Propeller spare choice

[Tequila]

I've had a look on line and in the forum and didn't quite get the answers to some questions about spare propeller choice.
I've read that carrying a spare propeller is advisable and I can see the sense in that. I've never had a problem with my old Seagull but they are pretty bombproof. Better to have the spare and not need it than need it and not have it! (Lots of rocks around Arisaig).
Do members routinely keep a spare on board?

I've also seen on the forum that the propeller supplied as standard is probably best match to engine.
I have a Yamaha F6CMH powering a light air deck 3.2m SIB. The Yamaha web site gives a choice of five propellers as compatible to the 4hp-6hp F range. They obviously range in diameter and pitch with the one supplied for the 6hp is 6E0-45952-00-00 7¹/₄ x 8¹/₄ inch giving the second highest pitch after the 6E0-45954-00-00 7¹/₂ x 9 inch prop.
The other props are 7¹/₄ x 6¹/₂, 7¹/₂ x 7 and 7¹/₂ x 8 inch.
From reading on line I understand that a higher pitch will provide a higher speed assuming you can plane (or have a long narrow canoe ) and conversely a lower pitch gives more acceleration with a danger of over revving the motor.
Given that SIBs are a light boat and therefore not likely to have the inertia of a conventional boat I don't see acceleration as an issue. It seems logical to me that the 9" pitch prop may be worth considering as a spare. Has anyone experience of trying a higher pitched prop on SIB? Does it affect the ability to plane? Will a lower pitch aid getting on a plane with the boat due to the acceleration/power?
Given that all the props are compatible I assume that the rev range of the motor will not be exceeded whichever version is fitted.
Cheers Andy

[Martin]

Hi Andy,
Lots of people give their versions of how to size props or what you should be running based on their own experience.
In my opinion, the best way to size the prop is to fit a tach to the engine and load your boat up as you would normally do,
Your engine manufacturer will specify a WOT rpm for your particular outboard on its standard fitment prop, that’s maximum RPM at full throttle.
If you’re loaded as you normally do, and you’re just about hitting the max RPM at WOT (wide open throttle) then your prop is sized correctly.
If you’re struggling to hit the suggested RPM at WOT, then you need to decrease the pitch of your prop to suit. Generally speaking, 1” of pitch decrease will raise the RPM at WOT by 200-300 rpm.
This works vice versa too..

Think i got that the right way round

Martin

[Tequila]

Thanks for your reply Martin. I've had a play on the Vicprop calculator website
https://vicprop.com/planing_size_new.ph ... =calculate
It doesn't help a great deal so I'll look at measuring the revs on WOT once I've run the engine in. I think the supplied propeller is probably a good match as the SIB is light even with two people and gear (air deck).
With a guesstimate for weight and inputting the other data the Vicprop calculator suggests a higher diameter and pitch than the supplied prop. It's only if I put a very heavy load that it drops to the range of the props available for my outboard.
I'll feedback results at some point.
Cheers Andy