Next message: (no name): "Re: T&T: Locked vs. freewheeling drag"
In a message dated 5/9/2008 12:00:56 AM Eastern Daylight Time, trawl
writes:
My boat speed will drop from 9 mph to 7.5 mph on the GPS while maintaining
the same 1600 rpms with the 'dead' prop freewheeling. I've not done it long
enough to collect any fuel consuption data. With both engines running, I go
9 mph and burn 3.25 gallons per hour.
I just had a short cruise with a dirty (slime) bottom, into the wind, 75
degrees OAT, and some current (so this GPS SOG is not exact, but was
back-to-back); this might be helpful (42' Present with twin FL160s - ~34,000#):
one engine running at 1400 rpm and the other freewheeling = 6.1 knots
both running at 1400 rpm = 7.1 knots
Note - I thought I was running at 1500 but just found my upper helm tachs
were off by 100 rpm at this speed - I synched them by ear so they were running
the same, but slower (I love my new toy that came in last night after the
cruise - a digital infrared tach).
I do not have a flowmeter (want one!), but previous runs tell me combined
consumption at 1500 rpm is 4gph. So, let's say 3.6gph @ 1400 rpm both. I
believe that in these speed ranges prop loading extracts essentially the same
power for a given rpm, so the burn at 1400 rpm is the same whether the other
engine is running or not. Added comment, I noted that the engine speed did not
hunt/change when the other engine was brought up.
Therefore, economy for these conditions was 6.1/1.8 = 3.38nmpg on single and
7.1/3.6 = 1.97nmpg on both. As long as there is no damage to the tranny or
shaft packings, handling is acceptable, and added block time is tolerable -
why wouldn't I do this?!
Also - I can bring one engine to ~1600 rpm and get the higher 7.1 knots, but
I do not really know the fuel burn. I guess it is about 3.0 gph or so (2.4
kmpg). And this is better for my engine life...
I normally run harder (1600/1600), but this was a fun run and I was in no
hurry.
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