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
Rick Willoughby
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?
Rick Willoughby
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
Rick Willoughby
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
Rick Willoughby
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
Rick Willoughby
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
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