[PSUBS-MAILIST] Alec's test

Fri Apr 14 09:36:44 EDT 2017

Hi Hank,

Good points. We might be calling two different things "vents." First, the
issue with the tubing on the top of the tanks that releases the air for
diving... If air were all that were in those when submerging, I agree there
should not be a problem if they ran "downhill" a little due to listing.
However, these are small tanks and the tubes in question are only 1/2"
Swagelok. I think what's going on is that the water sloshing in the tanks
will occasionally reach the entrance to those tubes and get sucked in along
with the air, so you end up with a water block if that water goes downhill.
At the top of ball valve that all these little tubes feed into, it's nearly
all air coming out while flooding tanks but it does spit out water in small
quantities.

Second, the openings at the bottom of the tanks. I already have downspouts
on them to prevent burping. They are only 5" long, but again because the
tanks are small it would take a large angle of heel to burp past them and
they did seem to prevent burping just fine. However, there was one thing I
was unhappy with in this area. When blowing the tanks, I'm used to stopping
the blow as soon as bubbles come out the bottom of the tanks. In this case,
the bubbles are appearing prematurely because some tanks get to empty while
others are still blowing down. I can reach the normal flotation line by
keeping the blow going, but it feels wasteful to be dumping air out of the
already-empty tank to let the others finish. Maybe next time out I'll just
temporarily extend the downspouts by putting short lengths of hose on them.
D'you think that would help? A more sophisticated approach might be to take
some video from underneath and analyze the order in which the tanks are
emptying, then put orifices on their air supplies to even things out. But
that's surely a refinement for later.

I agree the ideal situation would be to have valving that separated the MBT
into quarters, so I could control either fore-aft or port-starboard
independently. Currently I'm set up fore-aft and have no way to add or
remove buoyancy specifically to port or starboard. My logic in doing so was
that the boat is symmetrical side to side, but that longitudinal trim was
more likely to be thrown off by differences in the weight of the crew (up
forward) or if diving solo by my moving between sitting up in the CT or
crawling forward. When I re-plumb the tanks, however, I might actually
change it to port and starboard. I have two ball valves to work with on the
CT, and going to four would be a much bigger job than just re-routing
tubing. It'll also be much more convenient space-wise to plumb all the
tanks on one side to the ball valve on that same side. I'll just think of
them as saddle tanks, which is pretty much what the raft arrangement is
anyway - a "squashed" set of saddle tanks up high.

I'm afraid I didn't end up taking enough photos to update the project page,
or any video. Here is a downsized photo attached on which you might be able
to make out the tubing above the tanks and the downspouts beneath. I'm not
sure what the size limitation on attachments is, so if it doesn't make it
through please advise and I'll reduce it further.

Thanks,

Alec

On Fri, Apr 14, 2017 at 5:44 AM, hank pronk via Personal_Submersibles <
personal_submersibles at psubs.org> wrote:

