Boat buoyancy question
A fellow biologist recently bought a 4-stroke motor for his duck boat. Because the motor is much heavier than his previous motor, he was concerned with the back of the boat riding extremely low in the water (this is definitely a concern in rough water). Anyway, he (and a couple other guys here) seems to believe that if he fills under the seats with styrofoam (or a similar light and buoyant material) that it will make his boat sit higher in the water. I (along with a couple others) disagree with him. I tried to explain that since the foam wouldn't actually come in contact with the water, that it wouldn't increase his buoyancy, but rather would make the boat sit slightly lower in the water (foam isn't heavy, but would probably weigh a couple pounds). I told him the reason styrofoam floats is because it's lighter than water. In order for it to lift the boat further out of the water, it would have to be under the hull, not above it. For some reason, I think he thinks it's going to work like helium in a balloon. We've had some heated discussions over this topic earlier today, and I'm just about ready to set up a small scale "experiment" in the lab. What does the spoonhouse think? I know there's some engineers out there, but try to keep your answers brief.
posted by Jim Brannick at 10:46 AM
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10 Comments:
The foam is good if the boat gets "swamped" it'll keep the boat from sinking to the bottom of the lake. As far as helping it float it won't do shit. You have to increase the amount of water being displaced by the hull. And the only way to do that is buy a bigger boat. You can buy foam strips that physically mount to the exterior of the hull just above the water line that run lengthwise. My dad installed those on his old duck-boat and it seemed to work well. I forgot what the name of them but Im sure you could find them by searching the net.
Okay, here is how boats work. The only reason that a boat floats is because the boat has less mass than the water that it displaces. The same thing as with helium and regular air. When two materials that are in a state of flux come in contact with each other the one that has least mass will 'float' to the top, eventually. Your friend could hypothetically fill the boat with foam and not make a difference. Point out to him that since foam is mostly air and some sort of polymer (which is probably denser than water) he's better off with nothing under the seats.
I would make it a priority to add foam under the seats as a saftey measure. When you're duck hunting in cold windy weather wearing heavy cloths in a boat thats loaded down with decoys people dogs and gear it would be a good idea to have a little safety factor if indeed the boat started to go under. My old man swamped a layout-boat in the middle of saginaw bay during a wind storm, the only thing keeping the boat afloat was the foam ballast in the hull. Lucky him.
I don't think that it gets that cold in Texas, not like Michigan duck weather. But your point is still very valid, boats without neutral buoyancy tend to not float so well when they are capsized or swamped.
Ndammitt had the correct explanation. A good lab experiment involves a bucket, a cup and foam you know floats on it's own.
Put the small cup in a the bucket of water. Mark the water line. Than fill the cup with foam. The weight of the foam will push the cup down further in the water. Even thogh the foam floats.
Jim, did your friend end up making a decision on what to do about his boat yet?
Thanks for all your comments on this subject. Before I created the post, I tried to explain all of this to him, I guess I was looking for a simpler way to pound it thru his thick skull (besides a simple experiment). Here's the skinny, he still thinks the foam in the seats will increase buoyancy, and I'm tired of trying to convince him otherwise (I'm no longer surprised that he's never finished his Master's). But, at the very least, his boat should be safer (like Simon said), which is a good thing since I'll probably hunt with this dolt a few times in the future.
Oh, and NDAmmit: it gets plenty cold here in NE TX in the winter... enough to where you get hypothermia should you spend some time in the water.
What a tool. Oh well one of things that my professors always tried to drill into us was the our intuition was faulty. For the first year and a half they'd do demo after demo that showed we didn't really understand physics very well.
Oh well, at least his boat will be safer.
Just goes to show you can't convince stubborn people, regardless of the evidence you present.
It's like trying to convince someone that "The Next Generation" is better than "Star Trek".
In many ways it's superior but will never be as recognized as the original
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