Prop barrel slightly smaller than the gearcase

[Joshto]

This is interesting, as my boat planes quickly with the current aluminum 4-blade 14.25 x 21P (4-3/8 barrel), and stays on top at ~3200 rpm. I guess I don't pay too much attention to performance or top speed, as it's mainly a family boat.

To clarify, my aluminum prop is 4-3/8 inches at the base (milled barrel), otherwise it is approximately 4-9/16 inches.

 

[Chris1956]

How about a temp fix? Clean up the edge of the gearcase hub, and the recessed part of the prop. Wipe them with some solvent.

Now rub some wax, candle etc. on the edges of the gearcase hub. Install the prop. Now run a precise bead of silicone caulk where the gearcase meets the prop. Let it dry, and then run a single edge razor to make sure the silicone doesn't stick to the gearcase. I would think that will seal it well enough, not add any drag and not add anything that could damage the hub or gearcase.

 

[jlh3rd]

I wanna clarify my "hub ring" statement.
The '05 Merc. 60, 4stroke I had was missing that ring. There was a lot I didn't know with the boat purchase in 2017.
My new re-power, 2021 Merc. 115 hp, I think (ass-u-me d) that it has that ring. I bought it brand new so I never looked, I don't know.
However, the gear case diameter is 4 3/4". If a prop hub is 4 1/4" , that prop will not work, as evidenced by the 2 props sitting on my shelf. Another lesson learned concerning propping a boat.
I have no idea about a merc stern drive, but thought worth mentioning.

 

[Texasmark]

yep. The exhaust exits through the prop hub instead of around the outside.

Interesting. I don't know what I expected to see but apparently that works.

 

[Texasmark]

That was the Mercury 'Chopper'
View attachment 411901

I think the physics behind this is similar to ported props. As soon as you reach a certain speed, water pressure from new water forces the exhaust to hug the hub, rather than being spread out over the blades like is the case at lower speeds where slip is significant.

 

[Texasmark]

yep. The exhaust exits through the prop hub instead of around the outside.

I replied to your post but it wound up several listings below.

 

[Joshto]

How about a temp fix? Clean up the edge of the gearcase hub, and the recessed part of the prop. Wipe them with some solvent.

Now rub some wax, candle etc. on the edges of the gearcase hub. Install the prop. Now run a precise bead of silicone caulk where the gearcase meets the prop. Let it dry, and then run a single edge razor to make sure the silicone doesn't stick to the gearcase. I would think that will seal it well enough, not add any drag and not add anything that could damage the hub or gearcase.

That's some McGiver Sh@t haha. I'll keep searching for a gearcase-mounted ring (aka weed ring), if I can't find any....I'll give this idea a shot. I don't want to keep the prop if it affects min-acceptable performance.

 

[Joshto]

It appears the part number 350626 might work; it's a weed ring (exhaust ring) for Renegade Bass props on Evinrude V6 (which seems to share the same diameter as the Alpha One gearcase).

Only 15 bucks, so I ordered one to see if it fits.

 

[Joshto]

I got the Ventilation/Exhaust Ring 360626, and after making some modifications, I fitted it snugly on the gearcase. The gap left it's about 1/16 (way better than my AL prop)

Tested it in the lake. I love the prop, great hole shot, speed (40 mph - could have been better if the lake wasn't choppy), sharp turns, and super smooth (virtually no vibration). However, the engine won't reach WOT (it peaked at 4,100 RPMs)....So I guess it's not for my boat. Need to drop the pitch.