A prop pitch question for twin 15hp Johnsons

[rustygoose]

Hello all and thanks in advance for any advice.

​I have a '56 14 foot Aluma Craft FD with twin Johnson QD-15's (10 hp). They look great, run very well. I should probably leave well enough alone....

But, I also have a pair of FD-10's (15hp) that I keep eyeing to replace the 10's. With the twin 10's mounted I can make about 22mph tops (about 9-10 with just a single motor), these motors have the original props. My ocd kicks in and I think it would be more satisfying to have the motors year of manufacture match the boat, let alone the designation. But mostly I would like to get a little speed out of it as well. On to the FD-10's, one has the original prop, the other an after-market one. Obviously I would like them to be identical. I am not sure what the original prop's size and pitch are, the other one is marked D-1 9x9 311484.
​Now finally my question- should I source another original prop to match? Can anyone tell me what the other prop is and it's particulars? Since I'm running two motors should I find two completely different props with a higher pitch?

 

[jimmbo]

Does the boat have Tachometers for both engines?
Looking in my old Michigan Catalog, your 15hp engines use the same props as the 18, 20, and 25 from the 1950s to 1977. Pitches were from 8 inches up to 12 inches
My 1981 OMC accessary catalog says 7 inch to 12 inch. Neither chart shows recommendations for twin engines, but my guesstimate would say 11 inch or 12 inch props would be close, and put you in the upper 20s, close to 30 for speed

 

[rustygoose]

Thank you for the input Jimmbo. I have been looking at the 11" props, they all seem a bit priceyx2 for guesswork! I am looking for a pair of Airguide 653 tachometers for this boat, hard to come by with the pick-ups. Right now I just tune them by ear!

 

[Sea Rider]

Buy 2 good quality induction tachs, before blindly venturing in changing props...Once installed go for a wot spin on flat calm, no wind water cond and check max wot rpm achieved on each OB.

Happy Boating

 

[steelespike]

As I recall the 10hp is rated for about 21 at wot.The old motors are very thirsty. Your 15s are rated for about 27 at wot.
You might try raising the motors.Just add a 1" block at the top of the transom.
You may be able to get some good speed, carefully setup maybe run only one tank.

 

[rustygoose]

I have it configured so I can run a single tank or two. If I plan to be out all day or make some real distance up the Sac river I use two tanks. Yes they do tend to be thirsty compared to the modern 4 cycle super quiet one my buddy has on the back of his K boat.

 

[Sea Rider]

How much do you throttle on those, medium, 3/4, full throttle on your outings ? If you can dial a prop for OB to run full wot rpm factory stated as usually loaded, will need to throttle less, consequently will consume less fuel at good plane speed. Is that set-up tiller driven or boat has a center conole with dual remote boxes ?

Happy Boating

 

[rustygoose]

No Title

I throttle back to cruise, like you said about 3/4 throttle while it is on plane. That knocks the speed back to about 18mph for cruising. I don't know if you can pick it out in this picture, there are dual Johnson simplex controls and steering forward. The whole things seems to list to starboard for some reason, I guess my first mate better put on some more weight!

 

[Sea Rider]

For dual OB installation, there's a minimum distance to consider between both OB's, does OB factory states this minimum distance ? Both OB's seems bit appart from each other. Try with a heavier first mate if boat still lists to starboard ? Definitely a bad weight distribution issue. That combo looks real nice and eyecatching, congrats.

Happy Boating

 

[Sirrobin53]

You bet. I love them motors