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Old 23 June 2004, 16:37   #1
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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, 17:09   #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, 17:32   #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, 14:07   #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, 15:41   #5
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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, 15:47   #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, 16:19   #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, 16:26   #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, 16:37   #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, 17:17   #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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Old 25 June 2004, 05:30   #11
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my performance

my 4m avon with its 50hp will give about 32knots with two people and 50L of fuel onboard. The prop is an aluminium 11 1/8 X 13 G if that helps. I have good acceleration since fitting my doel-fins and increased top speed. I know there are those on here that think / have noticed a drop in top speed, but i got a 2 knot increase after fitting them. These are GPS speeds, done over 2 runs.
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Old 25 June 2004, 07:30   #12
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IHMO you should call Julie at steel developments and ask her opinion, general rule is though that if your reaching max revs you can increase the props pitch by 2 inches either way, going up a pitch will lose you top end revs but should give you a better top end performace, but will be slower to accelerate onto the plain.
If your running a light fast boat then there's no reason why you cant increase up and gain better top end, to give you an example i'm currenly running a 50hp 2 stroke evinrude on a 4.7 Avon, my prop is 11/1/2x19 inch and one up on a flat sea with no wind resistance i can get 30 knotts, steel devs have said that the prop is over sized for the engine but becuase the boat runs with a relatively light load its ok to pitch up for better speed, my only problem comes when the boat gets loaded with people and kit, two weeks ago we went out 4 up with a load of kit and full of fuel and i could only just make 22 knots and it took ages and lots of weight adjutment to get up on the plain.
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Old 25 June 2004, 14:29   #13
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Thanks, for all the advice.
I emailed Steel deveopments and Julie said I could go up a size or they could re- pitch my current propeller. I would probably go up a size so that I would still have a spare propeller.
I would probably loose the initial acceleration but I could probably gain some back by fitting some doel fins. The boat is usuallly lightly loaded so I would not struggle to get on the plane.
However, I took the boat out today and according to the GPS I acheived a maximum speed of 30.2 knots. I dont know how I did it though! When I tried again I managed to get 27 knots. This seems fast enough for me although I will borrow a 14" prop to see if it will go any faster!
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Old 25 June 2004, 15:05   #14
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For what it's worth, my 4.1m with 40hp 2/stroke has a max speed of 25knots. I too am looking for some more speed. Would a stainless steel prop help?
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Old 25 June 2004, 15:21   #15
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Stainless props do make you go faster I think it is because the water runs of the blades faster. Steel Developments would be good people to ask about the technicalities.
There are two reasons why I am not considering a stainless steel proppeller. Firstly, they are bloody expensive!
Secondly, I launch from Christchurch and I am forever pushing my luck with the tide tables and touching the bottom! With aluminium being a soft metal I just get the odd scrape but a stainless prop could damage the gear box.
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Old 25 June 2004, 15:22   #16
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tim what wrong with 25 knots

last time i was out i had two large men and my top crew bethany going against the tide and had 25 knots from my vailant set up which i was very pleased with i can keep up with the sub 5m lot as we have talked about this before with phils avon searider the cost of making your boat go fast would be better spent getting a bigger boat if you want to go very fast if you mess with what you have you will lose something in the end

but what do i know
dan
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Old 25 June 2004, 15:33   #17
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I can see where your coming form Dan, 25 knots is plenty fast enough especially in 4 metres worth of boat. However, my friend who I regularly go cruising with has a 12 foot speed boat wwith an elderly 40Hp on it and he gets over 30 knots. Its very annoying when I cant catch him! Haviong said that when I went out this afternoon in a rough sea I thrashed him!

Alex
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Old 25 June 2004, 15:42   #18
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ADS has answered your question Dan; other people go faster than me with similar sized boats, I want to keep up!!!
If your boat can go at 25knotts with three people in it (and yours is pretty much the same as mine with the same engine) then in theory I definatly should be able to go fast (that's 25 knotts with only me, 25l of fuel, on flat water, with the engine fully trimmed out.)
Since my finances won't stretch to a new boat, I want to make this one go as fast as possible.
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Old 25 June 2004, 17:33   #19
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you could always put stripes on the side!
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Old 27 June 2004, 15:14   #20
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There's always someone out there with a bigger faster boat than yours, where does it end, and at what point does it turn from being fun to becomming a mission ?
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