simple...but perhaps confusing....prop question ('53 15HP Super Fastwin)

Hi folks,

I've done a ton of searching here at iBoats, but I haven't found any threads that address my seemingly-simple question...

I just finished restoring a '53 15HP Evinrude Super Fastwin; The prop on my engine is a.) old, b.) not in great shape, and c.) of unknown origin & spec. I'm going to want to get a replacement.

My Evinrude service manual (Publication EM5thED, revised Dec. '60) says that the prop for my engine is a 3-blade 9 x 11. Simple enough....

So why are different props available? This link shows the availability of 9" props with pitches of 7, 9 & 10 inch, in addition to the 11 pitch that Evinrude said to use.... 15 HP Evinrude Outboard Boat Propellers 1953 - 1956 - iboats

Why would I want to use anything except the 'right' prop? Help me understand what I clearly don't.....

Thanks!

Re: simple...but perhaps confusing....prop question ('53 15HP Super Fastwin)

The "right" prop was the one that the engine came with from the factory. Because the manufacturer did not know the use to which the engine would be put, it is USUALLY an "average" prop. Yours appears to be a light load prop.

These smaller engines almost always did not have tachometers, so there are charts describing correct props for different uses, that is, Low pitched prop for heavy loads and high pitched for light loads. Average pitch for average load. If your engine is on a 12 or 14 foot tin boat and you do not carry a lot of people or gear, that is a light load. carry four people or a bunch of fishing gear and you now have a heavy load. Similarly a fiberglass 15-18 foot boat is a heavy load and you would want a low pitches prop.

Re: simple...but perhaps confusing....prop question ('53 15HP Super Fastwin)

OK, props are selected for the motor and for the boat size/weight and the typical activities.
The idea is to be able to operate within the motors rpm range at wot.4000-5000. You would prop for closer to the upper number.
The 11 inch prop is the base prop and could be exchanged for a prop better suited for the application.
Very basically higher pitch props 11" would be for light reasonably fast boats not likely to be carrying heavy loads.
Lower pitched props would be suited for heavier applications or perhaps water sports.
To do it right you need a tach,Tinytach or other induction tach will do,and a gps.
Try to find your present prop number near the base of the blades or possibly on the hub.
Hopefully you can make a lightly loaded wot test run get us the rpm a gps speed with a description of the boat.
This will give us a base line to start from.
All this fuss is to be sure the motor is able to operate within its rpm range. Once setup you can of course operate at whatever rpm/speed you want.

Re: simple...but perhaps confusing....prop question ('53 15HP Super Fastwin)

Ah - HA! Perfect sense! Thanks!

So if this engine is on a 120 pound, 14' Aluminum V-hull with just me....the 11 pitch might be great....

But if the boat is a 17' Lapstrake Lyman, tipping the scales at 400+ pounds, with 4 adults sitting in it, the 11 pitch doesn't stand a chance, and the engine will bog.........best to go to a lower pitch. The lower pitch will allow the engine to RPM where it ought to, but the cost is speed.

Is that it? Something like that?

Re: simple...but perhaps confusing....prop question ('53 15HP Super Fastwin)

You got it with this slight difference.The 11" prop wouldn't plane with the load you described the correct lower pitch would hopefully plane ,a bit of a stretch on a 17ft lapstrake,thus in this case produce a faster speed.