Cox's Bay Skimmer

Gary Baigents Cox's Bay Skimmer

I think this boat is one of the neatest small monohulls I've ever had the pleasure to see. It seems like it could be easily adapted to be a great candidate for the Everglades Challenge-maybe fitted with an asy spin or two and DSS.....
Designed and built by Gary Baigent it is a simply stunning small boat! I think this thread could be real interesting and at least it will save me from searching multiple threads to get a "hit" of the inspiration it provides.
Hopefully Gary will post the detailed specs...

 

And of course Doug you've actually seen it in action right? And you've charted out the course or the EC, complete with tidal interacttions...

Can you show us that chart?

 

Cox's Bay Skimmer wasn't designed to do the EC, just a simple knockabout boat with some performance to daysail and race with locals on the Waitemata ... but with a few modifications, could be adapted to do so. There are two bunks, well, two mean, bare areas below forward side decks that would allow some dry rest - but would require shifting the main hatch behind the fore mast to two sealed entries closer to the cockpit - and disappear/re-deck the original hatch. To enter the original position under sail would be a precarious manoeuvre because, even though the forward hull sections are beamy to carry the forward rig, gets tippy up there.
Boat is simple to sail, can be singlahanded but better with two - because 22m2 sail area requires some movable intelligent? ballast. But the big but low rig can be reefed quickly. Boat is fast planer and to our surprise, exceptionally close winded - because of the airfoil masts perhaps. Also the daggerboard is deep.
Measures 5.5 x 2.25 metres, wing masts 6 x 0.40m, boat weight empty 120kgs.
Doug likes the idea of DSS and I've thought about it but as an EC competitor and carrying gear, probably unnecessary. A bowsprit and reacher would be worthwhile though.
Last photograph is with freestanding rig, since changed to conventional 3 and 4 point stayed versions, on ball and socket bearing - which allows easier lowering. The transom rudder has also been changed to underhung.
The perspective after end view is of a 6.5 metre version with DSS.

 

And Doug has how much full scale sailing experience in a boat like this? Be wary of folks giving you free advice (mine included) its usually worth less than the price.

DSS would add a host of complications and extra weight to the boat, and given that you've already sized the sailplan for the RM you have, DSS will mostly generate drag and be something more to hit snags with...

Underhung rudder gives you better control, but basically precludes doing the EC.

 

Skimmer

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Gary, how important do you think it is to have the two rigs close to each other? I'm wondering if you've done any experimenting with separation between the two?
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I think if the hull can be designed to work with DSS properly(and it may be suitable already), the advantages would be most apparent in the EC and other uses around Florida. I've sailed and raced in many different areas of this state and there is no other lifting foil system that has the potential that DSS does-particularly in shallow water.

 

OS7 said that although the double main rig setup works well on Cox's Bay Skimmer, he implied that on the wind, with the same rig on a faster multihull, it would not be so efficient. And he may be right.
However the Skimmer is not a slow boat beating in decent wind and even so, Chris White has a near similar rig on his fast cruising trimaran Juniper - and has not complained about performance to my knowledge.
And Jim Young's 42 foot trimaran Bladon Racer (see below), designed in the late 1970's, with the true double main rig also said the boat sailed well on all points of sail.
Beating on the Skimmer, the wind indicator on the forward mast points high while the after, main mast indicator is maybe 3-4 degrees lower; the main is sheeted hard. Both mains are cut flat. The rotating wing masts also must help in going to windward.
A taller una or conventional sloop type rig would undoubtedly be the better - but on the unballasted Skimmer, would quickly over power the boat - but with the high sail area, but lower CoG on the "sketch" rig, we can stack out and carry this power well. Main has to be played of course.

 

Gary, do you think it would be worth it on a new boat to make one or both masts movable to test separation? On a smaller boat, like below, it seems like it wouldn't be too hard to set up. My idea is to bring both rigs right down to the deck(foot sealed) and have the crew sit up between the two rigs. In order for that to work they'll have to be more separation between the rigs than there is on your boat. What do you think?
click:

 

Doug why are you asking Gary to spend effort reinventing the wheel? Ketch and schooner rigged boats are well understood. the slot effect is well understood.

Basically from 65 degrees off the wind down to about 165 degrees the twin masts will outperform the same sail area carried on a single mast. but from 30degrees TWA to 65 deg TWA you will see a single mast higher aspect ratio sail plan work better. Particularly when rigged with a headsail. why? because with an overlapping or almost overlapping sail plan you get constructive interaction between jib and main.

With two non-overlapping and separated mast rigged sails you do not gain this benefit, but you pay the penalty of redirected flow (upwash from the fwd sail) and you pay double the penalty for the drag of the mast.

That Bladon "sailed well" doesn't mean it would sail AS well as if it had a single mast

now most cruising boats don't spend much time beating, so twin masts actually make a lot of sense from a sailhandling perspective. That's why Tabberlies Pen Duick was a Ketch rig for one. But it is much more expensive since your sail inventory is higher and your standing and running rigging costs are higher.

