TIRE TUBES FOR FLOTATION???

[sailboatmike]

This whole "How much floatation do I need for positive buoyancy" thing has long had me wondering about the calculation.

Now the has positive buoyancy yet I would think would only have just over 1 cubic meter of foam, and the boat weights say 1,500Kg or 3.300lb.

Now we all know that 1 cubic meter of foam on its own cant lift that weight,

So I tried to work out other factors that need to be taken into account.

The main one I think is that the hull would still has a rather large displacement even if full of water, that alone dramatically reduces the amount of buoyancy one needs for positive buoyancy , and as we know the boat wont float unless displacement of water is greater than the weight of the object,

So before the boat actually starts to really sink it has to take in water (weight) to overcome the normal positive buoyancy of the hull weight carrying capacity, know I know on my 20 footer I have had 8 adults and a couple of kids on the boat and she only dropped a couple of inches below her normal waterline, and thats a tiny displacement compared to the Mac (quick calculation 6 x adults @ 175lbea = 1408lb plus a couple of kids@ 90lbs ea = 1588lb total plus the outboard and normal bits and bobs maybe another 500lb at least, So as you can see the displacement of the hull has significant extra capacity.

As far as my logic takes me even when full of water the hull will still have significant displacement

If Im barking up the wrong tree here please feel free to correct me as this is one of those things I would love to get my head around just for my own knowledge base

[Russ]

Are you really worried about sinking your boat? And what reason do you believe would cause the boat to sink? Prop shaft pulling out maybe? Hitting a reef? What will that look like? Clinging to a hull barely above the water. I don't get the sense of safety folks feel because their boat won't sink. On the open sea in rough waves, what will that floating carcass of fiberglass look like.
I guess it makes salvage easier, but you're still in the water hopefully wearing a PFD on the floating VHF calling for help.

I've seen so many find comfort in an "unsinkable" boat. I wouldn't put much stock in that.

My friend who's been sailing around the Caribbean over the winter was on his way home to Key West from Puerto Rico when he holed his boat. Hit a rocky shore on a remote island. Had to deploy his EPIRB and the Coasties sent a helicopter to pick him up.
http://coastguardnews.com/coast-guard-r ... 015/05/22/

You see, the thing that holed his boat allowed him to jump off onto it.





Chopper view

[seahouse]

Mike-

In my first post above I said “neglecting lots of other things”. That statement replaced a pile of considerations that I started to list, but just kept on growing, that I deleted, LOL. It's an essay topic, but I will briefly spew some data and let you feel free to ask to clarify any of it...

I expect your X would have closer to 1.5 cubic meters of foam in it (to get the specified floatation) a cubic meter on its own will float around 2200 lbs.

Pretty well all solid plastics (except polyethylene and polypropylene) sink in water. Solid polystyrene sinks in water too, unless it's in the form of expanded polystyrene (EPS), which is what the foam floatation on our boats is.

Using that same logic, fibreglass sinks too, unless it is laminated with something less dense, like wood or foam. And that is only in a few select areas of the boat where extra strength is needed. Swamped, all on its own, with no air in the ballast, and with the fixed ballast in place, the hull itself will sink. There are lots of things fixed to the boat that float (gas tanks, companionway hatch, foam) lots that sink (anchor, motor, batteries), and some that float now, but will be sinking soon (or sooner or later, like some cushions)! Some things trap air and release it readily, some things don't. Air can get trapped inside the cabin and help buoyancy too, for a length of time depending on boat attitude and conditions.

I expect that the distribution of foam in the Mac is set in all three directions; up/ down; forward and aft; port/ starboard, so it gives a centre of gravity that allows the swamped boat to float reasonably upright in most conditions.

Anything that floats and is not attached to the boat will not contribute to its buoyancy, it will just float around inside the cabin (unless it's trapped on the ceiling) or cockpit, or float away. People with lifejackets on floating in a swamped boat fit that category as well, unless they are standing high and dry, like on top of the cabin. Then they will contribute to negative buoyancy.

Once the boat is fully swamped, the hull displacement no longer has any effect. In a hull breach an equilibrium will be reached where the water level inside the boat will equal the water level outside the boat, and all buoyancy from hull displacement of water will be nil, and then it's the net buoyancy of what remains that counts in keeping the boat afloat.

