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

[Atfirst]

Couple of things:

1] The numbers in the OP are arbitrary. All of us sailors know that 10 knots under sail is unrealistic. Doesn't affect the question.

2] Your particular sailboat is not designed to go fast under power. Some saiboats are designed to exceed 20 knots under power. Indeed, people can, and do, use them for waterskiing.

Not to the point. Going under sail at 10 knots and adding motorpower that without sails up gives 5 knots will not result in a boatspeed of 15 knots.
Also going faster than 10 knots under sail is a piece of cake for racing sailboats.

 

[A.T.]

So has this thread come to any consensus at all about the op?

Power requirement for a speed increase increases with speed, so you cannot expect the same speed increase at different speeds from the same power.

 

[gleem]

Not to the point. Going under sail at 10 knots and adding motorpower that without sails up gives 5 knots will not result in a boatspeed of 15 knots.

From post 40, I estimated that it would increase the speed 0.4 kts with the provision that the boat is designed to go 15 kts and that the propeller pitch be increased from that used at five knots.to accommodate the much higher speed. To get to 15 kts from 10 kts would require an engine 19 time more powerful than that use at 5 kts. using my approach.

Also going faster than 10 knots under sail is a piece of cake for racing sailboats.

The term racing boats is too generic to make that statement since they vary greatly in design and size too much. 10 knots speed upwind is only obtainable by long waterline racing boats where planing hulls loose their advantage wrt speed but can compensate somewhat by canting keels or shifting water ballast.. Down wind 10 knots readily by smaller planing hull boat and by many displacement hulls under the right conditions.

For your viewing enjoyment I have posted a video of the Volvo Ocean Racing "sled" "Pirates of the Caribbean - Black Pearl" A 70 ft light displacement 31,000 lb planing monohull sailing in a fresh breeze on a beam reach (wind at right angles to the course sailed). my favorite sailing video. The boat has a canting keel that can tilt to windward to help keep the boat upright an important consideration in attaining max speed.This is a boat that can attain 10 knot easily and over 20 down wind. Sorry about any pop-ups.

 

[Cutter Ketch]

This thread seems to have gone on some irrelevant tangents. It makes no difference what the sails do when the motor either speeds up or slows the boat down. It doesn't make any difference what the hull speed of the boat is (except in that clearly it is higher than 10 knots because somehow the boat got there). The original question said nothing about increasing the speed to 15 knots. It doesn't matter how the boat got to ten knots. Etc etc.

The question asked was: given a motor and prop that can propel a boat to 5 knots by itself, if the boat is made to travel at 10 knots will the motor add or subtract? I.e will said motor be a thrust or a drag when it is moving through the water at ten knots. All we are interested in is the sign and all that matters is the motor and prop except perhaps to argue that the 5 knot limit was somehow indicative of something about the design of the motor.

At five knots the thrust from the motor equaled the drag from the boat. That is clearly not zero thrust. The drag from the boat is not zero, so the thrust from the motor is not zero. There is no reason to think the thrust will suddenly drop to zero if the speed through the water is increased. The drag of the boat would increase and the motor doesn't have enough thrust to overcome the increase in drag, thus the top speed, but the thrust cannot and will not instantly drop to zero. if the boat is forced through the water at say 5.1 knots by some other agency whether it be sails, a separate stronger motor, or the boat is being towed behind a battleship, there is absolutely no question that the motor will continue to provide thrust at some speed which is at least slightly higher than the speed the motor alone could achieve.

There is certainly a speed higher than 5 knots at which the answer to the question which was asked is positively yes, the motor will provide extra thrust.

There is a speed through the water where the rotational speed of the prop and the pitch of the prop give a negative angle of attack. The prop cannot provide thrust at a negative angle of attack. There is certainly a speed high enough that the answer to the question which was asked is positively no, the motor will not provide extra thrust. It will be a drag.

Therefore there is a speed higher than 5 knots at which the answer changes from yes to no. The only question then is will that speed be above or below 10 knots.

The only way the boat enters into this question is what it's drag is at five knots. That gives us a single point on the motor's thrust to speed curve, although I'm not sure even that is much help. Everything else about the boat is irrelevant.

Instead of talking about sailboats and hull speeds and points of sail, this question should focus on propeller and motor design.

I feel very strongly that it is possible to create a plausible situation where the thrust of the propeller equals the drag of the boat at 5 knots and yet the thrust of the propeller is greater than zero at 10 knots. So I don't believe that there is a fundamental reason that the propeller can't have positive thrust at ten knots.

I am equally sure that it is possible to create a plausible situation where the propeller turns to drag below 10 knots.

The argument then is not about what is required by first principles, but rather what is likely by engineering choices of the designer of the propeller and motor, not to mention the choices of the boat owner who doesn't always buy the motor and propeller best optimized for how he intends to use it.

There is a perfect example curve here: http://atljsoft.com/html_help/Example, 34' Sailboat.htm

For the weakest prop the thrust crosses the drag a 7 or 8 knots. The thrust curve is rather shallow. They didn't draw much of it and it surely has shape making extrapolation iffy at best, but you can see it is heading off to hit zero at MUCH higher than the 8 knot top speed. Maybe 14 or 15 knots? Could it be double the thrust equals drag point? Sure. Could it be less? Sure.

