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