Easiest way of making attachment hardpoints on a carbon fiber mast?

Not really. I think it is a matter of preference, available equipment, and personal skills. Several solutions will work.

 

Yeah, suppose so As I understand you'd prefer that metal ring solution. Just curious, how would you solve the electrolytic action issue, and delamination due to different thermal expansion, for example when boat is stored for the winter, temperatures dropping to -25°C (-13 F)..?

 

When the temperature is -25, I wouldn't worry about it since the water is frozen. There are many ways to isolate the metal from the carbon. Thickened epoxy would probably be the easiest. However, I don't think there are vertical forces as large as you think. I have built several lug rigged skiffs with wooden masts and halyards to the partners with no problem. My preference is wood because of cost and the time to build one; about one hour with a planer.

 

1. Connect the mast partner to the bottom of the boat.

 

t occurred to me yesterday,while I was reminiscing about something entirely different,that there may be a simple alternative if the mast tube is not capped at both ends yet.I was thinking about a man i used to know who fulfilled the orders for the wooden caps that were fitted to Mirror dinghies when metal masts became an option.He turned them from iroko and they had a slot for a sheave and a lower end that was spigotted to fit inside the tubular section.Which made me think that maybe this mast might be created from a length of uncapped tube and not a home made alternative.This would give access from either end for a simpler solution.If this is the case then I would suggest a Tufnol insert which could be tapped to suit the fasteners used by the cleat and curved to suit tubular section.The technique would be to drill the mast at the required centres and through each of the holes,drop a thin line,take the two lines to the end of the mast,tread them through the holes in the insert and tie them together.Smear epoxy on the bonding surface and carefully tighten the lines to pull the insert into place.Once located tie the external parts of the lines together and roll the mast-when horizontal- until the insert is at the lowest position and hang some weigh on the lines until a cure has taken place.

The technique has worked for me in the past when adding reinforcing pads beneath the double bottom of a Moth.In the instance of a mast tube it might mean creating a curved sanding pad on the end of a batten to prepare the internal surface for bonding.There may be arguments for creating the tapped holes before sliding the insert into place or using an undersized hole and tapping the thread when bonded.These are matters to be decided by the man doing the job.

 

That is a pretty smart way of gluing in an insert. I had an idea of doing something like this, but in my case the mast is capped, in fact, not even hollow. The idea is to 3D print a permanent male mold / low-density core, and then coat it with 3 layers of carbon fiber. Theoretically the core could be removed afterwards, making the mast uncapped, but the weight savings are minimal, and the procedure is too difficult to make it worth it. I did, however, consider leaving a slot in the 3D printed core for such an insert, place it there, and then wrap carbon fiber as normal. Meaning the insert is already inside by the time carbon fiber is added.

However, either for your method or mine, the issue is still the same - to access that insert, you still need to drill holes in the carbon skin, which compromises it, doesn't it?

 

Yes,it does slightly compromise the mas tube,but there is some compensation in that the Tufnol insert bridges the holes you have created and I wouldn't think a couple of small holes,say M5 maximum would be that great a difference.For your proposed printed core,it wouldn't be too difficult to print a recess to receive the insert,the difficulty might be locating it after the carbon is in place.

 

Perhaps, I am not yet sure. I've seen so many warnings all over this and other forums, "never ever drill any holes in carbon spars", that I truly do not know how big of a hole beings to actually matter, especially if there is a backing plate that reinforces the mast in that area.

As for locating the holes, I already have a technique in place for this, since I'll use this build method for the entire boat, and there are many pre-defined strongpoints all over the boat for attaching hardware. The idea is to leave a small pilot hole at the center of each location of a desired hole (in this instance - in the plate), and when carbon skin is vacuumed, the vacuum will cause the skin to bridge and sag over that pilot hole, leaving a visible indent, that can later be easily found and drilled through.

 

The 'in mast' insert idea with pre drilled pilot holes in the mast and block could be made more secure with an outside curved plate to clamp both sides of the tubing, so wider friction clamping, once the threads have been made. I like the string pull through alignment idea - nifty. Do whatever you're comfortable with laukejas. Everything is a learning experience for the home DIY person.

 

I would think the simplest way to dilute the stress raiser effect of a hole ,or holes,would be to apply an additional layer of the same carbon with the fibres orientated at +/- 45 degrees to the axis of the spar and extending perhaps 20mm beyond the footprint of the cleat base.It would be much more feasible than hoping to wrap cloth around the edges of the cleat base and hoping for it to follow the contours of the shape.I hope it is already understood that creating a hollow radius in the base of the cleat to permit a close fit is obligatory.I'm not sure I would enjoy the prospect of laminating the spar,but we all have different levels of tolerance for various processes and mine isn't particularly high where big laminating projects are concerned.

 

That's so true. I tend to overthink these things. But once in a while it pays off and I come up with something good

If I understand this right, you're talking about the backing plate here? Add a recess in the mast core, add the plate, wrap carbon on top, and add a few additional layers of carbon with that 20mm extension over the area where the plate is? And that would probably offset the loss in strength when I drill the holes through? If so, then it is definitely an easier solution than gluing the cleat onto the surface of the mast without any screws.

Yeah, lamination is messy, I don't enjoy it too much either, especially for the entire spar. Infusion would be a lot neater here, but because of the shape of the spar it's not really an appropriate method (would result in a lot of wasted material due to the length of the bag, tubes, etc.). I will probably not even vacuum the thing but use shrink tape instead, and mark the locations of the holes with a yawl before the composite cures so I can drill them later (if I go with that method).

 

If you are set on using fasteners, the easiest way is to through bolt opposing cleats with a compression tube in between.

 

Well, in a lot of places in this boat, I am gluing in short lengths of carbon fiber tubes into the 3D printed core, like for example in the rudder casing, for the rudder blade pivot hole (cross section view, one tube uncut here for clarity):



The idea is the same as a typical epoxy plug, but with more strength.

So perhaps I could do the same on the mast... Run a couple of such tubes through the center, lay more carbon layers around these tubes to compensate for the loss of strength due to holes, and then run bolts through these tubes to attach opposing clamps like you suggested... What do you think?

 

I don't think it will make much difference if the additional ply is inside the mast,half way through the laminate stack or on the outside of the spar.The point is to distribute the loads into the laminate.

 

True. I'm just thinking that if I go with a plate, then it would need to have tapped holes to accept bolts to attach the cleat. And regardless of whether that plate is on the inside or the outside, it would still need to be drilled and tapped after gluing (otherwise it would be really difficult not to get any epoxy into the holes/threads). I suppose it is possible to glue the plate to the outside of the mast and then drill/tap a blind hole with a shortened tapping bit, to avoid drilling into the mast skin, but this is getting too complicated for comfort