Prop barrel slightly smaller than the gearcase

[Joshto]

Hey there,
My 4-blade Aluminum prop got a hit and bent the tip of one of the blades (nothing serious). I got this SS prop from a friend with similar specs, but the barrel is slightly smaller than the Aluminum propeller at the base; 3/8~1/2 inch less in total diameter.

Everything fits perfectly: the original forward thrust washer fits perfectly, and the shaft length/diameter/spline (15) in the hub. Once tightened, the prop rotates freely, without hitting or scraping anything.

Should I be concerned with the barrel diameter difference? Note that the aluminum propeller is tapered at the base, so it also sits into the gearcase like the SS. The only difference is that the aluminum barrel diameter gets bigger outside the gearcase, while the SS maintains the same diameter throughout the length of the barrel.

 

[jlh3rd]

what motor do you have.
Different motors can have different gearcase diameters, like my Merc. CT.
the problem is the exhaust gases will exit over the blades and cause ventilation problems.

 

[Joshto]

what motor do you have.
Different motors can have different gearcase diameters, like my Merc. CT.
the problem is the exhaust gases will exit over the blades and cause ventilation problems.

Motor in my signature. 3.0. Alpha one gen one gearcase 4 3/4

I thought of that but the prop barrel goes all the way in. If one looks through the prop exhaust cavities, I cannot see even light. So I doubt gases will take that path. I guess I'll have to give it a try in the water or unning the engine on the mufflers can also give me an idea if exhaust gases come out through the gap.

 

[jlh3rd]

yes, the gases will take that path.
do what you want, but 41/4" prop hubs don't work with 4 3/4" gear case diameters.
That's why they make props with 4 3/4" hub diameters.

 

[cyclops222]

Do not mix the different sizes. Unless ...YOU....take all the responsibility.

 

[Joshto]

Do not mix the different sizes. Unless ...YOU....take all the responsibility.

Lol...did I ask anyone here to take responsibility?

 

[jimmbo]

While there are instances of using the smaller Hub on the Large Case, it is not done on Recreational Boating. The Props would need to be engineered for it. The one you are showing, is not

 

[Mc Tool]

Im thinking that even if the exhaust is sealed inside the gearcase the sudden change in dia as the water flows from over the case and on to the prop will cause turbulence and drag . And why is there so much thread showing on the prop shaft ..... unless you haven't got the nut etc fitted

 

[Joshto]

Im thinking that even if the exhaust is sealed inside the gearcase the sudden change in dia as the water flows from over the case and on to the prop will cause turbulence and drag . And why is there so much thread showing on the prop shaft ..... unless you haven't got the nut etc fitted

Yes. Nut wasn't installed when I took the pic. When fully installed, it looks just like the other prop.

 

[Chris1956]

Some modular props came with a plastic "cap" that fit the recessed end of the prop, to make it's diameter 4-3/4". This cap allowed the prop to fit more motor applications.

You might go on the prop OEM's web site and see if they have one.

 

[jlh3rd]

jesus buddy, you get on here and ask questions and for help then start dissing all the answers.
So just go run the prop, it's that simple.

 

[jlh3rd]

Some modular props came with a plastic "cap" that fit the recessed end of the prop, to make it's diameter 4-3/4". This cap allowed the prop to fit more motor applications.

You might go on the prop OEM's web site and see if they have one.

and also, my 2021 Merc 115hp has a plastic ring that snaps into the gear case for exhaust gas sealing. Maybe that's in sterndrives also?

 

[Joshto]

and also, my 2021 Merc 115hp has a plastic ring that snaps into the gear case for exhaust gas sealing. Maybe that's in sterndrives also?

Thank you, this def a great suggestion. They seem available for alpha one too.

And I'm only dissing the unsolicited legal counselling.

 

[Texasmark]

While there are instances of using the smaller Hub on the Large Case, it is not done on Recreational Boating. The Props would need to be engineered for it. The one you are showing, is not

Along those lines, I "think" I have seen high performance props where there is no "tunnel" for gasses to exit behind the prop. What I don't remember seeing is what the LU casting looks like right in front of the prop......I recall early low HP Mercury racing hydroplane short shaft engines that had exhaust elsewhere and there was a smooth transition between the LU and the front of the prop hub.

 

[Texasmark]

and also, my 2021 Merc 115hp has a plastic ring that snaps into the gear case for exhaust gas sealing. Maybe that's in sterndrives also?

You mind posting a picture of that? I find that most interesting....in short, what does the engine do with its exhaust gasses as a result?

 

[Chris1956]

There are high performance props without exhaust hubs, that fit on motors with hubs. The theory is that the exhaust on the blades gives some slip when accelerating, and when the boat reaches higher speed, the exhaust gasses are pulled away by the receding water, so the prop bites.

I tried a 4-3/8 hub prop on my merc V6 with a 4-3/4 hub. The result was a lot of slip under acceleration, which was expected. The prop did catch as the boat went faster, but I never thought the change to the smaller prop hub was an advantage.

 

[jimmbo]

Along those lines, I "think" I have seen high performance props where there is no "tunnel" for gasses to exit behind the prop. What I don't remember seeing is what the LU casting looks like right in front of the prop......I recall early low HP Mercury racing hydroplane short shaft engines that had exhaust elsewhere and there was a smooth transition between the LU and the front of the prop hub.

That was the Mercury 'Chopper'

 

[flyingscott]

You are probably going to cavitate the snot out of that prop. Exhaust is supposed to go through the prop not out around the blades.

 

[jlh3rd]

You mind posting a picture of that? I find that most interesting....in short, what does the engine do with its exhaust gasses as a result?

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

 

[Joshto]

and also, my 2021 Merc 115hp has a plastic ring that snaps into the gear case for exhaust gas sealing. Maybe that's in sterndrives also?

More details here. The SS prop is exactly 4" at the barrel base.

The aluminum I've been running is 4-3/8.

I haven't found a ring (gearcase-mounted) compatible with Alpha One. Starting to guess they don't exist.

This one is for Mercury Outboards Part Number: 8M0214998, but it won't work for Alpha One (It's for gearcase 4 1/4). Installation:

The exhaust seal rings that go on the prop (such as 878421 Exhaust Seal Ring For V6 Gearcase Trophy Plus Propellers) wouldn't work for this SS prop either, as the barrel isn't milled/tapered, so the ring wouldn't hold up in place.

I tried a 4-3/8 hub prop on my merc V6 with a 4-3/4 hub. The result was a lot of slip under acceleration, which was expected. The prop did catch as the boat went faster, but I never thought the change to the smaller prop hub was an advantage.

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.