How to think about boat structures to increase panel stiffness and torsional rigidity without resorting to excessive weight. This is possible through structural efficiency even when building light structures.

Others in this discussion took a more lateral thinking approach with the suggestion the sailor was a better source of weight reduction!

Could a diet make the boat faster?

One of the forum members suggested that we go on diets!

That could take weeks!!!!!!! Much easier to build the boat light.

And a light boat can carry more beer with less degrade in the performance. Now THERE’S an incentive!!!

Distortion of plywood sailboat hulls while sailing

While weight is a consideration, so is resistance to oil canning and torsional rigidity.

Oil canning (panel flexing) will slow you down, and the wracking (twisting) forces exerted by the sails pushing against the blades and hiking crew can twist a hull requiring more rudder to hold a course–also slow.

Oil canning (inadequate panel stiffness) or twist are problems that can be resolved through structural efficiency. The materials can still be very light.

There are many boats that carry much greater torsional loads (bigger masts and sails, crews on leaning planks or wings) with no problems. The picture on the right is lifted from the video of the boat going upwind in around 13 to 15 knots – at times the sailor (me) is at full stretch (for me – I am getting older) and exerting the maximum righting moment and thus the greatest load on the mast. Note that there is no visible twist in the hull.

We did take a photo of the boat standing in the driveway with Peter and I standing on diagonally opposite corners of the boat. – much greater twisting load than will be experienced when sailing – and no visible flex.

Our OzRacers have come out of three sheets of ply – no extras – are fabulously torsionally rigid and the hull panels are adequately stiff despite being built of 3/16 (4mm) ply. (as a previous post pointed out a few dollars extra to get 1/4 inch – 6mm – ply would have eliminated the need for bottom runners in timber – a better choice, but the rest can still be 3/16 without any problems.

There are sailboats made of thinner ply that are loaded much much more.

Moth Class dinghies in Australia and the UK were built of 3/32 1/16 ply and display no significant torsion while sailing despite 20ft masts and the crew perched on wings 5 ft from the centre of buoyancy – rather than the 3ft of the geese and racers.

They also go a lot faster so there are big slamming loads to test the panel stiffness. – some will manage bursts of 15 to 18 knots (International Canoe, International Moth and many others). Slamming loads are proportional to the square of the speed (ie kinetic energy) – so these boats are generating 4 to 6 times the panel loads of the Oz Racers (kinetic energy is the square of the velocity. Though the Geese are similarly loaded

Enclosed boxes as part of the structure are the secret of torsional resistance

Geese going upwind. Blue boat is 5 years old. It finishes regularly in the top three.

In the case of the OZ Racer mk2 the bases of the rigidity are the three buoyancy tanks which as closed boxes are tremendously resistant to bending and torsional loads. These make up a substantial proportion of the length of the boat. Pic Below.

With the Mk2 there is only about 2 1/2 ft (750mm) of boat that is not so braced – but the side decks and centrecase structure is used to prevent the rectangular cross section in that area from distorting, so providing a girder that links the front box with the two aft ones.

With the Oz Geese and OzRacer RV we use a full length box down each side to define the structural rigidity.

Before the boxes got their permanent lids the boat was really prone to twisting. Whack the tops on and it is a different beast.

Structural efficiency for small boats is the key.

Rigidity is not about weight it’s about structural efficiency.

It doesn’t add more work – apart from the thinking – it makes the boats easier to lug around on shore and easier to lift for when the neighbours’s kids want to borrow the boats.

Lighter materials and less timber also save cost generally.

In the end we all build the sort of boats we imagine – as the wide range of puddle ducks displays – a variety mind numbing compared to the narrow views of the more conventional sailing classes.

Each to their own. 🙂 As you say – we get excited about all this stuff. We are having heaps of fun – you should see the reflections in the surfaces of the centreboards and rudders that I coated last night. The hollow tapered timber masts are looking pretty too.

Goose Plans information is here

More articles about structure here