Gap between gear housing and front of prop. Could it cause cavitation?

[comstox]

I gave an old 16ft boat to my son-in-law and he fixed it up and got it going for about $200! The engine is a 4 cylinder, 1990 100Hp Mercury outboard that has that two pistons work at lower RPMs until you punch the throttle. (Can't remember what that feature is called?) Anyway, When he does punch it he almost instantly cavitates (spelling?) I looked at his prop and it has an almost 1/4" gap between the front of the prop and the gear housing. Could this cause cavitation?
See pics. The hydro-foil also seems small for this outboard.
What say ye???

 

[TunaFish389]

I wouldn't think it would. I would look into prop slipping on hub, mark both and see if the line up moves. Or maybe adjusting outboard height it prop is not slipping.

 

[comstox]

Or maybe adjusting outboard height it prop is not slipping.

By lower it further into the water?

 

[dingbat]

That prop has seen better days. Worn edges, cav burn. Needs replaced.

While your at it, check to make sure you have the correct thrust washer for the prop.

 

[racerone]

Agreed---- check for the correct thrust washer.

 

[Faztbullet]

Yes that gap will cause problems....

 

[ahicks]

Engine exhaust can come out in front of the prop, causing the prop to cavitate. Need fairly close tolerance there. It looks like it's just a matter of getting the correct thrust washer to correct the problem.

 

[Bondo]

Ayuh,...... Looks to be the wrong prop to me,....

Looks to be for a smaller gear case motor,.....

 

[jimmbo]

It might be the wrong Thrust Washer. Might be the wrong prop. Or it might be a brand of Prop that can be fitted on many engines and uses a ring on the front of the prop for a better fit on some gearcases
As for the bolt-on fins, remove them and see how far you can throw them from the boat

 

[ahicks]

It might be the wrong Thrust Washer. Might be the wrong prop. Or it might be a brand of Prop that can be fitted on many engines and uses a ring on the front of the prop for a better fit on some gearcases
As for the bolt-on fins, remove them and see how far you can throw them from the boat

Good Lord, I missed those. Couldn't agree more. Not only are they just about worthless, they help keep welders in business welding the cavitation plates back on after a whale tail setup like that has broken them off. Cavitation plates are not designed for that kind of extra stress at all!

 

[Texasmark]

No. Have run such and no biggie. Think about props that are mounted on "thru hub exhaust" engines but there is no "tube" on the prop and it's made for high performance applications where you want all the "bite" you can get.

Just like the "ports" in most high performance SS props used in Bass Boats, for good hole shots on heavy stern loaded boats with a high pitch prop made for the WOT 65 MPH run, once you get up to speed, the water pressure running along the side of the gearbox just seals off the exhaust gasses from the blade tips.

 

[QBhoy]

Hi
that prop is pretty poor. Blades when worn and thin actually loose their pitch under load and especially acceleration.
The gap isn’t too much beyond the norm, but perhaps a little. I’d check the thrust washer isn’t the wrong way round or that the prop shaft nut isn’t caught the wrong way and not recessed if applicable

 

[Texasmark]

No. Have run such and no biggie. Think about props that are mounted on "thru hub exhaust" engines but there is no "tube" on the prop and it's made for high performance applications where you want all the "bite" you can get.

Just like the "ports" in most high performance SS props used in Bass Boats, for good hole shots on heavy stern loaded boats with a high pitch prop made for the WOT 65 MPH run, once you get up to speed, the water pressure running along the side of the gearbox just seals off the exhaust gasses from the blade tips.

The instance was a 14" diameter Turning Point Hustler prop (which I still have in the original box) that I ran on my 4" gearbox on my 90 Looper. I had to move the prop back from the gearcase as it was made for a 4 ¼ (I think is the size of the 150 gearbox) and the shell was at 4" also....same diameter as the gearbox housing, not smaller so that it could slide up inside as is the normal case with 13+" standard props for that engine. I couldn't allow it to touch the engine gearbox housing so I spaced it out with about a ⅜" gap.....also required cutting a notch in my narrow trim tab. I have recordeddata on the run since I was comparing it to my existing 13+ prop and it ran just fine, nice and smooth power across the rpm band.