Multihull microcruiser design considerations (capsize survival)

[Robert Biegler]

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.

I think you are re-inventing a rig first described in 1963, re-invented by Phil Bolger, and possibly others. Skip Johnson may tell you of his experiences with the rig. In Catalyst 3, John Norwood described a proa built to a sketch by Phil Bolger, with such a rig. He describes the rig, but not how it handles. There is a more detailed analysis in Catalyst 8. The rig has significant flaws.

Proa File | Multihull Boats https://proafile.com/ has featured articles on proa rigs.

 

[OrionSailor]

Some years ago there was a thread here, about a model catamaran for high speed and self righted. can't find it but the cat was red coloured,
it has some extreme stepped hulls, more like wings with came out off the water at higher speed.
Maybe a design idea, if we can find it.

found the thread, link. It's about a RC model with foils and self rightened.The concept of a two stage floating ama could also be used on yours.

 

[montero]

This idea kept me awake. First I was thinking about a solution with a one-way valve in the bottom of the leeward 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

What kind of CO2 patron do you mean ? I'm starting to get a little worried about your project

 

[CT249]

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?

Are you talking of horizontal bow-mounted hydrofoils? They have heaps of drawbacks, as listed in the Crowther Shockwave thread.

 

[magwas]

What kind of CO2 patron do you mean ? I'm starting to get a little worried about your project

The one used to make "soda", aka sparkling water.

 

[magwas]

Are you talking of horizontal bow-mounted hydrofoils? They have heaps of drawbacks, as listed in the Crowther Shockwave thread.

Thank you for the reference. As I understand the main problem is "once the angle of attack goes into negative the boat nosedives so instantly and violently". Duly noted.

What if I evolve from that foil by filling up the space above that, and actually make it part of the hull shape? Kind of a step above waterline which adds lift and buoyancy if the angle is high, but when it becomes too high, it does not make resistance and the buoyancy still lifts the bow? A sketch of transverse cross-section:

 

[magwas]

I think you are re-inventing a rig first described in 1963, re-invented by Phil Bolger, and possibly others. Skip Johnson may tell you of his experiences with the rig. In Catalyst 3, John Norwood described a proa built to a sketch by Phil Bolger, with such a rig. He describes the rig, but not how it handles. There is a more detailed analysis in Catalyst 8. The rig has significant flaws.

Proa File | Multihull Boats https://proafile.com/ has featured articles on proa rigs.

This Catalist is really an invaluable resource. As I understand you are editing it. Thank you!

So I see I reinvented the Bolger rig, which is just like aerospike engines in space technology: theorethically really cool, but in practice no one could make it work. Yet. There are some proposals to move forward with it, but not all issues are addressed yet. I take it as a challenge.

 

[CT249]

Thank you for the reference. As I understand the main problem is "once the angle of attack goes into negative the boat nosedives so instantly and violently". Duly noted.

What if I evolve from that foil by filling up the space above that, and actually make it part of the hull shape? Kind of a step above waterline which adds lift and buoyancy if the angle is high, but when it becomes too high, it does not make resistance and the buoyancy still lifts the bow? A sketch of transverse cross-section:

View attachment 200231

That's the old-style concept of multihull design. Whether it is a pro or a con depends on things like the speed of the boat while nosediving. The step WILL increase resistance when the bow goes down, and that slows the boat so that apparent wind increases and the boat nosedives. Because of the wider foredeck, the bow catches and over you go.

Decades ago the Nacras and other cats showed that it's better to ensure that the bow resistance remains as low as possible, even when the foredeck is a metre or so underwater. The volume still increases because more length of the bow is immersed, but the boat doesn't slow or trip as much. It's actually quite disconcerting to see how deeply the bows on such a cat can go underwater without the boat capsizing - the boat slows, tips 30 degrees in the air, and then pops back up.

Look at A Class cats for the extreme in these low-resistance bows.

Increasing the bow buoyancy for your boat could work if it's slow enough that the momentum and change in apparent wind are less important than the increase in bouyancy, but I have no idea where the dividing line between the two approaches is.

 

[Skip Johnson]

This Catalist is really an invaluable resource. As I understand you are editing it. Thank you!

