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A-frame mast construction for the KD 122 Italy two

 

Hi Bernd as I told you I started building a section of the A-frame mast.
As for some time and in my head I decided to do it in an oval shape, to abandon the idea of PVC pipes (too much fiberglass and sanding work on the pipes) but all in plywood and Douglas fir strips.
13.20m long it will be built entirely in a single block also with the internal crosspieces which will always be made of an oval shape but a little smaller 10cm x 25cm and afterwards it will be all bundled. The base is 43cm x 16cm and 8cm x 25cm at the top all in the shape of a pyramid.
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I care a lot about this discussion and I would like the whole forum to participate in the discussion to improve and make it strong on all aspects.
 

Domenico

Can you make a sketch to understand things better?
The oval shape is the way to go.?

Byer
 

Yes Bernd I have always thought it since the problem is the lateral thrust (from bow to stern) while the internal part is supported by the internal crosspieces and 4mm steel cables.
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The oval shape is more beautiful to look at as well as functional but I thought not to make it the same from the base to the top but pyramid-shaped, 45cm at the base and 25cm at the top.
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I always made the internal beam of the mast in a pyramid shape 15cm at the base and 8cm at the top.
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The two Douglas fir strips are 7.5cm at the base and 4cm at the top, the latter are then tapered 6cm at the base and 2.5cm at the top.
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I used the floor to tie the central thread, after the two side rows with the measurements and in the end I drew all the pieces
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Once the pieces have been cut, you need a straight bench 60cm x 13m long, this is very important.
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I made a sketch we hope you understand ....

A-frame.jpg
section A- frame.jpg

 

Just a question: why build an oval section instead of circular section, in general the wind will come from 30¡ã-135¡ã degrees?
 

Qiuseppe: One Builds it oval as wind resistance has a bigger impact on course and speed the more forward the wind is. i.e - when you're beating to windward the higher the wind resistance the less you can point and the slower your speed to windward will be, whilst when reaching or running wind resistance hardly matters or can even increase your speed the more aft the wind is..
 

The other reason for the oval mast instead of round is that the secondary structure of the A frame provides support from side to side but no support from front to back (bow to stern). So the masts need to be very strong fore and aft and not so strong side to side. So for the same weight, an oval provides more strength where it is needed.?

Patrick
 

OK Ivan, but in my experience the wind comes form the direction that winglet shows, so not on 0¡ã but from at least roughly 35¡ã-40¡ã to 140¡ã, so the circular section is the more conservative option, where is my error?
 

The reason to use an oval in this direction is, to dissipate the longitudinal load.

Bernd
 

Hi Giuseppe, do you think I didn't think about building it circular?
Do the internal crosspieces of the mast, always oval, serve no purpose in your opinion? Do you know that there are also cables in the middle, between the first crossbar and the second?
To make it circular, not only does it come heavy (having experienced the weight of the 6m x 180mm tubes of the Duo900) but you have to create a rhombus with four stainless steel cables from the base to the top, in the center at 6.50m a cross of 120cm to prevent it from bending . So between strllo forward, central cable, cables in the middle of the central crosspieces and more the rhombuses on the two profiles, it not only becomes heavy but a crap to look at it aesthetically. In my opinion, better central tree like Patrick does.
The oval structure (just see the photo on the balance) which does not flex from bow to stern, does not need reinforcement. In addition to the inner central cable (which starts from the base of the deck to the top) I added two dyneema cables from the top to the stern of the two hulls. These two cables (in my view) will help the upper part with carrying sails such as gennaker code 0 and avoid hauling the central cable where there is the mainsail.
However this section that I built weighs under 60kg and looking at the photo on the scale where all the 60kg force on the scale and the only way, the mast without the various carbon and glass fibers did not bend. Imagine it circulating at 13m how many reinforcements you need to put in to make it stiff.
Merry Christmas
 

On Thu, Dec 23, 2021 at 06:29 PM, Bernd Kohler wrote:
The reason to use an oval in this direction is, to dissipate the longitudinal load.

This was my concern Bernd and the choice to make it oval but pyramidal upwards, makes it even stronger.
Merry Christmas
 

Yes Patrick you have faced the problem first of all and to make it rigid it is too heavy to circulate. Am I right?
Merry Christmas ;-)
 

OK, thanks, that's the answer I looked for; it should be interesting to calculate It in carbon circular section.

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