Designing a fast open deck catamaran

Is it necessary to use metal screws with epoxy construction ?
I would look at composite fasteners, much kinder on saws and planer blades.

 

Probably when you start looking at modifications to the hull form.

It's certainly possible, although it also seems possible that length might mitigate that tendency (much like in cars). Just a guess.

Right, the V hull is suboptimal and woods abandoned it long ago. The question is how bad is it? Is it worth designing a new boat to avoid it? Maybe in the case of the rounded V it's just a bit of extra wetted surface that hurts light air performance? Certainly wharram designs are hurt by the V hull as well as lack of foils and crap rig. But Ondina just has the V hull and that's somewhat mitigated by the round bottom.

I'm sure you could mount the rudders on the transom if they aren't already. Many of the woods designs I've seen are daggerboard or "low aspect ratio keel," with the builder choosing.

I wouldn't assume the daggerboard case (assuming thats what it is) is structural. I'm sure the interior could be modified to suit your needs.

Certainly Ondina meets that requirement.

The other reason I suggest not designing your own boat is I knew two separate people who designed and attempted to build their own large catamarans. They spent a lot of time second guessing their design decisions, reinforcing things that didn't need it, etc. It was very inefficient, and in an odd coincidence both of them passed away around the age of 60, neither having finished their boats. I also knew a guy who built a Pahi 42. He stuck to the plans and finished in a couple of years. So I think there can be huge benefit to having plans that you trust and can stick to, although that really depends on you.

Admittedly your hull looks looks a lot better than the two self designers I knew, and as oldmulti said you have the benefit of having constructed a fairly large boat before, so clearly you know what your are getting into.

 

Putting the screws in is the easy part. 5,000 screws at 20 seconds each is 29 hours. Rot proofing the holes is the tricky bit. All the screws should be removed (maybe another 29 hours) and the holes filled with epoxy using a syringe (and another 29 hours). Otherwise, any untreated damage to the surface and rot will happen.

I don't have scientific proof, but years of looking at screws in plywood boats and the rot which invariably starts from them leads me to believe that the damage can easily come from microcracking and/or water incusion caused by differential expansion of the glass, resin, ply and screws.

 

Then I get hulls big enough for comfortable living, load capacity of around 3 tons and at the same time great waterline length. Basically a 15 m cat with cabin space and weight of typical 11 - 12 meter fat bridgedeck cabin cat.

Rhea 40 cruiser with central cuddy by Woods Designs https://web.archive.org/web/20110522013153fw_/http://sailingcatamarans.com/rhea.htm

sheet ply or grp flat panel hulls
or foam sandwich round bilge hulls, ply or foam decks

DISPL (empty) 3.5T
DISPL (loaded) 5T

at 10% 6.5

at foam or even strip cedar less than 3.5 so..3000 kg loaded.

 

One hull for living and one for storage can be tricky for weight distribution.

I suggest drawing up a table of weights.
Galley, fridge, water, nav gear, clothes, bedding etc. on one side.
Tools, spares, water, food, basically everything else on the other. This will be heavier.
After comparing, you can work out a loading plan to locate the weights evenly across the boat. Keep weight out of the ends!

 

That's what I will do when I have figured out how I would like everything arranged then give realistic weights for every piece, maybe slightly on the heavy side to remain on the conservative side and to account for filleting, paint and similar weight gain. I'm slowly going through The Multihull structure thoughts, it is a treasure trove of good info.

I'd like a pivoting centerboards to avoid breaking boards when hitting something. Essentially a vertical daggerboard case would have to be turned 90 degrees so pretty big modification. I dislike low aspect keels because they add permanent extra draft, extra wetted surface when sailing downwind. It is so nice to be able to anchor in water so shallow you can just walk ashore.

Interestingly that wasn't my case when I built my previous boat. It took something about 1500 - 2000 hours which seems to be pretty average for similar amateur built foam strip plank boats. A similar size plywood Wharram cats are claimed to take around 1000 hours so there are more incentive to go with plywood construction because it could take maybe two years off the build time for bigger boat.

Problem is hulls are too short to have everything for long term living inside one hull. I plan to sail a lot in colder climate so efficient heating is fairly important. Heating three separate spaces would waste a lot of fuel and make heating system unnecessary complex. Catamaran hulls already have poor internal volume vs surface area ratio compared to short fat monohull. Heating two hulls would make this problem even worse.

 

That definetily has to be taken into account. I presume compact and heavy stuff like batteries and water can be placed in a living hull without taking too much room.

 

Ahoi @Mulkari - how would you name her? Titanic II?

