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Lets talk about foam!
This one keeps coming up, so I thought I'd post up some thoughts on the subject.
Common misconceptions:
Foam will make my boat sit higher in the water. False.
Foam will cause my boat to capsize if I take on water. Maybe.... (what???)
Foam will make my boat sit higher in the water.
Foam will not float your boat any more than air. It is actually LESS bouyant (foam weighs a LOT more than air..) However, it has a specific gravity less than water, hence its tendency to float, not sink.
It won't make your boat float MORE just because it is foam. Ask yourself this question: How much foam will it take to float your boat all the way out of the water? Right. Not going to happen.
What foam WILL do for you is prevent your boat from sinking in the event of a hull breach. Using a sealed compartement with just air works fine, but if there is a hole in it, it will fill with water if you hit something and down you go. If there is foam, and it is in a properly sealed compartment, the water has no where to go. So it is a bit of a safety net. It also provides excellent sound damping.
Here is a great quote from the tubes that explains bouyance and boats:
http://www.newton.dep.anl.gov/askasc...9/phy99x34.htm
"..law of bouyancy, discovered by Archimedes, and basically says:
any material or object immersed in a fluid will tend to rise through the fluid if the fluid density is greater than the material density. The force associated with buoyancy is the difference between the weight of the displaced fluid and the weight of the immersed material.
Density is the amount of and object (weight or mass) there is an a given volume (the amount of water the object displaces).
So objects which weigh a lot and don't displace much water (have a low volume) will sink, while objects which are light and displace a lot of water will float.
The relationship, then is how well the object will float. If it floats, it is less dense than water, and if it sinks it is more dense than water.
You might think something like a ship is more dense than water, after all, it is made of metal which is certainly more dense than water, but, the ship is filled with air, which is much less dense than water, and makes up the difference. That is why, when a ship gets a hole in it, it sinks. The water pushes out all the air and makes the total density of the ship greater. When the water, metal, and air inside the ship become more dense than the water outside the ship, the ship will sink. "
Make sense?
Foam will cause my boat to capsize if I take on water!
Maybe. What causes boats to capsize is a shift in the center of gravity, not what is used as bouyancy. If you are taking on water, and your hull is truely sealed with foam, the water will collect in the boat, and settle on the deck. This might cause you to capsize if enough of it gets in there and overcomes the bouyancy of the hull (provided by its shape). At some point, the center of gravity shifts from below the water to above the water. Tip the boat just a bit with a wave or a moving body and over she goes. This isn't the foams fault. The same thing would happen if you had a bilge filled with air that was properly sealed up. The air would want to come to the top of the water just like the foam. So, in this set of circumstances, neither one is going to cause or hinder a capsize.
Some folks like to add foam to the gun'ls to prevent a capsize. This is surely a good idea in the sense that it displaces air and causes a given chamber to be sealed, vs being filled with air (which we know will get filled with water if you let it). However, the risk of a capsize is still there. It all depends on how much weight you keep above the water. Have a small runabout with not much above the deck? Chances of a capsize are small, even with a deck full of water. Boston whalers made their name for this. There is a bilge filled with foam and not much up top. Fill it with water and the boat still wont go all the way under. Have a fishing boat with downriggers, a big hard top with metal bits and radar, kayaks, whatever. ya, your chances are much higher that that stuff wants to go over. I think the value here is that if the boat does go over, depending on how sever the situation is, you might get the boat to recover if the center of gravity is below the water line. If not, you are still going to go for a swim.
One thing I like to think of as a peace of mind is the fact that even if your boat capsizes, the foam still wants to float! If you have enough of it, and enough wood, fuel, and other things that can cumulatively have a specific gravity less than water, you will at least have something to hang on to while you wait for a rescue.
This one keeps coming up, so I thought I'd post up some thoughts on the subject.
Common misconceptions:
Foam will make my boat sit higher in the water. False.
Foam will cause my boat to capsize if I take on water. Maybe.... (what???)
Foam will make my boat sit higher in the water.
Foam will not float your boat any more than air. It is actually LESS bouyant (foam weighs a LOT more than air..) However, it has a specific gravity less than water, hence its tendency to float, not sink.
It won't make your boat float MORE just because it is foam. Ask yourself this question: How much foam will it take to float your boat all the way out of the water? Right. Not going to happen. What foam WILL do for you is prevent your boat from sinking in the event of a hull breach. Using a sealed compartement with just air works fine, but if there is a hole in it, it will fill with water if you hit something and down you go. If there is foam, and it is in a properly sealed compartment, the water has no where to go. So it is a bit of a safety net. It also provides excellent sound damping.
Here is a great quote from the tubes that explains bouyance and boats:
http://www.newton.dep.anl.gov/askasc...9/phy99x34.htm
"..law of bouyancy, discovered by Archimedes, and basically says:
any material or object immersed in a fluid will tend to rise through the fluid if the fluid density is greater than the material density. The force associated with buoyancy is the difference between the weight of the displaced fluid and the weight of the immersed material.
Density is the amount of and object (weight or mass) there is an a given volume (the amount of water the object displaces).
So objects which weigh a lot and don't displace much water (have a low volume) will sink, while objects which are light and displace a lot of water will float.
The relationship, then is how well the object will float. If it floats, it is less dense than water, and if it sinks it is more dense than water.
You might think something like a ship is more dense than water, after all, it is made of metal which is certainly more dense than water, but, the ship is filled with air, which is much less dense than water, and makes up the difference. That is why, when a ship gets a hole in it, it sinks. The water pushes out all the air and makes the total density of the ship greater. When the water, metal, and air inside the ship become more dense than the water outside the ship, the ship will sink. "
Make sense?
Foam will cause my boat to capsize if I take on water!
Maybe. What causes boats to capsize is a shift in the center of gravity, not what is used as bouyancy. If you are taking on water, and your hull is truely sealed with foam, the water will collect in the boat, and settle on the deck. This might cause you to capsize if enough of it gets in there and overcomes the bouyancy of the hull (provided by its shape). At some point, the center of gravity shifts from below the water to above the water. Tip the boat just a bit with a wave or a moving body and over she goes. This isn't the foams fault. The same thing would happen if you had a bilge filled with air that was properly sealed up. The air would want to come to the top of the water just like the foam. So, in this set of circumstances, neither one is going to cause or hinder a capsize.
Some folks like to add foam to the gun'ls to prevent a capsize. This is surely a good idea in the sense that it displaces air and causes a given chamber to be sealed, vs being filled with air (which we know will get filled with water if you let it). However, the risk of a capsize is still there. It all depends on how much weight you keep above the water. Have a small runabout with not much above the deck? Chances of a capsize are small, even with a deck full of water. Boston whalers made their name for this. There is a bilge filled with foam and not much up top. Fill it with water and the boat still wont go all the way under. Have a fishing boat with downriggers, a big hard top with metal bits and radar, kayaks, whatever. ya, your chances are much higher that that stuff wants to go over. I think the value here is that if the boat does go over, depending on how sever the situation is, you might get the boat to recover if the center of gravity is below the water line. If not, you are still going to go for a swim.
One thing I like to think of as a peace of mind is the fact that even if your boat capsizes, the foam still wants to float! If you have enough of it, and enough wood, fuel, and other things that can cumulatively have a specific gravity less than water, you will at least have something to hang on to while you wait for a rescue.
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