Designing a fast open deck catamaran

A good question. In fact, one hull is always more draggy than the other as soon as the buoyancy is shifted by the sail pressure - on every boat.

Traditional proas with crab claw sails are known to produce a strong weather helm on a reach with the sail wide open - and the opposite when sailing close-hauled. Nevertheless, their sailors manage to steer by weight shifting and sail position alone... I don't know.

 

No experience with proa, I like crabc sails .Trying to build cc sail cat . I am not really interested in ama-vaka proa configuration at the moment.
Of course proa is invention of Pacific indigenous people , thats why its so important. It definitely has its unique properties .
Unfortunately my designs never consider using them.

 

Both correct in my experience. Thanks for mentioning them.

Mulkari,
You can tack a proa, but depending on the length of the small hull, need to be careful of a diagonal capsize, so light- moderate wind only.
The C50 lee hull bows are higher and more buoyant than most similar length trimarans, and way less cluttered (drag when immersed) than catamaran forebeams. The latest rudders are more streamlined than the video version. The ones I am building to try on the cargo proa are more streamlined, simpler and lighter again.
Sudden shifts in wind direction are no problem. The mains are sheeted direct to the windward hull. If the wind is from the wrong side, the sails weathercock, the boat stops and drifts and you steer onto the new course. Far less dramatic than the same thing with an overlapping headsail or a spinnaker.
All multihulls have different levels of hull drag. It is no more a problem on them than it is on a proa.

Montero,
To reef, the sheet is released and the sail weathercocks, regardless of point of sail. The reefs are then pulled in as per normal, but with none of the stress and highly loaded ropes involved when reefing a cat or tri while sailing. It is made even easier with the simple halyard lock (essentially a rope loop around the mast which 'catches' at each reef point). Trip the lock by slightly tensioning the halyard, it drops to the next reef point and the luff and leech lines are tensioned.

I cannot think of a more difficult way to build a carbon mast than the one in the video. Ours are way simpler, and lighter.
Crab claws are great, but not suitable for cruisers. Attached is a vid of me sailing one in the Marshalls. And another of a modern version of the boat which we taught the locals how to build. Notice the lack of use of the steering paddle in the second vid, the difference in payload and the logic of having crew to windward. Both sails are well over 20 years old, have had a hard life. Both boats sailed at similar speeds, but the traditional one was far more exciting.

 

My experience with a 'conventional proa' in light winds is that overall minimal wetted surface dominates. The least amount of wetted surface occurs when as much displacement as possible is transferred to the lee hull where the added displacement is absorbed by a slight increase in draft which is smaller than the lessened area of the smaller windward hull, a squared/cubed relationship.

 

Thank you Rob for the replies and cool videos. I posted the video of the mast construction as a curiosity.

 

That's a concern because with outboard motor it is not too difficult to get overpowered. Motorsailing and tacking with mainsail only is my go to solution if facing strong headwinds in channel too narrow to reliably tack upwind using sails alone. Although doing it with heavily reefed sails on proa probably would greatly reduce possibility of flipping it and water would be fairly calm.

I wonder what is the solution if some kind of jam happens at the end of the boom when reefing? When let go boom flings out over water so you can't reach it. Drop the sail completely and just pull the boom in over the deck to sort it out?

Great video. Modern proa is actually quite similar how you would sail normal catamaran of similar size by having crew sit on windward hull to counterbalance.

It also looks like there is far too little carbon. It's more like carbon reinforced wooden mast.

 

I'm looking through both most detailed C50 build blogs

The Harry "Hendon" Saga https://harryhendon.blogspot.com/
C50 - Brazil — Harryproa https://www.harryproa.com/blog/c50-brazil

It looks like while infusion itself is fast it takes a lot of time to make molds and arrange everything correctly for infusion. Hendon builder often had problems with infusion process leaving dry spots. Having a dry or semi dry spots in thick layup carbon parts is increasing chance of failure significantly.

