Cruise RPMs

[Mike Schmidt]

I just had my 6BTA props repitched, and now I’m happy to say I’ve gone from 2800 WOT to 2950 WOT. So now what should I shoot for in terms of cruise RPM? I come up on a good plane around 2200 or so.

Topic

[J Dragon]

I have a 315 6bta with the denso pump.

I am propped to make 2950 easy.

Assuming I am also safe to cruise 2100-2400 (the range Tony cites above), aside from the added fuel burn modestly decreasing engine life, is there any other concern cruising at the higher end of this range (2350-2400)?

I’ve historically stuck to 2100-2200ish.

I have the 160 stat and SMX redundant temp alarmed for 190 and the water flow alarm at my mixer.

I don’t have an EGT gauge but plan to add this winter…

Thanks 

[Rob Schepis]

I agree on the tabs as to making to boat easier to plane thus reducing the load. Your are decreasing the hump the boat needs to climb. If 2,850 is absolute worst case when she’s heavy and dirty and she makes an accurate 2,850 and makes it easy then pitching back is not that critical but if you do I’d go only a 1/2″. could wait until you have the props off for a routine check/balance or some other maintenance.

[Roger Franklin Williams]

Rob, I am verifying tachs which where serviced a few years ago. Right now best is around 2850. So I may need to take out an 1″ but will wait till I get tachs confirmed and going to haul clean and paint in next few months.

Guess my question may be that the larger tabs seem to make the boat plane easier at slower RPM’s I have assumed the load on the engines is reduced?

What do you think?

[Rob Schepis]

Roger,

What do you end up with as to WOT when things get heavy and dirty?

Have you verified the tachs with a laser meter?

If the rpms are verified and you settle in down to something around 2,900 I’d say your a okay.

[Tony Athens]

If you have 6BTA 5.9 330 Diamonds (rated in a test cell at 315 BHP / 2800RPM), and your tachs are accurate, you are as good as it will get under load.. The 370 Diamond versions ( rated 355 BHP at 3000) should reach about 3150 when set up right.

Tony

[Roger Franklin Williams]

I would like input from others. Have the same 6BTA’s on 24,000 lb cruiser. I have installed 2-12×12″ additional trim tabs years ago. Boat comes up on plane easily at 16-1700 RPM hull clean boat light to 19-2000 RPM needed hull dirty or boat loaded. I seem to think the extra trim tab area results in less strain on the engines. I have increased the trim tab area on all my boats for the past 40yrs. as they seem to perform much better.

I can only get WOT 2950 with a clean hull. The creek area I am in and after using the best bottom paint, tried many, still get considerable soft growth in 6 months after cleaning.

I understand repitching props but also believe the tabs make a big difference as well?

Comments?

[Tony Athens]

If a 330 Diamond (CPL 1975), then you are now good to cruise at 2100-2400 RPM all day long.

Tony

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