Model testing the Almaguin, part 3: The towing rig

Previous posts on model testing the Almaguin have looked at the construction of the framework and the hull surface. Today, we'll take a look at the towing rig. But first, the close-to-finished model:

Why orange? Well, I happened to have some orange paint left over. And test models should be high-visibility colours; white and yellow are popular- you want good contrast between the hull and the water around it.

Why does it look kind of rough? Because most of what you see here will be completely clear of the water once the boat starts to move. A perfect finish would look better, but would make absolutely no difference to performance. Being short on time these days, I simply didn't bother to do additional fairing in the above-water areas.

The actual running surface isn't quite mirror-smooth, either: it's fair, but slightly textured. That probably seems odd, since this area really is critical for performance. The rationale has to do with the scaling from full-size to one-quarter-size. Froude number, the ratio used to match wave-making characteristics (and therefore to determine model and full-scale speeds), scales with the reciprocal of the square root of the waterline length. But Reynolds number, the ratio used to determine whether you're dealing with laminar or turbulent flow, scales directly with length. That means that in the model, we'll have laminar flow in places where the full-size boat has turbulent flow. Careful texturing of the surface can help to induce turbulent flow where it would otherwise be laminar, allowing a better match between the model and the full-size boat (which, by the way, should have a carefully faired and sanded smooth underside.)

A test tank is a large, expensive piece of equipment, and I don't happen to have room for one in my backyard. But I do have a lake nearby, and I do keep a small powerboat in the garage....

A basic tow test rig is actually quite simple to make. A few bolts, a bit of lumber, and the better part of an hour will produce something like this. It can be extended up to about 2 m from the side of the tow boat, getting the model well clear of the wake.

The model is towed from near its centre of gravity. (My previous towing rig was similar to the one designed by Tom Lathrop for his Bluejacket; there are many ways to configure it, but the important thing is to tow from a point at or slightly ahead of the CG. Towing from the bow can artificially damp out some of the undesirable behaviours that the model test is supposed to check for, such as yawing or porpoising.)

The connection is as simple as it gets- just a rope through a hole. This allows the model to pitch, roll, yaw and heave as it pleases. (The tow height is adjusted on the fly by heeling the tow boat slightly to port or to starboard.) There will be two more lines, not seen here- slack lines to the model's bow and stern, to ensure that if it does start to yaw, it won't go out of control.

This isn't an instrumented setup, obviously. Resistance and power predictions for a boat like this can be done much more easily, and with good accuracy, in computer models. The physical test model will be used to verify what I'm already pretty sure of- that the Almaguin will have a smooth, stable ride in a broad range of load conditions- and to check for extreme use cases that could induce porpoising, chine walking or other undesirable behaviours.



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