I'm also following build of plywood Dix 470 cat
https://www.youtube.com/@saltlightdix4704/videos
which is bigger and more complicated boat than C50 and it is in 3 years at a more complete stage than C50 in similar time although it has advantage of being kit boat so less time spent cutting parts out of plywood sheets. Granted we don't know hours spent on each boat, but if they are comparable then there is no major time advantage to using molds and infusion. Hendon guy also spent weeks of sanding and fairing because some issue with mold. Infusion would definitely win if mass producing boats because molds would be needed only made once, but for one off boat it maybe more down to how familiar each builder is with each method than any inherent speed advantages of one over the another although infusion is less messy and makes better quality laminate if done correctly than hand laminating.

 

This is a drawback of the ship's outboard on the tender. If you are doing a lot of motorsailing, it is not a good solution. On the cargo proa, I have a 6kw electric motor with a fairly big prop on the end of a steerable strut through the toybox. It can be used to move the boat in any direction. We are using this instead of the tender as it was a gift (in lieu of grant money owed to us) and because the tender was included in the design as a boat which could access shallower water than the mothership. This is not a problem, so we don't need the tender.

The sail is unlikely to jam while reefing as a) the rig is head to wind when reefing and b) there is only one line for all the leech reefs. If you did manage to foul it, dropping the main and swinging it inboard would be an easy solution as the sail is contained in the bag under the wishbone and it does not need lazy jacks or for the boat to be head to wind to re-hoist it.

Yeah. Sitting to leeward and pumping water ballast requires a mindset that I don't have.

The Hendon proa builder is almost 70, started 2 years ago and had to build a shed and a house before he could start. He has no helpers and is a perfectionist. I have no idea how that compares to the Dix builder, but I would be amazed if he was as quick. In his "End of 2021" video at least one hull is up and part covered in ply and he talks about local helpers, so closer to 4 than 3 years, I think. In his "reality" video he spent 3 weeks fairing the bow blocks and the chainplates. It will take him a lot longer to do the entire boat once it is glassed.

Infusion is not just about the the time saved, it's the ease of construction. The sticky, difficult jobs requiring contortions or a lot of strength/helpers are done dry on the infusion table. Plenty of time to think and get it all correct.

Whether you choose ply or infusion is up to you. But before you choose , try infusion. Spend a couple of hundred bucks on an E Bay pump and a starter kit from your local supplier. Or visit the 20m Harryproa builder in Norway and give him a hand for a couple of days. While there, have a look at his spare leeward hull, maybe make him an offer on it. He and his brother were building a boat each, but his brother died in a freediving accident after the first 2 lee hulls were built. These were the biggest amateur infusions ever done, may still be. They are also compounded shapes, infused in one shot, a fairly unique achievement.

 

I usually sail if at all possible, but occasionally in very narrow channels I have had situations in strong winds where only way to do reasonable speed upwind was with motor and mainsail working together. Like 20 - 40 m and you have to tack and again and again. When shunting normal outboard motor would be usable only half time and in very narrow channels it would be impractical to lift it up and deploy all the time.

Yeah it is very hard to compare directly without knowing hours spent on a project. I'm definetly not a perfectionist. If it is strong and looks like nice workboat then it is good enough for me.

That sounds like good idea. I know I would be extremely stressed if I had to infuse large parts with potential to waste weeks of set up time and thousands of euros in materials or introduce hidden weakness if I screw up something with vacuum setup. Maybe infusing something simple and cheap like homemade XPS foam paddle board would be good start.

 

Mulkari. What are the exact length and width dimensions of the plywood sheets you're thinking about using? Also, there have been some questions you've asked that haven't been answered thoroughly along with some statements you've made that haven't been explored. Are you still looking for feedback or have you finalized a plan?

