Hi Folks,
I just spent
the whole day yesterday diving Snoopy with Greg Cottrell,
Dan Lance, and Les Sonnenmark (who designed Snoopy’s thruster controls) at Bainbridge quarry in
Pennsylvania. It was a blast, and we made perhaps twenty or more dives safely. Here’s how it went.
These were Snoopy’s first dives with saddle tanks
and with a crew of two. The tanks turned out really well and
helped Snoopy come off her trailer much earlier than she
used to, which was good given the quarry didn’t have a proper boat ramp. The tanks also added about four
inches of freeboard, which doesn’t sound like much but feels like a substantial difference when you consider it used to be eight inches. There’s better stability on the surface. But the
main purpose of the saddle tanks was to facilitate “riding a bubble” since Snoopy no longer has
a VBT. We certainly did a lot of bubble riding, and with two people it
was a challenge. I had been wondering
whether we would really need to add or remove ballast to adjust for different crew weights, or whether it might be an option instead to
permanently install ballast for one person, and ride a large bubble when operating with a crew of two. Who wants to ship or
unship two hundred pounds of weights? So we tried it. The
saddle tanks, when fully blown, have 440 lbs of lift. Well, my impression is
that it is POSSIBLE to do so, but mighty tricky, at least in conditions of
low visibility and uneven bottoms. We got better at it with
practice, but I think it will be worth it removing the extra
weight when a second crewmember comes aboard.
Here’s how it would typically go
with the “large bubble” approach:
1) Flood
MBTs on the surface until neutral
2) Motor
down
3) At perhaps 20 or 30 feet, you realize you’re dropping fast even after turning off the thrusters
4) You watch your rapidly decreasing altitude on the
depth sounder, and start blowing air into the MBTs to level
off before hitting bottom
The problem
is that the sub has a lot of momentum, and the air you inject takes a while
to slow it. If you keep blowing until you stop falling, you find yourself rising a moment later, and would need to start dumping air. Thus
you
have to stop the blow while you’re still falling, which is logical yet hard
to judge.
Let’s return to that item #1.
The flood valve on the new saddle tanks is large (1” diameter). This had the
desired effect of speeding dives and allowing rapid dumping
of an expanding bubble on ascent, but it also had a side
effect I had not anticipated. Standard procedure is to open the flood valve and let out MBT air until the apex of the dome is level
with the surface, at which time you close the valve and find yourself neutrally buoyant. But when I did this with
the large bore valve,
I found the boat went right on past the apex of the dome. The speed of the air
release had set the boat bobbing up and down, making it very hard to
find the sweet spot. The frequency
of oscillation was very low, but the
amplitude was considerable, about a foot. The take-away is that the
big valve is great for quick dives if you are correctly ballasted, but
if you are going to ride a bubble, only crack the valve
instead of opening it full bore.
Now, if you
have weighted the sub relatively accurately, and have little
or no bubble, things are dramatically easier than with the large bubble. So long as your buoyancy is within the power limits of
your thrusters, the thruster speed control knob becomes your depth
adjustment knob. Start with the boat slightly positive and
motor down. Reduce the thruster speed and you rise, increase
it and you descend.
The area of
the quarry we were diving in is used for SCUBA classes. It’s full of buoys, mid-water platforms, and all the lines holding
them down. There were sunken boats, and especially lot of trees. The terrain was both sloping and
curving, and the visibility very poor. Which is to say, we
certainly got a workout trying to navigate among all this. I found the best approach was to have the
person lying down in the bow control the thrusters, and the
one sitting in the conning tower control the buoyancy. The
person looking through the bow viewport can best see
obstacles ahead, but the person in the coning tower can look up at the
surface, and also has better access to depth gauge and depth
sounder. This division of labor seemed to be complementary
rather than cause any interference.
I’ve also updated the project
page, and you’ll see Snoopy is looking less and less like a
K-250.
Cheers,
Alec