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Old 15 December 2019, 14:05   #1
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Country: UK - Scotland
Town: Ayrshire
Boat name: The Gaffer
Make: Avon Deluxe
Length: 5m +
Engine: Yamaha 90
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SR 5.4m Prop/Cav plate

Advice needed please.
SR 5.4m Deluxe and almost new Yam 90a.
I have taken as much weight out as poss and now just have 2 side by side seats and 2x53l fuel tanks immediately behind.
Im mainly interested in speed and need advice on props and engine height.
1 hole up on transome seems the view, is there to much cav if move up 2 holes?
Props SS would be my pref
17@5500 38.5kts
19@5500 43kts
20@5500 45kts
These are theoretical max speed with no slip.
Do these boats really have such little slip as it would seem?
Can I get away with 13 diameter or is 13.25 going to give better bite/less slip.

Ive been through the trial and error route with my Magnum 27 which was costly ( although got a couple of really good set ups) but hope to crib off others with the SR.
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Old 15 December 2019, 14:54   #2
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Country: Australia
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An idea on what prop is currently on it and what max rpm the engine is reaching with that prop would be handy. Raising the outboard by each hole will often increase rpm 50-100rpm due to less drag. The difference between aluminium and stainless is marginal but the price difference is quite considerable 6/1, also aluminium is easier on the gearbox if you do a lot of in out gear changing.
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Old 15 December 2019, 15:25   #3
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Haven’t tried it yet so don’t know WOT RPM is.. I guess as boat is so small the SS prop advantage might be marginal.. that makes it a lot cheaper to do some trial and error. Gut feel is to begin with 13x20” and see how high I can get rpm then 13.25x19” .
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Old 15 December 2019, 20:58   #4
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Country: Ireland
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Hi Bizzarrini,

What are those 53l tanks you are using? Would you have a link where I could find them?

Best
A.
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Old 16 December 2019, 03:33   #5
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Quote:
Originally Posted by jonp View Post
An idea on what prop is currently on it and what max rpm the engine is reaching with that prop would be handy. Raising the outboard by each hole will often increase rpm 50-100rpm due to less drag. The difference between aluminium and stainless is marginal but the price difference is quite considerable 6/1, also aluminium is easier on the gearbox if you do a lot of in out gear changing.
Even though ribs are light in comparison to other vessels, the tubes can cause considerable drag depending on the manufacturer.

An example is my 2014 zodiac 550 pro tour is the lightest in the 550 models at 315 kg, yet I've had to drop to a 13x13 prop to reach the Yam f70 max rpm. The boat before was also relatively light with the latest Suzuki 90 which I needed to drop from the 21 pitch down to a 17 to reach max rpm.
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Old 16 December 2019, 11:56   #6
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Country: UK - Scotland
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Hi Bizzarrini,
In Ayrshire too with a now converted 5.4 jockey seat set up from a Deluxe set up with a 70HP Yamaha. Best idea I can advise without photos, is to move the tanks to the front under the Bowdodger - which is where I placed mine. It will flatten the "stern-down attitude, lessen issues around drag and improve planing." I now have 100l of fuel tanks in the mid-section under the seats which makes a big difference. Final point is AntiFoul - I now must do mine next year as it's on a mooring! Best wishes Iain
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Old 16 December 2019, 12:09   #7
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RIBase
My SR5.4 with Mariner 90hp 2 stroke made 39.7knots with a 20" Laser II Prop.
Would have gone faster if I'd moved the engine up a little - my Cavitation plate was below the bottom of the boat.
As a rule of thumb, the faster a boat is the further aft your LCG should be. You might not want to put your fuel up front as suggested by another poster.
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Old 16 December 2019, 13:28   #8
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Your tanks should be just centre and forward of the mid position as also the battery. The SR is not a beamy hull so dont take kindly to inbalance.
39 knots is doable with the right prop. 90 Yamaha cleaver prop 19pitch max
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Old 16 December 2019, 14:54   #9
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Hi can’t seem to attach link but marinestore.co.uk have them.
Best Jamie
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Old 16 December 2019, 15:10   #10
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Thankyou for all your replies. I get the concept that more power to weight the further back weight should be, so felt just behind seats would be a good starting point. I do have a 40l bow bag tank for longer trips and I can play with that to find a balance!
Is the SR hull sophisticated/fast enough to gain benefit from a cleaver prop? Would have to have engine really high I would have thought.... I know I said speed was main aim but with flooding hull will it not struggle to get up, very happy to be proved wrong though if it’s been tried...
Thank you all for this help and Ian I hope to see you out on the water, a pal of mine has just got an SR4 and is also very interested excited to have a play!!
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Old 17 December 2019, 00:15   #11
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Country: Ireland
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Hi Jamie,

Thanks and found it: https://marinestore.co.uk/Merchant2/...t_Code=pl27270

Best,
A.
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