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251 Convincor Cummins Diesel

O

osr1

New member
Hello to all. Thank you for the warm welcome in my intro. I hope this is where this can be posted. I'm having some issues sorting out prop selection or possibly looking at drive alternatives. I didn't get to run it with the 502 as I got the boat in salvage condition. I have since installed a 12v cummins Diesel from a Truck. I cant complain much other than it seems as if the prop isn't in the water enough, and also that a gear ratio change (maybe complete drive change) may be in order. It has a Bravo 1. I don't want to flood you with info so the short story is, as I trim the bow down it feels faster and faster but I run out of travel. At speed, raising the bow even a tickle of the switch makes the motor rev higher and the boat looses speed. The current prop is a 4 blade stainless 30, motor red line is 4k and around 36-3800 rpm (steady tach signal has yet to be refined, its defueling nearing goverend rpm) and about 46-48mph. Thank you in advance for you time.
 
Cool project!
Pictures?

Your prop is cavitating (sucking air) as you trim it up.
You can have "cup" added to the trailing edges of the blades. This puts a lip on them and will eliminate the vast majority of the slip, assuming your drive isn't too high.

As you add "up" trim, the hull should free up and the boat should pick up speed.
When at speed, if you trim up and the tach climbs but the speedo either stays they same or drops, that's is propeller slip.

Be careful w/the drive too.
They are meant for horsepower more than torque.
I have a friend that tried a similar project and the torque of the diesel destroyed (quickly) everything he screwed to it.
 
The boat will be coming out of storage in a week or so. It was pointed out to me that the drive seems very high in relation to the bottom of the boat. I realize leaning on the torque capabilities of the drive is a problem, especially if I get into gearing it for more prop RPM. Thats why if we can determine if height is the problem I may look into options for replacement. Thanks for the help thus far.
 
BTW this is an awesome project. I just read the whole S&F thread. If you are thinking of putting a transmission behind the boat, you can use the Imco SCX-T like PPE used behind their duramax. That is a pretty stout drive when used with a trans. Internal shifting drives are the weak link.

Can you put up a pic of your transom showing your drive now? Checkmate's do have a fairly high X-dimension but that is where speed comes from. You don't want your prop and drive completely burried in the water. My propshaft on my 253 is 6" below the keel and that is probably fairly typical.
 
Rons on the right track with his first post.. the drive ratio needs to be 1.1 to 1 or around there.

Cool project hopefully you get it straightened out.

I've considered doing something along the same lines and known the drive ratio was going to be the major hurdle. Also gettig a drive to hold together will be a whole other issue......
 
ndaniels said:
Rons on the right track with his first post.. the drive ratio needs to be 1.1 to 1 or around there.

Cool project hopefully you get it straightened out.

I've considered doing something along the same lines and known the drive ratio was going to be the major hurdle. Also gettig a drive to hold together will be a whole other issue......
Click to expand...


Psshhhh..... Like you know anything about a Cummins....
 
ndaniels said:
Rons on the right track with his first post.. the drive ratio needs to be 1.1 to 1 or around there.

Cool project hopefully you get it straightened out.

I've considered doing something along the same lines and known the drive ratio was going to be the major hurdle. Also gettig a drive to hold together will be a whole other issue......
Click to expand...

This fool hasn't logged in in months, hears Dodge Diesel and boom, there he is.
Next thing you know there'll be a floating Neon and he will really be happy.
 
I may have the same problem as the guy your talking about. It's my Cummins obsession that brought me to this stage. Out of all the pics I have, not one really shows the back of the boat. As soon as it's out of storage I will get a better one, I'll post the best I have (it's poor)
 
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