flotation....how much is enough?

[bendoo]

Thinking I will buy some of those collapsible 5 gal water jugs and blow them up and stash them forward and aft.
They need to overcome about 250 to 300 lbs of lead and mast etc in class configuration and 500 to 550 lbs in PHRF config.
How many would I need???

[Kevin]

Ben,

An option for you to consider as an alternative to the water jugs. IKEA makes some kids bean bag type things. Look like animals usually (mine were hedge hogs, but I digress). Instead of filling them with beans though, they sale an air bladder. The things are super tough. My kids jumped (and I mean full flight landing 75-80 pounds on the things) for years and they never even lost any air. They are now in my church basement and still going strong in the nursery there. I have 2 tied in aft of 169 in my boat as added flotation. I'm not exactly sure of the capacity, I'll need to look that up somewhere. I figured some was good and I wanted a little extra as a safety factor just in case things went crazy. So far I've been out in 15-18 and not had any water issues. So, so far they are just for piece of mind.

Chad hopefully will chime in with the formula you need. I know he has that knowledge at his figure tips if it's not already at the forefront of his brain. I need to do the volume math on my bunks and then figure out I need to add myself to be legal.

Cheers, Kevin.

Kevin.

[Mist]

A half-way rule of thumb for floatation is; a empty gallon container requires about 8 lbs of force to submerge. Therefore, 240 pounds of floation requires the volume of about 30 gallons.

[Kevin]

Ok, I'm backing into this factor from our rules which Chad did the math for. Tim K's 8lb per gallon rule is pretty darn close.

255 liters of air = 570 pounds of flotation which reduces to 2.235 pounds of flotation per liter.
3.7854 gallons per liter equals 8.46 pounds of flotation per gallon of air.

63.33 pounds of flotation per cubic foot of air(570 / 9 from the rule) is another way to think about it. That's 28.3333 liters per cubic foot.

Now time for some geometry to get the volume of those bunk and bow compartments. I'll need that for my measurement form I'm sure.

Kevin.

[Mist]

I’ve a little more prospective. I find it pretty amazing that there is very close to 7.5 gallons to cubic foot. So if you had 4 cubic feet of floatation you’d easily overcome 240 ponds. Also, 4 cubic feet equals 80 empty beer cans. I would need help drinking that many.

I'm planning to have three approximately 4 cubic foot air bags in my boat. Two on either side just forward of frame 169 and one as close to frame 18 as possible. This will be sufficient floatation and stability for a swamped boat. I don't like the idea of relying on the voids under the bunks because they will eventually leak and they are very low in the boat.

[Chad]

This all depends on what you want to achieve- "just" flotation, or the ability to recover and continue home without help. The proposed rule is aimed only at preventing boats sinking to the bottom. If you want additional protection for your boat, then additional flotation should be fitted.

The materials that make up the boat will displace about 6 cubic feet of volume, which at water's density of 64#/cubic foot will be providing about 400 pounds of flotation, against the boat's 800 pound target weight (just round numbers here, salt is different than fresh water, and most boats will weigh more than 800#, and most will also be built from materials that displace a little more than 6 cubic feet).

The rule added about 50% to the flotation requirement beyond the 400# deficit, rounded to the nearest whole cubic foot. This amount of flotation should keep the shear just above the water, if there isn't any crew standing in/on the boat.

To recover from a capsize and swamping, crew will likely need to stand on the boat to bail it out, and this would require additional flotation- about 3 cubic feet per crew person. All the flotation also needs to be substantially below the lowest deck opening to keep the water from flowing back in.

Also remember that the flotation needs to be distributed fore and aft- placing it all in the area behind F169 will allow the bow to sink until the boat stands bow down and almost vertical, for instance. Best bet (until somebody does some real life testing) is to place the flotation so that forward moment and the aft moment are equal, assuming a CG around f110. So (bow flotation X distance to f110) should approximately equal (stern flotation X distance to f110).

I'll have a look at the bunk volume of a standard boat and get back to you those numbers.

[Chad]

So the bunks in the plans, running from f53 to f169, excluding the 13" gap in the middle, displace about 8 cubic feet, to provide about 500 pounds of flotation.

i550_bunks.pdf

[bendoo]

ok...found the SWEEEEET solution!!
Big 5 sells an inflatable camping mattress. The double bed size is 60" x 80" x 8" or 22 cubic feet.
Any way you calculate it, that is over 1200 lbs of float. (1386 if you use 63 lbs/cu ft).
It is heavy duty material so should be fairly puncture proof.
It comes w/ a little electric pump that inflates it in 2 min's.
I'll lose some capacity, but it will fit under the cockpit just behind the keel. Along w/ making the area forward of bulkhead 18 watertight, I should have LOTS of float. And only cost $30!!
And guess where I am sleeping when we camp this summer!!!!
Gotta love double duty!
Ben

[Chad]

The forward 18" of the boat displaces just less than 2.5 cu ft of water, about 160 pounds. So, if your mattress only needs to lift about 400 pounds, and the mattress's CB is centered about 35" aft of the boat's CG, the bow needs to provide enough buoyancy to counter that moment. So the mattress is providing around 400X35 = 14000 inch-pounds of moment, in the ass-up direction. If your sealed bow area is about 100 inches forward of the CG, and displaces 160 pounds, its 16000 inch-pounds of moment will just counter the mattress. Just. I'd add more to the bow.

