propeller cupping

[bosun]

Hi All
Can anyone explain propeller cupping to me ?
I have several stainless props for my engine, the one I'm using at the moment is a michigan wheel 17'' pitch and has a definate cup or curve on the tips and edges of the blades, but I bought another new spare evinrude stainless prop off a dealer on ebay (also 17'') which is painted black and from its part number is described as 'double cupped' but it looks to have much flatter and narower blade than the michigan wheel one. Also the wot revs are higher on the black one? like they are an 18 and a 17 (or 17 and 16) rather than two slightly different 17's
Also the michigan wheel props have a vent? slot in the hub in front of the blade.
Whats this for?

[Andy B]

The cup is a curved lip on the trailing edge that allows the prop to get a better "bite" on the water by holding the water on the blade longer.
The vents in the hub allow exhaust gasses to lubricate the prop blades at low revs, this give the prop enough "slip" to spin.
At higher revs the water pressure stops gasses escaping through these holes and exits through the hub

As for the different performances of the two props,
Are they the same diameter?
Are they the same type of blade shape?
Bottom line is that they are two different props from two different manufacturers so they are bound to perform differently!

hope this helps

[bosun]

Thanks for that,they are the same diameter and pitch but the blade shape is radically different, the michigan wheel looks like the shape of a standard merc/mariner prop and the evinrude is like a standard honda prop. As to the cupping what you describe is obvious on the michigan wheel version but not discernable on the painted one ,perhaps it's been reconned and the cupping taken out (inside of spline looks new though)
cupping seems to make a big difference, like an inch or two of pitch?

[jwalker]

Originally Posted by bosun

Thanks for that,they are the same diameter and pitch but the blade shape is radically different, the michigan wheel looks like the shape of a standard merc/mariner prop and the evinrude is like a standard honda prop. As to the cupping what you describe is obvious on the michigan wheel version but not discernable on the painted one ,perhaps it's been reconned and the cupping taken out (inside of spline looks new though)
cupping seems to make a big difference, like an inch or two of pitch?

My understanding is that prop bades work similarly to an aeroplane's wing. A wing is horizontal so the lift acts upwards, a prop blade is vertical so the lift is horizontal. Just as lowering the aeroplane's wing flaps increases the lift of the wing, the cup on a prop blade increases the lift of the blade. The benefit to a boat is that the prop still retains its low pitch but since the cup has more influence at higher blade speed it has a similar effect to having a small increase in pitch at higher speeds. The amount of cupping will determine the increase but it is generally accepted a typical cup will be approximately equivalent to one inch of increased pitch.

The vent holes allow exhaust gasses to aerate the water in the vicinity of the prop at low speed so the engine has less load to drive, its revs therefore increase so provides more power to drive the high pitch prop.

[m chappelow]

Its just a glorified self tapping screw ,,LOL

[jwalker]

Originally Posted by m chappelow

Its just a glorified self tapping screw ,,LOL

So the blades would be better as angled, flat plates. It would certainly make propeller design simpler.

[Nos4r2]

Originally Posted by jwalker

So the blades would be better as angled, flat plates. It would certainly make propeller design simpler.

á la British Seagull?

[jyasaki]

Originally Posted by bosun

(inside of spline looks new though)

The hub can be replaced when a prop is reworked (and, I think, needs to be if the hub has been spun), so that's not necessarily an indication of a factory-new prop.

jky

[bosun]

Thanks all for your most useful replies.

[gotchiguy]

Originally Posted by Nos4r2

á la British Seagull?

The Silver Century hydrofan that I'm working on at the moment is made from cast aloominum, the blades are so thick that even ten mintues with the grinder removing the paint and smoothing the edges has no effect on the pitch!
oh and dont you mean à la British seagull?!