Directional Stability, was Looking for used prop

E

Ed Price

Guest
The props push water to the rear, causing a forward pressure to be exerted on
each propeller shaft housing. If both props are the same size and pitch, and
turn at the same RPM, then an equal thrust is developed from each prop. These
equal thrusts push the Amphi hull through the water.

The water exerts a drag on the moving Amphi hull. Since an Amphi is symmetrical
left to right, the amount of drag on the left matches the drag on the right. So,
your Amphi moves along, balanced by equal thrusts and drags. If one prop no
longer provides thrust (like one or more blades shear off), then the thrust
forces no longer balance the drags. The thrust from the good prop will be larger
than the thrust from the bad prop, and the Amphi will steer toward the bad side.

You may be able to steer the rudders (ah, the front wheels) to counteract this
imbalance of forces. OTOH, all is not lost if you find yourself with one bad
prop and you can only make sharp or sweeping turns (you can't turn the other way
or even go straight). This is hard to describe, but just make as wide of a
sweeping turn as possible, then go to dead slow and make a very sharp turn, just
not coming about a full 360 degrees. Stop turning short of you intended course,
and then proceed with the least sweeping turn course possible.

Your course plot may look like a drunken turtle, but if you have room for the
turns, you can get back to the ramp. Ah the tricks we could do if we only had
independent control of forward and reverse for each prop shaft! We could spin an
Amphi in its own length, steer without the front wheels, or cross-steer wheel
thrust and prop thrust to let us crab sideways.

There's a couple of other factors in directional stability. Torque and shaft
angle.

Torque is the force needed to rotate the prop shaft against the mass of the
water. The Amphi props rotate counter-clockwise (viewed from the stern), so the
prop rotation imparts an opposite torque on each prop shaft tube. This combines
to create a force that wants to twist the Amphi clockwise in the water. This is
what causes the rudders to work better in one direction than the other. (If the
props rotated clockwise, the Amphi would turn better opposite to what it does
now.) This problem is eliminated in larger boats which have twin engines. One
prop shaft turns CW, and the other turns CCW. The torques cancel out, making
directional stability a lot better.

Shaft angle refers to the angle which the prop shafts "dips" into the water. In
traditional boats, the drivetrain is simplified by angling the rear of the
engine downward, allowing for a straight driveshaft to the prop. This angle puts
the prop thrust out of line with the boat hull's direction through the water.
Fortunately, the Amphi's prop shafts are just about level, so this is a very
minor consideration for Amphis.

Regards,

Ed
67 Rust Guppy
El Cajon

----- Original Message -----
From: Gary Grieshaber
To: amphicar-lovers@yahoogroups.com ; amphicar-lovers@yahoogroups.com
Sent: Tuesday, August 06, 2002 1:51 PM
Subject: Re: [amphicar-lovers] Re: Looking for used prop


Would someone please explain, when both props spin in the same
direction anyway, why taking one away would cause the car to travel in
circles?

>
> Going in circles occurs when one prop sheers ofe and is lost. I have
heard
> only one vane missing doesn't matter too much.
>
> Same thing happens when I prop is completely tangled with sea weed.
>
> Only fix for sea weed I have found while in water was to reverse and
prop
> seems to then rid itself from seaweed.
>
>
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A

antique459@aol.com

Guest
My goodness ED! By any chance, were you the scarecrow in the wizard of OZ?

Thanks for the great explanation.


[Non-text portions of this message have been removed]
 
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