Gear ratio

[renken86]

Can you go from a 21 20 gear ratio to something like a 21 18? It's a stringer outdrive in a v6. The boat doesnt seem to go to fast and wondering if I change up the ratio it might go a little better just dont want to tear up any gears and get stranded.

 

[Star64]

Have you tried a different pitch prop may be cheaper than gearing

 

[southkogs]

How fast are you trying to go? How fast are you currently going? And what kinda' boat is it? What kinda' water are you on?

There's a lot more in making a boat go faster than gears and HP (though, they always help). And a Stringer is a system I'd probably not try to push too hard.

Welcome aboard.

 

[Bondo]

Can you go from a 21 20 gear ratio to something like a 21 18? It's a stringer outdrive in a v6. The boat doesnt seem to go to fast and wondering if I change up the ratio it might go a little better just dont want to tear up any gears and get stranded.

Ayuh,..... Welcome Aboard,..... What are the rpms at Wot,..??

What pitch prop ya got,..??

 

[renken86]

The boat i have is a 1984 cruiser sea devel. It has a 3.8 liter. Right now the propeller is stamped 15 x17 and when its at full speed it only goes abot 12. Just looking to see about how to get it to go a little faster.

 

[southkogs]

Something's way wrong ...

3.8 is a six cylinder, so we're in a boat that's under 25', right? Even propped wrong, 12 is kinda' crazy for that setup. It should be up in the 20s at least. How long has it been going like this for you? Anything happen that you can recall?

 

[renken86]

I got the boat about 2 months ago. Its 22ft and I know the gears are 20 21 in the upper unit. I thought it was kinda slow but don't really understand why. Now I'm just trying to find how to fix this issue.

 

[southkogs]

I'll have to look tomorrow at listings for what the correct ratio is for that drive (don't have them here at home), but everything working together your boat should be doing 25MPH up on plane. 12MPH you're not even planing.

Are you turning a lot of RPMs? As in the engine is winding away, but the boat isn't going anywhere? Or are you not even getting the RPMs out of the engine?

My first suspect is a drive shaft that's kinda' notorious on these. But there's other things it could be.

 

[renken86]

I have the upper unit apart because I hade to change the drive shaft that stripped thats how I know the gears are 20 21. The ram if I remember right was somewhere between 35000 and 40000. Sorry can't remember the exact rpms.

 

[southkogs]

So, you were getting 12 MPH @ 4000 RPM before the drive shaft stripped. And you're still getting 12 MPH @ 4000 RPM after the shaft has been replaced?

I don't have the ratio numbers on the 3.8. I thought I did, but all I have is electric shift numbers. I would think your upper ratio should be 21:19, transposed of what you wrote.

Have you checked for a spun hub?

 

[Scott Danforth]

did you replace the vertical shafts with new?

how is the prop hub?

that motor should spin 4400 RPM

you have the tilt all the way down correct?

and the trim all the way in for the holeshot, correct?

 

[renken86]

Yes I did replace both shafts. The shift the go through the impeller and the one it slides into. Prop is good not bent up or broken. The tilt was all the way down and the trim. When I got the boat the guy said it wasn't a very fast boat but I never thought it was going to be this slow.

 

[southkogs]

Not a question of the prop being bent or broken - is the hub of the prop shot. The hub on that prop and the screw should be two pieces. If the hub is spun, then the motor will over-rev the hub on the shaft but the screw won't turn enough to move ya'.

 

[renken86]

The prop is good. If I turn the prop hard enough the motor will turn with it.

 

[southkogs]

Nope. Still not it. Google "Spun Hub" if you don't know specifically what it is or how to test for it.

You test by marking the outer prop and the prop hub and then running the boat (short version).