Fuel consumption: sail/motor. are you serious?!

[Adelie]

Originally Posted by Lee Jerry

A. "Fuel consumption is going to depend on boat length, hull design, displacement and speed."
This is correct, but even more simply, just displacement and speed (design or cruising) can tell you the fuel consumption within +/- 20% (at least on the low end, there's almost no limit to ways to make things less efficient). The displacement could be broken down a little more to payload and range if desired. Everything past that is just details.

However, I'm going to have to challenge / disagree with you on several other of these statements.

B. "...so displacement vessels are designed for more living space at the cost of efficiency."
No, this goes to the displacement / payload above. I suppose this depends on how you define efficiency, but IMHO deciding to carry more stuff isn't less efficient. (Maybe wasteful, but not less efficient.) For example, a school bus may be less "efficient" than a sedan in terms of fuel economy, but if you have to get 30 kids to the same place at the same time it's "more efficient" than 30 sedans. Efficiency is better used when comparing two things doing the same thing.

C. "These hull shapes are still much lower drag than planing hulls."
That depends, as you've indicated previously, entirely on the speed - displacement has lower drag at "low" speeds (~<hull speed) while planing has lower drag at "high" speeds (~>hull speed).

D. "Because sail power is generally significantly less than engine power their hulls are designed..."
Every sailboat I have owned, from 25ft sportboat (similar to Melges 24) to 38ft racer/cruiser to 52 ft heavy cruiser, and probably every boat I have ever sailed on (many, many more within that same range), has achieved its max speed under sail. This is without considering surfing. Drag is a function of speed, so that means more power (thrust) is generated by the sail plan than the engine.

E. "WHO’s means that sailboats under motor typically get significantly better mph than most displacement motor vessels."
Even though it's probably something simple and obvious, I can't figure out what you mean by "WHO." Also, did you mean "mpg" or "mph?" Regardless, I would say that a heavy cruising sailboat (Oyster, HR, Passport, etc.) have both very similar speed (mph) and fuel economy (mpg) to a trawler - i.e. two boats designed for a similar purpose.

A. You can't ignore length. For example:
A 41' displacement power boat that weighs 43,000lb is going to use about 95hp to go 8.3kt.
You won't be able to get a 30' displacement boat that weighs 30,000 to go even 7.5kt no matter how much power it has. Maybe if it's a planing hull with 1,000 HP of turbine power. Even that would be iffy.

B. Yeah a loaded school bus is more efficient per passenger than a similar number of cars. But let's be real, most vessels are crewed by a couple, whether it's a 40' sailboat or a 40' powerboat. So for a given length boat you are getting more living volume and room for toys in a power vessel than in a monohull sailboat for the same couple. It will cost more in fuel to push the power boat the same distance at the same normal cruising speed (around SL-1.0 or so) than to push the sailboat. You aren't going to get better per passenger fuel economy, but you will be able to carry more toys and other amenities.

C. If you feel that you need to make planing speeds or near planing speeds, a displacement hull will never work and no comparison is possible. Also the fuel consumption becomes almost astronomical. If you have the money for the fuel and want to go fast, fine. If you don't then you need to decide whether it is more important to you to travel shorter distances at high speeds or go longer distances at slower speeds.
Let's assume you are content to go displacement speeds, are similar length and displacement power and sailboats going to use similar amounts of power?
The Nordhavn 41 has a 40' LWL, is 43,000lb and will hit 8.3kt using 93hp. 170hp is max engine output.
The Formosa 44 has a 36' LWL, 8.0kt hull speed, is 36,000lb and came with an 80hp engine installed. In flat water it was probably using about 50-60hp to go 8.0kt.
The extra 8,000lb of the Nordhavn is not going to require that 33-43hp difference, at least half of that is hull shape. That 8,000lb weight difference is another part of sailboats optimizing for low drag.
https://nordhavn.com/models/n41/
https://sailboatdata.com/sailboat/formosa-44/

D. Yeah, every sailboat I've been on has been under sail when it hit its all time op speed. That's because during storms there is a LOT of power available from the wind. But most of the time, there really isn't that much power available, most sailing is done when winds are in the 5-15kt range. Also sailboats tend to have a lot less power installed per ton than power vessels to optimize for the lower weight needed to sail well in lighter winds.

