GOP Home Build

Pictures of the boat out of the water, particularly showing the engine area, might help. What kind of props are you running, stainless or aluminium ? The latter will give a lot of cavitation and/or ventilation problems on cats. The smoothness of the hull is not a factor for this. Even if your cav plate is running level with the bottom, and there is no appendage causing turbulence, alloy props usually let go, particularly in turns.

EDIT: I just looked at the pics in post #173. Seems to be aluminium props. Unfortunately testing with stainless won't be cheap. Some people have resorted to five-blade stainless on some cats. Three-bladed worked well for me though. Check you are getting the revs into the recommended max rpm range before choosing any props, you may need to come down in pitch.

 

They are Aluminium Props. I want to go to four or five blades, stainless would be good, but as you say expensive. Ill get some pics next time I take her down. I don't really care for speed, just want a reasonable cruise, and lots of bite. Can the ventilation be that noticeable with rubbish props?

 

Yes, aluminium is far from ideal where there is a tendency for the prop to lose grip. Is it happening in turns only ? If it is, I'd say stainless will be the answer. Don't spend money on more alloy. Stainless gives you better turning, more trim range. The difference is surprising.

 

Ok will do. I'll need to start calculating pitch/diameter vs revs, and that's going to be hard if I keep getting Ventilation.

 

To re-iterate, the main thing is when this problem occurs, is it happening in straight-line running, once on plane, or only in turns, no matter if they are gentle turns ?

 

Straight line when I throttle up too much. At low speeds no issue, as soon as it comes on the plane seems to lose bite and then start to over rev and lose speed. On one motor no cavitation. In turns cavitates at the same rate as throttle up.

 

You mean only one of the engines does this ? If that is so, you may have a slipping prop hub, which is easily repaired by a marine shop. To test that, clearly mark both the rubber hub and the propellor boss at the same spot on the circumference, if after use the marks don't line up, it is slipping under load. Alternatively, that prop may have blade damage. Inspect for that.

 

Nah, I mean it does not cavitate when under power with one motor only.

 

I'd have thought it would not plane on one motor.

 

Correct, it's not planing on one motor.

 

Gone to single outboard and she is going really really well. 30 knots WOT stable as. Hole shot is flat and fast. She leans a bit in the turns, taking some getting used to.

 

G'day again. How many hours have you done, any rough water use, and how is the structure holding up ?

 

Haven’t been bold enough for the rough water yet. Not many hours at all. Been on a go slow. Bit of chop on the lake but no open ocean yet. Structure is good. I did throw a lot of fibreglass at it and foam filled so hopefully strong enough. We’ll see in time. She rides very smoothly.

 

Thanks for the tip that's about exactly what I was thinking of doing some day. I guess maybe it'll be more like 5 or 10 horsepower. Just keep it in Swift displacement mode.

 

It seems pretty light, the way it accelerated away, do you have a hull weight ? Whilst being light does have its advantages, with planing cats it can be a mixed blessing, when you get up and running fast at certain angles to a wave train, it can be unnerving, and sometimes get away from you.