I'am joking, of course - but reading about "fairly cold places" i must immediatly think on the Arktis. In fact, you have much more sailing and boat building experience than me - so propably i can't give practical advice - but some thoughts popped up in my brain that you might find useful.

First of all, these are the books by Wolfgang Hausner - you must be related ;-)
The long story in (very) short - as a young man (in the 1960s) he migrated from Austria to Australia - lived there in some Crocodile-Dundee-Style - became fascinated by the idea of a boat as a living space and transport vehicle at the same time - started to learn sailing and began at the same time design his boat - coming up with "Taboo" - a selfdesigned and selfbuilded 10m Cat - sailed out, and rounded the globe one and a half times. If I remember correctly, this was the first circumnavigation with on a sailig catamaran.

The boat was wooden, with U-frames, shallow keels, and a bridgedeck-cabin with seating headroom. Some years later, Taboo found her reef, sunk - the guy survived somehow - started building a new boat - coming up with ... what do you think? ... a new 18m cat, flatdeck, V-hulls - as a living place for two, sometimes some guests.

If you don't know his books, reading is a must. Beyond the very exciting and modest rather than exaggeratedly described travel experiences - you will find a constant critical examination of the boat and equipment - by a seafaringly experienced and technically brilliant mind.

He is still living and sailing in the Philipines, and some times acceptig charter guests:
Wolfgang Hausner der Abenteurer, Weltumsegler und Buchautor https://wolfgang-hausner.com/

The Second ist the story of Gleda.
Gleda is a Tiki 38, built with love and passion in Great Britain, the entire building process carefully documented on the Internet. The boat, the skipper, and his wife were ready to go, the living place given up, the jobs terminated - but due to the weather they could not make the passage to Spain before winter came ... and had to survive a harsh british winter on the boat in the harbor.

Survival is the right word. Although they had prepared very well - the boat and the deck tent were extensively insulated and heated - they constantly had to fight against moisture and mold. Everything that could mold, molded, and had to be completely replaced in the spring.

The point is - ever since people started insulating their houses, they have run into the same problems. There are solutions for this, e.g. sophisticated diffusion profiles so that moisture that has penetrated or condensed can also escape again - and this can probably be achieved on a large boat - but it is not as easy as it looks at first glance.

The Third is - you seem to be a lucky man whose boat is not limited by the size of her place - unlike most of us Central Europeans.This allows for "unusual" approaches, and I would like to draw your attention to an unusual idea:

Future - Cat for Two https://multihull.de/future/ and
Future - Segelspass für Zwei https://multihull.de/future/seiten/konstruktion.htm

Please do not look at the particular design, but at the idea behind it. Enlarge them to your own needs. Judge the concentration of space, masses, surfaces - as well as how 4 people live together - compared to a 2m wide tunnel, which is still limited by lots of insulation material ... but the most important thing is the size and the style of the hulls. They generate dramatically lower forces than an "inhabited" hull in their interaction with the waves ... that's why beach cats or amas of trimarans are built like this.

Maybe you don't like the concept at all - but if you do, we can discuss the last point in more detail.
^^ ropf

 

Ok, got it. I'm assuming you designed your last boat yourself? In that case you definitely know what you're getting into! Just more of the same, but maybe 4x the work volume (hopefully mitigated by the used of plywood)

I agree with oldmulti, continue on and consider using the scantlings from Ondina.

Right, I got that. I was just saying in the boats I've seen the daggerboard case was optional and could be omitted without compromising structure.

Have you seen the Woods Salish? Much like your design it's meant to be a fast and spartan with simple plywood construction. Notably, Woods chose to use dory hulls which at first seems like an odd choice for a racing boat. It was intended to have a good chance at winning the race to Alaska (at the time it was designed) where there are sometimes light winds and it would need to be rowed. Of course the added surface area of the dory hulls hurts most in these exact conditions, but obviously Woods didn't think it's an issue. This makes me wonder whether you need the four panels on the bottom of your boat and whether you'd be just just as happy with the performance of a simpler hull like Salish.

Also, have you tried using michlet? It's free and it's not hard to use. I managed to get it up and running when I designed a very simple row boat some time ago.

 

I can't remember if I've posted this before, but the late Malcolm Tennant designed a whole bunch of open/podcats in the 12-15 metre range, some more cruisy than others. Getting the plans would be the difficult part, but as you've already achieved a design/build project you might not need to.

 

Will check them out , sounds pretty interesting, really like the technical part.

