Getting Screwed!

[Renegde_Sailor]

I have a propeller question...

Right now I am sailing my Spencer 42 to Alaska, she has a 12-8 pitch 19" 3 blade prop, I have noticed every time we wind up pushing through some waves running against us that the prop cavitates and the boat slows way down.

As well with my 40hp engine, it can easily spin the prop, I never have to run full throttle to get max speed because if I push it too far, it just cavitates.

I cannot go any bigger due to the aperture limiting the size, but I could go to a 4 bladed pitching prop like a MaxProp.

I am curious if anyone more knowledgeable than myself might have some insight on doing so, right now at 2200 rpm where the engine runs real nice, she will cruise at 6 knots, if I push her a bit harder she will clip along at 7.2 above that the prop just cavitates.

Or maybe it is not even worth messing with, but it would be nice if I could get a bit more speed under motor out of her without having to rev her so hard, it feels to me like she is under-propped, and I am wondering if going to a 4 blade would put her in better order when motoring against waves and current?

[Lee Jerry]

I have more questions first...

The size of your prop is not clear to me. It is 3 bladed. And the diameter is 19", right? So the pitch is what?

What is the condition of the prop? Is it clean, without growth on it? Any bent blades?

What is the maximum rated speed (rpm) of the engine? What the max eng speed you can reach, either limited by excessive cavitation or at WOT?

What type of prop is it: fixed, folding, feathering?

[Renegde_Sailor]

Originally Posted by Lee Jerry

I have more questions first...

The size of your prop is not clear to me. It is 3 bladed. And the diameter is 19", right? So the pitch is what?

What is the condition of the prop? Is it clean, without growth on it? Any bent blades?

What is the maximum rated speed (rpm) of the engine? What the max eng speed you can reach, either limited by excessive cavitation or at WOT?

What type of prop is it: fixed, folding, feathering?

Fixed, I believe 19" and the number stamped on the side is 12-8 so I am assuming a 8" pitch? Prop is clean, was serviced and balanced last summer while the boat was out of the water.

[Renegde_Sailor]

I should add it might be a 18", I measured it when I pulled it, but that was about 50,000 projects ago!

[Pete7]

Is this a new problem, or has it always been like this?

What is the clearance from the prop tip to the hull? should be at least 10% of the of the prop radius.

Problem with props is its a black art.

Pete

[George_SD]

you may still be able to get a larger prop by replacing your shaft for a longer one (IF you have space)

[Renegde_Sailor]

Originally Posted by Pete7

Is this a new problem, or has it always been like this?

What is the clearance from the prop tip to the hull? should be at least 10% of the of the prop radius.

Problem with props is its a black art.

Pete

Well before this I had mostly sailed protected waters do I don't know as far as bashing through waves and having it cavitate.

The bottom job on my boat was hot trash and slowing it way down, she is much faster than what she used to be, I gained a knot and a half from when I pulled her out of the water to strip her.

[Renegde_Sailor]

I just pulled out my old survey, it says 16" 3 blade 12 pitch, but 12 seems pretty steep to me, the prop had numbers stamped on the side which I believe were 12-8

[Renegde_Sailor]

Originally Posted by George_SD

you may still be able to get a larger prop by replacing your shaft for a longer one (IF you have space)

Keel hung rudder with an aperture.

[Panope]

What symptoms occur during the events you describe as "cavitation" ?

If the answer includes "a rise in engine rpm and loss of boat speed", then I would consider the possibility that the clutch in the gear box is slipping (assuming your gearbox has clutches), and that there is nothing wrong with the propeller.

[Jim Cate]

Another critical bit of info needed: what is the reduction ratio in the gear box?

Jim

[Renegde_Sailor]

Originally Posted by Jim Cate

Another critical bit of info needed: what is the reduction ratio in the gear box?

Jim

Yanmar 4JH3E 36.8HP @ 3650 rpm (I run at 2200)

2.36 reduction

[Renegde_Sailor]

Originally Posted by Panope

What symptoms occur during the events you describe as "cavitation" ?

If the answer includes "a rise in engine rpm and loss of boat speed", then I would consider the possibility that the clutch in the gear box is slipping (assuming your gearbox has clutches), and that there is nothing wrong with the propeller.

Happens when I hit a wave, you can hear the bubbles on the prop under the boat.

Same as if you were starting from a standstill and gun it hard, you get the bubbles noise then as the boat comes up to speed it stops. Don't really see a rise in engine RPM.

[Panope]

Originally Posted by Renegde_Sailor

Happens when I hit a wave, you can hear the bubbles on the prop under the boat.

Same as if you were starting from a standstill and gun it hard, you get the bubbles noise then as the boat comes up to speed it stops. Don't really see a rise in engine RPM.

Ok.

Rather than "cavitation" (the boiling of water due to extreme low pressure areas on the propeller face),

consider the possibility

that what you are experiencing is actually "ventilation" (air entering the propeller stream).

A friend of mine had an Islander 37 that had the propeller mounted very close to the surface. In waves the propeller would ventilate due to the boat pitching.

[Renegde_Sailor]

Originally Posted by Panope

Ok.

Rather than "cavitation" (the boiling of water due to extreme low pressure areas on the propeller face),

consider the possibility

that what you are experiencing is actually "ventilation" (air entering the propeller stream).

A friend of mine had an Islander 37 that had the propeller mounted very close to the surface. In waves the propeller would ventilate due to the boat pitching.

I mean it is possible but I didn't think we were ramping that hard, these were 5-6' swells.

Here is a profile drawing of the boat.




Possible it could be venting as you said, I am going to have to pay more attention to it.