Suggested prop size for a 1997 90hp Yamaha autolube 2 stroke

[Jeff J]

Can anyone offer any ideas on the above question it would be greatly appreciated.
Cheers

[Chris]

What boat is iti on?

[Downhilldai]

And what will the boat be used for? How heavily loaded, etc.?

[Jeff J]

Sorry should have said

Its on an 18ft Wilson Flyer (sorry not a RIB) fishing boat that carries between 2 and 3 people. I recently changed to the yamaha from a 60hp evinrude vro. However the yamaha does not seem to give me any more speed than the evinrude did. At present the Yamaha has a 13.25 x 17 on it and was wondering if this was correct of not.
Any help received would be appreciated.
Cheers
Jeff

[Chris]

You need to check the RPM you are achieving with the engine at WOT (Full throttle) and also trimed out for maximum speed.

I am unsure what the target RPM is for that engine at full chat but I would have throught it would be towards 5,600 to 5,800 RPM.

I am assuming you have a rev counter?

Chris

[Downhilldai]

How fast does it go at WOT and what RPM are you hitting?
Is the engine running ok and is the prop in good condition?

[yorfuoj]

even without the RPM figure, it seems evident that 17" is too much, should you have the possibility to try a 15" shoulld give a solution

[Nos4r2]

Max WOT rpm for that engine IIRC is 5500.

Try a 15".

[Jeff J]

Thanks for all the suggestions.

RPM seems to be around 4000 and top speed around 28mph. Will try a 15" prop and see if that cures the problem also have a mate coming down to clear out the carbs just in case.

[Jeff J]

Hi All

Changed the prop to a 15" and things have remained pretty much the same any ideas.

Many thanks

[Nos4r2]

does it 'hit a wall' at 4000 rpm?

[doggypaddle]

Maybe the tacho is wrong and its hitting the revlimiter??
if you changed the prop pitch and the RPM and speed are exactly the same then thats really wierd Are the props all 13.25 diameter? are they all ali? and are they all cupped the same?
material and cupping changes aparent pitch AFAIK.

[spartacus]

Is the engine compression good on all cylinders? Even accounting for the extra weight of your Yamaha 2-stroke you should see some difference with an extra 30 horses onboard. 90hp Yamaha (20" shaft) weighs in at 119kg and is generally regarded as one of the best engines for power-to-weight ratio (2-stroke).

What about the outboard set-up. Is it off-set or raised on the transom? Anti-cavitation plate should be level with the hull.

[Downhilldai]

Did your mate clean the carbs?

[Jeff J]

Hi All

Yes it seems to hit that wall at around 4000 revs. Both of the props tried are 13.25 in dia.

The chap came out and checked all the relevant bits etc (not that i know what is relevant) but he is sound and genuine. It all fits fine on the boat central to the transom and the caviation plate is in exactly the right place.

Have no idea what to do but am gutted as i thought it would be a fair bit quicker than the rude but its about the same and cant help but feel something is clearly not right.

Forgot to add that compression is fine on all cylinders.

Any more thoughts.

Regards
Jeff

[Nos4r2]

Originally Posted by Jeff J

Hi All

Yes it seems to hit that wall at around 4000 revs.

Forgot to add that compression is fine on all cylinders.

Is your trim gauge working correctly?

Might be the rev limiter kicking in because it thinks you're trimmed out too far or still in neutral. (Not sure if your engine has a limiter though)

[doggypaddle]

well perhaps 4000 rpm is on the rev limiter and the tacho or the rev limiter are wrong?
maybe its worth either checking the tacho calibration or trying a 19"?
also check you are getting full throttle and the ignition advance is working properly.
if the carbs are clean does squeezing the primer bulb make any difference? Ie fuel starvation.
I am sure NOS4r2 will have some more ideas?
did you check the cupping on the prop? its possible i think to replace a plain 17 with a cupped 15 and get the same apparent pitch? someone will correct me if i am wrong!!

if all else fails put the rude back on and i will take the yam off your hands

[Jeff J]

Hi thanks again for your thoughts.

Can you explain to me about the rev limiter what it does and how it works etc please.

If it was indeed something to do with this would disconnecting the tacko from the engine show a change in performance.

[doggypaddle]

I dont know how the rev limiter works NOS4R2 probably will, i guess it stops the sparks or retards the ignition timing. worth a try disconnecting the tacho, if its driven off the coils i suppose it could affect the spark quality and hence possibly RPM.
even so if your engine hits a wall at 4000 rpm with a 17inch prop @28mph, if it only gets 4000rpm with a 15" then that should only be about 24.5 mph.
theres something strange going on there! you shouldnt have the same speed and rpm with 2 different props.
I dont know for sure but i would have expected that engine and boat combination to be OK with a 19" or even a 21" prop, and would have expected to get 30-35 knots.
Most people on here with 5.4 m seariders get around 36-38kts with a 19" pitch i dont know how the drag on your boat would compare though, but obviously it should be a heck of a lot quicker than the engine you took off.
I would double check tacho calibration, and consider a 19" to try.

[Nos4r2]

Should just cut the sparks at the limit of the revs. Not sure where it is on the Yam 90 though without sticking my head under the hood.It won't be tacho dependant though.
Have a look round the saddle and unplug the wires from the trim gauge or any microswitches you can see then try again-but really this needs to go into a workshop to sort it out. It's not really something that can be diagnosed without seeing it.