Gear ratio and performance implications

[chrispallen]

Happy New Year all.

I am looking for some real world feedback on performance changes when changing a lower.

I have an 89 Checkmate Predictor with an 86 XR-2, last year I made some pretty significant upgrades, SeaStar Hydr steering, new 6 inch jackplate, removed the can, drilled the mid, added bigger jets, put on the ever controversial velocity stacks :),oil injection is removed, plugged the top holes on the pickup, I am running the stock 2:1 lower with no cone. I am about 1/2 below the pad and spinning a 26 Trophy Plus at around 5800 rpm and getting a consistent 68 mph, and on a perfect day 70. I had the prop labbed by Merc Racing and am expecting 300-400 rpm pickup and hopefully 3-4 mph consistently.

at the end of the season, I weighed the truck and trailer on the scale at the town transfer station (dump) and then went back through after I pulled the boat out of the water, and the net weight of the fully rigged boat, motor, life jackets, about 1/4 of a tank of gas (full tank is 12 gal) was about 1400 lbs.

My question is, what impact would I expect to see if I swapped out my lower for a different gear ratio?

any suggestions or advice are greatly appreciated.

[chrispallen]

I should add that my goal is mid to high 70's consistently without killing the motor or myself....

[DanielC]

Obviously, if you change the gear ratio, you know you will have to change the propeller pitch to run the same speed, at the same rpm.
However, generally is it true that all other things being equal, a higher pitch propeller, turning slower, is more efficient.
But since the gearcase is the same size, a lower ratio (2.3 to 1, 2 to 1) means you have to make the pinion gear on the driveshaft smaller, so the gears on the propeller shaft can be larger, to make it possible to have the low gear ratio.
The smaller pinion gear tends to be a little weaker.

You are already running a 2 to 1 gear ratio. I do not think you really want to run a 2.3 to 1. And I believe going to a 1.87 to 1 gear ratio will hurt, not help your quest for more speed. I think the nose cone, on your present lower unit would be a good thing to add to your boat.

[afr]

imo the whole gear thing is mis leading bs either you turn a bunch of rpm,s or gear up and adjust the motor for it
once i get handle on it ill let you know
3 liters are 162,s IO are 152/ 1.5to1 etc you just got have the torque to swing the thing
and sure a small pinion may break
the whole thing for speed is the prop rpm not motor rpm
again just my thoughts on the subject

[chrispallen]

help me get my head around the gearing for a second...

in a truck, a 4:10 rear end is a lower gear than a 3:73 so the lower gear has more low end grunt but falls off at the top end and the higher gear has better top end but takes a little more to get there???

so in a boat would a 2:1 be more low end grunt and a 1:87 give you more top end? (all things being equal)????

[afr]

open for debate wait till i get my research done

help me get my head around the gearing for a second...

in a truck, a 4:10 rear end is a lower gear than a 3:73 so the lower gear has more low end grunt but falls off at the top end and the higher gear has better top end but takes a little more to get there???

so in a boat would a 2:1 be more low end grunt and a 1:87 give you more top end? (all things being equal)????

[petehubbell]

"so in a boat would a 2:1 be more low end grunt and a 1:87 give you more top end? (all things being equal)????"

Yes<!-- BEGIN TEMPLATE: ad_showthread_firstpost_sig --><!-- END TEMPLATE: ad_showthread_firstpost_sig -->

[chrispallen]

and (back to my original post), i am ok running the rpm's up to higher than i am now... if it means i can get more speed out of a higher gear.

and when trying different props, a 25 LazerII runs higher rpms, but doesn't give as much speed. I have tried everything from a 21 LazerII to a 25 LazerII, to a 24, 26, 28 Chopper and by far the best for this particular boat is the 26 Trophy Plus. I even tried a 23 Mirage and a 26 Fury

[DanielC]

Think of it as this way. Your lower unit gear is like the rear axle ratio of your tow truck. The propeller pitch is like the tire diameter.

You put huge tall tires on your truck, your truck will accelerate slower, but will be faster, at maximum engine engine speed, providing the engine has the power to turn the tall tire. In reality, the truck may actually have a slower top speed, because the engine cannot get to max RPM with the tall tire.

If you change to a 1.78 to 1 lower unit, from a 2.0 to 1, with the same propeller, your boat will accelerate slower, and probably have a lower top speed. but you could gain a little more distance before you run out of fuel.

[afr]

my preliminary conclusions is as follows a 2/123 might get beat by a 2/328 jury is still out

[laserguy]

I would just play with props and set-up for now. then go to the nose cone when you are in the mid 80s and running a different power plant. No help with gearing unless you want to waste time and money to go play. When you get into high horse hot dogging at 100 + mph, I would consider the 1.87 unit as you won't have to spin the engine as hard to brake triple digits. Your stuff isn't running hard enough to go there.

[lpugh]

i would just play with props and set-up for now. Then go to the nose cone when you are in the mid 80s and running a different power plant. No help with gearing unless you want to waste time and money to go play. When you get into high horse hot dogging at 100 + mph, i would consider the 1.87 unit as you won't have to spin the engine as hard to brake triple digits. Your stuff isn't running hard enough to go there.


right on

[chrispallen]

thanks for the suggestions, i feel like i have hit the limit setup and prop wise... i was exploring lowers and possibly shaved heads next, before i go to a larger displacement block.

i do appreciate the feedback

[Mark75H]

Daniel is correct in his explanation, except for what he left out.

Just as rotation speed and pitch are related, so are prop blade tip speed - via rotational speed and diameter.

It is possible to overload a prop and your motor with excessive tip speed. You can correct this by reducing diameter or rotational speed of the prop (gear ratio change).

Back in the late 1950's Mercury was having their butt handed to them by another maker in the 20 cubic inch class both in acceleration and top end. The top end problem was a tuned exhaust problem they worked out, but the acceleration was solved by using a larger diameter prop and a gear ratio change. With the gear ratio change, they did pitch change that made up for the ratio change. Without the gear ratio change, the tip speed of the larger diameter prop would have been too much for the motor.