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Old 20 November 2011, 16:27   #1
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First run on new RIB

Hi All

So today was the first run on my friends new RIB, 5.2m with 90 merc.

Straight off the bat it is porpoising all over the place, flat calm and its like being on a bucking bronco !

Also lots of water shooting up between the shaft and transom.

When we got out of the water and put the boat on the trailer I checked where the cav plate was sitting in relation to the keel of the boat and discovered it is set up about 3 inches below the keel

I am guessing that there are 2 issues here.

1. The engine needs to be raised, I am certain this is what is causing the water shooting up the shaft and most likely causing the porpoising.

2. The tank at the moment is in a locker at the stern of the boat, I am thinking that if we move this forward it will also help.

Do you ever have to ballast a RIB, I am thinking a bit of extra weight in the bow might also be a possible solution.

Cheers
Stephen
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Old 20 November 2011, 16:35   #2
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Old 20 November 2011, 16:45   #3
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Yes, probably raise the engine. More weight up front.

Is the boat rated for 90hp - is it a four stroke merc?
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Old 20 November 2011, 16:53   #4
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Hi Willk, Yes, rated for 90, its the 2 stroke merc Optimax.
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Old 20 November 2011, 16:53   #5
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Sounds like the motor is too long for the transom. What's the height of the transom at the centre and is the motor std, long, or XL shaft?
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Old 20 November 2011, 16:57   #6
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Quote:
Originally Posted by Downhilldai View Post
Sounds like the motor is too long for the transom. What's the height of the transom at the centre and is the motor std, long, or XL shaft?
Its the correct size, 20" from top to keel and its a long shaft 20" leg.

I think he has just bolted it on too low on the transom.
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Old 20 November 2011, 17:09   #7
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Originally Posted by sburt10 View Post
Its the correct size, 20" from top to keel and its a long shaft 20" leg.

I think he has just bolted it on too low on the transom.
He wouldn't be able to create a 3" spacing from the cav plate to the keel in these circumstances.
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Old 20 November 2011, 17:17   #8
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Sounds like an XL shaft. Wrong length for the transom... Ideally the Anti ventilation plate should be level with the bottom or possibly and inch or two above depending upon engine, prop etc.
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Old 20 November 2011, 17:28   #9
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The merc shafts are a little over 20"

The transom is approx 20"

The only other options are to go for a short shaft at 15" (approx)
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Old 20 November 2011, 17:30   #10
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No, its not an XL shaft guys.
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