RPM + - rocker switch on dash...what's the right way to use this feature?

C

Ckroeger

New Member
Sep 7, 2011
4
Charleston, SC
Boat Info
2008 40', twin Raymarine E120's
Engines
Twin Cummins 5.9 QSB
Hi,
I have a 2007 Sea Ray 40 with twin Cummins QSB 380's. On the dash switch panel, I have a rocker switch that is labeled RPM + -
OK, I feel stupid asking this question, but what is this toggle switch used for? Does it set the cruise control? Does it limit engine RPMs?
There is nothing in my owners manual that states proper use of either the cruise control or the RPM +- rocker switch.
So...does anyone use this feature?
Thanks, in advance, for your reply.
Sincerely,
Charlie
 
I just bought my SR two months ago and am still getting used to everything. I use the RPM +/- switch to up the idle speed after I start the engines so they warm up faster. Every time you press the top of the button the RPM increases by 20 or so. I get them up to around 900 RPM to warm up for 3-4 minutes, then bring back down to idle. Mine also has a "slow idle" feature; when engaged the engines idle at 550 instead of 600 which is handy when you want to go slow. With both engines in gear at 600 RPM my boat will get up to about six knots which is sometimes too fast.
 
OK, great information. I'm working through a problem in which my engines will not achieve max RPM (and never have). They used to get up to @ 2900 and Sea Ray told me the reason it couldn't achieve max @ 3060 is because the bottom was fouled. OK...fast forward to today, now I'm at 2700 RPM max and I get the bottom maintained every month. The engines are well maintained, after coolers are clean, turbos are spinning up fine. I'm pretty sure that the boat is over propped (I have 21 x 23 pitch nibral) and that I have to pitch down...but I want to make sure there's nothing electronic going on first. Can the RPM +- be used to limit the top end or is it solely active when the cruise control is on? Unfortunately, the weather down here is going to keep me off the water for a few more days.
Anybody out there have a 40 Searay with cummins QSB 380's? I'd love to know what your prop spec is...
 
Ckroeger said:
OK, great information. I'm working through a problem in which my engines will not achieve max RPM (and never have). They used to get up to @ 2900 and Sea Ray told me the reason it couldn't achieve max @ 3060 is because the bottom was fouled. OK...fast forward to today, now I'm at 2700 RPM max and I get the bottom maintained every month. The engines are well maintained, after coolers are clean, turbos are spinning up fine. I'm pretty sure that the boat is over propped (I have 21 x 23 pitch nibral) and that I have to pitch down...but I want to make sure there's nothing electronic going on first. Can the RPM +- be used to limit the top end or is it solely active when the cruise control is on? Unfortunately, the weather down here is going to keep me off the water for a few more days.
Anybody out there have a 40 Searay with cummins QSB 380's? I'd love to know what your prop spec is...
Click to expand...

It sounds like you may be way overpropped, which can be very damaging to the engine. On an electronic engine like yours the best way to determine that is by comparing your fuel consumption in the mid-range RPM to the performance curve that Cummins created for that engine, using the instantaneous fuel consumption on your SmartCraft screen. You can find the performance curve at Boatdiesel.com. It looks like you should be consuming no more than 4.9 gal/hr at 1800, 7.1 at 2000, and 8.7 at 2200 RPM. Burning more fuel at those RPM levels is an indication the engine is overloaded, because you are overpropped.
 
I don't believe the RPM +- can be used to limit your engine RPMs other than when the Cruise Control is engaged or when at idle. If you have your Cummins Owners Manual there is (at least there was with my QSC's) a write up on the functionality of both the RPM +- and the Cruise Control. As for your rpm's at WOT, it very well could be that your props are spec'd correctly but need to be tuned up, especially if they've never been done. If you've got a clean bottom that's the next thing I would check. Have you checked the max rpm while in neutral? Not sure if that is a recommended practice or not but it would give you some indication that the engines can get to rated rpm.

Ckroeger said:
OK, great information. I'm working through a problem in which my engines will not achieve max RPM (and never have). They used to get up to @ 2900 and Sea Ray told me the reason it couldn't achieve max @ 3060 is because the bottom was fouled. OK...fast forward to today, now I'm at 2700 RPM max and I get the bottom maintained every month. The engines are well maintained, after coolers are clean, turbos are spinning up fine. I'm pretty sure that the boat is over propped (I have 21 x 23 pitch nibral) and that I have to pitch down...but I want to make sure there's nothing electronic going on first. Can the RPM +- be used to limit the top end or is it solely active when the cruise control is on? Unfortunately, the weather down here is going to keep me off the water for a few more days.
Anybody out there have a 40 Searay with cummins QSB 380's? I'd love to know what your prop spec is...
Click to expand...
 
I think there is a limit switch, will not let rpm's above 1000. Agree with Brian, no high end limit. Check props next.

mark
 
The QSB 380's came with 21x23 props. The QSB 425's came with 21x24 props, both Nibral. Just becasue your props are spec'd right doesn't mean they're within those specs. I'd have them checked, the engines need to be able to get to rated RPM at WOT otherwise you are likely overloading them at cruise.
 
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