Fuel Economy - Engine speed or Boat speed?

[Irish Rover]

I've admitted more than once on the forum I'm a bit of numpty when it comes to technical stuff and I was hopeless at physics at school and not great at maths apart from pure business stuff. So I'm always confused about stuff like displacement speed etc.
My boat is a power Cat with 2 x 75HP Yanmar engines. For the most part I only ever use one engine at a time. The previous owner advised me that the most economical use was achieved at 2200rpm producing 7 knots. So I usually set @ 2200. Depending on the sea state and how clean the hull may be my speed can vary from around 6.5 to 7.5kn. So my question - should I be aiming to maintain my engine speed at 2200 or my hull speed at 7Kn or is it possible neither is right.
By the way I'm not concerned about the fuel consumption which works out at around 3.5 Lph. I'm just curious.

 

[BruceK]

I suspect the the term most economical use would be more accurate as best balanced compromise. As a compromise the balance will always shift according to conditions.

 

[Irish Rover]

I suspect the the term most economical use would be more accurate as best balanced compromise. As a compromise the balance will always shift according to conditions.

Thanks. I'm not sure I understand but thanks anyway for taking the trouble to reply.

 

[BruceK]

The most advantageous compromise would be a ratio of fuel burn per distance covered over time. Most economical would be just fuel burn per time. The latter would probably be achieved at far lower revs. So if conditions were against you the distance element gets skewed and the ratio changes.

 

[ST840]

If your not bothered by the fuel consumption and your happy with your speed then I don’t understand why the question. I’d of thought that two engines working at 7knots would give better fuel consumption or economy if you like than one engine because that one engine has to work harder than two working together !

 

[Refueler]

The question is in two 'flavours' ....

What is best economic for distance traveled vs What is best consumption of engine at what rpm.

The two are not the same.

Or as Litres per KM (Gallons per mile) or Litres per hour (gallons per hour).

You can increase engine speed and so increase litres per hour - but your boat speed increases and you get better distance covered in that time ... there is a balance when you get best distance per litre vs time.

Next hulls are designed to work at speeds commensurate with hull shape. A planing hull will obtain better conmsumption once the hull planes and is suffering less drag.
A sailboat - the more upright she is - the better but a compromise sought where sails drive is balanced by heel at such angle to not create too much extra drag.

To try and decide boat speed through water vs engine rpm ... is not really the way to go .. because weather, pitching rolling of boats etc. all have their say in it. Basically as an owner gathers time and use of a boat - he gets to know where to put that throttle ... and also noting how much fuel he puts in after trips etc.

 

[Portofino]

I've admitted more than once on the forum I'm a bit of numpty when it comes to technical stuff and I was hopeless at physics at school and not great at maths apart from pure business stuff. So I'm always confused about stuff like displacement speed etc.
My boat is a power Cat with 2 x 75HP Yanmar engines. For the most part I only ever use one engine at a time. The previous owner advised me that the most economical use was achieved at 2200rpm producing 7 knots. So I usually set @ 2200. Depending on the sea state and how clean the hull may be my speed can vary from around 6.5 to 7.5kn. So my question - should I be aiming to maintain my engine speed at 2200 or my hull speed at 7Kn or is it possible neither is right.
By the way I'm not concerned about the fuel consumption which works out at around 3.5 Lph. I'm just curious.

Need more specific info ps .
When you say “ power cat “ what exactly is that in terms of L ?
what other engine options do they do in the same modal ? Eg is there a 2x 300 Hp version ?

A pair of 75 Hp does not sound a lot .Appreciate a pair of canoes might drag less than a mono hull .
Whats it’s design speed with both running ? And max WOT speed and it’s rpm @ WOT .....with both and one engine .

Max displacement speed is with a mono hull approx sq root of the water line length x 1.34 a CAT might have a bigger multiple.
Up to this water is moved sideways only .

Go over ( with enough HP ) and in attempting to get on top of the water a huge bow wave forms and the stern dips adding to the drag which because of the extra Hp , you can get a disproportionate increase in fuel burn ( to create the additional Hp ) compared to the increase in fwd speed .

