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Boat Construction Techniques Discussion 
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Joined: Tue Jan 15, 2008 2:31 pm
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Building a strong boat is important. It's important because when you're battling a very heavy sea, the last thing you want is for the boat to start coming apart and to eventually kill you. I helped my father build out boat many many years ago, in fact, the only time that I EVER wagged school was to do just that. I don't claim to be a boat builder, or even an engineer, but I have a few clues about making things strong and understanding stress. I welcome discussion on this matter, from anyone here who happens to think they have a clue about this topic.

Our Boat

A short description of the construction of our boat.

Primarily bonded with epoxy, screws and diffused staples. Frames made in a fairly traditional way, with beams of wood meeting at the keelston and chines, supported at those junctions by plywood and a spacer. On top of the frames lay stringers. I guess they're about 25x35mm in cross section, made from clean grained native New Zealand wood. On top of the stringers lays 3/8" ply wood, 3 layers on the bottom, two on the sides. These laminations are done with epoxy troweled on with a saw pattern tool and stapled together with diffuse staples that pull it tight together. The front deck is similar, though I honestly do not know how it was built. I do know that it has beams and an inner skin of thin ply covered in "front runner". The cabin sides/pillars are solid pieces of wood, likely laminated, and attach to the roof, which is similar to the front deck, IE, hollow, thicker skin on top, thinner skin on bottom, beams and insulation in between.

In summary: A fairly classic approach.


A boat has to take four main forms of physical abuse:

  • Landing into water off of waves/smashing into waves head first
  • Smashing into wharfs while docking or docked
  • Supporting its own weight on a keel or the chine or both, high and dry, including impacts while it's not quite dry, and not quite wet, with waves.
  • Foreign body impacts into random parts of the outer surface

How does our boat do? Pretty well. It sits on the bottom OK, and has bounced off a few rocks etc from time to time. It smashes into wharfs OK and has rarely or never been damaged from it. It's been smashed into by various things, and punctured once by another boat's bowsprit, and dropped onto a cradle, and come out of all of those well. But in its early life, it had frames every 1m, and at some point a bunch of them ended up cracked/broken from a head-sea beating. Since then, those were repaired, and more were added in between to supplement them. The new ones only come up to the cabin floor, which is sufficient, the bottom takes the most stress under such circumstances.

Why did it fail? Rigidity and fragility. It's strong, but it's not flexible and it wants to flex. To have something robust you have two choices:

  • Let it flex and deform and rebound
  • Make it rigid and strong enough to not deform and break

It was neither.


Because of the natural amount of bracing within a boat, walls, floors, etc, flexing isn't an option. The boat must be rigid, and strong, and deform only in unison, if at all. What happened to our boat was that the basic hull skin, ply, stringers, they flexed, and the frame did not, and it instead cracked. The ultimate goal of construction therefore has to be to distribute load from the skin of the boat with the surface pressure from the water, through to structural members in a way that does not move or flex.

One thing that the frames of our boat lacks is triangulation. Up the centre of the entire cabin is a set of removable floors, and once removed, the two halves are not connected. In effect this makes the entire cabin and forward section much like a piece of tube, subject to flexing. The exception to this is the divider between the front cabin and the main cabin, which is pretty stiffly braced. The rear deck is more contiguous too, and thus much stiffer and less prone to failure.

So then, the interior structure of the vessel is just as important as the skin itself. If you're to have a hatch way or open space, the edge of this space must be strong and well connected so as to pass force between the two sides. As many frames as possible should be braced as high up as possible with ply/foam/ply laminate, and beams to triangulate the structure from chine to chine, and from centre to keelston. If possible, the frames should extend all the way up to the roof of the main cabin contiguously such that the roof becomes structural, rather than a rain proofing and dinghy storage area. This will improve safety in the extreme event of an open ocean roll over in storm conditions, too.

Equally important as good well braced frames are the chine, keel and keelston which keep the boat stiff fore and aft. Many boats, especially twin screw boats, neglect to use a keel in order to save wetted area, which is a choice I could never make. It only occurs to me now, though, how important a good keel is, in terms of being the real backbone of the entire vessel. At the chines, the skin likely provides enough stiffness from front to back. But in the centre the keel is the majority of the meat. There is no real structure above it from front to back. In our boat, the engine sits in the middle, and two huge beams run fore and aft through the centre 1/3 or more of the boat (and possibly all the way aft?), this is super important too in terms of both stiffening the hull, and distributing the load of the engine during a fall into the blue stuff. They're tied into every frame securely too. Thinking about it now, she's pretty solidly built. And when she was new, she weighed in at about 7 tonne flat. Not bad.

High Tech

Composites are something I don't know a whole lot about, but am going to make an effort to become expert in over the coming years. If/when I build my dream boat I want to build her in a higher tech way. I want to use foam sandwiches with light skins where feasible for surfaces and for stiffening. The way the skin is built, the skin meets the beams, and the beams are constructed will be crucial to it being good and not a disaster. It's important that if it is punctured, it not fill with water, so small compartments of skin are a necessity. It's important that if it's impacted and doesn't puncture, the core is not delaminated from the surfaces. It's important to realise a weight saving and stiffness/strenght increase over wood, or it's not worth the effort. Any advice/thoughts here welcome.

Further thoughts as I think them.


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Fri Oct 04, 2013 8:42 pm
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Joined: Wed Jun 19, 2013 7:11 am
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Nice, any CAD designs? I also plan to build sometime my own boat, but think little smaller than your Idea.
How long and wide should it be? Engine, and engine Power? Some one use Bmw Engines for it, in german we say "marinisieren", here a example :)

Tue Oct 08, 2013 7:26 am
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Joined: Tue Jan 15, 2008 2:31 pm
Posts: 15346
Location: Home sweet home!
No, no CAD design as yet, however as I mentioned in the other thread, I would like to have a parameterised model of it. In that way I could look at weights of components and the structure itself versus floatation in salt and/or fresh water with full and empty tanks and full/empty ice chests and see how it sits in the water balance wise. That's one of the big challenges of designing such a vessel: making sure it floats level. The other huge one is making sure it drives fairly level at speed. Then you have to fit all of that together into one big design. If I could set rules and relationships between parts and then just move sliders to change tank volume or engine weight or length overall, or whatever, that would be nice.

I don't foresee myself doing this build for a very long time, maybe another 5 to 15 years, however in the mean time I might undertake to build something smaller that lives on a trailer as practice. I might also integrate FreeEMS into that equation by using a car or bike engine and a custom drivetrain for it rather than a very expensive outboard. Another thread if/when I think about that more. Such a thing would likely be around 6 to 8 meters... OK, I've started thinking about it, new thread soon :-p Damn you. EDIT: viewtopic.php?f=13&t=2311

For the big boat, 38 to 45 feet, no more than 50 feet, 12 to 15 meters. Engine would be a single large inboard turbo diesel (6 to 10 litres, 6 or 8 cylinder) with mechanical injection (must continue to function submerged). Power in the 300 to 500hp range. The word in English is "marinised" meaning cooling systems have a heat exchanger, exhaust manifolds are water jacketed, intercooler is air to water, etc.


_________________ - where Open Source means Open Source, and Free means Freedom - the open source engine management system
FreeEMS dev diary and its comments thread and my turbo truck!
n00bs, do NOT PM or email tech questions! Use the forum!
The ever growing list of FreeEMS success stories!

Tue Oct 08, 2013 9:47 am
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