Self righting multihull?

[rob denney]

Good questions

umm, how to ensure lee hull floods only in a capsize?

The inlet holes need to be well above the waterline on the lee side when the boat is flying a hull. The outlet holes can have flaps on them unless the false floor is above the hull flying waterline. The flaps can be gravity, pressure or crew activated.

How to ensure that the capsize remains "in plane" and doesn't fall off bow or stern?

A pitchpole where the boat ended up bows down would be a problem. The self righting would not work. There are a couple of solutions i am playing with, just need time. However, I have pitchpoled my 7.5m test boat ELEMENTARRY – Harryproa http://harryproa.com/?p=1753 a few times. It always ends up on it's side with the mast pointing upwind, even when the true wind is close to dead astern. Consequently, I am pretty confident that a pitchpole resulting in both bows going under would be pretty unusual (the boat would have to be well by the lee) and easily avoided by sailing a little higher.

The mast lee stay seems to have a very acute angle.

That is a function of the roughness of the rig on the model. Apart from foiling Bucket List, which is a test bed for the Volvo Proa , all harrys have unstayed masts. There is more on that boat and it's rig at http://harryproa.com/wp-content/uploads/2017/10/volvoproa.pdf

But you are right, if all goes well it is self-righting.

Ta.

 

[Stephen Ditmore]

Great thread. How about I drop in a thought about a truly self-righting possibility as well as indicate interest in further exploring how to re-right an inverted multihull at sea (manually and unassisted).

Imagine a high-dihedral trimaran. If the amas were high enough relative to the main hull, and if it had a deep but retractable centerline lifting keel, it wouldn't take much weight at the end of that keel to bring the CG below the CB of the amas when the keel is in the down position.

If the above keel could also be designed to cant in a way that a person could operate it should the boat invert, that would open other possibilities, as would the ability to cant the rig if the boat is on its side.

Concerning racing, my thought is to do what the IMOCAs and Class 40s do (+ allow multihulls and allow a greater beam). Last I checked they require the boats to have a certain RM at 90 degrees and a way to re-right the boat from inverted, then they give top naval architects the freedom they need to find the optimum solution. I feel like writing a class rule is a little like doing a google search - ask for the thing you actually want and the computer (or naval architect hive-mind) will do the rest!

I'm having trouble finding an article on it, but I think Walter Green built at least one catamaran, perhaps the 1984 Sebago, with one cored hull and one single-skin hull so that the single skin hull could be sunk & then air blown into it to bring the boat upright from being turtled. I think he had in mind being able to do that himself, at sea, if necessary. Boats - Greene Marine - Maine sailboats, catamarans, trimarans http://www.greene-marine.com/sailboats.htm

There have also been proposals to sink either the sterns or the bows and flip a multihull end-for end. I don't know what became of them.

 

[Russell Brown]

Great thread. How about I drop in a thought about a truly self-righting possibility as well as indicate interest in further exploring how to re-right an inverted multihull at sea (manually and unassisted).

Imagine a high-dihedral trimaran. If the amas were high enough relative to the main hull, and if it had a deep but retractable centerline lifting keel, it wouldn't take much weight at the end of that keel to bring the CG below the CB of the amas when the keel is in the down position.

If the above keel could also be designed to cant in a way that a person could operate it should the boat invert, that would open other possibilities, as would the ability to cant the rig if the boat is on its side.

Concerning racing, my thought is to do what the IMOCAs and Class 40s do (+ allow multihulls and allow a greater beam). Last I checked they require the boats to have a certain RM at 90 degrees and a way to re-right the boat from inverted, then they give top naval architects the freedom they need to find the optimum solution. I feel like writing a class rule is a little like doing a google search - ask for the thing you actually want and the computer (or naval architect hive-mind) will do the rest!

I'm having trouble finding an article on it, but I think Walter Green built at least one catamaran, perhaps the 1984 Sebago, with one cored hull and one single-skin hull so that the single skin hull could be sunk & then air blown into it to bring the boat upright from being turtled. I think he had in mind being able to do that himself, at sea, if necessary. Boats - Greene Marine - Maine sailboats, catamarans, trimarans http://www.greene-marine.com/sailboats.htm

There have also been proposals to sink either the sterns or the bows and flip a multihull end-for end. I don't know what became of them.

