DIY Inflatable Catamaran

[FlyingMo]

Hi

I have a DIY project, designing a inflatable catamaran with a trampoline deck held together by an aluminum frame.

There are some comercial products like this: HappyCat, MiniCat, SmartKat. However, these seem to be optimized to be "good" sail boats, but take way too much time to set up. I wanted to simplify this.
First I created a design, where the frame can be folded up after detaching some stay lines. But this quickly turned out to be just as complicated, and there are lots of swiveling joints that need to be welded into to tubes of the frame.

So I looked at sizing all the brackets and tubes such that no stay lines are necessary. This turned out to be not much heavier, because the joining locations are lighter because fewer plates need to be welded in. The size is 2.5m width x 4.6m length with 98kg mass (53kg metal frame).
For simplicity, I designed a trampoline that is supported by only two bars, and not on 4 sides, with tensioning cords in the front and the back.
I dont know what I will do for the sail. The priority is quick assembly, so I though I might use a head sail design for the main sail. what do you think about this?

I found a couple suppliers in China who make the custom inflateable hulls, but I have not found anyone who makes cheap sails and trampoline. Can you recommend any?

I don´t know much about boat building, so any advice, even seemingly obvious things, are welcome.

 

[montero]

4 side tensioned trampoline will be nice frame stiffener . Why give it up?

 

[SolGato]

I agree that the trampoline should be tension-able in all directions. It will add more rigidity and reduce stress on connections.

For the sail, consider a simple boomless roller furler-able hybrid with a cut that allows for headroom on deck.

Something similar to what you might see on a Hobie Kayak which would be easy to adapt for testing.

You can also check out the link below for more traditional rig inspiration and kits that are purchaseable:

Sailboats To Go » Sail Rigs for Dinghies and Rowboats

Sail kit for dinghy, rowboats fishing boats, Porta-Bote. Clamps on. Folds. Complete sailing conversion with steering and upwind capability.

www.sailboatstogo.com

 

[FlyingMo]

4 side tensioned trampoline will be nice frame stiffener . Why give it up?

Yes, it also would provide a larger more stable area, but there are some problems.

1) The front and rear bars (especially the front) need to transfer torsion loads. I chose sqare tubing because it is easy to create spigot connections that transfer torque. with round tubing I would need to introduce additional torque transferig features, like bolts, that might also lead to additional stress concentrations. I can not use simple and fast sleeves on the trampoline for attachment on sqare tubes.

2) Tensioning the trampoline becomes more of a hassle, because I need to tension along at least two lengths of the cloth.

3) Assembly becomes much more complicated. Simple sleeves would not work anymore.

4) The corner connections of the current design already provide most of the stabilizing effect

 

[FlyingMo]

I agree that the trampoline should be tension-able in all directions. It will add more rigidity and reduce stress on connections.

For the sail, consider a simple boomless roller furler-able hybrid with a cut that allows for headroom on deck.

Something similar to what you might see on a Hobie Kayak which would be easy to adapt for testing.

You can also check out the link below for more traditional rig inspiration and kits that are purchaseable:

Sailboats To Go » Sail Rigs for Dinghies and Rowboats

Sail kit for dinghy, rowboats fishing boats, Porta-Bote. Clamps on. Folds. Complete sailing conversion with steering and upwind capability.

www.sailboatstogo.com

Thank you, that is a useful link :-D

Do you know how I specify such a sail? Can you recommend any manufacturers in Asia?

 

[montero]

Inverted aft tube cones = less drag under sail ?

 

[BGW]

Are you saying you have found a company in China that will make you a one-off set of inflatable hulls? Or is this a plan to manufacture these in quantity?

I can't recommend anyone for making sails or trampoline, but the very simple sleeved 'Smart Rig' I believe it was designed by Gary Hoyt and made by North Sails. It was fitted to the now defunct Escape line of rotomolded dinghies. It was a very simple, efficient and easy to rig system.

Two part unstayed composite mast, roller reefing built in, sleeved sail and minimal control lines. It took just a couple of minutes to assemble the mast, slide the sail on and set it up. Maybe worth taking a look at those. Supposedly Catalina yachts used the smart rig for a similar line of dinghies.

What about a locking hinge mechanism on the tubes that tensions the trampoline when

 

[BGW]

Are you saying you've found a company that will make you a one-off inflatable hull set? Or is this intended for production?

