macjazzy

Not sure if this is the right term for them, the 3-4in tubes that mount vertically above the prop to help air the props out so the engine can get into its power range sooner and help plane out.

My friend has a Fountain 38 Express cruiser with Merc Cummins and Bravo III xr drives. His boat always takes so long to plane off, and if he has a crowd onboard it can be pretty frustrating. It seems it is just until the turbo starts boosting, as soon as you hear the turbo it jumps up on plane. But there is so much "traction" with the Bravo III's it has a hard time spinning up fast enough to start making boost.

So I was wondering if the "Ventilation" tubes I see on some boats with 6 drives, or Arnesons, would help and if anyone makes them for bravo drives.

lakematdude

http://www.performanceboats.com/html...ight=blackhawk

http://www.performanceboats.com/html...ad.php?t=57688

But you should wait for others to chime in here before doing something like that blackHawk.

There is one thing you could check in the mean time on your friends boat, Make sure the celery stick is in the correct spot.

The "celery sticks" = the trim insert, Trim Adjustment stick, If it's in the wrong location on any bravo style drive it will make your boat not plane out correctly, See number 30 in this link below for an idea of what and where it goes.
http://www.mercruiserparts.com/Show_...nd+Drive+Gears

Also see page 3 - 5 in this next link for directions of how to install the celery stick between the bravo's 1 thru 3.

http://www.boatfix.com/merc/Techbk/97/97hs3.pdf

macjazzy

I do know about the "celery sticks" although didn't know that was the name for them. They are in the correct position, we have had his drives off a few times doing services.

But regardless, even if they were on wrong I don't think that would help out on this particular issue.

I really think if we could get the props to slip enough to get the engine up into the rpm range where it starts making boost this thing would jump up on plane.

I think the issue is the Bravo III's have so much "traction" the engine can't spin up fast enough for the turbo to boost. I believe Volvo fixed this by exquipping their engines with Superchargers that boost at low rpm's untill the turbo can build up enough boost to take over. Obviously this route would be much much more expensive in his Cummins engines.

looseconnection

What model cummins? Some are know to have a long lag before turbos kit in. My father has a convertible with 450 8.3CTA's and it is 1900 to 2100 rpm before the turbos come on strong.

LC

Airpacker

That BH drive is mine and yes, installing the tube made the world of difference in getting the motor up into its torque curve.

macjazzy

I am not sure of the model, they are 320hp inline 6 cyl, mechanical fuel injection, 4.2L ( not positive on the 4.2L part)
It does seem to be aorund the 1900-2100 rpm mark that they do finally take off. But as I said, getting them to spin up to that can take awhile sometimes.
So I am trying to think of ways to get the engines into their power band quicker.