RPM/Knot Performance

[wetpaint]

I understand no two boats have the same engine performance or performance for that fact......Question....Two 36 Sedan Bridge boats with same cummins diesel engines (QSB 380)....One boat will max out at 2700 RPM and 22 knots (mine)......The other will Max out at 3000 RPM and 28 Knots.......My boat in did not get a lot of use in July so maybe growth may contribute to the problem......I have never opened the boat up in two years to 100% load and throttle....I did notice that at 90% throttle and 100% throttle read 2700 RPMs...Any thoughts

 

[Four Suns]

My first thought is always the bottom/running gear are dirty. Have a diver look at it..

I generally haul my boat in May for it's annual painting and get a quick haul in Sept because the running gear just gets crapped up and removes some speed from the boat. It also allows me to check for any damage as I have one of those God awful cored bottoms.

 

[B-Rock]

Hey wetpaint....
I had a similar issue when I took delivery of my 36 Sedan Bridge Gas version last year where one engine was not turning full RPM's, and there turned out to be a voltage issue with the e-plex electrical system causing the system to limit the voltage to the engine in order to keep them running. They replaced a module, and all the batteries, and that turned out to solve the issue. Not sure if this would be the same issue with the diesel engines....but it is the same electrical system....so the chances are good.

 

[wetpaint]

Thanks Gary and Brian......The boat will be hauled Thursday to see if I have a ton of growth on the bottom......I will also forward your thoughts Brian to Hyannis......Heading anywhere this weekend Brian.....I will prob hit the Vineyard after the drop the boat back in the water on Thursday

 

[Sea Gull]

While I am not a WOT kind of guy you MUST run your egnines up to rated WOT once in a while. I do it several times per season just to make sure that everything is OK.

The inablility of your engines to acheive rated WOT in one of the best indicators that something is wrong. But the only way you are going to know is to start with a clean bottom.

 

[B-Rock]

Hey wetpaint.....we will be at Oak Bluffs for the fireworks this weekend (friday through sunday) along with D-Rob (Double the madness) in a slip......be nice to finally meet up with you for a mudslide or 6! Let me know if you are heading over!

 

[promocop]

Does anyone with a 2005 SR 390 Dancer with 8.1 gassers have a chart or opinion about the optimum speed/performance/fuel burn chart? I'm running at about 3,000 rpm/14 knots and was wondering about 'efficent' this was versus running at about 3,500 rpms etc. Any ideas?

 

[wetpaint]

Brian....

I think I will be in Vineyard Haven (calling for space now).....I will need alcohol as it is a dry town....I will try to shoot to Oak Bluffs and support the local AA meetings and try to catch up with you. If you are in town at the fireworks, I might just come aboard and use the head, see whats in the fridge and take a nap...

 

[DaveL]

Hey Gary,
I know you've got a chart of your speed vs. fuel consumption laying around somewhere for your boat. Can you share what you've seen as optimal?

Having run our boat for only one season and also just come back from NE while playing along the way, I noticed a couple of things.

1. Faster = more fuel (stating obvious.)
2. If I put in a lot of trim, the boat consumed less fuel and ran faster than no trim, as some have suggested at higher speeds. Pushed less water?
3. Although I'm honing in on "ideal", the boat seems to "like" to be at about 21 - 22 knots and burns about 32 gph at about 1800 rpm.
4. When I go above 21 - 22 knots, then the square law takes over and fuel consumption starts to climb exponentially.

Dave

 

[dgrumm]

A buddy of mine has a 390DA w/8.1's. I don't remember the exact numbers, but I'm pretty sure he said he gets better MPG at 19knots than he does at 16knots.