to a room-full of sailboat captains.
Sorry...but I am having a distinct feeling that my questions are not in the right area...please excuse me, but I am obviously in the wrong crowd...for God help me if I talk of installing Tuna Towers on 35-footers...I think it would fall on deaf ears.
I've been around more salt water than fresh, and worked on building hundreds of vessels, designing many - and many features, from expanded-metal dive doors, to special transoms for high horsepower apps. I've fore-weighted more boats than you can imagine, to regain CG balance after installing 'big motors' caused them to nose-up and transom-flood.
My guess would be that after 30-years of drawing a paycheck for such work, I've been around more water than most of who are replying here...and I really must admit, I've not been talked down to so much since probably Marine Corps boot camp, many decades ago. I really can't figure out if you guys are that 'special' (legends in your own minds) or really don't have a clue as to the scope of the question I was asking. Something tells me that my answer is closer to being right, than your pomposity for thinking you know what you're talking about really is.
Overall...a few here have convinced me of one thing...you don't know as much as you think you do, and are convinced you know so much that there is nothing left to learn...I have surely found a forum to avoid.
Enjoy, gentlemen...you have the comm...
http://www.yamaha-motor.com/assets/products/otb/bulletins/bulletin_4stroke_midthrustjetport_al_sa53be.tmp.pdf
http://www.yamaha-motor.com/assets/products/otb/bulletins/bulletin_4stroke_highthrust_d-t_sa9f3c.tmp.pdf
WOW...I just checked your offered PDFs there...
Those are 18 and 19' boats, with 300-lbs of people on them...Now, add 10-more-ft of boat another pontoon/fuel cell, and 1200-lbs of gear and equipment...oh, yeah, and another 4-inches of draft because of it...also, they are only carrying 24-gallon cells, while mine is 60-gallon...so add about 200-lbs more fuel, too.
Again...great idea, and I see the flyer, but unless you have crystal clear flat waters, you are NOT going to get that speed from a pontoon...sorry, but that is NOT going to happen.
I am thinking that this is a 'barge driver' trying to explain such vessels to a room-full of sailboat captains.
Sorry...but I am having a distinct feeling that my questions are not in the right area...please excuse me, but I am obviously in the wrong crowd...for God help me if I talk of installing Tuna Towers on 35-footers...I think it would fall on deaf ears.
I've been around more salt water than fresh, and worked on building hundreds of vessels, designing many - and many features, from expanded-metal dive doors, to special transoms for high horsepower apps. I've fore-weighted more boats than you can imagine, to regain CG balance after installing 'big motors' caused them to nose-up and transom-flood.
My guess would be that after 30-years of drawing a paycheck for such work, I've been around more water than most of who are replying here...and I really must admit, I've not been talked down to so much since probably Marine Corps boot camp, many decades ago. I really can't figure out if you guys are that 'special' (legends in your own minds) or really don't have a clue as to the scope of the question I was asking. Something tells me that my answer is closer to being right, than your pomposity for thinking you know what you're talking about really is.
Overall...a few here have convinced me of one thing...you don't know as much as you think you do, and are convinced you know so much that there is nothing left to learn...I have surely found a forum to avoid.
Enjoy, gentlemen...you have the comm...
Submarine Tom
12-15-2010, 04:04 PM
Roger, out.
kerosene
12-15-2010, 04:12 PM
My actual craft goes a bit faster than 4.0 kts, but again, no one is able to get out of quoting textbooks to give a valid answer, does it really matter?
You have been given numerous answers. An people are naysayers because you are wrong when ignoring speed. Would you really just be happy if people said yes even when you are wrong? that seems to be the case.
I have given you 20-HP, gas motor, BOLLARD PULL of 320-lbs. This value seems to be also represented in other publications, for a 20-HP OEM Mercury, with OEM general general use aluminum propeller. Now...
Can you answer this?
As I already have a hypothesis that the answer is 3-quantity 115-lb thrust trolling motors...is this right? Is this wrong? If wrong, then what is the right answer, and how did you come to such answer?
You are absolutely right - in bollard test you can have them match if those values you have are correct. However both setups will NOT have the same top speed. Most likely the electric will fall way short of your current top speed.
To Heckle as Submarine Tom does, does not impress. I am asking a valid question that seems to draw contempt. If my question, and following statement above is incorrect, all I ask, legitimately, is the corrected answer with justification of how you came to it, be provided.
Sure Tom is heckling - but you are not much better when you call propeller and speed stuff "nonsense" or irrelevant. When IT IS NOT. You have been given numerous explanations (which happen to be correct) - instead of taking a minute of what is being said you brush it off as "text book nonsense". Your writing skills suggest you are a normal person so use your head.
YES at zero speed you can match the thrust
NO at 3 knot speed you will not have matching thrust
also:
How many horsepower equal 400 lbft ? you ignored my post with transmission analogy when its totally relevant.
WOW...I really figured there were people here on this board trying to design new craft, and overcome concept limits...but clearly, there are 90% nay-sayers trying to explain theory and how it 'can't work' while the other 10% are left scratching their heads...
Should they say it will work when it actually will not?
You have been given numerous answers. An people are naysayers because you are wrong when ignoring speed. Would you really just be happy if people said yes even when you are wrong? that seems to be the case.
I have given you 20-HP, gas motor, BOLLARD PULL of 320-lbs. This value seems to be also represented in other publications, for a 20-HP OEM Mercury, with OEM general general use aluminum propeller. Now...
Can you answer this?
As I already have a hypothesis that the answer is 3-quantity 115-lb thrust trolling motors...is this right? Is this wrong? If wrong, then what is the right answer, and how did you come to such answer?
