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Old 14-06-2013, 06:41   #1
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diesel horsepower

Would a 120-140 h.p. perkins diesel push a 28 foot boat at about 8-10 knots?
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Old 14-06-2013, 06:53   #2
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Well, my 62 hp can push my 42', 30000+ lb boat at 7+ knots.
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Old 14-06-2013, 06:56   #3
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Re: diesel horsepower

No Unless you can the boat on a plane

Theoretical maximum hull speed = 1.34 * (LWL)1/2

LWL = Length of waterline.

Not knowing our water line, i used 28 feet. Max hull speed is 7.09 knots. You waterline length is probably 10% less, meaning your max hull speed will be about 6.3 knots

Don't know what kind of boat you have but a 120 HP diesel will weigh so much that the back end of the boat will be deep underwater.
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Old 14-06-2013, 06:57   #4
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Yes it will. Ski boats go that fast all the time.
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Old 14-06-2013, 07:02   #5
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Re: diesel horsepower

Quote:
Originally Posted by kingyellow40 View Post
Would a 120-140 h.p. perkins diesel push a 28 foot boat at about 8-10 knots?
Yes, it would also be an overkill situation where having a lesser horsepower engine would be more economical.

Also, the hull speed of your boat is around 7.0 to 7.3 knots, depending on the length of your waterline. So if your boat is a full displacement boat, like most keel boat sailboats are then your goal should be to get an engine with just enough horsepower, or perhaps a little more horsepower to get you to around 7.5 knots at most Trying to go faster would be very inefficient.

Above hull speed and your bow starts lifting, which requires a great deal more energy. The slope of the power to speed curve increases quite a bit.
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Old 14-06-2013, 08:03   #6
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Re: diesel horsepower

The OP doesn't say what kind of "Willy" this boat is. A picture would be nice.
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Old 14-06-2013, 08:05   #7
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Re: diesel horsepower

Sailboat, no. Semi displacement power boat, yes.

What kind of boat and how much does she weigh?

I have a 28', 8,000 lb. semi displacement downeaster and it probably takes 80-90 hp to push it to 10 kts.

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Old 14-06-2013, 08:50   #8
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Re: diesel horsepower

A voluminous hull may become unstable. A flat one will go on the plane and reach/outdo the proposed speed.

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Old 14-06-2013, 09:07   #9
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Re: diesel horsepower

Hull speed = 1.34 x square root of LWL.
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Old 14-06-2013, 09:43   #10
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Re: diesel horsepower

Would want sufficient HP to maintain way at 50 to 70% of hull speed with stiff head winds and short choppy waves on the nose. This is greater than the HP required for hull speed in flat water.
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Old 14-06-2013, 14:02   #11
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Re: diesel horsepower

I'm guessing we are talking about a single engine trawler not a sailboat.

With that assumption and the assumption that the waterline is 28', I would estimate 8-9kts. Based on similar boats. Ie: the old Mainship 34' with a 160hp and 32.5LWL, will run at 10-11kts even though the theoretical hull speed is 7.6kts.

Three thoughts:
- Ask for a test run and have them open it up and check the speed. That will eliminate the specualtion.
- You don't want to run wide open or you lose all the fuel economy benefits of displacment operation.
- The 1.34 in the theoretical hull speed changes based on the hull shape so at best it gives you a rough idea for typical monohull sailboats.
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Old 14-06-2013, 14:16   #12
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Re: diesel horsepower

Quote:
Originally Posted by kingyellow40 View Post
Would a 120-140 h.p. perkins diesel push a 28 foot boat at about 8-10 knots?

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Old 14-06-2013, 14:20   #13
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So, what exactly does 'hull speed' mean? How does it differ from actual speed?
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Old 14-06-2013, 14:39   #14
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Re: diesel horsepower

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?
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Old 14-06-2013, 14:41   #15
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Re: diesel horsepower

"Hull speed" is the theoretical maximum speed of a boat hull and it corelates strongly to the water line length of the hull but is also influenced by other factors such as the length to beam ratio (the reason multihulls can often exceed hull speed by large amounts) among other factors.

In reality, there is not a true hull speed as a boat will keep going faster the more power you apply. The problem is horse power needs increase with the cube of speed so every time you double the speed it takes many times the horse power. Eventually you need to add massive amounts of horse power to get even a tenth of a knot more speed. Theoretical hull speed usually correlates to the point where adding more power starts to become silly as you are buring lots more fuel for very little gain.

For example our 34' boat will happily chug along at 3kts with just our 5hp dingy motor at half throttle. With the 25hp primary outboard, she cruises easily at 6kts at half throttle. Give her full throttle and we only get up to around 7.5kts. Before we replaced the old outboard, we had 40hp and could make 8kts wide open. Do you see the pattern? This is why when we repowered, we dropped to 25hp as hull speed was the limit.
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