Lower unit question

[SHickey]

Hello,

I have been tinkering and adjusting my 87' Blue Fin (aluminum) 2.3 liter Cobra and I just can not get it to top out properly. The motor struggles to get to 5000 rpm and there is no appreciable speed increase (trimmed out) from 4500 to 5000. Top speed is about 31 MPH (GPS) I have the recommended 13 1/4 X 17 P and have another 19 P prop but don't dare to put that one on because the boat seems to barely handle the 17 P.

I looked at my owners manual and the original LU had a model # 984540 and on further inspection, the model # on the bottom of my LU (near the trim tab) was 985583. I can tell the LU was replaced before I bought the boat but would assume the LU is correct for my motor and out-drive set up. Could I be mistaken??

I am wondering if the LU I have was from a different (more powerful) set up and may not be geared properly for my 2.3. If the LU is ok for my set up, does anyone have any suggestions for achieving better top end, or am I asking too much for the set up I have.

Other than top end the motor is smooth and the drive is very consistent ( 0 - 25 mph piece of cake)

Thanks for any responses!!

 

[HT32BSX115]

Re: Lower unit question

I looked at my owners manual and the original LU had a model # 984540 and on further inspection, the model # on the bottom of my LU (near the trim tab) was 985583. I can tell the LU was replaced before I bought the boat but would assume the LU is correct for my motor and out-drive set up. Could I be mistaken??

Howdy,

Yes. because your boat is pretty aged, someone could have changed and/or rebuilt that drive with different gears many times in it's life.

I am wondering if the LU I have was from a different (more powerful) set up and may not be geared properly for my 2.3. If the LU is ok for my set up, does anyone have any suggestions for achieving better top end, or am I asking too much for the set up I have.

Yes again. So you need to determine what (ratio) you have, and that other "things" are correct before you change props or do other "stuff"

The most important performance indicators are your Tach and speed indicator. Since you're using GPS, you have HALF of the equation.

Now you need to ensure that your Tach is accurate. (Some of them can be as much as 500 RPM OFF at higher RPM's)


I don't have a drive ratio chart in front of me but I am pretty sure that a 2.3L engine requires a roughly 2.00:1 ratio. (1.98:1 I think)

[Anyone want to jump in here?]

So to easily determine your ratio, just pull your spark-plugs (to easily [and safely] turn your engine by hand), put the drive in gear (while turning the engine by hand)

Then turn the engine exactly 2 revolutions. Have your helper observe the prop make exactly 1 revolution.

If it's a LOT less than 1 turn, you don't have a 2.00:1 drive. You can also get more accurate by turning your engine exactly 10 turns. The prop should turn 5 etc.....

Once you know you have the right ratio, you can look at the "sticky" message up at the top of the message list for more info on being unable to reach max correct WOT RPM.


Cheers,


Rick

 

[SHickey]

Re: Lower unit question

Thank you Admiral for that insight.

When I get home tonight I will do the ratio test and see what my ratio is. If you don't mind answering another question, can you explain how the ratio's work in relation the motor size. For example, if my 2.3 requires a 2:1 ratio, what would a 3.0 or a 4.6 require for ratio. I'm thinking the ratio I have in my LU is for something beefier that the 2.3 I have.

Thanks

 

[HT32BSX115]

Re: Lower unit question

Thank you Admiral for that insight.

When I get home tonight I will do the ratio test and see what my ratio is. If you don't mind answering another question, can you explain how the ratio's work in relation the motor size. For example, if my 2.3 requires a 2:1 ratio, what would a 3.0 or a 4.6 require for ratio. I'm thinking the ratio I have in my LU is for something beefier that the 2.3 I have.

Thanks

I found this list of ratios right here in an iBoats forum message,

2.3L: 2:1

3.0L: 1.86:1

4.3L: 1.68:1

5.0L: 1.59:1

5.7L & 5.8: 1.41:1

7.5L: 1.43:1


Notice the low torque/ HP engines require a rather "low" gear ratio

 

[SHickey]

Re: Lower unit question

Admiral,

If I could be so bold and ask a final question??

If my LU gear ratio was changed to a 1.86:1 or a 1.68:1 before I bought this boat, then I would probably need a smaller diameter or lower pitch prop to reach 5000 rpm assuming my tach and GPS are both accurate??

Am I making any sense??

Maybe I should be happy that my boat starts, idles, shifts and trails easily but to get that last 1000 rpms with out the motor sounding like it is laboring would put my mind at ease. Even if it means I will probably lose some top end speed.

Thanks

 

[HT32BSX115]

Re: Lower unit question

If my LU gear ratio was changed to a 1.86:1 or a 1.68:1 before I bought this boat, then I would probably need a smaller diameter or lower pitch prop to reach 5000 rpm assuming my tach and GPS are both accurate??

For the most part, YES. lower pitch is usually what you would do but those props are designed to get max efficiency turning a particular range of (prop) RPM.....hence the different gear ratios for different engines.

Maybe I should be happy that my boat starts, idles, shifts and trails easily but to get that last 1000 rpms with out the motor sounding like it is laboring would put my mind at ease. Even if it means I will probably lose some top end speed.

no. ALL engine manufacturers recommend a prop pitch that gives you a WOT RPM at the top of the recommended range of WOT RPM.

Laboring (lugging!) a marine engine at any RPM is asking for eventual damage.


Cheers,


Rick

 

[SHickey]

Re: Lower unit question

Well,

I did a gear ratio test this am and discovered the LU gears are not the 2:1 ratio they should be for my 2.3. I was getting a rough ratio of 1.5:1 or for every 1 revolution the motor turned the prop turned @ 1.5 times. Looking at the chart, the gearing may have been from a 5.0 which would explain why it has trouble pushing at higher RPM's.

Given the hand I am dealt and the fact that I would not replace the LU with the "correct" one (because I am cheap) could anyone suggest the best prop size and pitch for my set up? Currently I am running a 13.25 X 17 aluminum and I know this question should go to the "prop" dept but you guys seem to be much nicer.

If I ever need to replace my LU, I will be sure to get the right gearing for my motor and I will already have 2 props to use (17P and 19P).

Thanks again for all of your help!!

SH

 

[Bondo]

Re: Lower unit question

Well,

I did a gear ratio test this am and discovered the LU gears are not the 2:1 ratio they should be for my 2.3. I was getting a rough ratio of 1.5:1 or for every 1 revolution the motor turned the prop turned @ 1.5 times. Looking at the chart, the gearing may have been from a 5.0 which would explain why it has trouble pushing at higher RPM's.

Given the hand I am dealt and the fact that I would not replace the LU with the "correct" one (because I am cheap) could anyone suggest the best prop size and pitch for my set up? Currently I am running a 13.25 X 17 aluminum and I know this question should go to the "prop" dept but you guys seem to be much nicer.

If I ever need to replace my LU, I will be sure to get the right gearing for my motor and I will already have 2 props to use (17P and 19P).

Thanks again for all of your help!!

SH

Ayuh,.... The logical answer is,.... Try a 15" prop,...

 

[SHickey]

Re: Lower unit question

Thanks for for the response. Stick with the same diam 13.25 and drop the pitch to a 15 (13.25 X15P) and see what happens. The 13.25X15P props seem to be an easy size to come by.

Thanks Bond-o