Multihull microcruiser design considerations (capsize survival)

[magwas]

I am thinking about building a multihull microcruiser. I got a boost of inspiration from Balkan Shipyard, thanks Rael! Of course I want it cheap, fast, easy to build and impossible to capsize I guess the main topic will be capsize here, as for me the main difference between a beach boat (which a small multihull usually is) and a cruiser is the difference between the severity of capsize in protected waters and open seas.

Yeah, I do understand that with a small boat the question is not really to make it resistant to capsize - as it is impossible - but to make it least painful to survive it and easy to right it. After capsizing my Hoobie 4 times, and being unable to right it by myself three out of them, I see how that design makes capsize a relatively easy experience, and how I do not want to leave protected waters with a multihull I cannot right.

Here is what I think I understood about protection against capsize, survivability of it, and about righting it. And I do have a lot of questions. (and sorry for my bad English). Please keep in mind that I am not as sure in what I write down as it seems, just always qualifying my level of understanding would make it too long. I am grateful for any correction of facts.

Capsize resistance:

The first thing I learned in the hard way that it is not a good idea to sail out with a sail which cannot be reefed and quite hard to hoist down (my F18 had that hook on the top of the mast). My answer to this is junk sail, quite in the style of what you can see on Why Not? I am wondering whether it is feasible to change it in a way that the ropes which keep the "vertical boom" (what is the proper name of that part which is tied to the top of the mast in one end and the other end is put to the current forward end of the waka?) to the battens are going from the endpoint the vertical boom sits at in one direction to the same at the other direction? The idea would be that this way the sail always keeps in the "good side", the hope is that the wind puts the sail right in the place relative to the boom as it is intended, and the question is whether it does, and whether it complicates shunting? I guess that will be one of the things I will test first in my 4/7 size test build.

As I understand, Polinesians ended up with an assymetric hull because that gives more room for optimizing the different parts of the hull for different functions. Putting the rig to the leeward hull makes sure that when the other hull starts to fly the center of the sails goes downward, and the sail gets somewhat into wind shadow. The banana shape turns the boat windward as it pitches forward as the wind blows it, and the center of effort with the water goes forward. The vertical leeward side means that it will slide on the water more as it rolls leeward more. In the meantime the windward hull pushes down the boat, which in times even helped by partially flooding it intentionally. As the windward hull is usually smaller than the leeward one, it naturally "heaves to" in lack of steering and sails.
What are the practices in a standard pacific proa for flooding the ama? Is is always flooded a bit, or when needed? If flooded underway, how?

I am also wondering whether giving some leeward tilt to the lee side of the waka to make it easier to slide is a good idea, and if so, how much would give a good compromise between keeping the boat in path and making easy to slide when the ama flies? I am thinking about a flat bottom with some small kiel bent the other way than the windward side of the waka to make beaching safe and to give some "backsteer", and probably I want to place it in a way that beyond some degree of tilt its bottom will be higher than the edge of the leeward side, so it does not interfere much with the sliding. Does it even make sense?

A feature meant to prevent capsize is the leepod. Which I also see as a nice place to put accomodation in as the rest of the hull is quite narrow. I tried to read up on it, but I found only one more or less serious conversation on it. My understanding is that as a device against capsize it might not be as useful as it seems at first, as when it reaches water the boat is already quite tilted, and it might even gives resistance against sliding out the tilt. I am playing with the idea of making some upside-down cone-like bulbs near -but not the very- end of the lee side of the waka to help sliding effort. I dunno whether it makes sense.

Another way I saw as a solution to the ballast problem is Harry proa: to keep the rig on the leeward hull, but put accomodation on the windward one, so the weight of people and stuff keeps the boat righted. Which is nice and dandy until all the crew have to go leeward to reef the sail. Which probably can be handled by easing the sail. Which makes it go leeward from the leeward hull, making harder to reach. For which the leepod also a kind of solution. I guess it is another question which I will seek the answer for by testing. Now I am more inclined towards the Harry proa way.

When we are talking about the windward hull, I am wondering whether tilting its windward side leeward helps as the wind will push the hull down. Seems like a so crazy idea I must also test it.

Now about surviving the capsize:

In my mind there are two phases: the first when the boat actually capsizes, and than the time interval between this and when the seas make it possible to attempt to right the boat.

