Sailboat Speed Question: Can a Motor Increase Speed Beyond 10 Knots with Sails?

[Hercuflea]

You'll reach some speed between 10 and 15.

But what about the drag caused by the propeller? If the motor cannot push the boat past 5 on it's own, that means thrust = drag. When the boat is traveling at 10 knots, drag is > than when it is traveling at 5. But theoretically the motor is still outputting the same thrust.

 

[DaveC426913]

But what about the drag caused by the propeller? If the motor cannot push the boat past 5 on it's own, that means thrust = drag. When the boat is traveling at 10 knots, drag is > than when it is traveling at 5. But theoretically the motor is still outputting the same thrust.

Yes. This is what I argued in posts 22 and 25.

In essence, how can a motor (that can only provide 5 knots in calm water) still manage to provide that same 5 knots when in 10 knots of flow?

 

[Hercuflea]

Yes. This is what I argued in posts 22 and 25.

In essence, how can a motor (that can only provide 5 knots in calm water) still manage to provide that same 5 knots when in 10 knots of flow?

I'm not sure if it would provide any knots. It could provide negative knots.

 

[DaveC426913]

I'm not sure if it would provide any knots. It could provide negative knots.

Yes. I can see that rationale, and argued for it.

But I acknowledge that others are raising pertinent factors, and the issue is more complex.

 

[Cutter Ketch]

I must apologize. The OP did originally ask if the boat will reach 15 knots. I did not take it to mean he seriously wanted to know if it will reach 15 knots. It was in the spirit of "what will happen? Will it go 15 knots, will it slow down?" but then I remembered it as "will it speed up, will it slow down".

As I said physics allows for slowing down or speeding up, but yes, you would need to know the details (lots of details) to know by exactly how much.

I agree with arguments that 15 is not possible. (Thrust at 5 equals drag at 5. Drag is nonlinear. Increase in drag from 10-15 is greater than increase from 0-5. Thrust is less at 10 than at 5. 15 is not possible)

Again, my apologies for misremembering the question.

 

[anorlunda]

Man is this thread lost in the woods of complications. Let's try taking @A.T. s advice.

How about looking at the actual physics? Any amount of mechanical power an engine can deliver can be turned into forward thrust at any speed[...]

Think of conservation of energy. Any power that the engine puts out in excess of its internal friction must go to increased boat speed. It can do that by decreasing drag, or by increasing thrust, or both. The only other possibility is that the extra power just churns water increasing the water temperature but not changing momentum of water or vessel.

If we all agree that more power necessarily makes some increment of more speed, that would be an accomplishment. If you don't agree, please explain how you think the energy is conserved.

Next step, how much more speed? That is the place where many complications come into play, and an accurate answer requires a much more detailed problem statement.

 

[Hercuflea]

Man is this thread lost in the woods of complications. Let's try taking @A.T. s advice.
Think of conservation of energy. Any power that the engine puts out in excess of its internal friction must go to increased boat speed. It can do that by decreasing drag, or by increasing thrust, or both. The only other possibility is that the extra power just churns water increasing the water temperature but not changing momentum of water or vessel.

If we all agree that more power necessarily makes some increment of more speed, that would be an accomplishment. If you don't agree, please explain how you think the energy is conserved.

Next step, how much more speed? That is the place where many complications come into play, and an accurate answer requires a much more detailed problem statement.

I do agree with you now. If the motor is fixed under water and the sails provide 10 knots, the energy spent by the motor must go into either forward movement or heat.

What more do we need to specify before we can get a tentative answer on how much? I am really interested to know the answer to the OP.

 

[Cutter Ketch]

hmmm ... that works in the case where the prop is always in the water and the motor is either on or off. I'll agree that the OP did not specify. However the more usual case for sailboats is that the motor is pulled out of the water when not in use. In that case dropping the motor introduces a large drag and the power from the motor has to overcome that before the force is net forward compared to the no motor case.

 

[A.T.]

What more do we need to specify before we can get a tentative answer on how much?

As a first approximation assume that the hull drag = const * v^2 and the propulsive sail force doesn't decrease when you add the motor thrust. Write down the force balance using P = F * v and the above drag model.

