Yamaha 30 Prop size vs rpm

[Fenlander]

It looks as if it will be just right... near enough... or a good start for even finer tuning. Be interesting to hear your speed and rev results from the next outing... I assume it's your original 12" pitch prop?

[Yabadabadoo]

Morning all

Thought I’d update this thread - no change as yet.

Boat set up:

• Weight in boat moved forward, really just anchor/chain, extra fuel 20 odd litres.
• One person on board.
• Boat dry in hull.
• Hull smooth and clean.
• Outboard raised as high as it would go. Sucks air in v lumpy conditions and on hard turn so might drop a pin. Fully trimmed in it behaves fine for now

WOT I’m still getting 16-17 knots gps (going with tide)
Revs @ 4400-4500

A fb group for these boats has a post of a similar hull shape and length to mine with same outboard and prop of 11 3/8 x 12-G. Same results.

Another owner of similar boat is running a cupped 10 5/8 x 12” and getting 20 knots @ 5200 with a 30 Yam.

Consensus on RIBnet is dropping to 10 or 11, I called Steel Development and they said 7, 8 or 9 as did a very time served mechanic I spoke to.

Now pretty confused at what to shop for. Been told the G in the 113/8 x 12G is the hub size of the existing prop so need to get like for like.

Does dropping prop diameter automatically change prop pitch or is that also a variable to decide on?

8” prop on a 30hp doesn’t seem right

[paintman]

This would suggest that the letter 'G' and others designate the engine & hub size for the Yamaha props:
https://www.bhg-marine.co.uk/propellers-21-c.asp
They show 'G' props from 8" pitch to 16" pitch in 1" increments.

IIRC a pitch change of 1" is around 150/200rpm up or down which would suggest as you need to go from 4400 to 5000 you need 600rpm so a 3" or 4" change which as the advice you've had from Steel Developments says could be an 8 or 9 pitch standard 3 blade prop.
Assuming all your figures are from accurate actual performance with normal load.

Think you've got to the point where you need to try props to see which one will get what you need. Perhaps an 8 or 9 as a starter & then see where to go.
Been through this with my own boat & ebay to buy & sell used ones worked fine - no problems selling the unsuitable ones.
Don't forget the 'List your spare props here' at the top of this sub-forum!

[Yabadabadoo]

Small update.

Bought a 12 1/4 x 9 genuine Yam prop - ex display/shop soiled so about half price [emoji1360]

Couldn’t run it with just me as Brother and kids (combined about 150kg) were up. Sticking them in the cuddy for trim and going with incoming tide we were at 4700 and 13.5 knots

With the 12” pitch + 1 adult I would see 12 knots and 4400 rpm

So am guessing rpm might go up a bit with just me.

Noticed fuel line is non marine grade and 6mm so will change that for 8mm

[Yabadabadoo]

Run test 19/11

12 1/4 x 9 prop
5000 rpm WOT
15.1 knots

11 3/8 x 12 prop
4400 rpm WOT
16.5 knots

Solo
One up from flat calm
Close to slack water

[Fenlander]

Is that on the rev limiter at 5000rpm with the 9"?

Seems you might need something nearer halfway between the two. I do like a prop collection.

[Yabadabadoo]

Late reply..

OB should rev to 5500

Am reading about cupped props, props with same pitch but smaller diameter.

Oh yes I can see the garage prop trophy cabinet now