The Escape line of dinghies had a very simple sleeved sail and roller furling 2-piece mast system well suited to a packable boat. North sails made the sails but don't know about the mast and furler.

Perhaps more complex to make, but easier to assemble, what about hinged transverse tubes that tension the trampoline as they straighten and lock in place?

 

[BGW]

Excuse the double post, I thought I had deleted the first in error.

 

[FlyingMo]

Are you saying you have found a company in China that will make you a one-off set of inflatable hulls? Or is this a plan to manufacture these in quantity?

I can't recommend anyone for making sails or trampoline, but the very simple sleeved 'Smart Rig' I believe it was designed by Gary Hoyt and made by North Sails. It was fitted to the now defunct Escape line of rotomolded dinghies. It was a very simple, efficient and easy to rig system.

Two part unstayed composite mast, roller reefing built in, sleeved sail and minimal control lines. It took just a couple of minutes to assemble the mast, slide the sail on and set it up. Maybe worth taking a look at those. Supposedly Catalina yachts used the smart rig for a similar line of dinghies.

What about a locking hinge mechanism on the tubes that tensions the trampoline when

Yes, they make one-off sets of inflatable hulls, and it is surprisingly cheap. Quality will be a gamble, of course.

Does the mast as a whole rotate for the roller reefing? Is that worth it? I thought just spanning a vertical line right next to the mast and have a commercial roller reefing system on that line.

Can you make a sketch of the trampoline tensioning mechanism?

 

[FlyingMo]

Inverted aft tube cones = less drag under sail ?

Yes, I thought it might be better if the waterline ended pointyer.

 

[montero]

That's the question . Probably you will reach different speeds . There are e.g. load and load distribution. The lower cone may probably be faster under higher loads.

 

[BGW]

The mast was integrated with the roller. I really like it for the simplicity, lay the mast down, slide the sail sleeve on, put the end of the mast in the deck and feed the two lines through the cleats and you're ready to sail.

Where are you located? My old Escape Mango hull developed a number of cracks but I kept the Smart Rig, thinking I would use it on another boat some day but that's not likely. It's in NM.

That is amazing that they will do one offs. I have been thinking of a catamaran tender for my catamaran that can be rowed, powered or sailed but am not ready to tackle it yet.

I am neither engineer or artist and doubt I could draw anythkng that would make it any clearer. Imagine the transverse frames were hinged in the center, with the trampoline fixed to them. Mount them to the hulls and press down on the center point. As they flatten they would be sized to tension against the hulls and tension the trampoline. Get everything nice and tight when flat quickly and easily. Lock them with a pin. Folding kayaks often use a variation of this to get the frame and skin tension.

And as a bonus, with the mast removed, folding the transverse members would make an easily stored size relative to the size in sailing trim.

 

[FlyingMo]

I thought about tensioning the trampoline with a lace like this:

This is with a single line going back and forth. I have seen this on boats but I don´t know what to do with the loose ends of the line. what about using two lines (or one going left and right) like shoe laces: alternating between every other hole and tied off against each other at the end?

 

[FlyingMo]

The mast was integrated with the roller. I really like it for the simplicity, lay the mast down, slide the sail sleeve on, put the end of the mast in the deck and feed the two lines through the cleats and you're ready to sail.

Where are you located? My old Escape Mango hull developed a number of cracks but I kept the Smart Rig, thinking I would use it on another boat some day but that's not likely. It's in NM.

That is amazing that they will do one offs. I have been thinking of a catamaran tender for my catamaran that can be rowed, powered or sailed but am not ready to tackle it yet.

I am neither engineer or artist and doubt I could draw anythkng that would make it any clearer. Imagine the transverse frames were hinged in the center, with the trampoline fixed to them. Mount them to the hulls and press down on the center point. As they flatten they would be sized to tension against the hulls and tension the trampoline. Get everything nice and tight when flat quickly and easily. Lock them with a pin. Folding kayaks often use a variation of this to get the frame and skin tension.

And as a bonus, with the mast removed, folding the transverse members would make an easily stored size relative to the size in sailing trim.

Im in Abu Dhabi in the UAE.

After reading more on it and looking at videos of the assembly I think that a simple sail on hanks might be easier than a rolling furling sail. because, as I understand, a rolled up sail can not be disassembled as such.