Model testing the Almaguin, part 1: The framework

Computer simulations are pretty good these days, but there's still a lot they can't do. And as nice as it would be to have my own full-scale prototype of every boat I draw, that's just not feasible- there's never enough space or money for that. Models, though, are inexpensive, don't take up much room, and are fun to build- and, if done carefully with the right mathematical backing, can offer a lot of insight into how the full-size boat will perform.

In today's installment, I'll cover the first part of the construction of a 1/4 scale tow test model for the Almaguin 400 and 500 runabouts.

The model is built using similar techniques to the full-size boat. This is efficient in terms of time and cost, and is also a good way to ensure that the boat will go together as planned. Here, I'm using 1/8" medium-density fibreboard for most of the parts. It's cheap, readily available, bends fair and can take tight curves without too much trouble. MDF is too floppy and weak for a full-size boat, but it works reasonably well for a test model.

A notable deviation from the full-size Almaguin is that the model has a solid central backbone. It could be built without one, but is quicker and easier this way (and it's not as if the space is needed for passengers or cargo). We begin with the backbone, bulkheads and hull panels all cut out, and loosely assemble the bulkheads to the backbone.

The hull side panels are then joined to the transom, just as for the full-size boat. Here, I've used small blocks of spruce/pine/fir as cleats to keep the joint at right angles.

The bulkhead positions are marked both on the side panels (as for the full-size boat) and on the central backbone. As expected, they line up just fine. The side panels are pulled in to meet at the stem; in the real boat, this can be a bit of a hassle as the plywood will tend to spring back out. The model's panels can easily be pulled into place by hand, and secured at the stem with a couple of zip ties.

Rough edges at this stage don't matter all that much. The critical lines are the bulkhead edges and the mating edges of the panels, all of which are within a couple of millimetres here. The sheer will probably be left rough for now, as it has no effect on performance. A few more zip ties on the model take the place of the temporary screws that would be used to hold these panels in place on the real boat. (Indeed, the zip tie method can scale to full-size too, if you can find big enough zip ties and don't mind filling in the holes later.)

The inner and outer V panels are now test-fitted. They won't stay in place yet, as the rest of the model is still pretty flexible. But when they're pushed down into place, they land right on the expected lines. The computer model and the physical model yield identical results, with the panels landing exactly where they should (to within the cutting tolerances)- that's a good sign.

With everything squared up and properly aligned, a thick mixture of epoxy and sawdust is used to fillet the joints. The 4 mm fillets alone will be strong enough for the test model; the real boat gets 19 mm fillets with 300 gsm / 10 oz fibreglass tape.

Coming to this site on the weekend: Fitting the inner and outer V panels and the chines, then fairing and waterproofing the outside of the hull.




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