Outboards engine slipping issue

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Aidan Borg

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I have a Mariner 20hp 1990 s , yesterday I was cruising normaly at 4500 rpms / 20/21 miles . At a sudden the engine begun to over rev the boat couldn't get on plane too . I took out the prop and found that the hub got burnt so I bought a brand you propeller eith the pitch and diameter. Today I went to try the new prop and at low speed it was working good but when I give it some throttle it looks like there a type of slip again , infact the boat is taking longer to plain and top speed is reduced . Before it was doing 27miles and now it s doing only 21 miles at full throttle can someone suggest something pls ?
 
Don't mariner engines have a pin through the shaft to stop the prop slipping ? My 15 hp did, though other engines have a rubber bush in the prop to prevent damage.
Possibly better to put this on the practical forum and also search as this was discussed a while back, though it might have been on the motorboat one.
 
Boathook said:
Don't mariner engines have a pin through the shaft to stop the prop slipping ? My 15 hp did, though other engines have a rubber bush in the prop to prevent damage.
Possibly better to put this on the practical forum and also search as this was discussed a while back, though it might have been on the motorboat one.
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Mine has a rubber bush in the propeller and it was worn out so I bought and new propeller the difference is that the original black max the one with spun hub has a 10 3/8 x 13 pitch and the new propeller is Solas 10.1 x 13pitch there is 5mm difference in diameter does it effect it to slip ?
 
Take the prop off again and check the splines are not worn, if its a small outboard it may just have a woodruff key instead and this might be damaged or even missing! They are designed to break if you hit something hard.
 
Chris_d said:
Take the prop off again and check the splines are not worn, if its a small outboard it may just have a woodruff key instead and this might be damaged or even missing! They are designed to break if you hit something hard.
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The splines are in a great shape I think the problem is with the new propeller that I bought (Solas) the original one is Black Max the difference between the 2 is the shape and the diameter about 5mm difference
 
Outinthedinghy said:
I hope this planing isn't taking place on the Thames.

Must be quite a small vessel to be getting 27 or 21mph out of a 20.
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I am running my motor on a centre console V hull fiber glass boat 13 ft and in salt water so the water is never flat there is always some a bit of waves
 
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