30 Sep 2011

Expedition Yachts # 2

The ATOA 64

An amphibious yacht

Maybe I should keep this to myself as I am partial. But the Atoa is one of the most interesting yachts I know of:

- She is unusually comfortable at sea, in any climate
- By concept, she is one of the safest yachts around
- She sails without a proper keel
- And she is designed to walk on land

Beyond the horizon

Properties like these obviously do not happen by themselves. In the case of the ATOA, they are in part the result of some extremely careful planning and engineering.

But, to be honest, to a great extent she is the result of coincidence. Pure luck, if you will.

The Atoa 64 was conceived as a competent expedition yacht – ATOA refers to Arctic to Antarctic. With a visit to the Amazon river on the way. The requirements were very specific:
·       Fast, at least 8 knots average offshore, under engine or sail, off the wind or to windward.
·       Draft limited to 1,60 m. Able to dry out.
·       Protected propeller, good for all sorts of conditions
·       Completely self-reliant. Foolproof keel, rudder and rig.
·       Immensely strong construction, capable of any weather without damage and able to go through thin ice.
·       A double-ended stern was desirable, if it did not detract from the basic qualities
·       Walk-in engine room.
·       A completely enclosed pilot house from which the yacht could be handled for long periods in adverse weather.
·       Cockpit as sheltered as ever possible.
·       Easily handled by one or two persons.
·       Three cabins, one of which could be used as a crew cabin. En-suite layout.

stability requires that the keel stays in place...
During the project phase, all sorts of different concepts were evaluated. Our client suggested twin keels – these were ruled out for their lacklustre performance. Ballasted swing keels were not enough foolproof. A lifting keel was regarded not fit for the south Atlantic. Even an ordinary fixed fin keel was ruled out as being too vulnerable. Spade rudders likewise.

Please note that all these concepts are fine for almost any boat and we use most of them all the time. Only, this particular yacht was supposed to be able to be fine and safe in the most remote parts of the world, on her own and under any conditions.

Boarding the boat

ATOA 64, with both dagger boards shown. One is used at a time
Finally, a somewhat unique concept grew on the drawing board.

The boat would be built with a long, very shallow keel, extending all the way aft to protect the propeller and support the rudder. This would be the backbone of the yacht and allow a reasonable position for approx. 12 tons of lead ballast.

The rudder would of course be balanced, in order not to strain the helm or autopilot too much.

section through engine room
On the outside of the engine room each side, through the side decks, there would be an asymmetric daggerboard. This would provide a lift to windward equivalent to a modern fin-keel yacht with 2,8 m draft.

Outside of the dagger boards, towards the hull sides and under the side decks, there would be space left for nothing. Unless we put ballast tanks there. But the boat wouldn’t be able to survive freezing conditions with water ballast, fresh or salt, so we put the spare diesel bunker tanks there. 2000 litres, to be half in each tank, or all on the windward side during a passage.

Pilot house with inside helm station

Downstairs, looking aft under pilot house into engine room

Apart from batteries, all installations in one place

Again, with the combined stability gained by the filled windward tank together with the lead keel, this would provide the equivalent kind of righting moment that one would expect from a modern fin-keel yacht with 2,8 m draft. Voilà!

If one of the daggerboards got damaged in the Antarctic, the boat would still be able to keep sailing. She would lose that last edge, that’s all. Same if the spare diesel had to be used.

So, essentially, we were creating a totally safe yacht with 1,6 m draft that would behave like it had a modern cruiser-racer fin keel 2,8 m deep. Alas, with slightly more drag.

As dry as possible

Maybe the real beauty of this concept is the ability to dry out. The dagger boards serve as perfect legs. And the yacht will in such case rest on its keel bottom, not on the hull itself. Drying out with a lifting-keel boat could be a nightmare if you discover you are sitting on a boulder. With the thick sole of the keel onto the sea bed, you will still be safe.

