Question about drive ratio

[tonyjh63]

Ok, for some background: I have a '99 Bryant 182 Limited with 4.3 Mercruiser and Alpha 1 Gen 2 out drive, along with a 19 in 4-blade prop. I took it out this summer a few times and had a great time with it, but it just didn't seem to have the "ooomph" I thought it should, either in hole-shot or top end. One of the boat's previous owners had apparently replaced the out drive, as the numbers didn't match up with my research on the boat - it had what appeared to be a 2.0 ratio drive on it, but we can't really be sure what the internals actually are. Well, I've got a guy winterizing it right now, and while in the shop, I asked him to determine the actual drive ratio. He took the top off the drive and counted the teeth - he said that both gears had 24 teeth! That would be a 1:1 ratio, right? I've never heard of any such thing, so my question is: is this really possible? Could the guy have counted the wrong sets of gear teeth? (Would my boat even been able to get up on plane with such a high drive gear and 19 pitch prop?)
Assuming the guy is correct, what can I do about this (other than buy another drive)? Can the gearing inside be changed - relatively inexpensively? Or should I just get a 13 or 14 in prop? Thanks in advance for any advice...
Tony

 

[alldodge]

Are you turning 4400 to 4800 rpm at WOT?

Having gears 24 and 24 is not an issue, your final ratio is determined in the lower half. Engage the drive and rotate the drive shaft and count the amount of turns the lower does.

 

[thumpar]

The 24 and 24 tooth upper is used on the 1.62 and 2.0 drives. Do as AD stated to find out your final drive ratio.

 

[stonyloam]

Here is some info: http://www.mercstuff.com/drivegearratio.htm. You need to gather some more info to determine the correct prop. You will probably want to go somewhere around a 15-17 pitch. Before you make your choice you need to make a couple of WOT test runs to determine engine RPM and speed (GPS) at WOT. Then you can make some calculations to determine your best setup. You will also want to decide if you want a strong hole shot or top speed or something in between. Also need the pitch and diameter of your prop, and decide if you want a 4 or 3 blade. You can find a lot of help in the "props" section.

 

[achris]

Final drive ratio is determined by the ratio in both the top and bottom boxes.

There are essentially 2 different lower box ratios, 1.62:1 and a 2:1 (there is a 3rd, 1.65:1, but that was superseded by the 1.62, and for all intents and purposes and our discussions, it the same as the 1.62)....

The top box has 4 different gearsets, 17:19, 20:24, 22:20 and 24:24....

Used with a 1.62 lower, the overall ratio could be, 1.81:1, 1.94:1, 1.47:1 or 1.62:1

The 2:1 lower was only used with the 20:24 and 24:24 upper gearsets, giving 2:1 and 2.4:1... That doesn't preclude someone changing a lower and/or upper after they left the factory, so other ratios are possible.

Determining drive ratio

Take the spark plugs out. Put the engine in forward gear. Have someone count the turns of the propeller as you turn and count the turns of the engine. When the prop has turned 10 times, divided the number of turns of the engine by 10.... That is the ratio.

For a V6 I would expect a drive with 1.62:1 or 1.81:1 drive... So the engine will have to be turned 16.2 turns or 18.1 turns to make 10 turns of the prop....

On to your actual problem. I assume your boat is 18'2". With the same engine as you have, my heavy 20' boat turned a 21" prop (with a 1.84:1 drive) and was still running over the top of the max RPM.... The current engine, with a 1.62:1 drive, will turn a 19" at 4600rpm, but it does feel 'heavy'... I run a 17" and it runs very very nicely. My previous engine, a carbed 4.3, is now in a 23" hardtop cuddy and turns a 19" at 4700rpm. Apparently at WOT it scares the current owner.

If you are running a 2:1, then you should require a prop in the order of 25"... Even with the lowest reduction, 1.62:1, I would expect that engine in that boat to spin a 19" prop and still be too small. Check the engie to make sure it's running properly (compression, timing, etc)...

Chris.........

 

[tonyjh63]

Determining drive ratio

Take the spark plugs out. Put the engine in forward gear. Have someone count the turns of the propeller as you turn and count the turns of the engine. When the prop has turned 10 times, divided the number of turns of the engine by 10.... That is the ratio.

Chris.........

