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  Lowdown on Cav. plate height

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Author Topic:   Lowdown on Cav. plate height
Vek posted 06-25-2002 08:43 PM ET (US)   Profile for Vek   Send Email to Vek  

Let me preface this non-whaler general question by mentioning I do own a whaler and enjoy the information I glean from this site.

My father owns a 20' welded aluminum boat (deep vee, coastal pacific northwest/southern BC type boat) and the deadrise is probably around 18-22 degrees at the transom. The boat builder set up the Evinrude 175 so that the top of the cavitation plate is planar with the keel line - so it is quite submerged while running. Thus dragging, we're swinging a 15-inch pitch prop and getting 5600-5700 rpms out of her. The 15 inch pitch prop has a large diameter (15.5? factory OMC/Bomb alum.) and is not far below the bottom of the plate. Can we raise it up a hole without blowing out? Is this one of those "try it and see" questions?


Vek posted 06-25-2002 08:46 PM ET (US)     Profile for Vek  Send Email to Vek     
Oh, a few more things. As far as the height goes, if you sight down the top of the Cav. plate, it is directly in line with the very bottom of the vee, along the keel.

Also, the boat sees use in predominately large sound chop, so blowout is of some concern.


Clark Roberts posted 06-26-2002 06:40 AM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
Vec, it's a "try it and see" situation! run it as high as possible without "blowing out" the prop on hard accelleration/turns and while maintaining adequate cooling water flow! Testing is the only way I know... Clark.. SCN
Tom W Clark posted 06-26-2002 11:00 AM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     

It sounds like the builder has the motor set up correctly. You can experiment if you like. There's nothing "exact" about where the cavitation plate is supposed to be relative to the keel. It can be below the water's surface as it almost always was in the old days, or above it as it is on fast high performance boats. Try it and see.

On the boat you describe and in our local waters, you probably are not trying for the absolute maximum top speed, but rather want good grip in the choppy conditions we experience so much of the time. To that end you neednít jack it up as far as it will possibly go.

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