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| WaterLines for a Box Barge / Scow |
WaterLines
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!
*****
IMPORTANT NOTE:
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).
So by my understanding, a triloboat with 12" draft upright will have a max heeled waterline 24"? Every inch one side goes down, the other side also comes up one inch? What is the height of your lee guard boards? I would guess about 30" above the chine, if your side copper goes up 24"?
ReplyDeleteHi Dennis,
ReplyDeleteYep. For a minute I thought the beam might change that, but no.
Keep in mind, thought, that it's a rule of thumb. Shape at the ends gets involved to throw it off, in theory. But in practice, I can't see that it leaves the ballpark.
Yes, our guards' bottom faces are 30" up from the bottom. This turns out to be half our total side height, so it's a bigger step up from the beach and a slightly shorter step over the gunnel. Copper is 24in.
BTW, your insulated sole sure looks toasty!
Dave Z
Posted on behalf of JOHN:
ReplyDeleteDave,
Would you further explain why lifting the windward chine on a square boat is so bad? I understand why putting the lee rail awash marks a major buoyancy/stability transition. And I understand why bringing the windward chine to the waterline is a handy indicator of degree of heel, and perhaps a good reference point to indicate the need to slow down. But I'm having trouble seeing that the boat's stability is dramatically reduced the moment the windward chine is lifted?
In some respects a barge is similar to a catamaran. Lifting the windward hull on a catamaran is certainly on the continuum between ultimate stability and capsize, but the few degrees between having the hull partially in the water and totally out of the water doesn't seem to me to represent a major change in stability (though, for a racing catamaran, it represents a major reduction is resistance)
John
Hi John,
DeletePicture the hull floating at 45deg. The windward chine is flying, and the center of buoyancy is dead center and you've lost all your form stability. All things being equal, it could fall over either way.
All things aren't equal... the boat's center of gravity is likely still to windward, so you'd still have some safety margin left.
So its not so much that those few degrees make such a difference. Rather you've engaged some big forces to get you that far, so things are woofy.
Past that point, righting forces start falling off quickly, giving you less time to take corrective action. Worse, once acclimated to that side, it's easy to think "I'm okay here... what's a leeeeeettle bit more?"
So it's a rule of thumb line that gives you a good working safety line, with good safety margin. Make a habit of sailing on the other side of it, and you're likely to go over at some point.
RE the catamaran, Wharram recommends never letting it fly, either. He under rigs his boat by modern cat standards and that conservative approach has proven its worth in millions of sea miles.
In the early days, cruising sailors would sail with one ama flying. Cats had a very bad reputation as inherently unsafe for the number of capsizes they suffered.
Nowadays, most cats are designed and sailed more conservatively.
Slower, but safer.
Dave Z
I understand you are making a proposition, but why not add some flare and increase reserve buoyancy in a blow? Not too difficult from a construction perspective, with benefits. Then you'd have a dory, with the stems cut off. Just sayin'. (what else would I say?).
ReplyDeleteHi Doryman,
DeleteCertainly, one could, and I'd be the last to stop you.
Remember, on any given OVERALL beam, square sections yield the greatest initial AND reserve buoyancy of any sectional shape (full 'prism'). On 8ft overall beam, say, a dory with moderate flare might have only 6ft bottom beam.
ADDING flare increases initial and reserve buoyancy, AND overall beam. Now ya gotta wonder if it would be worth widening the bottom to make the most of that extra width, increasing all that yet again. You could flare from there, but round and round ya go.
My inclination would be to choose the hull form to fit the intended purpose. Dories excel at being easily driven sea boats. Boxes barges excel at being simple and roomy on a given, overall footprint (or smaller footprint on given displacement).
But I think adding flare adds construction related penalties. More material is required (especially if beam is one sheet, the deck must be wider by the amount of flare, so more than one sheet), chines must be beveled (a rolling bevel in some cases) and interior structures meet the wall along an angle (rather than plumb), center of gravity is higher.If the sides come in, flare adds sheer, which complicates the intersection of sides with section of cylinder decks (curvature and flare have to work together. More material, more expense.
All in all, flare takes a simple box and adds cost, materials, head scratchin', layout and cuttin'. If it pays its way, great, but weigh it up!
(What else would >I< say??) ;)
Dave Z
PS. Forgot to mention that dories are cool as cool can be... always worth something right there!
DeleteDZ
There is some irony here - years ago, when I decided to build my 36 foot dory (thus the Doryman moniker), I had (and still do) a friend in Olympia, WA (Scott Hauser) who was turning out house barges at a regular pace. The one huge benefit to his designs, over conventional houseboats was that they had an outboard motor or two mounted on one end, so the authorities considered them boats, not floating homes and they could be moored in any marina that allowed liveaboards. His advert claimed you could take your home out for a spin anytime you liked.
ReplyDeleteI decided to build a floating home that was more efficient and could actually be used as a boat. Your designs (and others) weren't out there yet, or I might have gone in that direction.
I'm very pleased with my dory, in fact that big boat moves thorough the water with nary a ripple. Stability is still an issue, however, and many passengers feel uncomfortable with how tender she is. I could have, and should have, made the bottom wider, but on the other hand, I love the shape and the look of the boat.
You are spot on about difficulty. I built that boat by myself and the strain was sometimes overwhelming. Now, in my advancing years, would never consider taking on such a task alone.
Hi Doryman,
DeleteEvery vessel is such a complex amalgam of compromise that I find it hard them hard to compare. Any particular component (eg. stability) is easier, but even so, never as straightforward as it might appear.
Cruiser sized dories, I think are particularly challenging to design, the physical implications of their simple shape imposing sharp conflicts among competing priorities. But your fine vessel along with examples from Culler, MacNaughhton, Benford, and others prove that a balance can be struck.
And they are oh so loverly to behold! And slippery li'l devils, too!
Dave Z