E. WHO's, yeah I really like not catching autocorrect failures in my writing (sarcasm). Should have read Which.
MPH should have been MPG, that was not autocorrect, that was me being stupid by posting near midnight and not proofreading.
The Nordhavn 41 gets about 3nm/gal at 7.0kt. per above link
Robert Beebe's original Passagemaker 50 got about 3nm/gal at 7.5kt per his book
Willard 44 trawler - 5.3mpg @ 7.5kt https://www.cruisersforum.com/forums...ml#post3787788
x442 gets about 7.3nm/gal at 7kt https://www.cruisersforum.com/forums...ml#post3787685
Bavaria 42 - 7.6mpg @ 7.0kt https://www.cruisersforum.com/forums...ml#post3787821
Beneteau 43 - 11.9mpg @5.5kt https://www.cruisersforum.com/forums...ml#post3790019

[MHTinkler]

2011 Gemini 105Mc sailing catamaran with 27 HP Westerbeke 3 cylinder diesel. At cruising speed of 6 knots +/- use 0.6 US Gals per hour (2800 rpm)

[ranger58sb]

OP hasn't posted again here, no indication he's read the many responses, no indication he really cares what the answers may have been, no clue if he recognizes (or cares about) compromises that attend various capabilities.

-Chris

[Adelie]

Maybe not but 30min after a thread is posted it’s everybody’s thread and whether the OP continues to participate is immaterial.

The only exception is classified and crew ads.

[gonesail]

Originally Posted by mvweebles

Somehow you're superior because you sail. I was pretty sure that was the base of the oddball comparisons. Arrogance reminds me of Harley riders when I rode motorcycles.

of course the sailor is superior.

[ranger58sb]

Originally Posted by Adelie

Maybe not but 30min after a thread is posted it’s everybody’s thread and whether the OP continues to participate is immaterial.

Yep, good point. Lots of useful input here, even if OP ignores it all.

-Chris

[Lee Jerry]

Originally Posted by Adelie

A. You can't ignore length. For example:
A 41' displacement power boat that weighs 43,000lb is going to use about 95hp to go 8.3kt.
You won't be able to get a 30' displacement boat that weighs 30,000 to go even 7.5kt no matter how much power it has. Maybe if it's a planing hull with 1,000 HP of turbine power. Even that would be iffy.

B. Yeah a loaded school bus is more efficient per passenger than a similar number of cars. But let's be real, most vessels are crewed by a couple, whether it's a 40' sailboat or a 40' powerboat. So for a given length boat you are getting more living volume and room for toys in a power vessel than in a monohull sailboat for the same couple. It will cost more in fuel to push the power boat the same distance at the same normal cruising speed (around SL-1.0 or so) than to push the sailboat. You aren't going to get better per passenger fuel economy, but you will be able to carry more toys and other amenities.

C. If you feel that you need to make planing speeds or near planing speeds, a displacement hull will never work and no comparison is possible. Also the fuel consumption becomes almost astronomical. If you have the money for the fuel and want to go fast, fine. If you don't then you need to decide whether it is more important to you to travel shorter distances at high speeds or go longer distances at slower speeds.
Let's assume you are content to go displacement speeds, are similar length and displacement power and sailboats going to use similar amounts of power?
The Nordhavn 41 has a 40' LWL, is 43,000lb and will hit 8.3kt using 93hp. 170hp is max engine output.
The Formosa 44 has a 36' LWL, 8.0kt hull speed, is 36,000lb and came with an 80hp engine installed. In flat water it was probably using about 50-60hp to go 8.0kt.
The extra 8,000lb of the Nordhavn is not going to require that 33-43hp difference, at least half of that is hull shape. That 8,000lb weight difference is another part of sailboats optimizing for low drag.
https://nordhavn.com/models/n41/
https://sailboatdata.com/sailboat/formosa-44/

D. Yeah, every sailboat I've been on has been under sail when it hit its all time op speed. That's because during storms there is a LOT of power available from the wind. But most of the time, there really isn't that much power available, most sailing is done when winds are in the 5-15kt range. Also sailboats tend to have a lot less power installed per ton than power vessels to optimize for the lower weight needed to sail well in lighter winds.