 

[anorlunda]

hmmm ... that works in the case where the prop is always in the water and the motor is either on or off. I'll agree that the OP did not specify. However the more usual case for sailboats is that the motor is pulled out of the water when not in use. In that case dropping the motor introduces a large drag and the power from the motor has to overcome that before the force is net forward compared to the no motor case.

No, the more usual case is that sailboats have inboard engines. The props are always in the water. The OP didn't say inboard out outboard.

As a first approximation assume that the hull drag = const * v^2 and the propulsive sail force doesn't decrease when you add the motor thrust. Write down the force balance using P = F * v and the above drag model.

Sorry no on both counts. Reposting from #3

You didn't say which way the wind was blowing.

• It the wind is from behind the boat, then apparent wind speed is true wind speed minus boat speed. Turning on the motor makes the force on the sail less. Enough motor can make the wind force on the sail zero or negative.
• If the boat is heading into the wind (30 degrees is a close to the wind as it can get) then apparent wind is true wind plus boat speed. Turning on the motor increases apparent wind speed and makes the sail more efficient, allowing higher speeds and angles even closer to the wind.

You are also neglecting hull speed. That is a speed at which resistance of the water grows nonlinearly. It is not a brick wall to increase speed but it is very significant. Hull speed depends on the length of the boat, wind and or motor power don't matter.

If you want really rough approximations, consider hull speed as hard max limit on speed. A bit less rough, consider a region where drag grows $v^4$.

IMO, there is no way to make the "how much" problem easy enough to explain in a forum post.

 

[A.T.]

Sorry no on both counts.

You can say 'no' to all physical models. The approximation I proposed doesn't require any additional info, and already makes clear how little the speed will increase.

 

[gleem]

You can say 'no' to all physical models. The approximation I proposed doesn't require any additional info, and already makes clear how little the speed will increase.

Yes very little speed increase at the very best with all advantageous assumptions about 0.4 kts probably less.in practice.
.

 

[anorlunda]

The approximation I proposed doesn't require any additional info, and already makes clear how little the speed will increase.

Fair enough. We can leave it there.

 

[DaveC426913]

I sortta lost the thread of conversation in the last few posts, though you seem to have come to a consensus.

Is the approximation in post 69, and the summarized conclusion in post 72?

 

[anorlunda]

Is the approximation in post 69, and the summarized conclusion in post 72?

Pretty much Dave.

 

[gleem]

@A.T. Don't you think that I proposed your approach in posts 29 and 40. None of these posts were acknowledged or commented on as even relevant.

 

[Hercuflea]

This thread calls for some simulations.

 

[desertbike]

One of the things that all of these discourses have failed to take into consideration is maximum hull speed of a sailboat. The maximum that a boat can theoretically sail is the square root of the waterline x 1.34 for a displacement hull, x 1.43 for a non-displacement hull. So the square root of a 25 ft waterline, = 5 x 1.34 gives a hull speed of 6.7 knots. You can achieve a reduction on the rigging and sails by use of the motor and to even out the pulsations of the wind effect on the sails, but only by lengthening the waterline, usually by heeling the boat over 20-30 degrees, can you increase the hull speed of the sailboat. Once you go past hull speed the drag increases and the boat sits lower and deeper in the water and if speed is not reduced you can sail the boat under and lose it all.

 

[A.T.]

This thread calls for some simulations.

Yes, if you really want to get more specific then you cannot solve this analytically.

 

[topmast]

Zero power?? Spin free or lock shaft? Sailors empty the rum bottle on this topic . Knot that the twain shall meet .Meanwhile nary a mention of apparent wind, hullspeed and waterline length .

 

[ddjj77]

If the motor or engine is able to operate at any RPM, then it will be able to provide an increase in speed by 5 knots. The propeller will "catch up" with the ambient speed. Most of the drag on the boat is from skin friction, and the coefficient of skin friction doesn't increase with speed (as do viscous forces). However, the speed limit of sailboats is about the speed of the bow wave, and if the latter is less than 15knots, then the projected 15 knots may not be reached.

 

[DaveC426913]

If the motor or engine is able to operate at any RPM...

Can this condition be true of a motor that is specifed to only be able to push the boat from 0 to 5 knots?

