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Thread: New/Old Boat Designs for Maximum Efficiency 28 mpg

  1. #1
    Premium Member bmunsell's Avatar
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    May 2007
    Wasilla, behind 3 Bears on the Palmer Wasilla Hwy

    Default New/Old Boat Designs for Maximum Efficiency 28 mpg

    Given the extended distances we would like to travel here in Alaska and the high cost of fuel, I have been daydreaming about this type of boat ever since I first saw it in the March/April issue of the WoodenBoat magazine. This version is called Rescue Minor.

    In an article at the builder Robb White reports that the boat at twenty ft long and 76 wide, strip built out of tulip poplar and powered by a three cylinder Kubota Diesel tractor engine rated at 20 hp, will run 20 knots in six inches of water top speed and at 10.5 knots gets about 28.6 nautical miles per gallon of Diesel fuel. In the WoodenBoat article the builder says he hauled all the shingles needed for his thousand sq. ft. house in one load, still got on plane and burned about the same amount of fuel as always.

    An internet search shows that the original design was by William Atkin and was intended to be used for wartime shallow water rescue duty.

    Think about it, would a guy be willing to give up higher speeds for 15 to 20 mpg efficiency? I don't know about running a wooden boat on the river, even though I read that the old timers did becuase it was all they had, but if the boat can be built in wood somebody should be able to build it in aluminum.

    I just thought I would throw the idea out there. I know it is summer in Alaska and everybody needs something else to do between midnight and 6 when they can't sleep anyway.

  2. #2
    Moderator Paul H's Avatar
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    Apr 2006


    Renn Tolman, designer of the Tolman skiffs was intrigued by the design, and found the fuel efficiency claims enticing. So a few years ago he built his version of the design. I'll have to do some searching for details of it, and I know I've seen a few pictures. Anyhow, he wrote a letter to Wooden Boat (don't know if it was published) detailing his experience:

    Dear Editor

    I wonder if there aren't some bad numbers in Maynard Bray's article on the
    Hand and Hand-inspired boats (WB No. 208), specifically in regards to his
    own boat, Constanza? I remember the original article (how long ago?) on the
    first boat of that type turned out of Harry Bryan's shop in New Brunswick
    and run down to Maine by the buyer, who, as I recollect, painted a very
    different picture. First, the economy. Seems to me he was running a 25-hp
    Honda, which perhaps Mr. Bray is , too. (Wouldn't it be nice to know in such
    an article which bandies about performance figures?) Anyway, I was really
    interested in the performance figures and recall fuel economy to be around
    seven miles per gallon, much more reasonable than the 15 or so Mr. Bray
    claims. (The reasoning is a 25-hp engine burns upwards toward 2.5 gallons an
    hour WOT, and at 16 knots the engine must be burning fuel approaching that.)
    Secondly, there were performance issues, among them that at speed the boat
    tipped back on its rockered bottom, impeding the operators vision. This
    indicates the boat was being driven faster than it was designed for. . . If
    you would like to tell us what issue this original article appeared in, we
    could perhaps make up our own minds about this boat.

    But the real thrust of this letter is to report on the performance of a
    Seabright skiff I recently built, which may refute some of the "near magical
    characteristics" Mike O'Brien refers to in his article on Atkin Seabtight
    skiffs (WB No. 208). The Atkin Seabright model that inspired my design was
    the Rescue Minor, which W/B readers may recall was the model that also
    inspired Robb White (WoodenBoat, March/April 2006). Like Robb's mine was a
    radical departure in appearance from Atkin's design. It has roughly twice
    the freeboard, flaring dory-style sides, a well-rounded forefoot replacing
    the Rescue Minor's plumb stem, and two pairs of spray rails. In short my
    version looks much like my Standard model Tolman skiff except that I was
    careful to retain the long, slender hull and most importantly the exact
    dimensions of the tunnel the prop runs in. In addition, I matched Atkin's
    specs for the horsepower, the shaft speed, the gear ratio and prop diameter
    and pitch. Since my version was made of thinner plywood than Atkin called
    for and was built Tolman skiff-style without conventional framing utilizing
    longitudinal framing members and sewn-seam construction, I can assume it to
    be substantially lighter than Atkin's design. Okay shipmates, here comes the
    magic part. Atkin predicted the Rescue Minor would achieve 17 mph. My
    version maxed out at 13.

    So what's going on? Readers may recall how successful Robb White's iteration
    was: over 20 mph, fabulous economy. A couple of things, first weight. Robb's
    boat was minimalist, with a hull that barely came up to his knees as he
    stood to drive. It was strip-built out of light poplar wood with planking
    scarcely thicker than strip-built a canoe and like a canoe derived
    sufficient strength by having no flat panels, the chines rounded, the bottom
    slightly veed and the sides, again, with no flat surfaces, even at the
    stern. Furthermore, he had an extremely light-weight, high-reving automotive
    diesel with no reduction/reverse gear. His whole boat/engine/ trailer
    weighed less than 600 pounds. My skiff (with engine) alone weighs about

