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Posted: Wed Nov 10, 2004 7:02 am
by norbert
J Dower Posted: Norbert - I am getting a Yamaha 50 for an M so I was glad to see your post. What prop do you use, and what speed at cruisng RPM ?
sorry can't tell you my prop size, the boat is 2 1/2 hours away, and i never really cared. i'm crusing mostly with ballast full, rudders down, cb 1 ft down at about 6 knots. i think the engine will be rervolving at estimated 2200 rpm (rarely looked on my tach!). very quiet voyage so far.
if in a hurry i can attempt about 12 knots with ballast and 15 without (depending on number of crew), both with mast up and boat fully equipped for extended trip.
Ventilation?
Posted: Wed Nov 10, 2004 8:17 am
by Terry
OK Darren & Moe, You really brimg things into perspective, and I thank you, but at the same time pique further curiosity. About that ventilation problem, is there any hard & fast rule as to how much clearance there should be between the blade tip & anti-cavitation plate. I measured the radius from the centre of my prop shaft to the plate - 6.25" so theoretically I can swing a 12.25" diameter prop and still have 1\8" clearance. The dealer puts 3 X 12 X10.50 or 10.25 props on (Honda's) which provides 1\4" clearance. Since my 4 blade is 11.4 X 9 I have about a 5.70 radius which gives me a good 1\2" clearance. So the big question is, should we strive to get the blade tip as close to that plate as possible for maximum diameter and blade area or should we be making some allowance for a certain amount of clearance to address the ventilation problem.
PS. In retrospect, after digesting all this, If I could go back in time, I would choose a bigfoot or some engine that can swing a 14" prop. I am beginning to realize that a larger diameter is more important than a higher pitch for these boats. No wonder BWY is putting 709Hp engines on these boats.
Posted: Wed Nov 10, 2004 8:54 am
by Moe
Studying for a test today, so don't have a lot of time, but here are a few points:
Cavitation is air bubbles forming ahead of the prop as a result of low pressure ahead of it. The bubbles can cause erosion of the prop surface.
Ventilation is air from the surface being sucked down into the prop causing it to lose grip on the water. The plate above the prop is an anti-ventilation plate, not anti-cavitation.
A BigFoot has 1/2" clearance between the plate and the tip of a 14" diameter prop. I've always considered clearance here to be more important for keeping a line from jamming between them, and haven't really thought about its effect. Maybe Darren can add something here.
I agree that I'd have rather had a BigFoot than the Suzuki DF50 our used boat came with. There doesn't seem to be much, if any difference in top speed, though. Thankfully, a member here, Robert, has done extensive testing of DF50 props, and I'm not going to reinvent the wheel.
--
Moe