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Old 25 June 2004, 06:30   #11
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Country: UK - England
Town: Torbay
Boat name: Marlin
Make: PBS humber 5.5
Length: 5m +
Engine: 115 optimax!!
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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, 08:30   #12
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Make: Avon and Narwhal2.4m
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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, 15:29   #13
ADS
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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, 16:05   #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, 16:21   #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, 16:22   #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, 16:33   #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, 16:42   #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, 18:33   #19
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you could always put stripes on the side!
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Old 27 June 2004, 16:14   #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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