Engine Efficiency

[FlyWright]

I have been tracking different power settings on my 34 Californian with twin Perkins 4.236 engines over the past year or so. I have run 30-40 hours between refuelings at the same power setting to provide a good sampling for each setting.

I seem to have found the sweet spot I am comfortable with for now, a good compromise between efficiency, speed and noise. If I slow to 7 or 6.5 kts, I'll likely save more fuel and even eclipse 3 mpg but the speed feels too slow for me to endure for a 30 hour test period.

Here's what I have found on my 2800 RPM (WOT) engines:

2350 RPM (84%), 8.0 kts, 3.70 gph, 2.16 mpg
2100 RPM (75%), 7.6 kts, 2.79 gph, 2.72 mpg
2000 RPM (71%), 7.4 kts, 2.66 gph, 2.78 mpg

I'm sure these numbers would pale in comparison to single engine boats, especially the new technology diesels, but I'm happy with this level of efficiency from a pair of old school 1977 85 hp Perkins naturals.

 

[Dswizzler]

I'd be thrilled to see anything even near those numbers

 

[Nomad Willy]

FlyWright,
You wrote;

"2350 RPM (84%), 8.0 kts, 3.70 gph, 2.16 mpg
2100 RPM (75%), 7.6 kts, 2.79 gph, 2.72 mpg
2000 RPM (71%), 7.4 kts, 2.66 gph, 2.78 mpg"

Why did you mention 84,75 and 71%. And what % of what is it?
Also you mention that your Perkies are a 2800rpm engine but give no WOT rpm information.

I feel a little better about 6.15 knots than Marin feels about 8 and I've got about 1000 hrs of running time to get used to going that slow (slower than you) and am only 95% used to it. If you can afford to burn more fuel I'd DO IT.

 

[FlyWright]

My engines WOT is 2800 RPM and the engines are rated at 2800 RPM. The percentages shown are the % of WOT RPM. (probably unnecessary, but it represents a meaningful value in my way of thinking of power settings.)

 

[markpierce]

I'd be thrilled to see anything even near those numbers

Mark, we have to consider the differences in boat displacements. ... FlyWright's 7.4-knot GPM is better than the single-engined, 80-HP, 14-ton Coot's 7.3-knot-hull-speed consumption rate, but fuel consumption drops about 40 percent if speed is lowered to 6.3.

Wouldn't a ton-per-mile fuel-consumption rate be a more meaningful measure of efficiency?

 

[Nomad Willy]

The percentage of load that is also the percentage of the maximum amount of fuel that can be burned (by that engine) would be (to my way of thinking) a much more relative bit of information.

Interestingly at 7.4 knots your MPG is about exactly half of what Willy does. Not bad for your hull type and propulsion system.

Mark? "ton per mile"?

 

[FlyWright]

The percentage of load that is also the percentage of the maximum amount of fuel that can be burned (by that engine) would be (to my way of thinking) a much more relative bit of information.

Interestingly at 7.4 knots your MPG is about exactly half of what Willy does. Not bad for your hull type and propulsion system.

Mark? "ton per mile"?

Eric, maybe I should explore the 6.0 - 6.5 Kt realm sometime in the future. I'm still getting used to this retirement thing, but once I become comfortable with life without schedules, maybe it'll be a good fit. Right now, 7.5 n miles per hr, after life at 7.5 n miles per minute, feels good.

I bet your efficiency at 6.15 kt is tough to beat. What's your gph?

 

[Nomad Willy]

Fly,
One gallon an hour at 6.15 knots. It's really good compared to most here on the forum but Willy could easily be almost twice as efficient. For example you don't put 2 tons of ballast in a 30' boat if your'e looking for efficiency. After all we most often roughly calculate power required for a disp boat by how much power per ton of disp. A very light boat like the Camano 41 is almost as efficient as a 40 Willard. If the Willard 30 was optimized for efficency it would have a 24hp engine, no ballast, a bigger prop and a deep reduction gear, a stern that isn't so full aft, a sharp cutwater, a smaller rudder that is faired like a FG and foam sailboat rudder and a narrower hull at the water line. Then the Willard would burn about .6 gph and get about 10 mpg.

Our resident NA here, TAD Roberts has on his design board a boat (Yellow Cedar) that is 38' long w a weight and beam much like Willy that goes faster and on 28hp. Willy has 40.

