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Anke and I live aboard WAYWARD, and wrote about it's design and construction at

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Fair winds!

Dave and Anke
triloboats swirly gmail daughter com

Tuesday, April 5, 2016

Bent Oars for BIRDWATCHER-Inspired Hulls

If it ain't broke, don't fix it.
But if it is...?

Bent Oars for BIRDWATCHER-Style Hulls

In BIRDWATCHER, Phil Bolger introduced a true revolution in boat design. They're distinguished by an upper hull completely or mostly consisting of window.

Core concepts:
  • Waterproof to above the knocked-down waterline (should float on its side without shipping water).
  • Long gangway running the length of the interior (mid-line security for standing crew).
  • Self-rescuing from inside the hull (!).
  • All operations possible from within shelter (no outside cockpit necessary).
  • Easily stricken rig (to reduce windage for rowing).
  • Reasonable performance under oars (eliminates expense, and weight of motor, and allows extended cruising away from fuel sources).

Not all vessels styled after BIRDWATCHER hit all these points, but together they set an impressive bar. Each point interacts with others for high synergy.

Anke and I spent close to three months dinking around in our BIRDWATCHER inspired prototype T16x4 TRILOBYTE. It was a total hoot! But one problem was persistently irritating and borderline dangerous...

To keep the hull watertight (a must when sailing in heavy winds that risk an otherwise acceptable knock-down), the oars must be shipped and the oar ports securely dogged. Turns out, this is easier said than done!

Even in normal rowing, if we brought the oars inboard to rest, grab a bite or what-have-you, the inboard portion of the loom created an awkward obstacle course within the cabin. If the blade caught while the hull was moving, the inboard end would sweep and shove us. We called it 'the sword dance'. If we had to ship in a hurry, as in a sudden squall, things got positively dicey.

For BIRDWATCHER, Phil selected a double-ended hull type that rows well. Placing the (single pair of) oars aft of the maximum beam allows oars to temporarily trail aft, clear of the the hull inside and out. But a small, double ended hull gives a lot away in terms of interior volume.

In most other types – notably barge/scow/pram shapes – that approach doesn't work. The port openings fetch up against the loom and don't allow them to swing fully parallel; if let go unshipped, they angle out and aft, dragging through the water.

To address this, I proffer a bent oar proposal:
Bend a 45deg, angled section at the oar's fulcrum, offsetting the inboard loom from the outboard by enough to clear the hull and your chosen lock hardware. Boot the oar outboard (raingear sleeves?) to waterproof and retain the oar from sliding out. Stowage can be arranged using single-horn cleats for blade and grip, with bungee tie-downs.
Know anyone with a pipe bender?


These oars would never (seldom) be dismounted. They'd trail when released and stow mostly clear of cabin spaces.

Oar Boots  can be affixed around a lipped oar port opening, similar to a kayak skirt lip.

A stiffish rubber sheet, close fit around the oar would make a pretty fair lock in this situation.

BTW, TriloBoats has a new, BIRDWATCHER inpired offering, the T20x4 SCUTTLEFISH.


  1. There are several forward facing rowing systems which have bent oars. Might give them some thought.

  2. Second Suggestion: Mirage Drive by Hobbie.

  3. another idea. It might fit in the 16 easily.

  4. Hi Everitt,

    All three of those ideas could be adapted, though the fwd facing systems I've seen would present some extra challenges.

    I've been very interested in the Hobie Mirage Drive. The challenge for BW types is that there's rarely a raised sole or seat in which to set the through-hull mount. But if that space can be spared...

    The Power Fin sculling system (your YouTube link) and several others like it look intriguing, and could be arranged to be operated from within the cabin.

    Thanks for the input!

    Dave Z

  5. Two thoughts come to mind:

    1. The bent oar thing is only useful for small boats (but you knew that already ;-)

    2. I wonder if there is any mechanical (i.e., mechanical engineering) reason to think that the hinged yuloh in the video would work better than a normal one? To me, just using simple reasoning, it would seem that the hinge would cause the fin to exert less force on the water, thus reducing the propulsion. That said, maybe I'm looking at it wrong. It might make "yuloh"-ing easier, which might be the goal.

    Any ideas?

    1. Hi Yoda,

      RE 1... not only small boats, but those which, for some reason or another, one can't simply lift 'em out normal like.

      RE 2... I'd say yes. Think in terms of vectors. All sculling gets its push from the blade working at an angle relative to both its motion through the water (lateral) and direction of thrust (generally longitudinal).

      As it is worked laterally, water is 'bounced' aft and outbrd. The aft bounce (vector) produces the equal, opposite reaction we know and love as thrust. Same with props, BTW.