So I see I reinvented the Bolger rig, which is just like aerospike engines in space technology: theorethically really cool, but in practice no one could make it work. Yet. There are some proposals to move forward with it, but not all issues are addressed yet. I take it as a challenge.

The definitive explanation for the multitude of pros and cons of the rig are here IMO Proa File | Proa Rig Options: Bolger Rig https://proafile.com/multihull-boats/article/proa-rig-options-bolger-rig
I wish you well, I enjoyed my trip down that particular rabbit hole but have found more productive trails since.
FWIW if you do manage to corral the beast the "sliding tack" I came up with might be of some value.

 

[magwas]

So I see I reinvented the Bolger rig, which is just like aerospike engines in space technology: theorethically really cool, but in practice no one could make it work. Yet. There are some proposals to move forward with it, but not all issues are addressed yet. I take it as a challenge.

After giving some thoughts to the idea, I realized that in the process of shunting the sail must go through the point of its maximum area perpendicular to the wind, so it is like gybing a bermuda rig without first pulling the main into the wind. While the other problems probably can be solved, this one can only be overcame by reducing the sail area before shunting. The only solution I can think of which does not make the center of the sail - thus the momentum - too high is dropping the sail before shunting. If other aspects of handling the sail can be made easy, this might be a viable compromise. The Catalyst article actually mentions this in passing, but I think it is important to emphasize that it is a fundamental property of the Bolger rig with no way around.

Now I am thinking about a lazyjack like solution so the sail does not end up in the soup every time when dropped, and flat battens which bend to shape by the force of the sheet(s). The best solution I came up with so far gets away without using any boom, but there are still four ropes to handle in a shunt, the shunting process must be followed properly to avoid big forces. But at least reefing is not a big deal on a dropped sail.

 

[gonzo]

fter giving some thoughts to the idea, I realized that in the process of shunting the sail must go through the point of its maximum area perpendicular to the wind,

That is why they used a crabclaw sail. It weathercocks while shunting.

 

[Skip Johnson]

After giving some thoughts to the idea, I realized that in the process of shunting the sail must go through the point of its maximum area perpendicular to the wind, so it is like gybing a bermuda rig without first pulling the main into the wind. While the other problems probably can be solved, this one can only be overcame by reducing the sail area before shunting. The only solution I can think of which does not make the center of the sail - thus the momentum - too high is dropping the sail before shunting. If other aspects of handling the sail can be made easy, this might be a viable compromise. The Catalyst article actually mentions this in passing, but I think it is important to emphasize that it is a fundamental property of the Bolger rig with no way around.

Now I am thinking about a lazyjack like solution so the sail does not end up in the soup every time when dropped, and flat battens which bend to shape by the force of the sheet(s). The best solution I came up with so far gets away without using any boom, but there are still four ropes to handle in a shunt, the shunting process must be followed properly to avoid big forces. But at least reefing is not a big deal on a dropped sail.

No, if your sail pivots around the axis from the head of the sail to the tack line, it will pivot around that line during a shunt. The issue, and it's a big one is transferring the from one tack line to the other; if it's cleated lines switching functions during a shunt, someday you will have a sail with two mainsheets rather than a tack line and a mainsheet. I promise you you will not want to experience this condition ;-) Read the bit referenced in Proafile, it's a gold mine of pluses and minuses. I got around the loose tackline issue by having a sliding tack much like a draftsman parallel bar turned on it's side. My problem was the rig was skitzophrenic, it was either very good or very bad, nothing in-between, and the good was only a small percentage of the time. A big part of the problem was probably me, I'm not that good a sailor but I've had reasonable success with other rigs, crabclaw, lugsails, bermudian and wingsails; the Bolger, aka AYRS or biaxial sail was beyond me.

 

[CT249]

No, if your sail pivots around the axis from the head of the sail to the tack line, it will pivot around that line during a shunt. The issue, and it's a big one is transferring the from one tack line to the other; if it's cleated lines switching functions during a shunt, someday you will have a sail with two mainsheets rather than a tack line and a mainsheet. I promise you you will not want to experience this condition ;-) Read the bit referenced in Proafile, it's a gold mine of pluses and minuses. I got around the loose tackline issue by having a sliding tack much like a draftsman parallel bar turned on it's side. My problem was the rig was skitzophrenic, it was either very good or very bad, nothing in-between, and the good was only a small percentage of the time. A big part of the problem was probably me, I'm not that good a sailor but I've had reasonable success with other rigs, crabclaw, lugsails, bermudian and wingsails; the Bolger, aka AYRS or biaxial sail was beyond me.

