Engine Sizing and Fuel Consumption

[chris07732]

Originally Posted by 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.

barnakiel: Could you post link to data pages you refer to. Checked their site, couldn't find them.

Thx

[europaflyer]

At the bottom of each page for one of their yachts, it lists a PDF file. Good find, will be interested to look through those. In so many ways HR show real thoroughness in yachtbuilding, just a shame they completely lost my trust with the substandard resin debacle with the HR37 a few years back, and the ensuing court case where they refused to accept responsibility. If I was buying a boat of that sort of spec, my business would probably now go to Rustler.

[europaflyer]

Wow, by the numbers the HR310 has a range of 1025nm at 2.1 knots and 1000RPM. 20 days motoring, and an utterly clogged engine at the end of it! I'm going to have lots of fun looking at this... probably says a lot about me...

[Tony B]

Europa

I see you are fairly new here and I don't know if you are new to forums in general. Usually filling out a profile helps people answer your questions because then they know what type of boat you have, your skill/knowledge level, the climate of your sailing area and your sailing area itself. Different sailing areas and different climates oftentimes require different solutions. Some people don't like filling out a profile and that's understandable also. Just a thought.
Tony B

[gjordan]

Just to add a giggle to this thread! Last night I was looking through Eric Hiscocks book Voyaging Under Sail. When he had Wanderer 3 built (30 foot) it was fitted with a 4 hp petrol engine. After his first circumnavigation he had the engine replaced with an 8 hp petrol engine for his 2nd circumnavigation and said it was a great improvment. He even admitted that 8 hp was a little small by modern (1959) standards. MY MY How things have changed.______Grant.

[donradcliffe]

Originally Posted by europaflyer

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

Nice curves, but they are just plain wrong--see my previous post.

[barnakiel]

Originally Posted by chris07732

barnakiel: Could you post link to data pages you refer to. Checked their site, couldn't find them.

Thx

E.g.

Hallberg-Rassy - Yachts - Aft Cockpit Boats

available for all newer models.

Cheers,
b.

[europaflyer]

Originally Posted by donradcliffe

Back to the original poster, the fuel consumption curves are for liters per hour versus RPM with a prop which is matched to require full rated engine power at the rated RPM. Do not make the mistake of comparing the fuel consumption at less than rated RPM with the MAXIMUM horsepower available versus RPM.

It is based on different assumptions--- the horsepower vs RPM which the fuel consumption curve is based on drops off much faster at lower RPM.

Sorry Don, I'm clearly being a bit slow here but... what?

[barnakiel]

Originally Posted by europaflyer

Wow, by the numbers the HR310 has a range of 1025nm at 2.1 knots and 1000RPM. 20 days motoring, and an utterly clogged engine at the end of it! I'm going to have lots of fun looking at this... probably says a lot about me...

Nowhere does the data say you have to motor 20 days on end!

The data implies that if you motor at X knots, then the total number of hours you can motor on one tank is Y and the total number of miles you can cover at this speed is Z. Somehow, the Z is highest for lowest X. Indeed, some find!

When best economy is equaled with 'max miles per tank' then go slow.

I missed the fun element but than again this says a lot about me ;-).

Cheers,
b.

[donradcliffe]

Originally Posted by europaflyer

Sorry Don, I'm clearly being a bit slow here but... what?

Take a look at http://www.cmdmarine.com/Products/Re...6B/fr90208.pdf

In this case its a different engine but they give you the full story--First the horsepower vs RPM for both the propeller demand curve and maximum available power curve. Second the give you fuel consumption vs RPM for BOTH cases. In the Beta case, you were using the maximum HP curve to get horsepower, but the fuel consumption curve was based on propeller demand. The propeller demand curve is based on RPM cubed, and is much lower than the maximum HP curve at reduced RPM.

[europaflyer]

Originally Posted by barnakiel

Nowhere does the data say you have to motor 20 days on end!

The data implies that if you motor at X knots, then the total number of hours you can motor on one tank is Y and the total number of miles you can cover at this speed is Z.

I realise a long motoring jaunt is not a requirement of owning an HR310... I was merely expressing surprise that a part of the performance envelope of a sailing yacht under power allowed a 1000+ nm range.

Originally Posted by barnakiel

I missed the fun element but than again this says a lot about me ;-).

Saying I was going to have fun was a little introspective sarcasm, I do have a penchant for nerdy number crunching but 'fun' is perhaps an exaggeration.

Originally Posted by barnakiel

Somehow, the Z is highest for lowest X. Indeed, some find!

Sorry, I've got bogged down here, are you now being sarcastic? I've always thought that slow equals better mileage.

Originally Posted by donradcliffe

Take a look at http://www.cmdmarine.com/Products/Re...6B/fr90208.pdf

In this case its a different engine but they give you the full story--First the horsepower vs RPM for both the propeller demand curve and maximum available power curve. Second the give you fuel consumption vs RPM for BOTH cases. In the Beta case, you were using the maximum HP curve to get horsepower, but the fuel consumption curve was based on propeller demand. The propeller demand curve is based on RPM cubed, and is much lower than the maximum HP curve at reduced RPM.