Clear?

[seahouse]

Russ --

I don't think there was ever a boat made (well, a U-boat maybe) that was deliberately made "sinkable". It is purely for practical or economical reasons that any boat is made “sinkable”. It's a major design flaw that many boats, other than Macs, have; being designed from the outset to “sink” if they were ever swamped or overturned. No rational sailor, given the choice, would choose a boat that sinks over one that doesn't, all else being equal.

Consider the case of two boats, one sinks fast from a hull breach (say, from hitting a shipping container) in the middle of the ocean, no time for life rafts, loses everything, entangles some crew in the rigging and takes them 5 miles down with it, crushing them under the pressure (ouch, I bet that would hurt). The remaining “lucky” ones might be floating potential shark food (cue the jaws music), waiting for rescue for who knows how long (“I thought YOU brought the EPIRB”!).

On the other hand, a second boat floats, but is swamped. Everything is still with them, they have a haven from predators and sun, have shelter, provisions, all their food and supplies, and lots of time to call rescue, or effect a plan for repair and self-rescue. The family is all together and able to stay safe for a while, and are a large target for SAR to locate. They fight over which I-Tunes get played in the mean time. The Dad is T'd off because his beer is warm.

Sure, under the conditions that Macs get used it's an unlikely scenario... well at least the middle of the ocean part is. But being able to float while swamped is an important feature that most manufacturers, for a number of practical reasons, are unable to achieve.

I think it's a natural instinct for survival and self-preservation that attracts most folks to the second boat.

[Ixneigh]

Wow was the guy able to salvage his boat?

There are inflatable boat flotation devices available.
Several are needed for keel boats but probably only one for Mac.

Ix

[Tomfoolery]

Putting a few numbers to it, FRP has a density around .06 lb/in^3, or 104 lb/ft^3. The weighs about 2100 lb empty (though that does include mast and hardware), so its volume would be around 20 ft^3 of FRP.

Water has a density of around 62 lb/ft^3, so a fully submerged, fully flooded would displace around 1250 lb of water, assuming all cavities are filled with water (no air pockets).

You need another 850 lb of net buoyancy to balance that, plus whatever additional weight is added (like the OB, minus the weight of water that extra weight displaces), so something over 1000 lb of water displacement when submerged, plus a little more to keep the cabin roof above water a little. Plus a couple or few people.

So an additional cubic yard of foam, with very low density, like closed-cell polyurethane at 2-3 lb/ft^3. A cubic yard of that would weigh 80 lb on the outside, and displace 1670 lb of water. A cubic yard isn't all that much foam.

Just thinking out loud.

[TaZ-n-Izzy]

Keep in mind that that stuff (EPS) is flammable, so, once it's placed, you don't want it more exposed to that risk than it needs to be. In a house it has to be covered (like by drywall) to meet building codes. You can test a small piece with a match if you are in question about it.

yes that's my other concern, but we are a smoke free boat

I will start cutting foam and fill the boat... let u know the progress!

[Russ]

But being able to float while swamped is an important feature that most manufacturers, for a number of practical reasons, are unable to achieve.

I think it's a natural instinct for survival and self-preservation that attracts most folks to the second boat.

I get it. It's attractive. I just don't understand the benefit vs. probability. I remember in the 80s there were kiosks in airports to sell life insurance for your flight. I suppose lots of people bought one flight policies. However, what are the odds of a Mac sinking? Is it worth the trouble? To each his own. If it makes you sleep better, why not.

[Russ]

Wow was the guy able to salvage his boat?

There are inflatable boat flotation devices available.
Several are needed for keel boats but probably only one for Mac.

Ix

He abandoned it. I suspect the hole was minor, but he couldn't get it off so he left it for an insurance claim. Want the GPS coords? Go get it.

[TaZ-n-Izzy]

Macgregor 19 Owners MANUAL say



Macgregor/Venture V21, V22, V25 Owners MANUAL say



it doesn't bother a little more safety

just saying!!!!

