On my maiden voyage this year I tore a nice chunk out of my prop. I hit
something underwater with the propeller hard enough to kill the engine,
although we were barely above an idle. Note that I NOW have the depth
finder installed...

As a matter of complete blind luck, and even though I was not looking for
one, I found a prop at the flea market for a low enough price to where I
said "maybe this is the right one" (Trust me, the price was THAT low).

Sure enough, the prop went right on and fit perfectly. Except it is a larger
diameter. (The outside dimension, of course. The spline, etc are correct for
my application). This new prop is about 1.5 inches larger in diameter. Not
to the point where it interferes with anything, it still has plenty of
clearance.

Due to my lack of experience, I have to ask, will this be an issue? I am
trying to see how a larger diameter prop can be anything but better than a
smaller diameter prop. The pitch appears to be very close, if not the same.
Due to the larger diameter, though, there is more surface area on each
blade.

What are the pros and cons of a larger vs a smaller diameter?

Application is a 19 ft cuddy with a 165 hp Merc with the Alpa outdrive.

Thanks!

"TSC" wrote in message
...
On my maiden voyage this year I tore a nice chunk out of my prop. I hit
something underwater with the propeller hard enough to kill the engine,
although we were barely above an idle. Note that I NOW have the depth
finder installed...

As a matter of complete blind luck, and even though I was not looking for
one, I found a prop at the flea market for a low enough price to where I
said "maybe this is the right one" (Trust me, the price was THAT low).

Sure enough, the prop went right on and fit perfectly. Except it is a
larger diameter. (The outside dimension, of course. The spline, etc are
correct for my application). This new prop is about 1.5 inches larger in
diameter. Not to the point where it interferes with anything, it still has
plenty of clearance.

Due to my lack of experience, I have to ask, will this be an issue? I am
trying to see how a larger diameter prop can be anything but better than a
smaller diameter prop. The pitch appears to be very close, if not the
same. Due to the larger diameter, though, there is more surface area on
each blade.

What are the pros and cons of a larger vs a smaller diameter?

Application is a 19 ft cuddy with a 165 hp Merc with the Alpa outdrive.

Thanks!


My understanding is that the pitch of the prop is more important than the
diameter (within reason), as long as it fits. A larger diameter with the
correct pitch equals more overall efficiency.

Eisboch

My understanding is that the pitch of the prop is more important than the
diameter (within reason), as long as it fits. A larger diameter with the
correct pitch equals more overall efficiency.


And more drag, as well. Assuming equal pitch, the larger diameter prop
would be more appropriate for situations that require more low rpm
efficiency and torque than high rpm speed, such as in an industrial work
boat, slow cruiser, or trawler . The smaller diameter prop would be more
appropriate for a high speed application such as a performance boat. A
medium sized diameter would be a compromise of the two. A good example
would be MerCruiser's line of outdrives. The Bravo II swings a relatively
large prop and is typically used on industrial boats and some slower
cruisers. The Bravo I or Alpha dives use a much smaller diameter prop and
is used on sport boats, faster cruisers, or runabouts. The Bravo III drive
uses props about the same size as the Bravo I or Alpha, but gains efficiency
of blade area and other advantages by having twin counter-rotating props in
line.

Theory aside, there is most likely very little leeway in the choice of prop
diameter for a given application due to clearance, fit, and parts
availability for the given drive system.