Taking all aspects in account, we knew this design was as safe and amphibious and fast and simple we could come up with for a 40-ton, world cruising 64-foot sailing yacht.

It never turns out the way you expect

Atoa as she was built. The keel protrudes only 0,5 metres (1' 8") below the hull
The ATOA was built beautifully in Enkhuizen, Holland. During the build, however, a decision was made not to build the daggerboards. We were very concerned, convinced that she would become a mediocre motorsailer kind of yacht.
Motoring out of Enkhuizen, last days of December
As it happened, the test sails with ATOA proved us all wrong.

Reaching under reefed main + jib
In blustery, freezing conditions on the Ijselmeer she reached out of Enkhuizen at good speed, 9,4 knots, under reefed main and 106% jib. This was all expected, because even if she is on the medium to heavy displacement side of the scale, she is a slippery boat with a long waterline and a very fine entry.

She was easy on the helm and felt nimble to handle. Everybody perched in the forward sheltered part of the cockpit or inside the pilot house.

The cockpit is well protected forward. The are doors to reach the side decks

As we headed up close-hauled, the speed dropped to 8,2 – 8,5 knots.

The yacht had approx. 98 degrees between the tacks, counting leeway. Thus, she wasn’t very close winded, but she compensated more than well in speed. We didn’t even try to sheet harder and head up more – she had the potential, but speed seemed to be ATOA’s thing more than close-windedness.

Check how she goes. In this video, ATOA is close-hauled.

Still, looking at the polar, with such speeds ATOA’s ability to windward would definitely take her anywhere, with panache.

The wind was a steady 24 knots, occasionally topping 28. Still with a reef in the main, we hoisted the mizzen. The speed increased by perhaps a tenth, she needed a little more helm and if the mizzen was sheeted hard, the pressure on the wheel increased but still not enough to make steering unduly heavy.

A little later, the diesel ballast was tested. It took around 4 minutes to pump the 2000 litres to windward, during which she righted herself from 18 to 14 degrees. She certainly felt powerful bearing away, again increasing to 10 knots, with little heel.

Understanding ATOA

Her performance on the wind was a surprise to everybody. Somewhat later we had the opportunity to test shoal draft keel concepts at Chalmers University of Technology, in Göteborg, Sweden. The study was conducted by Andre Sauer under Professor Lars Larsson and Michal Orych, comparing a thoroughly modern cruiser / racer with a deep keel and the same boat with a very shallow keel.

This time, the keel was given a slightly more sophisticated shape, with a bulb turning into an end-plate back at the rudder

The results, in short, are that the boat with the shallow keel still sails rather well. Even to windward. As expected, her VMG (windward ability) is certainly a few percent inferior to the deep-draft boat.

Still, I am not convinced that such extremely shoal draft would work as well for any boat: Our hypothesis is that it only works well for relatively large and slippery yachts. That the efficiency of this inefficient keel is entirely speed-dependent. The study was not able to put enough light on this so there is room for further research.

I am not advocating anything here. Personally, I have a very soft spot for fast and responsive boats. On the other hand, giving away half a knot may be acceptable if you are going at around 8 or 9 knots anyway – especially taking into account the way most boats are used, very shoal draft keels as these could certainly be a serious option.

There are lovely cruising grounds with limited water. The Bahamas, the west side of Florida… being able to enter more or less any harbour may be worth a lot more than losing that half knot. 

It is interesting to contemplate this as you take a look at the market: More or less every boat has a kind of deep draft fin keel – be it fixed, lifting or swinging. Out of a thousand sailing boats, none is equipped with a fixed keel of such shoal draft that it is almost non-existent. You may draw your own conclusions from this.

But speaking of ATOA, it must be remembered she is not all about shoal draft. This particular design has a number of virtues and maybe has something to offer for any yachtsman contemplating that particular Swan, Hallberg Rassy or Oyster cruising yacht.

There will be a third expedition yacht published on this blog. You can read more about the ATOA here