That's a good article in Proafile- thanks for the link.

A couple of points about the rig and article seem worthy of comment. For one, it refers to the "elliptical shape (that) provides optimum span loading, meaning that unlike conventional rigs the top third of the sail can operate at full power" which seems to ignore the fact that an elliptical outline does not equate to elliptical span loading, and that (as far as I have learned) in conventional rigs the top of the sail not only can but should "operate at full power" to achieve something pretty close to elliptical span loading.

The article also notes that the top of the sail was 2% deeper at the head which would seem to indicate that, all else being equal, the sail did not have an elliptical span loading but rather a span loading that was even more heavily oriented towards the upper sections.

The "permanently curved battens (that) give ironclad control over foil shape" would seem to act against "allowing precise tuning and twist control" because if a permanent curve is in the battens then it is harder to move the sail draft away from the shape forced in by that permanent curve.

It seems that the rig development may have been driven by unconscious belief that sailing is more of a steady-state activity than it really is; that things like the draft and twist should be locked into one position almost without regard to wind strength.

While as Tom Speer and Mikko Brummer have shown here, masts are actually a drive producer rather than the negative they are often claimed to be, it's obvious why the concept has attractions for proa sailing. But what I really wonder about is the forces that would have to be put onto the downhaul of the leading edge in order to keep it straight enough to maintain proper twist. Looking at the gear we used to load cat and windsurfer downhauls, cat mainsheets and yacht Code Zero halyards makes me wonder about the magnitude and management of the forces needed to get a truly efficient luff shape into the proa rig.

 

[magwas]

No, if your sail pivots around the axis from the head of the sail to the tack line, it will pivot around that line during a shunt.
The issue, and it's a big one is transferring the from one tack line to the other; if it's cleated lines switching functions during a shunt, someday you will have a sail with two mainsheets rather than a tack line and a mainsheet.

My point was that it is indeed inherent problem with this sail, and I see no other way around it than eliminate (drop) it during shunt.

When I said that the shunting process should be followed properly, I had exactly the two mainsheet, aka now we have a spinakker sideways situation in my mind. It is very important to tighten the new tack line and ease the new mainsheet before hoisting the sail after a shunt.

So I guess we do agree in this point. Whether this is a hard no, or just "this is the procedure and other properties of the sail worth it for me" is a question of personal preference, which of course depends on other characteristics of the sail, and especially on how easy it is to actually follow the procedure. For which I will only have my own answer when I have tried it.

A big part of the problem was probably me,

Definitely not. With the exception of specialized racing equipment anything on a boat should be straightforward to handle and work predictably. Which is probably not the case with vanilla Bolger rig, and definitely not the case when the knowledge of "do not leave it up when shunting" is not as deeply understood as "do not gybe with a loose mainsheet" with bermuda rig. And we are talking about an experimental rig with nowhere as much experience with it as with bermuda.

I guess I understand the pros, the cons and possible solutions as much as they could be understood without actually sailing it (which is arguably not much), and right now I see some possibility of liking this rig with some changes to the existing implementations. And it is also possible that experimenting with it will convince me that it is not good. Only one way to find out.

 

[Skip Johnson]

While as Tom Speer and Mikko Brummer have shown here, masts are actually a drive producer rather than the negative they are often claimed to be, it's obvious why the concept has attractions for proa sailing. But what I really wonder about is the forces that would have to be put onto the downhaul of the leading edge in order to keep it straight enough to maintain proper twist. Looking at the gear we used to load cat and windsurfer downhauls, cat mainsheets and yacht Code Zero halyards makes me wonder about the magnitude and management of the forces needed to get a truly efficient luff shape into the proa rig.

Yes, exactly. One of the major issues contributing to abandoning the rig approach was the very 'unproaish' rig tension requirement for a craft that was otherwise a very low key easy does it approach for sailing quickly.