derekwarner_decoy wrote:

> Hi PD's -

Hi Ph.D.'s ;-)

> from a purely mathematical, scientific or academic point I am not sure

> if it would make any real difference from point A to point B on the water

Well, it makes some difference, depending on your premises:

Whether you keep engine torque, engine speed or engine power constant

while reducing the number of floats, and also depending on the slip

of the wheel (i.e. whether you hold the boat fixed, for example

when doing bathtub testing, or whether you let it run freely

during normal operation).

> 1) let's assume that the output shaft power & egine RPM is/are equal

And here you get a problem: You can't keep both output power AND

speed constant at the same time!

If you reduce the float, you reduce the torque at given rpm, and thus

you reduce the power (torque times rotational speed).

> So from this with RPM & power output being constant, the result is (...)

Therefore, I'm sorry not to agree fully with your following theorems.

First, let's look at the simple case (bathtub testing):

- If you keep the TORQUE constant (as a steam engine would),

half the floats would result in engine speed and power both increased

to 141% (square root of 2!), but the pushing force (being directly

proportional to the torque) would be equal.

- If you keep the engine SPEED constant (as a geared electric motor

would approximately do), half the floats would result in

torque, pushing force and power consumption being all reduced to 50%.

Big difference, you see!

- If you finally keep the engine POWER constant (as by thermic

restrictions of the motor or restrictions of the power source such as

battery or boiler), half the floats would result in torque and pushing

force reduced to 79% while output rpm being increased to 126%.

Differences aren't this big however if you look at the moving boat.

Let's assume for the original configuration a ratio of float and

hull resistance producing a wheel slip (difference of circumferential

wheel speed and boat speed, divided by circumferential speed) of 20%

(typical value).

Half the floats would then result in an increased slip of 26% and

thus less efficiency of the drive.

- If you keep the TORQUE constant, half the floats would result in

engine speed and power both increased to 108%, but the pushing force

as well as the velocity of the boat would be equal.

- If you keep the engine SPEED constant, half the floats would result

in torque, pushing force and power consumption being all reduced

to 85%, while the velocity of the boat would drop to 92%.

- If you keep the POWER constant, half the floats would result in

torque and pushing force reduced to 95%, output rpm being increased

to 105% and boat velocity slightly dropping to 97%.

So finally, after all these calculations, for a free running boat I would

tend to agree to

> So all things being near equal ....

But for a towboat running at much higher slip, things would significantly

different.

Regards, Moritz