Mercury Thunderbolt 7.5hp 2 or 3 blade prop?

[Lee1]

I have acquired 2 vintage Mercury 7.5hp thunderbolts.

A long shaft with a three blade prop (slightly earlier model than the short shaft and only good for parts)

A standard shaft with a 2 blade prop. All there with 165psi compression but not running yet!!

I've seen on YouTube vids that all the standard shaft 7.5 and 9.8 thunderbolts have the 2 blade prop not the 3 so I'm guessing the 3 blade was fitted only to the long shaft for thrust use on large displacement sail boats? Or I'm wrong and the long shaft I have has had a different prop fitted (it is painted red not black) and the long shaft had a 2 blade prop originally fitted as well? If the 3 blade prop on the long shaft is the original prop Mercury must have done some testing and decided to use a 3 blade not a 2 if so I wonder why as no other outboards of these sizes use a 2 blade prop?

I've advertised the outboard for sale but I'm considering keeping them and if the standard shaft is an easy fix (when I look at it in depth tomorrow) I will keep it and use it as my back up for this season and something to enjoy using and testing out both props on a 2.7 v honwave sib.

Also if the standard shaft is a keeper what prop will give me the best top end speed? I'm guessing the 3 blade.

Also any suggestions for a better prop for use on this Mercury Thunderbolt 7.5hp standard shaft for SIB use other than the 2 props I already have, many thanks

[Searider]

I've seen both 2 and 3 blade propellers on those old Mercs.
The number of blades will have less effect than the pitch and the condition of the propellers.
You'll need to try both and see which one performs best.

That being said, the two blade will be more efficient as it has less blade area to drag through the water.

[Lee1]

Unfortunately this one won't be running, see my post on the engine thanks for your reply

[9D280]

My 60 is a similar vintage and pushes my humber along nicely.

If it works for your setup chill and enjoy the boating.

[Lee1]

Very true!