yamaha 25hp prop help

Billybruiser · 11 July 2017, 17:54

Hi lads, Im fixing up a tatty yamaha 25hp 2stroke, late 90s model, it needs a new prop on it, the old prop has numbers on it but i don't understand what they mean? anyone advise me on what prop size would be a good start on this outboard? its pushing a small avon rib. the numbers on the old prop are....
10 5/8 x 8 1/4 - F thanks for any help :-)

Poly · 11 July 2017, 18:30

First no is diameter, second is pitch. The f is to do with the range of engines it fits. 8.25" seems quite fine pitched, unless the boat is relatively heavy.

jeffstevens763@g · 11 July 2017, 18:49

Agree with poly pitch is too fine my 25 4 stroke came with 10 1/4 x 11 I would think that would over Rev on a light boat unless it been used on a displacement craft

Billybruiser · 11 July 2017, 18:56

Thanks for the quick replies, the outboard used to be pushing a big 4m avon sailing club sib boat, my little rib is much lighter so any recommendations on the pitch needed and where do you buy them from? eBay? Thanks

jeffstevens763@g · 11 July 2017, 19:15

Yes eBay good start I got one for my Suzuki few months back for £60 inc vat & post think the company was SSI marine I would look up the prop selection for the engine first make sure the 10 5/8 dia is correct

Billybruiser · 11 July 2017, 19:48

Hopefully someone on here has used a yam 25 on a small rib, and can give me there advice on size & pitch etc, it's a mine field buying a prop as they're not cheap and wouldn't want to buy the wrong size

jeffstevens763@g · 11 July 2017, 19:55

Quote:

Originally Posted by Billybruiser

Hopefully someone on here has used a yam 25 on a small rib, and can give me there advice on size & pitch etc, it's a mine field buying a prop as they're not cheap and wouldn't want to buy the wrong size

Gurnard on here runs a mariner 25 2 smoke on a 4.2 m sib much the same engine I think but stand corrected if not, being a rib you would proberbly get 2 knots more than him at WOT with same prop you right though trial and error

Poly · 12 July 2017, 08:31

Quote:

Originally Posted by jeffstevens763@g

Gurnard on here runs a mariner 25 2 smoke on a 4.2 m sib much the same engine I think but stand corrected if not, being a rib you would proberbly get 2 knots more than him at WOT with same prop you right though trial and error

I think you'll need a different prop to get two extra knots.

jeffstevens763@g · 12 July 2017, 09:10

Quote:

Originally Posted by Poly

I think you'll need a different prop to get two extra knots.

you might be right poly lots of factors to consider best he starts with the one he's got find the RPM and go from there [as always] i looked on the solas site briefly but the 10 5/8 dia didn't seem to be on offer might just be them.
someone must have a yam 25 on here very popular

jeffstevens763@g · 12 July 2017, 13:50

Ron Hale Marine Ltd - Yamaha Propeller 20-30hp Two and Four Stroke

Poly · 12 July 2017, 18:02

Quote:

Originally Posted by jeffstevens763@g

you might be right poly lots of factors to consider best he starts with the one he's got find the RPM and go from there [as always] i looked on the solas site briefly but the 10 5/8 dia didn't seem to be on offer might just be them.

someone must have a yam 25 on here very popular

I have a 20 on a rigid hull. I have a selection of less than perfect yam props all roughly the same size (+/-1") . The diameter is not critical - focus on pitch. Assuming a 3.8-4.2 m boat, with one or two people on board I would be starting at 11" and then adjusting based on observed revs/speed.

Billybruiser · 12 July 2017, 20:04

Thanks for your help lads :-)

jeffstevens763@g · 13 July 2017, 06:12

Ron Hale Marine Ltd - Yamaha Propeller 25-60hp Two and Four Stroke

there's some difference in the two i would ring Ross at ron hale marine very helpful.
i disagree that diameter is not critical as 1 inch in diameter is equal to 2 inch of pitch.

Poly · 13 July 2017, 07:02

Quote:

Originally Posted by jeffstevens763@g

Ron Hale Marine Ltd - Yamaha Propeller 25-60hp Two and Four Stroke

there's some difference in the two i would ring Ross at ron hale marine very helpful.
i disagree that diameter is not critical as 1 inch in diameter is equal to 2 inch of pitch.

Those are G series props, they won't fit if he has F series prop at the moment.

It's not true to say 1" in diam = 2" in pitch. There is an effect of changing diameter of course, a smaller prop is easier to turn so the engine can spin faster but if you follow that logic we would all have high pitch low diam props to get top speed without over working the engine, and of course life is not that simple.

However what I really meant by don't worry about the diam, was that all the F-series Yam props are about the same diam anyway. Unlike in the big engines you don't get the same choice of diam, and for any given pitch size you will usually only have one to pick from. But the key point is he needn't worry if it's say 10+1/4 diam rather than the 10+5/8 he has. If he thinks it has to be exactly the same he will have a long and possibly expensive hunt on for no benefit.

jeffstevens763@g · 13 July 2017, 08:18

https://www.propeller-shop.eu/propeller-info/

ah missed the [F] if thats correct for the engine.
diameter info above my explanation some what lacking 1 inch of diameter has the same effect as 2 inch of pitch on RPM.

as a side i am after a stainless prop and talking to steel developments if they prove to be as good as some say i will report back might be talking to them will sort prop choice

Bartol · 25 July 2017, 16:16

I have a Mercury Ocean runner 340 rib and Mercury 20hp 2stroke with carburetor from 25hp and am reaching 6200rpm with 10 3/8 x 13p prop at 28 kts. It really depends on total weight but you should be looking at pitch range from 11 to 13 inches. If you have a chance to get test props, do a couple of trial runs.

Roflhat · 27 July 2017, 06:55

A lot of prop selection depends on your boat weight and desired operating speed. We ran a 10x13p ali prop on our tohatsu 30 on a Ceasar Surfcat at ~34 knots. Then we went to a 10x15p chopper which was still really too small.

As a general rule the higher pitch you run the less diameter you need, you'll notice this looking at mercury props - for an 11p it will have around 11.5" diameter (50hp props) but if you go to a 15p prop it's only 10" diameter. More diameter helps push a heavy boat along at low speed, but at high speed the extra blade area will hold you back.

Not sure if Yamaha changed the gear ratio's in those engines but worth checking, the close your ratio is to 1:1 the less diameter you require. Even going from 1.86 on my Tohatsu 50 to 1.71 on my Yam 70 I've noticed it prefers smaller diameter props by around 0.5"

Bartol · 27 July 2017, 08:55

Sounds interesting, Roflhat.
Perhaps I should try a 14.5 pitch on mine, as I am currently well above the recommended top RPM.