I will have to consider that. My current boat have 25 mm foam sandwich construction and condensation mostly happens on aluminum crossbeams where they go through hulls. With standard fan type electric heater it is quite easy to dry out the inside, just have to run the heater and leave the hatch a bit open to encourage air circulation. With diesel heater and small wood stove these problems should be manageable. I plan on having around 50 mm insulation so that would be twice as good as I have now.

I have thought about pod cats. Some of the features I really like on my current boat is big open deck space between the hulls. It's kinda like mobile backyard. Pod cat or trimaran lacks that. Boom is about 1.3 m from deck giving excellent access to mainsail although you can bash your head into it if not careful. If there is some snag with reefing or something it is super easy to fix . Now imagine at night in 30+ knot wind and 3 m waves having to climb on the roof of a pod to sort out some problem. Same thing applies to most bridgedeck cabins too. Not sure I understand why there would be significantly less forces on a pod cat. Aren't that dependent on overall weight, length and beam?

Yeah, my current boat is of my own design with a bit of a help from other multihull builder.

Wouldn't flat bottoms at the bows slam annoyingly? Even my cat with V bottom about 1/3 the length from the bow occasionally does a pretty good slam when jumping off a wave with half the boat length leaving water and coming down hard. Dory hull is simpler, but 4 flat panels seems still be very simple compared to round bilge hull made from hundreds of foam pieces while probably 95% as good from performance point.
Have not yet tried michlet , but will give it a try.

 

It's a dory for most of the hull and veed in the forward section, like in the image below. I would imagine there is a lot of time saving on fitting panels, filleting and taping, interior fit out, etc. And if a boat like Salish can get away with it then maybe it's not such a huge performance issue.

 

Ah, this image makes it more clear, I initially thought bottom was just one flat strip of plywood, but it is twisted actually to form V shape at the bow. Although not sure if 12 - 15 mm plywood needed for a bottom of bigger boat will accept that much twist. Maybe would have to be built from 2 or 3 layers of 6 mm ply. Looking at the image there still are 3 panels per hull side as my design. First bottom, second for angled lower side, third for vertical upper side. If eliminating third vertical panel and continuing second angled panel upwards then two panel construction per side would be achievable. Maybe I should model several bottom shape variants to my target displacement of 9 tons and see how much wet surface area change.

 

Eliminated extra chine at the bottom and gained only around 0.12 m2 of wet surface area. So yeah, extra chine is not worth bothering with. Any eliminated part is build time saved.

 

You are right, open decks are great. However, I think in smaller dimensions - like the cockpit of a Tiki 26 ;-)

I completely agree with you that in reality it is mostly like that - but I think that it does not have to be in general - especially not in your dimensions.

How much living space do you have in two typical 16m hulls?
If designed for "comfort" - maybe two double cabins each between the pantry, navigation, sanitary facilities - let's say at shoulder height a good 14m² per hull - in the foot area it is significantly less - and also significantly less if the design is supposed to be "fast".

Now imagine a simple box - as a rough simplification for the pod - just to estimate the dimensions. You get a box measuring 7m x 4m x2m - which you can divide up to your liking - from floor to ceiling.

The hulls would be roughly 1m wide - then you have 1.5m of space on the side between the pod and the hull - and 2x 4m x 7m area in front of and behind the pod - as a trampoline or slatted deck, or whatever.

So far a generous rough estimate - in practice you would probably choose the pod smaller.

OK, I imagine it. The roof of the pod is a mostly flat surface with an area of 4x7m - located in the central part of the ship that is exposed to the least movement - looks to me like a completely safe platform - with an gorgious view ;-)

Of course, they must provide the possibility to get up quickly and safely (a staircase?)

First - look at your design or another - look at different sections to see how much volume is under and above the water surface.

Now drive in front of a wave - the stern pushes up with the full force, the accomodation-intendet Volume creates - pushing the bows into the water - which REQUIRES a large reserve volume in the bows above the water surface...

.. now imagine the same thing with the slim ama of a tri - it provides MUCH less lift in the rear - just enough to carry the ship - therefore the reserve volume in the bow can also be much smaller.

... now they turn around and drive against the wave. A slender bow "cuts" through - a fuller one "boxes" through - in any case, a large part of the bow volume is under water producing buoyancy - which not only catapults itself upwards, but also the other bow.

--> This force creates a strong torsional moment that is transmitted via the beams to the second hull, forcing it to rise as well. This is, alongside the rig tension, the largest and most determining load for the entire structure.

Yes and no. In a static view you are right - the masses pressing areas and volumes below the waterline - creating forces ... in a dynamic view - waves create forces on areas (pressure and impact) and volumes (bouyancy) - no matter wich masses insides. The the masses (in this case) only determines how fast themself are accelerated.