A Question For You Engineers Out There

greenspun.com : LUSENET : Open-water rowing : One Thread

Archimedes Principle states that the buoyant force on a submerged object is equal to the weight of the fluid that is displaced by the object. Since most pulling boats don't displace much water, presumably, the buoyancy thrust of the water doesn't lighten the pulling load much. Is this true?

Still, I can row 500 lbs of boat and passenger, but I'd be hard pressed to push a 500 lb object, even if it was on wheels or ice (and certainly not for miles at a time). Is this nothing more then the lever arm advantage provide by the oars? Obviously, a 250 lb man and 50 lb child can balance themselves on a seesaw if the fulcrum is properly placed. Anybody out there with a good grasp of the force dynamics in rowing?

-- Marc J. Epstein (tercon@superlink.net), November 30, 2001

The buayant force has nothing to do with the resistance to forward motion of the boat. The reason baots are so easy to move is that the baot is resting in a liquid, buch like a baloon. The resistance to motion is only the viscous nature ot the liquid. The thicker the liquid, the greater the force required to move it and the slower the vessel will move. So an airplane can fly at hundres of mile per hor in air, but water is quite a diffent animal. I tar, a vessel would move even slower and would require even more force.

Boats in water reach what is called "hull speed" and up to this speed they are very easily move. Above this speed, they must "plane" or you will not be able to ecxeed it without adding very much more power.

Hull shape has more to do with resistance than weight. It will, however, take longer to get a large boat moving due to enertia, not friction.

Obviously there is much more to this, but maybe this helps?

-- Jim Reineck (JMRandSon@aol.com), December 08, 2001.

I think Jim has touched on some of the issues, but I'm not quite satisfied yet. Clearly, the viscosity of the liquid (water vs. tar) most affects displacement-type vessels and hull speed is a function of their speed to length ratio (in water). Is the small amount of water displaced by most pulling boats enough to throw them into the displacement class? I can't quite see it. A planning speedboat has an abundance of power and lift so the water has less affect on its speed, once it's up and out of the water. "Stiction" is a combination of inertia, stickyness and friction. You have to overcome stiction to get a boat moving. Still, it's a heck of a lot easier to row a 500 lb boat with all that wetted surface then to push a 500 lb block of cement, even on wheels or ice. At the moment I'm left with the mechanical advantage of the oars. But I know of a guy who could row a 36 ft sailboat (albeit slowly with gigantic oars). You can make the case here that the buoyancy of the water reduces the weight of the boat to make it movable. Anyway, it would be interesting to understand what's going on here.

-- Marc J. Epstein (tercon@superlink.net), December 09, 2001.

OK, I'll try and add what I know to this question. Yes, rowing boats are displacement boats. Most rowing boats are very narrow for their length (High length/beam ratio). As this ratio increases, it makes the boat easier to move in two ways: it decreases wetted surface area and increases waterline length. Rowing boats, particularly the faster rec and racing shells are a type of hull form known as a "destroyer hull" - long and thin like a Navy destroyer. These hulls can achieve hull speed with very little power and with sufficient power can travel faster than their calculated hull speed (hull speed is NOT a hard and fast number - it is a useful estimating tool only). The biggest factor in determining how much power it will take to move a displacement hull is its frictional and residual (wave- making) resistance.

As far as being able to move a heavy boat with relative ease, yes the tremendous leverage of the oars is a big factor. it is actually pretty easy to move a large object on a low friciton surface AS LONG AS you have sufficient leverage and traction. I can move my 1500 lb sailboat on it's trailer around as long as I'm on pavement and it's level with no problem and I only weigh 120 lb. Also, you get a big help from the run or glide of the boat, and you're pretty much always on a flat surface.

If you want to learn more about what makes boats tick, go to the library and try and find an old copy of Skene's Elements of Yacht Design or the Nature of Boats by Dave Gerr (Should be easy to find)

Hope this helps!

-- Brooke Longval (newfound77951@yahoo.com), February 07, 2002.

The resistance of a hull in displacement mode is a function of the square of the speed at which it is moving. The force required to move even a massive boat, very slowly, is finite. Theoretically, on a completely calm day, with no other motion, a child could push a super tanker away from the pier by leaning against it for long enough.

Doug

-- Doug Kidder (dgkidder@pacbell.net), March 04, 2002.

You are overestimating the difficulty of pushing a 500 pound object.

With wheels, on a hard smooth dead level floor, moving such an object is not at all difficult. Warehouse workers do it all the time with pallet jacks.

Water is dead level (no hills to climb, no significant bumps - though waves can be a factor), and smooth and slippery. Pushing an object on land IS a good analogy - if you have wheels and a smooth level surface.

gbb

-- George Butler (gbb132000@yahoo.com), December 31, 2002.