Prop performace with gear ratio change

[guck454]

Hi guys. I have an 84 Tower of Power 115hp on a 17.5 VFR, redesigned to be more like a Tyee. I always thought I was getting horrible prop slip, 6000rpm at 42.5 mph with (perfect shape) 13.25 x 19P Laser II. However recently I discovered I have a 70's lower unit on it that came with it, and not only that but confirmed I'm running 2.33:1 gear ratio instead of the 2:1 it should be. This lower unit leaks some, so instead of rebuilding it I'm going to get a good correct 80's 2:1 ratio lower unit that should be on my motor. Now, my question is on paper at 42.5mph with 2:1 ratio and maintaining my (realistic) 8.27% slip I'll max out at 5150 rpm, or will I be able to carry more rpm and speed because I "should" have more torque at those rpms than very high? Anyone ever play with gears in these towers?

I guess I'll find out soon but was hoping to get some insight in case I need a smaller prop. I'm hoping that's not the case because I love that laser and they don't make less than than 19p!

 

[jimmbo]

Almost all inline 6s from 1972 on used a 2:1 ratio. The 1972 1150 used a 25:14 ratio. The larger 4 cylinder engines 650, 800, 850, 80hp, and 75hp used a 2.3:1 gearcase. The 2.3:1 ratio was also a dealer installed 'Altitude option' for the 1984 and later 115hp. If you have a 2.3:1 ratio instead of a 2:1, you will need a higher pitch prop to make up for the slower propshaft rotation, in relation to the engine rpm.

If you change the case to a 2:1 ratio, you may indeed end up at 5100, which is near the bottom of the 5000 -5500 WOT range, then you will need to decrease pitch. However, 1984 was the first year Mercury updated the transom mount which allowed engine height adjustment. You might find some gain there, and with careful trimming...
However it would be easier just to keep your 2.3:1 and try a 22 in pitch.
One thing get rid of any unneeded weight. Kids, wife, and especially Mothers in law, are to be left ashore. Mistresses and Girlfriends can remain aboard.

 

[guck454]

Thanks for the response. That's very interesting about the high altitude option. But this is definitely 70's, casting 1623-5886. Must be from an inline 4 like you mentoned. But I think you're right, might just be easier to keep what I got, and even get like a 21P Laser. These don't mind upper 5xxx rpms, but at 6k she really starts to scream! I already have a 3" manual jackplate (obsessed much?) and got her dialed in right where she likes it.

It just eats me alive knowing I'm running something "wrong". But maybe I can talk myself into it! BTW with this 2.3:1 and solid plugs in the laser she'll throw you out of the boat on a holeshot if you're not holding on. Always thought that was odd for an inline, especially with solid plugs.

And don't ever put my mother in law near my boat again!!!

 

[jimmbo]

Here was my 84 115

 

[guck454]

That's a sweet Rooster!

 

[jimmbo]

It's called overtrim. It has a tendency to starve the engine of cooling water