1974 25' Tournament Fisherman with twin 140 hp inboards ( need prop info )

Dave
Dave

I am trying to find out what the original factory specs were on the props as installed at the factory. I currently have 12 X 10 3B LH props with a slight cup.

My issue is that the boat peaks at 3600rpm and all engine issues have been eliminated. The boat runs great,just not at the engine spec of 4200-4600rpm.

Dave

Gary Roberts
Gary Roberts
I have the exact same issue. The boat came with 12 x 10 from the factory. I am not sure if they were cupped or not though.

I ran a 11 x 8 with out a cup. Totaly overreved and had alot of slip. I tried some 12 x 10 with a cup 3600 rpm and 25 mph.

put the 11 x 8 on with cup 4600 rpm but the same speed 25.

Let me know what you had figured out.

Paul
Paul
Right now I don't know if you are running a direct drive setup or if there is some slight reduction like a 1.15:1 or something like that. It sort of appears there is simply because the drag factor seems high, but this may well be because of two props and drives being in the water as opposed to one.

I realize this is a twin engine application, so my comments are more of a math analysis than a real world knowledge, but I toss this out for review and discussion.

You have given some good data of prop, rpm and speed. Taking those numbers I do my own math on the 12x10 cupped prop you say is 3600 rpm and 25 mph.

The 10 cupped will behave like a 10.5 so that is what I factored in.
10.5 x 3600rpm = 37800 inches forward movement in a minute, x 60 = 2268000 divided by 63360 = 35.79 miles per hour on paper BEFORE DRAG is factored in, and I normally use 15% which in this case is 5.37 miles per hour reduction, or a total on paper speed of 30.42 miles per hour. If I use a 20% drag factpr I get to 28.64 mph and now you can see why I suspect perhaps there may be a slight gear reduction in play.

IN ANY CASE............drumroll please........if a 12x10 was the stock prop and a 12x10 cupped is limiting your rpm down to 3600, I would jump back onto a 12 x 10 quickly. That will run your rpm back up and take a lug off the motor, and you will still be around the same top speed, perhaps a bit higher.

I am not all that familiar with the outdrives and especially twin outdrives so I stick my neck out on this one. Hope this helps in some way. Your comment about the 12 x 10 being the stock prop already has helped loads.

Regards,

Paul

gary roberts
gary roberts
Paul,

This is a twin inboard application. It has paragon gears and they are 1 to 1 they are both right hand rotation.

Paul
Paul
Very interesting Gary. These are GREAT BOATS, by the way, Very cool. Jerry Namken has one like it now and he is intent on installing a pair of small block Chevy motors and all new drives.

Back to the stock boat, however, with two drives in the water (and I couch my comments based on the speed you noted) there must be a larger slippage and drag factor. All bets are off if your speed was taken from a pitot tube speedometer, if it was taken by GPS then I would trust it more but not totally.

Based on the comments it looks like the stock 12 x 10 would be prop you want. Although a cup makes things seem okay too, it puts a little more lug on the motors and spinning at rpm generally doesn't really hurt a motor at all as long as you don't exceed the limits. If you go to the 12 x 10 you would probably be running the exact same speed but just a little easier on the motors, letting them spin a little more freely.

Send in some more photos of these boats guys, they're of high interest to the group, we do not see them very often.

Regards,

Paul
Last edited by FEfinaticP on October 7th, 2011, 2:09 pm, edited 1 time in total.