      In the Power Fin, it's the small, angling portion dead aft that provides the thrust. The fwd part of the fin has been cut away (could be even more, to my mind) to reduce lateral bounce, wagging the dog.

      Note that a running line controls the sweep of that aft flap's 'tiller', allowing one to adjust its 'pitch'.

      Yulohs allow control of all this via initial geometry and various, adjustable hand/loom/lanyard positions.

      Hobie Drive 'sail' fins allow a set amount of flap, which angles the fin surface during its lateral sweep.

      Unless using a variable pitch system, a prop's pitch is fixed... high pitch (angle or blade twist) for low RPM, high torque engines (typically fit for motorsailors); low pitch for high RPM, low torque engines.

      Another approach to all this is via the screw (inclined plane), with the pitch determining how much travel per revolution (discounting friction effects).

      Once you see it, it's everywhere! 8)

      Dave Z

    2. Dave,

      RE 1: OK, small boats ... or ... people with really long arms. ;-)

      RE 2: (Clearing throat - uuuuhumm)

      Hey, diddle, diddle,
      The cat and the fiddle,
      The cow jumped over the moon;
      The little dog laughed
      To see such sport,
      And the dish ran away with the spoon.


      I'll attempt an exercise in distillation:

      All of the useful thrust of a yuloh (or similar thingee) occurs during those parts of its movement/arc when the yuloh is angled away from, but moving toward the centerline. Otherwise, the thrust is lateral and counterproductive. The hinged paddle (power fin) maximizes angled time, and so, maximizes thrust.

      Is that close?


    3. Aquatic critter tails bend.
      Perhaps instead of using a hinge (to simulate the bend) the fin could be made from a flexible material(as is the mirage drive)

      note: no reason fin has to be vertical. Tail flukes are not.

      or more exotically the 'ripple rousers" similar to the cuttleFish

      doesn't have to be just one fin

    4. Everitt,

      All of those are very cool, but I especially like the cuttlefish/Squidbot and the bi-flapper models. The cuttlefish seems to have the least amount of wasted (non-thrust) effort, but the bi-flappers strike me as more likely for adaptation to an actual boat. Augmentation of existing manual methods, etc.

      I wonder what the mysterious doohickeys are between the basic electric motor and the yuloh handle are? They'll probably have the words "cog" or "rotating" somewhere in the name ... along with "doohickey" of course.


    5. @Yoda,

      Your description is pretty much on, though power is generated across the whole, lateral sweep (not just that toward the centerline).

      I'll try to keep from running away with the spoon! 8)

      Dave Z

    6. "Your description is pretty much on, though power is generated across the whole, lateral sweep (not just that toward the centerline)."

      Do you mean when using 1) a normal sculling oar (non-bending), or 2) a modified power-fin type of oar, or 3) both? I assume the answer is # 2, which is what makes the power-fin type of oar worth doing in the first place. If I'm wrong, please explain.

    7. Hi Yoda,

      I mean both... the aft tip of both systems angles at the beginning of its lateral sweep, and maintains an angle throughout the sweep across the CL to the opposite side. This angle is modified by the loom angle relative to the CL, so you get an average.

      But water is 'bounced aft' through the whole sweep, only half of which is moving toward the CL.

      At the end of each sweep, the angle is reset to mirror its opposite.

      In sculling systems (vs fin arrangements) the angle also produces a downward vector, pressing the loom against the fulcrum. Some techniques require no lock, for this reason, and Asian systems often use a pin/dimple.

      Because the PowerFin and related produce no downward vector (only lateral and aft) that extra energy goes toward thrust.

      In thinking about all this, it helps me to picture the planes involved around the 2 or 3 axies involved. I ask myself where thrust vectors are generated across these planes as the blade moves.

      Dave Z

  6. Another thought on the Mirage Drive:

    Doesn't each 'fin' of the Mirage Drive look much like a lee board?

    Suppose the Mirage Drive were to be reverse engineered, slit into two pieces. One on one side of a TriloBoat and the other on the other side. In exactly the same place that a lee board would be? Possibly on the inside of the hull.

    Of course the Hobie Mirage Drive fins are much too small for a lee board...make em (much) bigger.

    Make some kind of cable apparatus to power them from a rowing machine type apparatus (or not), Thus the arms, back AND legs could get into the act.

    The fins could be 'lockable' in any position through 180 degrees, from flat outboard, like wings, to inboard hugging the hull bottom. Thus they would be self stowing and if attached right never get in the way of the windows.

    The could be made as long as desired, and could provide a LOT of propulsion.

    and they wouldn't take up any floor space.

  7. Dave:
    Imagine a birdwatcher style Triloboat with twin bilgeboards.
    At the rear of each bilgeboard is a powerfin.

