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Old 19 August 2009, 10:45   #1
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Country: Ireland
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Its probably been asked before..Avon SR5.4 90hp 2 stroke, how fast & which prop?

I have recently purchased an Avon Searider 5.4 & I have fitted a new Mercury 90hp 2 stroke outboard. Its an extra long shaft (25") which fits the boat well as the transom is level at a height of 25" as it was formerly outfitted with twin engines.

The engine came supplied with a four blade 13 3/4 x 19" aluminium propellor which provides great lift & acceleration but a top end speed of just 32kn with two adults on board. Adjusting the engine tilt has a negligible increase in speed. I was hoping to get a higher top end speed than 32kn.

The engine max RPM is according to the engine plate is 5500, after the engine was fitted at full ahead the tacho read 8000 RPM, clearly something wrong! I brought the boat back to the "technician" to have the tacho calibrated correctly, it now shows 6000 RPm at full ahead.


Two questions:

1:Could anybody with the same boat & a similar engine set up please let me know what size & type of propellor you are using?

2:Is it possible that an outboard can run at an RPM above its rated maximum or should I suspect that the tacho is still not calibrated correctly by the "technician" or is there a general error parameter with a tacho, in this case 10%?
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Old 19 August 2009, 11:20   #2
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I am no expert but i would think a 3 blade prop would be better for top speed? and 4 blades usaally better for skiing/wakeboarding?
propped correctly you should get nearer 40knots in millpond conditions with 1 up and little fuel with a 90. I get more than 32 knots with an 80 2 up.
can you "borrow" a few 3 blade props to try? say a 19 a 17 and a 21?
i would try a 21 or first and see what WOT rpm you get and go from there, my reasoning for this is you are getting overrevving with a 19 and a 3 blade will have less drag too.......
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Old 20 August 2009, 10:02   #3
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If everything is perfect I'll get just under 37 kts with a 17 inch pitch standard Yammy 3 blade and a little over with my 19 inch same. I would think that you will probably be over propped at 21".

If you go stainless etc you will prob get an extra kt or 2.

Ian
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Old 20 August 2009, 10:47   #4
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Ive just gotten a loan of a Mercury 13 1/2 x 17" prop so ill see what results I get over the weekend..
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Old 24 August 2009, 10:42   #5
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Quote:
Originally Posted by walruz View Post
If everything is perfect I'll get just under 37 kts with a 17 inch pitch standard Yammy 3 blade and a little over with my 19 inch same. I would think that you will probably be over propped at 21".

If you go stainless etc you will prob get an extra kt or 2.

Ian
but he already has a 19" and is overrevving........
is the mercury gearbox a different ratio to a yam?
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Old 24 August 2009, 17:30   #6
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5.4m Seariders with Mercury 90's always get fitted with Laser 2 Props, 20" pitch.
should do about 38knts at about 5400-5600rpm.
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Old 24 August 2009, 18:06   #7
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13 1/4 x 19 and 38 knots, ally @ 5500 rpm
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Old 25 August 2009, 08:45   #8
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The gear ratio for the Merc is 2.3:1.

The 17" prop I got a loan of didnt fit (13 splines instead of 15) but I tried a three blade 13 x 19" aluminium prop over the weekend & got 35 / 6 knots but I suspect the engine is over revving as it shows 6300 on the tacho when the supposed maximum is 5500.
I asked the guy who fitted the engine & tacho & he said that an error of +/- 10% is acceptable which sounds like bullshi% to me.

Is a Laser II a stainless steel prop? I think a 20" prop would be worth trying & should have the engine in the right RPM range.

Where does one buy a Laser II prop? From the UK or US & does anyone know which website?
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Old 25 August 2009, 08:52   #9
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http://www.propline.com/Select-Boat-...llers/main.htm

edit: sorry that's an american site.
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Old 25 August 2009, 08:54   #10
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There's a S/H one for sale here http://forums.hotribs.com/showthread.php?p=2559
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