Cavitation, Spun Hub, Wrong Prop or Something Else

[marksa1458]

I have a 14' runabout with a 1964 Merc 500 short shaft on the back. Engine runs great and pushes the boat to 35 on the old Airguide gauge that I have. My problem is that when I give it full throttle, take a turn or hit a sizable wave the engine RPMs will increase with no forward movement. If I back off the throttle and then increase the throttle slowly the engine will be fine. I thought I had a spun hub, so I had the prop rehubbed, which has reduced the problem considerably - before I had the prop re-hubbed it was very difficult to get the boat going and I had to very gently increase the throttle.

At this point, I am a bit stumped. I have tourqued the prop nut to 55 lbs.

Any thoughts and help would be appreciated. Thanks,

 

[Alumarine]

Re: Cavitation, Spun Hub, Wrong Prop or Something Else

Is that a 2 blade prop?
I thought most 50's would use a 3 blade.

 

[marksa1458]

Re: Cavitation, Spun Hub, Wrong Prop or Something Else

It is a 2 blade, I have two of them... I am also looking for a 3 blade for a Merc 500 Longshaft project that I'm working on.

 

[Texasmark]

Re: Cavitation, Spun Hub, Wrong Prop or Something Else

Nope, in those days, it was a 2 blade.

Mark

 

[CharlieB]

Re: Cavitation, Spun Hub, Wrong Prop or Something Else

Your motor mounting height looks close to perfect, although it is difficult to be sure by the pictures.

The A/V plate looks to be correctly at the same height as a line drawn back from the bottom of the hull.

Is this the trim position of the motor when the RPM increases without any change in boat speed? Or is the motor trimmed up, possibly loosing 'bite' in the water?

A 3 blade prop may have more holding power than the old style two blade.

This may be a good question for the guys in the prop forum, they really knkow a lot about perfecting set-up.

 

[Texasmark]

Re: Cavitation, Spun Hub, Wrong Prop or Something Else

Looks like you are a tad high on the engine mounting vs the bottom of the hull and your engine is trimmed/tilt pin pushed out more than might be required. This is great for performance, but bad in rough water and heavily loaded starts. You have to decide what you want and adjust accordingly. Personally, I'd keep it like it is and suffer through the undesirables....can't have your cake and eat it without spending a ton of money.

The problem you mention is ventilation (prop sucking in surface air) whereby the prop sucks air producing your problem. You can tuck the engine in somewhat and reduce the probability, but that will put your bow down into the waves (nice and smooth, but reduces performance and top end speed). Lowering the engine on the transom will also reduce the opportunity for the problem and your performance.

A cupped 3 blade SS high performance prop with a high rake could solve the problem but you can drop $400 in the deal.

Balls in your court as to what you want to do.

Mark

 

[marksa1458]

Re: Cavitation, Spun Hub, Wrong Prop or Something Else

Thanks for the replies - there is no change in this problem between the second and third mounting holes. The current position does ride better in the water though. Without modifying the transom, there is no way to lower the engine any further. I will try to get my hands on a 3 blade and see if it makes any difference. Appreciate your thoughts!

 

[Texasmark]

Re: Cavitation, Spun Hub, Wrong Prop or Something Else

Not being there to see 1 on 1 what's going on, a cupped prop, preferably a 3 blade, will bite more water and reduce the opportunity for cavitation for a given set of conditions with approximately the same pitch as what you have.

Mark