When the boat capsizes, I want to be already on the top of the windward hull, and want it to happen slowly enough that I have time to jump to the underside of the leeward hull or the structures in between. Fortunately I have practice with that and of course in such conditions one is already in the most windward part of the boat except if it happens when reefing. The speed is determined by the resistance of the rig against the water, the mass of the windward side and the air resistance of the structures between the hulls. And the heavier the windward side is, the faster the speed partly because it will happen later in bigger winds/waves. I guess more resistant the multihull against capsize, the trickier to test capsize in a safe manner. If anyone have ideas how to do such tests, I am eager to hear.
I guess while having a solid structure between the hulls is nice in good weather, I want a net there as it hurts less.

When talking about the rig and its behaviour in a capsize, I am inclined towards a mast tilted leeward, and having a junk rig on the leeward side on a vertical boom, as it gives a lot of free space for the sail. It gives some of the freedom of a unstayed mast, while giving the strength to the mast itself with stays and shrouds. But it works if the leeward shroud is at least somewhat optimized out from the picture. As the mast tilts leeward, it is theoretically not even needed, as the stays can pull against the windward stay. But of course we are not talking about entirely rigid things, this is why some rigs feature a rod as the windward shroud. Which I do not want to see breaking in a capsize and getting injected to something or someone. So probably a short leeward shroud would be the solution. And a hollow mast, so the mast itself have some buoyancy. It works nice with Hoobies, though I doubt it would be enough to keep a heavier boat perpendicular to water. But it can slow down the capsize process.

Ok, now we are hopefully sitting on a capsized boat with only minor injuries, all we have to do is to wait a couple of days until sea state allows us to right it. Scantling rules and common sense say there should be some dry space in the boat in such cases to weather it out. Which is probably a bit hard to achieve with a very small boat. But some buoyancy can be added by making the deck bulging. I love flat decks for a couple of reasons, but probably not a good idea with a microcruiser. And making the hull more close to being cylindrical also helps in the righting process. A consideration here is the position of hatches. Harry proas usually have entry in the leeward side to make it more sheltered in the upright position. But that becomes windward and down when upside down. A hatch on the top is best for capsize survival at first sight, but fresh air should be available in such a small confined space. Probably a window between the normal and upside-down waterlines?

Righting the boat:

Pacific proas are traditionally built by roping the parts together. Which means that it can be easily dismantled, the hulls individually righted and then the whole thing can be roped together again. A bulging deck and generally closer to cylinder form makes it easier. I can see how can I test that. Probably first with no stuff on board, then modelling the stuff expected to be onboard, to see how the bread gets wet and electronics gets fried without making the actual damage.

I am very interested in any corrections, ideas, opinions and facts about it.

 

[Robert Biegler]

The idea would be that this way the sail always keeps in the "good side", the hope is that the wind puts the sail right in the place relative to the boom as it is intended, and the question is whether it does, and whether it complicates shunting?

Your idea is not clear to me. Could you draw it? I think you are looking for a way to have the sail always to the lee of the last, like a dipping lug, but still have the battens attached to the mast.

I have no idea how to achieve that and still maintain some area of the sail in the front of the mast. I once doodled this hybrid of the gunter and the junk rig. I haven't built even a model, so I don't know whether it would work:

The best source of informed comments is The Junk Rig Association - HOME https://www.junkrigassociation.org/

Putting the rig to the leeward hull makes sure that when the other hull starts to fly the center of the sails goes downward, and the sail gets somewhat into wind shadow.

To the best of my knowledge, that is not how it works. The best explanation I know is in the most recent issue of AYRS Catalyst, that I just finished editing. John Perry explains how angle of attack changes as a boat heels.

As the windward hull is usually smaller than the leeward one, it naturally "heaves to" in lack of steering and sails.

As I understand it, whether the boat is stable heaving to depends on whether the centre aerodynamic drag is to leeward of the centre of hydrodynamic drag. A leeward rig helps a lot with that, I am not sure what relative size of hulls does.

What are the practices in a standard pacific proa for flooding the ama? Is is always flooded a bit, or when needed? If flooded underway, how?

If you mean traditional boats, must of them used a solid log as an ama, so there was no hollow that could be flooded or not. I don't know whether the few designs that used an actual windward hull ever flooded it.

