Merry Fisher 635 going Faster advice needed

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nrgbentall

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Hi
Could anyone please advise if re propping the old dog will speed her up
Currently running at 3000 rpm at 19 knots 200 rpm over her recommended 2800 running a 19*18 3 blade
Looking at dropping the revs and maybe increasing the speed by fitting a new prop of 19*21 anyone done this on the Nanni 85hp 5 cylinder
Any info most appreciated as this could be an expensive mistake

Thanks
Nick
 
I have a similar boat 6.6M with 72 hp inboard. I was considering putting a larger prop on it and I after a bit of research I was told that some boats are fitted with "compromise props" where they make sure that the boat in a marina environment amongst pontoons isent going too fast at tick over, ie you dont want a boat doing 5knots at tick over, but that is precisely what you want for fuel economy and a greater top speed.
And if you changed the prop and it did not do what you wanted would it really be an expensive mistake, as after all if it worked you could put the old one on Ebay, and if it did not work you would also be able to sell your new one.
 
We had a 635 as our first boat some years back and from memory 19knots is fast. Ours too had the 85hp nanni. We used to cruise about 12-13k happily all day. Fastest I ever saw was about 17k when she had a clean bum and not much clobber on. So if you are getting 19 that is pretty good.

Hamble propellers are another possible source for info for prop suitability.

Good luck.
 
it does rather sound like the boat is both extra clean and polished as well as empty of ... everything really.

initial reaction is that unless the above isn't the case you probably have the optimum prop for speed and fuel efficiency already - you could try a private message to member Nautibusiness who has wide experience as a dealer and user.
 
I have driven literally hundreds of 635s and if you are getting 19 knots from a Nanni 85Hp in a 635 then I would be absolutely over the moon.

The book speed is 18 knots and even then I only ever saw them do that downhill and with the wind and tide behind!

Most are 16 to 17 knot boats, cruising at 12 with that engine.

Very capable little sea boat but never intended to be fast.

If you do manage to get even more from yours then I'd love to know the final prop size.

Tom
 
nrgbentall said:
Hi
Could anyone please advise if re propping the old dog will speed her up
Currently running at 3000 rpm at 19 knots 200 rpm over her recommended 2800 running a 19*18 3 blade
Looking at dropping the revs and maybe increasing the speed by fitting a new prop of 19*21 anyone done this on the Nanni 85hp 5 cylinder
Any info most appreciated as this could be an expensive mistake

Thanks
Nick
Click to expand...
On thinking about it if you did decide to fit a bigger prop I could be interested in buying the old one. I am going to put the numbers which are on my prop on the foreum to find out what size it is. What would be useful is if a propeller manufactrurer did a "Propeller Hire Service" say "£100 deposit" £30 a week rent. Then we could try a different propeller without the capital outlay.
 
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