Please visit our home site at www.TRILOBOATS.com.

Anke and I are building our next boat, and writing about it at ABargeInTheMaking.blogspot.com. Access to the net comes and goes, so I'll be writing in fits and spurts.

Please feel free to browse the archives, leave comments where you will and write, and I'll respond as I can.

Fair winds!

Dave and Anke
triloboats swirl gmail daughter com

Monday, May 6, 2013

The comPromise of Speed



There was once a young fellow named Bright
Who exceeded the Speed of Light.
  He left early one morning
  And returned  without warning
At dusk of the preceeding night.


Some things aren't meant to be rushed.


Speed is Relative.
-- Albert Einstein



The comPromise of Speed

Speed under sail is a glorious thing. But it isn't the only thing.

All boats are a balance of numerous compromises. One virtue at the expense of others. A vice tolerated for net gain. A good design is a good match with its owner's priorities. Such a balance is as precise, magical and rare as is to be found in any good relationship.

There are only two broad considerations concerning speed:

First one is easy. Generally speaking, displacement monohulls are limited to their hull speed, a mere one and a third times the square root of their sailing waterline length.

The only way to raise the hull speed ceiling is to lengthen the waterline. But a curious result of the math, here, is that short boats are faster for their length than longer ones. A long hull is an expensive way to gain speed. Cost rises exponentially at construction time, in maintenance, heavier gear and rig and linearly in harbor fees. Speed gains diminish exponentially with length, however... each extra knot of hull speed will cost you dear.

In effect, because hull speed is strictly relative to waterline length, it doesn't usefully distinguish fast boats from slow.

The second, more pertinent question is how easily does the boat reach its hull speed? Most boats achieve low fractions (to about half) of hull speed with little energy input. But diminishing returns apply as one approaches full hull speed. A fast boat will start ghosting sooner and reach hull speed sooner than a slow one. This is one reason that the crew's skill and attentiveness is such a big factor in any competition.

The faster boat, adjusted for length, is simply the one that, for whatever the reason, spends more time at higher fractions of the narrow range between zero and hull speed.

Hull shape plays a major role. Easy entrance (at the bow) and exit (aft) minimize plow and drag. Well placed curves ease water aside and return it to equilibrium with minimal disturbance. Hull shape may be optimized for specific conditions; flat water, steep chop or anywhere between. Wetted surface area (friction) from keels and such is a liability, balanced against lateral resistance and optimal ballast placement.

But these are hard to construct, and carve away volume and displacement. Angles of heel and motion are affected. Easy lines can make for relatively tender hulls, tamed with expensive ballast that reduces payload. Hull smooth and clean? No brainer, but what are the costs of keeping it so?

Then there's the rig. Bigger rigs supply more power, all things being equal, but size must be balanced with expense, stability and handling issues. Fail-safe vs fail-dangerous. Maintenance issues. Windward ability? Another round of compromises struggling for a point or two.

*****

Anke and I have a set of priorities for live-aboard sailing that runs in clusters approximately as follows:
  • Ease and economy of construction -- If we can't get it built, it's a pipe dream.
  • Ease and economy of maintenance -- A) We're lazy. B) We'd rather be sailing).
  • Ease and economy of operation -- Ditto.
  • Fail safer -- Hedge our bets.
  • Ultra-shoal draft -- Opens up our world and safe harbors.
  • Single handling in all weather -- If one of us is sick, hurt or otherwise occupied...
  • Reliable windward ability -- We count on being able to make good to windward shy of storm force winds.

  • Relatively heavy displacement -- We carry a lot of stuff.
  • Relatively high interior volume -- For inside projects and hangin' with friends.
  • Speed and efficiency under sail -- Suckin' hind teat.

Our solution has been box barge hulls sporting lee (off-center, actually) boards and simplified junk sails (on two+ masts).

It can be fairly said that each and every choice along these lines makes it just a little harder for us to achieve hull speed. It takes a little more wind or a little more time. Both of these, however, are free. We also tow a 16ft dory, which sucks up another erg of energy. One of these days, I'd like to take SLACKTIDE out for a spin without it, just to see what she'll do. But, around here, round-trips on windy days are never guaranteed.

Still and all, we don't find that our choices cut too deep.

Off the wind, we're surprisingly fast -- we objects in your mirror which are closer than we appear. On the wind... well... we'll get there, slow but sure, though it's often more pleasant to wait for it to turn around. In fact, it's only the last fifth of hull speed that we're reaching for. If we have wind, we cruise with a decent average for our size. We won't be placing in any races, but we get where we're going.

We do sometimes dream of speed, I won't deny it. A Bolger ROMP. A Wharram catamaran. A Brown JZERRO.

If we owned a magic wand, instead of a hammer, maybe things would be different.



