Quick advice needed - enlarging a long blind hole

[BlueBell]

Since you didn't explain how you would ensure concentricity and axial alignment,I couldn't see what would be usefully achieved other than local enlargement.Simply increasing a dimension is of little use if the result is a banana shaped bore having a diameter that can't actually accept the mast from top to bottom.If a diagram should have been attached,adding it now would be helpful.

It's too simple to explain wetfeet
you've overcomplicated the whole procedure
I think we can just leave it at that
all the best

 

[trip the light fandango]

Was it possible Laukejas?

 

[laukejas]

Hey guys, many apologies for only replying now, because I worked pretty much every spare minute getting that boat ready for the launch and the first regatta. I took your advice and tried my best to enlarge that hole, but unfortunately nothing worked. Like I wrote, I tried the teethed hole saw, but it wandered horribly. So next I tried the fly cutter with guide discs, but trying it out on a spare piece of that pipe, I realized I can't get a straight entry. By the time the cutter is deep enough for the top guide disks to engage, the cutter has already made a mess of the hole. I tried again again and again, but the results were so horrendous that I just couldn't bring myself to do it on the boat.

In the end I decided to bite the bullet and re-build the carbon mast from scratch, this time in the correct diameter. Had to go 48 hours without sleep, but I managed to build it, just in time. I didn't have the time to fair and paint it, so it looked quite horrible, but the epoxy cured just in time (cooking under sunlight for 3 days straight with 50°C surface temp) to get it to the launch in one piece.

Together with my friends, we christened the boat with a bottle of champagne and launched her. She's called The Feather, owing to her very light hull (40kg), which was quite a challenge to achieve for a 4.5m (14.75 ft) LOA.

The boat handles like a dream, and is insanely fast. I had the opportunity to participate in a local regatta just this weekend, in an open class, but all classes started at the same time, so I could measure myself up against Lasers, Laser 2000s, Finns, 470, F18, several all-carbon performance catamarans, several daysailers, etc. - a fleet of 25 boats. I crossed the start line 8th I think, and the race took 20 minutes to complete. Not only did my boat overtake every single one of these 25 boats, but she finished with a 5 minute lead on the next boat (the 470). Alas, this was in quite light winds (6-8 mph), but still, a massive achievement for a DIY boat. Checking the GPS course, sailing upwind on best VMG (I think), she tacks through 80 degrees, which was a big surprise for me, considering that I went with a lug rig again.

The downside is that I totally botched the carbon mast build (both the old (too large) mast and this new one). Both masts are ridiculously bendy, despite using a lot of carbon sleeves. I had assumed that using large diameter carbon sleeves that stretch to a reasonable fiber angle (~35°) and a 2mm wall thickness (for a total mast weight of 5kg) would yield a reasonably stiff spar, but it turns out that my understanding of composites was ridiculously naive. Not only is the mast WAY too bendy, like a wet noodle, but after sailing for an hour or two it stays deformed like a banana.

I've never seen anything like this. I've seen many carbon yield stress graphs, and when the the yield point is approached there are audible cracks and then a sudden catastrophic break. I've never seen carbon get deformed and stay deformed like some metal wire. No cracks, no breaks, no visible damage, just bending into a curve and staying there even when the load is removed. Funnily enough, I can straighten this mast by bending it in the opposite direction (by sitting on it for 10 minutes or so). And I actually had to do that between the races in order to get the rig to behave. Of course I had to race with almost zero downhaul tension, which made the sail into a mess, but I guess the magical hull was still good enough to secure me that sweet 1st place prize.

In any case, it is clear that I was humbled with my naive attempts at making a low-cost carbon spar, and I will obviously have to rebuild it once again, probably with wood, birdsmouth construction again. I said it before in this topic that it is incredibly difficult and ridiculously expensive to get quality lumber right now in my country, bordering on impossible, but I will have to try it anyway over the course of the winter, because clearly I lack the facilities and know-how to make an efficient carbon mast...

Anyway, I digress. What I wanted to say, is that despite me not succeeding in enlarging that hole, I learned a lot from your advice, and I am very grateful for the wealth of info you all supplied over such a short amount of time. Even if indirectly, it helped me get the project finished and out on the water, not to mention the great amount of info you provided on the other topics I created regarding this little project. Thanks to you all, I consider it a huge success, and even with this joke of a mast, I have a very capable boat that I will be able to upgrade many times over

 

[BlueBell]

Good call, I never thought of that option.

