TOPIC: Speed prop on a 75 hp?

I recently took the 16' aristocraft funliner with the 75hp v-4 to the coast to cruise the ICW and to play in the inlets. I noticed with just me on board the old girl would run 36 mph taching 5000 rpm with a 10x11 prop. My Dad told me many years ago that 36 mph was as fast as he ever managed with the boat even when new and that was with a 10 1/4 x 10 prop. I was pleased to see the boat would still run it's 'new boat" top speed. So I got to thinking that I'd try a 10x12 prop as I found one on ebay for 9.99 plus shipping. I thought what the hey and pulled the trigger. I haven't had the chance to try it out yet as I did go to the lake yesterday,but had three people with me so I took the nineteen instead. Anybody tried a 10x12 prop on one of these motors on a 600lb boat??

This is an OMC propeller booklet from 1965 with boat size recommendations relative to pitch / diameter for the 75 HP:

www.fiberglassics.com/library/OMC

And some tips for selecting the right prop:

Thanks Andy,the first chart tells me the 10x12 should do alright with just me in the boat which is kinda what I was thinking. I know that would be too much prop with my boat loaded with 3 or 4 extra people. I just figured I'd try the 10 x12 and if it don't pan out , I'll sell it.

Jim,

Chances are that an 11 pitch is going to work the best for your boat. Most important in the choice of a propeller is the engine RPM - the motor MUST be able to reach 4,500 to 5,000 (some models 5,500) rpm at full throttle. Any full-throttle full speed rpm lower than that means the engine is being overloaded. Experts on these crossflow engines have found that overloading them will cause severe carbon buildup behind the piston rings, eventually causing them to stick and destroy the cylinder walls.

The main problem problem with these OMC V4 lower units is that the propeller speed is nearly the same speed as the crankshaft. They are geared to spin like the blades in a kitchen blender. These lower units were designed more like a racing lower unit than one for an everyday pleasure boat. The high speed of the gearing would not allow the engine to "wind up" without a low pitch prop. The lower pitch propeller gave better out of hole power but caused a penalty on the top end speed, due to the engine hitting redline earlier. So the the choice of "correct" propellers for these motors was limited to just a handful. OMC engineers apparently realized this fairly early, and they designed the large "100" HP lower unit with the gearing much different, and utilizing a large propeller. They probably looked at the Mercury outboard lower unit for guidance. This slower turning large propeller was a much more efficient design, and it enabled a much wider range of propellers for all applications. They also held the boat on course much better than the small gearcase models.

So with the small gearcase lower units, we are stuck with 9", 10", 11", and 12" pitch stock selections. Of those, 10" and 11" will usually give the best overall performance.

If you can get your hands on a Good aftermarket bronze propeller like an OJ or a Michigan Feather Weight, your will probably see a performance improvement. The large hub Michigans need to be avoided, they weighed a ton & are very bad on the gears, bearings and clutch. I ran an OJ 2 blade 10-1/4 X 14 on my Reinell with a Starflite III and it ran beautifully - through slightly overloaded. a two bladed 10-1/4X12 Cup would likely have been perfect for that boat.

Bruce,the 10x11 does the best with my wife, and 1 or 2 more aboard. I'm going to try this 10x12 with just me in the boat. I 'm 80 percent sure it's way too steep a prop with a load of people push. Right now,the tach is reading 5000 to 5100 at wot with the 11 prop.

Bruce Gerard wrote:

Jim,

OMC engineers apparently realized this fairly early, and they designed the large "100" HP lower unit with the gearing much different, and utilizing a large propeller. They probably looked at the Mercury outboard lower unit for guidance. This slower turning large propeller was a much more efficient design, and it enabled a much wider range of propellers for all applications. They also held the boat on course much better than the small gearcase models..

Thanks for the great info Bruce.

I have a question. I'm wanting to get a 1960 75hp OMC running and it is a short shaft model......which is what I need for the boat it's to go on.

Is the 100HP lower unit going to work out for a short shaft model?

If so, I'm shopping!

Thanks,

Dan

Well, I finally had a chance to sneak off down to the lake solo today. Man is it different during the week !! Smooth as silk. maybe 25 boats on the whole lake. Anyway, the 75hp johnson did pretty good with the 10x12 prop. Speeds varied, but would consistently run 36 mph at 4900-5000 rpm. She did touch 37 mph a couple times. I know that's not at all fast compared to some of you guys boats,but for my little funliner,it was as fast as I've had her to go,so I was pleased with my $10.00 prop

I wish my prop was only $10.00 . Mine was $400.00 (and a week later someone had the same prop in great used condition on Craigslist for $100.00 - dag-nab-it!

73Crestliner wrote:

I wish my prop was only $10.00 . Mine was $400.00 (and a week later someone had the same prop in great used condition on Craigslist for $100.00 - dag-nab-it!

That's usually how my luck runs. Every once in a while I find a good deal to make up for some of the "not so good deals"

Quote"If you can get your hands on a Good aftermarket bronze propeller like an OJ or a Michigan Feather Weight, your will probably see a performance improvement. The large hub Michigans need to be avoided, they weighed a ton & are very bad on the gears, bearings and clutch. I ran an OJ 2 blade 10-1/4 X 14 on my Reinell with a Starflite III and it ran beautifully - through slightly overloaded. a two bladed 10-1/4X12 Cup would likely have been perfect for that boat."
Bruce, I'll see if I can find one of those 2 blades. Thanks for the info.

VinTinFan wrote:

Thanks for the great info Bruce.

I have a question. I'm wanting to get a 1960 75hp OMC running and it is a short shaft model......which is what I need for the boat it's to go on.

Is the 100HP lower unit going to work out for a short shaft model?

If so, I'm shopping!

Thanks,

Dan

The 100 lower unit is only electric shift and will not work on a 1960 manual shift engine. Short shaft 100 lower units are not as common as the 20" models - so they can be very difficult to source for short transoms

Jim L wrote:

Bruce, I'll see if I can find one of those 2 blades. Thanks for the info.

Look on ebay - the 3 blade are fine too and easier to find than 2 blade versions. Look for an OJ - and be patient. Also, pay special attention to the pin holes on the snout of the drive bushing. If the holes are egged out, chances are that the hub is messed up. The V4 OJ's are stamped "V75" on the flat of the hub - where the thrust washer for the shear pin goes. When the plastic "prop nut" is in place, the stamping can't be seen. These props use the smaller sheer pin and nose cone that the later V4 engines were equipped with - not the early large hub ones