When we are talking about the windward hull, I am wondering whether tilting its windward side leeward helps as the wind will push the hull down. Seems like a so crazy idea I must also test it.

That was proposed in the 1960. I am not aware of any boat being built accordingly. You could build two models, compare them, and tell us how the idea works.

Ok, now we are hopefully sitting on a capsized boat with only minor injuries, all we have to do is to wait a couple of days until sea state allows us to right it.

Even if you sail in warm waters, a few days waiting on rather than in a capsized boat, in rough seas, has a good chance of killing you.

Scantling rules and common sense say there should be some dry space in the boat in such cases to weather it out. Which is probably a bit hard to achieve with a very small boat. But some buoyancy can be added by making the deck bulging. I love flat decks for a couple of reasons, but probably not a good idea with a microcruiser.

I don't think the decks need to be round, they just need enough volume to float the boat as high as possible. From what I have read of larger multihulls capsizing, a major cause of structural damage and loss of equipment is lots of water sloshing about inside the hulls. Have watertight compartments, and thick decks that can float the capsized hull. Perhaps even trigger an airbag on deck that lifts the deck above water.

Pacific proas are traditionally built by roping the parts together. Which means that it can be easily dismantled, the hulls individually righted and then the whole thing can be roped together again. A bulging deck and generally closer to cylinder form makes it easier.

Even a beachcat-szied boat would be hard to assemble while swimming, harder if you want to avoid being squished by waves that push components together while you are in between. I am not saying it is impossible to design a boat for that purpose, only that I have no idea how. And I think there are alternatives.

In the 1980s, a racing cat was designed to have one hull with high enough density to sink when flooded. I don't know how that was supposed to right the boat beyond about 90 degrees. However, I wrote an article on adapting the idea to proas. You can find it in Catalyst 22, which you can download for free from Catalysts – Amateur Yacht Research Society https://www.ayrs.org/catalysts/ I only did calculations, I didn't build a boat. An AYRS member is building a proa that is intended to be rightable, I think using a similar scheme.

 

[montero]

@magwas Any dimensions ?

 

[Skip Johnson]

Capsize as in having your craft rotated 180 degrees is something I have no experience with and hope to keep it that way ;-). There's some hope on that front as I usually sail in shallow waters or on a deeper lake that's not likely to make large enough wave to cause such a capsize.

My limited experience with proas (I've only sailed a few 100 miles in such) has fortunately not provided an opportunity to deal with a 90 degree knockdown though I did come close once.

My current camp cruiser proa QB sails with an inflatable beach roller on the leeward side and in theory as well as model test will pop back up from a knockdown. I plan on testing same after the water warms up around here.

Hopefully Rael with provide some insight on his thinking. It's an interesting topic and I'd like to hear about other peoples experience, I don't know of any proas that have turned turtle but some probably have.

 

[magwas]

@magwas Any dimensions ?

I am thinking about 7m LOA. The test platform will be a 4m one.

 

[magwas]

Your idea is not clear to me. Could you draw it? I think you are looking for a way to have the sail always to the lee of the last, like a dipping lug, but still have the battens attached to the mast.

Thank you for your detailed reply, I am still processing it. Here is a picture of what I am thinking about:

The blue lines are the two states of the vertical boom. The yellow ones are ropes attached to the battens of the junk rig, which is drawn by black.

 

[magwas]

In the 1980s, a racing cat was designed to have one hull with high enough density to sink when flooded. I don't know how that was supposed to right the boat beyond about 90 degrees. However, I wrote an article on adapting the idea to proas. You can find it in Catalyst 22, which you can download for free from Catalysts – Amateur Yacht Research Society https://www.ayrs.org/catalysts/ I only did calculations, I didn't build a boat. An AYRS member is building a proa that is intended to be rightable, I think using a similar scheme.

Thank you, this seems like a viable direction.

The best explanation I know is in the most recent issue of AYRS Catalyst, that I just finished editing. John Perry explains how angle of attack changes as a boat heels.

Exactly which issue is that?

 

[montero]

Actually I am working on my experimental catamaran. 7.5 m LOA . I didn't consider selfrecovery in this project.
It's possible by many ways all of them are complicated and dangerous . Special solutions should be considered on the project stage . Exceptional strength of build is highly recommended .

