Easy Sail Rig for Centered Mast

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FlyingMo

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Hello,

I posted about this inflatable catamaran in the Multihull Forum, but here I need some help with the selection of the right kind of sailing rig.

I want to design a boat, that I can pack compact enough to fit in my car, but I can also assemble it quickly. Currently I am exploring a mechanism like an upside-down inverted umbrella for the deck between the hulls of the catamaran. However, with this design the mast ends up in the middle of the boat.
Ideally I want to avoid any stay-lines, with a pure cantilever mast. If possible, I would also like to avoid a boom for safety.

I think a conventional cat-rig would end up too far back, so the CE of the sail would push the boat into the wind all the time.

Therefore, I think a Lateen-rig may be the best choice. what do you think?


Screenshot 2026-08-10 094822.webp

Screenshot 2026-08-10 094919.webp
 
Two conventional cross beams would do the same. It would be smaller, lighter, less complicated and cheaper.
 
wet feet said:
Before you take the concept too much further,might I suggest taking a look at this website about the Catapult?An inflatable sailing catamaran that has been around for quite a while and appeals to a number of people.It could save you some of the work in re-inventing the wheel. Catapult catamaran racing: Early design https://www.catapultcats.com/boat.early.html
Click to expand...
Thank you.
I also looked at other inflatable catamarans currently on the market, and recently at some russian DIY designs. The one that I find most interesting is the MiniCat 420. However, even though they claim 30 minute assembly, any uncut assembly video that I have seen runs more than an hour. The assembly steps that consume most time are the trampoline, the stays and lines. There are some unstayed designs that might be better.

I thought it might be an interesting design project, to optimize for assembly speed (which happens to also be the main hurdle for me actually using it). possibly at the cost of sailing performance.

This design unfolds like a upside down inverted umbrella. This should speed up the assembly of the frame and the trampoline. But I need to then insert the transom/rudder mounts in the rear (simple square spigot with retaining quick release fastener), attach the keel (one quick release fastener), and assemble the mast and sail (have not optimized this, yet. so still a lot to do.
 
would it make sense to split the lug sail at the mast like this? The idea is that I get a proper camber line for both tacks.

Screenshot 2026-08-11 112853.webp
 
gonzo said:
Two conventional cross beams would do the same. It would be smaller, lighter, less complicated and cheaper.
Click to expand...
Instead of the 'X' bridge. The trampoline would hold the square shape and you could put your mast/keel in the forward bridge beam.

However, if you like the more complex 'X' and centered mast, for your "umbrella" folding action, you could do a single Genoa staysail to move the CE forward. Just add a split backstay.

I think, with the "umbrella" construction, you want to flip the frame so the pivot joints don't wear on the canvas of the trampoline.

-Will
 
Having owned a number of boats and bicycles that fold or dissasemble, I think your focus on speed and ease of assembly is the way to go.

I think you can achieve a similar degree of support with just the two transverse members being folding. Then you can position the mast forward and simplify the rig at the same time.

You could add a third transverse folding member or a longitudinal drop in brace down the middle, though I think either will cause more issues than benefits.

Bear in mind that an unstayed mast will require a stout and deeper mount than a stayed mast. Perhaps a dolphin striker assembly the mast foot rests on. That could be made with dyneema or cable and would not be an issue with assembly or dissasembly.

Have you looked at rowing frames for rafts? May give you a bit of help in figuring out frame design and attachment.
 
I though about a simple rectangle, and making two sides of the rectanble foldable. This would allow a conventional spacial arrangement, with the mast on the front beam, and the transom, and rudder on the rear beam. However, if I make the front and rear bar foldable, then I have to put hinges near the mast attachment, which are loaded in both bending and torsion. possible, but It would become quite bulky. I might return to this solution if this umbrella solution runs into a roadblock.

On the other hand, If I make the bars on the side foldable, then I run into another problem: I need to tansfer bending in two axis, so there is no obvious single hinge direction, this would require some heavy duty locking mechanism.

This umbrella mechanism has the advantage, that I need a single lock with relatively low loads, and the hinges are mostly loaded uniaxially and the bars are mostly loaded in bending and also axially. The torsion loads on the bars are small (only second order effects).

