New old boat!

[GreenLake]

Hmm, this can't be right:

I think you're wildly optimistic on your estimation, kokko. I did the math a while ago, and to make the boat unsinkable, you need 95 noodles, which no one has ever been able to fit in.

http://forum.daysailer.org/forum/viewtopi ... e&start=30

"According to the formulas here: http://www.glen-l.com/weblettr/weblette ... ation.html which seem reasonable, and assuming 1.8 as the specific gravity of solid fiberglass, I'm coming up with a negative buoyancy for the 575 pound daysailer of about 255 pounds (not accounting for the higher negative buoyancy of the mast, rigging, etc etc).

Assuming a 2lb density for the swim noodles, which seems reasonable (I've seen similar looking polyethelyne rated at 1.7), a 2.25" diameter 60" long noodle with a 1/2" hole in the middle is about 0.04 cubic feet. The net buoyancy of a noodle, then is 2.68 pounds (62 lbs per cf for fresh, 64 for salt water, less the foam's weight = 60 lbs per cubic foot net buoyancy). Which means you need at least 95 noodles to make a daysailer unsinkable. That's a lot of noodles."

At merely 2.68lbs buoyancy, these noodles would not be a lot of fun in the swimming pool! Fortunately, when I use your numbers, I get nearly 3 times the volume (.131 cubic foot or nearly a gallon).

The flotation comes to around 7.8 lbs per noodle in water, closer to 8 in saltwater, which seems more reasonable when you consider how they act in a pool. In terms of length, you get about 2.1 oz per inch of tube. (I then had to do a "literature search" and came up with this nice report, and the numbers match - assuming theirs is a 2.5" tube and not 2.25" - see page 12)

Under these revised assumptions you need about 35 noodles for neutral buoyancy, or half the total of what people report (75), which is about right when you consider that one tank is always out of the water in a capsize.

If your tanks are waterproof (or at least leak slowly) you should have additional buoyancy from the interstices between the noodles.

[ctenidae]

My math sucks. How did I get that small of a number for the number? I ran the math a couple of times. Rerunning it, though, I get buch closer to your answers- about 8 lbs of flotation per noodle.

Man, I suck at math.

However, GreenLake brings up a good point: on its side, not all of the DS is underwater, and so the portion above water will have its full weight, no positive bouyancy. Assuming 1/3 of the boat is in the water (wild guess), you end up having to support 468 pounds of negative bouyancy, which requires 7.8 cf of foam to float, or about 60 noodles. Of course, if you floated the boat entirely on its side that high, it would most likely turtle. In the end, the point of floatation is to keep the boat from going to the bottom, so 35-40 noodles should be sufficient. Probably.

[GreenLake]

My math sucks. How did I get that small of a number for the number? .

Dunno. Our answers do seem apart by something close to a factor of pi. Could that have been it?

Anyway, the noodles are for reserve buoyancy. Your tanks should be tight and provide primary floatation using their whole volume.

[ctenidae]

Anyway, the noodles are for reserve buoyancy. Your tanks should be tight and provide primary floatation using their whole volume.

That's the hope anyway, right?

[seandwyer]

My tanks breathe. When I sit down they exhale, when I get up they inhale. I can hear it but have never quite figured where the air is coming and going from. Hopefully it is just the drain plugs. They are old and I really should replace them.

[GreenLake]

The breathing is a sign that they are not fully tight, but also a sign that they are so tight that air (which is compressible) takes a bit to get through the cracks. (Same sitch for my boat, by the way).

The hope would be that this means that in the event, water will take some time to fill up the volume. Full buoyancy will aid in rescuing your boat, but the reserve should be enough to make sure you don't lose it altogether.

If the buoyancy isn't reduced to the utter reserve right away, because the leaks are (relatively) small (and perhaps localized) you would get more buoyancy during the first few minutes of rescue attempts. Which would be good.

If it's just your plugs "whistling" (and I suspected mine while they were still cork) then you might try to test your seats.

Here's an idea for a test apparatus - haven't tried it yet, but, let me describe it anyway (in the next post).

[GreenLake]

If your flotation tanks are tight, they should hold a small bit of overpressure for an extended period. How to measure this, without special equipment?

Here's an idea. Take a length of bicycle inner tube, with the valve attached. Close off one end. (Superglue might work, otherwise the glue from the tire repair kit, or folding and clamping).

Now fit the other end to the 1" drain holes on the tanks.

Possible way to do that: use a 1" rubber cork, drill a hole, glue the tire to the outside.

All of this probably works best with a tire from a road bike, because it's closer to the desired diameter.

