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I recently bought a 1984 Montauk 17. Previous owner took the 100HP for his pontoon and the boat now has a 1993 40HP 20" Mercury 2 stroke on it now. It also had a 9 pitch prop which ran 6200 RPM at 22 MPH (like a tractor). I'm looking for a 100 HP, but in the mean time, after some research, I put a Mercury 13 pitch prop on it (which is a little steep, but I opted for a little lower RPM at WOT)

It ventilated badly, so I lowered the motor one notch, which is as low as it can go with the current bolt setup. The water now runs at almost even with the cavitation plate (after it rises some), and measures with a board coming off the keel to about 1/2" below the plate. But it still ventilates badly, and still only goes 23. I was able to set it at 3500 and it was biting very nicely, and it increased to 4500 RPM all by itself, planed and was going 23 at that RPM (which is about 16% slip) and was cruising very nicely. It's very happy and has LOTS of headroom left at that speed. But any throttle / upset / wave / steering from there and it will ventilate and will go to 5600 RPM at WOT and still go about 23, maybe 24. Once ventilation happens, it's pushing air and water, and slip goes very high. Then you can back off and start over, but it's going to vent unless you're super careful and running slow. There is only one proper trim setting at speed, and it's going to be running very ventilated if you're going high RPM. There are no items in front of the prop other than the hull, which has some bottom paint, but not bad.

It's a 3 blade 10-3/8x 13" pitch prop, a Mercury BlackMax. The 9" pitch prop was considerably larger diameter and didn't ventilate with the motor 1" higher (but went way past redline). It had very little slip.

I would estimate that I could probably get 29 MPH out of this motor and prop if I could get it to not suck air. This would be OK with me while I search for a bigger motor. The motor runs fantastic.

1) Do you have any suggestions on a prop that might work on this motor given that I can't lower it?
2) What direction should I go? Lower pitch, different brand, cupped, 4 blade, larger diameter, larger anti-ventilation plate? I don't want to put much more money into this motor but I figure I can sell a prop that is almost brand new and only lose a few bucks. I do want to go faster than 23, but I only want to try ONE more prop.
3) Anyone know of a 100HP oil injected Mercury I could get?

THANK YOU!!!

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For a 4 cylinder 40 hp Merc your redline is 5500 which you probably know. Aluminum black max props have very little cup at the blade tips and it sounds like that is what you need. The boat is way under powered so it will be dicey getting it propped right. I'm assuming you were running the engine at a reasonable trim angle. For this application, the engine running straight vertical or a little tucked in is how she is probably going to run best. I don't like spending other people's money but I highly recommend looking at a Stilleto stainless prop for this in a 10"-12" pitch range.They are fantastic props for these engines with the 2:1 ratio small gearcase. The props that Mercury offers for these engines are lost in time and have little if any advanced engineering to offer. You should not be ventilating that much unless you are over trimming so make sure of thing. If someone ran this engine with the wrong thrust washer behind the propeller, the prop can dig into the gearcase and create additional clearance behind the prop and have exhaust blowing over the hub instead of through it. Rare, but I've seen this.

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SOLVED (but not tested yet)

Thanks Fitz, you were close!!! REALLY close!

I went to a place today whose mechanic I trust... He said bring it over so he can see it. He spotted it right away. There was a gap axially between the prop and the area where the exhaust comes out. (there is always a radial gap there of maybe 1mm, but mine had a space axially of about 1/8" which allowed exhaust gas to exit out the sides and give the prop air, in addition to it exiting the prop end.

So we took the prop off and guess what? TWO thrust washers in there, which made the gap he noticed. When I changed the prop from the 9 pitch, it looked like a machined surface the other (rounded) thrust washer should sit on. Since the old one was a one piece and the new one had a hub to install, I completely missed that I should remove the old thrust washer first, and thought it was part of the shaft! DUH!

So we're going to the lake tomorrow afternoon and we'll see what happens!

Thanks for your response, you were definitely on the right track!

The engine looks to be sitting at the correct height, with water just below the anti-cavitation plate, so I think I'll leave it alone for now.

Guess: 29.3 MPH at WOT, 5300 RPM :-)

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I have a 2004 Mercury 90 hp 2 stroke that would be perfect for this boat. I have it listed check it out and let me know.

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JSharpe, I sent you a couple of emails... Please call me. Thanks.

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So how did she run without the ventilation issue?

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No issues! We did about 100 miles over the holiday on the west side of Michigan and didn't have that problem once. Top speed is 27.8 at 5550 RPM. Cruises nice at about 22.

But the prop has about 2mm of axial play. From what I have read, this is normal, but it doesn't make sense to me. So I might put a 3mm washer in it to take up the slack (washer on the nut side of the prop). I also might have wrecked the hub by putting 2 on it, but I doubt it since I only torqued it to 55 Ft*lb.

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Great news!
That boat runs a lot better than I thought it would with 40hp. As far as the axial play, do you mean that you are seeing the propeller slide up and down on the propshaft with the nut tightened or do you mean the whole assembly including the propshaft has front to back movement? The latter is very normal and what you are feeling is the clutch dog spring that pushes from the shift cam to the cross pin the pushes the clutch dog into neutral and reverse, the shift function is spring loaded into forward gear, because of another opposing spring that allows the clutch dog to engage in forward gear from the back side of the cross pin, once the shift cam swings out of the way. There is no mechanical connection between the shift shaft and the clutch dog besides the two springs pushing against the cross pin. It's a very simple reliable design. So that's what you are feeling under normal circumstances.

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<<< Yes. I've read in several places that this is OK with this hub assembly and it's designed that way, but I don't think the old one did this (not a replaceable hub), and it doesn't make sense from a wear standpoint to have the prop rattling around on the spline like that. It rotates in that slop maybe 2 degrees also. I think I'll put a shim in it and have it tight.