# angle resistance

Discussion in 'All Things Boats & Boating' started by cutting edge, Aug 1, 2017.

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### Mr EfficiencySenior Member

This thread is hurting my head !

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### TANSLSenior Member

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### cutting edgeNew Member

TANSL
My comments have been about approximate values. There is data about the tool-angle of chisels and the vectors of splitting and compressing . Would the general principle apply in similar form to ship's bows being sharp and blunt?

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### TANSLSenior Member

The general principle, physics, applies to all the phenomena of nature. But the bow of a ship acts like a knife trying to make its way, evenly, through a fluid and you are studying the impact (instantaneous effort) produced by one solid object over another. Naturally, the same impact force, depending on the surface of the impacting object, will give rise to a greater or lesser pressure on the object struck. In my opinion they are different phenomena, one governed by the principles of hydrodynamics and another by the "amount of movement (mass multiplied by speed)". I do not know the formulas for this last phenomenon and I do not have more knowledge to give an in depth opinion on the subject. Some will say that if I do not know to shut up, but I have only tried to help. Maybe Gonzo ....?

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### Mr EfficiencySenior Member

The OP has a hobby-horse theory that sword-wielding foreign interlopers from gawd-knows-where, reached the back-blocks of inland Australia, many hundreds of years before the accepted "first encounter". I don't credit it, unless they also had fire-arms....they would have been liquidated by the spear-wielding warriors of the aboriginal race, long before reaching those inhospitable parts. There is no doubt, Australia was settled at the point of a gun, and it encountered fierce resistance. The swordsmen would not have stood a chance, even if they managed to scrape a living from an unfamiliar and generally inhospitable landscape. The war-like peoples that settled New Zealand, never established a beach-head on the long Eastern Australian coast, despite what must have been many land-falls, on a target much bigger than NZ. They'd have been slaughtered to the last man.

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### alan craigSenior Member

I can't help but the picture and story are very interesting; This is my inexpert version:

The poor guy was struck over the left eye which cracked the left side of the left eye socket. Then while on the ground he was was struck on the right eye socket, deep enough for the wound to continue to his cheek bone. Not only did he survive but he got away and lived long enough for the bone injuries to heal. That's amazing by itself but it seems that the inference is that these injuries were made with a sharp wooden implement, sharp enough to have a cutting edge not destroyed by use on apparently hard bone.
How did they make and sharpen the hard wood weapons?

I think the OP might have to get some skulls from the abattoir, make some tools - and send pictures of the results! You might have to include a picture of a boat.

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### Mr EfficiencySenior Member

Spear heads were hardened by fire, and no doubt endless trial and error involved in selection of suitable timber, and working the material. Ditto other weapons probably.

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### TANSLSenior Member

Guayacan is a really dense and hard wood.
Very good point.

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### Mr EfficiencySenior Member

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### cutting edgeNew Member

TANSL
Someone noticed that flint cuts. A sword strike is not just Momentum of instant force , it's cutting and a cut distance at speed takes time . "As you cut material it gets compressed and springs back after the cutting edge passes. A steep side clearance angle gives plenty of room for the material to expand and prevents heating and burning." _ description of circular saw blade. There is an element of shear and of compression. Chisel blades are designed from 15degrees to 60degrees with increasing thickness due to changing force ratios due to angle. This resembles water resistance even if at the crudest factor of less / greater .
In the same way , bones fracture either straight through the cortex or by comminuted depressed fracture . The first is by a sharp blade, the second by a blunt instrument .

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### cutting edgeNew Member

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### TANSLSenior Member

The resistance to advance of an object in the water, if that's what you mean (action of the bow on the water, etc...), has many other variables and components that have nothing to do with what you explain. Thank you, really, for your explanations.

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### gonzoSenior Member

You need to look at completely different equations. Search online under strength of materials and machining. What you are needing is similar to the calculations for cutting any material at a machine shop.

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### cutting edgeNew Member

A bow wave is caused by compression which exists in cutting wood and bone .
The bulk modulus of bone is about 6 times sea water and .09 steel . With marrow being about 40% water the inner trabecular layer is compressible as images show. 1 year old bone has about .12 stiffness of adult bone . Images of fracture by wood ceiling- fans and ice hockey pucks give Newtons / sq cm sliced bone as max arm-speed for cricket / baseball / ice hockey stick is known . Boats have a sharp end and the other is not . Axes are sharp with a blunt part which is not used in wood-chopping competitions.

The Perfect Edge: The Ultimate Guide to Sharpening for Woodworkers
Ron Hock - 2010 - ā€ˇCrafts & Hobbies
... the blade must compress wood fibers before cutting them, before they fail. ...

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### gonzoSenior Member

That analogy is completely wrong. The question about the force, etc. to cut into bone has been solved a long time ago. It has nothing to do with bow waves or any other boat related design. It is a matter of materials engineering. That book is wrong about compression. The action of a cutting tool is through shear stress.

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