|
Hi Glen. I like the lift bag idea and believe it has real merit.
On shearing off a thruster motor, it just seems to me that a release
"mechanism" would be more dependable. I think Brent is working on a design to
allow his side thrusters to break free.
The best approach might be to prevent the sub from getting snagged in
the first place. I have a couple of pictures of Hugh's new Comsub and it's a
beautiful boat. Very sleek and judging by the few photos I have, it's got very
few places where it can get tangled up.
The advantages of making the boats smooth include not only snag proofing
but more efficient ( faster ) and less power to move them. This helps conserve
battery power for longer dive durations on a charge.
The Minnkota thrusters I'm using are the 101's, but I'm not sure how that
rating actually relates to "push power". It says 101 pounds of thrust but even a
small line like heavy fishing line could stop that.
The most vulnerable snag point on my sub is the rear thruster/rudder
components. There are two of them side by side and they swivel as a unit giving
me vectored thrust. I have prop guard rings built as rudders/tangle guards and
will be installing additional tangle guards in front of the components, but the
idea of "jettisoning" them sounds pretty good.
The swivel point on them is basically just a 1 inch stainless rod mounted
vertically in a stainless tube with a 1/2 inch bolt holding them in the tube. If
I replaced the bolt with a removeable pin through the rod, the thruster/rudder
assembly could simply drop out and fall away from gravity.
The side thruster/dive plane assembly is not much different, except it's
horizontal and uses 2 inch stainless bar stock and tube rather than the 1 inch
like the back.
Here again, a 1/2 inch bolt retains the connecting rod in it's tube so
replacing the bolt with a pin would be fairly simple. Getting the assembly to
slide out of the tube should be just a matter of "rocking" the sub a little
until it gets pulled out by whatever it's snagged on. Vance has a good idea
here, and I'm seriously considering making that change. The addition of three
small thru-hulls for a hydraulic cylinder to pull the pins isn't much work and
making the wire connection as a "break away" won't take much either. The rest of
the sub is very smooth and I can't see where anything can hook on. The keel/drop
weight is smooth, and the FRP fairings cover up all the other little nooks and
crannies where something might get stuck. These fairings are very strong with
most of it at 3/8 inch thick ( 9 layers of hand laid glass ) and the stress
points and impact zones run up to 1-1/2 inches thick.
I did a study of how and where it's possible to smash into something, where
the impacts can occur, and designed the areas open to impact to transfer the
forces into the support structures so this little bugger should be able to take
a real beating without getting damaged.
I like to use the word "robust" when describing the design. It really is a
tough little guy.
The sub weighs about 7000 pounds, and when you add the ballast tanks full
of water, there's a LOT of inertia there if it hits an object like a rock that
isn't going to move.
Not much of the pressure hull is exposed, the ballast tanks which make up a
lot of the exposed area are 12 gage stainless and shaped so they will dent
rather than rip, and the FRP fairings are strong but also quite flexible. I put
"bumper" impact ribs along the most likely areas to get hit, built up the
fiberglass to 1 inch thick and added gusset stiffeners behind them. It's pretty
amazing just how strong these things are.
The flexibility of the glass makes it a good choice for this application,
and the light weight relative to strength keeps the center of gravity very
low for the sub.
On the scale model tests I did, the sub was held under the water upside
down and when released it immediately flipped over before it had risen an equal
distance to it's height.
Now model testing can only get you so far, so open ocean sea trials will
show me just how well the design actually performs, but I'm pretty sure it will
be stable and should survive some serious impacts without damaging any critical
components. If the FRP gets hit hard enough it WILL break, but it's pretty easy
to patch that stuff, and using the old one as a mold, it would be easy to make a
complete replacement fairing.
The dive plane wings which are the most likely to get damaged are built
with a " break away " zone and also there to protect the side motors. I didn't
put prop guards on the side motors because the Minnkota props are plastic and
should shear off fairly easily if impacted, plus they are Minnkota designed to
cut weeds or kelp and are pretty much tangle free in their shape. Even if a rope
was to get tangled around them, they will either snap the flute or shear the
small retaining pin holding them on. The dive plane wings are mounted in such a
way as to protect the motors from direct impact, the FRP wings can break off
leaving the motor attached to the sub, and the motor mounts with the 2 inch bar
stock swivel joints are plenty strong. Now if I was to add the ability to
release the motor assembly, the possibility of a snag or entanglement would be
minimal.
Frank D.
|