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Old 10-22-2017, 09:29 AM   #1
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Dinghy Prop Confusion

I have a 4HP Merc 1cyl outboard on my dinghy. After a batch of bad gas it was running poorly so I removed, cleaned and re-installed the carb. On a quick test ride, I brought it up to full speed. It ran great!

Until I hit the log. The engine kicked up and there was no visible damage to the prop, but I apparently spun out the hub of the prop. The engine still ran great, but at high speed the prop would start to slip.

It's clearly marked as 7 pitch, and the manual provided the diameter, 7.8". Amazon to the rescue, ordered my Merc 7 pitch 7.8" diameter and popped it on.

Now it seems to cavitate a lot at any speed and can go maybe half the speed as before the log incident. By sound (no tach) it only revs up to maybe half the prior RPMs.

Careful comparison shows both props are indeed the same diameter, both are marked as 7 pitch, but the blade shapes are different. The old one seems to have longer, thinner blades.

Any ideas what's going on here?
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Old 10-22-2017, 10:06 AM   #2
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Did the new prop come with a new hub to? How were the splines on the shaft before you put the new prop on? How deep does the motor sit in the water? If the blades are a little longer maybe the motor is not that deep in the water.

Did you look into a new hub for the old prop?
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Old 10-22-2017, 10:20 AM   #3
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I wouldn't trust an Amazon prop to be exactly the same as the OEM Mercury prop. It probably will cost a bit more, but google Mercury outboard parts and order the prop from them.

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Old 10-22-2017, 11:15 AM   #4
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Cap Tom,
You wrote;
“Careful comparison shows both props are indeed the same diameter, both are marked as 7 pitch, but the blade shapes are different. The old one seems to have longer, thinner blades.‘

The blades of the new prop probably have too much blade area. So the engine gets overloaded like like over-propping. The new prop could be cut down to the same blade area as the old restoring the engine’s propper loading. A prop man would be much more likely to do this accurately. There could be other prop design features that contribute to cavatation like prop tips pointy or blades raked too much or too little ect. But it sounds like mostly a blade area issue.
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Old 10-22-2017, 11:34 AM   #5
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It’s possible that one of the props is miss labeled. Take both your props to a merc dealer or a prop shop for comparison.
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Old 10-22-2017, 07:45 PM   #6
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Quote:
Originally Posted by tiltrider1 View Post
It’s possible that one of the props is miss labeled. Take both your props to a merc dealer or a prop shop for comparison.
Or, just return the prop to Amazon and try for an OEM prop from an online supplier, I've used Boats.net: Outboard Motors, OEM Marine Parts, Boats for Sale for Yamaha parts, props included. Have had decent experiences with them. I'd look up the parts list for the correct model and year of your outboard, then compare the recommended prop to what you've got, as well as the old one. There is typically a dia. X pitch for the prop, just matching the diameter is no guarantee it'll match yours.

If you cruise full-time, it'd be a good idea to have a spare prop, they're not terribly costly for the hassle it saves having one on board.

I never thought it was necessary to carry a spare until I whacked a reef on Conception Island. Might as well have been on the moon with trying to find a replacement!
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Old 10-22-2017, 08:52 PM   #7
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Thanks all! Yes, they're the same diameter. It's just that the old blades seem to "wrap" around farther, and are thinner.

They both have the same 12-spline hub, splines are good on both shaft and prop. Both have rubber hubs, no shear pins. Haven't found any indication on-line that the hubs can be replaced, nor did I think it would be worth the effort for a $65 part ($85 at a brick-and-mortar shop.)

The prop I got IS an official mercury part number. Fulfilled by a "real" prop shop but sold via Amazon. Yes, they'll take it back (I pay to ship.) Both are clearly stamped "7" for pitch.

My working hypothesis, which I didn't want to poison the discussion with at first, is that it was way over-propped with the new one. It sure felt that way running it.

I had no idea these things weren't plug-and-play. I looked up the Merc part number in the manual that came with the motor and ordered the genuine article. I figured that would be a slam-dunk.

It's the same motor on the same boat, so no issues with shaft length or anything like that. The prop is the only change.

Next stop is a real prop shop. Geez. It's a stupid 4HP! Who knew it would be so complicated? I didn't even know there were different shapes for the same-pitch props on a motor this size.

Old prop is going into inventory as a spare. It still works, just can't hit high RPMs.
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Old 10-23-2017, 07:33 AM   #8
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Before "guessing" over propping, a thouroughly misunderstood topic by most, including me.....

There are other reasons the prop may be running differently....the best answer "might be" as posted before, not all good deals really are.

The options are fiddle with it with no satisfaction, a huge waste of time, and now an unreturnable prop.....or return it and as previously advised, buy OEM.

That way you know you have the correct prop so if symptoms are still there, you can rule out the prop.
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