> Hi Alec,
> Congratulations !!  your issues are pretty minor in the grand scheme of
> things.  The problem with the venting is suspicious to me.  If the vent
> tubes do not have water in them, they should still vent air, even if they
> are below the water line because the weight of the sub is creating pressure
> inside the tank witch will force the air down hill so to speak.  I was not
> there to see it of coarse, so I could be out to lunch.  From what you
> describe, changing the lines may not do it, I think you will need
> individual valves, not per tank but per side.
> I had a similar problem with Gamma, but in reverse.  When I surfaced the
> sub would list to one  side badly because I have a common vent for the
> front and back.  Ideally you want front and back and side to side.  I
> corrected this by putting extension tubes on the bottom of the ballast tank
> vents, this forces the water level to self balance because one tank will
>  not vent air out the bottom until the sub is level.
> It sounds to me like you have a similar situation. If you have the ability
> to control venting from side to side, you will also avoid the problem I
> had, and I suspect may still show up.  Your big tall CT could create this
> effect while transitioning.
>
>
>
> Hank
>
>
>
> On Thursday, April 13, 2017 8:45 PM, Alec Smyth via Personal_Submersibles <
> personal_submersibles at psubs.org> wrote:
>
>
> Hi Greg,
>
> Well here's how it went - much better but there's one more issue to
> resolve.
>
> The change to a raft configuration for the MBTs has solved the surfaced
> stability issue. It was great, I could walk around to any edge of the deck
> without excessive list. The longitudinal trim was slightly down by the
> stern when empty, and trimmed out to perfectly level when a person was
> lying in the front half of the boat (stability was calculated for the boat
> with crew). She floats in water only just over waist deep, 38 inches, and
> the freeboard is 24 inches. The thrusters and their controls worked
> beautifully, and the "tank drive" is really, really intuitive.
>
> After launching I drove around on the surface a bit. I didn't get a
> measurement of speed but would say while slower than a K350 she's decidedly
> faster than Snoopy. I tested a prototype kort nozzle by mounting it on one
> of the stern thrusters only and then giving both thrusters equal throttle,
> to see which way the boat tracked. Surprise, the un-shrouded original prop
> was more efficient because the boat would turn toward the side with the
> kort. So I'm just going to put on standard prop guards, at least for now.
>
> Part of the surface running I did lying down and looking through the bow
> dome. The view is ridiculously good! From the CT it wasn't bad either, and
> I was surprised how the flat domes made objects appear closer, something I
> didn't recall from the flat bow dome in Snoopy. In this one, the leading
> edge of the deck as seen through the CT viewports seemed only a foot away.
> The dome is something else entirely, and optically seemed to have the
> opposite effect of making things appear further away, but maybe that was
> just in contrast to the CT viewports I'd been looking though moments before.
>
> And so here is the new problem. The raft MBT consists of a collection of
> aluminum tanks, each of which has SS tubing coming out the top and
> gathering at a manifold, which is piped to the ball valves on the CT. As
> some of the tanks are off on the edges of the raft, some of the tubing runs
> side to side at an angle (up to 90 degrees) to the centerline. When you
> start flooding MBT, invariably one side will begin to fill slightly faster
> than the other. The side that is flooding faster will be lower in the
> water, and the effect of this list on the opposite side is that the tube
> connecting tanks to their manifold is now sloping downhill instead of up.
> This blocks the high side from letting out air, which exacerbates the
> initial list. It's really obvious when you see it, I should have thought of
> this effect. But luckily the solution is obvious too. There are two ways to
> fix it; remotely actuated valves right on the tanks, or new manifolds that
> are high enough to keep all tubing going uphill at reasonable angles of
> list. My initial impression is that the simpler of the two methods is to
> modify the plumbing.
>
>
> Best,
>
> Alec
>
>
>
>
>
> On Thu, Apr 13, 2017 at 10:10 PM, james cottrell via Personal_Submersibles
> <personal_submersibles at psubs.org> wrote:
>
> Did anyone hear how Alec's test went today?
>
> ------------------------------
> *From:* Alan via Personal_Submersibles <personal_submersibles at psubs. org
> <personal_submersibles at psubs.org>>
> *To:* Personal Submersibles General Discussion <personal_submersibles at psubs.
> org <personal_submersibles at psubs.org>>
> *Sent:* Thursday, April 13, 2017 9:34 PM
> *Subject:* Re: [PSUBS-MAILIST] Oil Compensator
>
> Jon,
> an easy solution is to take the thruster, or that section of the thruster
> in to a plumbing merchant or hydraulic repair shop & ask for something
> compatible with a barbed hose fitting. Sometimes a metric option will
> fit in an imperial thread, & so they may know of not so obvious solutions.
> Alan
>
> Sent from my iPad
>
> On 14/04/2017, at 12:53 PM, Jon Wallace via Personal_Submersibles <personal_submersibles at psubs.
> org <personal_submersibles at psubs.org>> wrote:
>
> Hi Hank,
>
> No, I don't have the shaft, I purchased just the lower head.  A 36 inch
> shaft is $28 but like you said, I really only need a few inches of it so
> don't really want to purchase it that way.
>
> Jon
>
>
> On Thursday, April 13, 2017 8:27 PM, hank pronk via Personal_Submersibles <personal_submersibles at psubs.
> org <personal_submersibles at psubs.org>> wrote:
>
>
> Jon,
> Do you have the shaft that used to screw into the motor?  if so, you can
> cut it down to a couple inches long and put a waterline compression fitting
> on that.   Then reduce from the compression pipe thread.  Or you can thread
> the inside of that stub shaft with a pipe tap and reduce from that,
> providing it is the heave fibreglass shaft.
> Hank
>
>
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