relative fuel economy

H'mmm<g>
I guess its sea trials then - as the formulas come out saying the same as you guys - it depends!!
It seems a trivial question - if you use half as much fuel at low speed, but have to stay out twice as long, the answer is it's the same total fuel cost. But the math says this simple result assumes that the fuel vs knots curve is linear. It can't be that simple, however (is anything?).
I suspect, as with all such problems, that for any given passage length, there is an optimum speed to minimise fuel used. I just can't get any reliable data to support (or refute) this idea.

Its a long summer, so if we get the weather, I guess sea trials is has to be...

Thanks to everyone who replied

PS the boat is a Nelson 29 by Seaward, twin Yanmar 170s on shafts

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Well I suspect the most economical speed will be max hull displacement speed which is the square root of the waterline length x 1.25, so for you will be about 6.25kts. Suspect though that you will want to travel a bit faster!

After that it will probably be around 85% of max engine revs, which I expect will give you about 25kts at a guess.

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Most boat tests in the boat mags have fuel consumption v speed tables for a variety of speeds. Try to find a test on your own boat (use the search feature on the MBY site) or a similar boat with the same engines
Alternatively get the spec sheet for the engines fitted to your boat and you should find a graph showing fuel consumption in litres or galls per hour against rpm. Do some sea trials and note engine rpm against various speeds. From this data you will be able to calculate approximate fuel consumption at any speed for your particular boat

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agree with your first but not your second.
Maximum dispacement speed without wake will give one efficency and minimum planning speed is likely to give the other. 25 knots is going to be well above fully planning.
Turbo's on those engines come in at 1700 prm I think so it should be propped for the resistance hump at 15-1550 relating to the hull and, if a planning hull, be up and clear at about 2100 light - 2300 loaded. Max fuel efficency in that area whatever speed it equates to - which may or may not be better than back at 1100 - 1200 revs 'ish overall.

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Planing - Don't agree that mimimum planing speed is most economical - not in all circumstances. I can plane at about 17-19 knots, but most efficient is about 27knts

<hr width=100% size=1>Err, let me know if Depsol enters the forum, I'll go and hide
 
Ah but it's not a planing boat! It's a semi displacement where once it is in semi displacement mode it's then down to the most efficient revs for the engine which I think is around 85% of max revs. Won't make a whole load of difference to fuel efficiency, but probably marginally better. All IMHO.

Seaward 29 is a great boat, should go really well with twin 170s.

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Is it the Fyfe Boat show? I am supposed to be working all weekend, but am trying to take sunday off so as i can go over to Cumbrae and sit my Vhf licence.
On monday weather permitting hopefully we shall get another couple of hour on the clock.
If we get a chance we shall be over at the festival, do you know what they have planned?




<hr width=100% size=1><A target="_blank" HREF=http://www.hamptonandmcmurray.co.uk>Hampton and McMurray</A>
 
Yeah Honestly!! I've just managed to get sunday off, so i'll be in Cumbrae doing my vhf course, Have you done yours?
If not you should do it at the same time.



<hr width=100% size=1><A target="_blank" HREF=http://www.hamptonandmcmurray.co.uk>Hampton and McMurray</A>
 
Alistair, good luck with the course. Unfortunately, I'm off back down to Manchester Sunday A.M. - Birmingham Monday Morning (Should be fun, they're handing out 500 redundancy letters to some of the Brummy Staff today!)

Are you based at the shop here in the big G? If so, we can meet for a brew (of whatever kind...)

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Well,i hope your job is safe, is it?
We came over to Largs last night, Those Fyfe yachts are beautiful.. And WHAT a size. Was looking outfor you but couldn't find Stingray, Where are they hiding you now?????????
Am usually based at warehouse in whiteinch, but occaisionally work out the shop, like tomorrow unfortunatly.
If you are out tomorrow you had better be careful i think the clyde will be busy, what with the fyfe regatta race and also some race to Arran.



<hr width=100% size=1><A target="_blank" HREF=http://www.hamptonandmcmurray.co.uk>Hampton and McMurray</A>
 
With a planing boat the most economical (fast) speed is just a bit faster than when the boat is fully planing. Ie for a 29' boat about 20ish knots.
10 knots will produce less MPG than 20 knots.

With a semi displacement boat generally speaking the faster you go the less economical it is. There is not usually a "sweet spot" higher up the speed range.
10 knots will be more economical than 20 knots.

However, the benefit you get with a semi-displacement boat is that you can drive at any speed wheras a planing boat generally is very unhappy until fully planing.

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