So Willy is economical but could be far more economical. And my guess is just a guess. The boat I described may burn LESS than half the fuel Willy burns.

 

[FlyWright]

Wow! That's impressively low fuel consumption.

Even with fuel at $5 per gal, improvements to that level come with diminishing returns. If you improve efficiencies a whopping 40%, you're still only saving $2/hr. The cost of the efficiencies can greatly outweigh the benefits.

 

[FlyWright]

Eric, I don't know if this helps in the discussion, but here's the performance curves from my engines.

 

[Conrad]

I have been tracking different power settings on my 34 Californian with twin Perkins 4.236 engines over the past year or so. I have run 30-40 hours between refuelings at the same power setting to provide a good sampling for each setting.

I seem to have found the sweet spot I am comfortable with for now, a good compromise between efficiency, speed and noise. If I slow to 7 or 6.5 kts, I'll likely save more fuel and even eclipse 3 mpg but the speed feels too slow for me to endure for a 30 hour test period.

Here's what I have found on my 2800 RPM (WOT) engines:

2350 RPM (84%), 8.0 kts, 3.70 gph, 2.16 mpg
2100 RPM (75%), 7.6 kts, 2.79 gph, 2.72 mpg
2000 RPM (71%), 7.4 kts, 2.66 gph, 2.78 mpg

I'm sure these numbers would pale in comparison to single engine boats, especially the new technology diesels, but I'm happy with this level of efficiency from a pair of old school 1977 85 hp Perkins naturals.

I would say that those are very nice numbers.

Our 1981 era 32' Ennos Sapphire 11,600 # (Single Volvo TAMD40A) gives us a consistent 10 litres/hr at 7.5 knots, which we are happy with. Having twins on a slightly larger vessel with similar performance figures is great to see.

I can't give you exact HP ratings on our Volvo as anything I've seen published gives a range of 130 -165 HP, so it's somewhere below your total HP.

 

[Tad Roberts]

Wouldn't a ton-per-mile fuel-consumption rate be a more meaningful measure of efficiency?

Yes it would, Naval Architects refer to this as Transport Efficiency or Et.

TransportEfficiency takes into account not just the power required (in effect MPG), but also the weight (displacement in the case of a boat) being moved, and the time required to move that weight (speed).



One definition runs thus.....

Et = W * V / 326 * P

W = Weight in pounds (displacement)
V = Speed in knots
P = total shaft Horsepower

Plot this against Volume Froude Number,

FNv = v / (g * disp.^.333)^.5

v is speed in feet per second. (knots * 1.6889)
g is acceleration due to gravity, (32.2)
disp. Is displacement, this time in cubic feet.


[FONT=&quot]Displacement hulls operate at a FNv of 1.3 or less, semi-displacement hulls at FNv of from 1.0 to 3.0, and planning hulls at FNv of 2.3 or more.[FONT=&quot]

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[Codger2]

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As much as I would love to know what all this means, I'm not going to understand it from this chart. Any chance you can boil it all down for the common sailor?

 

[Nomad Willy]

I think what he's say'in Walt is that if I took the ballast out of Willy it would be less efficient. Because I'd be pack'in less weight I would'nt be getting as much work done. So when you step aboard my boat it's Et goes up.

Fly Wright,
Yes. Notice that at the high end with a given increase in rpm you increase propeller loading more than you increase power. As you back off throttle and rpm the decrease in propeller loading is more profound than decrease in power. I think that if you coupled that engine to a faster propeller (smaller as a direct drive) the unloading would be more gradual. So w a deep reduction and big prop if you dropped 20% in rpm you'd loose more load than if you dropped 20% rpm and had a direct drive. So the bottom curve would be straighter and flatter w less reduction. I'm not sure if got the right curve but it looks like that.

I'm surprised your engine's continuous ceiling is 400rpm down. Must be part of the "high" speed rating.

 

[Tad Roberts]

As much as I would love to know what all this means, I'm not going to understand it from this chart. Any chance you can boil it all down for the common sailor?

I can try, I may be juggling too many things today.......

If we look at Fly's original numbers he mentions 7.4 knots at 2.66gph...

2.66gph is about 52 horsepower/hour....

Published displacement for a California 34 is 18,000 pounds.

So Et = 18000 * 7.4 / 326 * 52

Et = 7.85

Volume Froude number is a dimensionless way of comparing different hulls using speed and displacement.

