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Jeff Gilbert

Design Philosophy

(23' Catamaran)

Scarper Flo
(31' Gaff Cutter)

by Jeff Gilbert jgilbert@dynamite.com.au

November, 2000
Fast Passages

Hello, folks.

If there's one thing that we all agree upon, It’s that there are limits to the joys of sailing.  And that the best route across an Ocean is the one that takes the least time.

How do you ensure a fast passage in a vehicle with very finite limits on its speed? One hot summer, 20 years ago, three of us went on a cycle tour which had no particular itinerary, but finished up as 1200km through the mountains of Victoria, Australia. Our daily average for the trip was about 100km, and we did it in an erratic fashion, taking odd easy days to stop at a folk festival, and once to escape the rain in a pub. One pattern that emerged was that as we got fitter we covered a lot more distance in our 6 or so hours per day riding. As we were in the mountains, our riding consisted of hours of plodding up climbs, sometimes walking really steep bits, followed by breakneck downhill sections. So most of our time was spent climbing, and we got better at it. We sped up our trip by cutting time where most of it was squandered, in the slow sections.

Take a 63 nautical mile Coastal passage, to make the sums easy. If you are making 7 knots you are happy, you get there in 9 hours. If you start the motor to give you 9knots in the same conditions, you’ll save another 2 hours if your hull speed is up to 9 in the first place. Further to take a 10 ton yacht from 7 to 9 knots needs, using standard approximations, 32hp to pick up those measly 2 knots you don’t need. Its not worth it.

On the other hand if you are struggling along in fluky airs at 3 knots, you are facing 21 hours, and crossing a harbour bar exhausted in the dark. If you pick up 2 knots you'll get there Ok, in 12.6 hrs,

picking up over 8hours. Furthermore to get your same yacht from 3 to 5 only needs 16hp, and at the commonly applied rate of 18hp/g/hr it costs you 11gallons of diesel. That’s worth doing, and most of us would start the motor knowing that without recourse to this great rigmarole of mathematics, or even needing a log. For the record its 11gals to save 12hrs, versus 22 to save two. And as a bonus you may just save running aground.

However, what about crossing the Atlantic, 2880nm from NY to the Old Dart, as I’m reliably informed by something printed. You have the best boat you can afford, say 33ft WLL but with a pretty high prismatic coefficient giving her a handy hull speed of 8knots. All loaded up with intrepid crew, tucker, water, rum, ship’s dog and 100 gals of fuel you’ve kept her to 10tons. For the purposes of this exercise, make that 25,000 lbs, you’ll soon see why. You study other folk’s logs and discover a boat like yours is doing OK to make it in a month.

The length of the trip is such that you won’t get plain sailing, there will be a range of conditions including some calms. So you might break it down to a rough mathematical model like this….

5days @ 7knots VMG, 10d @ 5k, 10 @ 3k, 5 @ 1knot. That adds up to 2880 NM in 30days.

If this seems slow, note these are VMG, and you’ll have to go quicker through the water to make them.

OK how can you do it quicker without a longer waterline boat??? Use the logic above. You don’t want to waste fuel, so run the Bukh 48 at an easy half throttle during the 1 & 3 knot blocks of sailing.

i.e. for half the trip.

Applying 24hp to the yacht at 1knot gives 5, at 3 it gives 5.75knots, and you get there in 21.5 days which is a significant saving, though not as spectacular as a light wind day at the coast. Or is it?. The period between days 22 and 30 is a lot longer than between cast off and day 8.

So why don't people do it, motor sail the less windy half of the time at half-throttle?

Because it takes, using our same arithmetic, 344 gallons of diesel, which is why I added a ton to the boat with the hundred-gallon tank. This amount of diesel needs a space of 4 x 4 x 3 feet to store it, and it should be near both the waterline and C of G because emptying alters your boat weight by 13%..

Since you wont find this space in any 10meter cruiser I've seen he only way is to scatter it around in containers so that wherever you go you smell diesel. This is not working after all, is it?

What’s the answer? I think its still gestating in the form of electrics. Solar panels are becoming more efficient, cheaper and sturdier. In time the entire deck, cabin and hull sides above the waterline will be collecting power, as will wind generators at anchor. Some types will be vertical deploying from the mainmast once the sail is furled round or inside the boom. Some of the windage on hulls can be converted…we could do this right now with a two foot hole right through the forr’d fine sections of a yacht bow containing a wind turbine. We may see solar cells built into the sails themselves. With solid sails proven in the little Americas Cup, these are bound to gain popularity, especially doubling as sites for solar cells.

Storage batteries are not such a problem as sailboats require the ballast anyway. Gel batteries are stable at any angle and immersed. Propulsion units are OK already, they just need the acceptance required to see them marketed for a reasonable price. A Laurent Giles customer has boldly commissioned this venerable firm to redesign a classic13ton 43ft cruiser for electric auxiliary and a modern build. The projected cost of 380 thousand English pounds is not a testament to its futility, merely its uniqueness.

The cost per unit of electric propulsion is already less than fossil fuels. The one remaining barrier is range, longevity of supply. Once this barrier comes down, electro-sailers will be the leading cruisers, and the sort of passage times currently only achieved by the most extreme racers will be available without the extreme conditions that currently go with them. All this not by sailing faster, but by filling in the blanks with a reliable, quiet, odourless and pollution-free electric motor.

Jeff Gilbert,


November 2000

NOTE: In the spirit of fun and exploration, I've taken a leap into the future, and designed an Electro-Sailer, called 2033 after its probable year of appearance. Its 43 feet on deck, & developed in such a way that a prototype could be built today, but only at great expense. If solar development continues, it may be a standard for the future. To view the sketches and design proposal visit the Designs section of Duckworks and look for 43ft Electro-Sailer by Jeff Gilbert, or click HERE. I think you’ll get a kick out of it. Id be interested in any feedback as long as people realize my crystal ball could be a little cloudy.


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