3 Bladed fixed propellers

[Ken Juul]

I had a cambell sailor for a while.  Can't remember if it was over or under pitched, but it wasn't right.  After a couple tries at repitching I went back to the Michigan Sailor 3 blade.

[Robert Mann]

The sizing is very different from the more conventional 3 bladed sailor props.  14x8 is the size that is recommended by the company supplying the CS prop. This also shows up on the various other forums as the size for 34 a 36' boats with M35B.  Dropping in diameter and pitch is a little bit counterintuitive.

[KWKloeber]

  Dropping in diameter and pitch is a little bit counterintuitive.

CS has it down.   The blade design lets them do a smaller section. It's all about efficiency of the foil design and thrust vs the larger "flat" blade props.


KK

tidbit, Speaking of efficiency of foil design, how about those Wright Bros' prop -- designed w/o an iPad, excel, microsoft, or an HP calculator -- manufactured using axes and spokeshaves?

[Robert Mann]

Yes but Wilbur and Orville didn't need a haul out to do a re-pitch

[KWKloeber]

Yes but Wilbur and Orville didn't need a haul out to do a re-pitch

True, but in those days a trip off the Hawk and back and forth to the bicycle shop was a lot more involved than our haul!   

[Noah]

Any "good" diver should be able to swap out a prop cheaper than a haulout. But if you want to do it out of the water, you could do a quick roundtrip "out-and-in" while still in the slings.

[KWKloeber]

Any "good" diver should be able to swap out a prop cheaper than a haulout. But if you want to do it out of the water, you could do a quick roundtrip "out-and-in" while still in the slings.

How do you properly lap fit a prop underwater?

kk

[Noah]

His concern was changing the pitch if he bought the wrong pitch.

[KWKloeber]

Agreed.  And a prop should be properly lap fitted when installed.

So how does the diver reinstall the repitched prop underwater and lap fit it?  And do many divers know how to do that?

One could make an argument that it was fit once, so it doesn't need lap fitting when reinstalled.  But I wouldn't chance it if it were my brand new prop (and have a chance of loosing it to Davy Jones.)  It might not need regrinding -- but I wouldn't install it without at least the prussian blue fit test.

kk

[Noah]

I am not an expert at lap fitting, but I would conjecture that once the prop was properly fitted to the shaft, later repitching and reinstalling wouldn't impact the fit on the shaft. However, I am "an expert" at changing props in the water. Back in my yacht delivery days, after long distance ocean races, it was standard practice to race with a small 2-blade folder, then change to a 3-blade fixed for the long upwind slog back home. I used to change half a dozen in a day after long Mexico races and the Transpac--I never lost one to Davey Jones or had any issues.

[KWKloeber]

Let me clarify....

I'm not an expert at either one, just yielding to our guru (RC.)  Maybe he'll say once done, not necessary again.   
But I'm just saying regardless, it's not worth the chance to me.

I know someone who lost theirs -- all was fine, until -- and most likely all else equal there won't be a problem losing it (until there's a problem, say hitting something submerged) and then all of sudden the fit becomes the critical link that fails. 

Hell, there's plenty of Universal engines out there still with exploding alt brackets and no fuses on the harness -- none are a problem in themselves -- until something else unanticipated happens, and then....

I have a "risk and safety bucket" that I fill up (and sometimes overflow) when I get the chance -- and there's been times that I feel that I have drawn my luck down, but the excess I had paid into the bucket saved my butt.

It's everyone's own boat though, and their own choice as to what risks they want to accept vs cost.  -:))

k

[Ken Juul]

The nut that holds my prop on has a cotter pin through it.  Aren't they all that way? 

[Robert Mann]

Its new, it needs lapping to the shaft.  I would also re-blue it and check it if it was re-pitched just to be sure.  It's just one more potential vibration point to eliminate. More importantly my slip is 100' deep and "there be monsters down there".  I'd rather drop it on my foot than into 100" of empty un-divable space.

[KWKloeber]

The nut that holds my prop on has a cotter pin through it.  Aren't they all that way?

Yes (nearly all? never say never) have a castle nut and should have a thinner locking nut.  But the C30 who lost a prop had the castle/cotter in place the last time it was splashed (reportedly.)  Go figure?

Ken