infusing with contour balsa

Yes I meant to mold the bend...

So if using a half female mold, whats tye pros and cons of left and right halves, vs front and back halves?

 

My thoughts -
1) If you are to bend the mould you will need a LHS and a RHS ie two moulds
2) if straight we usually make a single mould but put the taper at both ends saving a lot of work
3) I like having the joint transverse vs long. This means the track details can be built properly vs having them on the joint which can lead to trouble
4) I presume its a one off so I'd consider building a mandrel so the whole mast has taper, build the laminate on the outside, pull it off, then fair it. This gives you a one piece mast which will be lighter then a joined mast
5) If joined having the join on the transverse means the extra bits contribute to wall stiffness, if the join is in the long direction the extra bits don't contribute to wall stiffness or prime buckling stiffness
6) Thats about it from a quick brain dump... Peter

Additional - I have worked through a way with a client but he jasn't done it yet. It was decided to build a mandel but the layup would not go completly to the "back". This allows the wrinkles to be burnished out and the release is easy as the part is a "U". So now we have a mast section minus the track bit. The track bit is built seperately and eased into the back and bonded in closing the section out. I think this process has merit, but I'd shot a one piece on a mandrel for a one off myself, the joint has always been the problem area, it adds weight, is the weakest bit, its gets complicated with tracks etc. . cheers

 

Cheers peter,

The mandrel method would be nice, but I worry about fiber crimping doing it that way... As the entire preform pulls down on the mandrel the total diameter must become smaller and therefore fibre waviness must develop. In a compression loaded mast, this can't be a good thing...

Another experienced mast builder advised the front and rear female half method also, so it definitely sounds like the way to go.

I would like to design the mast without spreaders if possible. The preferred stay configuration is a 3 point upper and lower type - upper and lower aft swept shrouds, inner and outer forestay. This gives 3 points at the masthead and 3 points at 50% diving the mast into 6m panels. The problem is that the aft chain plates have a rather acute angle, the longitudinal distance from the mast step is quite small. On the other hand, the forward stays have a very large angle. The mast step was moved aft from standard on this boat...

Was thinking of running the upper and lower shrouds as a single piece of synthetic fiber, attached to th mast head, down through a pulley floating a little above deck level, then back upto the lower cheek on th mast. a pulley block on the deck to adjust the shroud tension. This way, the uppers and lowers automatically balance tension.

How to go about determining required moment of inertia from this mast?

 

Just thinking out loud groper............... sounds like less control of the mast, maybe then you have a 12 meter panel? if you want a single peice you can seize it above a bullseye/deadeye.
Jeff.

 

Hi Groper - hall spars make all their masts/spars on a mandrel. No spreaders no problem, but the idea of having a continuous upper and lower is not good a) the stiffness of the shroud is proportional to its length and you have now made the length about 2x longer and stiffness is what you need b) there is no advantage in having them balanced as they do different jobs c) the idea of having them fixed at the deck is to create two triangles which is an absolutely fixed geometry. If you allow one to slide and stretch I'm afraid you will allow the mast to perhaps behave as a single span (12m) vs two spans (6m) (they are usually called panels) firstly determine the heavy ship weight of the boat multiply this by half its beam to calculate its righting moment at hull lift. Multiply this by a suitable safety factor which if a crusiing boat maybe less then 1.0 for instance as it will never lift a hull? or just use the boat heavy ship weight as the nominal design column load or Rm and use Eulers equation to calculate the required inertia of the mast. Use E=80GPa for the modulus (CF 70%UD 30%DB and Rw=30%) . Then you check this by calculating the possible lift of the sail plan at the design max speed say 25kn or 30kn and check the sails can't blow the boat over. The only thing this doesn't give you is if the section is thin it will locally buckle not globally buckle (eulers first buckle) have to do an FEA to determine if it will locally buckle. If you go to my website gallery there are lots of images showing global and local mast buckles. Cheers Peter S

 

Hi Groper - what is the angle from athwartships to the chainplate? 20degs is conservative less then 10degs not advised. So perhaps it may not be as bad as you think? Peter

 

Here is some rough guidelines on carbon mast layups.

http://www.aes.net.nz/comment19.html

My idea for a one off mast was to lay up a thin flat sheet of double bias. Then wrap it around a few light foam carbon bulkheads and then finish the lay up. With this method a light carbon tube could be run up the centre for halyards thus allowing the rest of the section to be sealed for flotation.
Would probably only be doable for a thickish section.

 

Great topic, I may be interested in doing a mast at some point. A couple of years ago my son and i attended one of Derek Kelsalls 3 day kss workshops where we infused and then shaped one half of a 45ft cat hull as well as built a section of what was to become a 15m wing mast where an airex foam core was used, i will have to go through my notes and talk with Jason to refresh my memory on the details. Peter, you use the half beam from hull centerlines, not overall beam, right? This is going to be a really interesting project to follow Goper. What cat design did you buy?

Steve.

 

Yes Steve,
To hull centres. Be careful with using cores in masts, they lead to some interesting unintended failures. If it is a construction aid on inside only thats fine but a sandwich is correctly used for out of plane bending not in plane loads. Cheers Peter s

 

Hi Steve and Groper - read "Rotating Wing Mast – theoretical discussion" has lots of good stuff on this subject. Peter

 

Sorry haven't had time to reply... Still in this, just need a tad more time to reply in detail...

Peter, just quickly, as it's a catamaran, the athwart ship distance is good, from mast to chain plate is around 3.1 meters. It was the longitudinal distance which was I referring to, only 2.2m aft.

 

OK so arctan 2.2/3.1=35degs which is very stable. Is this what you mean't? cheers Peter s

 

No what I was concerned with is that the forestay angle is 25.85degrees whereas the shroud angle is 16.45degrees @ 35 degree aft. I beleive this means a lot of compression to get forestay tension... not so bad with the main sail roach tension?

 

Seems OK angles to me... Your main leach tension will be more affected by if its a pin top or a square top main. If square or highly roached sheet load will be high & therefore forestay will be high. No matter what the forestay angle (within normal range) to get forestay tension is relative. Do you mean as high as an 18 footer skiff or just normal cruiser type load? The forestay design load is usually about 30-40% the BS of the wire. If you know the probable wire size then that will give you the load. Or if you know the sailmaker and they can give you the sag of the sail I can calculate forestay load. If you are not trying to bend the mast heaps this will reduce the load etc.... Through much to work for you young Groper... Peter

 

Ok, if all the stay angles seem normal then I won't worry anymore about it...

So now onto the compression load... How to define it?

I know it's often done, but I don't like to define the loads by using 100% of the righting moment... unless of course, it's reasonable to expect sailing on 1 hull in a 40ft cruising cat without a super high aspect ratio rig! In this case it's 18000kg/m. But none the less we have to start with something, so be it...

So at mast tip, we have 14.9m height from waterline. So that's, 18000/14.9= 1208kg at the mast tip.

So does compression equate to (shroud length/distance from step) * tip load?
= (13.7/3.8)*1208=4355kg?

Then do ewe take this to eulers as 4355*9.8m^s^s = 42680N

Then transpose to I = (P.L^2)/C.Pi^2.E
= 7.7E-6 m^4

Have I got it right thus far?

Next is to figure the I for a wing section...?