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Old 01-14-2013, 05:49 PM   #1
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Engine Efficiency

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.
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Old 01-14-2013, 06:44 PM   #2
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I'd be thrilled to see anything even near those numbers
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Old 01-14-2013, 07:55 PM   #3
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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.
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Old 01-14-2013, 08:06 PM   #4
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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.)
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Old 01-14-2013, 08:22 PM   #5
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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?
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Old 01-14-2013, 10:27 PM   #6
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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"?
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Old 01-14-2013, 10:54 PM   #7
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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?
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Old 01-14-2013, 11:46 PM   #8
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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.
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Old 01-15-2013, 12:04 AM   #9
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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.
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Old 01-15-2013, 12:40 AM   #10
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Eric, I don't know if this helps in the discussion, but here's the performance curves from my engines.

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Old 01-15-2013, 01:16 AM   #11
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Quote:
Originally Posted by FlyWright View Post
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.
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Old 01-15-2013, 03:07 PM   #12
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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.


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.


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Old 01-15-2013, 03:47 PM   #13
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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?
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Old 01-15-2013, 04:00 PM   #14
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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.
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Old 01-15-2013, 04:48 PM   #15
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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?
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/p...d-function.pdf
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Old 01-15-2013, 07:09 PM   #16
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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?
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Old 01-15-2013, 07:15 PM   #17
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Mark? "ton per mile"?
Yes! ... Hoping our boats aren't hauling extra weight unless adding to their strength, capability, comfort, and accommodation.
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Old 01-15-2013, 10:14 PM   #18
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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
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Old 01-22-2013, 08:52 AM   #19
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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.
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Old 01-22-2013, 11:04 AM   #20
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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/p...d-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?
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