There's something wrong with your resin or the cf is contaminated.
What you describe are very odd characteristic for cf.
Was that a guard dog in the photo or did a christening pee follow?

 

[laukejas]

Good call, I never thought of that option.

There's something wrong with your resin or the cf is contaminated.
What you describe are very odd characteristic for cf.
Was that a guard dog in the photo or did a christening pee follow?

After I discovered that my first (too thick) mast was so bendy, I made sure I used a fresh batch of CF and epoxy, checked my scales, checked the mix ratios, 2 minute mixing rule, second cup transfer, etc., to make absolutely sure everything is right. And epoxy did cure rock hard, no tackiness, no anything. I used that stuff to build the entire boat without any issues. Same methods, same materials, same tools, same everything...

Luckily the only liquids that the boat received was a bit of champagne followed by a lot of water

 

[wet feet]

I was wondering how the boat turned out.Before going too far with a new mast project,you might perhaps do some tests with a solid piece of wood of about the size you intend to use and compare it with the prototype carbon mast.If it isn’t significantly better the next option might involve major surgery to take the foredeck off and install a larger tube below the deck.No small job but better to find out in advance than to make yet another mast and then discovering no improvement,and then having to do the surgery to the hull.

For a boat with so many novel concepts to have just a single weak point isn't a bad outcome by any standards.

 

[laukejas]

I was wondering how the boat turned out.Before going too far with a new mast project,you might perhaps do some tests with a solid piece of wood of about the size you intend to use and compare it with the prototype carbon mast.If it isn’t significantly better the next option might involve major surgery to take the foredeck off and install a larger tube below the deck.No small job but better to find out in advance than to make yet another mast and then discovering no improvement,and then having to do the surgery to the hull.

For a boat with so many novel concepts to have just a single weak point isn't a bad outcome by any standards.

I have built several wooden masts before, one square at 60x60mm, and one round birdsmouth at 68mm. They are both WAY stiff than this 78mm carbon mast. So I assume that if I made one from wood again, it would be at least as stiff as these previous two masts, most likely more stiffer. But your point still stands, it never hurts to test before committing.

One interesting option is making a birdsmouth mast and putting a biaxial carbon sleeve over it. I am getting conflicting opinions on this. Composites guys say that this is a dumb idea, because:
1. Carbon is so much stiffer than wood that it will initially take all of the load, but because 1 or 2 sleeves can't handle it, it will break before the wood has any chance to do it's part.
2. And if you add more carbon so it can take all of the load, the wood is now just dead weight.
3. Also biaxial sleeves provide very poor fiber orientation (typically 45°, can get as low as 30° if you get an oversized sleeve and stretch it), whereas carbon needs close to 0° fiber angle to work well, even 10° is already very bad. Of course you can lay unidirectional carbon, but that is more complicated to do in DIY workshop.

However, there are multiple people who still did biaxial carbon sleeves over birdsmouth mast, and claim that it added a lot of stiffness, like this guy:

.

So I am really puzzled on whether adding carbon sleeves on birdsmouth does work, or if it's just an myth...

Of course I could always just go with a 80x2 mm aluminum tube. Cheap and fast. I threw together a FEA comparison between a birdsmouth wooden mast and aluminum tube, and they come out almost identical in strength, similar in stiffness, and equal in weight (both 6kg). For comparison, my banana carbon mast was 5kg, and even that felt a bit heavy for a boat this narrow... Maybe I could shave off 1kg or so if I went with thin-walled birdsmouth + carbon sleeve, IF that works, but like I said, I am really unsure about that, since theoretical explanations of composites folk and anecdotal claims of
mast builders like in that youtube clip are wildly different. Any comments?

 

[Skip Johnson]

The biaxial carbon sleeve on the birdmouth cedar mast on my old P52 proa stiffened up the mast significantly and worked quite well until it snapped in two in Corpus Christi Bay; under stayed and seriously overloaded. King of the lightweight masts was a carbon over birdmouthed foam for a sailing canoe experiment. Most of my masts have been stayed aluminum tubing.

It's true that braided carbon is less "efficient" than unidirectional but OTOH wouldn't require any hoop reinforcement so its not all bad.

I considered going all out with pultruded carbon rods or rectangles over foam with braided sleeve hoop reinforcement at one time for ultimate lightweight but I'm too old and heavy to go down that road now ;-)

 

[wet feet]

My first comment is that you should always do your own research.Too many people offering to solve your problems are either selling something-and they only ever recommend their own range of products-or they don't fully understand how things work in your production situation.

