# Sail Problem

Discussion in 'Hydrodynamics and Aerodynamics' started by Murat124, Oct 20, 2013.

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### Murat124Junior Member

Sailing A sailboat is running along a straight course with the
wind providing a constant forward force of 50 lb. The only other
force acting on the boat is resistance as the boat moves through
the water. The resisting force is numerically equal to five times
the boat’s speed, and the initial velocity is 1 ft sec. What is the
maximum velocity in feet per second of the boat under this wind?

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### PARYacht Designer/Builder

So how long did your instructor give you, to work through this problem and how much will you actually learn, if someone just tells you?

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### Murat124Junior Member

This is not a home work Mr designer, it is a maximization problem as an application of differential equation, if somebody would give attention I would try to develop this problem for control and Automation on sail operation

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### philSweetSenior Member

so what have you got so far?

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### TANSLSenior Member

Murat124, with all due respect because I do not understand what you're getting, why do you think this issue may be of interest to us?. Whatever it serves, it is clear that is a simplification so great that it will be useless. Sorry, this is my opinion and would love you to get me out of my mistake. Thanks.

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### Murat124Junior Member

may be I will be witness of an invention e.g. ?? may be somebody's imagination will be triggered and start on this issue

I have posted this problem here that it was so interesting for me

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### Murat124Junior Member

it is not more simple than your designs. <rude remark towards other member removed> It is necessary to develop yourself, read and think about control theory, aerodynamics and mathematics so as to understand what I am getting.

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### TANSLSenior Member

Murat124, from the first moment I raised my ignorance regarding the issue you are raising. Again I tell you I do not know what it is or what it can serve for and so I ask explanations. What I do know is that a problem in which, apparently, the resistance of the boat only depends on its speed is an oversimplification. So in my opinion, whatever the outcome, not good for anything practical.
Do you propose a mental exercise to demonstrate the intelligence of each one?. Mine certainly does not reach the appropriate level.
I insist on my ignorance and so, I return to ask you to better define the problem (whose approach I do not understand) and, please, to give us the solution.
My ignorance does not allow me to be proud, therefore, with humility, I ask answers. Thanks

P.S. although your problem involved the wind I think I will not need to study aerodynamics but hydrodynamics or kinematics. Do you agree?

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### Murat124Junior Member

Was that the best way to express your <insult removed>? silence a little bit might be better ??

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### Boat Design Net ModeratorModerator

Getting reported posts from this thread -- if this thread is to remain open, let's please try and avoid further personal attacks/insults. Thanks.

11. ### El_GueroPrevious Member

You are missing so much information, you cannot build a simple, much less a complex equation from this.

And why would a differential equation be considered for this?

Where did you come up with your 'assumptions?'

If you want engineers to help you with this project, you need to give better data. At a minimum you need things like:

Weight of boat - displacement

LWL

BWL

Wetted surface area.

Propulsive force

50lb is not propulsive force, you need something like number of knots per hour for the wind speed times the area of the sails - AND THE DIRECTION the force is applied relative to the direction travelled. And if you are being thorough, you need an efficiency of your sails ....

Remove your attitude, and give some real data, and you might get some engineers and architects to teach you some things about your subject.

wayne

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### PetrosSenior Member

This appears to just a problem to help develop problem solving skills.

at terminal speed: 50 lb = 5V

therefore max V = 10 ?

But what sail force is always the same no matter the hull speed? and why would the Resistance always be equal to 5 times speed? Not very realistic.

Beside what about the units? resistance has to be in lbs, as is your driving force, but V is in ft/sec? Ft/min?

Seems there is not enough information, if you start at 1 ft/sec and you end up at 10 ft/sec you would have to create an equation for the acceleration demising to zero when 10 ft/sec is reached. In an equation it would take a very long time to reach 10 ft/sec because the accelerations would drop off more and more the closer you get to 10. You can usually assume you reach your speed when you are within 2 percent of terminal speed (just to cut off the time at a reasonable limit). You need to integrate the acceleration equation.

You can work out the rest.

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### daiquiriEngineering and Design

The mathematical equation to be resolved, with V as the unknown:
5 V = 50

What differential equations do you need to solve that expression?

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### jehardimanSenior Member

Perhaps he should read Zeno's "Achilles and the tortoise" proof.

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