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post #457 of Old 08-08-2013
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Re: Pros and cons of steel sailboats

Originally Posted by mark2gmtrans View Post
Okay, let's address the math here....

First we need know that to figure the force of the impact, we are going to have to calculate the Impulse of Force, which is the force of the impact in PSI in this case over the period (length of time) of the impact. In order to know the force we have to know the weight, the speed and the size of the impact point, and to get the Impulse on that we need to know the duration.

From there it is a simple little calculus problem, and we can get it all done in just a few minutes. So, we figure that the length of one of Brent's keels is about 60 inches long by 6 inches wide at the point of contact. ( I tried to look up some of Brent's designs on like I do with Bob's or Roger Long's but I found this photo, and I am just kind of guessing on the actual length, at the end of the story it won't matter.

So we have a weight (per Brent) of 20,000 lbs
A hull impact area of 360 square inches maximum.
We will designate a forward hull speed of 10 knots which is 26.465 per sec
A downward force of 954417 N on the keel, and that is not a full calculation, but there are too few here who would understand the math to make it worth the effort.

So that translates to 214561.47 PSI

Score Fukushima Debris Field 1 BS hull Zero

In other words a full on 10 knot collision would poke a hole in your boat, and that force is the force that would occur along the entire 360 square inches, angle it slightly and it goes way up. I know my math is dirty on this because the formula I used was not the full formula, which gets to be fairly complex because I would have to estimate too many of the numbers. I used

Impact Force(F): 2 m vt

Which is really too simple. The actual formula for calculating keel impact force is longer and more complicated and I would have to calculate too much stuff after a very long day at work.

However for those interested the whole formula is here....

Guide Specifications and Commentary for Vessel Collision Design of Highway ... - Aashto - Google Books
It seems to me you are speaking apples and oranges. Brent is simply giving a calculation that is perfectly correct. His statement is only to give the impression that the keel is strong. And his numbers are correct.

On the other hand, you are providing a trivial collision calculation that is quite far from reality and does not reflect what sailboats do. For instance, my boat is 20,000 lbs. Real-life, I hit a sandbar at about 8 knots, stopped the boat dead. Threw us both head over heels, but did no damage to the keel or the hull.. not even a crack in the keel to hull joint. Your calculations indicate our boat was holed!!!

You are failing to include the dynamic effects of deformation of the boat or the item collided with. Inertial effects are poorly represented.
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