(these are rough numbers, everybody's CB will be different for various reasons such as cockpit and cabin shape, etc.- I'm just giving a rough idea how to make sure the flotation does what you expect it to. If Ben straps it in well enough, if it's tough enough, and if there's no sharp bits under his cockpit, Ben's mattress will absolutely prevent his boat from outright sinking, but the boat MAY need to be towed/salvaged with it's bow straight down, rather than being able to be self-rescued.)

[bendoo]

I keep the cooler in the bow...that should help some?
Hopefully we never find out how bad this idea is......
bd

[ryderp]

I'm new to the fleet - Hull 381. These forums are great for a newbie.

Is there any reason why I shouldn't just fill the space under the bunks with expanded polyurethane foam? I would think that this would add both floatation and some stiffness to the hull.

Phil Ryder

[jray]

Welcome Phil, glad you have joined in. One reason not to use expanding foam is it is not closed cell and will hold water and may cause rot in the hull after time.

[ryderp]

Thanks for the comments on the foam - actually the material that you linked to happened to be what i was looking at. I'll weigh the pros and cons - I'm just getting started, so I have time.

On a related note, a couple of oversized bumpers also will add 2 or 3 cubic feet of flotation, and they come in handy around the dock.

[Chad]

I'm dubious that filling the bunks with foam will add much hull stiffness- the area already has many "L" and "T" shaped plywood intersections, which provide lots of stiffness themselves. The bunks do provide a sort of box girder effect without the foam, but then the entire hull is a sort of box girder. My impression was that the bunk design was a left over from the boat's original micro-class origins, and I'm not convinced they do anything useful structurally, and left them out of my own hull. The hull bottom panel from chine to inner bunk stringer is at most 17" or so wide so the foam could provide some small amount of panel stiffness to the hull bottom panel, although I suspect higher densities of foam would work better for that. The idea of permanently hiding large areas of wood in the bilge of a wood boat has much larger downsides than any of the upsides I can see. Sort of "can't help, almost certainly will hurt"...

Except from a rules perspective, where it is very helpful (under the PDX rules, too) to concentrate as much of the boat's weight in the middle, while avoiding the appearance of adding the material for ballast purposes. Basically, the middle of the boat should be made as strong as you can reasonably justify. This will be a departure from what you have seen in most builder's blogs.

[M&S]

I will agree with Chad about not hiding area of the boat from inspection.

Every bit of reserve floatation can be regarded as "insurance". A point brought up about location of all these buoyant voids and foam blocks needs emphasis. I have had personal experience with an open water rowing shell with the floatation placed in the bottom of the boat. Unsinkable. But when the boat swamped there was too much weight above the bouyancy and the boat just rolled side to side and could not be bailed out. That was a long Morning.

I would tend towards placing the buoyancy away from the centerline and equal side to side aft. Forward of 53.5 to keep the bow from nosing down. the tricky part is for all of us to figure out how high the boat needs to ride in the water in relation to each boat's companionway to cockpit floor height when max water load is inside the boat. The point being that you can self rescue if you can keep the water from coming back in as you bail out.

Which brings another subject up. compaionway wash boards should have a robust method of keeping in place with the top open for water removal. Lot of food for thought and we are glad that so many thinking sorts are engaged here.
T

[admin]

Good thread.
I brought up the issue of wash boards and companionway issues as a matter of safety concerns a while back on the i550.org forum, don't think anyone chimed in though. Agree with M&S whole-heartedly. Haven't quite engineered the lock-in method for my boards, but I don't that's rocket science. They may just end up getting pinned in, but I'm sure someone will come up with some mighty creative solutions, maybe they've already done so (Chad?).

The subject of hidden areas came up recently when Tim K. the builder of "Mist" visited me last week. The question was, did I put in the flat panel that continues the bunk level, aft of frame 124 (i think?) and I said I had left it out b/c I did not want an area back there that would be closed for inspection if the bottom ever took a hard knock back in that area.

Do you think that leaving this out comprises rigidity and strength Chad? I can't remember who installed it and who left it out...it's a pretty big piece of ply, almost 4 x 4 feet, if I remember correctly. --tf

[Chad]

I remember proposing a rule to prevent downflooding way back when, but nobody else was interested. Basically, requiring any hatch below the level of the sheerline to be able to be blocked or sealed.

