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

[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.