Aluminum Mast Tapering

[Brian Duff]

hey riggas

so i have never cut a mast taper on an aluminum mast before, nor has my welder. the day always comes to expand horizons....

In order to learn the process (and to get a faster spar for my little wooden penguin) I have bought a DM-1 section and my welder and I are going to learn the process. I have spoken with a couple riggers I worked with in the past, and have been instructed to simply cut a straight sided wedge out of the front side of the mast, squeeze it shut, clamp something that will stay straight to the back edge and weld the thing up.

Anyone here have any advice?

while I am eager to learn the process , i would rather not trash a $200 extrusion through some stupid mistake.

thanks fellaz

[Douglas]

Ahoy Brian , three things come to my mind about welding long joints.

One is trying to prevent ripples caused by heat distortion, maybe also called puckering, on long linear welds.

Two, is that the welding heat causes a loss of temper near the weld.

and Three, is later corrosion that seems to start under paint around previously welded areas.

Please DO let us all know, how you decide to go about it .

Enquireing minds want to know.

Douglas

[Brian Duff]

Thanks Doug. We know about all that. We have done PLENTY of metal work. built spars, car bodies, signs , railing, boat fittings, underwater turbine intake grates,and on and on and on.

What i am after is details of the shape and placement of the cuts made to create the taper. well i can see from spars I have worked on where to do the cutting, but am not sure if the cuts should be straight sided or camberd, and weather I should do one on centerline or two cuts (one on each side). I guess cause its such a small spar one on front will be enough. but I am tapering alot of section away....


still searching...

[Brian Duff]

Penguin DM-1 Aluminum Mast Taper : I suppose I am cutting a straight sided V from the mast, centered on the front of the spar, to create this taper. The width of centerline will be 1-3/8". This taper is going to run 8' down to end in a point, which will have a 1/8" radius...then I suppose I will bend the sides in (almost 25*!) right at the aft mast 'wall' to close this V... and weld it? Ta-da

Hope this is right- speak up if you can, and will!

I am off to Florida to pull the rigs from a little ketch, so circular saw day is probably not till Thursday...of course that's also the first day of Boat Show

Too many fun things to do.

Thanks guys

[Brion Toss]

Hi there,
As I understand it, the taper is cut out of the side of the spar, not the front, so you cut a wedge out of each side. That way you won't be messing with two planes, or distorting the front's radius. And of course the aft face of the wedge is vertical, so only the forward portion of the mast bends back.
Fair leads,
Brion Toss

[Douglas]

Oh , very nice reply Brion , Heaps of intuative, information and insight there,,,, say what do you think about having a radiased curve at the apex or bottom of the vee cuts,,,,, on port and stb ?

This radiused curve looks like the bottom loop of a "J" !


Douglas

[Brian Duff]

Brion,

Thanks for the pointers. I suppose my agreement ( with the couple guys I talked to locally) in thinking that removing the material at the front is that the spar ends up betting more narrow in both planes, so that as gusts hit the spar can tip off to leeward as well as aft, to open the leach and spill air out the top of the sail. The effect desired is much that which Windsurfers use to control their sails in heavy air. These boats are quite sensitive to weight and gusts, so a soft rig can make them much easier to handle (so 'they' say)

Riggers,

That said, I can see how much more difficult it will be to keep the spar straight as I try to bend inwards and aft. Ummmm...

Wish some folks from Proctor hung out on this site. I have been talking with Lawton, but his interest is more in selling a spar, not telling me how to build one. Plus he is just in sales not spar building.....

I guess I need to get down to a few local dingy clubs and carefully look over the taperd spars I can find, particularly the Penguin spars. Umm....

I have noticed that Charleston spars uses that 'J' bottom to thier tapers, which are cut on the side of the spar near the aft quarter, and the J runs to the flat forward face at the bottom of the taper...

Eager to hear from more of you Riggers...

[Joe Henderson]

Dear Brian,

The side-cut tapers you are discussing are generally used on yacht size sections - 4 and 5 mm wall thickness typically.

For a dinghy-size mast with thin walls, the front- cut is the way to go.

Do not just cut a straight wedge there needs to be a bit of either convex or "S" curve in it to obviate a "garfish" look to the finished taper.

Some radically tapered one tonner masts in the eighties, had four cut tapers with very intricate curves and allowances for distortion. Even then the hydraulic press came in for some work to ensure a sraight topmast after everthing was welded-in.

I am not surprised that you can get no free specific information!

Mast builders will have already used up and scrapped several pieces of section to establish the right number and configuration of cuts for each profile ands they guard these data jealously.

I inherited a whole sheaf of section and vessel specific taper cut profiles when I bought a mast makers archive and, let me tell you, there is nothing straight about the cut lines produced by the tables of offsets!

Do not expect to get yours right first time. You may get away with stitch-welding the taper say 20 mm weld then 10 mm space then 20 mm weld again. leave the spaces they will not harm the bend characteristics.

I have seen dinghy tapers with riveted straps holding the taper closed and they worked ok!

This may be what you have to do after you have cut your existing section up in disgust and thrown it in the bin.

Good luck and regards,

Joe Henderson.

[Brian Duff]

ok joe, thanks. I got some extra section and will start trying. I understand (I think) how a straight cut can make a negative curve on the front side of the taper. Maybe I will draw out each 1' of the taper to loft up what curve I want...

Oy, nothing on boats is easy.

[Joe Henderson]

Dear Brian,

Thinking more on your tapering problem, I seem to remember that the extrusion needs pressing athwartships before cutting the taper.

We used to use a set of padded steel "crocodile jaws" that were set in the press. These took the shape of a pair of adjustable, pivoted arms jointed at one end and set the correct distance apart to take the section arranged athwartships with the hinge pin just below the start of the taper and the other ends free to close-up under the influence of the press.

After gentle pressing, with lots of checks to see that you are not distorting the luff groove too much, you end up with a section that is tapered athwartships and flared out fore and aft.

With care, and if the section is stable, it is possible to reduce the athwartships dimension at the tip to very close to that of the width of the luff groove reinforcing web. This is how we configured the Flying Dutchman and Flying Fifteen masts at Alspar way back in the mid seventies! We had a real job finding room for the halyard sheave and in the end just formed a horn on the top not unlike a longbow's tip that a loop in the head of the sail just hooked over.

It is easier to judge the size and form of the wedge to be cut out of the front of the section, and a LOT easier to close the two sides in, maybe with the press again.

After cutting, but before welding, is the time to straighten out any curve or fore and aft bend in the back of the section.

Good luck again,

Regards,

Joe Henderson.