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Cummins 5.9 Performance questions
I'm looking at a 3100 Open with Cummins 5.9 Diesels.  WOT is 30 knots at 2600 RPMs.  I'm a bit concerned about the 2600 RPMs as the spec says the engine should get 2800 max.  I would appreciate input from this group on those engines.  Also, if anyone has performance data for those engines on a 3100 they could share that would be helpful (WOT speed and RPMs, Cruising speed & RPMs, etc).
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I do not have specific feedback on those engines but diesels in general need to hit WOT plus 5% to keep them healthy. The fact that they wont reach WOT would concern me as well as how long has the issue been around? If the current owners were running it overloaded for a long time, there could be serious issues looming. Get a Cummins tech to do a complete work up on the engines. 200 RPMs down in a diesel is a lot...
[-] The following 2 users Like August Rush's post: (09-11-2021), Misdirection (09-10-2021)
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How was the bottom of the boat?? Clean and good running gear? A fouled bottom or gear will cost you a couple hundred rpm in a heartbeat. If everything was clean then I would suspect It to be overproped.
If overproped I would worry a little because that has probably been going on for a long time. I would have the engines checked out thoroughly ( but I would to that anyway, it saved me from spending $100k on my current boat).
If the bottom wasn't clean, have it cleaned and rerun it.
Sans Peur 2003 Tiara 50 Open
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Thanks. Yes bottom was relatively clean. The engines seemed to run well but couldn't make it above 2600, so maybe it is overproped. I have a mechanic checking it. So, if a diesel runs longer term at lower RPMs what kind of issues are created? Thanks.
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Over-propping means the engines are being asked to produce more HP than they should at any given RPM. Another way of looking at it is the governor is pushing more fuel than it should to achieve the desired engine speed. High exhaust temperature tends to stress the valve train and shorten engine life. In extreme cases valves can drop resulting in catastrophic engine failure. 

A lot of things can cause an engine to not achieve rated speed including fouled bottom and or running gear (as stated above) dirty air cleaner or restricted air into engine room, fuel restriction (filters, air leaks), props, etc. 

Check with Tiara to see what props were installed out of the factory. It may be the previous owner swapped them without having the prop shop dial them in. If you haven’t done so confirm rated RPM and HP via data plate on the engine. 

Hopefully you and or your engine guy can chase it down quickly.  As previous posts have indicated, ideal setup is to achieve Rated RPM + 2-3% with full fuel and load.
[-] The following 1 user Likes jclark003's post: (09-13-2021)
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For reference the boat should be propped to not be overloaded at the end of the season when its heavier and more drag is there due to growth. In other words have a solid cushion, even if it means sacrificing a knot or 2.
[-] The following 1 user Likes August Rush's post: (09-13-2021)
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Cummins 5.9 Performance questions00