1:78 ratio vs. 1.47:1 ratio ... which to choose?

[ezfriday]

What is the main difference in performance with the lower gear ratio?

The price of the drive is the same so the choice must have something to do with the type of performance you expect to get out of it?

Please explain to me how these gear ratios impact my decision on which to choose?

[Willallison]

This is not really something that you simply choose. It has to be carefully calculated along with the correct prop.
You could get a copy of Dave Gerr's Propellor Handbook which would allow you to calculate the best option.
But what sort of boat is this? A lightweight runabout or a big heavy cruiser?
What sort of power is installed?

[ezfriday]

Originally Posted by Willallison

This is not really something that you simply choose. It has to be carefully calculated along with the correct prop.
You could get a copy of Dave Gerr's Propellor Handbook which would allow you to calculate the best option.
But what sort of boat is this? A lightweight runabout or a big heavy cruiser?
What sort of power is installed?

It's a 2002 Chris Craft 262 open bow. Pretty heavy for this type of boat ... around 8000lbs. It originally had a 454 8.2GFi but that engine suffered a cracked block and was replaced with a 502mag. The performance guy I had at the time was very knowledgeable and recommended the 502mag because everything from the 454 would bolt right up to it. However, he also advised me to eventually go to the DPX outdrive when I could afford to. I have been running it with the original DP outdrive and it does ok so far ... though several mechs have been surprised to see a DP drive attached to a 502mag.

I currently have an option to buy a DPX with the 147:1 ratio at a good price, so I am trying to educate myself on understanding these gear ratios so I don't make a bad decision.

[Willallison]

If it's a duoprop leg then don't bother with Dave's book - there isn't any info in there on calculating ratios or props for counter-rotating drives.
Talk to a Volvo person and get their recommendation
Gonzo may be able to help...

[ezfriday]

It is a douprop ...

[Gilbert]

Just off the top of my head I would say the 1.47:1 ratio should work out well for you. An 8000 pound boat is not all that much for that size motor so I think that the 1.47 gears will give you the opportunity to have a nice cruising speed and still have the engine runnig at a reasonable rpm.
However it would be great if someone who has an 8000 pound boat with this setup chimed in here and said "yes, it works great" or "No, you will be happier with the other ratio."

[ezfriday]

Thx Gilbert ~

I've gotten mixed views on the matter of gear ratio. I've had a bit of a learning curve so far. My understanding so far is (correct me if I'm wrong) that a higher gear ratio would be better suited for a boat that has a powerful enough engine for the weight of the boat and a lower gear ratio is more suitable for a boat that needs the extra torque to get up to plane.

As I mentions in earlier threads my boat is as follows:

502GFI Mag
DP Outdrive (not sure of the exact ratio ~ but I know its higher than the 1:147)

Right now the boat gets to plane pretty quick and handles well. The only area I feel the torque drop is when coming out of a tight turn, at that time it sounds more like cavitation than power/torque, and it's pretty brief.

Of course what I don't want to happen is to put this drive on my boat only to see my top end drop from the 68mph I get right now.

Thx

Ez

[bananas]

The greater reduction (higher number, in this case 1.78:1) slows the propeller RPM and thus reduces the blade inflow speed. The lower inflow speed results in improved propeller efficiency, but also reduced load capacity. It will take a larger propeller diameter to achieve the same slip as the lower reduction (1.47:1).

Because of the non-linearities in slip versus efficiency this can be a difficult trade. The potential speed benefit of greater reduction can be easily be lost due to increased slip.

My clients who have been in the most trouble have been the ones that have been sold too large of a reduction and find that a sufficiently large propeller cannot be installed. They are faced with high slip, noise, vibration and poor efficiency.

www.softcav.com

[Gilbert]

Hi ez,
Your last post is helpful. You are happy with your performance now and so I suggest you do everything you can to find out what your present ratio is. Do it even if you have to resort to taking the spark plugs out with the boat out of the water and turning the engine by hand and keeping track of the revolutions while a buddy counts the propellor revolutions until he counts ten. Whatever revolutions you have on the engine will give you a very close approximation of your present gear ratio. And you should not change it very much, if at all, if you can avoid it. If the engine turns 15 x to 10 prop turns for instance you are very close to the 1.47:1 ratio you are considering.
The tight turn, near cavitation or actual cavitation may be more of a characteristic of your boat's bottom than it is a problem of your gear ratio-propellor combination. A really tight turn just may 'dig' a hole that makes your running trim angle too high to power out of easily. If you are actually having cavitation your engine rpm will increase dramatically, at least momentarily.
If you have a good thing going don't change the equation too much.

[longliner45]

We had 31 ft jc boat with twin volvo duoprops.we could get 20 knots in mild seas, with 6000lb payload,,,,,,we were told by volvo that no matter what powered the outdrives we would top out at about 40 knots,something obout duoprops haveing their own ranges.you have a smaller lighter boat> can you find out at what might be tops on your set up.and wht rpms would best suit you engine?I know you are basically trying to find this out now .but wernt aware if you realized the top out factor.

[gonzo]

What is your WOT RPM's right now? If they are within the normal range the ratio is correct.

[longliner45]

gonzo; please tell me what is WOT ,and what can you tell me about duo props topping out ,no matter the horse power? thanks,,,longliner

[Jango]

If the Prop Pitch is correct for the Drive ratio, "Wide Open Throttle" rpm (Engine) will be correct for whatever the ratio is. Higher ratio (slower prop rpm) will require more pitch. Lower ratio (higher prop rpm) - less pitch.

[gonzo]

The important thing is to get the right combination of boat, engine , propeller and gear ratio. There are more than one that will work. I think your engine is rated at 4200-4500 RPMs. If it is running in that range, the setup is correct. I'm not sure what you mean by duoprops topping out.

[longliner45]

gonzo dont get me confused with ez here . my question is pertanant but maybe less important, our boats ran 2 volvos at 3300 rpm .volvo told us that even if you increased the rpm to 5000 you would still go as fast as you would at 3300 ,something about the physices of the set up ;counter rotating props; sorry dont want to get anyone corn fused here, just trying to expose other optionss ,thanks longliner