 

[gonzo]

Thank you for your detailed reply, I am still processing it. Here is a picture of what I am thinking about:
View attachment 200181
The blue lines are the two states of the vertical boom. The yellow ones are ropes attached to the battens of the junk rig, which is drawn by black.

Is this a shunting system?

 

[magwas]

Is this a shunting system?

Yes.

 

[rob denney]

The first capsize rule is 'Don't!' Good seamanship is key, but you can also fit a fuse to release the sheets when a certain angle of heel or pitch is reached. These can be expensive electronic devices or a simple float on a stick with a cleat mounted on the rotational axis. The hull lifts, the float drops, the cleat rotates and the sheet falls out. Works best on self vanging rigs (wishbones, ballestrons, etc) where the sheet loads only alter the angle of attack and there is no traveller friction. To work on all points of sail, unstayed rigs are best and to minimise damage from flogging sails, mainsail only rigs, either one mast or two. The sheet should go directly to the cleat, ideally under the bridgedeck so it cannot be stood on. This also means it can be released when on a winch or wrapped around your ankle.

Proas make great self righters. They only capsize to 90 degrees (more if they go over the lee pod which is likely in big wave capsizes on small proas) if they have a sealed, buoyant mast. Once the mast hits the water, the boat blows around with the mast head pointing into the wind. This happens quickly. The windage of the hull, beams and bridgedeck may be enough to flip it up, or a float can be tied to the boom end and pulled under the water.
Or, if the lee hull is not required for accommodation, a self draining/flooding when capsized false floor will allow the boat to self right. See https://fb.watch/wYvNHzLxQo/ for a cheap nasty working model and explanation.
Pitchpoling running down wind can end up with the boat tripodding: standing on it's bows and the mast. This is to be avoided at all costs. ie, sea anchors, drogues, nothing on the foredeck (stays, headsails, nets, etc) and not surfing down waves unless you are going fast enough for the apparent to be on the beam, in which case the pitchpole will turn into a capsize.

If you are planning on water or movable ballast in the windward hull of a proa, make sure it is big and buoyant enough to support it.

 

[magwas]

In the 1980s, a racing cat was designed to have one hull with high enough density to sink when flooded.

This idea kept me awake. First I was thinking about a solution with a one-way valve in the bottom of the leeward hull, and tried to figure out how to make it small enough and durable as it would be in the most tortured part of the boat. Then I have realized that a pipe ending inside and going to nearly anywhere is good for letting air our when upside down, and a simple truhull on the top can let the water in. Now I am thinking about a pipe system which acts as bilge draining system with one pump in normal mode (if situations when a bilge pump is needed count as normal), can be used to let air out from the leeward ama when capsized, and pump air/CO2 when it turned over 90 degrees.

Then I looked at the design I was sketching, and realized that the wooden crossbeam and the shape of the accomodation pod on top of the windward hull would prevent the leeward hull to go under. So I made the pod much slicker and decided to go with aluminium pipe crossbeams and mast, where all the alu pipes are floodable. I am here now:

Then I was thinking about wind and wave action, and figured out that when the lee hull is flooded, the not-flooded hull will be dragged leeward, so the boat would not turn over 90 degrees. The solution to that could be to tilt the mast towards and beyond the lee hull and have a drag on its end. The sail seems to be a logical choice for that drag. With the mast in that position it could also act as the buoyancy source to help the boat to go beyond 90 degrees, though the speed and way of filling the mast with air/CO2 probably makes a crucial difference.

This way I can get away without using ball valves - except for the mast -, just having normal hand operated valves in the pipe system, with the ones for the lee hull and mast normally open, and the ones for the windward hull normally closed. I can route the whole thing to a pump for draining out water, and a thing where I can insert CO2 from a patron. I looked it up, a normal patron for making sparkling water contains 8.4g = 230 liter of CO2, so such a maneuver would cost around a pack of it.

Probably I will call the boat "Sparkling Water", and the test platform will be "Small Spritzer", or I may elect to use NO2 patron with similar properties for the extra fun

 

[magwas]

The first capsize rule is 'Don't!'

Sure. Just with a small boat in open seas the seamanship quality / harm from capsize ratio is too small for any amount of seamanship quality. And we are humans, we make mistakes. In this respect I would say I am more humane than most So while I see how the proa I am thinking about would be quite hard to capsize, I would like to be prepared for that contingency.