 

BBandit, I'm quite certain the wing masts make an important difference when comparing windward performance to other fixed rig, double mast boats. And we never fall off to leeward lining up with other conventional sloops; in fact we usually climb away. The aero masts DO have less drag plus there is a sweet curve feeding from mast to sail - and the masts are clean, no diamonds or extra rigging. So I think that gives us an edge.
However should another wing mast una or sloop appear, they would most probably beat us. Talking about boats in our size range and up to say, 25 feet.
Doug, we crew in the central position anyway - because I hate dragging sterns. The main (after) sail is sheeted to the track on the transom - so if you wanted to separate the rigs further, would have to have a bumpkin setup aft of transom - which I didn't want.
Originally I thought of canting the after rig to windward with block and tackle shrouds thinking that that rig would be in clearer air than an in line setup. But too much hassle and fuss. Happy with what we've got.
If, big if, I build the 6.5m Skimmer, would definitely have DSS - then you could carry proportionately even more sail with the added righting moment.

 

Wing masts definitely improve performance, but being unstayed means you are paying a size penalty in drag as well. Now whether you fall of to leeward of other conventional sloops isn't the really valid measure. The real question is whether the same sail area on a sloop rigged wing mast would drive your boat faster or let it point higher.

I strongly suspect it would allow higher pointing while also sailing faster.

Note that canting the rig will only give you marginally cleaner air. Think about what "leebow" in racing is used for. Essentially a boat that is 1-3 mast lengths ahead of you and 1/2 a mast height to leeward, creates enough upwash to force the windward boat to sail between 3 and 10 degrees lower, or 5%-15% slower.

And there is nothing magical about this effect, it is well understood. Coming off the line in J-24s, my tactician would call for "5 degrees of up bubble" ( ie the inclination bubble meter) which of course sucks for speed, but throws upwash another 5'-10' to weather of us and causes the boat on our hip to be sucked down even further into our upwash.

So heeling your aft mast to weather will give you 1-2 degrees of improved AoA, but not much more. And that will be at the price of some drive out of the sail (since you are now generating a vertical lift vector as well as a horizontal one, though with some heel this might be offset).

Whether DSS would work for you is unclear. its a fair amount of weight and complication. for something like WOXI its a no brainer, but for your boat, particularly since you crew in the very spot that DSS would essentially take 6"-12" out of the depth of the hull, it would likely be more hassle than it is worth.

 

BBandit, theory regarding disturbance to the after main rig has been well agreed upon ... but, the Skimmer goes very well to windward, end of story.
Yes, agreed, a similar but taller wing mast una or sloop would beat us, all things being equal. But such a beast has not revealed itself in our neck of the harbour yet.
Although the boat had unstayed masts initially, they have been changed to fine fibre stays and shrouds.
I have looked at where the DSS would be placed on the Skimmer and it would be just forward of the daggerboard. But not doing that.
But if I did, I'd make some major alterations at the same time, like lift the cockpit floor to just a shallow indentation on deck, seal the forward hatch and so on, beside placing the DSS case.

 

Skimmer

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I think there is potential for real speed with this configuration especially with DSS which adds practically no weight in comparison to the RM it generates. The instalation can be designed to take up very little room.
And the two rigs can carry at least as much SA as a "normal" sloop rig with maybe some advantages off the wind. The sail area is broken up in easy to handle sails that have a significantly lower CE than would a performance sloop. Besides, it looks so damn good! Great idea, Gary, that has inspired me since I first saw it.....

 

It's nothing new, Doug, look at this Baltimore clipper schooner from the 1840's, this from Chapelle.

 

Doug, you have never installed a DSS, never designed one into a hull, so I wonder how you can be so strong in your statements about what it does and does not take. All of the installs I've seen take up a lot of space. Essentially its two additional daggerboard trunks and a fair amount of gear to handle the need to move them during a tack.

And in fact this is probably the biggest factor against them. On a boat like WOXI where you have hoards of bodies and diesel hydraulic power, you can tack without much adverse impact, in a long distance racer like a mini 6.50, where boat handling in tacks and gybes is not as big a percentage of the time, it MIGHT (it hasn't shown yet) be a positive tradeoff,

But for a boat navigating narrow channels and thin water, its patently a violation of one of the most fundamental design and engineering principles:

KISS

Keep It Simple: Stupid

twin rigs only have advantages from 65 to 160 degrees and they carry a non-trivial penalty in both weight aloft , parasitic drag, pointing angle and in fact sail handling complexity (puff on with one boom I Trav Down or ease sheet. With two I need a complex linking system or I can only affect one sail, that in turn affects helm balance).

Doesn't mean its not sometimes the right choice - but its hardly a straightforward benefit.

 

BBandit, if I built such a boat I'd make the DSS a single case, which with a single asymmetric foil is pretty simple to operate; don't see any real problems there; one case, slide the board through depending on the tack or gybe you are on, pas de probleme.
I'm all for the Newick principle but don't see much complication there. And the power of righting moment is worth the minor problems of operation. Course I have no practical experience with DSS ... but maybe will learn later.