Prop Size

[SkiprJohn]

Originally Posted by Cheechako

I'd try that prop you have. Calculators are not perfect any way. Get some real world info by doing so too.

And you could change the pitch a bit if it isn't quite right.

[westwinds]

Originally Posted by weephee

I'll go with the two blade. Likely have to buy a new one as used props for 3/4 shaft are few and far between. Thanks for all the information. I've learned a lot and appreciate your help

I do not think there are any propellers for the Yanmar 2QM15 that will fit a 3/4 inch shaft. If you want that diameter, you would need a new propeller shaft with a 1 inch diameter at the propeller that was turned on a lathe immediately before the cutless bearing to a 3/4 inch size. The shaft would have to be Aquamet 22 for enough strength. If the 2QM15 is used in this boat, (or some similar diesel with a reduction gear) the original propeller shaft has to be one inch. Changing the cutless bearing and stuffing box to one inch with a new propeller shaft would be the normal way of approaching this. I would still use Aquamet to avoid pit corrosion which is a problem with stainless 304. If you do machine the shaft to 3/4, make sure there is a proper fillet (curved shape to prevent concentration of stress) where the shaft goes from 1 inch to 3/4.

[Rustic Charm]

Hi guys,
I was with someone today putting their boat back in after a repaint and I noticed he'd sharpened the edges to pretty much a knife edge. His yacht is a 32 foot cruiser. Apparently guy next door at the slip suggested it. Does anyone know if this is a recommended action. Mine is similar to what a new prop comes like which is reasonably blunt edge.

[Opie91]

Is he sharpening the leading and trailing edges? Is he trying to make a poor mans set of prop spurs?

Sounds like a good way to loose a finger when it comes time to clean the prop mid season.

[Cheechako]

Originally Posted by tedsherrin

Hi guys,
I was with someone today putting their boat back in after a repaint and I noticed he'd sharpened the edges to pretty much a knife edge. His yacht is a 32 foot cruiser. Apparently guy next door at the slip suggested it. Does anyone know if this is a recommended action. Mine is similar to what a new prop comes like which is reasonably blunt edge.

Sounds like a good way to loose a bit of your prop blade every year; sharped to an edge, the edge corrodes off leaving an uneven surface, grind and sharpen next year... etc. I wouldnt, what's the point?

[weephee]

Don't mean to disagree with you Westwinds but I have the original Columbia dwgs for my 1977 8.3 Columbia and they specify a 3/4 inch stainless steel shaft with a 2 blade RH 15 x 9 x 3/4 propeller for the yanmar 2qm15. The prop for the optional Volvo Md7a is the same only LH and the optional Atomic 4 is also 2 blade on a 3/4 shaft but 12 x 6 RH. I did find and purchase a 2 blade 15 inch prop RH for a 3/4 shaft from Bacon sails in Annapolis. The pitch is 10 inches instead of 9. I'm not sure if I should spend the money to have the pitch changed to 9 inch or try it for a season as is.

[Cheechako]

try it first.

[westwinds]

Originally Posted by Cheechako

try it first.

Yes, try it. The engine will probably turn about 200 RPM lower than what it would turn at maximum horsepower; however, the engine will have almost the maximum horsepower, the economy, slightly better. I remember reading somewhere in The Propeller handbook that 10% reduction in engine speed from maximum under full propeller loading will not damage the engine.

[Rustic Charm]

Originally Posted by Opie91

Is he sharpening the leading and trailing edges? Is he trying to make a poor mans set of prop spurs?

Sounds like a good way to loose a finger when it comes time to clean the prop mid season.

No, from what I noticed he simply ground the edges to sharpness, purely because the guy next to him told him to do it.. The first thought in my mind is that if he does this every year he's going to run out of prop altogether. I don't have the experience to know as I'm fairly knew to sailing.

[westwinds]

Originally Posted by tedsherrin

No, from what I noticed he simply ground the edges to sharpness, purely because the guy next to him told him to do it.. The first thought in my mind is that if he does this every year he's going to run out of prop altogether. I don't have the experience to know as I'm fairly knew to sailing.

If it is a stainless steel propeller, maybe there is some chance that hitting the bottom at dock will not damage it. Stainless propellers are slightly more efficient because of the thin cross section of the blades.