Fuel consumption - 50 hp diesel in a yacht...

Oen

Oen

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Greetings all,

I recall there is a 'rule of thumb' relating to fuel consumption for a diesel engine driving a yacht - and I have some rough figures for my new boat (42ft LOA, 35ft LWL, 50hp Volvo MD22 with a shaft), but I'd like to start refining the data I have. Can anyone remember what the 'rule' is, please? (I was astonished not to find it in Nigel Calder's books!)
 
My experience shows 50hp in a Westerly 33 gives around 1 gal per hour at 6 knts, perhaps al itttle more - up to 1.2 gal per hour.

Don't know about a rule though.
 
Rule of thumb for any constant running diesel (ie, not a vehicle) is each 10hp uses 1 litre per hour,

That is HP actually being developed, not the HP printed on the cover of the manual. ie. you probably only run your 50HP Volvo at 40 HP most of the time,

This fits with Gorb's 1.2gal = 5 Lt = 50HP

It follows of course, given a fairly standard rate of efficiency of combustion in most engines, x amount of fuel burns to give z wasted heat and y shaft power.
 
Our boat which is 39ft and nearly 12 tonnes with a 36HP (Bukh)) engine burns 2 litres per hour. A friend of mine whose boat is similar lenght but heavier at almost 14 tonnes with a 50HP yanmar burns 2.5 litres per hour. Both at 2500rpm
 
Best way is to just keep an accurate record of hours run and fuel used, as each boat will be slightly different, and it might be very usefull to know. We had to motor back to the Canaries once and had to make a very tricky call on if we would make it back or should we go on downwind to the next stop. Steering failed but we still had autopilot on quadrant, but using it needed electric, hence engine.

My 43hp Volvo turbo would get 2ltr/hour at 5.5knots.
 
The golden rule is that each gallon of diesel will yield 20 bhp-hours. The max. power of the engine - in this case 50 bhp.- isn't very relevant as most people use only a fraction of that power except in emergencies. Our VP2002 in a Sadler 29 does about 3.5 hours per gallon ie. at 2000rpm. we're using about 6 bhp to push the boat along. That gives nice restful progress at a bit below hull speed in smooth conditions. Rough conditions or strong adverse tides obviously demand more power.
 
Wow, a gallon an hour.

My Fulmar, 1 foot shorter and probably 1 tonne lighter only has an 18hp engine, and the last 6 seasons has burned a remarkably consistent 1.5 LITRES / hour.

How do you manage to use 50hp in a W33?
 
Re: Wow, a gallon an hour.

[ QUOTE ]
How do you manage to use 50hp in a W33?

[/ QUOTE ]

Well the short answer is you don't, anymore than you use the 18hp in your Fulmar. However the reserve power is there if it is need, 4 cylinders are much smoother and quieter and if your teeth can stand the bone breaking jarring then you can bash into pretty well any kind of sea. Tried it once going NE up through Alderney Race with a NE7 against a big spring tide, the spinnaker pole decided to jump ship from the sidedeck over the top rail and I could understand it's feelings, so we slowed back down and just let the tide take us through then started sailing again. We were motoring because the seas in the Race were stopping us stone dead under sail, plus we had a coaster in the way on one tack and a girt great chunk of granite on the other. On balance I was glad to have the reserve hp!

The standard engine in a W33 was a Mercedes 42hp. When ours (last boat) went terminal we replaced it with a Volvo MD22L 50hp (Perkins Prima) which was slightly smaller and lighter than the old engine but was still 4 cylinders. The prop pitch went up a size but as it turned the blades more in line fore/aft the drag if anything went down. The fuel consumption was 0.52gph at 1800 rpm and 6kts in a flat calm at which speed you couldn't hear or feel the engine running.

Our current boat is 41ft with a 44hp 4 cylinder Yanmar and Brunton Prop which at 2000rpm (max is 3,600) gives 6.5kts and uses about 0.58gph.

Diesels burn 225mls or 0.4pts per bhp actually used per hour so you can use that figure to calculate the HP used at cruising revs. On that basis our W33 used just 10.4bhp at the prop, say 11bhp at the engine. Our Sun Legende is using a bit more, say 11.6 at the prop or 12.5bhp at the engine. The figure on this one vary somewhat as the Brunton prop pitch self adjusts, like changing in/out of overdrive or low gear and consumption varies accordingly depending if we keep speed constant or revs constant.

