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Kayak and Canoe Design Bulletin Board
Re: Dynamic versus static weight
Posted By: Nick Schade In Response To: Re: Dynamic versus static weight (Nolan)
Date: Friday, 27 August 1999, at 10:52 a.m.
> Oh Nick! How could you step right in front of the force equation?! F=MA.
I am aware of the equation and taking it into account.
> The kinetic energy is the energy of the moving body. The energy required
> to obtain a change in velocity (direction included) is force.Maybe I didn't say it right but a boat in motion does not require more energy added to the system to stay in motion. An object in an ideal frictionless environment does not require a constant input of energy to stay moving. Any energy added to a constant velocity system is to overcome drag. BTW, the energy required to change velocity is not just Force (F). More properly it is work (F*Distance).
> This is why even in a nice vacuum, you cannot shove a sitting locomotive
> down the track, or stop one that is moving. Your ability to impart
> significant amounts of energy are negligible compared to the energy of the
> locomotive. Though you can cause changes to a light weight hand car
> sitting on the same tracks in the same vacuum.I can move a one ton sailboat with one hand without even breaking a sweat. Given a frictionless locomotive in a vacuum a mouse capable of creating 0.05 ounces of force could eventually get the locomotive up the speed the mouse is capable of running. F=ma does not put any limits on F, m or a. However, there is a lot of drag in a locomotive so I doubt I could make one move, but I have seen pictures of men pulling locomotives with their teeth.
> I certainly will not argue about the increased draft and drag, for those
> are important factors as well. But mass itself cannot be simply ignored.The mass itself is only a concern when there is acceleration (f=ma). This at the beginning and the end of your trip. The longer the trip, the less difference the mass makes.
> I would also agree with you that for the average paddler under average
> conditions that minute amounts of weight changes will not measurably
> affect speed, especially in light of the many other factors that will.
> But, for a paddler who is going to be paddling all day, weight very
> definately has a direct bearing on how well that paddler will be doing
> come dinner time. Will one pound all by itself be a phenominal effect? No,
> but 20 lbs may. And 150 lbs certainly will.I do not think that additional weight makes no difference, but it makes much less difference than you think. Imagine an ideal frictionless system with two objects moving at the same speed, one of mass m, the of mass 2m. Which requires more energy to stay in motion? Neither, they will both keep moving until some force acts upon them. Lets say there is a frictional force (Ff) proportional to mass (Ff = 1.5*m). Now obviously the work required to keep both objects moving with be proportional to the mass because the frictional force is proportional to mass. But what happens if the drag is proportional to some other factor?
Lets say the drag is proportional to the area of the bottom of the object. If the objects are cubes made of the same material, the mass=L^3 where L is the length of one side so surface area of one face (S) = L^2. So the surface area expressed in terms of mass is S=m^(2/3).
In this case a doubling of the mass increases the drag by a factor of 1.59. In other words the drag does not increase as fast as the mass (1.59 vs. 2). The case of a boat is analagous. In a racing situation we are generally talking a smaller differences. Using my example above, a 10% change in mass produces a 7% change in drag.
In real life on a boat, the drag is a more complicated function but it still does not increase proportionally to mass.
In a boat drag is the primary concern. A heavier boat will be harder to accelerate in proportion to the increase in weight, but once it is moving, the increase in weight is of concern due to the increased drag.
> In the strictest of math modeling, the carry factor doesn't matter. Only
> the lifting of it does, and that is negated as soon as one lowers the
> boat. You know the drill about work. And in real world stuff, the better
> balanced the load, the easier it is to carry, hence the price for really
> good back packs. It's also why U-haul went with that low frame truck.> But when it comes to moving a body in a horizontal direction, it does not
> matter who is floating in whom, for all the mass must be propelled. A 50
> lb person in a 100 lb boat must expell the same energy as a 100 lb person
> in a 50 lb boat if both are moving at the same velocity. All other factors
> being equal or removed.> That I certainly disagree with. For this greatly has to do then with a
> persons muscle structure, experience and loading, and therefore becomes
> highly subjective. Myself as an example, I am far more experienced with
> hiking and carrying heavy loads then I am with propelling myself from the
> shoulders. To cover a given distance, it would be much easier for me to do
> so upon my feet, using my legs for propulsion, carrying a balanced load
> that is properly distributed. Other peoples milage will vary of course.Try lifting a boat and standing there without moving, see how long it takes before you get tired. Now sit in a boat on the water and see how long it takes to get tired. Add weight to each case. People must do work just to support something, water does not. If you lock your knees you can hold the weight longer, but people don't usually carry things with their knees locked.
Also remember that hiking you are going up and down in elevation. Kayaking you generally stick to sea level.
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