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Subject: Re: We're in a quandary


Author:
Robin Carter
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Date Posted: 12:17:43 19/01/2009 Mon
In reply to: Alan Smith 's message, "Re: We're in a quandary" on 15:55:27 12/01/2009 Mon

>Thanks, Robin. That's really clear. Having talked
>and thought it through since Friday, I'm now moving
>away from the idea of replacing the engine. At least,
>not now. We've a lot to do, still, to get the boat
>shipshape inside and out and, although it's a bit slow
>for the tidal Thames, it's fine for mid-Thames, where
>we're based.
>
>We have checked speed with a gps and averaged out
>upstream and downstream speeds to give us a
>black-smoke-free speed of 6mph. With a bit of black
>smoke, she'll do 6.5. I'm starting to think that for
>a 45-year-old, 40hp diesel, that's not bad.
>
>Next time we're out in her, we'll check the revs with
>a tachometer while the engine's under load to get a
>more accurate understanding of the power being
>delivered.
>
>Thanks again to Robin and Ian

Alan
I got a bit confused with the 6mph to knots, its 5.2 Knts not 4.5, we're on the broads and the limits are 5 and 6mph and my log and plotter is in knots so I should know. At 6.5 mph, you're getting towards 6 knots and the hull speed is 6.5 so there's not a lot to be gained. As like mine you have a 1 to 1 box you also need to consider the shaft speed, with a bundle of revs you will exceed the design speed of the shaft and get increased wear. Mine is proped to run at a max of 2200 with a 12 by 8 prop. We have some quite strong tides here, I've seen 3.5 knots on the log with the boat stopped and ballanced on the tide and our boat will pull like a train against the tide using the torque of the engine. If you are producing black smoke its either over fueling or pulling a bundle of revs, I think you need to look at the prop, is it too short a pitch or worn. have fun.
Best regards
Robin

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