Engine Sizing and Fuel Consumption

[europaflyer]

Tony B - hope you didn't get the impression I was having a go at you. Couldn't be further from the truth. You didn't misunderstand the question, I am looking at a repower from 18hp.

The boat is a Contessa 32, 9,500lb, 32' overall and 24' on the waterline. The estimate of 10hp wasn't about engine size, it was about the actual power output needed to make 5 or so knots in light conditions. Clearly, motoring in to the chop in a F8 will need much more. The majority of boats have 18ish hp, new ones have 20hp. What I was really trying to find out is whether this is just a 'we're proper sailors so we fit the smallest practical engine' thing and in reality a bigger engine would use less fuel, or whether there's nothing in it apart from the obvious advantage of more grunt to windward in heavy weather. Motoring is generally something I try to avoid, but I was interested to find out if, when motoring along in calm conditions at a moderate speed (say 5kt), the boat with the bigger engine would really be using less fuel, which seemed odd but was implied by the Beta data. I think the general conclusion from the posts - and I'm happy to be corrected - is that this isn't the case, and the fuel consumption would be broadly similar unless the smaller engine was having to be run at excessively high RPM, say perhaps 3000+. This is what I always thought, but as I said, the manufacturers data seemed to go against this, which is why I asked the question! 30HP would be a little excessive I think, and not being a fan of motoring or a speed junkie, 20HP rather than 25HP will be all I need if there is no benefit to fuel usage. 20HP should be just enough to move the boat slowly in to the chop, or if things get really ugly to hold me off a lee-shore for enough time to get the sails up. Sensible conclusion or not?

[Adelie]

Originally Posted by europaflyer

Hi all,

I'm looking in to choosing an engine and I've come up against a rather interesting question. Here goes...

I'm looking at a repower from an 18hp diesel. The boat needs around 10hp by my estimation to be pushed at a nice cruising speed. I've been looking at Beta engines between 16-25hp.

To produce a given HP output, the larger engines need to run at lower RPM, ie. with a greater pitched or larger propellor. However, the larger engines seem to have lower fuel consumption to produce a given power. I know fuel consumption is not linear with RPM, but this seems surprising.

To put out the 10hp (which I estimate will produce a nice 5kt) these are the figures. The graphs supplied aren't the best, so these need to be treated with caution.

Beta 16 = 1900 RPM, 0.7 litres per hour
Beta 20 = 1700 RPM, 0.5 LPH
Beta 25 = 1400 RPM, 0.5 LPH

A little surprising the larger engines don't use more fuel, but hey ho. Putting out 16hp is where it gets interesting though.

Beta 16 = 3600 RPM, 3.5 LPH (full power)
Beta 20 = 2600 RPM, 1.8 LPH
Beta 25 = 2100 RPM, 1.2 LPH

The fuel consumption is lower with a bigger engine. Not just a bit - a lot. To prove a point, here's 20HP.

Beta 20 = 3600 RPM, 4.5 LPH
Beta 25 = 2600 RPM, 2.2 LPH
Beta 30 = 1900 RPM, 1.0 LPH

That last one particularly got me. Going from a 25HP to a 30HP engine HALVES the fuel consumption needed to produce 20HP.

I appreciate that an engine running at full RPM is not at its most economical power. But this seems to be true over the whole range.

So am I right? Because fuel consumption and RPM aren't linear over any part of the RPM range, it will within reason be more economical to fit a larger engine, and by a big margin? Surely not! Am I just reading too much in to sketchy graphs?

Here are the data sheets.
http://www.betamarine.co.uk/seagoing...6-SDS-0311.pdf
http://www.betamarine.co.uk/seagoing...0-SDS-0311.pdf
http://www.betamarine.co.uk/seagoing...5-SDS-0311.pdf
http://www.betamarine.co.uk/seagoing...0-SDS-0311.pdf

I think the l/hr curve is a bit misleading. What you want to do is construct a curve showing the gal/hr/hp or l/hr/kw. Yanmar used to include this curve on their engine performance charts but don't seem to be doing so now.

Generally the peak fuel efficiency for given diese is at about 60-80% max revs.

Optimally you chose your engine to be at 75-80% rpm at your chosen cruising speed. Then the question becomes what do you want for a cruising speed. Most boats will more than double their fuel economy by motoring at 4.5kt.

But if you pick a motor optimized for this speed you may not have the reserve power to get to hull speed.

Picking the motor to achieve hull speed you have the problem of carbon build up when you throttle down to a more economical cruise speed, say at 50-60%rpm.

What I would do is pick a motor to achieve hull speed or maybe 10% over that.