Fv = .86

So looking at the graph I posted above, look along the bottom to .86 and go up to 7.85 and you find Fly' operating near the top of the Displacement range, but not at the edge of efficiency.......Which makes sense given the non-extreme nature of his vessel. It's also likely that the real displacement is more than the published number, which would cause the Et to improve.

For more see.....http://www.tadroberts.ca/about/pdf/power-boat-design-form-and-function.pdf

 

[markpierce]

Fly,
One gallon an hour at 6.15 knots. It's really good compared to most here on the forum but Willy could easily be almost twice as efficient. For example you don't put 2 tons of ballast in a 30' boat if your'e looking for efficiency.

Exactly. Why have two tons of ballast unless one likes a snap-back roll? Or is your Willard looking for substantial sails to become a motorsailer? Hey Eric, isn't your Willard basically a motorsailer without sails?

 

[markpierce]

Mark? "ton per mile"?

Yes! ... Hoping our boats aren't hauling extra weight unless adding to their strength, capability, comfort, and accommodation.

 

[Nomad Willy]

Mark ... heaven forbid no. Willy is not a sailboat at all. There is a version of the 30' Willard "Vega" line of pleasure boats ythat is actually a sailboat. It's called the 8 Ton. It's got the same basic hull except that the keel extends down about 18" below where mine ends. So it draws 5' of water. It has a full length rudder that's almost twice as big as mine. And it has the same propulsion system including the same 18" prop. So even the 8 Ton is no thoroughbred sailboat but several people have traveled extensively on the ocean w their 8 Ton Willard's. But I would classify the 8 Ton as a motorsailer. Unlike most motorsailers the 8 Ton has a traditional low cabin common to most sailboats. Motorsailers usually have raised cabins for visibility in poor weather.

Then there is the Willard Horizon like our TF Doug had that has exactly the same hull as the Nomad (Willy is a Nomad), the Voyager the Horizon and the Searcher. All 5 are of the 30' Willards have exactly the same hull except the 8 Ton. The only one that will sail fairly well against the wind is the 8 Ton. Horizon's sail less well than most motorsailers ... you could say they are an auxiliary whereas the auxiliary power is it's sails and not it's engine.

I suspect there are some subtle sailboat elements of the Willard's designed by Rod Swift (30 and 40') and I suspect Mr Swift was basically a sailboat designer but that is a suspicion of mine only. Notice my SS vinyl coated wire "rails" to keep folks from falling overboard and that they don't go all the way fwd .... again like sailboats.

So you have a good eye Mark. There are elements of sail in the Willards but Willy is definitely not a sailboat.


Mark, Here is a 22'OB boat w a hull very similar to your Coot.
http://www.atkinboatplans.com/Oar/JogAlong.html

 

[hypersloth]

Next you need to run (I'm to lazy to do the math) only one engine and see if the numbers work for or against mileage. I typically shut one engine down and the boat speed, tach rpm hardly dip. Plus the haromics stop. However don't know if extra load on engine kills efficiency.

 

[Delfin]

I can try, I may be juggling too many things today.......

If we look at Fly's original numbers he mentions 7.4 knots at 2.66gph...

2.66gph is about 52 horsepower/hour....

Published displacement for a California 34 is 18,000 pounds.

So Et = 18000 * 7.4 / 326 * 52

Et = 7.85

Volume Froude number is a dimensionless way of comparing different hulls using speed and displacement.

Fv = .86

So looking at the graph I posted above, look along the bottom to .86 and go up to 7.85 and you find Fly' operating near the top of the Displacement range, but not at the edge of efficiency.......Which makes sense given the non-extreme nature of his vessel. It's also likely that the real displacement is more than the published number, which would cause the Et to improve.

For more see.....http://www.tadroberts.ca/about/pdf/power-boat-design-form-and-function.pdf

Large container ships must have Et numbers that are quite high then.

I may be missing something, but based on the CAT 3306 Performance Data chart for prop demand hp and fuel consumption at 1250 rpm(verified by the fuel consumption gauge), plus our speed , we move 130,000 pounds through the water at 7.2 knots using 50 hp, which I guess gives Delfin an Et of 57.4.

Have I made a mistake?

