Join Date: Feb 2011
Location: San Francisco Bay area
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Re: Adding Hydro Foil to Yamaha 8 HP 4 Stroke on 27 Catalina
Firstly it's not clear that you understand the concepts of displacement hulls, and hull speed.
Most sailboats have what are called displacement hulls, which are optimised for a mode where the water passes around the boat, instead of the boat riding over it like the planing hull of a speedboat. This means that your sailboat does not have the hull features which would make it plane - at ANY power (those features are large flat sections aft)
So you are stuck in displacement mode. A displacement hull has a concept called hull speed. This is the point where you are putting in more and more power, and not going faster. The power goes into the bow wave which gets bigger and bigger. As the bow wave gets bigger, the bow rises up it and the stern drops. This is your problem.
The fix is to go more slowly. I cruise at the point where the exhaust pipe is just above the water - about 6 kts on my boat.
I would expect your C27 to have a hull speed of about 6 kts, though you might get too much squat lower - as low as 5 kts.
You will also get much better fuel consumption cruising at the speed where the squat is small, as the power is going into moving the boat, not creating a huge bow wave.
Bristol 31.1, San Francisco Bay
Last edited by MarkSF; 04-02-2013 at 04:10 PM.