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Old 23 June 2004, 17:37   #1
ADS
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Another new ribber!

Hi,
I, ve been reading the forum for a while but this is my first post.
I am after a bit more top end speed, currently my 4m Avon when lightly loaded makes 25 knots @ WOT. The acceleration is very fast but the top speed seems slow in comparison to similar RIBS. I was considering changing the prop, the current prop is marked 11.5 x 13 and is marked with the suzuki serial number S1301. Would a different pitched prop make any difference or is this the fastest it will go!!!

Any advice would be great!

Alex
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Old 23 June 2004, 18:09   #2
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I think the important thing is the revs. Does your prop and loaded boat hit the max revs suggested, if so, stick with it. If you are not hitting max revs then you can think of changing the prop.
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Old 23 June 2004, 18:32   #3
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ADS

Hi and welcome to the forums, I can only suggest you try and 'Borrow' another prop to try as 25Knots does not sound like enough to me, particularly as you say the acceleration is very good. However Ebay is always a good source for cheap props.

Red Fox's comment about revs is a good one, but it would be quite unusual to have a revcounter on a Boat/combination of this size.

Perhaps your boat was set up for towing, a 40HP engine is plenty big enough to get a skier out of the water, but its easier with a prop set up for grunt rather than speed.

Good luck

Nasher
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Old 24 June 2004, 15:07   #4
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Revs

Nasher, thanks for the advice. Unfortunatly I do not have a rev counter but being a Suzuki 2-stroke it seems to make a lot of noise!
When at 3/4 throttle and increasing to flat out the boat still accelerates very fast whereas on other RIBS I have driven the last inch of throttle just increases the volume!
Does any body else on this forum have a similar size boat and engine combination and what prop are they running?
I would like to be able to cruise at around 24/25 knots and hit 30 flat out. Am I being a bit optimistic? I know SR4's with 50's can break the magic 30 knot barrier.
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Old 24 June 2004, 16:41   #5
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Country: UK - England
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Boat name: gazza's girl
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previous boat was a 4.95 Zodiac cherokee with a mariner 50 4 stroke on the back....

On a good day and two up we could just touch 30 knots, but it was happy to cruise at 24-25 knots all day...

How old is your engine?????


Gazza
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Old 24 June 2004, 16:47   #6
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Gazza, the engine is a late '92 model but is in mint condition. The RIB used to be the tender to a large 100ft yacht and the engine has not been thrashed. Judging by the condition of the cowling and inside it has not had alot of use, it has also been given a clean bill of health by the local outboard specialist. Maybe 30 knots is a bit optimistic especially considering yours was a newish engine.

Alex
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Old 24 June 2004, 17:19   #7
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I know this might seem a stupid question, but what fuel do you use???....

I found Shell Optimax gave the best performance both on the boat and in our rally car....

I suggest you give it a try...

In some cases it alledged that you gain 7-10% more power...

Gazza
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Old 24 June 2004, 17:26   #8
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Thanks Gazza,

I ve being running it on diesel the whole time. (only joking) I put the normal unleaded in it.
Thanks to my 'flexible' sixth form timetable I am going out tommorow afternoon so i will give optimax a try!
Being an older engine do you think it was originally intended for leaded 'four star' if so I've got some Redex somewhere in the garage would that help?

Alex
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Old 24 June 2004, 17:37   #9
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Quote:
Originally Posted by ADS
Thanks Gazza,

I ve being running it on diesel the whole time. (only joking) I put the normal unleaded in it.
Thanks to my 'flexible' sixth form timetable I am going out tommorow afternoon so i will give optimax a try!
Being an older engine do you think it was originally intended for leaded 'four star' if so I've got some Redex somewhere in the garage would that help?

Alex
I think the fact that its a two stroke should be ok.... Unleaded fuel in leaded engines only caused problems with the valve seats....

On a two stroke you don't got any....... Unless anyone knows differently..

Gazza
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Old 24 June 2004, 18:17   #10
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Word of advice

I seem to have had the same problems as you so I bought a new prop. I went up two prop sizes.

Now I have gone from bags of acceleration with max speed of 29 mph in all scenarios to having trouble getting on the plane with two people but masses of speed. I've gained 5 mph total but have real trouble getting on the plane.

I think I should have gone up only one prop size.

I have removed the new prop and am saving that for solo sailings.

When I save up some more money I shall get the next size up prop.

Nick R
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