Correct prop for Sunbird 1989 Fish & Ski with 90 HP Johnson O/B

[Fed]

Make it stop!
A pitch is not a unit of measurement.

 

[Derek Domino]

Imagine my confusion, I'm a guitarist.

 

[Fed]

These were the original factory props for a 1989 90 HP Johnson.

80 0382761 PROPELLER, 14 X 11
80 0391668 PROPELLER, 14 X 13
80 0391198 PROPELLER, 13 3/4 X 15
80 0391199 PROPELLER, 13 3/4 X 17
80 0390896 PROPELLER, 13 X 19
80 0389788 PROPELLER, 12 3/4 X 21
80 0390895 PROPELLER, 12 3/4 X 23
80 0389949 PROPELLER, 13 3/4 X 15-SST
80 0389948 PROPELLER, 13 3/8 X 17-SST
80 0389510 PROPELLER, 13 X 19-SST
80 0389512 PROPELLER, 12 3/4 X 21-SST
80 0392261 PROPELLER, 13 3/4 X 13-SST
80 0389792 PROPELLER, 14 X 11-SST

You won't go too far wrong with a prop pitch of 17".

 

[Derek Domino]

That's a pretty wide range. I guess that shows how important it is to try it on the boat you're using and the conditions that are most common for that boat.

 

[Sea Rider]

You can't possibly try them all, must test one that dials max wot rpm range as usually loaded under trial & error. Don't know if listed OEM props are hard to find, if so, go for Solas 13-15 pitches.

Fed, pitch is half part of the unit of measurement which comprises : Diam & Pitch, being pitch way more important that diam.

Happy Boating

 

[Fed]

How long is the boat Derek?

 

[Derek Domino]

The boat is 17 feet long.

 

[Fed]

Blaming & trying to correct with the prop pitch is wrong, your problem is something else & here are your options.

Belted up prop acting more like a paddlewheel than a prop.
Outboard mounted way too deep.
Tacho not reading correctly.
Overweight or overloaded boat.
Sick motor.

You will need to fix the problem before trying to fine tune with a new prop.

A prop with a 17" pitch should easily achieve 5000 RPM @ WOT with 90 HP on a your 17' boat.

 

[Sea Rider]

Recapitulating :

If boating with an old 14 x 17 installed on OB, would be ideal to make a hub slip test before going to a less pitch prop to check if it's doing its homework right, if found ok go straight for a lower pitch prop.

It's a waste of boating time and effort blaming all other mention parameters, OP mentioned test was done with 3 up so not overloaded, has a new induction tach installed, a unhealthy OB will not rev even at 4620 wot revs. All boat's hulls are not build same, doesn't matter if all being 17 footers, each one has it's own drag coeficient formula. Some floats less, some more. So can't put all 17 foot lboats on same sac while powered with same 90 HP-OB.

Did you know that after break in period has ended, boater should make a wot test run as usually loaded on flat water cond and dial a prop for OB to run at least middle wot range, although max wot rpm range is much preferred. That applies also for second hand combos. Most factory delivered props are medium pitched ones which don't know if perfom top for your combo. A prop maximization is a must do.

A less pitch prop will definitely pull lost rwot revs up, it's not a wow costly investment..

Happy Boating

 

[Derek Domino]

The new (13 3/4 x 13) prop came in today. I'll get it out over the weekend and let you all know how it goes.

 

[Derek Domino]

Amazing! I've never heard the motor sound so good. WOT 5450. It actually seems to be a couple of mph slower but it could not sound sweeter. With the trim just right it was gliding across the water. Thanks to all for the input. I couldn't have figured it out on my own.

 

[Sea Rider]

Congrats Derek on prop maximization and for posting final wot results too. If max wot range is 5500 and and only achieved 4600 with a 17 pitch there were 900 lost wot rpm. With a 13 pitch at max wot rpm of 5450, OB has achieved + 850 wot rpm, that's + 212.5 wot revs per one less pitch. So Rule of Thumb in this particular case is correct.

Amazing you dialed top prop pitch on first try. How loaded was combo on that test ? It's much preferred to lose slight top end speed on the befefit of gainig top hole shot....

Happy Boating

 

[Derek Domino]

We had our usual load, four people. Yes, I don't mind the loss of a couple miles per hour since the performance is so much better.

 

[Sea Rider]

Nice boating with 4 up, with less boaters on board you'll see how max wot rpm increases. OB's can run at 3-5% of their max wot range with no issues.

Happy Boating

 

[Fed]

As far as I can see all you've done is mask the real problem.

 

[Sea Rider]

What was the real problem, could you be more objective in your appreciation ? OB is running at near max wot revs with 4 up, can't ask for more, was definitely a prop issue, previous 17 was too much prop lugging the OB even while lightly loaded. Pitch is what dials top wot rpm along extreme fun....

Would be interesting to see how much wot rpm OB gains while with 2 up ?

Happy Boating

 

[Fed]

I have already listed the possible problems & the prop size wasn't one of them.

 

[Sea Rider]

I have already listed the possible problems & the prop size wasn't one of them.

If previous listed "possible problems" is what you considered being wrong, a proper prop maximization as in this particular case burried all those mentioned supposed problems. Most times spark plugs and props are highly overlooked when counting with a healthy OB and poor combo performance.

Happy Boating

 

[ajgraz]

Amazing! I've never heard the motor sound so good. WOT 5450. It actually seems to be a couple of mph slower but it could not sound sweeter. With the trim just right it was gliding across the water. Thanks to all for the input. I couldn't have figured it out on my own.

What was your speed in mph at 5450 rpm WOT?

 

[Fed]

You'll have to take my word for it Sea Rider, a 17" prop is right for this rig.
There's a definite problem having to run a 13" prop to get the rpms up.
Are you going to pay for the re-build when the lean carby scores the bore?
Get to the bottom of it Derek.

This is a 90hp motor on a 17 foot hull, it's not a 9.8 with manual trim on an inflatable.