IWO, with that limit on the motor specified, can it still be stated that the motor can operate at any RPM?

 

[A.T.]

an this condition be true of a motor that is specifed to only be able to push the boat from 0 to 5 knots?

IWO, with that limit on the motor specified, can it still be stated that the motor can operate at any RPM?

No real motor can operate at any RPM, but that is not required to provide positive thrust at 10 knots.

The specification of 5 knots pure motor speed alone, says noting about the RPM range of the motor, or the max speed at which the motor/gearing/prop combo can provide thrust.

 

[A.T.]

If the motor or engine is able to operate at any RPM, then it will be able to provide an increase in speed by 5 knots.

What is relevant is the power output. If the motor is working extremely inefficiently at 5 kts (far below it's optimal RPM), so that it's power output at 15 kts is multiple times higher, then it might add those 5 kts or even more to the 10 kts by sail. But this is a rather contrived case.

 

[Hercuflea]

I'm assuming the 5 knots no sails is occurring at the max cruising power output of the motor.

I'm still trying to think about whether this question would be the same if, say for example the motor were replaced with a tiny rocket mounted above the water that is capable of pushing the boat along at 5 kts.

 

[A.T.]

I'm still trying to think about whether this question would be the same if, say for example the motor were replaced with a tiny rocket mounted above the water that is capable of pushing the boat along at 5 kts.

Not quite the same, because the rocket force doesn't depend on velocity like engine-power supplied force does. But you still won’t get an 5 kts increase, because drag is non-linear.

 

[gleem]

I found a propeller manufacturer website that allows you to determine the propeller size for a given boat specs like LWL, beam, displacement, and gives the HP ( at full throttle) necessary at the prop for a desired speed.. It also gives the HP of the engine needed to move at hull speed.as well as the maximum speed for the actual HP of the engines used in the boat.

https://www.vicprop.com/displacement_size.php

I chose an actual yacht http://yachtpals.com/largest-yacht-3446 capable of 14.7 kts. with a 140 foot LWL 760,000 lb displacement 31 ft beam at WL, 7 ft hull draft (exclude keel) just a guess, 2 - 450 HP engines, gear ratio 1.5 (guess), 3000 rpm max of engines, 5 shaft bearings (guess) and a desired speed of 5 kts. this gave 47 Hp. The max speed for 2 - 450 HP engines is 12.81 kts.. If I add 47 HP to these engines (23.5 each) the calcs give me 13.04 kts and increase of 0.23 Kts.

 

[cjl]

One of the things that all of these discourses have failed to take into consideration is maximum hull speed of a sailboat. The maximum that a boat can theoretically sail is the square root of the waterline x 1.34 for a displacement hull, x 1.43 for a non-displacement hull. So the square root of a 25 ft waterline, = 5 x 1.34 gives a hull speed of 6.7 knots. You can achieve a reduction on the rigging and sails by use of the motor and to even out the pulsations of the wind effect on the sails, but only by lengthening the waterline, usually by heeling the boat over 20-30 degrees, can you increase the hull speed of the sailboat. Once you go past hull speed the drag increases and the boat sits lower and deeper in the water and if speed is not reduced you can sail the boat under and lose it all.

This isn't true at all, or boat racing would be a very boring sport. It's true that power required goes up pretty dramatically as you approach and exceed hull speed, but when you do, the boat definitely doesn't sit lower and deeper in the water. It starts to raise the nose instead, and depending on hull design, might even get up on full plane (at which point it can go almost arbitrarily fast if you have the power to do so).

 

[DaveC426913]

Once you go past hull speed the drag increases and the boat sits lower and deeper in the water and if speed is not reduced you can sail the boat under and lose it all.

Missed this before. How does that work?

 

[gleem]

As the boat speed increase a bow and stern wave are created and increase in height with a concomitant reduction in the water level between them. A sailboat has a greater percentage of buoyancy at mid ship where the beam is greatest with much less at the bow and stern. Thus relative to amount of the boat above the water level at the beam the bow and stern are more underwater. If the boat goes fast enough and has too little buoyancy at the bow you could submerge the bow. This usually doesn't happen under the boats own power but if it is being towed by a more powerful boat it has happened apparently.