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Old 23 August 2010, 06:11   #1
Country: Other
Town: Cebu Philippines
Boat name: JP
Make: LianYa
Length: 5m +
Engine: Yamaha
Join Date: Jul 2010
Posts: 45
5.2M need help mounting outboard...

I am mounting a MERC 90HP 2stroke in a 5.2M RIB....right now it is already mounted and I measured the cavitation plate to be 2.5 inches above the bottom of the transom....from what I have read, it should be inline with the bottom of the's really not too easy for me to do everything all over and lower the engine, but is it really nescessary, or I can just run it 2.5inches above the bottom of transom?

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Old 23 August 2010, 06:27   #2
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Country: UK - Isle of Man
Town: Douglas
Length: no boat
Join Date: Aug 2006
Posts: 337
You are very likely to get "ventilation" or "aeriation" (not cavitation as some believe !!) which is when the prop is not fully immersed in water.
Does the mounting bracket not have a series of vertical mounting holes which "should" allow you to lower the OB (but probably not by 2.5ins) ?
Probably the best thing is to try the engine as mounted and see how it behaves.

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Old 23 August 2010, 09:32   #3
Country: UK - Scotland
Boat name: Wildheart
Make: Humber/Delta Seasafe
Length: 5m +
Engine: Merc 60 Clamshell
MMSI: 235068449
Join Date: Apr 2007
Posts: 4,614
For what its worth, the cav plate doesn't neccessarily need to be in the water. The prop on the other hand....

Have a search - plenty of threads about engine height.

I'll second cypman's thought about test it first.
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Old 23 August 2010, 10:46   #4
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Country: UK - England
Town: Southampton
Boat name: Hissing Sid
Make: Ross Smith Cobra
Length: 6m +
Engine: 200HP Optimax
MMSI: 235038046
Join Date: Jun 2002
Posts: 3,803
Originally Posted by jpn View Post
or I can just run it 2.5inches above the bottom of transom?
Try it and see...
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