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