Hi all. I am currently building a Garden designed sailboat - 28 feet on deck, 23 ft waterline and displacing about 11,000 lbs.(loaded) as designed. I would guess more like 12-12,500 when finished (hull a little heavier than drawn) Cutaway forefoot on a full keel hull. The original spec calls for a 10 h.p. Bukh but conventional thoughts put this a little low for power.
I am thinking maybe the 20 h.p. Beta would be a better candidate. Prop size originally spec'd at 11x9 2 blade, but cut-out for prop not yet done so it could be changed. Any thoughts appreciated and thanks
Welcome to the forum. You are always better off on a full keel boat to have a larger slower turning propeller. A 3 blade prop will be much more efficient than a 2 blade. A 2 blade prop in an apeture is possibly one of the worst ways to power a boat.
The best drive using a Beta 20 will come from using a 2.6:1 transmission but this will call for a 15"x 11" three blade propeller with 50% blade area ratio.
You will get similar performance if you use a 2:1 transmission with a 13"x9" three blade (again with 50% BAR) but expect some cavitation and prop noise with the faster turning propeller.
Now it is up to you to make your decisions on drive vs drag amongst all the other decisions (compromises?) that you make when building a boat.
Hi Stanley, thanks for the reply. Upon looking at the drawings in daylight (best excuse I can come up with on short notice !) I see the prop is spec'd at 16 inches in diameter, not 11 as I posted. That is more in keeping with an efficient prop size for this installation I would think. Now to mock up an engine and bore the shaft hole...
Also, as stated, there is some room for modifying the aperture from original as it has not yet been cut.
cheers, Brian
Hi Brian, have you considered a feathering propellor? I repowered from a 34 HP Perkins to a Beta 50 with a 4 bladed Variprop. Due to my aperture size, a 4 bladed propellor was indicated. The performance isgreat!
