Deck Guns (2)

[netjaws]

Woooowww,

I started a thread like three years ago, asking about deck guns and their heeling effect on ships. Well, we never did answer the question - the interest sort of faded-

And then today, COMPLETELY at random, I come across this in my 1916 edition of Attwood. Ha! Ha-ha! Look at that!

The original thread: http://www.boatdesign.net/forums/boat-design/deck-guns-13649-1.html

[kach22i]

Looks like you had some help last time.

Hey those guys were pretty smart back in 1916.

[daiquiri]

It completely neglects the elasticity of ship's structure. That calculus considers the ship as a rigid body, which it is not.

[Ike]

How cool is that ? I'll have to look through some of my old Naval Architecture books.

[marshmat]

It completely neglects the elasticity of ship's structure. That calculus considers the ship as a rigid body, which it is not.
True. But given that warships tend to be of fairly hefty steel construction, would it be reasonable to surmise that vibrational modes of the ship structure itself are both small in amplitude and sufficiently high frequency as to average out to zero over periods comparable to the ship's roll period?

[daiquiri]

True. But given that warships tend to be of fairly hefty steel construction, would it be reasonable to surmise that vibrational modes of the ship structure itself are both small in amplitude and sufficiently high frequency as to average out to zero over periods comparable to the ship's roll period?

You're right, that sounds correct.

[kach22i]

If the answer has been answered, then can I expand the topic a bit?

Aircraft lose air speed when firing fixed forward aimed guns, right? Classic action, reaction (recoil energy) situation.

Question:
How do you calculated the sideways motion and rotation about the C of G of the boat hull?

I would assume you would calculate it as a zero friction body first (like a hovercraft), then calculate the resistance or counter force while the boat hull moves through the water and the superstructure moves through the air (counter force; opposite force).

Is that covered in the book data already posted?

[daiquiri]

Question:
How do you calculated the sideways motion and rotation about the C of G of the boat hull?
I would assume you would calculate it as a zero friction body first (like a hovercraft), then calculate the resistance or counter force while the boat hull moves through the water and the superstructure moves through the air (counter force; opposite force).


I'm affraid that a proper analysis is more complicated than that. You have impulsive forces here and, therefore, an impulsive sideways motion. Which mean that it is a dynamic and non-linear problem, full of vortex generation and pressure waves around the hull.

Traditionaly, methods involving the concept of "added mass" have been used, but I'm not aware of any handy analytical method available for the solution of problems like that one. As far as I know only the CFD can help, though it might as well be just a matter my ignorance.

Non-linear unsteady aerodynamics is a subject of current research efforts in rotorcraft industry, for example.
And, just for an off-topic info, it is also being currently investigated for the possible use with future insect-like spy micro-airplanes. Aerodynamics of tiny insect wings is not a small problem at all. ;)