This boat is one of the most interesting self righting try that I know of. Jan Gougeon designed it. The ama's are less than 100 percent buoyancy and the mast prevents turtling. This is a very competitive boat and it won the multihull division of the Port Huron to Mackinaw race this year. Jan also designed the G-32, which is very reliably self righting (I know this).

 

[Stephen Ditmore]

Is that Adagio? Adagio - Epoxyworks https://www.epoxyworks.com/index.php/adagio-2/. Does she still have the original John Roncz rig?

If you're the Russel Brown I think you are, you know a lot of designers & builders. Why do you think more have not been built like Adagio? Say you were going to propose a new class for boats that size to race in the Route du Rhum 4 years from now. What would you require? Could a Dragonfly, for example, be modified to qualify?

Also I wonder, for competitive ocean sailing, what if she were dismasted & turned turtle by waves. What would a single sailor standing on the underside do in that case?

 

[Russell Brown]

That's Ollie, which races against Adagio in all the major Great Lakes races. Adagio (35 feet) has been the fastest rated boat under 40 feet for most of the last 52 years. All three hulls on both boats are tortured plywood.

 

[Stephen Ditmore]

Interesting; thanks, Russell. What are you up to these days - will you be continuing to build hydrofoils?

 

[Robert Biegler]

Imagine a high-dihedral trimaran. If the amas were high enough relative to the main hull, and if it had a deep but retractable centerline lifting keel, it wouldn't take much weight at the end of that keel to bring the CG below the CB of the amas when the keel is in the down position.

Unless you reduce the buoyancy of the amas and akas by flooding, I think it would take a lot of torque, so either a lot of weight, or a very long lever. Without flooding, the torque would have to be similar to that needed to take the boat from upright to floating on just one hull by canting the keel. Less if the mast is buoyant, more if it is not.

If the mast is buoyant enough, and strong enough to remain intact through a capsize, then just flooding ama and akas might do the job. You would need enough buoyancy in the mast to overcome any residual buoyancy in the flooded ama and akas, but enough residual buoyancy and dihedral to lift the mast out of the water.

An alternative would be to fold the amas back, like on the Dragonfly trimarans, but mount the hinges so that they turn not around vertical axes, but axes that are tilted forward. When upright, that would lift the amas when swinging them back, which would be bad if you want to reduce the width in harbour and stay upright. If you swing the amas back after a capsize, that would lift the centre hull out of the water. Give the hinges on each side different angles, and you lift more on one side than the other, which shifts the weight of the centre hull more to one side.

Shrouds would not remain at the same tension unless you can invent a system for taking up the slack as you swing back. Therefore this might work best with an unstayed rig. Also, the forces when swinging the amas back, and later swinging them out again when the boat is on its side, would be large.

 

[Paul Scott]

This is probably the dumbest thing I’ve posted in a month or so, but why not a ballasted keel? My rationalization is based on the following-

without a keel, multihulls are heavy compared to dinghies-

monohulls have been catching up to multis, even with ballast, especially as foils get into the scene, and once you get into automatic self rescue, look at IMOCA 60’s and even a Beneteau….

When you right a capsized beach catamaran, for example, you put your feet on the leeward hull, and your head is in the middle, and the things come up…. eventually…but for me, for example, that’s 165 lbs, just a few feet under the tramp, rather than a bulb of lead sticking out a few feet beyond the windward hull.

So how much weight how far down in the middle of a catamaran would it take to make a two man cat non cap-sizable for a single handed sailor?

There are examples out there already of single center foil catamarans, some on the bleeding edge, some not, some with Moth style hydro…

how many cats are using water ballast for stability? This does mess with hull design efficiency- for which weight regime do you design the hulls? With a keel, weight would be constant, which has its ups and downs….

but if you’re willing to give up some performance for easy self recovery, the advantage is that the moment arm of a cat is almost always going to be farther out than a mono.

As far as structure, you could use diagonal hydro struts from the hulls down to a central bulb that could give some lift and leeway resistance.

How much performance would you lose? Take a Gougeon 32, for example, with water ballast in both hulls, 2 or 3 people aboard- get rid of the water ballast, add a central ballasted keel (bulb or not), get rid of the centerboards - what would the difference be in performance? Keep the floatation pod at the top of the mast. How much ballast how far down would you need in a central foil to have a self righting boat that you could keep in a regular slip, or on a trailer? I don’t have the design skills for that question.

A tri could have submersible outer hulls with a ballasted keel.

Cat construction with a keel could (?) make hull construction simpler, a ballasted tri, smaller, lighter amas, no centerboard trunk in the main hull. Depends on how much stability where in the stability curve. Weight savings in structure moved to a keel?

