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Discussion Starter #1
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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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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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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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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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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Discussion Starter #9
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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Discussion Starter #10
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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The Cal 29 has a 24' LWL and is really stable

The J24 has a 20' LWL not so much in the stable :)

The A4 motor is mounted at or near the max 15 degree angle BUT the outboard was level and the Cal still had stern squat were as the J24 had littel if any stern squat at the same speed

It just seems to be the nature of pushing that style hull VS the C/E from the sails being more or less in the center of the boat and if it squats under sail it does it level :)

The question is how much HP does a 400 to 500 SQFT sailplan make in twenty knots of wind as it takes some honking wind to break 7 knots on the Cal

The Cal will go above 6 knots under power BUT the water gets really deep in the cockpit and its NO fun standing in 12" of water and using 3 times as much fuel to go 1 knot faster

The Cal was built with and still has its ordinal 12 x 8 prop which is a bit big BUT does not seem to have hurt the motor as it still is within spec at 41 years
 

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Theoretical hull speed = (Sqrt of LWL)*1.34. You can reach that speed with a reasonable amount of energy (sail or mechanical) but to exceed that you have to have a lot more power. The exception is a planing boat or when you get lucky and can use following seas and surf down the faces (thats fun). Again that is theorectical hull speed and can be affected by any of the variables mentioned above, but every non planing boat has a definate point that it reaches this transition.
DD
 

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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?
Just for fun, compare a Farr/Mumm 30 One Design to a Catalina 30.
 
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