Recommended HP for a 20ft wooden motor launch

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picardy

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Some advice please......

I am in the process of finalising the specification for a new build 20ft clinker built motor boat with a 7ft 6" beam. The boat will be used in coastal areas along South Devon and Suffolk coast. An online calculator tells me max hull speed will be about 6 knots and I suspect the boat will have a dry weight around 1000-1200kgs but am not sure what engine HP to go for.

The builder would like to use Vetus because they supply an around the engine package (shaft, prop, gland etc etc) which makes installation easier. This means I can have either a 2 cylinder M2 18 which is 16hp or a 3 cylinder M3.29 which is 27hp.

I am leaning towards a 27hp because it has 3 cyl so will be smoother (quieter?) for not much extra cost in the scheme of the project, but what do others feel about the pros and cons of a larger hp engine. And can anyone tell me what the displacement of this boat might be?

Many thanks
 
I had a 20hp Beta in a Sussex beach boat, 16ft x 8ft ish, quite flat bottomed. It was, I suppose, too much but acceleration and braking (!) were quite impressive. I suspect the smaller of your two options might be the most appropriate. There must be a significant saving in £££s and weight.

Look at the Plymouth Pilot website, 10hp for 16', 16hp for 18' etc etc
 
Given that this is a displacement craft, a 27hp diesel will never get up to optimal rpm and will be seriously underworked. For comparison, a similar engine powers my heavy displacement, full keel, 7 ton displacement yacht at 6.5 kt. This is a case that bigger is not better. A properly installed smaller diesel is much the better option.
 
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Thanks for the comments - are there any benefits to having an over sized engine? eg Low down weight to improve stability , smoother running with 3 cylinders, greater torque when running against a strong tide or ......is it a total waste of money?
 
Someone I knew built two 20ft strip planked displacement double enders. First, he used the 2 cyl Vetus package, but for the second, he used the 3 cyl engine as 'more reserve'.
Personally, I thought it was overkill, given the hull speed, but he thought it worth it. Possibly if heavily loaded it would come into play.
 
picardy said:
Thanks for the comments - are there any benefits to having an over sized engine? eg Low down weight to improve stability , smoother running with 3 cylinders, greater torque when running against a strong tide or ......is it a total waste of money?
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You've stated the advantages, but the major disadvantage is reduced engine life because of continuous use below the engine's rated power. The other disadvantage is that you may not be able to fit a propellor capable of absorbing the extra power.

For comparison, I have a Moody 31 with a Volvo 2003 28HP engine. I rarely use full throttle; the boat reaches its hull speed at about two-thirds throttle. After that, all that happens is that the stern "squats" in the water without going any faster. The extra power is sometimes useful when heading into a bit of a chop; it makes no difference against the tide - that's an old wives tale. Basically a boat "sees" the speed through the water - if the water is travelling, then you simply do the vector sum of the direction and speed of the water and the direction and speed of the boat with respect to the water. But "plugging the tide" does not depend on power - it depends on speed, and more power does not equate to more speed. And using the extra power to make progress into a head sea is very uncomfortable!
 
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