Pilgrim Diaries #3 - 31 October, '08

How thick should the plywood be?
How many stringers and where, how far apart should the frames be and how thick?

Over the years I’ve been doing some panel deflection testing to ascertain stiffness and yield point relative to boat size, use and weight.  There is a lot of work in this research and I consider the results part of my professional assets so won’t be advertising the results too much.  But I thought that some of you might be interested in the methodology and objectives of the testing.

Any boat hull can be considered a series of stressed skin panels, those panels defined as areas of hull skin bounded by stringers and frames.   Pilgrim like most plywood boats being framed and stringered the secondary consideration will be the size of the panels between frames or other substantial members that support the stringers.

For the purposes of determining plywood or planking thicknesses, frame sizes and stringer cross section and spacing  I’ve  worked out a range of panel sizes typical of my construction methods, and built a section of curved hull three average frame spacings long, but three stringer spacings wide. The objective is to test the centre panel to avoid the free end effect. I then build the  hull section from whatever material and construction method that I am interested in, an after allowing it to cure, coating it with fiberglass or whatever would be typical of the finished boat I put a DTI behind it, and  pressure  it with a hydraulic jack with a load cell to measure the “push”. I use a small sandbag to spread the load appropriately.

For a very light harbour racer I allow deflection of no more than 6% of the shortest unsupported panel dimension.  That’s usually the short way across from one stringer to the next.  The load is equivalent to 60% of the loaded weight of the boat.  For a daysailer operating in sheltered waters, the deflection figure comes down to 3% at the same weight loading. For a coastal cruiser its still 3% but the loading goes up to 80% of the boats weight, and for a blue water cruiser its 2% and 90% which is a bigger difference than you’d think. For an exploration grade vessel that’s 2% deflection under a load equivalent to 100% of the vessels loaded weight. 

There are also tests of the entire panel between frames, the frames themselves and, although I have not gone that far in physical testing myself, the torsional and bending load resistance of the entire hull.  You would not believe the loadings that high performance rigs can put on a hull, and for that I work on an analysis of the loads and a theoretical approach to the materials ability to resist tension and compression.

With the above information, I can work out appropriate skin thicknesses, stringer sizes and frame spacings for a very diverse range of boats.  Its interesting how light the harbour racers and dayboats can be, and how rapidly the stiffness increases with comparatively small increases in stringer sizes and skin thickness.  The small boats in particular can have frighteningly thin skins, and considerations such as trailer or debris impact damage determine the minimum skin thickness rather than the sailing stress or the requirement of simply keeping the water out.

Pilgrim is about blue water spec in terms of the above, and I’ve used a thicker skin and fewer stringers to achieve that.  But I’ve closed the frame spacings up to make it easier for the builder to keep the structure fair when framing it up.  Those frames also form the transverse vertical components that shape and support the interior bulkheads, floors, seats and tanks. This not only contributes enormously to the boats rigidity, it subdivides the airtight compartments into cells that mean in case of the vessel being holed there will be intact buoyancy compartments in anything short of the boat being reduced to matchwood.

I want a really tough boat, but it has to be reasonably light.  Skin weight and framing is a large part of that.  I’m not 25 years old any more, prefer to work smart rather than work hard, and am not one for manhandling heavy weights if I can avoid it so Pilgrim is intended to be within one persons ability to launch and retrieve, to push off a beach, and not least, to roll over and move about when building.  It’s a balance, too light is not strong enough to stand being run up a beach in heavy surf, too heavy and its beyond one person to heave about.

John Welsford
Designer.