 

[Delfin]

Exactly. Why have two tons of ballast unless one likes a snap-back roll? Or is your Willard looking for substantial sails to become a motorsailer? Hey Eric, isn't your Willard basically a motorsailer without sails?

Don't know for sure, but I believe that removing ballast would increase roll rate, not decrease it. Adding weight above the W/L certainly decreases it, as any sailor who has been dismasted will attest, or piano player who uses a metronome.

 

[Nomad Willy]

hypersloth,
Let me guess. The sloth is because you have a trawler and the hyper is for the twin engines. Your'e a slow guy but no slug. Well I'm a slug w one engine and a speed of 6 knots.

Re your question I think there is no doubt running single on a twin saves fuel. But I wonder why there's so many asking about it. It seems to me there's lots of guys w twins that should have singles. Perhaps it's just that there are lots of guys interested in safety re the perils of loosing power.

 

[sunchaser]

Best not to confuse gph with engine efficiency. On gas and diesel engines the term Brake Specific Fuel Consumption or bsfc is the more relevant tool for measurement of engine efficiency. In general that will require running an engine at somewhere in the 65 to 80% range of power draw. Most of us can't do this since our engines are way too big for the vessel. It would appear Eric, Daddyo and Mark are closer to the best bsfc targets than the rest of us.

I'm the grinch on all this BS anyway, bragging rights on ultra low gph sounds good but has little to do with my fun on the water. If you really want to save on fuel costs cruise less or sell your boat! Obviously many do this with lots of boats for sale averaging less than 100 hours per year.

 

[Nomad Willy]

"Originally Posted by markpierce
Exactly. Why have two tons of ballast unless one likes a snap-back roll? Or is your Willard looking for substantial sails to become a motorsailer? Hey Eric, isn't your Willard basically a motorsailer without sails?"

This is an old post isn't it? I must have left this unaddressed. Sorry. Rod Swift designed the boat and I think he was in some way into sailboats. Whether or not that has anything to do w the ballast I don't know. But I think the main objective is to make the boat more seaworthy and to give it a slow pleasant feel in most situations. I've talked w Willard people that place great importance on a trawler having lots of mass below the WL and hull to go along w it. This mass and feel makes the essence of "trawler" they feel but very few trawlers share this feature. By previous boat was an Albin 25 Swedish boat that was very light (2 tons) and had a terrible "snap roll". One of the main reasons I left her. The solid feel of a heavy boat can't be matched by anything else. And a low CG is always good.

Fishermen have put rocks in their bilges for centuries to get a seaworthy boat they could live w on the rolling sea.


I see your post above Tom and like all you said very much. But I run my engine faster than ideal for best efficiency. I burn 1gph and aren't motivated to make any sacrifice to burn .975gph.

 

[Codger2]

I'm the grinch on all this BS anyway, bragging rights on ultra low gph sounds good but has little to do with my fun on the water.

Amen...

 

[psneeld]

...and some retire early and pinch pennies spent on fuel because the average number of hours underway dwarfs the vacation boaters hours ...
plus we have put thousands of miles and hundreds of ports under our keels before something medical or otherwise happens and the dream remains a dream...

 

[markpierce]

At my normal cruising speed of 6.3 knots at 1800 RPM, I'm getting 85% of the JD's 80 horsepower consuming about 1.7 gallons an hour.

 

[Craig Schreck]

Its nice to see that the OPs WOT rpm are on the money, proving that the propeller sizing and gearing are in spec, He is way ahead of the game compared to most

 

[FlyWright]

I'm the grinch on all this BS anyway, bragging rights on ultra low gph sounds good but has little to do with my fun on the water. If you really want to save on fuel costs cruise less or sell your boat! Obviously many do this with lots of boats for sale averaging less than 100 hours per year.

What it boils down to is that I'm interested in three things:
1. Maximizing the use and enjoyment of my boat
2. Maintaining safety on my boat
3. Maximizing the efficiency of my boat

Sorry if discussions of efficiency don't mix with your definition of boating, but it's part of our lexicon and deserves a place at the table. The more safely and efficient I can operate the boat, the more time I can enjoy operating it. It's all part of the equation of boating.

 

[Codger2]

3. Maximizing the efficiency of my boat

Sorry, Al, but I have to disagree with you on this one. Simply taking your foot off the throttle doesn't = efficiency. I had a gasser that was more efficient at 18 knots than it was at 8 knots. (I measure efficiency in mpg.)