prop/engine/boat setup

[rollover]

Hi Guys,
Joined today since every time I go looking for RIB info I seem to end up here anyway.
I have a 4.8m Chinook that I've put 2 Yamaha 55s onto. The (common?) 13" pitch prop took revs right up to the top end, but without much extra speed. I've put a 17" pitch prop on the starboard engine, which has given increased top end speed, but I now have a lot of water splashing over the transom.
I will adjust trim to see if things improve, but if someone here could explain the science behind prop/cavitation plate/hull relative positions I'd be very grateful.
Cheers, Roy.

[boristhebold]

There are a lot of decent articles on the internet, do a search, you will learn heaps.

Basic points are that determin performance.

Point 1, what do you want to do in your boat ? sounds a stupid question but if you want max top speed and dont pull water sports people like water skiers then set up of prop will be different to if you want to do mainly water sking etc.

Boat type
Hull Type and length
Engine hieght
Prop diamater
Prop pitch
Cupped or uncupped
Steel or stainless

all have a bearing.

If the 17inch pitch prop you put on had the same diameter as the 13inch pitch one I would have expected your top speed to go down and the revs to go down. Something doesn't side right.

Rule of thumb is what is the current prop you have on and what is the performance like with that, are you missing any revs at full throttle or not, then consider what you want to do in your boat, read all the articles and try a few different props witjin the range of your engine and boat performance.

Never expect when you buy a boat whether new or used that the engine has the best prop fitted especially if second hand.