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Old 14-06-2013, 13:44   #16
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Re: diesel horsepower

Originally Posted by Matt sachs View Post
Hull speed = 1.34 x square root of LWL.

There must be more to it than that. Using that formula, if I loaded my boat so it sat deeper in the water, it would be faster. Is this just a rule of thumb or is it really that simple?
To a degree, that is true. For a while the sail racing rules ranked boats based on there LWL, so racers started buying boats with huge overhangs fore and aft. When the boat was rated at the dock, it was rated as being slow due to the short LWL but when healed over, the overhangs were in the water effecitvely increasing the LWL and the hull speed.

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Old 14-06-2013, 14:47   #17
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Re: diesel horsepower

Speed vs power required graph of a small boat that will plane. If your boat won't plane the line will just keep going up closer to vertical instead of flattening out.

V/square root(L) at 1.3 is where a lot of people put the theoretical hull speed constant.

Hull speed - Boat Design Forums


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Old 14-06-2013, 15:05   #18
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Re: diesel horsepower

I don't get it all this advise about a boat we know nothing about? Is that what they call pissing in the wind??
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Old 14-06-2013, 15:36   #19
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Re: diesel horsepower

We are talking hp, so I thought this might be relevant. I don't know quite how to ask this question, therefore I can't readily find an answer. I'm sure someone on CF can jump right in. Does the depth of the prop change the characteristics of the prop and engine, e.g. I have a 55# thrust electric motor. If I submerge the prop deeper will I gain more thrust than if prop is located closer to the surface? Sorry to sound redundant.
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Old 14-06-2013, 16:08   #20
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Re: diesel horsepower

Operating at a knot below hull speed could save you 50% in fuel costs.
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Old 14-06-2013, 16:27   #21
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Re: diesel horsepower

Originally Posted by kingyellow40 View Post
Would a 120-140 h.p. perkins diesel push a 28 foot boat at about 8-10 knots?
There's nothing in the OP to indicate whether this is a displacement hull or not. For that matter, we have no idea whether the boat in question is a sailboat or powerboat. kingyellow40's profile states that he has a "28 foot willy." Any slang dictionary will define "willy" as a flaccid penis. I'm guessing there aren't too many production boat manufacturers who would call a boat by such a name.

The OP was kingyellow40's first and only post on this forum.

cruising is entirely about showing up--in boat shoes.
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Old 06-06-2015, 12:25   #22
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Re: diesel horsepower

need to have a brain first!
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Old 06-06-2015, 12:38   #23
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Re: diesel horsepower

Beside of theoretical aspect of the displacement hull speed, isn't there also a practical? Higher speed and bad circumstances can sink the boat, because the stern is tending to go down.

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