gearing....

[serious3]

ere's what i'm thinking......(its not much but hey!) the 6e9 twin 40hp that i'm running on my SLT cat has a gearbox that is 2:1 (2 turns flywheel to 1 turn prop) now i know that some yammy 40's had a 1.85:1 box and that 55 twin box will fit mine, now as that motor and box was designed for all types of boats and wieghts etc surely the gearbox is a compromise for it to work acceptably on most types of craft.
my SLT and me weigh all up at about 400lbs, so i was thinking of changing the box for one of the 1.85 ones, my thing being that it will spin the prop faster resulting in more speed!

[zul8tr]

generally the lower gear ratio requires a lower pitch all else being the same. If you put the 1.85:1 in there and use the 2:1 prop you might not be able to spin it at the same rpm as with the 2:1. So have you tried to max out your 2:1 with props, engine height, trim angle and set back? If not you should do it with a tach and gps to get a base line. The 1:85 is only about 8% lower than 2:1 so with the 1.85 if you can swing the best 2:1 prop pitch at the same rpm and if slip is the same the speed will theoretically increase about 8% or 4 mph from 50 mph to 54 mph, or about 3 mph from 40 to 43 mph (just examples not knowing what rpm and speed you are presently running) .

Cost might be an issue with what you want to achieve, borrowed props are cheaper at first try.

[serious3]

i'm running a 10.1/8x15p ballistic prop at 6600rpm she's running 46mph, considering the powertech 10.25x16p semi cleaver. she's just ventilating at WOT at current trim, its forecast flat calm tommorrow so ill trim it out a bit and see what happens

[zul8tr]

With the speed, pitch and rpm provided the theoretical no slip speed is:

6600rpm x 15"pitch /(1056 x 2 gear ratio) = 46.9 mph

The 46 mph speed you note is very good but darn close to the no slip speed and your calculated slip is only about 2%. Slip at 2% is really to low, I would have expected somewhere around 7 - 10% slip for a light rig. Are you sure of the numbers provided?

The 16 pitch that you want to try will pull about 200 rpm less but maybe a higher speed will result and no ventilating.

Post results.

[serious3]

6680rpm this afternoon(tiny tach) and running at 46mph dead(gps) boat weighs 165lbs, motor weighs about 125lbs, i weigh in at about 200lbs. prop is a 10.1/8x15p stainless, ballistic prop

[zul8tr]

Well your rig is 490# and I think at top form with that speed.

[serious3]

so i;m gonna need a bigger motor or more power.......

[zul8tr]

Maybe you can tweek a bit more out of it with props, height, trim and setback but if you really want much more speed you need Hp to turn more pitch at a high rpm.

So play with your set up some more.