Hydro vs Runabout gear ratio

[Syd Marsden]

I have a MK25 set up with a quickie lower unit and tower, I have used it on a older runabout class B with a 11" pitch prop. The unit has the 21/16 gears in it. This prop is the largest pitch B prop that I have. The boat will do 44.6 so far. I am also restoring a Furnal Flyer class D hydro. The boat is 11' long. I was thinking of trying this motor on it. I figure that this boat would need a larger prop than the runabout. But being that I only have some smaller ones and also have a lower unit with the 1 to 1 gears, I was thinking of swapping the lower units and seeing how that will work. Any input from the pro's? Thanks, Syd

[jeff55v]

My two cents.

I'm far from an expert.
But I recall, as a kid racing the B class stuff, my best B prop was a D prop. The D motor is 1:1, B motor 16:21. I used a bushing on the prop to fit the smaller propshaft of the B lower unit.
My point here is, I think you'll need a D motor to turn that prop at 1:1 ratio. Regardless of the style boat. The different gear ratios make a big difference.

[T Chance]

I think an 11' hydro is too big for a MK 25. I used to run an alcohol burning 20-H on a hydro that was 9'9". Also, it the prop works on a runabout with 16:21's it should work on a hydro, you may have to experiment with set-up a little bit. I wouldn't worry about 1:1 gears.

Props are funny, you never know until you try them. I ran the same prop both on Runabouts and Hydros on the above mentioned 20-H; and on a B Quincy Looper with 12:15's, a 4-carb D Konig and a C Konig with 14:15's, a Mod Hot Rod with 14:19's and a 4-cyl B Konig with 12:15's. After 15 years I finally wore it out and threw a blade.

Like Jeff, I had bushings for different prop shaft diameters, and a slot milled for the Hot Rod shearpin. It was a Seebold 112SST.

[sam]

16:21 gears also reduce prop tip speed allowing you to use a larger diameter prop with a motor that has less torque ... pitch is not the only difference between B and D wheels for 1:1. 1:1 B wheels are smaller diameter.

For playing with a heavier than racing weight rig, the 16:21's should give a good advantage.

44.6 is respectable speed for the Mk25 on any boat ... I would not expect any more on the D hydro

[johnsonm50]

Thats about right, I know of a 25/quicky, hydro that gets 49ish but mine on a C hydro is good for 42-45. Your D hydro should go about as fast as the runabout as is. Dont forget.. props, setup, props, testing w/ notes and props. I think 50 is possible. Are you going to Mays? Mike

[Syd Marsden]

If I do not have to work I hope to be at Mays Landing. I am not sure if I will have one of the boats there, but will have pictures. Thanks for everyones feed back. Take care, Syd

[RichardKCMo]

That's Fun!!!

16:21 gears also reduce prop tip speed allowing you to use a larger diameter prop with a motor that has less torque ... pitch is not the only difference between B and D wheels for 1:1. 1:1 B wheels are smaller diameter.

For playing with a heavier than racing weight rig, the 16:21's should give a good advantage.

44.6 is respectable speed for the Mk25 on any boat ... I would not expect any more on the D hydro

KG7 ratio is 16/21 Lu isn't quite racing shape , but if river racing is your forte it's as good as it gets, jack it up till it says NO.
Richard