You are absolutely right - in bollard test you can have them match if those values you have are correct. However both setups will NOT have the same top speed. Most likely the electric will fall way short of your current top speed.
To Heckle as Submarine Tom does, does not impress. I am asking a valid question that seems to draw contempt. If my question, and following statement above is incorrect, all I ask, legitimately, is the corrected answer with justification of how you came to it, be provided.
Sure Tom is heckling - but you are not much better when you call propeller and speed stuff "nonsense" or irrelevant. When IT IS NOT. You have been given numerous explanations (which happen to be correct) - instead of taking a minute of what is being said you brush it off as "text book nonsense". Your writing skills suggest you are a normal person so use your head.
YES at zero speed you can match the thrust
NO at 3 knot speed you will not have matching thrust
also:
How many horsepower equal 400 lbft ? you ignored my post with transmission analogy when its totally relevant.
WOW...I really figured there were people here on this board trying to design new craft, and overcome concept limits...but clearly, there are 90% nay-sayers trying to explain theory and how it 'can't work' while the other 10% are left scratching their heads...
Should they say it will work when it actually will not?
Joakim
12-15-2010, 05:28 PM
My actual craft goes a bit faster than 4.0 kts, but again, no one is able to get out of quoting textbooks to give a valid answer, does it really matter?
Yes, it matters, if you want to get the same performance. 6-6.5 is very different from 4.0 kts. The thrust needed for 4.0 kts is likely about 20% of the one needed for 6.5 kts. Thus you could likely get 4.0 kts with a 2.5 hp outboard. 2.5-6 hp outboards are typical for 28" 2 tn sailboats and they go 5-6 kts.
As I already have a hypothesis that the answer is 3-quantity 115-lb thrust trolling motors...is this right? Is this wrong? If wrong, then what is the right answer, and how did you come to such answer?
With the new information you have provided I think you will get about 4.0 kts with a single Torqeedo 2.0, which truly delivers that much of thrust. But you won't get the same top speed even with three of them.
Wow...I see you don't know a lot about energy. Ok, lead-acid batteries, standard deep cycle batteries (I prefer AGM-type) are around 72-lbs per, so six of them will weigh about 432-lbs. Six good AGM batteries would power a single 115-lb thrust trolling motor for around 20-25 hours.
I guess you have six about 100 Ah 12 V batteries. Torqeedo 2.0 takes about 100 A 24 V at that thrust. Thus theoretically your six batteries would be empty in 3 hours using just one motor. In practice much sooner, since Ah rating is not valid for such a high current and you don't want to use even AGM batteries 100% cycles. To run three motors three hours you need (more than) 18 batteries. That is 1300 lbs (600 kg), thus only 1/3 of the 2 tn I said earlier. That's because you only have 6 kW against 20 hp (15 kW). Your Mercury will run about 3.5 hours WOT with your 6 gallon tank.
Have you calculated how long it takes to charge the batteries from sun?
Yes, it matters, if you want to get the same performance. 6-6.5 is very different from 4.0 kts. The thrust needed for 4.0 kts is likely about 20% of the one needed for 6.5 kts. Thus you could likely get 4.0 kts with a 2.5 hp outboard. 2.5-6 hp outboards are typical for 28" 2 tn sailboats and they go 5-6 kts.
As I already have a hypothesis that the answer is 3-quantity 115-lb thrust trolling motors...is this right? Is this wrong? If wrong, then what is the right answer, and how did you come to such answer?
With the new information you have provided I think you will get about 4.0 kts with a single Torqeedo 2.0, which truly delivers that much of thrust. But you won't get the same top speed even with three of them.
Wow...I see you don't know a lot about energy. Ok, lead-acid batteries, standard deep cycle batteries (I prefer AGM-type) are around 72-lbs per, so six of them will weigh about 432-lbs. Six good AGM batteries would power a single 115-lb thrust trolling motor for around 20-25 hours.
I guess you have six about 100 Ah 12 V batteries. Torqeedo 2.0 takes about 100 A 24 V at that thrust. Thus theoretically your six batteries would be empty in 3 hours using just one motor. In practice much sooner, since Ah rating is not valid for such a high current and you don't want to use even AGM batteries 100% cycles. To run three motors three hours you need (more than) 18 batteries. That is 1300 lbs (600 kg), thus only 1/3 of the 2 tn I said earlier. That's because you only have 6 kW against 20 hp (15 kW). Your Mercury will run about 3.5 hours WOT with your 6 gallon tank.
Have you calculated how long it takes to charge the batteries from sun?
Joakim
12-15-2010, 05:34 PM
WOW...I just checked your offered PDFs there...
Those are 18 and 19' boats, with 300-lbs of people on them...Now, add 10-more-ft of boat another pontoon/fuel cell, and 1200-lbs of gear and equipment...oh, yeah, and another 4-inches of draft because of it...also, they are only carrying 24-gallon cells, while mine is 60-gallon...so add about 200-lbs more fuel, too.
Earlier you said "total mass approaches 1800-lbs", now it is much more. Why on earth are you carrying 60 gallon fuel for 4 stroke 20 hp engine? That would run 35 hours WOT on that and 100 hours for reasonable power on your boat.
Those are 18 and 19' boats, with 300-lbs of people on them...Now, add 10-more-ft of boat another pontoon/fuel cell, and 1200-lbs of gear and equipment...oh, yeah, and another 4-inches of draft because of it...also, they are only carrying 24-gallon cells, while mine is 60-gallon...so add about 200-lbs more fuel, too.
Earlier you said "total mass approaches 1800-lbs", now it is much more. Why on earth are you carrying 60 gallon fuel for 4 stroke 20 hp engine? That would run 35 hours WOT on that and 100 hours for reasonable power on your boat.
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