    Well, I can live with the diminished performance and so apparently can a few
    dozen would-be builders who have bought my plans. But sadly I was looking
    for the "all-water, all-weather" performance that characterizes my other
    skiff models, and this skiff has a major design flaw that renders it
    unsuitable in all but relatively calm waters. Close readers of Robb's
    article may remember that one of the clever aspects of the design was that
    the tunnel had a slight downward turn at the after end. This had the effect
    of deflecting the prop wash downward, which in turn depressed the bow, much
    in the manner of the trim tabs on a modern power boat, and caused the boat
    to run level, punch through a choppy seas and resist hobby horsing and
    pounding. In short, it substantially removed the curse of flat-bottom hulls.
    The down side of this feature is that unlike trim tabs it isn't adjustable,
    and exceeding the designed speed results in an excessive bow-down aspect
    which results in yawing (sometimes called bow steering or as Robb called it
    "rooting" ), which Robb found out when he pushed his boat past 20 mph.
    However, the trim tab feature is only effective when the chop is less than
    about 30 inches. Exceeding that, the flat part of the hull emerges, air is
    entrapped and the prop cavitates so severely virtually all headway is lost.
    One's only recourse is to throttle back or bear off (change direction). . .
    So the slightly veed bottom was the second bit of genius, intentional or
    unintentional, in Robb's version of the Seabright, which presumably negated
    the cavitation problem. And by the way, my plans go out with an addendum
    clearly stating the above and also suggesting the outboard power option (see

    In closing, I'll credit Mike O"Brien's article for suggesting outboard
    power. I measured a hi-thrust Honda 25 hp, which fits skeg and all above the
    bottom when its ventilation (cavitation) plate is hard up against the roof
    of the tunnel. I would steer with a rudder, same as with the inboard. Mike
    is a bit amiss when he says "appropriate engines are not so common." . . .
    They're everywhere, Mike. After all, what's in every small auxiliary
    sailboat these days but small diesels? It is likely in Atkin's day they
    used marinized gas engines, perhaps with a clutch but no reverse gear.
    Happily such engines have gone the way of the mechanical typewriter. The
    problem isn't engine availability but finding a ready-made unit with
    low-ratio reduction gear or better, simply a 1:1-ratio reversing gear. My
    research revealed no stock units, and I installed an after-market gear. Such
    is necessary to achieve the rapid shaft speed to drive a small prop, which
    is all there's room for given the restriction imposed by the tunnel. And
    shaft speed is what you need for hull speed. (Of course all sailboats are
    driven at displacement speeds, hence the high-reduction gears.) That clever
    Robb White achieved the high shaft speed with a home made belt drive
    employing equal-sized pulleys and saved a lot of expense, weight and power
    loss to boot. His son Wes finally let the cat out posthumously, telling us
    Robb got his inspiration from the drive in a common garden tiller. To be
    sure Robb never said otherwise, and he was way ahead of most of us in being
    able to build it himself. Quite a guy.

    Renn Tolman

    CEO (retired) Tolman Skiffs and Proprietor (still hanging in) of Kamishak
    Those that are successful in Alaska are those who are flexible, and allow the reality of life in Alaska to shape their dreams, vs. trying to force their dreams on the reality of Alaska.

    If you have a tenuous grasp of reality, Alaska is not for you.

  3. #3
    Join Date
    Sep 2008


    Quote Originally Posted by bmunsell
    In the WoodenBoat article the builder says he hauled all the shingles needed for his thousand sq. ft. house in one load, still got on plane and burned about the same amount of fuel as always.
    That right there should tell you he is full of it. You don't put a couple thousand pounds of payload in a relatively small boat and get the same performance as always.

  4. #4
    Member Sobie2's Avatar
    Join Date
    Jul 2006


    Atkins & Co Ninigret

    Been eyeing this boat since last fall in Woodenboat's Small Boats annual issue. 22' Planes out with a 30-40 HP outboard, small cabin. Seems like a good salmon catcher.


  5. #5
    Premium Member bmunsell's Avatar
    Join Date
    May 2007
    Wasilla, behind 3 Bears on the Palmer Wasilla Hwy


    I guess I should have targeted the thread a little better. I'm really thinking about river boats and any designs that would yield better efficiency than the 3 or 4 mpg our current boats deliever under optimal conditions.

    If you live in a village and fuel is $8 or more per gallon, would you be willing to motor along at 13 mph if you could get better than 10 mpg? How about 15 mph or 19 mph? Even for those of us who live in town, could we live with a 20 mph cruise speed if the boat was really quite and got 20 miles to the gallon of diesel fuel? If the boat could get 2000 lbs up on plane, run at 20 mph and get 20 mpg, you would only need to start out with 75 gallons of fuel at 500 libs for a 1500 mile Yukon River hunting trip. What are the possibilities?

    Right now we put way in excess of 2000 lbs in our jet boats and still get on step and can still get almost the same kind of performance. Sure, any performance statement is a relative thing. The performace of my Wooldridge with only a couple of people and 25 gal of fuel, say 600 lbs, on board is night and day different than having 100 gal of fuel and 6 people on board or 2000 lbs, but it still does get up on step and it still runs fairly shallow and it only runs a couple of mph slower, it still perfoms.

    I think Rob White was just saying that the boat he built was a capable, practical work boat for his application. He was living on a tidal flat and wanted a boat the he could moor out in wadeable water.

    What kind of boat do we want and what is our application?


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