[muised]

My unscientific mind led me to repower my Douglas 32 (full keel, similar specs to the Contessa 32) with a Beta Marine 25. Just finished the installation last week, and I don't think I would want less power. As I recall the difference in price between the 20 and 25 was about $400 which is not significant overall in a repower, I get hull speed at between 2200 and 2400 rpm. The Beta is half the physical size and weight of the Westerbeke 4-91 it replaced. Having some reserve power seems like a good idea, so unless you're on a real tight budget or are trying to reduce every bit of weight, I'd recommend the 25 based on my experience so far.

[europaflyer]

Here are the graphs for fuel consumption (left) and specific fuel consumption (right) as per Adelie. (disregard the first 3 values for the Beta 30, the data didn't exist but putting in 0 it made it easier for excel).

They both show that above 10hp the Beta 30 is the most economical,followed by the 25, then the 20, then the 16. The lowest specific fuel consumption for each engine is at 6hp for the 16 and 20, 12hp for the 25 and 12-16hp for the 30.

At useful power outputs, the larger engines all use less fuel. The Beta 25 would only be more economical than the 30 at 10hp output or less. Below 8 horsepower the 16 or the 25 are the most economical.

Normal engine usage would be, I would guess, in the 10-15hp range. By the numbers then, the most economical engine wouldn't even be the 25, it would be the 30. Eh??!! I'm missing something surely!

[europaflyer]

It didn't let me upload the graph before, here goes...

[Cotemar]

That is exactly what I was seeing when I compared the 20 hp engines with my 30 hp.
The 30hp is the way to go. The size and weight where not that much different.

[chris07732]

I question if your target "to cruise at 5 kts" as a goal is long term valid for engine sizing. It would seem to me to be more important to be able to cruise most efficiently at hull speed, with sufficient reserve for head winds, currents, etc to continue to make hull speed. Slow down to 5 kts as you want, but make sure the boat is capable of hull speed for extended periods. In the future, your boat will have more resale value being able to make hull speed for extended periods as opposed to hull speed only at 100% engine output in flat conditions.

[Tony B]

Europa
Sorry if I sounded too blunt. Sometimes I just get carried away in my writing and tend to write quickly because I am at work. My work comes in fits and spurts and when the radio calls my name, I have to get going.

Anyway, here are some more thoughts of mine. I think my formulas are fairly correct based on my Catalina 30 and Cheech with his 14 HP tends to confirm. Assuming I am correct, you need 15.5 HP to get you almost up to hull speed and that could even take a while. That is also based on no wind resistance, no current and no waves coming at you, no nothin'. Add any of those things above to the equation and you are now running your engine at max. RPM's and that's why you have a greater fuel consumption. As you are runnning your engine to the max. that is telling you you are vusing more HP. Me thinks that you are probably using your full 18 horses at that point and so 2 more ponies under the hood will not make that much of a difference.
Back to my Catalina 30. It weighed slightly more than your boat but not enough more to make a noticable difference in the calculations. I had a 25HP diesel. It was good most of the time. I ran just below hull speed at about 3/4 gal per hour at about 6 kts. A little bigger would have been nicer but cost is usually a big factor in engine sizing.
If it were me personally, I would not even consider the 20HP. I don't think you will be getting your money's worth. For me it would be an economical decision between the 25, the 30 or nothing. That's just me talking.
The 25 would be much better fuel economy from what you have now, not to mention the stress of running your engine on high RPM's and the stress on motor mounts etc. Your curves will almost bear me out if you interpolate and put your present 18HP in there.
Another note - with a full displacement hull, your theoretical hull speed means everything. Once you actually hit it at 6.5 kts your fuel consumption becomes noticably worse. Just 1 kt over it can possibly double your fuel consumption with any of the engines above.
Look at your curves above and study the HP lines vertically and just for a moment, look at what your estimated HP will be and then look at where that stands when comparing the shape of the curves for each engine. Are you near a slow rise or a sharp rise in consumption? That sharp rise in consumption also means a sharp rise in engine RPM's.
Hope this helps.
Regards
Tony B

[europaflyer]

Thanks everyone.

Looking at the graphs, I reckon that most of the time my power needs would leave the 30hp running at undesirably low RPM. Forget the data, common sense says running the engine at 1200-1500 RPM for most of the time just can't be good, either for the health of the engine or, whatever the graphs say, for the fuel consumption. I reckon Beta either interpolate between max revs and idle, or just extrapolate from tests at high RPM.

The 20 might be the usual engine for the boats, but while it would be OK in optimum conditions at less than hull speed, any chop and we're looking at uneconomically high RPMs. The 25 might just be a tad more than I need most of the time, but it doesn't weigh or cost much more. I reckon that at normal usage the 25 would use at worst the same amount of fuel as the 20, but would probably use less. Any time grunt is needed, the 25 would really come in to its own and would certainly use less fuel while also running at much more comfortable RPM. For people who are more keen on motorsailing than I am the 30HP may well be nearer the mark.

