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Fair winds!

Dave and Anke
triloboats swirly gmail daughter com

Monday, September 14, 2020

(Sea)HorsePower and Square Motorsailing


Full and Half-By with Todd Allen

Gentlefolk do not sail to windward.

-- I'd swear it was Sir Francis Chichester

(Sea)HorsePower and Square Motorsailing

Many years ago, Phil Bolger observed that, for the average sailor, most if not all sailing is done off the wind, with the motor being used for windward work. Furthermore, as many must sail on a schedule, he felt a faster transit to and from cruising grounds with leisurely, fun sailing once there was a plus for most.

As such, he began to design many of his vessels around that premise.

By average, one is sometimes tempted to think it means sailors with any sense at'all.

Note that by off the wind, one means the 180deg half-circle from a beam reach (90deg to the wind on one side), through straight downwind (wind on the stern), to the other beam reach (90deg to the wind on the other side). That's a lot of degrees of freedom... a lot of sailing!

In contrast, sailing to windward only allows half that from a beam reach to close-hauled (45deg to the wind on one side) and same again on the other. Plus, you're heeled over, often bashing into waves and spray, and the wind feels strong and cold.

So, despite motors being largely beyond my ken, I'll nevertheless share some musings...

Hull Design

Hulls that move through the water (as opposed to climbing on top of it) displace water ahead, downward and to the sides and are called displacement hulls. And all that pushed-aside water has to return to fill the hole left behind by the moving boat.

Some consequences for displacement hulls:

  • Speed is proportional to Length (S/L ratio)... the longer a boat's Water Line Length (WLL), the faster it can travel.

  • S/L = 1.34 x √WLL = maximum speed aka hull speed

  • It takes exponentially more power to approach maximum speed

  • Additional length provides diminishing returns in speed (longer is faster, but shorter is more speed per foot of WLL)

In other words and once again, small is beautiful.

Sailing boats heel (lean over) when sailing to windward. When they do, a wide, square transom dips its lower, lee corner under the water and drags. So we raise it to minimize the effect. Problem is, displaced water now has to travel more abruptly back to fill the hole we leave behind, creating turbulence and drag. Part of the price we square boat sailors pay.

But if we motor to windward, we do it upright (heeling is negligible). Accordingly, we can ease the aft curves by lowering the transom to kiss the water. You can see this in the picture above, and also see how little disturbance it leaves as it sails upright downwind.

So consider a transom with lower edge at the upright WL.

Another design element becomes practically negligible when not sailing to windward... Lateral Resistance. This is resistance to sideways motion, provided by dagger-, lee- and center-boards, keels, chine runners, etc.. 

Skip LR and all its many hassles.


The lion's share of rig complexity arises when sailing to windward. Sailing down and off the wind calls for little more than putting up and spreading a shaped sheet to catch the wind. Efficiency might be important, but for the average sailor, not likely. If we're going small, everything gets easier.

Here's what I'd look for in a rig:

  • Forward placement... Put the horse before the cart.
  • Easily mounted and stricken rig... When the wind is again' ya, take it down.
  • Easily handled... Why be fiddlin' while the sun burns?
  • Easily reefed... Just 'cuz we're loafing along doesn't mean the wind won't blow up!
  • Consider a free standing rig... No shrouds or stays to set up, take down or worry about in a jibe.

Quadrilateral sails (four-sided) spread a lot of sail on a short mast for easier setup and take-down.

Consider Ljungstrom Rig, which meets all of the above, and has infinite reefing, which can be dumbed down for smaller boats.

Mechanical Propulsion

I'm a fan of outboard motors over inboard for their ease of dis/mounting, lower installation complexities and non-perforation of the hull. Any fuel spills are relatively easily contained and reduce the risk of explosion. Many regulations for safe installation of inboards are avoided.

Options include electric, gas and propane (and natural gas)... each have their fans. Among electric motors, trolling motors are inexpensive and are designed for long run-times at lower speeds on a given charge.

Consider modest, outboard propulsion.

Remember that it takes exponentially more power to approach maximum speed, which in a displacement hull is in any case relatively low. This means you can run at some fraction of hull speed - say a half, two thirds or three quarters - with far less power input than for top speed.

Now, you hitch up a few horses (power is often measure in Horse Power or HP) and you can easily push a small load at low throttle, while sipping fuel (or Watts).

Consider backing down from top speed to radically extend your range.

Manual Propulsion

Me? I'd still carry a manual alternative to those finicky mechanisms. 

A sculling oar is simple and effective. An Atsushi Doi Power Fin is less simple, but powerful (could almost skip the motor). Oars are tried and true, though harder to handle. Even a paddle will get you somewhere.

Consider 'auxiliary' manual propulsion.


I'm going to present this f'r'instance cobbled together from correspondence with Todd Allen. He writes [my emphases]...

I've built my second Triloboats style boat, thought you may be interested.

This one is a micro version, 10 ft long, 47 inches wide (to fit in my utility Trailer!). Birdwatcher cabin, rockered hull with a taste of PDR shape at the bow. Cabin is 6 ft long, lots of storage under the decks. I added Oar ports on this one which I really enjoy using, both for rowing and for ventilation at night. 

I brought the lower edge of the transom down as low as I could to maximize displacement for supplies and to give as much buoyancy as possible to the rear section to support the weight of the motor. I got lucky and it's about right! 

Can maintain 2 mph easily under oars in calms. 

I mainly motor with my 2.5 hp Yamaha 4 stroke (24+ mpg), but have a small downwind sail I deploy as often as possible, using the motor as my rudder (turned off of course).

[I checked back with him about that great mpg...]

24 miles per gallon is not a typo! I love this little Yamaha 2.5 four stroke, one of the best things I have ever bought. I run it at just above idle most of the time, and this gets me to my cruising speed of 3.5 knots quietly. At just over half throttle I can get up to 4.4 knots, but louder and bigger waves. Full throttle digs a bigger hole, no real speed increase. 

One thing I'll note is my offcenter spritsail rig. Even though I set my boats up primarily for motoring, I find I sail as much or more than my traditional sailboat cruising friends because my sail is so easily set. 

I usually keep it up and brailed, so if a favorable breeze comes up, I drop the brail and I'm sailing, takes 15 seconds. Same for brailing it when wind dies. The slot top cabin of course makes this possible.
Makes me chuckle that mine is often the first sail deployed. Also chuckled on the last trip when a sailing friend complained that it took him so long to catch me going downwind in a light breeze in his 17ft Siren!

And there ya have it.