[seahouse]

Yeah, I get your point too Russ. And the use of foam is just so in sync with the overall Mac design philosophy. I think it also has other functions, like giving some areas some structural rigidity, based on the removal descriptions of some others here.

So Tazz, it sounds like it would be wise to locate your floatation higher up in the boat, to help it from turning turtle when swamped.

[grady]

Keep in mind that that stuff (EPS) is flammable, so, once it's placed, you don't want it more exposed to that risk than it needs to be. In a house it has to be covered (like by drywall) to meet building codes. You can test a small piece with a match if you are in question about it.

yes that's my other concern, but we are a smoke free boat

I will start cutting foam and fill the boat... let u know the progress!

Smoke free or not if it is flamable DO NOT USE IT!

[Russ]

Yeah, I get your point too Russ. And the use of foam is just so in sync with the overall Mac design philosophy. I think it also has other functions, like giving some areas some structural rigidity, based on the removal descriptions of some others here.

For many it is an extra sense of safety and that may be true. Better to be rescued (and spotted) from a floating lunk than from just a PFD bobbing in the ocean. I'm concerned, however, that folks may get too much of a sense of security about it being unsinkable. Below it reads:

"If the cabin of the boat is entirely filled with water, and the boat is dependent on the foam flotation to keep it afloat, it will be very unstable, and may turn upside down."

What I take away is the boat will be floating, but don't expect to be sitting in the cockpit or deck (like the brochure) all comfy waiting for help. The boat won't sink to the bottom making salvage much easier, but in a rough sea, you might not want to be clinging to a floundering hull that could tip over. Call me a pessimist, but the brochure photo with no engine, no ballast and five little guys standing on the deck in the marina is not realistic. The M with it's permanent ballast probably stands a better chance of remaining upright, but the other Macs will be flopping around. Add wind on the rigging and that boat is going over. I've pulled my boat way over simply by standing on one side of the deck.


Again, easier to be spotted clinging to a white hunk of fiberglass. However few of us sail blue water and can easily be located if close to shore with VHF with GPS capability. It's all probability and risk. I don't buy extended warranties, but others feel better with them. I know the warranty companies know they won't have to pay out and make tons of money on fears and the need for security. Same here. By default, I would recommend keeping flotation. It's one more layer of safety that doesn't cost anything. However, many of us have taken some of it out for storage and that's the risk. Out of 10,000 boats, I've seen 2 Macs that may have capsized to engage the flotation and one Venture that actually sank from removing the flotation (he later raised it). So I don't see these boats as easily sunk. It's my understanding that boats under 26' require flotation by law in many states, so that may be why our boats have it. Roger didn't add anything extra he didn't have to and the foam in my boat is all scrap foam from who knows what.

If the OP is planning on blue water sailing, add some flotation, and EPIRB and good PFDs. For most of us, it's never going to be an issue. And if you sleep better knowing your boat won't sink, then it's well worth adding the foam. If my boat sinks, I have insurance and in the meantime a nice storage locker over my dinette that once held a few scraps of Styrofoam.

[seahouse]

Keep in mind that that stuff (EPS) is flammable, so, once it's placed, you don't want it more exposed to that risk than it needs to be. In a house it has to be covered (like by drywall) to meet building codes. You can test a small piece with a match if you are in question about it.

yes that's my other concern, but we are a smoke free boat

I will start cutting foam and fill the boat... let u know the progress!

Smoke free or not if it is flamable DO NOT USE IT!

Some foam is treated with borax to improve fire rating (also helps keep insects away), but it still burns, just more slowly, and should fizzle out when the igniting flame is removed. It would be interesting to test a small piece (like a sliver) of factory foam with a match and compare it to ordinary foam. If you do this don't let any drip on you, and do it outside. I doubt there's a fire retardant in the factory foam, but you never know.

If you want to see how fast the flame spreads in untreated foam in real life, Google "Whitesnake concert fire" (or similar) and see the video. A lot of people died - one lung full of fumes and you're done. (Warning - it's a sobering video).

[TaZ-n-Izzy]

So Tazz, it sounds like it would be wise to locate your floatation higher up in the boat, to help it from turning turtle when swamped.

I'm a network engineer not a structural engineer!!!

well... back to the drawing board