Electric Motors

[K.C. Walker]

Every once in a while I muse about how much power it would take to make a DaySailer plane. With my 4 hp motor and a moderate breeze I can sail faster than I can motor. Well, maybe not straight up wind. Obviously, the DaySailer transom can't handle a motor powerful enough so I will never be tempted to find out. I figure once you reach sufficient thrust to start making a bow wake you don't need much more than that as a reserve. Half throttle on my motor is about as fast as the boat will go. I might get a little more speed by going full throttle but not much.

[jdoorly]

The old broshures say the DS can use up to 8 hp on the transom! But my guess is that you need more hp to get up to planing speed (to overcome inertia) and then a bit less to maintain it.

[GreenLake]

The old broshures say the DS can use up to 8 hp on the transom! But my guess is that you need more hp to get up to planing speed (to overcome inertia) and then a bit less to maintain it.

It's not inertia, but a peak in the drag vs. velocity curve. (Some skiffs no longer have this "hump" in the drag curve, but the DS certainly does).

There's a calculator that lets you estimate the max. speed on a plane for a given weight, horespower and hull form. 4HP should allow the DS to plane at 8knots, if loaded to 1200lbs total displacement.

However, the hump in the drag curve might be high enough for 4HP not to make it. That's a different formula - and with a bit of search you'll find that described somewhere.

[jdoorly]

To my thinking the calculation and graph (total hull resistance vs speed) you suggest only results in instantaneous results. In reality the boat has mass and that mass must me overcome by propulsive forces to accelerate to some speed and provide a data point for the drag vs speed graph. Once you have equilibrium between throttle forces and drag there is no acceleration and inertia becomes zero.

[GreenLake]

Take an example where the speed/drag curve doesn't have a hump: a body falling through air.

First the drag is low, and gravity will accelerate the body rapidly. As drag increases, the acceleration will become less. Near the terminal velocity, velocity gain smoothly decreases to zero - going through all the small values in the process. (In this example the "thrust", i.e. gravity, stays constant throughout).

Now imagine jumping off a building holding a (large) umbrella. Initially the drag curve is steep, but once you reach the point where the umbrella turns inside out, the drag-curve flattens out (and may, for a bit, have less drag than immediately before the umbrella flipped).

That's a closer analogy to a boat going on a plane. First the steep drag curve (displacement mode) then the hump (forced mode) and then the much flatter drag curve in planing mode.

Your motor (or your sails) need to generate enough power to get you over the "hump" in the drag curve. That same power, on the much flatter drag curve for planing, will result in a higher speed. And, yes, the closer you come to the max. speed, the slower the boat will accelerate.

If you really needed to calculate this, you'd need not only the drag curve in all three modes, but also various efficiency curves relative to speed for your motor and propeller. In general, the thrust will not be a constant. But overall, that's the picture.