Motor Boats - Fuel Economy at Low Speed

[Edd_King]

Hi All!

So I'm currently working in Spain on a contract that finishes in July - I've always owned sailing boats (most recently a Beneteau First 35) but my girlfriend and I are considering a French Canals trip home, with this in mind we're looking at motor cruisers...

We've viewed a Birchwood 33, a Cranchi 32 and a Sunseeker Martinique 36.

My question is really in relation to the Cranchi / Sunseeker, both of which I very much liked; Both have 2 x 200HP diesels and the brokers claim a realistic fuel economy of around 75L per hour in total at cruising speed (20knts ish)... but.. Is fuel economy significantly improved at displacement speed or will I need to sell my extended family to get through the canals at 5knts on one engine? As an extension of this first question.. is offshore fuel economy much better if we cruise up the coast at a relaxed 12 knts?

Many thanks for any assistance you can give!

Edd

 

[Chris_d]

None of those boats will use much fuel at 5knts, I've run loads of different boats at this speed on the Thames and 4.5L/hour is typical even with two large lumps.

Forget crusing at 12knts though, smaller planing boats do not work at that speed, you can basicaly do up to 7knts and over about 17knts anything in between the bow will pointing to the sky and mpg will be worse than going faster.

 

[Edd_King]

Hi Chris,

Many thanks for the reply - very useful!

Edd

 

[Elessar]

None of those boats will use much fuel at 5knts, I've run loads of different boats at this speed on the Thames and 4.5L/hour is typical even with two large lumps.

Forget crusing at 12knts though, smaller planing boats do not work at that speed, you can basicaly do up to 7knts and over about 17knts anything in between the bow will pointing to the sky and mpg will be worse than going faster.

excellent advice.

i would add that all of these boats are twitchy at low speed and will need constant input to the steering, particularly the 'seeker. Some get used to that and forget they are doing it, some find it wearing.

 

[burgundyben]

Forget crusing at 12knts though, smaller planing boats do not work at that speed, you can basicaly do up to 7knts and over about 17knts anything in between the bow will pointing to the sky and mpg will be worse than going faster.

I'm not sure that is correct, drag increases exponentially so the faster you go the fuel burn becomes more.

Basically, there are no free 'knots', faster you go the more you burn.

Planning hulls can have an uncomfortable motion at lower speeds.

 

[Elessar]

I'm not sure that is correct, drag increases exponentially so the faster you go the fuel burn becomes more.

Basically, there are no free 'knots', faster you go the more you burn.

Planning hulls can have an uncomfortable motion at lower speeds.

he is absolutely right on most planing hulls, particularly stern heavy outdrive hulls. Pure D speed is the most economical, but above that, 18 knots is usually more economical in mpg terms than 12. Often 30 knots is more economical than 12. (but not than 6)

 

[BruceK]

I'm not sure that is correct, drag increases exponentially so the faster you go the fuel burn becomes more.

Basically, there are no free 'knots', faster you go the more you burn.

Planning hulls can have an uncomfortable motion at lower speeds.

That's going to open a can of worms that'll have the mathemeticians out with their calculators. My understanding is the whole principle of a planing hull was one planing speed was achieved the amount of water displaced reduced hence less drag better economy. Just from experience if I dont go WOT to get on the plane and then back off to 3/4 throttle for a 20knt cruise, and just go 3/4 throttle, a) I dont get on the plane b) I dont get past 10knts C) dont make equivalent revs D) pretty damn certain I am burning more fuel to make those nice stern waves that animate the raggies so much.

 

[Mr Proach]

by no means an expert in this but if the coefficient of drag is similar to that of lift on an aircraft then it makes sense that if the surface area of the hull in contact with the wet stuff is reduced through speed then drag is reduced and so the engines are more fuel efficient.

 

[EugeneR]

Our last boat had a fuel computer which indicated we used the same amount of fuel in displacement at 6 knots, than we used on the plane at 16 knots. So, there are free knots once you get on the plane, absolutely.

 

[Momac]

If on twins you might not want to run on one engine all the time. I expect only one engine will power the steering.
I have kad32's in a 34ft boat which are probably a bit more economical than ad41's (200hp) and I estimate 4.5 to 5 litres per hour - if mostly sticking to 5 knots .

.
.

 

[GSL]

Several years ago we took our Sea Ray 400 Sundancer all the way from Southampton to the Med through the canals. Best trip ever, 4- 5 knots and I recall we used about 1,200 Euro of fuel all the way.

Steerage is no big deal, actually used autopilot most of the time and the left / right buttons to make adjustments as needed.

When we first opened up the throttles after the trip, tiny puff of black smoke and all perfect.

Do it, you really will not regret the experience.

Graham

 

[stuartwineberg]

Several years ago we took our Sea Ray 400 Sundancer all the way from Southampton to the Med through the canals. Best trip ever, 4- 5 knots and I recall we used about 1,200 Euro of fuel all the way.

Steerage is no big deal, actually used autopilot most of the time and the left / right buttons to make adjustments as needed.

When we first opened up the throttles after the trip, tiny puff of black smoke and all perfect.


Do it, you really will not regret the experience.

Graham

The "tiny puff of black smoke" is a reminder that if the engines are turbo they don't really like long runs at low revs but the famed Italian tune up seems to clear them

 

[rustybarge]

I've been looking at the Trawler forum for a while, and the consensus is that most boats up to 34' and 6-7 tons with a single engine can chug along at 2gals/hr and 6-7 kts.

once you start talking about twin engines boats that number changes to 4galls/hr but at a faster 8kts, running on one engine doesn't save much because the second prop causes a lot of drag.

