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Old 11-07-2016, 08:53 PM   #20
Peter Knowles
Peter Knowles's Avatar
City: Victoria BC
Country: Canada
Vessel Name: Geordie
Vessel Model: 1953 38' Monk Tricabin
Join Date: Nov 2016
Posts: 12

Thanks for your thoughts.
Although I'm relatively new to power boats I've given it a lot of thought.
My natural inclination is that semi displacement is a marketing term.
To me if you're not up on a plane, you're sitting in the water.
Now there are certainly better bottom shapes, and the slipperyest is a canoe. pushing the water apart at the bow and allowing it's natural return to pinch the stern along forward. Problem is if you try to exceed hull speed you create a big void because the water hasn't returned against the hull in time. Thus massive wake.
Monk and others at the time, developed better bottoms to allow their boats to be pushed past hull speed without so great a loss, and you are correct, mine is a very sweet shape with a deep forefoot and almost flat at the transom only drawing a few inches.
This being a good improvement the canoe which rolls like a bugger without too much loss of efficiency.
But in the end I never intend to operate at any speed over hull speed, 1/3 throttle gives me hull speed, wide open gives me 2 more knots. And a tidal wave of wake.
I completely agree with you that an ideal speed is a bit below hull speed, it's so easy to judge it by the wake.
And yes, I also agree with your 3 to 5 HP per ton for a displacement hull.
Where I'm confused is where you indicate more power for a semi displacement hull. Is that for the same speed or a greater speed that is potentially achieveable?
The question would be, is my semi displacement hull any less efficient at say a knot below hull speed than a full displacement. I suspect it is, but only marginally. Can it really need 1.5 the power?

If I use 5 HP per ton, 9.5 X 5 = 47.5 HP.

I know the old Chrysler Crown had well over 100 HP, but in that era, throwing fuel away to achieve another knot or two was irrelevant.
I think it was way more than necessary for hull speed or that magical sweet spot slightly slower.

The power curves for my Perkins gives 50 HP at just over 1000 RPM!
But sadly peak torque at about 1800 RPM.
Poor thing is just carboning up.

Now in the end, all things being equal, I'd happily pop in 90 HP, "just to be safe" but my budget and the available space make that difficult.
Those old flatheads were pretty short.

Perhaps your engine recommendations will help me with those issues.
I'll look intro it.

Thanks again, great discussion.

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