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Old 12-08-2014, 19:25   #46
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Re: Buying/Using a Sailboat Without a Mast

Quote:
Originally Posted by valhalla360 View Post
Don't know what your boat is. If it's a planing hull dressed up as a trawler with a couple of big engines, you are probably right but I said a traditional trawler which implies a shape very similar to a traditional monohull sailboat (that's one of the problems with marketers applying names that have little to do with the actual boat)
You can only compare the boats as you would say "If they were triditional" but we're talking modern style within the last 50 or so years..
There is NO possible way the effort it takes to move Beneteau FIRST 42 throu the water at 7 knots is the same effort it takes to move a 38 foot Nortic Tug throu the water at 7 knots. and use somewhat equal amounts of fuel doing so..
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Old 13-08-2014, 12:36   #47
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Re: Buying/Using a Sailboat Without a Mast

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Originally Posted by Randyonr3 View Post
You can only compare the boats as you would say "If they were triditional" but we're talking modern style within the last 50 or so years..
There is NO possible way the effort it takes to move Beneteau FIRST 42 throu the water at 7 knots is the same effort it takes to move a 38 foot Nortic Tug throu the water at 7 knots. and use somewhat equal amounts of fuel doing so..
There are still round bilge displacement trawlers being built and the rules still apply.

I clarified traditional as you see a lot of planing or semi displacement boats with 2 large engines labled trawlers and they are dragging two sets of running gear thru the water but even they can do pretty good if you back off on the throttle enough.

Other than to scream you are wrong, you have provided no data to the contrary:
- For example: I looked up performance for a Nordic Tug 34. Not a traditional hull and with a larger 260hp diesel. At 7kts, it is reported at just over 1gal/hr. Push a big beamy 34' sailboat up to the same speed and you are going to be pretty close to that consumption maybe a bit worse. Change to a true displacement hull and any slight advantage the sailboat has will go away.
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Old 13-08-2014, 15:02   #48
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Re: Buying/Using a Sailboat Without a Mast

Valhalla's point is correct. The assumption that a sailboat is more efficient than a properly designed displacement powerboat is missing several very relevant points:
1. A sailboat is carrying around thousands of pounds of ballast. That weight, which is not needed for a powerboat, significantly increases the power and fuel consumption required. Look up the formula, horsepower required for a displacement hull at a given speed/length ratio is a direct function of displacement.
2. Sailboats virtually always have small props. This is done to reduce drag under sail. But prop efficiency is significantly a function of diameter. A larger, slower turning prop is much more efficient, thus less power and fuel consumption is required to create a given amount of thrust.
3. A sailboats rig creates significant windage which increases power consumption when not headed downwind.

Bottom line - a sailboat with power is a compromise. In engineering, compromise designs very rarely perform as well in one specific area as a design specialized for that one area.

A comparably designed displacement powerboat, for the reasons above, will be noticeably more efficient than a sailboat.

The reason for the perception that powerboats are not fuel efficient is that most powerboats are big fat draggy hulls. Very few are designed as displacement hulls anymore. Most have huge superstructures with tons of windage and large engines to push all this thru the water, usually at planning/semi-planning speeds.

A displacement powerboat built to similar beam/length ratios as a sailboat, with similar superstructure and an efficient large diameter low RPM prop will need significantly less power than the sailboat and will give significantly better fuel economy.
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Old 13-08-2014, 15:33   #49
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pirate Re: Buying/Using a Sailboat Without a Mast

I must pass on a modest sea story. I know a guy, an experienced guy, that owned some kind of Swedish/Danish glass sailboat, about 30', full keel, low freeboard ... many ocean crossings ... a boat everyone would call a blue water boat. I could have had it cheap and sometimes wish I had but it was tiny inside as was the old custom.

Anyways, the owner swore he motored from Norfolk to somewhere, 77 miles on TWO QUARTS of diesel. For us math impaired, is this even remotely possible?
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Old 13-08-2014, 17:28   #50
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Re: Buying/Using a Sailboat Without a Mast

No. Not even close.

That said, it is possible to greatly increase fuel economy by slowing down. It is a generality that for every knot of speed you slow down (in displacement speed range) you double the miles per gallon. It's a rule of thumb but is often not that far off the mark.
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Old 13-08-2014, 17:34   #51
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Re: Buying/Using a Sailboat Without a Mast

I have read many times (accounts) of 40ft motor sailors and trawlers getting very efficient fuel consumption while "sipping fuel" and still cruising at about 6-7 knots. They usually have 100hp+ engines and rather large fixed props and use low cruising RPMs. As I recall, many quoted about 1 gallon per hour at about 7 knots. Even less fuel burn at slower speeds.
------
Here is a quote about fuel consumption of a very different boat, Dashew FPB64. Note that this is for a FULLY loaded boat that has enough fuel aboard to go 9,000 miles on power.

"Another part of this equation is payload, both personal, ships gear, and liquids. Within the definition of full load are the following allowances:

Fuel and water – 11,600 kg/25,600 lbs.
Spares, tools, ground tackle, personal gear – 3600 kg/8000 lbs.
Evaluation of how the design meets its goals has to be done in the context of a fully loaded condition, with the factors of safety inherent in the scantlings described above

Boat Speed, Fuel Burn, and Range

For recent real world data during four contentious passages, click here.

In smooth water, no wind, full load configuration the FPB 64 is hitting its numbers as predicted (we’ll just discuss the highlights here):

At 8.6 knots we are burning 11.8L/3.12 gallons an hour which gives smooth water range of over 9000 nautical miles.
At 9.7 knots fuel burn is 18.9L/5.0 gallons per hour with a range of 5700 miles.
Top speed at full load is 11 knots, not bad from a 236 HP engine.
So, we’ve got the ability to carry lots of payload, quickly and efficiently, with extra factors of safety discussed in the preceding section. We think this makes an excellent foundation for the rest of our evaluation.

You may be wondering “why you need all that range?” At 8.6 knots you could cross the Atlantic four times before running out of fuel."

"You will find this new design summarized below, and then detailed explanations of the various features on the sidebar to the left under the heading “FPB 64″. But first, some basic specifications:
LOD 64.95′ / 19.85m
LWL 63.6′ / 19.4m
Beam Deck 17.04′ / 5.22m
Extreme Beam (edge of rub rails) 17.72′ / 5.42m
Draft at half load (75,000 lbs/34 tons) Canoe Body 3.25′ / 1m
Draft at half load (75,000 lbs/34 tons) Prop Skeg 4.5′ / 1.37m
Full displacement 90,000 lbs /40,800 kg
Air Draft (top of masts-excluding whips) 25.75′ / 7.85m
Fuel Capacity 3400 US Gallons / 12,800L
Fresh Water Capacity 1800 US Gallons /6800 L
Minimum Range of Positive Stability 130-degrees (half fuel in one tank, full fresh water tanks)
Cruising Speed 9.25-10.0-knots
Top Speed 10.5 knots
Approximate Range 9.0 knots – 6400 NM
9.5 knots – 5500 NM
Note: speeds/ranges are for smooth water, 75,000 pound displacement, clean bottom."

Source of quote: SetSail» Blog Archive » FPB 64 – Theory & Reality
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