Model testing the Almaguin, part 5: Detailed behaviour

In the last few entries, I've covered the construction of the framework and hull for the Almaguin 400 test model, the towing rig, and some videos of the boat's behaviour. In this final episode of the series, we'll take a detailed look at what we've learned about this boat's behaviour from the model tests.

Thanks are due to Katy for taking the photos in this series, while I was at the helm of the tow boat.

Light Load

We'll start with the 1/4 scale model lightly loaded- not quite empty, but ballasted to 3.7 kg (237 kg scale), giving the weight and balance you'd expect with an engine, a fuel tank and one person on board. The model is towed by a flexible tether just ahead of her centre of gravity, allowing her to pitch, heave and roll as she pleases. The bow and stern lines are slightly slack; they're present to stop the model from spinning around when we manoeuvre the big boat at the dock. Scale speeds are double the true speeds at this scale. Once we're above 6 knots true / 12 knots scale, we can use the pitot tube speedometer (which, annoyingly, reads in statute miles per hour- so I've converted to knots here.)

Beginning at a dead idle, the scale speed is about 3-4 knots. The outer chine is clear of the water at this light displacement, which will make her a bit tender compared to common flat-bottom skiffs of similar size.

Accelerating to about 6 knots scale, the model is generating enough dynamic lift to get her forefoot clear of the water. The entry is just aft of station 2 and the bow spray is breaking around station 3. (On this design, as with many planing hulls, station 0 is the tip of the stem- in displacement hulls, station 0 is usually the stem/waterline intersection.)

Now at 8-9 knots scale, the bow is up, she's at a good slow planing cruise speed, and the keel is cutting the water around station 3. What little spray there is gets deflected outboard by the outer chine (and, in the full-size boat, the rub rail that protrudes another inch past the chine). The stern hasn't settled much, and the outer chines are just barely touching the water- the outer V is doing some work at this speed, as expected.

Now we'll take her up to 12 knots scale speed. The stern rises and the boat is fully on plane. The outer chines are now clear of the water and are serving only to control the spray, which originates around station 3 and doesn't leave the hull until aft of midships.

At 15 knots scale, she's really starting to take off. The hull is clear of the water from station 5 forward, the wake is clean and smooth, and the spray is leaving the hull around station 6. This is a great cruise condition for the Almaguin, corresponding to about half-throttle on the recommended mid-range engines.

We're now up to 20 knots scale. The boat is levelling out a bit as she generates more and more planing lift. There's no tension on the bow or stern control lines, and the model is now running mainly on her inner V; the outer V and chine are clear of the water but are still being hit by a fair bit of spray.

At 25 knots scale, there's not much change; she just rides a bit higher. There's no sign of porpoising or chine-walking.

Finally, at 30 knots scale, the model is starting to out-run her own spray. The inner V alone is enough to lift the boat at these speeds, and the wake is breaking cleanly away at the inner chine. We didn't take her much faster than this; in the full-size boat, you'd only be able to achieve this velocity with a skilled captain in calm conditions. We couldn't get the model to porpoise, but if you're not careful with weight distribution, it may be possible to do so in the real boat. In any case, 30 knots in a four metre boat is blazing fast, almost scary fast. But the boat can handle it just fine.

Heavy Load

The addition of a few more blocks of wood brings our model up to 7.5 kg, or 480 kg scale- essentially her rated safe limit under Canadian rules. This is equivalent to having three or four crew, some gear bags, and maybe a few nice fish on board.

Dead idle is about 3-4 knots scale, once again. We've made her pretty bow-heavy this time, as will often be the case when the Almaguin is heavily loaded. She digs a pretty deep trough for herself with this much weight! With the outer chine immersed, her motion will stiffen up considerably; her initial stability in this condition is pretty similar to typical aluminum flat-bottom skiffs of similar size.

Taking her up to the 6-knot range, the bow's starting to throw a nice chunk of spray. The outer chines (and rub rails, on the full-size version) catch it and deflect it away from the crew. The wake is breaking cleanly away at the transom, and she's starting to climb over her bow wave. This is close to the "plowing" speed range characteristic of most high-speed planing hulls, in which the boat throws a big wake and burns a lot of fuel. It's best to quickly accelerate through this range and get up on plane.

Now at 10 knots scale, the fully-loaded Almaguin is starting to climb on plane. This is a no-go speed in the real Almaguin and in most other planing hulls her size; fuel burn will be high and the wake will be large.

Taking her up to 14 knots, the model has levelled out and is fully on plane. She's still relying on her outer V and outer chines to generate lift in the stern, but the bow is clear of the water from station 3 forward and she's throwing a nice chunk of spray out to the sides. No spray has come aboard so far. She'll cruise comfortably at this speed for long stretches of time.

Now we'll go to 16 knots. The boat is running level, tracking straight, and hasn't shown any sign of porpoising or chine-walking. This would be a nice cruise speed, as long as things don't get too rough. Spray is starting to decrease as the hull rises.

The next image is at a scale speed of 26 knots. There's no real change in running attitude from about 18 knots on up. She's really flying here, and with typical engines you'd have a hard time getting her anywhere near this speed with a full load. The outer chines are still very much in play, as is the outer V from station 7 aft- after all, this is a four metre / 13 foot boat weighing half a tonne. We still haven't taken any spray on board.

Katy really liked the next photo, taken as the model fell off plane at about six or eight knots scale speed. To get a nice sheet like this requires a low Reynolds number (small characteristic length, low speed) to keep the flow laminar, and a low Weber number (again, small characteristic lengths, low speed) so that surface tension is a strong influence. You rarely see this on full-size boats, and when you do, it's usually just a little sheet near the bow.

There was one more test, already discussed in yesterday's video post: ramming this baby into a head sea. Not something you'd ordinarily like doing, at least not at 16 knots, but the boat stayed upright, in control, and reasonably dry. She did ship some spray, mainly from the tow boat's own bow spray, but after all those tests, this is all the water that got in:

And even that isn't really a fair assessment, since 90% of it came from the tow boat.

We didn't hook any scales up to the tow harness this time- the margins of error would be too large to make it worthwhile. And power calculations on a boat like this are mainly a regulatory matter (rated limits and all that)- in the end, owner-builders will just use whatever engine they happen to find that's more or less in the right ballpark.

Overall, I'm quite pleased with how the model performed. We couldn't get her to porpoise or chine-walk, she didn't ship any spray, she didn't stick her bow way up in the air, and she behaved more or less exactly as I had predicted based on experience and computer simulations. The same qualitative results will scale to the Almaguin 500 and to any future boats in the line, with appropriate adjustments to scale speed. The 500 is 25% longer, so her scale speeds will be 12% higher than those stated for the 400 in any given image (Froude number scales as 1/sqrt(L), sqrt(1.25)=1.12 ).



Add new comment