So we don’t know where the 2200 is as proportion of max Rpm , WOT or where the 7 knots is in terms of the D speed equation ....depends basically on the L and the other multiple than the std monohull 1.34 to use because it’s a CAT .

 

[Irish Rover]

Thanks guys but what part of me being a humpty did you not understand.
My boat is a FP Greenland 34 loa 10.3m x 4.45. As far as I know the standard engine configuration was 2 x 75 Yanmar but I know of at least one with 2 x 110 Yanmars. When clean and with 2 engines fully open @ 3500rpm it will get up close to 15 kts in a calm sea. I usually motor at 2200rpm and make 7kts with one engine. With 2 engines @ 2200 I make around 8.2.
Does this help?

 

[scottie]

As your propellers are probably designed to work best at a specific speed which is unlikely to be one engine at approximately 7 Knotts you are not going to get the ideal consumption without some major tweaking and in any case you are not concerned about consumption does it really matter
However keeping boat speed at non displacement might be the logical option
Is worth checking what different items are connected to each engine as normally you would not have calorifier Hot water take off ,house battery charging or power steering on both engines
The boat handling may be affected by running on one engine though
It would be good practice to give both engines a good hard run on a regular basis

 

[gordmac]

You might get better mpg at lower speed but will take longer to get anywhere! With a monohull you get the big bow wave/stern trough at what is called Hull speed and between there and planing (if you can get there) is not at all economical. With a cat you have long slender hulls which reduce the bow wave and therefore the Hull speed "hump". If you had a monohull 7 or 8 kn would be close to Hull speed, with a cat there may not be such a consumption penalty exceeding this.
The governor on your fuel pump will adjust the fuelling to maintain whatever engine speed has been selected, the actual boat speed will vary with tide, wind and wave size.

 

[ontheplane]

Do you have fuel flow meters on the engine??

If you do - you can plot a curve (on excel or paper) for litres per hour on the vertical axis and rpm on the horizontal axis....

if you are doing 2 engines on 2200 rpm giving 8.5kn and 1 engine at 2200 rpm giving 7.5 kn then it also sounds like the one engine is struggling and is having a lot of prop slip to create that speed - you may find it's burning almost as much fuel as the two running at 2200rpm. If you're having trouble picturing that - try riding a bike up a hill in top gear - although your legs are turning slowly, you are working massively hard so using vastly more energy (fuel) than your legs turning faster but in a lower gear.......

If you don't have a fuel meter this will certainly be hard - but I'd suggest that 2 engines running at 40 & 60% of max revs will probably give you the best economy and therefore range....

 

[ontheplane]

Forgot to say - using the graph of fuel / rpm you then plot again against speed and you should see a most economical point

 

[Irish Rover]

No fuel flow meters unfortunately [or maybe I should say fortunately as I now don't need to admit I couldn't plot a curve on a vertical and horizontal axis even if I know what it was].
7 knts suits me. I like the slow pace after a good few years with much faster sports boats. I know I burn 3.5 LPH with well over 90% of my motoring at 2200 rpm on one engine. If I used 2 engines together to achieve 7knts [lets say @ 1,900 rpm] is it likely my fuel burn per engine could be less than 1.75 LPH?

 

[MapisM]

If I used 2 engines together to achieve 7knts [lets say @ 1,900 rpm] is it likely my fuel burn per engine could be less than 1.75 LPH?

Impossible to tell for sure, but in principle I wouldn't be surprised if you'd burn the same or even less litres per nautical mile with two engines, at any speed.
Reason being that there are inefficiencies when using just one screw on a twin engines boat, due to additional drag of both the non-running prop and the constant steering.
And while this is true for any boat, it's bound to affect catamarans even more, for obvious geometric reasons: the single thrust in a cat is much more asymmetrical than in any monohull.
The only real advantage, if that matters to you, is that by cruising with one engine at a time you keep the hours count lower.
Not because this affects engines wear, which is actually more driven by load and fuel burn, rather than the hours clocked - in spite of the fact that for simplicity most maintenance schedules, particularly for smallish engines, are based on hours.
It's rather the perceived value whenever you will resell the boat, that might be affected by the hours clocked.
The only 2c that I would give you, should you prefer to keep cruising with one engine at a time, is to use the R/H prop on stbd and the L/H prop on port, to reduce a bit the asymmetric thrust effect.

