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Old 19 May 2012, 14:51   #1
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Do i need to modify my Transom / Get a longer Outboard?

RIB I have used to have Twin 60hp Mariner Bigfoot engines. When I bought it I had to find an engine so I have fitted a Single Mercury 75hp to her.

Today was its 1st outing and a great day it was so much so that I stayed out for 6 hours. However above 1800rpm there was alot of cavitation. On looking at the prop height I suspect I need to lower the engine / Fit a longer shaft engine to clear the hull is this likely to be what was causing the odd cavitation feeling?

Felt like it was revving but no more acceleration and alot of reverberation on backing off it would settle again but disappointing at 20mph.

I will let the photos do the talking:





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Old 19 May 2012, 15:05   #2
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Thats way way too high anti cav plate should be level with the bpttom of the hull. or so i am led to belive.
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Old 19 May 2012, 15:18   #3
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Yes too high but I do also remember discussing with you if a 75hp was enough to replace twin 60's ? I think you need at the very least a 90hp to get adequate performance and ideally a 115hp+

Are you re-considering ? might be worth deciding before cutting the transom as a 115hp could one in XL.

Plus you could then fit it correctly, bolted to the transom rather than sat upon it.

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Old 19 May 2012, 15:19   #4
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your right Jezza, as a general rule it need to level with the bottom of the hull, so drop the engine down till it is!!
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Old 19 May 2012, 15:29   #5
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I was told to have It an inch higher than the bottom of the hull on here
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Old 19 May 2012, 15:43   #6
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Pete I think the 75 pulls well enough and it was getting up onto the plane prior to the vibration. This one is bolted to the hull? Do you have anything in that would fit well without transom modification? Like to stick with this engine if I can spent a fair amount on servicing & its a good engine.

Open to suggestions on what to do. appreciate any advice.
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Old 19 May 2012, 16:00   #7
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level to an inch is fine, looks like you are 3-4 inches to high!! get your spanners out and get on with it!
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Old 19 May 2012, 16:05   #8
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Fair enough. Looks like cutting down the transom is the way ahead in that case. We have nothing better suited right now.

I know it's bolted to the hull but it's entire weight is also resting on the top of the transom, rather than being raised off it and bolted. Obviously not possible with a "long shaft" on that transom as it will be further up again. Your transom must be for a 25" shaft, have you measured it ?

These pictures show some on file fitted without it resting on the transom. See what I mean ?

The last pic is of similar motor, nicked from another Ribnet thread on here discussing the same topic. Have a read of it.

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Old 19 May 2012, 16:08   #9
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Quote:
Originally Posted by matt h View Post
level to an inch is fine, looks like you are 3-4 inches to high!! get your spanners out and get on with it!
Spanners wont work here unfortunately keen to modify the transom cutting a slot of the required height if its safe to do so as of course that will be the cheapest option.

Could fit a nice Stainless top to the transom then on refitting the engine for Anti Chainsaw purposes
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Old 19 May 2012, 16:09   #10
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Hi Tris




1" above the keel is good for this size boat in my experience, this is a good balance of grip in the turns to top end speed. As mentioned that is way too high and looks as though it has been used with a XL shaft, the 75 you've got I'm pretty sure is a long shaft?
If it's set up right it should push your boat along just fine.
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