how many lbs of thrust is equal to 1 hp?

[Joakim]

Originally Posted by Mr Efficiency

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.

[powerabout]

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

[Muerte]

Originally Posted by DCockey

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.

[Muerte]

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...

[kerosene]

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

[Submarine Tom]

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

[Joakim]

Originally Posted by Muerte

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/p...sa53be.tmp.pdf
http://www.yamaha-motor.com/assets/p...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/p...-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]

Originally Posted by Joakim

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/p...sa53be.tmp.pdf
http://www.yamaha-motor.com/assets/p...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?

Originally Posted by Joakim

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.

Originally Posted by Joakim

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]

Originally Posted by Joakim

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/p...sa53be.tmp.pdf
http://www.yamaha-motor.com/assets/p...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]

Roger, out.

[kerosene]

Originally Posted by Muerte

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?

[Joakim]

Originally Posted by Muerte

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.

Originally Posted by Muerte

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.

Originally Posted by Muerte

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]

Originally Posted by Muerte

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.