E. WHO's, yeah I really like not catching autocorrect failures in my writing (sarcasm). Should have read Which.
MPH should have been MPG, that was not autocorrect, that was me being stupid by posting near midnight and not proofreading.
The Nordhavn 41 gets about 3nm/gal at 7.0kt. per above link
Robert Beebe's original Passagemaker 50 got about 3nm/gal at 7.5kt per his book
Willard 44 trawler - 5.3mpg @ 7.5kt https://www.cruisersforum.com/forums...ml#post3787788
x442 gets about 7.3nm/gal at 7kt https://www.cruisersforum.com/forums...ml#post3787685
Bavaria 42 - 7.6mpg @ 7.0kt https://www.cruisersforum.com/forums...ml#post3787821
Beneteau 43 - 11.9mpg @5.5kt https://www.cruisersforum.com/forums...ml#post3790019

A – Did you see where I said “at least on the low end, there's almost no limit to ways to make things less efficient?” Your (somewhat ridiculous) example of 30,000 lb 30 ft’er doing 7.5 kt is the latter part. Let me re-phrase: Based on speed and displacement (or speed, range and payload) you can determine with reasonable accuracy what the minimum possible fuel consumption (and corresponding power requirement) is (are). You can obviously make design decisions to make them worse. For example, you may make changes to improve seakeeping, or perhaps to reduce draft, or simplify construction with chines. But you will be (or should be) doing this with the knowledge that they will increase power / fuel. This applies to any boat type (or plane or train or automobile) or size.


B – It’s not just about passengers, they simply represent the payload in the bus example. It’s also about comfort, lifestyle, etc. (i.e. more payload). So, while not everything needs to be exactly identical (like the size of accommodations, for example), to be comparing like to like (apples to apples), you have to put the same toys and amenities on the sailboat that are on the powerboat, i.e. the same payload. This would increase its fuel consumption in line with the powerboat. Conversely, you could remove them from the powerboat design and make it lighter, smaller etc. reducing its consumption. You do that, you get the same / similar efficiency between the two hulls. You let one have more stuff, then you know the efficiency will decrease simply based on the increase in displacement.


C – I assume you got the Nordhavn data from the sea trial report. If so, you don’t have them quite right.
93 hp gave an average speed of ~8.5 kt (they properly tested with runs in both directions to offset wind / sea / current effects), not 8.3 kt (yellow)
But there is a data point nearly right at 8 kt (7.975 kt, blue), so why not use that to match the 8 kt of the sailboat? That is at 77 hp. But the sea trial was done in Beaufort 3 (2-4 ft seas, pink). So what’s that add? Maybe 5%?

So take out the SS, and the extra 8000 lb (sic) of displacement, and then you’re pretty close to equal weight and speed (inputs) and getting the resulting equal power / fuel consumption (output) – as predicted when comparing apples.



You keep saying sailboats (regular old boats we use, not ACup etc.) are optimized for low drag. What form does this optimization take? (I have my thoughts but want to hear yours.) What evidence do you have to support that? What hull optimization do you think was done on the design of the Formosa?

Because we know what optimization was done on the Nordhavn 41. At the link you provided, you can scroll down to and download the "Hull Optimization Study" paper that tells you what was done. Spoiler alert (to above), this is unusual and way more than has been done for most boats. So don’t say the N41 is not designed for low drag.


D – You don’t need a storm to reach sailing speeds above powering speed (assumed to be hull speed). For most boats, 18-20 kt TWS will do it. Tight reach, say 70-90 deg TWA with white sails. Even easier with a spinnaker, at 120-150 deg TWA. Point is: sails provide more power than the AUXILARY engine.


E – I confirm the Nordhavn data in green above.

I didn’t read his book, but according to this article about Beebe, fuel capacity of 1200 gal for a range of 3200 nm gives 2.7 nm/gal. So a little under 3. No speed given though. 27 ton displacement (I think loaded), including 5000 lb of external ballast.

