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Old 19-09-2013, 04:26   #1
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Sailboat Exceeding Hull Speed under Engine

In Sail magazine there was mention of a new 38 ft motorsailor from Shannon yachts. I looked it up for curiosity. It can come with 2 x 75 hp or 1 x 160 hp engines. It's not, that I can see, a planing hull rather a long, shallow draft, full keel. The builder says that it can motor at 12 knots. This would well exceed (by some 50%) its hull displacement speed. How would it does this? I thought boats were physically limited to hull speeds because of the wave creation?
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Old 19-09-2013, 04:39   #2
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Re: Sailboat exceeding hull speed under engine

Obviously boats can exceed hull speed, otherwise how would you have power boats that plane like ski boats, Cigarettes, most fishing boats. Simple answer, boats are limited to hull speed by lack of horsepower to overcome the wave's created.

There are of course hull design issues that will help or prevent a hull from exceeding hull speed even with unlimited power. I have read that some of the old clipper ships, when caught by a gale and unable to take in sail quickly enough, actually sailed themselves under water. A case of enough power but a hull that could not climb out of it's wave.

On the other hand there are plenty of large power boats that are planing or semi-planing that significantly exceed their theoretical hull speed. Just takes power and the right hull shape.
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Old 19-09-2013, 05:00   #3
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Re: Sailboat exceeding hull speed under engine

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Obviously boats can exceed hull speed, otherwise how would you have power boats that plane like ski boats, Cigarettes, most fishing boats. Simple answer, boats are limited to hull speed by lack of horsepower to overcome the wave's created.
Cigarettes?? I did mention that it wasn't a planing boat. I'm aware that planing creates a different situation. I know that ships have a bulb nose which breaks up the bow wave. So what happens when a non-planing hull "breaks out" of the wave trap and how much fuel/horsepower (proportionally) would it require (the latter obviously relates to the size of the boat)
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Old 19-09-2013, 05:40   #4
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Re: Sailboat exceeding hull speed under engine

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Cigarettes?? I did mention that it wasn't a planing boat. I'm aware that planing creates a different situation. I know that ships have a bulb nose which breaks up the bow wave. So what happens when a non-planing hull "breaks out" of the wave trap and how much fuel/horsepower (proportionally) would it require (the latter obviously relates to the size of the boat)
I thought it would be obvious that a boat advertised with HP options far beyond that required to reach hull speed and a max speed also far beyond the theoretical hull speed would not be a strictly displacement hull design. That type of boat is designed to be able to exceed hull speed by rising over the bow and stern waves but not necessarily achieve a full plane either due to inadequate power for the size/shape/weight and/or because the overall hull form is not conducive to full planing speeds. The mechanism is the same as a planing hull IE the boat rises over the bow and stern waves based on sufficient HP and suitable hull form to do so.

There are many boats, planing or semi-planing, power, sail or motorsail that can exceed hull speed with enough horsepower or sailpower. Some may be designed to do so like the model you mention, some just happen to be able to do so.

The amount of power required to do this is a complex calculation based on primarily on the hull form, length and weight of the boat. Without getting into naval architecture, hydrodynamics and a lot of math the short answer on how much HP is, a whole lot, multiples of HP required for hull speed.

PS
By the question marks after Cigarettes do you mean you don't know what a Cigarette is in the context of boats or you don't understand the example?
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Old 19-09-2013, 06:23   #5
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Re: Sailboat Exceeding Hull Speed under Engine

McGreggor 26 with 50hp outboard sort of explains your theory
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Old 19-09-2013, 06:37   #6
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Re: Sailboat Exceeding Hull Speed under Engine

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Originally Posted by Srah 1953 View Post
I thought boats were physically limited to hull speeds because of the wave creation?
No, they are not. This is a common misconception, so don't feel bad. Nonetheless, it is just a misconception.

"Hull speed" is not an absolute limit on how fast a displacement hull can go. It is merely the speed at which it will take dramatically more horsepower to make it go any faster. That horsepower can come from engines, or it can come from the sails. Lots of sailboats, with displacement hulls, can sail--with no engine assist--in excess of their theoretical hull speeds under the right conditions. And I'm not just talking about flat bottomed racing boats. Heavy displacement, full-keel boats can exceed their theoretical hull speed also (and many have).
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Old 19-09-2013, 06:41   #7
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Re: Sailboat Exceeding Hull Speed under Engine

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No, they are not. This is a common misconception, so don't feel bad. Nonetheless, it is just a misconception.

"Hull speed" is not an absolute limit on how fast a displacement hull can go. It is merely the speed at which it will take dramatically more horsepower to make it go any faster. That horsepower can come from engines, or it can come from the sails. Lots of sailboats, with displacement hulls, can sail--with no engine assist--in excess of their theoretical hull speeds under the right conditions. And I'm not just talking about flat bottomed racing boats. Heavy displacement, full-keel boats can exceed their theoretical hull speed also (and many have).
Exactly. Many eyewitness reports have stated the old USS Enterprise exceed her theoretical hull speed limit on numerous occasions. One of the reasons was she was built with over twice as much power as she actually needed to make hull speed, as she was the first nuclear powered carrier and they overbuilt the power plant (eight nuclear reactors instead of the two that later carriers were equipped with).

