WOT test results 26M -propped correctly?

[JeffJuneau]

I finally carried out my wide-open throttle test (WOT) for my 2009 Mac 26M with a Suzuki DF70A, and have some questions about it. I had a full ballast, and with passengers, gear, water and gas, that works out to about 500 lbs of weight added to the stock 26M. My hull is bottom painted, but still had a little biofouling, and the rolled-on paint isn’t particularly smooth. I have a Solas 9411-138-13 prop, that I understand to be a 13 pitch by 13 ¾” diameter. The 3-blade prop is aluminum, not pristine, but doesn’t have any dings or dents in it. After a few directional tests, I am getting about 5,600 rpm at WOT with the top speed of 17knots. According to my manual, target WOT is at 5000 to 6000rpm.

Question 1: Does that sound like what experienced 26M operators would expect from this Mac/engine/prop combination?

I notice when I push the throttle forward a little aggressively the prop seems to cavitate, before grabbing. Or at least the engine revs up with no increase in speed (or very delayed increase in speed), so I assume it is cavitating. I have learned to modulate my acceleration so it doesn’t do that. After I explained how my engine overpowers the prop with the Solas 3-blade prop, the local Mercury dealer suggested I move to a 4-blade prop with 13.8” dia by 11 pitch, to stop the cavitation. This seems to be a prop frequently fitted to a pontoon boat.

Question 2: Does it sound like a worthwhile experiment to try that specific 4-blade prop? I will be out $300 +, … but its only money.

[Tomfoolery]

Might be ventilation, not cavitation. You need to look at the prop when you throttle up (have someone else at the helm) to see if it's sucking air from above. If it's staying fully under the water, then it would be cavitation.

[Starscream]

Well, I can't say too much since I have an X and an E-tec 90, but here are my comments based on my engine:


5,600 with full ballast sounds about right, since you'll add an extra ~ 500 RPM when unballasted.

My 90 turns a 3-blade 14x11 prop, but is geared at 2:1 while your motor is geared at 2.6:1. My E-tec runs at 4,500 RPM fully ballasted and 5,000 unballasted (that's per spec), so when loaded it's turning the prop at 2,250 RPM, while yours is turning at 2,153 RPM. That sounds pretty good for a 70HP considering I'm at 11 pitch and you're at 13 pitch.

With that kind of load I'll be at about 18 knots, so once again your 70HP sounds to be on point. Based on the performance numbers you give, I think your existing prop is perfect.


I have only had problems with cavitating like you describe once, and it was when trying to run high speed in big waves. Once it started cavitating it would over-rev pretty quickly, and not come back down until I brought the revs back down with the throttle, allowing the prop to bite once again.

Is there any room to lower the motor a bit on the transom?

My experience with rough vs. smooth bottom is that it made a huge difference on low speed fuel economy, but essentially no discernible difference at full throttle.

[Be Free]

5600 RPM for an engine that is supposed to top out at 5000-6000 RPM is just about perfect. I'm with Tom on the slipping. It sounds more like ventilation rather than cavitation. If the engine can be lowered a bit the problem will probably go away or you can just not "punch it" until the boat gathers a bit of speed.

IIRC some have complained that a 4 blade prop is noisy at low speeds on our boats (or was it high speed?). Anyway, they were noisy.

[Jimmyt]

One other thing to consider. Has your hub been spun? Once the hub has spun from running aground or hitting an object, it often behaves as you describe. Since it seems to be propped correctly, you might want to check your hub. https://www.boatingmag.com/how-to-tell- ... eller-hub/

[Jimmyt]

And if you haven't been pulling your prop and cleaning/greasing the shaft, you may be in for a real treat...
I had to surgically remove my hub from the shaft...

[Tomfoolery]

Jimmy - could the hub be replaced in that prop, or is it not worth the money?

It musta taken a while to cut that off the shaft without damaging it.

[adudinsk]

Isn't the Suz 70 a 3 cylinder?
Or is it a 4 cylinder?

AD

[Russ]

And if you haven't been pulling your prop and cleaning/greasing the shaft, you may be in for a real treat...
I had to surgically remove my hub from the shaft...

That's a thing?
Shoot. I'm afraid now to pull it, what I might find.

[Russ]

Isn't the Suz 70 a 3 cylinder?
Or is it a 4 cylinder?

AD

4 cylinders

[Be Free]

Jimmy - could the hub be replaced in that prop, or is it not worth the money?

It musta taken a while to cut that off the shaft without damaging it.

Replacing the hub is (usually) easier than changing the impeller in your water pump.

I have my doubts about a spun hub on your boat though. You are hitting about the right speed at about the right RPM. If the hub is truly "spun" the prop shaft and the prop are not mechanically connected anymore. You should not be able to move at all.

[Jimmyt]

Jimmy - could the hub be replaced in that prop, or is it not worth the money?

It musta taken a while to cut that off the shaft without damaging it.

Yes. The props are virtually universal and you buy a hub kit for your specific motor. If you've kept your prop shaft clean and anti-seized, replacing the hub kit is simple as Bill says. It is worth doing if the prop is still good.

Cutting the splined hub core off of the shaft was NOT the most fun I've ever had, but I can be very patient when it's necessary.

https://www.westmarine.com/propeller-hub-kits

[Jimmyt]

Replacing the hub is (usually) easier than changing the impeller in your water pump.

I have my doubts about a spun hub on your boat though. You are hitting about the right speed at about the right RPM. If the hub is truly "spun" the prop shaft and the prop are not mechanically connected anymore. You should not be able to move at all.

In fact, it is fairly common for a bad hub to slip under full load, but stay engaged under lighter loads. It will give exactly the symptoms he describes. See the article in boating mag that I linked https://www.boatingmag.com/how-to-tell- ... eller-hub/

[Jimmyt]

That's a thing?
Shoot. I'm afraid now to pull it, what I might find.

Don't panic yet. Mine was used exclusively in salt and brackish water. It should be done annually in that sort of use. Fresh water is probably very different.

[Tomfoolery]

That's a thing?
Shoot. I'm afraid now to pull it, what I might find.

Don't panic yet. Mine was used exclusively in salt and brackish water. It should be done annually in that sort of use. Fresh water is probably very different.

I pull mine every fall for winterizing, and I only once added some marine grease to the shaft. And even then, it was only because I thought I probably should, not because it was dry or the prop was sticking.

But I'm in Lake Ontario and the NY Barge Canal (Erie Canal). It only sees salt water once in a very rare while.