I didn’t look at the other links, but did check the displacement for the X442. It’s 21,300 lb per SBD. So less than half of the Nordhavn. And the Bene speed was only 5.5 kt.

Most of these do not seem to be particularly close to like to like with the Nordhavn, so I don't see your point.

[Adelie]

Originally Posted by Lee Jerry

A – Did you see where I said “at least on the low end, there's almost no limit to ways to make things less efficient?” Your (somewhat ridiculous) example of 30,000 lb 30 ft’er doing 7.5 kt is the latter part. Let me re-phrase: Based on speed and displacement (or speed, range and payload) you can determine with reasonable accuracy what the minimum possible fuel consumption (and corresponding power requirement) is (are). You can obviously make design decisions to make them worse. For example, you may make changes to improve seakeeping, or perhaps to reduce draft, or simplify construction with chines. But you will be (or should be) doing this with the knowledge that they will increase power / fuel. This applies to any boat type (or plane or train or automobile) or size.





B – It’s not just about passengers, they simply represent the payload in the bus example. It’s also about comfort, lifestyle, etc. (i.e. more payload). So, while not everything needs to be exactly identical (like the size of accommodations, for example), to be comparing like to like (apples to apples), you have to put the same toys and amenities on the sailboat that are on the powerboat, i.e. the same payload. This would increase its fuel consumption in line with the powerboat. Conversely, you could remove them from the powerboat design and make it lighter, smaller etc. reducing its consumption. You do that, you get the same / similar efficiency between the two hulls. You let one have more stuff, then you know the efficiency will decrease simply based on the increase in displacement.





C – I assume you got the Nordhavn data from the sea trial report. If so, you don’t have them quite right.

93 hp gave an average speed of ~8.5 kt (they properly tested with runs in both directions to offset wind / sea / current effects), not 8.3 kt (yellow)

But there is a data point nearly right at 8 kt (7.975 kt, blue), so why not use that to match the 8 kt of the sailboat? That is at 77 hp. But the sea trial was done in Beaufort 3 (2-4 ft seas, pink). So what’s that add? Maybe 5%?



So take out the SS, and the extra 8000 lb (sic) of displacement, and then you’re pretty close to equal weight and speed (inputs) and getting the resulting equal power / fuel consumption (output) – as predicted when comparing apples.



Attachment 276909



You keep saying sailboats (regular old boats we use, not ACup etc.) are optimized for low drag. What form does this optimization take? (I have my thoughts but want to hear yours.) What evidence do you have to support that? What hull optimization do you think was done on the design of the Formosa?



Because we know what optimization was done on the Nordhavn 41. At the link you provided, you can scroll down to and download the "Hull Optimization Study" paper that tells you what was done. Spoiler alert (to above), this is unusual and way more than has been done for most boats. So don’t say the N41 is not designed for low drag.





D – You don’t need a storm to reach sailing speeds above powering speed (assumed to be hull speed). For most boats, 18-20 kt TWS will do it. Tight reach, say 70-90 deg TWA with white sails. Even easier with a spinnaker, at 120-150 deg TWA. Point is: sails provide more power than the AUXILARY engine.





E – I confirm the Nordhavn data in green above.



I didn’t read his book, but according to this article about Beebe, fuel capacity of 1200 gal for a range of 3200 nm gives 2.7 nm/gal. So a little under 3. No speed given though. 27 ton displacement (I think loaded), including 5000 lb of external ballast.



I didn’t look at the other links, but did check the displacement for the X442. It’s 21,300 lb per SBD. So less than half of the Nordhavn. And the Bene speed was only 5.5 kt.



Most of these do not seem to be particularly close to like to like with the Nordhavn, so I don't see your point.

A. My “ridiculous” example of a 30’ boat not being able to get to 7.5kt where a longer heavier one can refutes your prior claim that length is not important. I used the extreme example.

B. You brought up the bus analogy indicating that a larger vessel could have a better per passenger mpg and I pointed out that most cruisers are couples so there really isn’t better per passenger mpg on that larger heavier boat just more space for toys & amenities and living volume.

