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Dave and Anke
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Monday, March 23, 2015

Form Follows Function: Comparing Bottom Profiles

Comparing two possible Triloboat hulls on the same footprint, plan and layout
Both deadflats end on bulkheads

I'm not bad, I'm just drawn that way.
- Jessica Rabbit

Form Follows Function: Comparing Bottom Profiles

Anke and I are building our new boat to the lines of the upper model shown.

It's the shape I typically draw for Triloboats - 1/4 aft + 1/2 deadflat + 1/4 bow - each given as a fraction of Length Over All (LOA).

This distribution maximizes interior volume and overall displacement, as well as rectangular storage areas. The long deadflat produces fully right angle carpentry throughout most or all of the interior, simplifying carpentry. It carries extra initial and reserve buoyancy at the ends, which dampens pitching, and makes the ends less sensitive to weight loading.

Of especial note, in this layout, the weight of the trunk cabin and contents of the large, under-the-cockpit hold (some may even have an engine in there) has more floatation in its vicinity. This allows heavier stuff to be stowed where the stowin' is good.

But it's not the slipperiest shape availbable to a box barge/scow.

The lower model shows a roll-up bow. This won't plow water when plunged into it, so doesn't slow the boat as much as the transom. If driving forward through water, the angle drawn (one among many) will develop kinetic lift. Cost is less reserve buoyancy (which helps lift the bow in short, steep seas, regardless of forward motion), and cuts some useful volume away (from, say, an anchor well).

The lower hull has had some of its underbody pared away, resulting in a hull that is easier to drive through water. Of the two, this should be the faster hull.

Its deadflat has been reduced to about 1/3 LOA (adjusted to land on bulkheads; not shown). This affects both bottom end curves, lengthening and 'easing' them. They are somewhat easier to construct, possibly avoiding the need to kerf. More importantly, they offer less resistance to the water, as it flows along the hull.

Lost volume is likely negligible, but lost displacement may cost you your collection of vintage bowling balls.

These changes drop roughly 2000lbs of displacement. Assuming all else is equal in terms of rig, gear, crew and outfit, that's 2Klbs of payload that comes out of your elective stores.

Depending on how you cruise, this might be a good trade; stuff for speed. After all, the barge/scow hullform - compared to many others - has carrying capacity to burn. It may well be that you can spare it.

Anke and I sail with a lot of food, tools, spares and books that see us through long spells between resupply. But most folks don't ask that of their boats. Instead of years, they're out for months, weeks, days or even hours. That 2Klbs is superfluous to the way their needs.

Of course, one could go to fully rockered bottom (no deadflat), and ease on down their road.

Both of these models are fairly well balanced, meaning that they should float fairly level, all things being equal. But other arrangements may not. For example, we use the upper forward and lower after end. The bow would then be buoyant, relative to the stern.

Not a problem, up to a point. In lading the vessel, we'd want to pay attention to weight distribution for trim. Heavy stuff amidships, and tending further forward than aft. Low and secured, as always, of course.

Point is, our needs lie along a spectrum. What shape we choose for our hull reflects how we see ourselves faring. And to shift the simple options of the box barge/scow isn't rocket science, but simply redrawing curves. Look at it from every angle you can think of... but you were going to do that anyway!

So, if you purchase one of our StudyPLANs - typically drawn with the higher capacity lines - please consider it but a starting point.

Remember the words of the late, great Dynamite Payson...

"If it looks like a boat, it'll pretty much act like a boat."

...and take up your pencil and play!


  1. we want a "junklette" !!! so cute. The idea demonstrated in this post of gaining 2000lbs extra carrying capacity is especially attractive to those of us floating(when the ice melts) around in "curvier dogs" can you wave your magic wand so we can throw out the mp3 player and bring aboard the turntable and lp collection. robin+karen, lac deschenes,quebec

    1. Ah, a challenge!

      I suppose you could tow a 'listening module'... sauce-pan bottom with LPs for ballast, weeble-style; acoustic cabin, windows and throw pillows all-round. Have to gimbal the turntable, of course, maybe even a gyroscope assist? Hmm...

      Anke tries to keep me AWAY from ideas like this! 8)

      Dave Z

    2. Or there's always that boring, traditional approach... repaint the waterline an inch or two up.

      You Curvy Dogs can usually afford it!

      Dave Z

  2. Displacement vrs speed dilemma nicely illustrated for the guy on the street. For folks bailing from the brainwashing of needing that curvy dog for the fabled round-the-world extravaganza, and realizing they'll probably stay a few hundred miles around home port (preferably in a archipelago!), the extra ton of goods aboard starts to make a "ton" of sense. After cobbling together 9 wooden boats, large and small, just the ability to work with right angles is ultra appealing. And the ability to spread out and really live once actually up and running as a floating home. I still maintain that a nice home base boat, like a trilo or a scow, with a little adventure sharpie auxiliary for exploring the true backwaters, covers the real needs of the majority of aspiring boat dwellers. Even those still cemented into that globe girdling reverie can build a trilo super tough and loaf along on the tradewind milk run (solar electric drive to get into the reef passes though). Oh boy.... here we go again..... armchair boat building and sailing..... so many boats..... so little time....

    1. Hi Robert,

      I feel your pain!

      Really like the idea you mention of big, base boat with an orbital, beachcruiser in tow.

      There's a point coming in the next decade or two where we can't lift our dory to carry to and fro the water. Then we may as well have a heavier, camper-cruiser.

      Being able to risk a smaller (and more nimble) investment in some of the wild and beautiful rock gardens guarding highwater beaches would really open up some great spots to watch the moon come up!

      So little time, but maybe enough?

      Dave Z

  3. hmm.......a "listening module" great idea, just imagine manoeuvring it into a secluded tidal pool for a session of attentive observation, music and contemplation! Could also be utilised as a guest pod if the guests could be trusted with the music collection and possibly the kegs of home brew stowed in the bilge.

    1. Hi Robin and Karen,

      The idea was used on land by Liz and Frans Reynders (two of the world's GREAT mimes), who built a home from surplus naval gun turrets of approximately beehive shape. They were all various sizes, and the smallest became their listening module.

      They were also across-the-board artists, and their home was a wonder!

      Dave Z