    1. Hi Everitt,

      All sounds promising, though beware of creeping complications.

      If you try any of that out, keep us posted!

      Dave Z

    2. "Beware of creeping complications".
      Very good point. In the military it's called 'mission creep'.
      I need to NOT do that. KISS. (keep it simple silly)

  8. I really like this concept. I think another approach for construction might be to build it a bit like the peg and block oarlocks of currach/curragh oars but instead have the handle on one side and the rest of the oar on the other. This would achieve the angle and prevents the oars from rolling in the hands which is inconvenient when you can't easily see them.

    1. Hi Tassiebush,

      That's a great solution!

      The system you're referring to is one of the variations of 'thole pins'. The block portion (drilled to fit over the pin) doubles - in this case - as an offsetting block. This could be DIYed from wood much easier than bent pipe.

      Generally, I like to 'feather' oars (roll the blade nearly flat on the return stroke), but with bent oars, that doesn't work well. Pinning them would therefore be no loss in that respect.

      Thanks for the suggestion!

      Dave Z

    2. I think I will give it a try with my build :)
      Thanks for giving me the system name. My searching the concept has been hampered for lack of the right wording.
      It just occurred to me that the inability to feather oars must be part of why the Irish ones often have square ends and are longer to compensate for the surface area (that's my impression anyway). Possibly too they're just a better shape for choppy atlantic coastlines.
      Oh well when I have settled on a choice of scuttlefish or trilobyte I have an idea of what the oars will be. I really like the idea of ones that just stow so easily.

    3. Hi Tassiebush,

      I've been thinking about the offset thole block attached to the oar and realized it wouldn't work for this application.

      Reason being, the straight oar will still cross the hull... when brought parallel, it wants to be either inboard or outboard. It's blocked from this by the port cut-out edges, so can never be brought parallel.

      A promising alternative would be to us a long-ish block offsetting and separating a short inboard loom from the longer outboard loom (zigged, not bent). A hole could be positioned in the mid-block to slip over a thole pin.

      The mid block would need to be longer to provide enough landing for the looms to handle the leverage involved.

      A couple has tried the block offset approach (minus thole pin), but I haven't heard back whether their prototype performed.

      Dave Z

  9. ScuttleFish looks interesting, what I can see of it

  10. Dave: I'm seriously considering building something much like your Triloboat Shanty as a retirement 'hide out'. I have many things I must work out first. I'm thinking that the size will be 8 x 16. (I spend a lot of time sitting on a 12 x 7 porch and thats a LOT of room , I spent the last 25 years living in a truck sleeper/cab which was about 7 x 8) I'm thinking I'll build a quarter size (4 x 8) PDR first. I've almost memorized much of what you have written.

    Keep writing. When is your book due?

    1. Hi Everitt and Yoda,

      Lessee, the book is due sometime in 2011! 8)

      It's mostly written, and I've been waiting for pics from WAYWARD for completion. From that point, it will take a block of free time (HA!) to complete.

      With luck, this coming winter? But you've heard that before...

      Dave Z

    2. Is that this upcoming Tasmanian winter? ;)

  11. "Keep writing. When is your book due?"

    Hear, hear!

  12. Dave

    What about a TriloBoat whose main source of propulsion is NOT sail?
    For rivers, and lakes?

    1. Hi Everitt,

      While my own prediliction is for sail, all the TBoats are adaptable to power, and many to sculling.

      One option to consider is a yawl boat... a smaller, powered tender to the bigger vessel. When you need to move, raft 'em up. Meanwhile, you've got a runabout with a lot more freedom of movement.

      Dave Z

    2. Now THAT is an excellent idea.
      A 'pontoon rear' would be excellent for that...the yawlboat could connect inside the poontoons, contain all steering, power etc.

      When anchored it's a runabout.


    3. That's an idea which holds interest for me as well, but one that I've yet to give much time to. One similar example -- though cruder than I'd adopt -- was used in the show "Yukon River Run". They used various boats to push log rafts down the river and as separate runabouts.

    4. Somehow I missed these... sorry!

      Phil Bolger (of course) has been there, done that...

      In his DOUBLE EAGLE catamaran, he designed two aft docking ports for SUPERBRICK tenders, powered by outboards. They run up into docking ramps which can be raised clear of the water for sailing or their aft ends lowered to motor depth for propulsion.

      My sister bought the only one I know of that was build, so I got to see it up close. Looks very practical... solves the issue of tying alongside in rough conditions with a yawl-boat. It allows two smaller motors to work together to propel the larger vessel, keeping them individually lighter and more fuel efficient and managable.

      Would work just fine on a pontoon or wide-ish scow hull!

      Dave Z