C. More posting too late at night. I screwed up the numbers. My point was that for similar sized vessels power boats install a lot more power on their vessels to achieve the same motoring speeds.
I am still looking for better vessels to compare. Will come in a later post.
Yes they optimized the Nordhavn to be a low drag trawler. That doesn’t mean they optimized it to be the lowest drag possible.
Trawlers are meant to have significant internal volume for their length. To achieve this they need to have higher prismatic coefficients. Beebe discusses this in his book. More efficient power vessels use a lower prismatic coefficient. As do sailboats.
Length to beam also affects drag. A higher ratio has lower stage. Multihulls and military vessels take this to an extreme. Because they tend not to be ballasted power vessels tend to be wider than monohull sailboats for their length using form stability to make up for lack of ballast. More on this in a future post.

D. Sails don’t provided more power than engines in all conditions, only in heavier conditions. Most sailing is in lighter conditions so sailboats are optimized for the conditions the mostly sail in. Some areas of the world consistently have higher winds and boats there tend to have features that reflect this, more ballast, higher ballast ratios and shorter masts.

E. Except for the Formosa I used those sailboat examples because they came from this thread and people could check what I wrote.
I have the Beebe book and looked at it last night. His “Passagemaker” had 1200gal and a 3200nm range with a 400nm reserve so really 3600nm for 3mpg. I believe it was at 8kt. I’ll check and include in a later post.
I found a bunch of better comparison power vessels in the Beebe book and will discuss in the future post.

[boat driver]

Originally Posted by RedneckRedcoat

Not even the engine manufactures go off gallons per mile. It’s gallons per hour at engine revs. You could be doing 3500 revs and go 1 mile in 4hrs or do 1500 rev and travel 5 miles in 1 hour, one would use far less diesel than the other.

We are not replacing sails or lines every year but on average they last about 10 years. Even at $20k that’s only $2000 over 10 years. We have traveled 1000 miles since December. If I sail 1000 miles in a year that’s $2 a mile almost half the cost and will be even cheaper if I clock up more miles were as in a the example above the Trawler the cost per mile is going to go up by $3.75 for every mile traveled

Sailing is way cheaper and economically

really? life is what you want it to be- and if rags for sails- then yes, I can argue below 6gpm to less. But find me a sail to do 10K mile plus travels annually and sails have a finite life. My discussion was supporting an understanding of the anticipated demand and then refining. As far as sails, find me the few 60; class vessels that cruise to explore even 5K per year and I will show the sail/power curve is very close. Its just simple physics- x energy to move y object- and length of waterline does create efficiency- anyone want to discuss what a 800 bulker ship burns a mile?
And for the RPM discussion- who cares what RPM burn is- its how dam far it gets you. When you buy a car- do you look at the rpm or the miles per gallon?

[Lee Jerry]

Originally Posted by Adelie

A. My “ridiculous” example of a 30’ boat not being able to get to 7.5kt where a longer heavier one can refutes your prior claim that length is not important. I used the extreme example.

B. You brought up the bus analogy indicating that a larger vessel could have a better per passenger mpg and I pointed out that most cruisers are couples so there really isn’t better per passenger mpg on that larger heavier boat just more space for toys & amenities and living volume.

C. More posting too late at night. I screwed up the numbers. My point was that for similar sized vessels power boats install a lot more power on their vessels to achieve the same motoring speeds.
I am still looking for better vessels to compare. Will come in a later post.
Yes they optimized the Nordhavn to be a low drag trawler. That doesn’t mean they optimized it to be the lowest drag possible.
Trawlers are meant to have significant internal volume for their length. To achieve this they need to have higher prismatic coefficients. Beebe discusses this in his book. More efficient power vessels use a lower prismatic coefficient. As do sailboats.
Length to beam also affects drag. A higher ratio has lower stage. Multihulls and military vessels take this to an extreme. Because they tend not to be ballasted power vessels tend to be wider than monohull sailboats for their length using form stability to make up for lack of ballast. More on this in a future post.

