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Almost any type of hull can be pushed through a canal at 5 mph with a fairly small motor, so long as it can pass the bridges and tunnels.
  The challenge we have met is to design a large craft that will also cruise swiftly and efficiently on the open sea using an ordinary outboard motor.
Teale's resistance graph

  Our hull is a trimaran, with three slim pontoons that always slice through the water in displacement mode (i.e. never climbing its own bow-wave to go into planing mode).
  There is a distinct performance region between displacement and planing modes which is extremely inefficient, often called "mushing," where the boat is attempting to climb its own bow-wave.
  The graph shown above is from a standard reference by John Teale, for a "typical" monocoque wide-beam craft. Note that the residuary resistance (often called "wave resistance") Rr rises steeply before plateauing (planing). This highly inefficient region is where the boat is mushing.
  A typical speed-boat or fast mono-hull luxury yacht, to achieve its high speed, is designed to reach that more efficient plateau where it can plane above its own bow-wave. To do so it needs a great deal of power, and it burns lots of fuel.
  Boats with much slimmer hulls, such as our trimaran, cut cleanly through the water and can cruise at higher speeds without "mushing." Our yachts are designed NEVER to plane.
  Using the best, latest experimental evidence, we have edited Teale's graph in red to show the wave resistances at higher speeds for narrow hulls.
  We have found that a speed-length ratio (SLR) of 1.7 or more can be reached by a narrower hull before "mushing."

    SLR is the speed in knots (1 knot = 1.2 mph) divided by the square root of the length at waterline (LWL) in feet.

  Our 77.5' yacht (LWL = 73.5' average) should reach 14.6 knots at SLR of 1.7, at which point its resistance (at 25 tons displacement) will be (from the graph: Rf+Rr = 25+41) 66 lbs/ton or 1650 pounds.
 You may wonder if this formula applies to our trimaran? Yes. Each of the three slim hulls carries a third of the 25 ton weight. Everything is proportional, and the resistance will still be a total of 1650 pounds.

  The next calculation will determine what horsepower motor is needed to power this yacht at 14.6 knots, and overcome the 1650 pounds of resistance?
  Brake horsepower (BHP) is the most useful measurement, and is typically 60% of an outboard motor's rated horsepower.

    BHP (brake horsepower) = 2 times velocity (in feet per second) times resistance (in pounds) all divided by 550. Feet per second is easily calculated as 1.68 times the speed in knots.

          2*(14.6*1.68)*1650/550 = 147

  By this calculation we arrive at 147 BHP. Recall that an outboard motor only applies 60% of its nominal power. So we will require a 245 HP motor to achieve 14.6 knots, and this will be near wide-open throttle, or about 5800 rpm engine speed.
  To cruise at about 14 knots with the motor running most efficiently at midrange, or 3500 rpm, one should have a larger outboard motor. Yamaha makes a fine 350 HP V-8 outboard motor that we can mount on the yacht for you. This motor can also achieve over 20 mph at full throttle.
  By similar calculations, we find that for lesser cruising speeds and better fuel efficiency one might select a 225 HP motor (for cruising at about 12 knots), or a 115 HP motor (for 10 knots).

 To overcome the occasionally intense tides and currents of the Atlantic, the Channel, or the Severn Estuary the 350HP motor should be selected.

 Using these same formulae, you will find that cruising at the speed limit of 8 kph (4.2 knots, and our SLR of 0.48) in the French canal system will require at least a 9 HP motor. We provide a 15 HP motor with our basic configuration. This smaller motor can also be used on the inflatable tender. It's also reassuring to have this backup motor in case, for any reason, the larger motor conks out while at sea. You can still make your destination port, if only at 5 knots, without an expensive tow. The exchange of motors can be handled by one person using the jib-crane and hoist.