English: Motor height - Cavitation plate?

[PiperBill]

The general rule that the cavitation plate should be just below the keel doesn't seem to apply to my boat. I have a 17.5 ft Castello Marine, made in Finland, with a 50hp Johnson. This is it here:

 

The hull is stepped at the stern like this .

As you can see the cavitation plate is not in line with the bottom of the keel but is actually 1.5 inches up. When I bought the boat the motor was even higher. I have dropped it one hole since then. The higher position is pretty much standard on this model when delivered new. This is a fairly fast boat for it's class and will easily do 30 knots. I have noticed some venting (cavitation) on turns or in choppy water. This might just be standard for this boat/motor combo... I dunno..

 

My question is.. Should the cavitation plate be just under the water or just over? I have taken 2 pics of the motor at about 3/4 throttle. Here

and here and this one looking out from the stern. There is some water spay on both sides.

 

Any thoughts and or opinions would be appreciated.

 

Bill Carr

Mysen

[Brutus]

Hi Bill

 

Hmmm, strange looking keel. It looks like when you increase speed, the waterstream from under the main keel is lower than the secondary "keel" where the low speed waterstream comes out. But if it vents in turns it's probably to high, it should be in plane with the waterstream/slipstream. So somewhere between those two keels. But not as low as in plane with the main keel.

 

There's a few more ways to cope with this problem. But they are a bit expensive.

1.You can mount fins om the cavitationplate. U can go lower but get more lift. Such as "Sharkfin".

2.Mount a cupped prop with a bit more bite when cornering.

3.Mounting a jackplate that you can trim on the fly, and you are getting the transom forwarded a bit too.

 

Brutus

Redigert 22.September.2008 av Brutus (see edit history)