Its probably been asked before..Avon SR5.4 90hp 2 stroke, how fast & which prop?

Zubenelgenubi · 19 August 2009, 14:45

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%?

doggypaddle · 19 August 2009, 15:20

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.......

walruz · 20 August 2009, 14:02

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

Zubenelgenubi · 20 August 2009, 14:47

Ive just gotten a loan of a Mercury 13 1/2 x 17" prop so ill see what results I get over the weekend..

doggypaddle · 24 August 2009, 14:42

Quote:

Originally Posted by walruz

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?

Turbodiesel · 24 August 2009, 21:30

5.4m Seariders with Mercury 90's always get fitted with Laser 2 Props, 20" pitch.
should do about 38knts at about 5400-5600rpm.

matt h · 24 August 2009, 22:06

13 1/4 x 19 and 38 knots, ally @ 5500 rpm

Zubenelgenubi · 25 August 2009, 12:45

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?

JSP · 25 August 2009, 12:52

http://www.propline.com/Select-Boat-...llers/main.htm

edit: sorry that's an american site.

JSP · 25 August 2009, 12:54

There's a S/H one for sale here http://forums.hotribs.com/showthread.php?p=2559

doggypaddle · 25 August 2009, 12:58

or you could try a 21 inch aluminium as i originally suggested, should be in the same ballpark as a 20 inch SS rpm wise.cheaper and wont cause as much damage if you hit anything.
I thought going from 4 to 3 blades would reduce the drag a bit!
Nos swears by Mitchegan props think its a match he uses(with a yam)

Nos4r2 · 25 August 2009, 23:45

Quote:

Originally Posted by doggypaddle

Nos swears by Mitchegan props think its a match he uses(with a yam)

It is, but the damn thing spun the hub while we were on holiday

Zubenelgenubi · 26 August 2009, 09:40

Thanks for all the info, excellent forum.

Ill continue to experiment with props, the top speed is all academic really, its just for my own curiosity to see how much I can get out of it!
I was out on Monday in Cork Harbour & the swell was such that I couldnt make good more than 20kn anyway.

david54 · 01 September 2009, 15:41

Yamaha 90 hp 2 stroke, 13 3/4" x 17" aluminium prop 38 knots @ 5500 rpm one on board with 91 litres fuel, 36 knots with two on board @5300 rpm same fuel load. When I fit yamaha ss prop same size top speed drops to 30 knots @ 4800. SS props are more aggressive so need to drop the pitch to 15" to get rpm 5200. If your engine is goes to 6000 rpm with 19" aluminium prop you should increase the pitch to 21" and rpm should come down to 5600. If you use ss prop 19" to 20" should be ok. Usually 1" of pitch = 200 rpm, go up in pitch to bring rpm down.

Zubenelgenubi · 02 September 2009, 13:30

What is the gear ratio on the Yamaha?

doggypaddle · 02 September 2009, 14:33

Quote:

Originally Posted by david54

Yamaha 90 hp 2 stroke, 13 3/4" x 17" aluminium prop 38 knots @ 5500 rpm one on board with 91 litres fuel, 36 knots with two on board @5300 rpm same fuel load. When I fit yamaha ss prop same size top speed drops to 30 knots @ 4800. SS props are more aggressive so need to drop the pitch to 15" to get rpm 5200. If your engine is goes to 6000 rpm with 19" aluminium prop you should increase the pitch to 21" and rpm should come down to 5600. If you use ss prop 19" to 20" should be ok. Usually 1" of pitch = 200 rpm, go up in pitch to bring rpm down.

Sure thats not a 19" rather than a 17"to get 38knots @5500rpm?

If not thats strange because my yam 80 went over 5700rpm @ 35knots with a 17" is it a K series?
I thought MattH tried 17K and 19K series too and found the 17 overrevved?
I currently run a 19 with the 80 and cant quite get 5500 rpm but i guess it would be around 38knots

doggypaddle · 02 September 2009, 14:45

Quote:

Originally Posted by Zubenelgenubi

What is the gear ratio on the Yamaha?

i believe its 26-13 ie 2-1.

Zubenelgenubi · 07 September 2009, 14:35

Got a loan of a 13 x 21" prop over the weekend, RPM down from 6000 rpm to 5750 rpm & speed up to 39.5 knots with one person on board & 65l of fuel.
Im going to try to get my hands on a 20" SS Laser II as recommended here (hopefully second hand), that should reduce the rpm by a couple of hundred, add a little more speed & problem solved!

Thanks for all advice received...

david54 · 08 September 2009, 09:49

Quote:

Originally Posted by doggypaddle

Sure thats not a 19" rather than a 17"to get 38knots @5500rpm?

If not thats strange because my yam 80 went over 5700rpm @ 35knots with a 17" is it a K series?
I thought MattH tried 17K and 19K series too and found the 17 overrevved?
I currently run a 19 with the 80 and cant quite get 5500 rpm but i guess it would be around 38knots

It's the 17k that came with engine as standard fit, also tried Yam ss pro 19" pitch, engine would only go to 4500 rpm.I keep trying different props, but always end up going back to the standard alu prop

Searider · 08 September 2009, 11:09

Quote:

Originally Posted by Turbodiesel

5.4m Seariders with Mercury 90's always get fitted with Laser 2 Props, 20" pitch.
should do about 38knts at about 5400-5600rpm.

That's what i had fitted to mine. 39.7knots was the best we got. can't remember at what RPM

90hp 2 stroke Mariner - same as Merc.