D. Sails don’t provided more power than engines in all conditions, only in heavier conditions. Most sailing is in lighter conditions so sailboats are optimized for the conditions the mostly sail in. Some areas of the world consistently have higher winds and boats there tend to have features that reflect this, more ballast, higher ballast ratios and shorter masts.

E. Except for the Formosa I used those sailboat examples because they came from this thread and people could check what I wrote.
I have the Beebe book and looked at it last night. His “Passagemaker” had 1200gal and a 3200nm range with a 400nm reserve so really 3600nm for 3mpg. I believe it was at 8kt. I’ll check and include in a later post.
I found a bunch of better comparison power vessels in the Beebe book and will discuss in the future post.

A - It's ridiculous on several levels, but let me just say I thought you were talking about "optimized" boats. Does 30,000 lb in 30 ft sound optimized?

And I didn't say length was "not important." Just of secondary importance (behind speed and displacement).


B - Again, not about passengers, but rather payload. No one mentioned passenger mpg until you just a couple posts ago. If all of the boats are carrying two people, that results in rather uninteresting data (they're all divided by two). You need something else to distinguish, something like, say, payload.


C - If (displacement) power boats install more power it is because that is their prime (only) means of propulsion, whereas in sailboats it is an AUXILIARY engine. I'm not convinced that happens though, for "equal" boats. The reason you say that is because "equal size" is usually not equal, the power boat is carrying more stuff (or going faster, or...). I would call that payload. (This is getting repetitive.)

The Nordhavn report says exactly what they optimized for:
"The objective of this study was to investigate and optimize the following:
• Wave pattern along the hull
• Wave resistance
• Flow lines along the hull
• Dynamic trim angle"

Once again, you may need to define what you mean by "more efficient." Are you simply referring to lower fuel consumption?

Prismatic coefficient is "used" in several ways. I would say higher speed (non-planing) requires lower Cp, and lower speed allows for higher Cp. Lower speed is more efficient (lower fuel consumption). Thus lower speed (i.e. higher efficiency), like trawlers, have higher Cp.

Quite simply, if sailboats were more efficient power boats than power boats then power boats would look like sailboats.


D - Sails are capable of providing more power than the engine. Engines aren't run at max power very often either.

"Designed for" is not the same as "optimized for." What makes you think that any common sailboat has been optimized for anything? And I shouldn't have to remind you that design is an exercise in compromise.


E - I noticed you didn't include Boatpoker's 38' Benford fantail. 9.9 nmpg @ 7.4 kt.

[Lee Jerry]

Originally Posted by Lee Jerry

A – Did you see where I said “at least on the low end, there's almost no limit to ways to make things less efficient?” Your (somewhat ridiculous) example of 30,000 lb 30 ft’er doing 7.5 kt is the latter part. Let me re-phrase: Based on speed and displacement (or speed, range and payload) you can determine with reasonable accuracy what the minimum possible fuel consumption (and corresponding power requirement) is (are). You can obviously make design decisions to make them worse. For example, you may make changes to improve seakeeping, or perhaps to reduce draft, or simplify construction with chines. But you will be (or should be) doing this with the knowledge that they will increase power / fuel. This applies to any boat type (or plane or train or automobile) or size.

Originally Posted by Adelie

A. My “ridiculous” example of a 30’ boat not being able to get to 7.5kt where a longer heavier one can refutes your prior claim that length is not important. I used the extreme example.

Originally Posted by Lee Jerry

A - It's ridiculous on several levels, but let me just say I thought you were talking about "optimized" boats. Does 30,000 lb in 30 ft sound optimized?

And I didn't say length was "not important." Just of secondary importance (behind speed and displacement).

Data is rather difficult to come by (as I'm sure you are aware), but I've found a much more applicable example of the length affect (or lack thereof) I'm referring to. It's not perfect data by any means (not test data, and not quite complete), but probably as good as we're going to get in general.

So the comparison is Nordhavn 52 and FPB 64 (Dashew). Two power yachts designed for extended low speed cruising.