Then pump up, until the section of tire is getting stretched noticeably from the air pressure. Measure circumference. Come back after a set time. Measure change.

Has anybody tried something like this? I haven't, and just brought my boat offsite for storage, so I can't play with the idea.

[algonquin]

I haven’t tried your method Greenlake. You can get the valve assembly from your local auto parts store and install one on each tank. Drill the hole and pop it in. Some aftermarket valves used on custom wheels have a fastener “nut” on both the front “outside” and one on the back “inside. What you will need is access like a opening port. If you don’t have a port drill the hole close to the existing hole that holds the tank drain so you can maneuver it through from inside the hole with your finger and set it with your free hand.

If you buy a general rubber replacement valve drill the hole and pull the valve through until it seats. It may need a drop or two of lubricant like dishwashing liquid to help coax it through until it seats.

Just read it with a tire pressure gauge. Brad

[GreenLake]

Now that's an idea --- as long as you don't inflate it at the gas station from the hose. I'd be afraid that you could easily overpressurize the tanks that way....

[K.C. Walker]

I think you do need to be really careful about over inflating. I put little bit of air in mine from my compressor to check its integrity and just a little bit too much in the tank starts making some gelcoat cracking noises.

I would think if you're going to try to check the air pressure you would want to keep it in the 3-4 pound range at most and even that might be too much. The only way to measure this accurately is with a low pressure gauge. I think it would be best to go at it very carefully and watch how much the tank bulges and only inflated so that it just starts to show.

[seandwyer]

While I like the simplicity of, and reduced cost of buying a valve stem (ones for truck tires come with rubber gaskets and nuts making it really easy to install them in any sort of hole of similar diameter) I wonder if Greenlake's idea might be just a little safer because the elastic nature of the inner tube would act as a buffer.

Definitely use a bike pump for this. I don't think I could really safely say that the pressure coming out of my compressor is something I can control at something as low as 3 or 4 pounds.

Maybe incorporating something that would be really elastic like a heavy balloon into the valve stem idea would be best. What about an inch piece of PVC (or 3/4 - I don't know what the OD is on either size) with a cap on the end. Mount the valve stem through a hole. Drill another hole and glue a barbed fitting for the balloon, then just insert the pipe into the tank. a bead of dried silicone caulk might act as a gasket and if the fit is tight enough you shouldn't need to have it mechanically fastened. We aren't talking about a of pressure - just enough to see if the tank is air tight.

[K.C. Walker]

It occurs to me that we don't really need to inflate the tanks to check their integrity. Or if we do we would only need moderate pressure like that from the exhaust of a vacuum cleaner. Someone mentioned recently that they heard hissing when they sat on their seat tanks, an obvious leak. When I was working on my boat and had the thwarts out I also heard this hissing through the screw holes that I had left open.

What I noticed was that the seat tanks were particularly soft when they were not airtight. That is, I was stepping in and out of the boat and onto the seat tanks repeatedly to work on the bilge area and noticed every time I stepped on the seat that they both hissed and felt squishy. When I sealed everything back up the seats became firm again.

The more I think about it the more I doubt we could measure air pressure inside the tank which would be something on the order of the pressure that would take to inflate an air mattress. So instead we would be measuring the convex deflection of the top of the seats or the top of the bow tank. Instead we could measure convex deflection by just putting weight on top of one of these surfaces.

I think for a quick check on the seat tanks to see if you have a significant leak you could just sit on your seat for a couple of minutes and measure the deflection with a straight edge before and after. We might need to be a little more creative for the bow tank. Also, if you open the tank with either the drain or your access panel and it doesn't feel soft compared to when you have it closed you probably have a significant leak.

I think a close visual examination along the seams should go a long way towards making sure you have no leaks. If you wanted to be absolutely sure that your seams where airtight or you had a leak you could not find, you could use a piece of hose like a stethoscope along all of the seams while someone sits on and gets off the tanks. Or you could use dishwashing soap and a spray bottle and spray the seams and watch for bubbles for smaller leaks.

Just some random thoughts for the end of November.

[algonquin]

You definitely wouldn’t want to use much air pressure. If you use a low pressure gauge to take your reading you may also want to check the ambient air temp to make sure the outside air temp is pretty much the same for your beginning and ending reading. Outside air temp will also have an impact on the pressure within an enclosed tank. Brad

[kokko]

I would not use the beaded syrofoam that O'Day used. After a time it soaks up water defeating the purpose of it. Use only closed cell foam or the soda bottle approach.

Unsinkable? Maybe not, but you would have to brach all three flotation tanks first, which is extremely unlikely. With the tanks intact, it woudl come pretty close to unsinkable, but could still capsize