Soda Bottles for Replacement Floatation

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Jan 19, 2010
12,362
Hobie 16 & Rhodes 22 Skeeter Charleston
a Never Ending Thread!

:D Maybe we could start a conversation about the effects of lightning strike protection on foam flotation. Some of you know what I'm talking about:dance:
 

walt

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Jun 1, 2007
3,511
Macgregor 26S Hobie TI Ridgway Colorado
Dont forget the effects of radioactive water also :eek:
 
Sep 26, 2010
808
Macgregor 1993 26S Houston
Speaking for myself, I am guilty of not doing my homework before posting, but this is usually a low key friendly type of place. Its been kinda dead for awhile and so it seems even that kinda stuff is good if it generates interest and gets guys posting.
Give em a break! It's all good.
Jim
 

Sumner

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Jan 31, 2009
5,254
Macgregor & Endeavour 26S and 37 Utah's Canyon Country
....One poster had several containers on board, water and fuel etc and figured there were usually some empty so that provided extra flotation. Fine but how about a What If game? What's the worst case? All full! Worst case (not usually) should be the design point....
That would be me....and I was commenting that a lot of the added weight to our boat would either be neutral buoyancy or would add to the buoyancy if the boat was filled with water.

I have no idea if our boat would float either before we started adding items or if it will float now. I saw the one floating in the Mac commercial tape after being filled with water, but that boat didn't have the items on board that even a weekend cruiser would have.

We just bought a second boat that weighs 20,000 lbs.. Will it float if filled with water? I doubt it, but ones like it have sailed around the world.

If every thing in my life had to be designed for 'worst case' then I probably wouldn't get out of bed in the morning. I'm just not that kind of guy, but I have no objection at all to you doing anything that you want in the name of safety.

c ya,

Sum

Our Trips to Utah, Idaho, Canada, Florida

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May 4, 2005
4,062
Macgregor 26d Ft Lauderdale, Fl
in general terms, boats are surprisingly hard to sink...

-the old adage is, never step down into a life raft. -plenty of boats have been abandoned by crew and were found floating in the ocean days or weeks later.

The only point I'd like to add is a half empty or empty soda bottle is not going to do much good if it pops out the top of the boat. Make sure that flotation stays put and helps lift the hull, vs floats away in the debris field.
 
Jan 19, 2010
12,362
Hobie 16 & Rhodes 22 Skeeter Charleston
DISPLACEMENT
Nobody seems to try to approximate the displaced volume of their vessel. It is a tedious time consuming task but it can be done and is not terribly difficult. Gotta make lots of measurements. Square the whole thing off etc. It doesn't have to be completely accurate. But at least you'd have a good first approximation. So instead of a WAG* you'd have a SWAG**. Give you a better handle on how much flotation is needed.


* Wild Ass Guess
**Scientific Wild Ass Guess
Personally, I like SWAG :D I think I'm going to steal that one (I teach chemistry at a small college and could use that in lab).

BTW It is not hard to determine the displacement of your boat. The literature that came with your boat (now a simple Google search) has the displacement. And if you want the actual displacement loaded down for cruising just drive your boat to a truck stop, weigh the boat on the trailer. Go put the boat in the water and weigh the trailer empty. Subtract the trailer from the boat-on-trailer weight.... VIOLA - displacement.

Now just add your crew's body weight plus a safety factor and you have a complete number to work with.
 
Jan 19, 2010
12,362
Hobie 16 & Rhodes 22 Skeeter Charleston
Or....

This is what they have.
I used the pink foam boards you find at Home Depot. The Lowes by my house did not carry the foam boards. They are easy to work with. 2-cents.
 
Jan 19, 2010
12,362
Hobie 16 & Rhodes 22 Skeeter Charleston
Anyway I wonder if my use of the word 'displacement' was in error. What I'm thinking of and was referring to is the volume displaced with the entire rig under water.
Rig? I'm not sure you mean rig. The rig is the mast, spars, and rigging. If that is underwater you are sunk. :dance:

But I think I might be splitting hairs. If the boat is full of water (and assuming zero buoyancy of the hull, rig etc.) then what you want is enough flotation (buoyancy) to lift the weight of the boat above the gunwale so you can bail. So I think I am correct in that you simply need to know how much your boat weighs. Then if your foam has a buoyancy equal to the weight of the boat, rig & stores + safety factor.... your good.
 
Sep 26, 2010
808
Macgregor 1993 26S Houston
A good bit of the flotation is above the gunwale is it not?
Seems like even in the Mac advertisement the transom/cockpit was under water.
No bailing there. They did say that it could still be sailed all but slowly like that?(yea right?)
It seems like once it gets that low, the water ballast would be neutrally buoyant, and won't act as a righting aid either,so you might not stay upright.
 
Jan 19, 2010
12,362
Hobie 16 & Rhodes 22 Skeeter Charleston
A good bit of the flotation is above the gunwale is it not?
.....the water ballast would be neutrally buoyant, and won't act as a righting aid either,so you might not stay upright.
We are talking about V222's they are swing keel with 800# cast iron ballast. In my V222, the flotation foam is under the V-birth, and wedged in under the cockpit. I also have some wedged in under the births port side. So all of it is below the water line (and gunwale for that matter).

