Non Compatible Propellers?

[tinker]

Ribcraft 4.8
Mariner 60hp 2 stroke

I have so far tried both 10 ¾ diameter 13" and 17" pitch props on my 60hp 2 stroke Mariner (3 ½ inch gearbox, 13 splines). The top end on both are more or less the same 31-32 knots although revs are a lot lower with with the 17", the hole shot is so......slow.

With this in mind I have just acquired a SS Michigan Wheel Rapture prop, which is 11 ¾ diameter with a pitch of 15. The physical fit is perfect, no snagging and clearance from the gearbox is around 40mm. Its the extra diameter that concerns me, I know in affect it will add a little to the pitch but what other considerations should be taken?

Ps The Michigan catalogue does not say it is compatible with my engine.

[Cookee]

Originally Posted by tinker

Ribcraft 4.8
Mariner 60hp 2 stroke

I have so far tried both 10 ¾ diameter 13" and 17" pitch props on my 60hp 2 stroke Mariner (3 ½ inch gearbox, 13 splines). The top end on both are more or less the same 31-32 knots although revs are a lot lower with with the 17", the hole shot is so......slow.

With this in mind I have just acquired a SS Michigan Wheel Rapture prop, which is 11 ¾ diameter with a pitch of 15. The physical fit is perfect, no snagging and clearance from the gearbox is around 40mm. Its the extra diameter that concerns me, I know in affect it will add a little to the pitch but what other considerations should be taken?

Ps The Michigan catalogue does not say it is compatible with my engine.

The RPM for all of the above props would help, but if it's not hitting the rev limiter with the 13" why go for a bigger prop? Also were they all Stainless props as this will also make a difference.

Larger diameter will bring the revs down as well as pitch.

[jyasaki]

As long as the propr doesn't hit the anti-ventilation plate, you should be OK as far as propeller diameter goes.

You should choose pitch (for a given diameter) based on WOT RPM's that you achieve with a normal load (it should be in the upper part of the manufacturers rev range.)

If you can't find a prop that suits that, the general rule of thumb is 200 to 250 rpm per inch of pitch, and 100 or so rpm per quarter inch of diameter.

Note that different blade contours will have an affect as well (different manufacturers.)

jky

[tinker]

Thanks for the replies. My Friday afternoon posting skills need a little brushing up

In my ignorance I would not like to cause damage to the engine or gearbox by fitting a prop that has a different design to the one mercury or mariner spec. Its not really the performance that concerns, at the moment.

Cookee
The 13" and 17" are both alloy, revs are 6000 + for the 13" and around 5000 for the 17".

jyasaki
The Prop is 40mm's clear of the anti-ventilation plate, but the shape and size of the blades plus the weight of the prop are of concern.

I visit Fowey in a few weeks and then I should have the answers on the right pitch.

P.S Anyone got a SS Solas 15"?

[jyasaki]

Weight should not really that much of an issue unless you're talking huge changes. The only force exerted by a heavier prop is the gravity pulling it down; while running, it is rotational (and is generally absorbed by the prop itself.) A heavier prop will resist changes in rotational velocity a bit more, both up and down.

In general, if you have a prop that fits the LU without interference, it will be OK from an operational standpoint. That doesn't speak to the suitability from a tuning standpoint, though.

Shape of the blade plays to tuning. The motor is supposed to have resistance; that's why it's there in the first place. The engine doesn't care if there's more or less resistance caused by the prop; it just sees more or less resistance, and the resulting WOT rpm varies down or up accordingly.

jky

[Turbodiesel]

Sounds like a 15" Ally prop is the one to go for. 6000rpm is way over the rev range of that engine and you will end up knackering the reed valves. I think from memory 5500 is the max rpm for your engine. Once you have the pitch right you can then start looking at stainless or 4 blade props etc.

[Cookee]

Originally Posted by Turbodiesel

Sounds like a 15" Ally prop is the one to go for. 6000rpm is way over the rev range of that engine and you will end up knackering the reed valves. I think from memory 5500 is the max rpm for your engine. Once you have the pitch right you can then start looking at stainless or 4 blade props etc.

What Turbo said!