Rotating Wing Mast – theoretical discussion

now im confused again... a paper ive read (cant remember which one right now) showed how much strength was lost when the fibres are not aligned with the load path, just 3 degrees off axis fibres made a considerable difference to the tensile stress it could cope with... And then your saying its better to stack it with a range of -+5 degrees fibres?

I can see where your going with this regarding toughness and ability to cope with off axis loads however... just many of these considerations seem to conflict with one another.

I neglected to clarify my previous comment also, i was referring to an unstayed mast whereby the mast is primarily in bending as opposed to a stayed mast primarily in compression - this is why the analogy wasnt clear. Eric sponbergs comments also refer towards unstayed bending type situations.

How would the laminate spec differ, in general terms, with respect to a stayed mast vs unstayed, in terms of fibre ply placement? - never mind the very different schedule for the completely different load, that goes without saying of course.

 

Well the next thing to do is design Sigurds mast. The gurit paper has a chart with off axis fibre ratios. www.gurit.com/files/documents/Gurit_Guide_to_Composites(1).pdf‎
There is no difference in the laminate for a stayed mast or a free standing. They both need maximum longitudional stiffness/strength for but different reasons. The stayed mast needs maximum compression strength and maximum longitudinal stiffness to prevent buckling. The free rig needs max longitudinal stiffness for rigidity and strength. So the 70%Ud 30%DB is quite good. The fibre I used in the example if the 0deg is 110GPa at 3deg is 103GPa Groper and others -I'm happy to answer a well formed question but it is hard to get into various complex issues in this sort of forum. Just have to take the answer on face value sometimes. Cheers Peter S

 

Hi Rob, what sort of sail will you put on it, it must be enormously narrow?

 

Hi Sigurd - To summaries the mast data - Total length 13m, luff 11m, bury 1m deck to boom 1m, 21m2 sail area, Rm=3T/m. I'll design a CF round mast as a bench mark as follows. Design at 1xRm for 15% deflection. The mast will be tapered 50% ie the top of the mast will be 0.5x the diameter of the mast at about 2m above the boom.

Results - CF laminate E=80GPa deflection at 1xRm = 1554mm or 1554/12000x100=13% very low stressed Dia200x5mm thick weighs 60kg. This does not predict local buckling. If this is what you are thinking of then I can do the buckling analysis. I also could figure out a wing that has the same inertia but has as small a "thickness" as possible. Your luff load is 0.4N/mm which is a bit low for a high performance sail but is close (usually 0.5-0.7N/mm) I figured this from the Rm not your sail area. This could be wound by CST in Sydney for instance, they also can do telescoping tubes if this works. Cheers Peter S

 

Hi Peter, just a quick correction: Total length 12m, not 13.
Here is the rig, I'll see if I can show more detail today.

 

Hi Sigurd - What is the width of your section? I'll adjust the model. What CAD is this done in? Can you send me the section shape? For the next step? cheers Peter S

OK I adjusted the model as follows. Corrected luff to 11m. Removed the taper. Changed the section to 180x4mm, weighs 45kg, Rm=3T/m deflection = 1757mm or 15%, stress = 300Mpa ie will break at about 2 or 3xRm.

Now how are you going to build this? I suggest that you make a cheap timber plug and taper it. Then infuse the CF on the outside, pull the mast off. This gives you a seamless mast section. What are your thoughts on building this? Lucky the AC is a bit late so I squeezed this in.... cheers and what lift coefficient are you aiming at?

 

Have you built masts like that before Peter? When infusing over a male mold, the fibres get crimped as the laminate compresses... How do you deal with that?

Also, demolding sounds difficult with such a shallow draft angle to release from. Perhaps wrapping in plastic film would be enough to solve that tho...I'm not sure... Some of the plastic would no doubt remain permanently stuck inside...

 

Hi Groper - Yes the fibres can get crimped but you have to use light cloth and wrap it hard. Then you pull the bag down slowly and massage the crimps out. If we aim at a 3 or 4mm laminate I think this is doable. If the laminate is much thicker I think it would be a problem. It can also be done in halves. So you make about 200 degs and then you have somewhere to push the crimps out, then you infuse, pull off the peel ply and layup the other half. Create a taper on the overlap. I havn't made a mast on a mandrel but have made various square and round tubes up to 4m long up to 10mm thick. Creases and crimps are a problem. Getting them off has never been a problem, even on parallel mandrels eg aluminium extrusions. Use 0.3mm mylar or shrink tube and good wax. Sometimes have had to use a 2T block and tackle but they do come off. If a good taper was put on this shape I'm sure it would pop quite easily. This is different but when I worked for Goldspar we used to make NS14 masts by dragging a round aluminium tube through a wing mast shaped roller. We did this by using the company truck in the carpark!! Peter

 

I havnt attempted it because of the crimping problem.

If it were possible, then it would be a simple matter to get the section hot wire cut from a block of foam, then infuse over it and not have to worry about demolding anything, the foam stays inside. Conduits can be put inside for the halyards, and same goes for the sail track etc. It simplfies the section shaping, as the section is CNC cut from the block of foam which ensures he gets his exact desired aero section. Last i checked, the local guy would cut this for me for about $300... i was going to do exactly this until i realized the crimping problem so i had to rethink it.

