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The prop is a Michigan Wheel Vortex 4x12x9" pitch. As reported here before, I've had problems with the prop losing grip on the water at much greater than hull speed. I originally got 7.2 knots at 4,000 rpm ballasted, and any throttle greater than that just increased rpm, all the way to near 7,000 rpm, with no increase in speed. That was followed by an unballasted test with average speed of about 9 knots and rpms as high as near 7,000. The hub, a Michigan Wheel XHS #115, is a copy of the Mercury Flo-Torque II, and shows no sign of a "spun hub."

The symptoms are typical of severe ventilation, however the motor is mounted as low on the transom as possible, with the antiventilation plate more than an inch below the keel. The boat when ballasted sits deeply in the water, which was near flat, and the test was in a forward direction, not a turn. The problem is not at all likely to be ventilation, and as a result I guessed at cavitation, although that usually doesn't result in such severe slip.

Upon closer examination today, I believe the problem is exhaust gas leaking ahead of the prop due to an improperly fitting thrust washer.

Note how deeply the stock Suzuki prop sits in the gearcase. It has approximately 1/4" of overlap inside it:





While it may be irrelevant to my problem, also note how the Suzuki thrust washer has a groove that sits over the inner ring of the exhaust passages:



Compare that to the Michigan Wheel thrust washer, which does not. Also note the front of the Michigan Wheel thrust washer is too large of a diameter to go into the inner ring of the gearcase around the prop shaft like that of the Suzuki thrust washer. The splines in the MW thrust washer prevent it from rubbing on the gearcase ring.



This is where I believe the problem is. The following picture is that of the Michigan Wheel prop sitting as deeply as it will go on their thrust washer. The leading edge of their prop barely gets between the inner and outer rings of the gearcase.



Here's the MW prop pulled out slightly so you can see how little of the leading edge overlaps the gearcase rings. Also note the Michigan Wheel thrust washer.



It appears to me the front of the thrust washer needs to be reduced in diameter and the splines cut deeper to let the thrust washer slide further up the shaft and the prop sit deeper in the gearcase.

I had noticed how far the prop sat out when I originally installed it, not only at the gearcase, but at the end of the prop shaft as well. Here's a look at the hub on the shaft without the prop.



Nevertheless, I double and triple checked to ensure everything was seating as far as it could, thrust washer on the prop shaft, prop on the thrust washer. The prop nut just barely covered the end of the shaft and there was no problem getting the cotter pin installed. It didn't occur to me at the time it could cause an exhaust leak problem and I was anxious to get the boat in the water.

I looked at the hub box again, and it is the one Michigan Wheel specifies for the DF40/50, and the hub is installed according to their directions. I guess I'll have to call their tech support Monday. They don't have an email address for support.

For now, I can't recommend this prop for the Suzuki DF50. And while Robert's Solas 4-blade prop sounds good, I believe they are also going to a one prop fits all interchangable hub system like this.
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Moe

You certainly have done your homework on this case. When you call them, if you can get an URL you should send them this whole posting. They couldn't ask for better info. Good luck.

Please share those pictures with Michigan Wheel. They should be happy to correct this problem very quickly in return for both a happy customer and the information needed to obtain a perfect fit for a popular outboard.
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Or, they sent the wrong thrust washer.
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Or, maybe you are supposed to use the Suzuki thrust washer. Through testing 9 propellers (Not including Michigan Wheel) I only use the Suzuki thrust washer with all props, aluminum, stainless, and composite.
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Similar experience when I first tried out the Piranha Propellers props. I might have been the first one fitting the Piranha Propellers Prop onto the newly released all new Suzuki DF50. Piranha Props was glad to help and glad to get the information. They overnighted an extra part machined down a little to fit inside the outer casing. I was also able to make the first one fit with a little bit of DIY machining with my hands and some sandpaper, Yours will require a machine to get that much work done.

Moe,
I wonder if you could venture some thoughts on a problem I have been dealing with. You piqued my interest when you mentioned gases venting forward. I'm one of the owners of a Mast 28 and have an Etek 75 pushing her. The motor vents fumes into the cockpit, enough to make us ill and set off the CO meter in the cabin. In talking with the other 2 owners of these Mast boats, I found that they had no similar problem. They both have Yamaha 4 strokes (75 and 90). They also get better speeds with their motors. I have the Evinrude prop (14X 11 and 14 X 13). Not settled on which I prefer yet because I sent the 13 out to be repitched as a 12 and it came back providing the same RPM's and speeds as the 11 (no gain here).
My primary concern, though, is the fumes we're getting from a supposedly user and environmentaly friendly rig. Any thoughts? Could it be the prop seating correctly?
Thanks ahead of time,
Jeff

Jeff, I could be wrong, but I don't think exhaust leaking ahead of the prop would cause your problem any more than exhaust going through it. Some boat designs tend to pull exhaust gas back toward the boat and cabin more than others, and at lower speeds, this may be dependent on wind speed and direction. CO is odorless so it can be happening without human awareness. My experience around an E-TEC is that while it's a far cry from carbureted two-strokes, it still has a little of the old two-stroke smell, where a four-stroke doesn't. Unless your fellow Mast owners have CO detectors in the same place as yours, they might actually have the same problem and not know it.

There is a potential for an exhaust leak in the powerhead (should cause rough running) or lower unit above the waterline, so I'd have the motor looked at carefully for this.

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Moe

Moe,
Thanks for the reply to my question. I'll be checking out the 2 items you posed . I did have one episode when I ran the engine for a time on low fuel and learned later that this can damage the engine by drawing oil into the cylinders?? Possibly some damage? Causing the improper flow of exhaust gases? Something I need to get checked. Thanks again for the feedback.
Jeff.