Thanks Don, that's probably one of the most important posts here then, sorry I missed it when you first posted. In other words, my initial figures and conclusions WERE based on a mistaken assumption about the graphs as supplied by Beta - as I suspected. It's some tricky stuff! Great to be on the receiving end of some proper engineering knowledge - cheers. I also found this article which explains how to interpret the specs quite well. http://seagrant.uaf.edu/bookstore/bo...el-engines.pdf

So... the original question of engine size v fuel consumption is actually a fairly simple one. Not too small it needs to be revved hard, not too big that it never really gets revved at all. As for something in between these extremes, there isn't any fuel usage penalty for fitting a moderately larger engine, so why not - more reserve power, more efficient than a smaller, over-revved engine when higher power than normal is needed, and quite possibly a longer service life as well from running at lower RPM. The weight of a modern diesel doesn't hurt the all-important sailing performance, so everyone's a winner.

The Beta 25 will allow me to go at normal speeds at 1800 RPM. This is the RPM used by most larger diesel generators as being the most economical in terms of service life and specific fuel consumption, so I reckon I've got it just about spot on. I found this page - Diesel Engine Overload: Diesel RPM Too Low? - the author reckons that as long as the engine is at full operating temperature, the lower RPM the better, which again supports my using the 25 as opposed to the 20hp unit.

The Beta 25 gives 1hp for every 380lb. Even 'modern' thinking on power usually states 1hp for every 500lb (whatever units you choose); maybe that's now a little outdated and improvements in marine diesels mean that what we've decided here is nearer the mark - part of an inexorable rise in the 'required' power for sailing yachts.

As Tony B says, it's been a fun ride. I think I can say now the question has been thoroughly answered! I really am grateful for everyones help.

[nes]

I realize that the OP has made a decision, but I still felt compelled to add my 2 cents worth here.
I have a 2002 Hunter 326 Full load displacement is 11,301 lbs (from the owners manual). It has a Yanmay 18 HP diesel, with a max RPM of 3400.
The LOA is 30' 10", the LWL is 28' 0" which makes the hull speed 7.09 kts, I have sailed in really good conditions and managed, 7 kts for about a half hour, so I know that this is a sustainable speed for my boat. (GPS and speed probe matched). Freshly painted hull, clean sails tweaked to perfection about 20 kts wind speed etc.
I keep my speed probe clean, which means that it is accurate by comparison with the GPS, so I can see my speed through the water and see if there is a current or not.
My practical real world experience shows that at 3400 RPM, I can get about 5.7 kts, at 3000 RPM, I can get about 5.5 kts. (the extra .2 kts is just not worth the extra 400 rpm).
As for fuel economy, I am only using about .3 - .4 Gallons per hour. Probably I could do something with the prop and get more speed and use slightly more fuel, but I can tell you that IF I were to re-power my boat, I would want enough power to get a heck of a lot closer to my hull speed than I can get now.
I was out on the water 39 days in 2011, and I used 25 gallons of diesel. Several of those days were the ones we all hate, which is I am here and I have to get there TODAY, and the wind is coming from there, or there is no wind.
My fuel tank only holds 25 gallons, and the gauge does not work, so I go by engine hours, and fill up when I get nervous. (I have never had to put more than 15 gallons in at a time, so you can see that I am a bit cautious).
Given a choice I would happily increase my fuel consumption to get that extra 1.5 kts.

[chris07732]

This has been an excellent discussion. Thanks all for participating. Learning about the wealth of fuel consumption data on the HR site was worth the price of admission

Europaflyer: Please come back to CF after you install your BETA 25 and let us know some real world data about your experience.

[S/V Alchemy]

Originally Posted by nes

My fuel tank only holds 25 gallons, and the gauge does not work, so I go by engine hours, and fill up when I get nervous. (I have never had to put more than 15 gallons in at a time, so you can see that I am a bit cautious).
Given a choice I would happily increase my fuel consumption to get that extra 1.5 kts.

In my experience, that last 1.5 kts. is a significant increase. Reaching hull speed under sail (a large, diffuse form of propulsion via the sails) is one thing, because the conversion of wind into useful work is literally in one's hands through the miracle of proper trimming.

But the prop and engine must be matched closely to do that under motor, and the relationship is not linear. Going from five to six knots in a 20 HP 32 footer can raise the hourly fuel requirement from .5 to 1.5 gal/hr or worse, and you are making more heat than thrust. It is the nature of a relatively small prop pushing a displacement vessel, and the tonnes of water that vessel displaces, out of the way.

So you may not get that last 1.5 knots regardless. Better, in my mind, to motorsail (which is my preferred way to get over six knots in light airs without redlining my engine), and to save maximum thrust for situations like crossing bars, fighting currents and full reverse to avoid collisions.

Just my .02, but the quest to motor sailboats at hull speeds is not something I can easily understand. You could modify a truck to run the Indy 500, I suppose, but at some point it's just easier to admit you should've bought a stock car instead and dropped in a different drive train.

[donradcliffe]

Originally Posted by S/V Alchemy

Just my .02, but the quest to motor sailboats at hull speeds is not something I can easily understand. You could modify a truck to run the Indy 500, I suppose, but at some point it's just easier to admit you should've bought a stock car instead and dropped in a different drive train.

Ah, but you have never had to race the herd into Annapolis to secure the last mooring--like I did today. Fortunately, I had 350 horsepower IN RESERVE, so those sailboats had to eat my wake, including the cats