I inverted the umbrella in order to make the trampoline deck more usable, non-inverted would mean climbing over the mechanism when moving on the deck.

The dolphin striker is kinda included in the umbrella, but with compression loaded spreaders, in stead of pairs of tension loaded stays.
 
Got it, you are clearly doing a lot of thinking. And it makes sense to me in some ways. Having the mast support built in and stayed in four directions is definitley a good thing.

My experience with that sort of system on tents or sun shelters has not been great for longevity. It creates a number of vulnerable failure point with the multitude of joints and locking points. Of course those are cheaply made but it does have a number of potential fail points.

But the more complex things are, the more likely they will fail. It may not be a law in physics but boats, especially in salt water, seems to fail at a rate that increases exponentially with increasing complexity.

I don't understand why the hinges need to be centered? They could be placed anywhere along the cross beams.

An alternative approach might be to design it to assemble partially or uninflated and use the air pressure to stabilize the various parts. Then you could do this with simple interlocking tubes and a couple of pins, or an external sliding sleeve that secures the joint, whether a hinge or a sleeve. Simple and robust.

Or assemble each inflated hull and half frame separately, slide the two tubes together and lock in place?

BTW, do you mind telling me what sort of quotes you were getting for having the hulls made? And made in PVC? Welded, glued?
 
BGW said:
Got it, you are clearly doing a lot of thinking. And it makes sense to me in some ways. Having the mast support built in and stayed in four directions is definitley a good thing.

My experience with that sort of system on tents or sun shelters has not been great for longevity. It creates a number of vulnerable failure point with the multitude of joints and locking points. Of course those are cheaply made but it does have a number of potential fail points.

But the more complex things are, the more likely they will fail. It may not be a law in physics but boats, especially in salt water, seems to fail at a rate that increases exponentially with increasing complexity.

I don't understand why the hinges need to be centered? They could be placed anywhere along the cross beams.

An alternative approach might be to design it to assemble partially or uninflated and use the air pressure to stabilize the various parts. Then you could do this with simple interlocking tubes and a couple of pins, or an external sliding sleeve that secures the joint, whether a hinge or a sleeve. Simple and robust.

Or assemble each inflated hull and half frame separately, slide the two tubes together and lock in place?

BTW, do you mind telling me what sort of quotes you were getting for having the hulls made? And made in PVC? Welded, glued?
Click to expand...

For the inflatable hulls I got 3 quotes:
China1: 550$ for one pair plus 200$ shipping
China2: 700$ for one pair plus 350$ shipping
Russia: inital estimate was roughly 1000$ for one pair including shipping (however shipping alone turned out to be more than 1000$, and the price was not updated). From my talk with him, I got the impression that the Russian guy makes the best quality, and the best shape.
All welded PVC

For a simple hanked 6m2 cat sail I got the quote 300$ from Thailand. I have not asked for shipping, yet.

For the trampoline i got the quote 150$ locally here in Abu Dhabi.

For a bulky rectangular frame (lots of spigot fittings) I got the range 3500-5000$ locally here in Abu Dhabi. The metal parts were all together 55kg of Aluminum. This updated umbrella frame is only 35kg of Aluminum so it might be cheaper. They claimed the price is driven by the raw material costs and the welding supplies, but I find that odd, because the raw material should be less than 1000$ and the indian welders earn less than 1000$ per month. Ill get another quote
 
Last edited:
FlyingMo said:
Changing tack seems difficult with this one.
Click to expand...
You could probably join Bernd Kohler's group chat thingy and ask that question, but I cannot see why the sail could not be temporarily shortened during course changes if it is difficult to manage. This link is only to point to the K-Designs chat group.


Member @brian eiland has spent a lot of time on here pondering catamaran rigs with the mast placed well aft. Deceased Australian designer Ross Turner used one on his Jarcat boats.


J

Thread 'Aftmast rigs???'

Can anyone comment on aftmast rigs. I read some of Brian Eiland's stuff and recently saw a motorsailer with a aftmast bipod flying a hugh genoa. I liked it a lot and want to rerig my 48 steel yawl this way if I can. Any help would be appreciated.

 
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