 

[Refueler]

Something that has to be remembered ....

Two engines does not double your speed. Nor does it halve fuel consumption on each, with total same as using only one.

What two engines do is increase the acceleration by that 2x thrust. But the pitch of the props has a speed it will make at an rpm ... meaning that once you get up to or near to pitch speed - you can add as many engines as you like but speed will hardly change.

The fact that OP gets 8.2kts with both engines at 2200rpm vs 7kts at 2200rpm on one engine is down to drag vs thrust ... the greater thrust of the two engines and also the lack of rudder correction to correct asymmetric thrust means a cleaner fwd movement and slight increase in speed and nearer to pitch speed.
Each engine will work less hard of course unless its the acceleration phase and stick pushed hard fwd ! So once speed is attained - fuel consumption per engine will reduce ......

But it still comes down to distance against fuel used as the real economic ....

 

[QBhoy]

I’d imagine the simple answer would be thay you are likely about perfect for being efficient as you describe. On that kind of hull...as soon as you start lifting the bow...you are usually becoming less efficient. Once you are up to around your 7 knots or so, it will take very little work from the engine to keep you there with that hull.
You mention running just on one engine, I’d say that it’s 6 and 2 3’s doikg that or running both at same speed but with less effort from the work being shared.

 

[MapisM]

The only 2c that I would give you, should you prefer to keep cruising with one engine at a time, is to use the R/H prop on stbd and the L/H prop on port, to reduce a bit the asymmetric thrust effect.

Sorry, I just noticed that I made a mistake.
The above setup is the normal one, which probably you already have.
What I meant is exactly the opposite, which is better for cruising on one engine at a time.

 

[Irish Rover]

Thanks to all for your replies. I think I've learned a little. I'm lifting my boat on Monday for painting. I'll fill the 2 tanks when I put it back in and do so experimenting over the summer using both engines instead of just one.
Am I right in assuming that displacement speed is the optimum speed and how do I calculate it for a catamaran?

 

[MapisM]

If by optimum you mean the most efficient in terms of fuel burnt for any given distance, yes, absolutely.
Calculating the exact maximum D speed is a bit of fine art though, because it's specific to any vessel.
The classic rule is that D speed in knots equals 1.34 times the square root of the waterline length (measured in feet).
But a lower beam to length ratio (among other considerations) allows for a greater than 1.34 factor.
I.e., D speed of racing canoes can be much higher than the D speed of a tug with the same WLL, just to mention two extremes.
Mind, for catamarans the beam to length is NOT meant as the total vessel beam, but is referred to each of the two hulls, which are typically much narrower than monohulls - therefore, for any given WLL, the max D speed of a catamaran is higher than a monohull.
But again, there isn't a magic formula valid for all boats.
Nowadays, with electronic engines, it's easy to find the optimal speed, because you can see the fuel burn in real time, not only in l/h, but also in l/nm, when the engine instruments are interfaced with the speedo.
And if fuel optimization is really crucial for you, you could get that on your boat even if your engines are not electronic, by installing fuel flow meters.
OTOH, since you said that the 7 kts ballpark suits you, I think you can just stick to that and not bother about more accurate numbers.
Btw, even if you would find out that you can increase your speed at say 8 kts before the fuel burn begins to increase exponentially (as it happens when you exceed hull speed), you would still burn less at 7kts anyway - and even less at 6 than at 7, for that matter...
Besides, 7kts burning 3.5 l/h means that you are in the half a liter per nautical mile ballpark, which for a mobo is pretty much like saying that you are burning nothing.
The next best thing is bound to be a sailboat, I reckon!

 

[Portofino]

Yep with a hull beam ratio exceeding 8 the calcs with cats becomes complex .
May be approach it from the engine side ....look at the fuel burn rates ( on line ) and do it as a fraction of WOT rpm .
2200 out of 3600 seems low to me .
Perhaps ease both together up to say 2800 and just accept the increase in speed .
It might be kinder in terms of longevity and lower maintenance bills all be it slightly more fuel ?
As others have said try a few options and experiment .

Btw does it actually plane with 2x75hp or I suspect just the canoes push water aside as opposed to climb up and out ?