How easily driven is a particular hull? - SailNet Community

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  #1  
Old 07-19-2011
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How easily driven is a particular hull?

ewdysar (Eric) posted that his Cheoy Lee Bermuda 30 will go 4.2kts at a 1200 watt load with an electric drive motor.
His Cheoy Lee while a beautiful classy design is probably does not have the ideal hull shape to reduce thrust needed to achieve hull speed.

How you you go about comparing different boats as to how much it would cost in watts, foot/lbs etc to push at hull speed?

What other issues do you think should be considered?

For example would a Catalina 30 push easier or harder than the Cheoy Lee?
What about a Catalina 27?
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Old 07-20-2011
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Displacement, including everything we load on our boats, and total wetted surface area are the biggies.
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Old 07-20-2011
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Having moved my Cal 29 with a 4Hp outboard and now the Atomic 4 is back in and healthy

At 8000# and not all that slick of a form the 4HP moved it at 4.5 knots

The A4 will take it up to 6+ knots but the stern squats so bad we don't go much over 5 to 5.5 knots at about 1400 RPM

So i can conclude that as you try and go above 4.5 knots the HP needed goes wau UP

The boat will sail that fast in 5 knots of wind BUT to reach 7 knots it needs to be in the twentys
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Last edited by tommays; 07-20-2011 at 07:53 AM.
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Old 07-20-2011
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Boats with longer waterlines for their length will be faster. Hulls with fair lines without rule induced bumps will be faster. Lighter will be faster. Narrow waterline beam boats are faster.

Long and lean will beat short and fat any day.
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Old 07-20-2011
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As a data point, my Islander 30 MkII will move at 6.4 kts (multi direction pass with GPS average) at about 10 KW (~14HP). Transom drops at least a foot with large (by sailboat standards) trailing waves. Roughly speaking power should be proportional to square of speed up to hull speed, which is close to 6.4 kts on my boat. By that formula, 4 kts should cost me about 3.9 KW.
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Old 07-20-2011
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Quote:
Originally Posted by shogan50 View Post
Roughly speaking power should be proportional to square of speed up to hull speed, which is close to 6.4 kts on my boat.
Actually, the power requirement goes up by the cube of speed as long as you are well below hull speed. At low speeds, frictional resistance is most important. As you approach hull speed, wave making resistance becomes most significant.
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Old 07-21-2011
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Quote:
Originally Posted by mitiempo View Post
Long and lean will beat short and fat any day.
Like the rest of life.
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Old 07-21-2011
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Dohhhh! Of course!
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Old 07-21-2011
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Quote:
Originally Posted by tommays View Post
Having moved my Cal 29 with a 4Hp outboard and now the Atomic 4 is back in and healthy

At 8000# and not all that slick of a form the 4HP moved it at 4.5 knots

The A4 will take it up to 6+ knots but the stern squats so bad we don't go much over 5 to 5.5 knots at about 1400 RPM

So i can conclude that as you try and go above 4.5 knots the HP needed goes wau UP

The boat will sail that fast in 5 knots of wind BUT to reach 7 knots it needs to be in the twentys
One would think that a boat could be sailed or motered pretty close to the same top speed yes?
In your case are you saying that top speed sailing is 7knots and motoring is 6knots.

If so is that because you have the wrong prop?
The A4 certainly has more than enough HP.
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Old 07-21-2011
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Quote:
Originally Posted by mitiempo View Post
Boats with longer waterlines for their length will be faster. Hulls with fair lines without rule induced bumps will be faster. Lighter will be faster. Narrow waterline beam boats are faster.

Long and lean will beat short and fat any day.
Would you mind mentioning a boat or three that pop into your mind in the 28 - 33' or so size.

Long waterline for lengh:
Fair lines:
Rule induced bumps:
Narrower vs. fatter.

I know you have been doing this a long time and I would like to look up some specific boats and see the difference.

It seemed to me that the Catalina 27 is a little better hull shape than the Catalina 30 according to this criteria, yes?
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