I haven't done anything brilliant with my cabin hatch or washboards yet. I was actually leaning toward the PDX-style canvas solution. And lots of air bags to keep the opening above the water line, however hard I may try to fill the boat...

Aft of frame 124, I think the best thing to do is to tie the cockpit sole to the hull bottom with full-height longitudinal "stringers" or longitudinal bulkheads/frames. You already have the two "flanges" (cockpit sole and hull bottom) of a very tall "I-beam", just waiting to have the "web" installed between them. I see no use at all to adding a large, horizontal surface back there between the already large horizontal surfaces of the hull bottom and cockpit sole. They might stabilize the hull side panels a little, but they connect just above the chine which itself already adds gobs of stiffness to the bottom of the side panels. Again, the bunks all appear to be left overs from the micro-class, either to meet their accommodation rules or the flotation rules.

I have no plan to sleep aft of f124, and for camping I'll be installing removable, drop in cedar-slat bunk surfaces between f53 and f89. I did put in some flat plywood at bunk height in the area between f89 and f124 to stabilize the mast step and the keel box. Lots of holes in the plywood though, for easy hull inside surface inspection.

[ryderp]

This topic has been quiet, but I'm close to gluing the deck to the hull and would like to address floatation beforehand. I'm really not a fan of just using air bags. That would be fine if we are sailing with others near by, but if you're out on the lake alone it's nice to know that your boat is not going to end up on the bottom if you get "holed".

My plan right now is to put floatation under the deck surface rather than in the bottom of the hull (to avoid worrying about rot primarily per the suggestions above). Is the pink foam from Home Depot good enough for this purpose? Can I just glue it to the underside of the deck? Alternately, I could use expanded polyurethane foam but that would take a bit more effort to build temporary containers.

Any suggestions?

Phil

[jray]

I was concerned about the same thing. My solution (not proven) was to create flotation compartments during the build. All have access ports, for inspection and open air flow during storage. I don't believe in a total sealed tank with no access. I have five separate flotation voids in the boat. First is frame 18 forward, three in the cabin deck between the hull and interior deck,the final one everything aft of frame 169.5.

On th the cabin deck and under the cockpit I fit two inch thick closed cell foam attached with Velcro strips to hold everything. The plus of having the foam is it floats first, it's easy on the knees and makes a great foundation for weekending.

It does add some extra work and expense but I'm confident with what I've done that I have more then enough flotation to (hope it never happens) keep the Twist afloat.

[ryderp]

Thanks for the advice guys. I like the Velcro idea - it makes the foam was to remove if you need access to that area for any reason.

[lohwaikin]

I am working on the "bunk" right now. Before sealing off the "floor boards" I am filling up the void space underneath with that "pink" foam (closed cell EP foam of approx. 35kg/m3). Just in case I run aground and puncture the hull under the bunk. At least the EP foam will prevent complete flooding of the "bunk" space.
I have purchased about 15 sheets of these EP foams. That adds up to about 590 lb of buoyancy. And I'm going to find ways to pack them all in....

Cheers,
Loh Wai Kin,
Singapore, Hull #437.

[micah202]

I am working on the "bunk" right now. Before sealing off the "floor boards" I am filling up the void space underneath with that "pink" foam (closed cell EP foam of approx. 35kg/m3). Just in case I run aground and puncture the hull under the bunk. At least the EP foam will prevent complete flooding of the "bunk" space.
I have purchased about 15 sheets of these EP foams. That adds up to about 590 lb of buoyancy. And I'm going to find ways to pack them all in....

Cheers,
Loh Wai Kin,
Singapore, Hull #437.

...I'd suggest sealing the foam in plastic and/or having airflow around it,,in case there's any moisture--definitely have some space between the foam and the floor.

..I found these useful for under the floor...if you know of any dead lasers ,you'll find 5 of these free inside!....http://www.amazon.com/Reliance-Products ... B001QBZI90

[TexLex]

Ok, for all if you mathematicians out there, how much floatation does each 5 gal container provide?

[Chad]

About 8.3#/gal, so 41ish pounds if fully inflated (with AIR!).

[Kevin]

..I found these useful for under the floor...if you know of any dead lasers ,you'll find 5 of these free inside!....http://www.amazon.com/Reliance-Products ... B001QBZI90

I used some air bladders from IKEA that they make for inflated kids toys. Similar idea only without the leaking air nozzle. They are super tough as my kids jumped on them relentlessly for years with barely any air loss. I strapped them down between 169 and 214. I'm sealed forward of frame 18 and under the bunks too. I'm not too worried about a hull breach through 6mm ply personally, but I understand the concern. I do want to add a bag in the bow to keep the boat bow up if things go totally to crap but that's not on the 2012 get ready for launch todo list.

Kevin.