Logo
FULL SITE MENU

Drive ratio, empeller speed, prop choice and cooling water suppliy

P

publicnuisance

New member
I was reading another thread about drive ratio choices (http://www.marineengine.com/boat-forum/showthread.php?385197-mercruiser-drive-ratio-choice) and it was mentioned that the empeller speed is a concern. I have a 1988 searay 30 with twin 5.7/260/alpha 1's and hope to change the sterndrives soon. I was wondering about changing ratios to tweek the fuel economy but had not considered the empeller speed. I think 1.47:1 is the standard for this engine setup. Is fuel economy more related to prop size and pitch rather than drive ratio? Also had considered installing crank driven sea water pumps and abandoning the supply from the alphas, maybe even removing the empellers. It never made sense to me to not be able to change the empeller with out dry docking boat, I don't have a trailer so am at the mercy of my marina @#$%^.

Your learned two cents would be helpful.
Nuse
 
This is from my rather limited knowledge on the subject:

The marine propulsion engineers will always gear a drive for the engine type/size, with an influence from the hull design.
In the field, there is usually NO substitute for a correct over-all drive ratio.... nada, zip, zero!
I've corrected these for owners who have inadvertently had the wrong ratio drive installed..... and it has always been an improvement.
I'd stay with the over-all ratio that the factory recommed for this hull!

Some of the exceptions are high altitude use....., light weight hulls....., and twin engines that are more than capable of moving a lighter weight hull.

.
 
publicnuisance said:
Does1.47:1 seem about right for a 10,000 boat?

Any opinion on abandoning the alpha fresh water source?

Nuse
Click to expand...
Nuse, I mainly do the AQ series Volvo Penta. So I can't recommend on a Mercruiser A drive ratio for you.
Your OEM manual may have this information.

As for abandoning the A pump sea water as a cooling source! The A pump sea water supply cools the upper gear box, so we can't completely abandon this pump.
We can divert the sea water elsewhere after it has cooled the upper gear unit......, and then use an engine mounted sea water pump with a thru-hull water pick up for the engine cooling.
Been there/done that!

.
 
publicnuisance said:
I was reading another thread about drive ratio choices (http://www.marineengine.com/boat-forum/showthread.php?385197-mercruiser-drive-ratio-choice) and it was mentioned that the empeller speed is a concern. I have a 1988 searay 30 with twin 5.7/260/alpha 1's and hope to change the sterndrives soon. I was wondering about changing ratios to tweek the fuel economy but had not considered the empeller speed. I think 1.47:1 is the standard for this engine setup. Is fuel economy more related to prop size and pitch rather than drive ratio? Also had considered installing crank driven sea water pumps and abandoning the supply from the alphas, maybe even removing the empellers. It never made sense to me to not be able to change the empeller with out dry docking boat, I don't have a trailer so am at the mercy of my marina @#$%^.

Your learned two cents would be helpful.
Nuse
Click to expand...

if you have the mcm 250 model 5.7L engine this would be 1.32 ratio. the mcm 260 5.7L is the 1.47 ratio
 
Last edited:
publicnuisance said:
Is fuel economy more related to prop size and pitch rather than drive ratio?
Click to expand...
fuel economy is up to the captain working the throttle....

prop pitch might have a slight effect...doubt it would be noticeable
17P & 19P are usually the most efficient pitches for single engine planing hulls, so if you have the right ratio drive, you probably have a 17 or 19P prop.
15 or 21 are close.

don't regear yourself out of those props
 
Alpha one gen one would be a 1.50:1 ration. The new ratio to replace the 1.50:1 ratio is 1.47:1.
Drive ratio is not really the concern, wide open throttle rpm's are. If you are not able to spin your engines up between min 4600 and max 4800 rpm (boat loaded and trimmed out) you need to change your prop pitch. Below 4600 (Too much Pitch) you are over working your engine and destroying it slowly. Over 4800 (Too little pitch you stand the chance of damaging your engine due to over speed.
 
if you have the mcm 250 model 5.7L engine this would be 1.32 ratio. the mcm 260 5.7L is the 1.47 ratio
Click to expand...

Ayuh,... The 1.32 gears haven't been available since they quit putting BBCs in front of Alpha drives...
A 350 SBC turns 1.50:1 gears...
 
Docksidemarineservices said:
  1. Drive ratio is not really the concern, wide open throttle rpm's are. If you are not able to spin your engines up between min 4600 and max 4800 rpm (boat loaded and trimmed out) you need to change your prop pitch. Below 4600 (Too much Pitch) you are over working your engine and destroying it slowly.
  2. Over 4800 (Too little pitch you stand the chance of damaging your engine due to over speed.
Click to expand...
  1. I respectfully disagree! Drive ratio is very important, and is part of the equation to obtaining a correct WOT RPM! Like the one member suggested, "don't regear yourself out of those props!"
  2. Yes, if you are a "knuckle-head" and run your engines at WOT RPM! Just note that this is NOT an RPM to be sustained. WOT RPM is to be used as a test/check to see if we are geared correctly, and propped correctly.

Always error on the side of a high WOT RPM, rather than a too low WOT RPM.
 
Rick,

As my post says, there should be a 1.50:1 drive on a 5.7 ltr. After the correct drive ratio is installed then you need to adjust your WOT with the correct prop so you are not overloading your engine at cruise RPM.

I guess I was not clear enough.
 
The question publicnisance had was about changing these drive gear ratio's , since he plans on getting new drives. I agree that he should not make and changes to this gear ratio . thats what most agree with . If he stays with the 1.50 , why mess around with prop pitch ? nothings gonna change.
 
Back
Top