I think a 5kts 1-1.5 gals/hr would be possible at idle speed on a single, not too sure about twins but maybe 2.5-3 gals?

http://www.trawlerforum.com/forums/

 

[bammylovechops]

"once you start talking about twin engines boats that number changes to 4galls/hr but at a faster 8kts, running on one engine doesn't save much because the second prop causes a lot of drag."


Often had that thought, is it better to chug on one engine or two???

 

[sap_2000]

I've been looking at the Trawler forum for a while, and the consensus is that most boats up to 34' and 6-7 tons with a single engine can chug along at 2gals/hr and 6-7 kts.

once you start talking about twin engines boats that number changes to 4galls/hr but at a faster 8kts, running on one engine doesn't save much because the second prop causes a lot of drag.

I think a 5kts 1-1.5 gals/hr would be possible at idle speed on a single, not too sure about twins but maybe 2.5-3 gals?

http://www.trawlerforum.com/forums/

I have a 37' displacement trawler with a cummins QSB 5,9 230 HD engine. I average about 6-7 liters an hour during the season.
I typically cruise at 6,5-6,8 knots using about the same amount of diesel an hour, so about 1 liter/ nm. If I increase one knot to 7,5 the consumption doubles to 15 l/hr. Going to 8,5 knots almost double it again.
5,5 knots give 0,8 liter/ nm.

My experience from planing twin engined boats are similar up to the point when you exceed the hull speed.
I find it very useful to run on one engine for several reasons. You put only half the hours on the engines, the consumption is reduced as the 2nd engine really isn't doing a lot of useful work at just about idle and the maybe most important reason is that the remaining engine gets more load as it you will run it a few 100 rpms faster and more favorable running conditions.

About the steering, you let the AP do the work and fire up the 2nd engine when entering a harbor, the weather conditions require it or you want to cruise at planning speeds. Alternating the engines as needed to keep the clock at the same readings.

I know a guy with a Phantom 43 with 74p's that does 80-90% of his cruising one one engine at 6-6,5 knots. His consumption is about 6,5-7 liters an hour.
Another guy has a Phantom 50 with D12s that runs about the same pattern with a consumption not a lot higher that the Ph43 at displacement speeds.

 

[mlines]

Rather than theory discussions, here is an actual performance set from our fuel flow computer.

It shows the boat coming over the hump from displacement to planing speed at around 10knots

(5.7 litre 350 MAG )

Looking at the "Miles per litre" column you can see the efficiency decrease as it pushes the hump and then increase again as it starts to plane

 

[Chris_d]

Rather than theory discussions, here is an actual performance set from our fuel flow computer.

It shows the boat coming over the hump from displacement to planing speed at around 10knots

(5.7 litre 350 MAG )

Looking at the "Miles per litre" column you can see the efficiency decrease as it pushes the hump and then increase again as it starts to plane

Thanks mlines, think that pretty much proves what I said in post 2 :encouragement:
That characterstic is typical of smaller planing up to about 35ft that are sterndrive or outboard driven, a shaftdrive boat is better balenced and may have a more progressive transition through the hump speeds. But generally really slow or proper planing speeds are best.

 

[BruceK]

Those are very interesting figures :encouragement: I rather was expecting and believed something quite different. Thanks for posting.

 

[rustybarge]

I have a 37' displacement trawler with a cummins QSB 5,9 230 HD engine. I average about 6-7 liters an hour during the season.
I typically cruise at 6,5-6,8 knots using about the same amount of diesel an hour, so about 1 liter/ nm. If I increase one knot to 7,5 the consumption doubles to 15 l/hr. Going to 8,5 knots almost double it again.
5,5 knots give 0,8 liter/ nm.

My experience from planing twin engined boats are similar up to the point when you exceed the hull speed.
I find it very useful to run on one engine for several reasons. You put only half the hours on the engines, the consumption is reduced as the 2nd engine really isn't doing a lot of useful work at just about idle and the maybe most important reason is that the remaining engine gets more load as it you will run it a few 100 rpms faster and more favorable running conditions.

About the steering, you let the AP do the work and fire up the 2nd engine when entering a harbor, the weather conditions require it or you want to cruise at planning speeds. Alternating the engines as needed to keep the clock at the same readings.

I know a guy with a Phantom 43 with 74p's that does 80-90% of his cruising one one engine at 6-6,5 knots. His consumption is about 6,5-7 liters an hour.
Another guy has a Phantom 50 with D12s that runs about the same pattern with a consumption not a lot higher that the Ph43 at displacement speeds.

I think a gallon of diesel produces about 20hp in a modern diesel, or about 15hp in the old 2 stroke detriot diesels.

so 30hp or so to drive your single engine dipl. Boat at 7kts sound plausible as it's a very efficient design.

....but I'm thinking 30hp won't push a Phamtom 43' twin engined boat very fast at all!
im going to stick my neck out and say not 7ltrs/hr but more like 14ltr/hr at 7kts: at least!

 

[sap_2000]

He is doing around or just above 6 knots, not 7. Numbers verified with both floscan meters and fill up after a longer trip on only one engine.
I am not surprised by the numbers a Ph43 has a longer waterline length than I do. My boat is hitting the wall at about 6,5-6,7 knots so I keep my cruise speed there in calm conditions.