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Old 11 September 2005, 16:02   #1
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Why won't my RIB go as fast as it did?

Last year I got 40mph out of my RibX 575 with 90HP suzuki on a flat calm day in southampton water. Yesterday I could only get about 34 out of it on the same stretch of water in flat conditions. Any one one give a clue as to why this is?

As far as I know the trim was in the same place. I had my two fairly light teenagers onboard last year, yesterday three heavier adults. I was going in the opposite direction to the tide (can't remember what the tide was doing last time). Would the tide/extra weight make much difference?
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Old 11 September 2005, 16:05   #2
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IMHO, the tidal flow (direction and speed) could make a 6mph difference, if you were going with the flow last year and against it this year.

Also, bit of a personal question ( ) but have you got a dirty bottom?
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Old 11 September 2005, 16:09   #3
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How is this speed being measured? If by boat speedo, then any kind of restriction in the pipe or blockage throws it completely. Does the boat feel slower? My guess is that it's not. A gps reading would be more accurate.
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Old 11 September 2005, 16:17   #4
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I lose about 4 knotts when two more adults are on board, but weight is a killer of speed as is barnicals on ye bottom.

There are loads of factors involved from tide speed to wind direction, water in the hull and air temparature. I wouldn't worry about it unless your revs are excessivly down or the outboard is running rough.

Andy
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Old 11 September 2005, 17:19   #5
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Have you drained the bilge recently?? I got about 5 gallons out of mine last time I drained it.That much slows you down

Jon
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Old 11 September 2005, 18:02   #6
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Quote:
Originally Posted by Cork Rib
Have you drained the bilge recently?? I got about 5 gallons out of mine last time I drained it.That much slows you down

Jon
I would check into this,

My Rib has a bilge pump which works fine, but doesn't reach down to the very bottom of the "V".

After a few months quite a bit of rain water eventuallly gathers there, but a quick dry out, and out with the plug...hey presto an extra 8 knots.
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Old 11 September 2005, 19:35   #7
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Quote:
Originally Posted by Hightower
I lose about 4 knotts when two more adults are on board, but weight is a killer of speed as is barnicals on ye bottom.

There are loads of factors involved from tide speed to wind direction, water in the hull and air temparature. I wouldn't worry about it unless your revs are excessivly down or the outboard is running rough.

Andy

The revs are the key. Same revs or lower? Weight will slow you down most followed by dirty bottom. GPS = speed over ground - pitot tube speedo = speed through water. Tide is therefore a factor dependant on which speed method is used.
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Old 12 September 2005, 01:52   #8
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and the prop....

Not as much as you are talking about....
But I got nearly 3 knots back by letting a propmaster have a look and balance the prop...hit any whales lately ;-)
best
j
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Old 12 September 2005, 08:49   #9
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Thanks for the replies.

I recover and wash of the boat every trip so my bottom is nice and white.

GPS for recording the speed.

I don't think the bilge is a problem. It has an automatic blige pump but I undo a drain at the very bottom of the transom after every trip because the dealer told me to - I can't remember why he said to do that but I guess that is the bilge drain plug.

I can't remember what the revs were before but I can't say as I noticed any change in engine performance.

The prop has a few nicks around the edges - would that make any difference?

I read somewhere on here that some people get more performance out of their engine by using that super plus unleaded - is that worth a try?

Peter.
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Old 12 September 2005, 09:31   #10
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The Prop

Well,
I would have thought some other people on here will tell you balancing and
repairing the prop is a good idea regardless and should give you a few mphs back, and surely a good prop guy will fix that for a decent fee if you have one around there....
FWIW, YMMV
janne a.
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