Propeller choice - huge rpm difference

[Sealfie]

Hello!

Ive got a Suzuki DT65 on a hardtop-boat with a weight of about 550 kg including engine.

Recently i had a propeller 11x14. My RPM was 4800 max and doing 30 knots.
4800 rpm is abit low so I replaced the propeller to a 11 1/2x13. Now its revving to much on full throttle (5800 rpm is max) and doing 27,5 knots. How can the difference be 1000 rpm by going down just 1 pitch (and abit bigger propeller)?? The acceleration is much better though but I dont like that I can overrev on full throttle, I would prefer it to have the max on 5400-5500 to keep me in the safe zone.

Apparently the standard propeller is 11 1/2x15 but I guess that the engine wouldnt have the strength to handle that. Any suggestions what propeller to try out?

[barrell]

Better to have the rpms high then to be overpropped. Are you sure 5800 is max rpms ? On the larger motors its 6200-6300.

[bladenbullet]

I would guess the 14 pitch prop also had some serious cupping in the blades...pitch, diameter and cupping all count....

[Sealfie]

According to the tachometer I have 5800 rpm on full throttle. From what I found on google the max recommended rpm on this engine is 5600. So its abit to much. I just dont want to damage my engine.

The 14 pitch propeller was not in perfect shape, thats correct, it had some minor damage on the edges but not serious

[Stijn_T]

5800 RPM is a bit high indeed, most likely your first 14 pitch propeller was damaged a bit too much. That could have caused lower RPM. Normally for this kind of engine one pitch smaller should result in a bout 200-400 RPM extra (see here). I would try to take a new one with pitch 14.

[Solarman]

It sounds like the 13" may need to have a bit more of a cup in the blades, or possibly the 14" blade had too much cup? Take both props in and explain what's happening, you don't need to run over 5400 rpms on the 2 stroke. The 6k rpms are for the newer 4 stroke motors.

The pitch of those props could be the cause.

Good luck, post back when able to let us know what you find out.

[2014DF25ARS]

Were your two props made by the same manufacturer? My DF25A motor came with an 11-pitch aluminum prop. I bought a SS prop from another maker, and took both props to a shop. The guy measured them with a gauge, and said the Suzuki prop was actually a 12-pitch prop, though it was stamped with 11. The SS prop measured true, but had a small amount of cupping in it.

A bigger prop shop might measure a prop with a fancy laser scanner that measures the entire blade surface and calculates the pitch & cup. My guy used a traditional metal gauge - like a protractor that lays on the blade.

Bottom line is that you can't necessarily trust what's stamped on the prop, though I would hope it will be fairly close. And 1000 RPM is a big difference. Could it be that the engine is high, and what you're hearing is "ventilation" (a.k.a. cavitation), i.e. air being sucked into the prop? That would explain a big jump in RPM. I raised my motor 1 1/2 inches a year ago, and got similar 1 MPH gain in speed with about 150 RPM increase. After raising it, I find that my motor will ventilate in a sharp turn, presumably because the engine is being raised further above the water in the turn. I believe the amount of cupping in a prop can change the height at which the prop will ventilate.

[2014DF25ARS]

Sorry about my response above. I had read the other thread about "raising the motor", and confused it with this one. So my comments about "what you're hearing" make no sense, as you didn't say anything about hearing a difference with the new prop!

But I would still consider getting the props measured. I was surprised when my stock Suzuki prop measured differently than indicated, and I watched the guy measure both props, so it's hard to write it off to a mistake.