Anyway, just a thought. And I know, ballast in a multi.. (But I do see arguments for a tri, for example, that involve the observation that there’s more weight to windward when a tri is up on one hull than a cat ….)

 

[oldmulti]

Paul Scott. Please refer to ballasted cat of old https://www.boatdesign.net/threads/ballasted-cat-of-old.45769/page-2
Golden Miller was an old cat but worked reasonably well. The designer/builder did a later version without keels. There have been other attempts but I suspect most people just made boats wider for the same weight which gained more power and more accommodation. The larger/beamier a multihull gets the greater the stability.

 

[Paul Scott]

I couldn’t remember the name of that cat- misty Miller? Golden Miller? (and I didn’t want to violate copyright) So I didn’t include it. That aside, she wouldn’t fit in a normal slip? I was looking through my meager sailing library for some performance reports on her, and couldn’t find any.

Does building wider mean a multi won’t flip? Difference between initial stability and ultimate stability I guess….

I should divulge that I have a narrow light Mono, 40’ by 10’ 8.5 draft 9600 pounds that is very satisfying-like 22 knots off the wind main only- could a narrow ballasted cat ~12’ beam be built lighter with better easy performance and automatic self righting?

 

[oldmulti]

Paul Scott. Golden Miller (GM) was 21 x 10.5 foot and displaced 2500 lbs with between 265 to 420 square foot of sail. According to a sailing report by Micheal Henderson GM would do on the wind in a force 3-4 breeze she would sail at 4 knots, on a reach it would go 6 knots. In a force 6 GM could reach 10 to 12 knots. In long offshore JOG events it averaged about 100 miles per day. Not bad for an 18 foot waterline. It could self right from 90 degrees unaided.

Compare this to a Jarcat 6 which is 20 x 8.2 foot plywood trailable cat that weighs 800 lbs with 220 square foot of sail area. The cabin is about about the same internal dimensions. These cats can reach up to 17 to 18 knots, go to windward well and work in light airs. A few have capsized but in relation to the number built its a low ratio.

Reality. As you understand how much sail to carry in strong winds to get the best from your boat, most multihull sailors know the limits of there boat and don't over drive them. A slightly different mindset.

Next stability in multihulls comes from either wide beam and light weight or narrow beam and heavy weight for the same sail area. The heavier the multihull the slower the boat on average (does not always apply). But another factor that hurts performance is how close the hulls are set. The wave interaction between the hulls of a "narrow" catamarans can degrade performance. 12 foot beams on a 40 foot cat can be a real performance problem.

Finally a better solution may be buying a Farrier F 32, 33 or 36 foot trimaran which can fold to 10 foot for mooring but can easily unfold by one person in minutes to 20 plus foot for sailing. Some of these models can do 20 knots at peak speeds and are very good in light airs. Yes capsize is an issue but if you are sensible it is unlikely to very unlikely to ever happen. There are people who have sailed 21 x 12 foot 2000 lbs cats around the world. Not recommended but possible.

Attached are some jpegs of a Jarcat 6

 

[SolGato]

I couldn’t remember the name of that cat- misty Miller? Golden Miller? (and I didn’t want to violate copyright) So I didn’t include it. That aside, she wouldn’t fit in a normal slip? I was looking through my meager sailing library for some performance reports on her, and couldn’t find any.

Does building wider mean a multi won’t flip? Difference between initial stability and ultimate stability I guess….

I should divulge that I have a narrow light Mono, 40’ by 10’ 8.5 draft 9600 pounds that is very satisfying-like 22 knots off the wind main only- could a narrow ballasted cat ~12’ beam be built lighter with better easy performance and automatic self righting?

Watch the beginning of this video to see that boat in action. I assume it had ballast in it based on the conditions and narrow beam, and as you can see at one point they were able to very easily recover by shifting passenger weight when blown over.

 

[Paul Scott]

But another factor that hurts performance is how close the hulls are set. The wave interaction between the hulls of a "narrow" catamarans can degrade performance. 12 foot beams on a 40 foot cat can be a real performance problem.

Finally a better solution may be buying a Farrier F 32, 33 or 36 foot trimaran which can fold to 10 foot for mooring but can easily unfold by one person in minutes to 20 plus foot for sailing. Some of these models can do 20 knots at peak speeds and are very good in light airs. Yes capsize is an issue but if you are sensible it is unlikely to very unlikely to ever happen. There are people who have sailed 21 x 12 foot 2000 lbs cats around the world. Not recommended but possible.



