It depends on relative L/D for the target speed.

As an example one of my pedal boats displacing 90kg will have a a drag around 35N at 6kts.  Lift to drag is therefore 25.  In my experience it is challenging to get a foil in this speed range to give L/D better than 20.  Hence it is better staying in displacement mode for this particular example.  In fact I once worked through a full design case and arrived at 14.4kph (say 7+kts) for the break even speed for a slender hull and practical foil.  That required about 250W.  Not many people can sustain 250W for very long.  If you wanted to outperform a rowing shell over 2000m then a hydrofoil would be the way to go - the Flyak has already demonstrated this.

With the Hobie tandem the displacement drag rises quite steeply over 5kts.  So if you are aiming to get more than 5kts there is a reasonable prospect of a hydrofoil being more efficient.  

As an example say you are aiming for 8kts, which is near enough to 4m/s.  Also take displacement as 200kg and foil L/D as 20.  That means the hydrofoil drag is near enough to 100N.  Hence required power on the foil is 400W.  Taking an overall propulsion efficiency of 60% you would need to suck 615W from the battery to get that condition.  

If you have more detail on your displacement and power available then I can run through numbers that are more meaningful to you.  I expect it would be very unlikely that you could pack enough solar panels across the hulls to get the power to foil more efficiently than simply staying in displacement mode.   If you are spending lots of money on panels and electrics to get that sort of power you would be better off using a more efficient hull.

Some years ago I did a prop design for a small electric foiler that turned out quite well:

I did not do the foil design but I had the L/D design data.  This gets above 12kts with 1HP.  However everything was carefully designed to minimise weight.  They used a high efficiency outrunner motor fitted with a planetary reduction and I think lithium batteries to keep weight down.  The V-foil had a design L/D of 30+ and it achieved close to that.

There are also some purpose designed solar powered hydrofoils being raced in The Netherlands that outperform displacement boats while the sun shines:

I did a single person displacement solar race boat design that was not built in full scale but it was tested as a 1/6th scale model.  It actually flew, not as a hydrofoil but in ground effect, which I was using to reduce displacement so similar to a hydrofoil in that regard:

The full scale would be 7.5m long with enough room on the wing shaped deck to get 2.5kW of solar panels.  The fellow already had 5kW of lightweight panels but the race was cancelled through lack of interest from anyone else.

Rick

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