Small catamaran out of plywood

Discussion in 'Boat Design' started by skatun, Apr 18, 2020.

  1. skatun
    Joined: Sep 2010
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    Location: Aachen

    skatun Junior Member

    Hi all,

    We are building a small catamaran powerboat our selves. So currently we are in the design phase but we have a rough layout of it as shown below. Its just under 5m long and 2.4m wide. The two hulls is to be held together by 40*40mm square aluminium tubes and some steel wires diagonally.

    The hulls are to be made of 4mm plywood with 12mm ribs to form the shape. The question is this is going to be strong enough? Are the aluminium profiles to small? And how many layers of fiberglass should we do on the inside and outside of the plywood?

    I do have the model in CAD and I can therefore run a FEM analysis but for that I need to know the boundary and load conditions, something I have no Idea about. So if anyone can highlight this to me would be great. The boat will only be used in calm waters on the fjords of Norway.

    Also another important aspect is how to attach the two 15hp outboards motors to it, to make sure the forces are handled properly. Any feedback on the design would be great. image.png image.png image (1).png


    image (2).png
    bajansailor likes this.
  2. JimMath
    Joined: Jan 2015
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    Location: E.Cdn

    JimMath Junior Member

    Sailing Catamarans - First Choose a Design

    You could invest a small fee to get a proper pair of Power Cat Hulls from the designer above , for 2 - 15hp OB's ,maybe make a pair of Sk 20 Hulls .

    At least use the hulls and beams ,talk to the designer about your ideas , he'll help , probably save your life from hypothermia in those cold Norweigian waters , been there , beautiful & cool. Good Luck , plans come on line ,easy to follow and you and your family & friends will be safer.....
    bajansailor likes this.
  3. bajansailor
    Joined: Oct 2007
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    Location: Barbados

    bajansailor Marine Surveyor

    Hi Skatun,

    I agree with Jim's comments above - please do not start building the design that you have posted above. There is just so much 'wrong' about it.

    If you invest US$ 130 for a full set of plans from Richard Woods re his 18' Skoota power cat (which should be very close to what you want your boat to be able to do?), it will be money that has been very well spent.
    And this $130 will be a tiny fraction of the cost of building the boat.
    Link below -
    Sailing Catamarans - Skoota 18 trailable 8ft wide daycruising or fishing

    Re your 4 mm plywood thickness for your hulls - please note that Richard only mentions 6 mm and 9 mm plywood in his materials list.

    If you really want to add a superstructure to the hulls like what you have shown above, you could ask for Richard's advice - or even ask on here.
    Will the tubular framework of the superstructure shown be supporting a canopy at the forward end of the boat?
    And what is the 'box' shaped superstructure at the aft end - I am guessing it might be a toilet compartment, or a store cupboard, or ????

    Do you already have the two 15 hp outboard motors, hence why you want to make use of both of them?
    If not, then it would be much less expensive (and much more simple) to just purchase a single 20 hp O/B motor as per the Skoota.
    If you do already have the O/B motors, then yes, it should be possible to fit them on to the transoms of a boat like Skoota - but again ask for Richard's opinion on this first.
    JimMath likes this.
  4. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    As a general rule, two motors on a cat is preferable, a single may work to a degree, the most likely time it won't work 100% well, is when you most want it to, in rough conditions.
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  5. skatun
    Joined: Sep 2010
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    Location: Aachen

    skatun Junior Member

    Yes, I saw that he uses 6mm and 9mm for bulkheads, he also says he glass it with 200gm biaxial fabric, however Richard did not say how many layers he uses of the fabric.
  6. skatun
    Joined: Sep 2010
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    Location: Aachen

    skatun Junior Member

    I have 1, so thats why I thought about just getting another one, I see Richard uses 10hp, so I might get two 10hp instead
  7. skatun
    Joined: Sep 2010
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    skatun Junior Member

    The Idea was that the aft bit will hold a small toilet.

    The tubular structure is square aluminium beams. Front beam will hold the trampoline and aft one will support the engines, the rest will support the living area and stiffen the two hulls. Thanks for all the feedback. I will update the design in the following months.
  8. skatun
    Joined: Sep 2010
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    Location: Aachen

    skatun Junior Member

    So here is a bit background information about the project to give a better understanding of the whole project. The aim is to make a 15m catamaran in the end with the specification given here. But start of building a cat like that without taking it in smaller steps does not make sense to me. So hence we will make this prototype in scale 1/3. We have already made some smaller protypes and simulations of various parts but we would like to get some hands-on experience and hence we would like to make a small power cat. We also need a small boat to go to work and the store to save time so that’s a big motivation as well. These trips will be under 4NM and in calm waters. If the weather is to bad, we can always drive and catch the ferry.

    So this means that we need some kind of close apartment with diesel heater.

    Back to the topic, we were asking for some structural advice, either as loadcases for FEM simulation or some tips of how many layers of fiberglas on top of 4/6mm plywood as well as strength of the beams holding the two hulls together. We seem to find little information about this around the web.

    If you read various blogs/vlogs most timewise only a small amount of time is used on the hull. So with that in mind lets start with blank sheets and remember we are in 2020 where laser cutter, 3d print and wood cnc mill is available at almost any makerspace around the globe. The problem is that I can not find anyone that sells a step file of a design that fit our requirements. Richard sells drawings, but these are not digital and basically I have to remake it from scratch to be able to use state of the art production technology.