7 comments:

  1. I was afraid you've been eaten by a shark or a polar bear...well, welcome back on the wouaib !
    My little scow is on the way, become 3D, i hope to lunch that box this summer :

    http://voilierfluvial.blogspot.fr/

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    1. Rumors of our demise have been greatly exagerated! 8)

      I checked out your SCOW 450... sweet! Looks like your coming right along with it and doing great work!

      Happy building!

      Dave Z

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  2. Hi Dave

    I would like to link you to that thread. It contains responses of designers because I wanted to build something like that.
    Warning, it contains responses that could be upset you.
    I hope you find it at least entertaining. I dont know what to think about responses like that.

    http://www.boatdesign.net/forums/boat-design/would-catamaran-ever-sail-46932.html

    my email: john5346boat@yahoo.com

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    Replies
    1. Hi John,

      Thanks for the link, and don't worry... ya can't design boxy boats without growing a thick skin! 8)

      Here's a comment that represents the gist;

      Many novices have come to this forum with designs that they came up with that have squarish hulls (no curves) because, I think, they assume that square shapes are infinitely easier to build than curved ones.

      There's nothing outlandishly difficult about building a curved hull. But those "corners" are awfully inefficient when pushed though the water.

      What is so difficult about making the hull curved (sides and bottom, fore and aft)?

      *****

      My response is that I essentially agree... there is nothing OUTLANDISHLY difficult in adding simple curves. Our Bolgeresque Advanced Sharpie LUNA did just that.

      But.

      Curves take more skill in both design and construction, involving considerable lofting, layout, cutting and spiling. They reduce interior volume and displacement (on given dimensions). Bevels, constant and possibly rolling are introduced.

      Custom professional design work can more than double the cost of the vessel(!). Waste materials (offcut from curves) reduce the return on your material investments. Increased construction time adds to cost in site rent and other overheads.

      I'm the last person to say that a boxy hull fits all or even most needs. But it IS suitable for many, and excels within certain use scenarios.

      For OUR needs, we find ourselves circling back to the box barge. Not because I design them, nor because we lack the skills for more complex vessels. They are simply economical relative to our needs, and sufficiently able once built.

      So to those who WISH to invest their time and energy in more complex hulls, I say 'good ON ya!' To those who require a simple vessel for simple needs, I say 'consider a box barge/scow'.

      And VIVE LA DIFERENCE!

      Dave Z

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    2. One more comment RE lofting:

      The biggest obstacle to lofting for the amateur builder, as I see it, is not so much the geometrical aspects (they're challenging, but fun and instructive). Rather it's the creation of a LOFTING SURFACE.

      You know... smooth, dry, level and as big as the boat's footprint. For those of us building on stumps in a rainforest, that's a tall order.

      Sure, it can be taken up and stored in pieces to one side, but that adds a LOT of material and physical labor. It can also be built solid as a building surface, but that's redundant and little can be repurposed for boat structure. Half or third scale lofting is possible, but I've been around some top professionals who made some annoying mistakes that way.

      The pieces can be used, later, for interior structure, but since it remains useful throughout, we run the risk of premature repurposing.

      In contrast, box boats allow what lofting there is to be done full sized and right on the material. In TriloBoats, this is most often confined to a single sheet at each end. I'd say this cuts at LEAST a week off a (non-'instant') lofted ply hull, all told, and as much as a month off a Curvy Dog. [Don't forget to count the time earning the money for lofting floor materials!]

      In short, I think those who dismiss lofting as 'easy' are understating the case.

      Dave Z

      PS... Note that many designers (Bolger, Michalak, Benford and others) specify lofting right on the materials. This class of 'instant' boats IS relatively easy to loft (a day or two's work, give or take).

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  3. "We do sometimes dream of speed, I won't deny it. A Bolger ROMP. A Wharram catamaran. A Brown JZERRO."

    Ha Dave, I too have dreamed of all three of these, I have a special fondness for the Bolger Romp,such a great shallow draft design with an aggressive and seaworthy look, almost a bolgerised Bristol Channel Cutter! My brother has a wharram styled proa. It's a very interesting boat as well. But he is gradually converting it to a Jzero style rig, showing just how well Russel Brown did with his first design.

    I've just sold Snow Petrel, and bought a bigger alloy boat that I can retrofit a lifting keel to to hopefully follow you into the shallows oneday.

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    Replies
    1. Hey Ben,

      Our big problem RE multihulls is that, given the amount of stuff we want to carry, we need a whopping big one or we wallow. Sigh.

      The ROMP and its descendants, said Phil, are barge mid-sections with the ends pulled out fine. All the great advantages of the barge shape, with slippery ends (at the cost of increased length). Plus all the other curves. Sigh.

      Double congrats on the ebb and flow of your vessels! We'll be looking for you from the shoals!

      Dave Z

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