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Go Back   SailNet Community > On Board > Gear & Maintenance
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  #11  
Old 10-29-2007
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I understand that they are Swedish. I have seen one of them, a twin. It looked well-built, but probably with expensive spare parts.
The triple will be inherently better balanced than the twin. Triples are polular on motorcycles for that reason.
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Quote:
Originally Posted by GBurton View Post
Is there a recommendation for HP/LBS of boat?
Like most quick rules of thumb this number, pounds per HP needs some common sense to use effectively. The amount of horsepower expressed this way varies quite a bit with length. And surprisingly it decreases as the boat gets larger. The best way to look at it is compare similar boats and in your case compare to other Westsails 32’s.

If you want the base numbers to draw a chart start with,
500 pounds per HP equals a speed length ratio of 1.3
1,000 pounds per HP equals a speed length ratio of 1.05
1,500 pounds per HP equals a speed length ratio of .95
2,000 pounds per HP equals a speed length ratio of .85

If you select 2 HP per 1,000 or 1 HP per 500 pounds you can expect a speed length ration of 1.3 which is high for a Westsail 32 but let’s go with that number and work it out, so with a waterline length of 27.5 feet you can expect a speed of 6.8 knots. This is all based on shaft horsepower not advertised power.
All the best,
Robert Gainer
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Old 10-30-2007
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Robert...

What am I misunderstanding?

How does one get a figure of 6.8 from the two figures 27.5 and 1.3 ?

My ship weighs 23,000 lbf, and I have about 35 hp (Volvo say). What speed should I expect? The ship is a double-ender... a Union 36.... with a big footprint in the water and a wetted length of 33 ft, apparently.
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I am sure Robert (as a Naval Architect) can explain this much better than I can (being a Marine Engineer), but I suspect he calculated the hull speed using an equation like:

Hull Speed (kts) = 1.3 to 1.4 X Square Roof of Waterline Length (in feet)

Hull speed in a displacement (non-planing) hull is when the resistance due to wavemaking basically limits further speed increases. Below that speed, the frictional resistance dominates. The very rough rule of 2 Hp per 1000 lbs generally provides sufficient power to get to hull speed plus a little margin.
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OK, so it's 1.3 (say) times the square root of waterline length.

I got it.
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Quote:
Originally Posted by Rockter View Post
I understand that they are Swedish. I have seen one of them, a twin. It looked well-built, but probably with expensive spare parts.
The triple will be inherently better balanced than the twin. Triples are polular on motorcycles for that reason.
They are Danish, not Swedish. And due to rotating counterweights they do not vibrate much. I have a new (2004) DV24ME and really love it compared to the Volvo's and Yanmar's I've seen. But I've only owned boats with Volvo's (and no engines) before. One of my friends installs these in rescue boats WW and is VERY proud of the product, which was one of the reasons for me to choose that engine (having your own mechanic is also great )
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I have found out that Bukh sells a turbocharger for the DV24 (The engine that I have) This might solve the problem that I have with the engine being too small for my boat (Westsail 32)

What are the pros and cons of turbocharging a sailboat engine?

Quote:
Originally Posted by JomsViking View Post
They are Danish, not Swedish. And due to rotating counterweights they do not vibrate much. I have a new (2004) DV24ME and really love it compared to the Volvo's and Yanmar's I've seen. But I've only owned boats with Volvo's (and no engines) before. One of my friends installs these in rescue boats WW and is VERY proud of the product, which was one of the reasons for me to choose that engine (having your own mechanic is also great )
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Quote:
Originally Posted by GBurton View Post
I have found out that Bukh sells a turbocharger for the DV24 (The engine that I have) This might solve the problem that I have with the engine being too small for my boat (Westsail 32)

What are the pros and cons of turbocharging a sailboat engine?
The downside is shorter engine life and more likelihood of unexpended failure. I donít feel the need for very much power on a sailboat and in fact sometimes sail without an engine of any kind. Maybe you should try the boat before changing things.
All the best,
Robert Gainer
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The thing is that the engine is not installed. I have to make the decision on whether to install the engine or not.
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Quote:
Originally Posted by GBurton View Post
The thing is that the engine is not installed. I have to make the decision on whether to install the engine or not.
What's you displacement ?

Raven is 34' displaces about 8 tons in the old imperial scale, 7 tonnes metric.

Relatively easily driven hull, fin keel.

Our Bukh DV24 is quite sufficient for our needs. Yeah, maybe I'd go up in size if buying a new one but its basically cool as. No turbo. We run a three bladed fixed prop btw. Would prefer folding but can't be bothered to pay the dosh.
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