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Gear sets and ratios.

8

87exciter

Member
Hey everyone. I just picked up a spare drive for my 87 Exciter. Upon researching what it would it take to swap gears from 1.84 to 1.5 I noticed there are 2 others that may give me a bit more top end. 1.47 and 1.32. I just read on another forum where a guy wanted to put the 1.32's in his big Sea Ray to help keep the Alpha alive behind the big block. He went on to say because of the way the 1.32 set was made they were naturally beafier. He also went on to say that in turn they would spin the prop slower and put less strain own the drive. I get that principal but it seems to be backwards from what I though and what the prop slip calculator says when plugging in different ratios when all else is the same. Is he just mistaken or am I? If he's all wet would the stronger 1.32's if in fact they were stronger be a better choice? Or should I stick with the 1.5? I should mention the reason for the spare drive is because there's a 383 going in soon. It's a light boat so I think I'll be ok long as I don't go nuts out of the hole. Thank you for the help! :)
 
Stern Drive Engineering, claims that the larger teeth on their gears help the drives take extra punishment.
 
In my experience, gear ratios don't do what people think in regards to performance.
I built a hot rod O/B for a customer and we ran a Merc Sportmaster on it, initially with 1.62 gears before I changed it to 1.87's.
I ran a truck load of props on both sets and it worked out exactly the same as changing 3" in pitch in the prop calculator.
We were both shocked.

The motor had enough balls to pull both so that helped where if it were peaky enough to not get to the sweet spot in the powerband, clearly it would have been different.

As to your question on drive strength, it's torque that kills them. Not rpm.
 
Instigator said:
I ran a truck load of props on both sets and it worked out exactly the same as changing 3" in pitch in the prop calculator.
We were both shocked.

.
Click to expand...

It's the same as changing tire size and rear end ratios in a car. It's the pairing that matters, not either one in particular. For instance if you jump up in tire size, you put in higher numerically gear ratios in the rear end, and your RPM ends up the same at a given speed. Eventually, however, you need a ratio so high you can't make the pinion strong enough.

Similarly with a boat, if you make the prop shaft spin slower by putting in different gears, you make each spin of the prop move more water by putting in a higher pitch prop.
 
jumbofordman said:
It's the same as changing tire size and rear end ratios in a car. It's the pairing that matters, not either one in particular. For instance if you jump up in tire size, you put in higher numerically gear ratios in the rear end, and your RPM ends up the same at a given speed. Eventually, however, you need a ratio so high you can't make the pinion strong enough.

Similarly with a boat, if you make the prop shaft spin slower by putting in different gears, you make each spin of the prop move more water by putting in a higher pitch prop.
Click to expand...

I get all that and agree. I matched tire size on my 440 six pack Super Bee to gears and target RPM.

On boats though, particulary O/Bs I have read it made a difference in torque transferred to water.
I'm a huge believer in a prop calculator and it said otherwise and it was right.

Another theory and one I believe is, when pitch gets too high, the paddle wheel effect (pulling the boat sideways due to the near flat blades) starts increasing. As in a 26" wheel is more efficient than a 31" at the same boat speed.

I'm a huge fan of alternate ratios in tuning and as an OMC guy w/no alternate ratios avail, I built two OMCs w/Merc gear cases so we could switch gear sets to tune.
Saw nothing from either.
Where I did see it was when chasing top end #s.
An OMC @ 7500 x 30" pitch is a 100 and your done.
A Merc Sporty w/1.62's is like adding 3" pitch to the same motor.

Ultimately, the motor still has to have the balls to turn it though.
 
The 1.32 gears will spin the prop faster than the 1.50 gears at the same engine rpm. The gears we are talking about are at the top portion of the Alpha and this is the weak point of the alpha. Usually when they break because of too much torque, it's these gears that break. I don't know about the 1.32's being made heavier but it's definitely possible. I remember seeing the 1.32's behind big blocks from the factory in the mid 80's boats before the bravo came out.
 
Gear sets and ratios

Thanks for all the replies. I'm gonna experiment with that spare drive I bought. I'll run the 1.32's and record what happens. On a different note I'm ordering my rotating assembly today for a 383 and have heard a vast range of what's a safe compression ratio. I can order the assembly now cause I can fine tune the compression with heads and head gaskets I run. I'm going to make a new post on this with all the specs. Boat weight water temp etc. Thanks again for the help!
 
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