Later they only actually ran four of them on the Enterprise at one time.
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Old 19-09-2013, 07:08   #8
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Re: Sailboat Exceeding Hull Speed under Engine

Speed versus power is not a straight line, there is not a discontinuity where the graph goes vertical at theoretical hull speed. Theoretical hull speed is the point where the wave made by the boat is the same length as the boat. Going faster makes a longer wave, so the boat is now climbing uphill on its own wave. A convenient point.

From:
Wave-making resistance - Wikipedia, the free encyclopedia

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Old 19-09-2013, 08:49   #9
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Re: Sailboat Exceeding Hull Speed under Engine

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Originally Posted by denverd0n View Post
No, they are not. This is a common misconception, so don't feel bad. Nonetheless, it is just a misconception.

"Hull speed" is not an absolute limit on how fast a displacement hull can go. It is merely the speed at which it will take dramatically more horsepower to make it go any faster. That horsepower can come from engines, or it can come from the sails. Lots of sailboats, with displacement hulls, can sail--with no engine assist--in excess of their theoretical hull speeds under the right conditions. And I'm not just talking about flat bottomed racing boats. Heavy displacement, full-keel boats can exceed their theoretical hull speed also (and many have).
Cherabinis can go faster than most other monos ,not sure why ,something about the hull design and it being able to push ahead of the bow wake?? Any way they can go fast comparing to other monos...
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Old 19-09-2013, 09:58   #10
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Re: Sailboat Exceeding Hull Speed under Engine

I only see artist renderings of the boat on their website, no photos. Have they actually built one?

The 12kts might simply be some combination of marketing exaggeration & uncalibrated speedo's & current (easy to go 12kts with the current in the cape cod canal ).

This calculator suggests they need 674HP to achieve 12kts with a 'displacement hull'. Shannon suggests this is a 'special patented hull shape' but with 6,000lbs of ballast I would think it has to qualify as a 'deep V'. But if it is actually a 'flat V' hull shape (again I doubt it with 3' of draft) the calculator says it only needs 77hp to achieve 12kts.

Their description of sailing potential seems perhaps a little puffy also "Sails at a fast 7 knots at a 35-40 degree angle into the wind in just 15 knots of wind". (note: this is a 38'er with only 3' of draft)

And I am a bit skeptical about the quoted displacement - 14,500 lb. Pretty light given 6,000lbs of ballast.
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Old 19-09-2013, 10:45   #11
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Re: Sailboat Exceeding Hull Speed under Engine

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I only see artist renderings of the boat on their website, no photos. Have they actually built one?

The 12kts might simply be some combination of marketing exaggeration & uncalibrated speedo's & current (easy to go 12kts with the current in the cape cod canal ).

This calculator suggests they need 674HP to achieve 12kts with a 'displacement hull'. Shannon suggests this is a 'special patented hull shape' but with 6,000lbs of ballast I would think it has to qualify as a 'deep V'. But if it is actually a 'flat V' hull shape (again I doubt it with 3' of draft) the calculator says it only needs 77hp to achieve 12kts.

Their description of sailing potential seems perhaps a little puffy also "Sails at a fast 7 knots at a 35-40 degree angle into the wind in just 15 knots of wind". (note: this is a 38'er with only 3' of draft)

And I am a bit skeptical about the quoted displacement - 14,500 lb. Pretty light given 6,000lbs of ballast.
Which boat?
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Old 19-09-2013, 10:47   #12
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Re: Sailboat Exceeding Hull Speed under Engine

^^
Shannon 38 HPS

Shannon has been trying to reinvent the motor sailor
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Old 19-09-2013, 10:52   #13
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Re: Sailboat Exceeding Hull Speed under Engine

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Originally Posted by denverd0n View Post
No, they are not. This is a common misconception, so don't feel bad. Nonetheless, it is just a misconception.

"Hull speed" is not an absolute limit on how fast a displacement hull can go. It is merely the speed at which it will take dramatically more horsepower to make it go any faster. That horsepower can come from engines, or it can come from the sails. Lots of sailboats, with displacement hulls, can sail--with no engine assist--in excess of their theoretical hull speeds under the right conditions. And I'm not just talking about flat bottomed racing boats. Heavy displacement, full-keel boats can exceed their theoretical hull speed also (and many have).

Sure. I've exceeded hull speed on both the boats I have owned in recent history (not counting the goofy little boat we had in grad school). There are too many variables for it to be reduced to a simple formula. Don't get me wrong -- I don't know what those variables are, but I know that they have to be there, because I got my little Irwin up to 6.5k more than once, and this boat over its supposed hull speed more than once also.
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Old 19-09-2013, 11:04   #14
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Re: Sailboat Exceeding Hull Speed under Engine

Well, the boat appears to almost have waterline equal to it's length. Figuring 36 ft waterline, it's "hull speed" is a little over 8 knots right? I dont imagine with that HP it would be super diffcult to push her to 12 knots...? whats that fuel sucking sound? haha. I like the concept of these boats though.
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Old 19-09-2013, 11:23   #15
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Re: Sailboat exceeding hull speed under engine

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PS
By the question marks after Cigarettes do you mean you don't know what a Cigarette is in the context of boats or you don't understand the example?
Thank you all. It was worth posing the question as I'm now better informed. Unfortunately, I've never got to hull speed on my boat except with the tide.

No, I don't know what a cigarette is in boat terms and a quick Google search didn't enlighten, but I'm wondering if it's possibly some kind of long thin racing powerboat.
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