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When I run my engine over 4000 rpm's it "spins" sporadically, as if it was cavitating. Reducing rpm's to 3500 stops the spinning. Any ideas? Thanks.

Lloyd - please post up your prop diameter and pitch to start on this problem

COuld be that the prop is ventilating (sucking air down into the porp)
another possiblity is a spun hub. Der rubber senter thing dun mebbe come loose from der propeller metal hub.

We had the same thing happen with our X and Suzuki DF50 this past weekend (our first use), when ballasted. Prop would grip up to 7.2 knots at 4,000 rpm and beyond that would start slipping. Wasn't the hub, but still don't know what it was. Didn't seem to happen unballasted, but then I wasn't looking for it then.

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Moe

I'm also experiencing cavitation with my Honda 50 at 5000 rpm and above, full ballast, 4 blade X 11.8 X 9 pitch. At 5000 rpm the GPS speed is about 10.2 mph. I'll try unballasted next. I sure hope someone has found a fix for the cavitation.

Darrell

Thanks guys for the replies. I will be at the boat on Monday and I will measure prop. How does one determine pitch? Also, the rubber "thingy" is where, how do you get at it, and is it replaceable? And why would filling the ballast stop the problem? Prop deeper with more weight in boat, I guess? Seems like that would only deal with the symptom. Thanks again.

The rubber thingy is the entire center of the propeller. It is there to protect the drive train of the outboard in case the prop hits something hard and stops instantly. Kinda the same function as a shear pin.

I think the other comment was that the prop ventilates when the boat is plowing with full ballast. Probably something to do with the boat trying to climb onto its bow wave and maybe this puts the stern of the boat near the wave trough, making it more susceptible to sucking air into the prop.

yes....avoid having your standard prop become a surface piercing drive.....

But..

I have had a Michigan prop in the past where the rubber hub was slightly spun.....it eventually failed totally.... like 5 minutes....but it wore the outer surface of the rubber hub where you could eventually spin it stiffly by hand.

There seem to be as many potential causes as there are replies. But thanks for the hints. To restate my situation however, the spinning occurred when ballast was empty, not full, and water was flat.

Can someone lay out a suggested series of steps that may lead to a more definitive answer? I realize there are many variables, but maybe we could try.

This issue makes me realize how valuable this board is. Lots of good advice (from owners, not salespeople), lots of discussion, and it's free!

Thanks again. Keep those cards and letters coming.

I tried out a lot of props on my Suzuki DF50. I had the:

When I run my engine over 4000 rpm's it "spins" sporadically

problem twice.
..
1.) Piranha Proppellers 13 pitch prop would either suck air or the composite blades might have yielded under the load, more likely sucking air. This prop was returned.

2.) Solas Proppelers 3 x 12.5 x 9 pitch stainless prop, sucking air. I was told by Solas that I could either lower my outboard mount by one bolt hole or get the prop cupped slightly. This discussion once went into a lot of detail about motor height and I moved my motor mount up two bolt holes from resting on the transom. I wanted the higher position to better keep the prop off the botom and to get the performance discussed here. So I sent the prop back to Solas for a slight additional cupping.
..
Prop Pitch Identification 101:
a.) Become familiar by looking at some props that you know the pitch of.
b.) Find the stamped in pitch numbers on the props in a. so you know where to find your stamped in numbers.
c.) know that when the outboard dealer (or Mac dealer ) does not know of a major problem with the prop pitch included in the box with your outboard you will get the standard issue prop. Or if they just don't want to bother helping you get to the right pitch. For the Suzuki DF50 and Honda BF50 you get a 13 pitch prop. 13 pitch on these motors is suited for pushing a large jon boat 30+ mph and it does not allow your Mac26 installed outboard to achieve the required for warrantee wide open throttle RPM range.

I starting having this same problem and was able to correct by moving some of weight more towards the stern of the boat. Try rebalancing your load to see if this corrects the problem.
Hubert

Recently I saw a discussion on "jack plates" somewhere around here.
My mind & time is pretty much off this issue for now, but I do consider it to be important.
Experienced the same with full ballast, 2003 M, Honda 50, 11 3/4 X 10 Aluminum 3 blade prop. I believe the problem is 'ventilation' not 'cavitation'. The prop begins to suck air - the motor races - and you slow down.
It only happens once in a while, but it just isn't cool.
The whole reason I'm lugging this monster transom ornament around is to douse the sails and run for shelter when we need to, slowing down because of air bubbles shouldn't be a prime concern at that point.

Hubert's answer is the most logical and straightforward - move weight to rear.
Jack plates entered the equation for two reasons; my engine is already as far down on the transom as it can go, (without slicing & dicing the only way down is to go back first). And if moving the prop farther down into less disturbed water solves the venting, the ability to move it up in shallows is the added benefit. [A byproduct of the jack plate idea is moving the single heaviest weight rearward by ~6 inches as well].

Of course the only way we'll ever know is to try it. And the last thing I want to do is adversely affect the boats sailing performance by throwing it's balance off.

Joe 26M Time Warp wrote: ... [A byproduct of the jack plate idea is moving the single heaviest weight rearward by ~6 inches as well].
... Of course the only way we'll ever know is to try it. And the last thing I want to do is adversely affect the boats sailing performance by throwing it's balance off.

Both points are valid. But, I cannot imagine the aft-weight "byproduct" is an advantage for the Mac's general buoyancy or sailing balance, can you?

Also, seems to me that weighting the stern to lower the prop is, at best, a temporary test for the prop's performance? Remember that you're also changing trim angle pretty significantly. If results are at all positive, the real answer is to lower the outboard on the transom.

Moe wrote:Ventilation MIGHT be fought somewhat with one of those lower unit hydrofoils such as a Doel-Fin or StingRay, enlarging the size of the antiventilation plate.

I remember reading somewhere (26X owner's manual, perhaps) that strongly urged that these hydrofoils not be used on this boat. Can't remember the reason given.

--Mike

Went to the boat yesterday after meeting of our sail club, accompanied by an experienced Mac owner. He had diagnosed my spinning problem as a spun hub as soon as I described it. When we pulled the prop, the hub surface did look worn and shiny. Not a lot of friction left. I took the hub to WM for a hub rebuild today. I will report back when I re-install.
As to the sail club, if anybody is in the Miami/Homestead area, we sail on Biscayne Bay. You are always welcome to join us. Our club has mostly Mac's, but a few other trailerables are represented. Thanks