I now think it would be better to get the foam cut into a 2 peice female mold and infuse it in 2 halves, with a matching bonding flange provided on each side to glue them together... no more crimping problem...

 

OK Sigurd - I've been looking at your ovoid slotted mast and concept. I've been involved in many unusual masts and rigs. And all of them have not been successful. The developers had high hopes of revolutionising the rig world, but didn't. A couple of suggestions. 1) If you are really committed to this concept you have a few technical and logistical hurdles to overcome. Get a laser and build a suitable rig and benchmark it against the normal rig. Will tell you lots at pocket money cost verses chequebook size losses. 2) It has worked out that to gain the 10% or 20% extra drive that this sort of thing offers its always been easier to build a 10% bigger conventional sail 3) These slot type arrangments are very fussy and require constant trimming. If you want a boat that is easy to sail then its not the type of rig for anyone. 4) You are not doing an aeroelastic analysis and therefore your results are very biased, they maybe nowhere near what happens in reality. Airflow through a slot only has to flutter or if you are modelling the boundary layer wrong and the results are all wrong. You must consider that real on the water air is turbulent so hopefully you are modlleing at correct reynolds numbers and also the boat is hobby horsing so chopping up the air. 5) Often the solution lies in the rig that is least senstive to all of this type of thing ie a std rig! 6) plus you have the problem of telescoping etc. Seems to me too many compromises to be made. But happy to continue chat. Cheers Peter S

 

Be careful with styrofoam. It can callapse under vacuum. Even if its sealed, if it has a micro hole and the foam is not completely consolidated then it will collapse locally. Get the high density stuff not the normal 17kg/m3 stuff seems to be better. styrofoam is made from beads and cooked in a big steamer. Often the beads don't fully connect so there are voids insode the structure. This allows the vacuum to generate internal negative pressure so it collapses. If you use an envelope bag you can crush the low density EPS totally or in local spots. Peter

Hall Spars make all their masts on aluminium mandrels. They use low weight UD prepreg and vacuum debulking then into an autoclave. Right up to superyacht masts so it is doable.

 

Didnt even consider styrofoam... I had urethane in mind...

if infusing on a make mandrel is possible, id like to read up on it so I can try it. Any advice or links woukd be great. Details about the resin and vac line placement etc. I cannot find anything about it anywhere.

I do know of a meth8d to infuse a laminate inside a full female split mold in one shot however.... havnt tried it yet but im dead certain it will work...

 

Theres nothing around in writing just consider it as a flat object rolled up. But there is a good trick. You need to fill long objects from the centre out. So I use 13mm irrigation pipe into the middle, this turns into spiral wrap then back to 13mm solid. Have good vac at both ends connected to laminate via a brake, Say make the spiral about 1/3 the length. Have resin feed in from both ends. Otherwise the rest is same as if it was flat. Urethane won't cut with a hot wire??

Same as for inside female. Figure out the overlap, stack one side with taper off. The other side the mould has to have an extension to support the overlap. Use spray tack to hold it all togther. make sure the corners are well debulked/burnised. When both moulds are stacked, remove extensions (I call them wing moulds) then turm it over or onto side and slide them together. Get the bag through the middle pronto and then get vac on. Once bag is down its all the same as a flat panel. Pull down to required pressure and turn on the resin taps. I've made 100x100x6 square tubes like this 3m long. I've also done 350x250x10 x2m long rectangular tubes. Make overlaps as long as possible and they will be fine. The 100x100x6 tubes were tested to bending destruction in both directions and we couldn't pick the difference. Cheers Peter S

 

Hi Peter, I am not sure of the thickness, in the drawing it is 150mm bottom and 180mm top, but that's a bit arbitrary. With 3/1 chord thickness it is about 3.5m2 bare and collapsed. A vane could be put on the boom in this case.

Regarding the fluid modelling, I take it with a good pinch of salt. The software I have that can do multiple elements is Martin Hepperle's Javafoil, and he's told me max CL results can be wrong. I have compared it a bit to xfoil, and they don't agree very much always.
But it is nevertheless fun to analyze various configurations.

But I am more concerned about being able to pull down the sail without rotating the mast square onto the wind.

It would definitely be cool to build a small one.

About building the bottom section. I can build carbon planks in a mold. These can be glued together over bulkheads and get +-45' around them. The planks would be bent and twisted to conform between the bearing round and the flanges of the spar. Then the skin is made from mostly +-45 and a little 0, bagged flat then bent over bulkheads.

CAD is Rhino 3d.

 

I didnt get this at all peter, can you have another go at explaining it? You say fill long objects from center out, but then say feed resin from both ends, you also say have good vac at both ends? Did not get it at all im sorry...

local guy up here does it, like i said i had a quote for him to do it, 2.4m sections id have to glue together for the length of the mast...

How do you get the infusion consumables out after its cured? Or do you leave all the resin feeds, flow media and plastic inside the finished mast...?

Also, there is reported difficulty wetting out heavy CF laminates via infusion and locking out some fibres etc, how do you deal with that?