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Old 23 April 2006, 14:36   #1
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Offset and cav plate height

I looking for advice from all who know about these things , I am about to have the transom beefed up a bit due to a previous owner cutting the transom down to accomadate a short shaft engine , my engine is a long shaft and we have run out of transom to put it in the correct position it is also not mounted true so we have a slight list at certain trim angles .
What I need to know from all the experts is the correct amount of offset (do I ask Humber? ) and where should the cav plate be in relation to the bottom of the hull ? Above level or below?
The new transom will be made from welded alloy sheet with suitable infill material to restore the transom to it origional shape . This will enable us to re drill the holes as at present it looks like swiss cheese with so many holes that have been filled !
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Old 23 April 2006, 16:07   #2
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someone told me that the cav plate should be roughly 1" below keel
but i might be wrong
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Old 23 April 2006, 16:24   #3
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When I had my 90 rigged last year it was put central on the transom (no offset) and Cav plate level to the bottom of the hull.

My understanding was that the Cav plate level with the hull bottom was a good place to start, but with certain set-ups the outboard could be mounted higher (fitting a Stainless Steel prop).
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Old 23 April 2006, 16:36   #4
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The higher, the less drag you get and the more speed. But you also loose grip in the water. Going with almost empty boat, you can lift the engine quite high. We lift our engies 2". But you normally start at cavitation=keel, then you may lift if everything is ok except top speed.
Instead of making the transom higher, you cold also change the mid section on the engine, making it a 20" or what ever you want. It sound like less work, but then you have this cheese-problem.
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Old 23 April 2006, 16:51   #5
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Quote:
Originally Posted by Fjordrafting
t
It sound like less work, but then you have this cheese-problem.
Good for a Norwegian......... to have cheese problem

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Old 23 April 2006, 17:54   #6
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Quote:
Originally Posted by LURCHER
I looking for advice from all who know about these things , I am about to have the transom beefed up a bit due to a previous owner cutting the transom down to accomadate a short shaft engine , my engine is a long shaft and we have run out of transom to put it in the correct position it is also not mounted true so we have a slight list at certain trim angles .
WTF don't people fit a Jack plate instead of mutilating a boat!

BTW it's not a Cav plate it's an anti Ventilation plate
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Old 04 June 2006, 18:12   #7
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Prop Swop

I have now got the boat in the water and the mods to the transom have really worked .The engine in now in the correct place no list 1.5 inches offset and cav plate is about 1'' above the bottom of the hull , we have also fitted a 19'' Laser 2 stainless prop . There is now vitually no cavitation at all which is great but I think I need a 17'' prop as we cannot pull more than 5200 WOT . Does anyone have a 17'' laser 2 prop they want to swop for a 19'' ?
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