In neither case is the bhp actually used in a flat calm very high because in neither case is the boat driven at hull speed. Hull speed for the W33 would be about 7.2kts and on the SL41 about 8.1kts. Driving either at or near the hull speed would push the fuel consumption steeply up, at full throttle/maxbhp the 44hp would use 2.2gph and the 50hp 2.5gph. In practice flat out full power is never used, but to run at 8kts on our SL41 would probably use 1.75gph at 2600/2700 rpm.
 
Just another bit of empirical data:

Oyster 39 - about 12 tons in cruising mode
6.5kts @1800rpm on Thorneycroft 154 (2.5 litre)
About 1 gallon per hour. [Wish it was less!]

Regards,

Tam
 
If the short answer is \'you don\'t\'...

.. gorb must have been chasing leaks 'cos he said he was using 1 to 1.2 gallons per hour. P'raps he was trying to get a skier up!

My 1.5 lit. per hour includes everything from going full chat -motorsailing - going nowhere charging the batteries at 1400rpm - switching the engine off and still burning diesel in the Mikuni.

Useful calculation in you post, on that basis I'm using on average 6.7Hp and have a reserve of at least 2.5 times more power if I need it.

That'll do!
 
Re: If the short answer is \'you don\'t\'...

[ QUOTE ]
My 1.5 lit. per hour includes everything from going full chat -motorsailing - going nowhere charging the batteries at 1400rpm - switching the engine off and still burning diesel in the Mikuni.

Useful calculation in you post, on that basis I'm using on average 6.7Hp and have a reserve of at least 2.5 times more power if I need it.


[/ QUOTE ]

Beware though because in our case we NEVER run the engine except under way, never to charge batteries nor do we use the Ever Busted. Our figures pretty well are for constant operation at cruising revs and I measure them from full to full because our fuel guage doesn't and the tank cannot be dipped, we use engine hours as the fuel used indicator. Any light use will make increase engine hours (what you divide by) and also decrease fuel burnt per hour (what you divide).
 
Re: If the short answer is \'you don\'t\'...

We are a wee bit over-propped, which does not help. And checking the figures (based on hrs run meter and actual litres into the tank) consumption is just Under 1 gal per hour at our normal cruising speed of 6knt. Still the original Merc engine, so now nearly 30 years old.
 
We have a 75hp turbo engine on 43' Yanmar max revs 3800. When new it motored at 10kts (almost max revs).

Would anyone like to venture an opinion on most economical cruising revs /speed. Without doing exact measurements I have tended to do 6-7kts in calm conditions and use same revs but consequentally less speed going over the speed bumps!!
 
Re: If the short answer is \'you don\'t\'...

We had our prop cut down from 17" to 16" dia on the W33 with the Mercedes in order to just pull max revs (and therfore hp) in gear under load. When we re-engined with the MD22L we had a new prop to match, also 16" dia but different pitch and luckily got the dia/pitch right first time. Westerly always had a tendency to overprop, getting ours right reduced the prop kick and the engine felt livelier. Ours was a fin keeled W33 BTW which might make a difference too and we always had a clean bottom. I seem to remember we motored at 2200 rpm with the Mercedes, not sure though.
 
I'm not particularly bright when it comes to HP and CC.
I have a Ford transit 2.5 litre (2496cc) marinised engine in my boat. In order to work out the fuel consumption and follow the replies in the question could someone please tell me what HP my engine is

Many thanks


David
 
The rule of thumb is 5 gallons per hour per 100 hp developed. You should cruise a diesel at around 75% rpm which, with a fixed pitch prop, gives around 50% rated hp. Thus a 50 hp engine run at 75% rpm will develop about 25 hp. Therfore the consumption will be around 5/4 = 1 and a quarter gallons per hour. The problem with most modern yachts is that the are supplied with an engine too big for it, so you end up cruising at 75% rpm too fast and too much fuel, or you run it slower and risk cylinder bore glazing, resulting in eventual low compressdion and poor starting anfd high oil consumption. I wish builders would stick to the conventional 4 hp per ton!

Pat Manley
 
But it sounds like there are still plenty of old mercs going strong - my 636 is nearly 30 yrs old, and is a 50's design. From your formula it is well over size for my boat, but still runs sweetly.

Something to be said for a good "agricultural" design?
 
Re: If the short answer is \'you don\'t\'...

I,d go with Robin on this, Westerly33 in cruising mode, aprox 2000rpm, 6.3 Knots, just under .5gph. with a OM636. Flat out @ 7.2 Knots. Flat water (Amersterdam/Willemstadt 17hrs) approx..8gph. Ref post on Transit Diesel, I think you will find it about 85bhp, and full revs usage approx 1.7gph. HTH
 
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