Hi all, thanks for the replies! Chris, could you please explain in a bit more detail the quote above in bold? I take the spark plugs out, engage forward gear...now, HOW do I turn the engine? Is there a way to turn it via the flywheel or something? Or if I have to do this by myself, is there an easy way to partially disassemble the lower section and count the gear teeth, since I already know that the upper section has a 1:1 ratio? Thanks again to all who have replied!
BTW, the reason that I'm even asking about the condition of the engine and/or drive ratio is that my boat never got close to reaching 4400-4800 rpm at WOT (I want to say maybe 3600 rpm max, and a top speed of around 32-34 mph?), so I'm trying to figure out what the problem is...:-(

 

[thumpar]

19 pitch prop, 3600 RPM, 34 MPH with a prop slip of 15% calculates out to 1.619 or pretty dang close to 1.62 so if those numbers are close I would say the drive is the right ratio and you need to figure out why the RPMs are so low.

 

[achris]

Hi all, thanks for the replies! Chris, could you please explain in a bit more detail the quote above in bold? I take the spark plugs out, engage forward gear...now, HOW do I turn the engine? Is there a way to turn it via the flywheel or something? Or if I have to do this by myself, is there an easy way to partially disassemble the lower section and count the gear teeth, since I already know that the upper section has a 1:1 ratio? Thanks again to all who have replied!
BTW, the reason that I'm even asking about the condition of the engine and/or drive ratio is that my boat never got close to reaching 4400-4800 rpm at WOT (I want to say maybe 3600 rpm max, and a top speed of around 32-34 mph?), so I'm trying to figure out what the problem is...:-(

I also wrote that you have someone else count the propeller revolutions as you turn the engine.... I'm yet to work out a way of doing it solo. If you do have someone to count prop turns, you can turn the engine with a wrench on the harmonic balancer bolt.

If you remove the drive and set it up on a bench, you can do the same thing, put it in forward gear, turn the input yoke (and count the turns) until you have 10 turns of the propeller shaft.

I've already said, if that is an 18' boat with a 19" prop and you're only getting 3600rpm, look at the engine, not the drive or propeller... Make sure it's actually opening the carb up fully, make sure the engine is in good condition....

Chris.......

 

[SeaDooSam]

19 pitch prop, 3600 RPM, 34 MPH with a prop slip of 15% calculates out to 1.619 or pretty dang close to 1.62 so if those numbers are close I would say the drive is the right ratio and you need to figure out why the RPMs are so low.

Not to hikack the thread, but
Could you explain how you calculated that ?

 

[alldodge]

Find it here
http://bblades.com/propeller-slip-calculator/

 

[achris]

Not to hikack the thread, but
Could you explain how you calculated that ?

Slip is the difference between the theoretical speed (as if the prop was turning in a non-compliant medium) and the actual measured speed.
Theoretical speed is the pitch (in inches) times the rotational speed of the prop (engine speed divided by drive ratio)...

So, if we have a prop with a 19" pitch (moves forward 19 inches per revolution, given no slip), and engine speed of 3600, a drive ratio of 1.62, then the theoretical speed would be 19 x (3600/1.62) inches per minute... That's 42,222,22. Change to miles per hour is just multiply by 60 and divide by 63,360 (or just divide by 1056).... So, we have a theoretical speed of 39.98mph, just call it 40mph. Actual speed is 34mph. The slip is theoretical minus actual, divided by theoretical times 100. (40-34)/40*100=15%....

Hope this helps....

Chris.....

 

[SeaDooSam]

Slip is the difference between the theoretical speed (as if the prop was turning in a non-compliant medium) and the actual measured speed.
Theoretical speed is the pitch (in inches) times the rotational speed of the prop (engine speed divided by drive ratio)...

So, if we have a prop with a 19" pitch (moves forward 19 inches per revolution, given no slip), and engine speed of 3600, a drive ratio of 1.62, then the theoretical speed would be 19 x (3600/1.62) inches per minute... That's 42,222,22. Change to miles per hour is just multiply by 60 and divide by 63,360 (or just divide by 1056).... So, we have a theoretical speed of 39.98mph, just call it 40mph. Actual speed is 34mph. The slip is theoretical minus actual, divided by theoretical times 100. (40-34)/40*100=15%....

Hope this helps....

Chris.....

Thank you very much. Very clear post. That should be a sticky!

I'll have to get better numbers in the spring, but based off memory numbers, I calculated 20%

 

[achris]

Whatever your slip is, is what it is. The usual numbers referred to are about 15% for a light runabout, 20% for a cruiser and upto 25% for a heavy cruiser or houseboat.

My preference is to ignore slip and calculate the propellers 'angle of attack'. But that involves quite a bit of trigonometry, and is beyond the scope of this forum. If you do find the formula and are looking for the 'right' number, it's accepted as between 4? and 6?.

I've written a small visual Basic program that I just plug the numbers in and it spits out the answer.

Chris.

 

[thumpar]

20% is kind of high. Did you take the MPH off of GPS?