I think I could bail but I sure hope I never have to find out.:eek:
 
Jul 29, 2010
1,392
Macgregor 76 V-25 #928 Lake Mead, Nevada
Floatation issue settled once and for all. Stuff the damned thing with ping pong balls!:cussing: Fair Winds and Full Sails (and floatation)
 
Oct 24, 2008
424
Macgregor 25 (1984) Wildomar, So. Cal.
And if THOSE happen to escape?!?

"This just in to our newsroom: officials are confused by the apperance of thousands of ping-pong balls floating iinto the harbor and washing onto the shore early this morning. Boaters and angleras alike were surprised to see what looked like a virtual slick of the little air-filled toys bobbing in the current just out side the breakwater. Sea lions have been mistakenly swallowing the balls, thinking them to be some sort of eggs.
Harbor officials and wildlife experts suspect a container of the balls fell off a Chinese cargo ship in the night".
 
Jan 22, 2008
198
Montgomery 17, Venture of Newport, Mirror sailing dinghy, El Toro sailing dinghy Mound, MN -- Lake Minnetonka
And if you want the actual displacement loaded down for cruising just drive your boat to a truck stop, weigh the boat on the trailer. Go put the boat in the water and weigh the trailer empty. Subtract the trailer from the boat-on-trailer weight.... VIOLA - displacement.

Now just add your crew's body weight plus a safety factor and you have a complete number to work with.
Actually, you don't need nearly that much floatation. Even though your boat may weigh, say, 3000 lbs in air, it will weigh considerably less if fully submerged. The submerged wieight is what you need to compensate for. Of course, if you had only that amount of floatation the boat would have neutral buoyancy. It would not necessarily float at the surface. But every bit of floatation above that amount will keep the boat from sinking. And if you have 3000 lbs of floatation down low on a 3000 lb (in air) boat she should float at her waterline even if flooded.

Everything on the boat, including the hull, keel, bulkheads, etc. has a specific gravity that is either more or less than that of water. The bulkheads, for example, are made of wood and would float if submerged. And, as Sumner stated, a full gas tank is very heavy in air but weighs less than zero in water. It has some positive floatation. An empty tank, depending on its weight, can have even more floatation than the equivalent volume of foam.

To illustrate, life jackets do not provide 200 lbs of floatation to support a 200 lb person because he does not weigh 200 lbs in the water. I believe life jackets are made to have 35-50 lbs of floatation because that is all that is needed. The weight of the boat and its gear WHEN SUBMERGED is the relevant number, not the weight in air. Concrete, for example, loses about half its weight when submerged; that is why it is not usually recommended for mooring anchors.

On the other hand, if you base your floatation amount on the weight of the boat in air you are building in a large safety factor.
 
Oct 16, 2008
512
MacGregor/Venture 25 Mesa AZ
Heinzer: When your boat sank; was the floatation removed?

BTW: many years ago they pumped ping-pong balls into a boat off Redondo Beach CA to refloat it. So it seems to work - if you can coral them so they don't escape the boat.
 
Sep 26, 2010
808
Macgregor 1993 26S Houston
Just for fun, I tried to find the weight of fiberglass. The closest I could come is fiberglass sheets used for construction. It has a specific gravity of 1.87.
If I used my Mac's listed weight to work from and ignore the fact that there are other some amount of materials on the boat I could estimate the cubic ft of foam I would need to be neutral.
So, the boat is 1650 lbs. divide by the SG of 1.87=882 lbs. So the boat weighs 882 lbs under water. Then,I looked up weight of styrofoam. One place said it weighs 6.24 lbs./cu. ft. ( I think it's actually lighter but I'll use that)
Fresh water is 62.5 lbs/cu.ft. So 62.5 less 6.24=56.26 lbs of flotation you get from 1 cu. ft. of that foam.
882 lbs of sunken fiberglass needs 882/56.26=15.67 cu ft of foam.
16 cu. ft. of foam seems like a lot. If I used foam at 2 lbs/cu.ft. like it seems like it should be, then 62.5 less 2=60.5. So, 882/60.5=14.5 cu. ft. of foam. Still a lot.
There you have it!
Jim
 
Sep 26, 2010
808
Macgregor 1993 26S Houston
I'm trying to imagine where they would stuff 15 cubic feet of foam in my boat, and It doesn't seem like it's really there. Of course my calculation leaves a lot to be desired but I thought it would be in the ball park at least.
Maybe I should test it? Not!
I think I'll take their word for it, but it's fun to play with the numbers.
 

Sumner

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Jan 31, 2009
5,254
Macgregor & Endeavour 26S and 37 Utah's Canyon Country
I'm trying to imagine where they would stuff 15 cubic feet of foam in my boat, and It doesn't seem like it's really there.....
Sitting here in the boat I can see quite a bit. What is above the sink, what is ahead of the v-berth, what is above the v-berth and what is in the cockpit coamings. Maybe not 15 cf, but maybe not that far off.

Maybe on the boat in the commercial they don't have the ballast tank full :redface:. If you start to go down blow the ballast tank ;),

Sum

Our Trips to Utah, Idaho, Canada, Florida

Our MacGregor S Pages

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