 

I made the three planks that sit on the cross beams of QB with some XPS foam with a Gpet cap reinforced with uni top and bottom covered with 18 oz cloth. Infused one at a time it was a fairly easy learning experience. Need advice on infusion/vacuum line arrangement https://www.boatdesign.net/threads/need-advice-on-infusion-vacuum-line-arrangement.69702/

 

Plywood I plan to use is available in various sizes including sheets up to 4 m long. With 4 m sheet length there would be only 3 stitches on 15 m hull.
https://www.finieris.com/products/riga-shipply/

• 1220 / 1250 mm × 2440 / 2500 / 2745 / 2750 / 3000 / 3050 / 3340 / 3660 mm
• 1500 / 1525 mm × 2440 / 2500 / 2745 / 2750 / 3000 / 3050 / 3340 / 3660 mm
• 1830 / 1850 mm × 3050 / 3340 / 3660 / 3850 mm
• 2150 mm × 3050 / 3340 / 3850 / 4000 mm
• 2290 mm × 4000 mm

My design is still in progress, I'm also contemplating Harryproa, but if I decide on that I would start only after one of the C50 currently in progress is finished and test sailed in stormy conditions similar to what would be expected in North Atlantic. I also have to secure the build site, owner of the hangar I plan to rent space will be back in Latvia after few months, the place needs some cleanup from various junk and I need a guarantee that the place it will be available to me while the boat is built to a stage where it can be put in water. It would be no fun to have to relocate half built hulls and finish who knows where.

Possibility of dry spots freak me out. In carbon it is hard to see where it is infused properly and where it may not. It is absolutely critical for masts and beams. Having a hidden dry spot could easily lead to failure later.

 

4m sheets of ply are handy, albeit difficult to move around without help.
You don't have to infuse the carbon, you can wet it by hand and vac bag it, which is the traditional technique.
There are several ways to make infusing carbon reliable.
Do a test sample first. Infusing half a metre of mast at full thickness, weighing it and cutting it up will ease your worries.
Use carbon made from thicker filaments, which allows easier resin movement. eg, Zoltek
Use bleeder material between layers. Some of this is structural (the off axis glassfibre), some is light weight tissue or continuous roving.
Infuse via infusion medium from the mould side of the laminate to the inside. The resin travels through the laminate and into the peel ply on the inside surface where it is visible through the bag. The resin must wet out all the fibres on it's way from mould to surface. You sacrifice the mould finish (not the fairness), but you lose this anyway when you paint the mast so it doesn't matter.
Lengthwise breaks allow it to be infused in sections, further ensuring complete wet out. The breaks also work when infusing the hulls and beams.

 

Sorry disturbing this thread by trivialitys but that is simply not true. Neither the generality of the claim nor the generality of the "acceptance".

Where does this bad-air argument come from, let's have a look at it:

1) Reducing the view to "classic rigs" with lots of drag making parts - stay, shrouds, diamonds, spreaders, halyars, lazyjacks ... some cruisers really look like a jungle of strings. Of course, nothig get's better by doubling something (aerodynamically) bad - but that doesn't apply to sleek aerodynamic designs like the Haryproas.

2) Isolated consideration of a sail alone. Of course, the sails influence each other - and if two boats sailing close together this is sometimes disadvantageous for one of them. But virtually any boat with a jib proves - in total, both sails together can generate more propulsion than the sum of both sails alone.

3) A commonly heard argument for a high single rig is reduced induced drag. But that is completely dependent on the basis of comparison - so let's compare a single rig with a schooner rig based on the same mast height.

Assume two sails very far apart, so that their mutual influence is negligible. If each produces half the power of a single sail of the same height, then each produces only a quarter of the induced drag - the half in total. Of course, this advantage decreases when you bring the sails closer together - and disappears when you combine them into a single large sail.

--> To achieve the same induced drag single rig MUST be higher than a comparable schooner rig, assuming the overall aerodynamic quality is similar. With all the disadvantages, such as greater forces on larger lever arms, or the more difficult handling of a single large sail area ...
-----------------------------------------------

I'm NOT saying that multi-mast rigs are always better. (In my area, with lots of bridges and often limited maneuvering space and, they would be totally impractical. But I'm always surprised when someone writes about a schooner "oh, she did surprisingy well". The only surprise is the over-simplified application of aerodynamic by people who should know better.

 

For large cruising boats two masts may be ok to reduce the area of individual sails to be more manageable. Balancing sail forces easier with two masts. There are some advantages there, but pure performance probably not or we would see some high end racing boats with more than one mast.
Schooners definitely can go fast downwind at angles where all sails get good wind.


Going to windward is where they loose to similar sail area boat with one tall mast.