    So what can we do to make a faster build, remember this is prototype and the finishing details is not so important.

    Number 1

    Curved surfaces add a lot of extra time, but it’s the traditional way of making a bridgeway look cosy and nice, but look at sunreef or silent yachts, they have straight walls and it looks decent.

    Number 2

    The infrastructure to run wires, ventilation and plumbing takes up a lot of time. So how can we reduce this? Instrument on bus system, lights on bus system, wireless switches is a no brainer. Then we spent quite some time on brainstorming this topic, what if we can have cable channels and air vents made out of the structural components in the boat? So the aluminium beams that you see in the design is for that purpose, we run hot air in some and cables and hoses in some other ones. No need to make holes in cabinets, covers to make it look nice etc. Another huge time saving.

    Also by having the design in cad and using a cabling module you can cut and install all the connectors on cables in your workshop instead of being at an akward position at your boat.

    Number 3

    Windows are always tricky, so how can we do it cost effective and fast. Generally we want to have as much light in the boat as possible without heat loss. For the prototype with a small volum this is not so important but for the fullscale it is. Lexan or arylic have their advantages, but there are few problems: huge thermal expansion,price scratch and UV resistance. So a normal glas are better on all those aspects but suffers lower visibility and higher weights.

    When it comes to instalion of glass is where the time skyrockets. You can use tape as Richards suggest, however the time is mostly spent in alignment, cutout and make the cutout waterproof. A window will leak at some point and the core needs to be sealed of properly. So in plywood design you could add a plastic trim to the edge, thicken epoxy or other method. But its gonna take hours to do so.

    So our idea is just glue the glass with tec7 in between aluminum beams, no sealing of edges because aluminium is already waterproof, but the thermal expension of aluminium which is way lower then acrylic requires a few mm of elastic sealent.

    Number 4

    Here I completely agree with Richard, outboard engine no doubt, easier to install and no risk of water leakage. The tank should be plastic and and placing it in a self draining comparting will make it easier , because then you don’t need make hull through for filling. A 30 liter tank for a 10hp engine is more than enough. I guess that should give range of at least 100NM.

    Number 5

    We want to have solar panels, I guess no ones design a boat these days with out a solar panel. So to save some time, design ur boat around them instead of spending hours later to place them correctly(I remember Richard saying this about his scoota 28). Most solar panels has standardized sizes and frame. So to save time this will be the roof of the boat. The idea is to 3d print some downlight holders, glue them on the underside of the solar panel, sprayfoam the whole underside, use hot wire to trim excess foam and glue 3mm bamboo strip on the underside, drill holes for low depth downlights and then weld the panels in place to the aluminium frame.

    Number 6

    Make the floor simple, no hatches or anything where water can get in, a simply flat floor with vinyl glued on top of it.

    Number 7

    It takes some time to install all the cleats, stanchions etc, if going through plywood the holes needs to be drilled bigger, filled with thickened epoxy and reinforced below deck. So by welding these directly on to the aluminum frame we will save some time.

    Number 8

    As said the hull is not the time consuming part, but with new technology you can join plywood with dovetail cut out on a laser cutting machine a cnc mill instead of the baffled joint. Also the design should have as many straight lines as possible and uses as few plywood sheets as possible. Holes for hatches etc should be made in CAD. For performance a flat bottom is not ideal and hence hotwired cutted foam blocks can easily be added to a flat bottom to make the hull have curvature at the bow and below the waterline. To save time the inside of the hull should be glassed with peel ply in vacuum bag while on the workbench. Then after the inside is cured it can be cutout and when the hull is glued together on the inside its just the seams that’s need to be glassed.

    Number 9

    Oh glorious sanding… It takes time to make a glossy finish. Below the waterline is where it should be smooth, however this is not where you see it. So I am currently unsure how to finish it. But the design needs to be single curvature going through the waterline for sure. This way we can either use mylar to get the perfect finish above waterline when it comes out of the vacuum bag or we can use film afterwards. The surface below the waterline, which is double curvature shape which means we need to cut the fiberglass and also join the sides of the hull here, will be finished of with peel ply and hopefully not to much fairing compound before the antifouling can be applied.

    So these are my thoughts so far, I rather use several hours in CAD to save a few hours building it, because these hours I will also get back when making the fullsize version. I know the design look bulky, but if we want to have 2m high ceiling in full version we can not simply scale it down to 70cm on this version because then we will not be able to sit in it. Also a flybridge does not make sense on the protype. And also some dimensions has to be scaled so that it fits inside the 120*240cm plywood sheets. And my workbench is just 150cm*600cm. So there are some limitations with the prototype.

  9. Dejay
    Joined: Mar 2018
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    Location: Europe

    Dejay Senior Newbie

    Are you looking to get experience building a boat or experience designing a boat with the prototype?

    It might be smarter to focus on building the tender for your boat. Rob Denney has a neat concept for his harry proas so the tender fits seamless inside the deck and also works as the motor for the cruiser. He has plans for his catamaran T60 tender that you could take a look at, he has smaller ones as well.

    I wish Rob would design a power trimaran with his intelligent infusion approach! :) I don't have experience myself but from all I've read it sounds like the fastest, easiest and least labour intensive building method and produces very lightweight boats. He's also designing so the hull has slots so interior elements like bulkheads or shelves can be easily glued in. The idea is that a lot of preparation can be made by simply cutting fiberglass and foam before it is infused, saving time. Maybe this gives you some ideas.
    bajansailor likes this.
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