Empathy List Archives

DC

Dave Cooper

Wed, Aug 30, 2006 9:28 PM

I guess we must qualify as a "narrow boat" at 63' overall and 12'10" of
beam. At 92,000 lbs as weighed on the travel lift when we left the BVI I
guess we are "heavy & narrow". Our mileage wasn't spectacular at ~ 1.6nm
then again the seas and wind weren't too kind either ;-)

So I think you need to be long, narrow and "light" to achieve the higher
fuel numbers we see from the unsailboat and others who get or claim higher
NM/G. Perhaps a better metric is tons/waterline, aka length/displacement.
We're 52' WL so we're .88 tons/ft.

Mark, I expect is 42' on the waterline at 16 tons. This is .38 or so tons/ft
The unsailboat is 80' waterline and if memory serves me about 30 tons all
up. I may be wrong on this please correct me if you know what it actually
weighs. If correct then she's .37 tons/ft. Again very light, IMO.

So maybe we can compile a list here from real numbers that we know from our
own performance of our boats. I could be a bit eye opening as I think it is
the weight i.e. tons/ft, not the beam that uses the fuel ;-)

YMMV
Cheers

Dave & Nancy
Swan Song
1974 Roughwater 58
Caribbean Cruise 2006

I guess we must qualify as a "narrow boat" at 63' overall and 12'10" of beam. At 92,000 lbs as weighed on the travel lift when we left the BVI I guess we are "heavy & narrow". Our mileage wasn't spectacular at ~ 1.6nm then again the seas and wind weren't too kind either ;-) So I think you need to be long, narrow and "light" to achieve the higher fuel numbers we see from the unsailboat and others who get or claim higher NM/G. Perhaps a better metric is tons/waterline, aka length/displacement. We're 52' WL so we're .88 tons/ft. Mark, I expect is 42' on the waterline at 16 tons. This is .38 or so tons/ft The unsailboat is 80' waterline and if memory serves me about 30 tons all up. I may be wrong on this please correct me if you know what it actually weighs. If correct then she's .37 tons/ft. Again very light, IMO. So maybe we can compile a list here from real numbers that we know from our own performance of our boats. I could be a bit eye opening as I think it is the weight i.e. tons/ft, not the beam that uses the fuel ;-) YMMV Cheers Dave & Nancy Swan Song 1974 Roughwater 58 Caribbean Cruise 2006

RR

Ron Rogers

Wed, Aug 30, 2006 10:20 PM

Larry could best deal with this. I think that weight or mass doesn't have
much to do with it once you get it going. At that point you have momentum
proportional to mass, so the more mass, the less she will be slowed by seas.
Of course, if she is slowed, it will take more power to get her going again.
But, she will not have lost as much momentum as a lighter boat.

The unsailboat is a wave piercer and has a long waterline and reasonable
displacement. She is designed to make ~ 10 knots. So that speed contributes
to momentum.

Idlelwild is long and narrow and built of aluminum (except for the wheels)
and she got good mileage. The Diesel Ducks claim good mileage and they are
relatively narrow in relation to their length.

Ron Rogers

Larry could best deal with this. I think that weight or mass doesn't have much to do with it once you get it going. At that point you have momentum proportional to mass, so the more mass, the less she will be slowed by seas. Of course, if she is slowed, it will take more power to get her going again. But, she will not have lost as much momentum as a lighter boat. The unsailboat is a wave piercer and has a long waterline and reasonable displacement. She is designed to make ~ 10 knots. So that speed contributes to momentum. Idlelwild is long and narrow and built of aluminum (except for the wheels) and she got good mileage. The Diesel Ducks claim good mileage and they are relatively narrow in relation to their length. Ron Rogers

JH

John Harris

Wed, Aug 30, 2006 10:45 PM

Ron,

Since weight is directly related to wetted area and wetted area is directly
related to viscous friction I think that the boat weight has a great deal
more to do with fuel consumption than it would in a locomotive for example
where the rolling friction of the train is nearly independent of weight.

John Harris

Ron, Since weight is directly related to wetted area and wetted area is directly related to viscous friction I think that the boat weight has a great deal more to do with fuel consumption than it would in a locomotive for example where the rolling friction of the train is nearly independent of weight. John Harris

RR

Ron Rogers

Wed, Aug 30, 2006 11:36 PM

John,

OK, I'm getting dizzy. Larry, help! And, doesn't a longer boat have a
greater wetted surface? Is this why naval architects have to know fluid
dynamics?

Ron Rogers

----- Original Message -----
From: "John Harris" JohnPH@Comcast.net

| Ron,
|
| Since weight is directly related to wetted area and wetted area is
directly
| related to viscous friction I think that the boat weight has a great deal
| more to do with fuel consumption than it would in a locomotive for example
| where the rolling friction of the train is nearly independent of weight.

John, OK, I'm getting dizzy. Larry, help! And, doesn't a longer boat have a greater wetted surface? Is this why naval architects have to know fluid dynamics? Ron Rogers ----- Original Message ----- From: "John Harris" <JohnPH@Comcast.net> | Ron, | | Since weight is directly related to wetted area and wetted area is directly | related to viscous friction I think that the boat weight has a great deal | more to do with fuel consumption than it would in a locomotive for example | where the rolling friction of the train is nearly independent of weight.

HW

Hal Wyman

Thu, Aug 31, 2006 2:24 AM

If you look at the graphs in Beebe's "Voyaging Under Power", he gives an
empirical way to calculate fuel consumption given merely waterline length
and displacement versus speed. His results, while obviously approximate,
seem to be reasonably accurate, i.e., within 20% or so in the examples I
have checked.

Hal
(or see http://www.halwyman.com/rangecalc2.html)

So maybe we can compile a list here from real numbers that we
know from our own performance of our boats. I could be a bit
eye opening as I think it is the weight i.e. tons/ft, not the
beam that uses the fuel ;-)

If you look at the graphs in Beebe's "Voyaging Under Power", he gives an empirical way to calculate fuel consumption given merely waterline length and displacement versus speed. His results, while obviously approximate, seem to be reasonably accurate, i.e., within 20% or so in the examples I have checked. Hal (or see http://www.halwyman.com/rangecalc2.html) > > So maybe we can compile a list here from real numbers that we > know from our own performance of our boats. I could be a bit > eye opening as I think it is the weight i.e. tons/ft, not the > beam that uses the fuel ;-)

passagemaking@lists.trawlering.com

Re: [PUP] Unsailboat fuel consumption