I reckon the 25 is the way to go. Not everyone will agree I'm sure! Although most of the time the stock 20 is without doubt OK for the boats, as there is only 20lb and $400 in it I see no reason not to go for the 25HP.

[S/V Alchemy]

As a rule of thumb, I use the "last knot of speed" rule.

If your hull speed is just over 6 knots (probably), and your prop is pitched correctly for your boat (like a two-blade Gori folder of perhaps 12 x 8), you want to "motor" at 4.5-5 knots/1900-2100 RPM for maximum fuel economy.

Want an extra half-knot? Throw up the main. Now you are motor-sailing.

I find that RPMs are a greater guide in this regard than SOG, which is subject to wave height, current, weight and other considerations. My new Beta 60 was sized to replace a Westerbeke W-52, and will turn a slightly larger prop in terms of area, a four-bladed feathering prop. My goal was not motoring speed, but fuel economy to transit the doldrums and to maneuver at low speeds, as it's a large steel full keeler with plenty of inertia.

I determine my "sweet spot" in calm water for economically cruising was about 4.7 knots SOG, at which point my consumption drops well below .75/gal./hour, or between 2.5 and 3 litres.

That means if I run at that speed in calm conditions (otherwise I'd have canvas up, right?), I have a range of some 1,100 NM or six days of straight motoring. More if I have jerrycans lashed on deck.

So I think in terms of fuel economy and range, not speed. If I thought much about speed, I'd take flying lessons, not Safety at Sea.

[S/V Alchemy]

Originally Posted by europaflyer

Thanks everyone.

Looking at the graphs, I reckon that most of the time my power needs would leave the 30hp running at undesirably low RPM. Forget the data, common sense says running the engine at 1200-1500 RPM for most of the time just can't be good, either for the health of the engine or, whatever the graphs say, for the fuel consumption. I reckon Beta either interpolate between max revs and idle, or just extrapolate from tests at high RPM.

The 20 might be the usual engine for the boats, but while it would be OK in optimum conditions at less than hull speed, any chop and we're looking at uneconomically high RPMs. The 25 might just be a tad more than I need most of the time, but it doesn't weigh or cost much more. I reckon that at normal usage the 25 would use at worst the same amount of fuel as the 20, but would probably use less. Any time grunt is needed, the 25 would really come in to its own and would certainly use less fuel while also running at much more comfortable RPM. For people who are more keen on motorsailing than I am the 30HP may well be nearer the mark.

I reckon the 25 is the way to go. Not everyone will agree I'm sure! Although most of the time the stock 20 is without doubt OK for the boats, as there is only 20lb and $400 in it I see no reason not to go for the 25HP.

Get the double PTO option if you have room to run two small alternators, which may or may not be the case. You can dedicate one to house and one to start bank, or switch as needed. It's nice to have the redundancy and the extra HPs get used to charge...maybe you can go to a mooring and save money!

[barnakiel]

There is a very interesting twist at HR homesite where they published data for:

- same engines/various props/various cruising speeds/resulting ranges-

of some of their designs.

The tables show clearly how engine/prop/speed match results in lowest fuel/mile consumption.

Cheers,
b.

[Tony B]

My best guess is that under nornal conditions you will probably be motoring at 18 to 20HP range getting you to about 90% of hull speed or roughly 6 MPH. See if your can find charts that will convert your HP to RPM for the 25 and the 30 HP motor. From the RPM charts you can find the charts that convert RPM to fuel consumption. From these charts, you can then be able to calculate whether the 25 or 30 HP is more economical in the long run. Figure your fuel consumption at 6 MPH and how many miles per year do you expect to be motoring, cost per gal. etc. etc . etc.

For you Motorheads: I'll bet you are wondering why fuel consumption is such a big thing with a sailboat. Mainly because as much as sailors want to be sailing there is either not enough wind or wind in the wrong direction a good amount of the time.
Usually on very long trips, the wind never cooperates. Sailors have an old expression about passage making: Long periods of boredom punctuated by short periods of sheer terror.

[Tony B]

Originally Posted by europaflyer

Thanks everyone........I reckon the 25 is the way to go. Not everyone will agree I'm sure! Although most of the time the stock 20 is without doubt OK for the boats, as there is only 20lb and $400 in it I see no reason not to go for the 25HP.

I think you are making a wise decision.

The last three or 4 posts were pretty much all written at the same time. Mine was delayed due to work. Hate that when that happens.
Anyway, I was wondering about the underloading of the 30 HP model and that is why I suggested looking at the RPM range.

Best of luck and keep us informed of your progress.

[Tony B]

Happy Fathers Day to all.

It's been a fun ride.