-The Gougeon 32 does beg the narrow beam question, esp with 2 sailors and both water tanks filled.

-we had an L 7 tri, which was a slider, and it was fiddly to collapse, esp at the end of a boisterous sail. Granted part of it was aesthetic, with the tramp folded, catching gunk and growing spider webs and other gunk which spilled over to the main hull.

-How much weight does a narrow cat, for example, save over a wider cat? I think the sensible sailing argument is made about unballasted dinghies? German lake cruisers are fun….

 

[oldmulti]

Paul. there is one qualification about narrow beam multihulls. If the length to beam ratio on the hulls are better than about 16 to 1, in short very thin hulls, they dont drag a large surface wave and can have minimal wave interference between hulls (G 32 32 hulls are about 22 to 1 length to beam). But with very narrow hulls you dont have much displacement which is great for speed but bad for stability if you have a reasonable sail area. Everything is a compromise.

Yes you can reduce the weight of a narrow cat but it is only in the wing bridgedeck area (generalized statement) the major weight reductions come from a light weight structures (often expensive) not just reduction of surface area. The lake racing cats in Europe can have a narrow beam but have large hiking racks to get stability back.

If you want some monohull comfort that sails well in a narrow beam multihull that is uncapsizable, yes it is possible but as one person found out, righting a light 32 foot G32 with a mast head float is possible, a guy tried a masthead float on a 40 foot cat and it fully capsized because the mast broke. The anticapsize mast head float was to much for the mast to sustain the force of the capsize.

Ballasted narrow cats may work but it has been tried and rejected by many. Once capsize used to be feared but now designs and sailing skills have improved to a point where most multihull sailors would prefer speed over the weight of ballast and there are other solutions to making a multihull narrow for a slip and wide for sailing. PS Farrier tris folding systems are a lot better than a sliding folding system. Some people with big enough tow vehicles really do have 32 foot Farrier trailer sailors that they take and launch for a day sail. Not many but they exist.

 

[Paul Scott]

Paul. there is one qualification about narrow beam multihulls. If the length to beam ratio on the hulls are better than about 16 to 1, in short very thin hulls, they dont drag a large surface wave and can have minimal wave interference between hulls (G 32 32 hulls are about 22 to 1 length to beam). But with very narrow hulls you dont have much displacement which is great for speed but bad for stability if you have a reasonable sail area. Everything is a compromise.

Yes you can reduce the weight of a narrow cat but it is only in the wing bridgedeck area (generalized statement) the major weight reductions come from a light weight structures (often expensive) not just reduction of surface area. The lake racing cats in Europe can have a narrow beam but have large hiking racks to get stability back.

If you want some monohull comfort that sails well in a narrow beam multihull that is uncapsizable, yes it is possible but as one person found out, righting a light 32 foot G32 with a mast head float is possible, a guy tried a masthead float on a 40 foot cat and it fully capsized because the mast broke. The anticapsize mast head float was to much for the mast to sustain the force of the capsize.

Ballasted narrow cats may work but it has been tried and rejected by many. Once capsize used to be feared but now designs and sailing skills have improved to a point where most multihull sailors would prefer speed over the weight of ballast and there are other solutions to making a multihull narrow for a slip and wide for sailing. PS Farrier tris folding systems are a lot better than a sliding folding system. Some people with big enough tow vehicles really do have 32 foot Farrier trailer sailors that they take and launch for a day sail. Not many but they exist.

I’ve seen many towed Farriers! Always looks messy. But as far as center lead fin ballasted cats, although I’ve looked for years, besides Golden Miller & DL’s experiments, I’ve not seen even pics of one. If you’ve links, I would be eager to see them. I’ve got a 135 lb 3.75 (below the waterline) long bulbed keel foil hanging around, and I’m getting tempted to try it on a 18’ cat of some sort, but it would be really useful to see what has been tried. (Kind of an Arrow cat sort of setup with lead). I used to sail a Sol Cat 18, which by modern standards was pretty porky, but it was still really fun to sail, which got me thinking about a 2 man modern (ish) cat, and using the weight savings of sailing solo see what the performance would be like, as well as capsize behavior.

(FWIW, Russell capsized his G 32 at the S end of the Island we live on. I think he had both tanks filled? Gets pretty sporty down there….)