I got the Nordhavn data from their website and this article (linked from site):
LOA 54'-4"
LWL 48'-3"
Displ 90,000 lb
Power 266 hp John Deere
Fuel Cap 1733 gal (6586 l)
Max Speed 9.5 kt
Cruise Speed 7.5 kt
Range @ Cruise 3000 nm
Avg Consumption 1.73 nm/gal

The FPB 64 data is from this listing (click on the full spec link toward bottom):
LOA 65'-2"
LWL 63'-8"
Displ 88,185 lb
Power 236 hp John Deere
Fuel Cap 3160 gal (11,960 l)
Max Speed 11 kt
"Normal" Cruise Speed 9.6 kt
Range @ Cruise ~5500 nm (this may be at less than "normal" cruise speed)
Avg Consumption 1.74 nm/gal

So two boats designed for the same purpose, similar displacement, similar operating speed, very different lengths (>20%) - one "short and fat" and the other "long and lean." And yet they have almost the same fuel consumption at cruise. Go figure.

[ranger58sb]

Originally Posted by Lee Jerry

So the comparison is Nordhavn 52 and FPB 64 (Dashew). Two power yachts designed for extended low speed cruising.

I got the Nordhavn data from their website and
Cruise Speed 7.5 kt
Range @ Cruise 3000 nm
Avg Consumption 1.73 nm/gal

The FPB 64 data is from
"Normal" Cruise Speed 9.6 kt
Range @ Cruise ~5500 nm (this may be at less than "normal" cruise speed)
Avg Consumption 1.74 nm/gal

So two boats designed for the same purpose, similar displacement, similar operating speed, very different lengths (>20%) - one "short and fat" and the other "long and lean." And yet they have almost the same fuel consumption at cruise. Go figure.

Significant difference in cruise speeds, though, and that should 'splain part of it...

-Chris

[Adelie]

The 88,000lb for the FPB64 is full load. 5500nm @ 9.5kt is for 1/2 load at 75,000lb.
That implies that lightship condition is 62,000lb.

https://www.berthoninternational.com...yachts/fpb-64/

The FPB can go 6400nm @9.0kt at 75,000lb. For 3160gal of fuel that would be 2.03mpg.
9.0kt is S/L-1.13. From reading Berne’s book that’s normal cruising speed.

Using linear extrapolation at 8.0kt it would have an 8200nm range so 2.59mpg.
Really it wouldn’t be linear so let’s say 9100nm at 8.0kt. That’s 2.88mpg.

The S&S Swan-65 was 65’LOA, 70,000lb lightship or perhaps half load. Hard to say. Comparable to the FPB.
https://sailboatdata.com/sailboat/swan-65-ss/

8.0kt would be about S/L-1.15 for this Swan so the most likely cruising speed when motoring.

This site indicates the Swan could go in excess of 1,500nm on 1,000L of fuel.
https://uk.boats.com/reviews/swan-65...%2C500%20miles.

1,000L is 265gal. So that would be at least 5.66mpg.

That’s double the MPG at the same speed at about the same weight.

[mvweebles]

Originally Posted by Adelie

8.0kt would be about S/L-1.15 for this Swan so the most likely cruising speed when motoring.

This site indicates the Swan could go in excess of 1,500nm on 1,000L of fuel.
https://uk.boats.com/reviews/swan-65...%2C500%20miles.
1,000L is 265gal. So that would be at least 5.66mpg.

That’s double the MPG at the same speed at about the same weight.

There are a few boats where the fuel consumption data is accurate. Nordhavn and FPB are two. It's because the designers and builders have a tremendous amount of long distance experience and know their customers will use them in a similar manner. When PAE built the Nordhavn 40 for a circumnavigation, they very carefully calibrated the fuel burn on several non-stop trips from Dana Point CA to Seattle and back. Magazine reviews, marketing brochures, and 95% of owner's reports are wildly optimistic.

I'm a past delivery skipper who delivered dozens of Nordhavns between Alaska and Florida. For other boats - including sail (especially sail) - I used to say that if I believed the fuel-burn numbers owners gave me, I'd still be floating somewhere in the Pacific.

[newhaul]

Originally Posted by HankOnthewater

How long is a piece of string?
.

Actually there is a correct answer to that question and that answer is
Twice the distance from the center to one end .