So despite all the noise the answer is "physics doesn't tell you from the information provided". It can easily be engineered to happen either way, and the thrust vs speed curves I've been looking at suggest 10 knots would be on the edge of where the answer changes from yes to no.

 

[gleem]

The question asked was: given a motor and prop that can propel a boat to 5 knots by itself, if the boat is made to travel at 10 knots will the motor add or subtract? I.e will said motor be a thrust or a drag when it is moving through the water at ten knots. All we are interested in is the sign and all that matters is the motor and prop except perhaps to argue that the 5 knot limit was somehow indicative of something about the design of the motor.

The OP said the motor should be capable of moving the boat at 5 kts.. nothing is said of the prop. We do not have to know anything of the motor just its HP.. If the pitch of the prop is appropriately modified for the higher boat speed and its rpms also appropriately adjusted the engine should incrementally increase the speed. If not but the gear box is changed to increase the prop rpm to an appropriate rate you still might get some thrust albeit less but this I have no idea how to even estimate it. If the engine and prop are left as is then I would expect some drag since the prop would be spinning inappropriately slow for the boat speed.

 

[DaveC426913]

It doesn't matter how the boat got to ten knots. Etc etc.

It matters. Once you turn on the engine and change speeds, it will affect whether the boat can still sail at 10 knots. There is an interplay between the two. It factors into the OP's question, to-wit:

the sailboat is capable of sailing at 10 knots under the current wind conditions using only its sails

Apparent wind conditions change under power.

Instead of talking about sailboats and hull speeds and points of sail, this question should focus on propeller and motor design.

You seem to be ignoring that the motor cannot push the boat to 15 knots. Thus, the fact that it is moving at 10 knots already - factors into the ability of the motor.

 

[Cutter Ketch]

It matters. Once you turn on the engine and change speeds, it will affect whether the boat can still sail at 10 knots. There is an interplay between the two. It factors into the OP's question, to-wit:

No, nothing can change the sign of the contribution. For example if you say the boat speeds up and therefore the sails become inefficient so it slows back down it can't go back through 10 knots (or even to 10 knots) because that is the speed where the sails are more efficient. That's circular logic. More can only mean more. The details of the boat can only affect HOW MUCH more.

 

[Hercuflea]

Let me try to make the assumptions of the OP clear.

1) The boat is sailing on calm water.
2) The boat is traveling downwind at full sail.
3) The wind speed is constant, and given the sail configuration, the current wind speed can push the boat at 10 knots.
4) The boat has a propeller which can propel the boat at a max cruising speed of 5 knots under it's own power.
5) Let's assume the propeller is not lifted in and out of the water when it is on or off. (Although that might be interesting to look at)

OP question: If the motor is turned on full power when the boat is already sailing at 10 knots, will the boat reach 15 knots? If not, what will happen?

Also, I do think it is important that this is a sail boat, because unlike other methods of propulsion, there is an interaction between the motor's power output and the thrust of the sail. The sail's thrust is a function of how fast the boat is going, whereas that wouldn't be the case if it was being tugged for example.

 

[DaveC426913]

No, nothing can change the sign of the contribution.

No one has suggested otherwise.

That's circular logic.

Indeed. The very art of sailing is that interplay between competing factors.

(Example:
more wind => more heel, more heel => more spillage, more spillage => slowing down, slowing down => less heel

A newbie sailor will cycle between fast and slow. A seasoned sailor is one who can balance on the knife edge of that hump.)

More can only mean more. The details of the boat can only affect HOW MUCH more.

Yes. The question was whether motor and sail are 100% additive.

To reach 15 knots, the sail would have to continue to supply 10 knots while the motor provided a cumulative 5 knots, adding to 15.

I grant that boat can exceed 10 knots, it;s just not apparent that it can reach 15.

 

[DaveC426913]

OP question: If the motor is turned on full power when the boat is already sailing at 10 knots, will the boat reach 15 knots? If not, what will happen?

No, it will not reach 15.

The apparent wind speed will drop as the boat is accelerated by motor. The sails wil no longer provide thrust to push it alone at 10 knots.

They are additive, but not 100% additive.

You'll reach some speed between 10 and 15.

 

[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.

 

[DaveC426913]

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.

Ah. I see.

 

[desertbike]

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).

You are only talking about specially designed racing sailboats. That can only be true on certain designs of sailboats, ULDB's (Ultra Light Displacement Boats) which are primarily off the wind sailboats with little in the way of keels and hulls designed specially to plane, they don't go to weather well at all, or multi-hulls, these days usually catamarans. Multi-hulls have come up with designs and systems which allow the boat to sail faster than "apparent wind" speed, and have incorporated foils to raise up out of water which is what you suggest. But the typical sailboat is a displacement hull and doesn't follow your premise. I've sailed well over hull speed in hurricanes unintentionally, and pushed a 60 ft boat deeper in the water in the process, increasing drag and load on the rest of the rigging and structure.

 

[desertbike]

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.

As I stated in another post on the subject I've nearly sailed a boat under before in a hurricane down south, but we reduced sail to keep from plowing it under. I know of boats under tow being pulled too far past hull speed and being submerged, and have read historical accounts of it occurring during battles and hurricanes. It's not an issue with multi-hulls or special hull designs made to plane or surf. They have their own bad things which can happen when going too fast, but the engine (remember, this all started about the engine!) can help mitigate the pulsations of sail power and provide a better smoother sailing experience, but it's not going to let you get too far past hull speed.