Good seamanship is key, but you can also fit a fuse to release the sheets when a certain angle of heel or pitch is reached. These can be expensive electronic devices or a simple float on a stick with a cleat mounted on the rotational axis. The hull lifts, the float drops, the cleat rotates and the sheet falls out. Works best on self vanging rigs (wishbones, ballestrons, etc) where the sheet loads only alter the angle of attack and there is no traveller friction. To work on all points of sail, unstayed rigs are best and to minimise damage from flogging sails, mainsail only rigs, either one mast or two. The sheet should go directly to the cleat, ideally under the bridgedeck so it cannot be stood on. This also means it can be released when on a winch or wrapped around your ankle.

I do not see how this would apply with the rig I am thinking about, and there is also the possibility that wave action alone causes the boat to capsize. I am thinking about making the windward shroud intentionally week though, so it would snap before the boat would capsize. Thank you for the suggestion still. I see this is applicable for some conditions. Probably it will help someone reading this thread.

Proas make great self righters. They only capsize to 90 degrees (more if they go over the lee pod which is likely in big wave capsizes on small proas) if they have a sealed, buoyant mast. Once the mast hits the water, the boat blows around with the mast head pointing into the wind. This happens quickly. The windage of the hull, beams and bridgedeck may be enough to flip it up, or a float can be tied to the boom end and pulled under the water.
Or, if the lee hull is not required for accommodation, a self draining/flooding when capsized false floor will allow the boat to self right. See https://fb.watch/wYvNHzLxQo/ for a cheap nasty working model and explanation.

Quite interesting. How a self-draining hull different from a hull with an intentionally low buoyancy in this respect?

Also, though I will probably use a mast which is larger diameter which is strictly needed with the forces there, I guess that in a capsize event with such a relatively heavy (for such small multihull) boat I would not count on its buoyancy for self-right, and the best I can design for is still having the mast somewhere, not having the mast and all the shrouds and stays intact.

Pitchpoling running down wind can end up with the boat tripodding: standing on it's bows and the mast. This is to be avoided at all costs. ie, sea anchors, drogues, nothing on the foredeck (stays, headsails, nets, etc) and not surfing down waves unless you are going fast enough for the apparent to be on the beam, in which case the pitchpole will turn into a capsize.

This is again the seamanship thing. Seamanship is all-important, but I think that boats should be designed for simplicity of handling and to allow for human errors. I know I will make some.

 

[waterbear]

The hobie is relatively easy to capsize because it's very narrow, has a lot of sail area and has low volume in the ends of the hulls. The low volume makes it easy to bury the bow and capsize diagonally. The hull you have drawn also looks to be very fine in the bow and low volume, especially in the ama. I would think this would make the boat easier to capsize and more prone to pitching.

I'm sure you're aware of the tiki 21 that circumnavigated, as well as the hinemoa that survived a hurricane ( see Thomas firth jones book) these boats can and do capsize, but it's orders of magnitude less likely. I've only heard of 2 or 3 accounts of Tiki 21s capsizing. There are many many accounts of people crossing oceans and making long coastal voyages in craft like these. I don't recall any stories of people coming to grief except for a man who fell off his Janus 22 off the coast of England.

The biggest hazard in sailing is not capsize or sinking, but falling off your boat. If you build a boat that is easy to capsize you are increasing your chances of going overboard and getting separated from the boat. You have to rescue yourself first before you can rescue your craft.

I suspect, and this is just my gut, that you are better off safety wise with a small catamaran, not necessarily a wharram. That would be much harder to capsize than a proa and on the off chance that you did capsize you can live on your inverted craft while you summon a rescue with your epirb. Just my 2 cents, I don't expect you to listen to this!

 

[magwas]

The hull you have drawn also looks to be very fine in the bow and low volume, especially in the ama. I would think this would make the boat easier to capsize and more prone to pitching.

I see. I am a bit undecided between making the hulls wider, or just adding some wing-like structures near the ends of the windward side of the ama above the waterline. The first would increase the weight and drag, but also would add more buoyancy, while the second one just increases lift when needed without much speed penalty. I guess that the wing solution is not widely used. Is it because there are some drawbacks I am not aware of, or just because lifting surfaces are a relatively new thing in boat design?