|WaterLines for a Box Barge / Scow|
Mostly, in boats, we hear about THE waterline. Where the water is, right? It's a clear picture, held in common by most everyone.
But there are all kinds:
- A waterline -- A closed line formed along the intersection of a hull and the surface of water.
- The waterline -- Mostly what you'd think... where the water surface actually touches the hull. Or sometimes the painted stripe that's supposed to mark where it usually is.
- Design WaterLine (DWL) -- Waterline where the designer thinks it should be. That implies the boat loaded with all its outfit and crew should float right to there. Any more weight sinks it lower (raises the waterline), while any less floats it higher (lowers the waterline). For any given hull, the DWL determines its designed draft, displacement and freeboard.
- Upright WaterLine (WL) -- The waterline while the boat is sitting upright. A designer draws this in end and profile views as the DWL. In either view, it looks like a straight, horizontal line.
- Heeled WaterLine -- The waterline when the boat is heeled. A designer might draw this in end view as the maximum allowable heel. It looks like a straight, canted line.
Designers of 'Curvy Dogs have it rough. They need calculus or planimeters and other advanced figgerin'. Poor saps! Designers of Square Boats have it easy.
Once you've decided your draft, the Rule-of-Thumb method - shown in the lead illustration - works well enough to answer important questions.
Simply draw the end view, split as shown or one for each end. Draw the upright WaterLine and the vertical midline. Now draw angled lines running through one chine and the intersection of water- and mid-line, and carry out beyond hull.
Now check your transoms, paying attention to their lower, outboard corners.We're trying to avoid plowing the bow and dragging the stern. All four corners should clear the heeled waterlines. By a fair margin at the bow and as low as you can stand aft. Dragging a small V aft probably won't hurt much, and the lower the better for an easy release.
If you have an outboard motor considered its placement for depth when heeled. If leeboard guards, see that they clear on the high side. It's convenient to place their undersides at the top of the heeled WL, so you have a visual check for maximum heel.
Okay, pin a medal on yourself. You passed this course!
Looking at the immersed triangles when heeled, we can see right off that the hull is quite stable at this angle, and still has some margin of safety.
But once the windward chine leaves the water (starts to 'fly'), the situation changes rapidly!
Do NOT sail with the chine clear of the water!!! Turn up and reef down, instead.
Hear that? We do not sail them chine a-flying or lee rail under. Not unless you're racing in a drysuit!
Square boats get an undeserved bad reputation as unstable. Let's compare to a dory of the same overall beam, which has an undeserved reputation for stability*.
What happens is that the dory goes over soon, but with slowly increasing resistance. By the time it's on its ear, the crew is feeling tippy and works to correct before knock-down.
The square boat goes over late, and feels rock solid till past our maximum. Once that chine comes out of the water, though - shortly after that chine goes flying - there is a rapid reduction of resistance to knock-down. The crew has little time to correct from that point.
Another folly; often the dory will have high sides (it needs them), while a square punt will not (doesn't need them). Bubba wants to show off by standing on the rail. Might make it in the dory. Puts the punt rail under. Apples to oranges.
At all points, the square hull has more stability than an otherwise equivalent hull with cutaway shape. It is the speed of the transition which catches the unwary. Knowing this, we act early to stay on the safe side of the flying chine. Meanwhile, we harvest all the advantages of that extra stability.
In cruiser size, you've got to work to put them over!
For solid analysis by a real naval architect on a nearly square boat, see Spray: The Ultimate Cruising Boat by Bruce Roberts. See especially his discussion of stability.
*Not to say the dory is a bad hull. On the contrary, designed to fit its purpose and well handled, it is versatile and able... a whopping great hull form. It's reputation for stability confuses less, late acting reserve buoyancy with stability. In fact, dories knock down easier than most. This is why their sail rigs, if any, are prudently kept low and small, OR they are built with an extra large dose of ballast stability (e.g., Benford Dories).