Monday, February 1, 2016

Mast Hinge Idea

I have been kicking this around for quite some time but have been reluctant to broach it the with boat-building community because I have never seen it done before with a sailing mast.  It is essentially a means of joining two pipes or tubes with  flanges and radially spaced bolts.

It's done with aircraft - in some examples to hold on a cantilevered wing - and with high pressure piping used in oil and gas .  It  makes complete sense from a structural standpoint since the lines of stress - in tension as well as compression - remain in roughly a straight line through the joint, unlike a tab and fork style hinge where the stress lines must make abrupt right angle turns through the joint and in greater force since they wind up well inside the radius of the structure.  In these examples where I have seen such a hinge used on an free standing mast the tab/fork hinge weakness is taken out of the equation by a sleeve that slides over the whole shebang.

aluminum pipe shown shiny - steel shown red

In my fantasized hinge the aluminum schedule 40 6 inch pipe slides over a steel mandrel that forms the upper part of the hinge assembly.  If the fit is tight, and both the aluminum and steel are properly etched, epoxy primed and fit together with pro-seal there should not be any corrosion problems because of the dis-similar metals.  Steel and aluminum are mated in this manner in aircraft all the time.

The lower part of the hinge assembly is made of steel as well, and continues on through down to the keel (or my equivalent).  Eight 1/2" high strength bolts hold the flanges together and a 1/2" hinge pin allows the hinge to open with the bolts removed.  A bracket could be included to accommodate a gin pole as well.

I selected steel for the hinge structure because it can be easily fabricated and welded up with steel plate and pipe or tubing.  In order to use aluminum the upper hinge mandrel portion would have to be machined from a solid billet at great expense.  The reason being  is that welded aluminum joints are weaker than the aluminum itself and that would preclude making the piece up with aluminum plate and tubing.  It would have to be machined from one piece.  You will never see welded aluminum used structurally in an aircraft with conventional and traditional construction.

So I picked steel for these parts because steel, unlike aluminum welds up as strong as the host material.

I can't see why this isn't viable...

Meanwhile, framing up Autarkia continues...

I'll clean her up with a belt sander before planking on the plywood.

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