Should I swap gear ratios?

[Uncle Binky]

I have been running a 1.62 ratio in my Alpha 1 Gen 2 drive behind my Trophy 2352 with a stock 260 HP 5.7 Mercruiser.
I'm building a new engine for it that will be putting out about 300 HP. I've never been able to get the performance or fuel economy that other owners of the same boat say they get. I use a 15 X 19 Mercruiser 3 blade prop, and at WOT I'm getting about 4600 to 4800 RPM. I do a lot of running offshore with a heavily loaded boat. I know that most 5.7s have a 1.5 or 1.47 ratio drive behind them. What is the functional benefit to swapping gear ratios, and is the expense of swapping gears or drives to accomplish this worth the expense, or should I just mess with finding a prop that works best with the existing drive ratio and new engine? What am I missing out on by using the 1.62 ratio?

 

[harleyman1975]

Re: Should I swap gear ratios?

My thoughts are like a truck that is heavily loaded you will need a lower gear to make it go efficently. ( think of this as weight vs h.p.) so with a more powerful motor and a 5% increase in gear ratio you will be basically breaking even. If you simply leave the 1.62 gears that you have now one might be able to back off the throttle a wee bit and thus be running more efficently...so build the hp and leave the gears in my opinion. adding blades to a prop will make it come out of the hole better but will slow it down some. stainless will make a prop come out of the hole better than a comp alum because it does not flex. try a 15x20 ss 4 blade and see if you like that any better. you are pretty close to the sweet spot already. I wouldn't want to be turning a SBC 5000 rpms all day long. I think that your higher hp motor will be happy with the taller prop though. (I am just speaking from personal experiance though).

 

[HT32BSX115]

Re: Should I swap gear ratios?

should I just mess with finding a prop that works best with the existing drive ratio and new engine? What am I missing out on by using the 1.62 ratio?

If you're currently using a 19p prop, you might just see how it does with that prop loaded where you run the boat on a regular basis.

If you're up against the rev limiter or you still have throttle left at MAX RPM You might consider a slightly 'taller' SS prop. (maybe a 21 or a 23)

You will want the engine to turn at MAX RPM at WOT when you're fully loaded. (if you were at 4600 before and it does 4900 now, I would leave it)

 

[Uncle Binky]

Re: Should I swap gear ratios?

No rev limiter. I am planning on 5000 RPM max. Are there pros/cons to 1.5 vs 1.62? As most of my boating is long pushes out to the canyons at 3600 RPM +/-, is there a prop speed that is more efficient? I have always been told that a slower prop is a better prop, and if the drive can handle the torque, why wouldn't I (or for that matter everyone) use the 1.62 ratio?

 

[achris]

Re: Should I swap gear ratios?

The ratio supplied by the manufacturer is based on the power of the engine (obviously) and the size of the boat most commonly having that particular engine installed. The idea is to try and keep the propeller pitch in the 15" to 23" (ish) range. High reductions on big engines in small boats will require a propeller too high and conversely, a small engine in a large boat will require too small a prop. So we end up with a bunch of different ratios to accommodate these variations.

In your case, your boat is on the largish side for a single V8, so using a drive geared down slightly will be of benefit. And you are correct, a lower spinning prop is slightly more efficient.

Chris.......

 

[HT32BSX115]

Re: Should I swap gear ratios?

No rev limiter. I am planning on 5000 RPM max. Are there pros/cons to 1.5 vs 1.62? As most of my boating is long pushes out to the canyons at 3600 RPM +/-, is there a prop speed that is more efficient? I have always been told that a slower prop is a better prop, and if the drive can handle the torque, why wouldn't I (or for that matter everyone) use the 1.62 ratio?

There's only about 300 (prop) RPM difference between the 1.5 and a 1.62 at 5000 RPM.

You can probably easily make up the difference by going to a 21p prop (if required). If you were going to put that engine in a "GO-Fast" (er) boat, the 1.5 would probably be a better choice. Your Trophy probably doesn’t fit that description.....

Since you're going to turn the engine approx 300 RPM more, you might just be RIGHT where you need to be with your current prop and the 1.62 drive.

 

[Bondo]

Re: Should I swap gear ratios?

I have been running a 1.62 ratio in my Alpha 1 Gen 2 drive behind my Trophy 2352 with a stock 260 HP 5.7 Mercruiser.
I'm building a new engine for it that will be putting out about 300 HP. I've never been able to get the performance or fuel economy that other owners of the same boat say they get. I use a 15 X 19 Mercruiser 3 blade prop, and at WOT I'm getting about 4600 to 4800 RPM. I do a lot of running offshore with a heavily loaded boat. I know that most 5.7s have a 1.5 or 1.47 ratio drive behind them. What is the functional benefit to swapping gear ratios, and is the expense of swapping gears or drives to accomplish this worth the expense, or should I just mess with finding a prop that works best with the existing drive ratio and new engine? What am I missing out on by using the 1.62 ratio?

Ayuh,.... Put it together with what you've got,....

Then Test run it,....

No sense in throwin' away $1500. or more, just on a theory,...

 

[wca_tim]

Re: Should I swap gear ratios?

I agree that it's best to run it and see how it goes...

...but the 1.62 gear are the weakest set of them all (smallest teeth, hardest to get shimmed perfectly, etc...). other than that, I'm betting that your performance will be more dependent on the right prop than which of the two gear ratios you're using. The only other difference is that the 1.62 ratio will spin the drive impeller a little slower slightly reducing the water pressure - but I've never seen that be a problem even with higher ratio drives... Just some things to think about...