I'm interested to learn that your initial mast was quite heavy and presume that much of it was attributable to the 3D printed core remaining in place,while contributing little to the stiffness.At the very least you will have gained the knowledge of how thick the chosen carbon laminate really is and you should make a note of this for future reference.Include the weight of carbon and the type of weave.

I can understand the doubts about any gain from a biaxial sleeve,as the fibres won't be running in the optimal direction,but I do think it wold be feasible for an amateur to apply unidirectional carbon,perhaps in several stages,around the birdsmouth core.If doing so,I would probably try to add a final wrap of very light woven carbon to resist any tendency for individual fibres to peel.As for mixing materials with different properties,we do it all the time with sandwich construction.

If I needed to go sailing in a few days,as you did,I'd put an alloy tube in and make a heel plug from wood and a head plug with a sheave to see if there was any discernible performance difference.In flat water,it would be much less of a disadvantage than in waves.

 

[laukejas]

It is quite difficult to understand for me why a single layer of biaxial sleeve of carbon would stiffen up a wooden mast significantly, yet 4 layers of such sleeve over the 3D printed core (that's how I made my banana mast) resulted in such flimsy spar. It suggests that the carbon and wood still share the loads, despite what the composites guys said. I would very much like to understand how exactly this all works, if anyone knows...

Additionally, if carbon sleeve does stiffen up a birdsmouth mast, it stands to reason that I could reduce the wall thickness of the birdsmouth mast by some amount and save weight. For example, right now, the octagon birdsmouth mast I designed is 78mm in diameter at the heel, 52mm at the top, with 12.5mm stave thickness, which corresponds to 16% of OD. It is a bit lower than the typically recommended 20%, but I ran the numbers and it should be fine, because the mast is very thick anyway, actually too thick than strictly required. So, let us assume that 12.5mm wall thickness will result in sufficiently strong and stiff mast. But now, if I add biaxial carbon sleeve on top, it will stiffen and strengthen the mast, so the question is, how much I can reduce that wall thickness until the mast is again at the same stiffness and strength as it was before adding the sleeve? Is there any ballpark formula to guesstimate this?

EDIT: out of curiosity, I asked ChatGPT to estimate how much I can reduce the wall thickness if I use carbon sleeve on top of the birdsmouth mast. If you're interested, check it out: Wall thickness reduction https://chatgpt.com/s/t_6890c4cbc6fc8191b494dd8b30423125. Intuitively it seems to me that the AI is way too optimistic with these numbers and doesn't take other modes of failure into account, but I am not knowledgeable enough to tell if the math checks out or not. If anyone could comment, that'd be interesting.

 

[gonzo]

One way to stiffen the mast is with a cable or low stretch rope inside. It will also let you adjust the stiffnes. When the mast bends, the cable touches the side and makes it stiffer. However, it also may cause earlier buckling because of the tension.

 

[Skip Johnson]

It is quite difficult to understand for me why a single layer of biaxial sleeve of carbon would stiffen up a wooden mast significantly, yet 4 layers of such sleeve over the 3D printed core (that's how I made my banana mast) resulted in such flimsy spar. It suggests that the carbon and wood still share the loads, despite what the composites guys said. I would very much like to understand how exactly this all works, if anyone knows...

The wood acts as a shear web much like the web on an I beam. The printed parts shear characteristics would vary wildly depending on type of filament, amount of infill, type of infill and orientation. I suspect it's an area I'll have to investigate if I build another boat.

Now that you've got some time ;-) and good wood is hard to come by you might look at using foam core for your mast. The aircraft and model airplane guys do composite wings with a hotwired XPS core all the time. A couple of years ago I designed a 48' needle of a hull 15" wide that was built carbon fibre over a hotwired foam core. Thinking back to the foam core mast I made several years ago I think I had to cover the care in stages , was way too bendy with just the foam. I laid a single strand of 12k carbon on opposite sides of the mast as it was held straight and then repeated the process at 90 degrees after the first strands cured. The mast blank was then quite stiff but had no strength to speak of but was easy apply the braided carbon for a very still extremely light small mast.

 

[fallguy]

I’d venture the mast was NOT fully cured. Needed an oven..

 

[wet feet]

I'd agree that the cure may not have been perfect,but good luck finding post cure recommendations for an amateur using a general purpose resin.Several hours of exposure to 50 deg C would have done something,but it isn't comparable to following a carefully programmed post cure cycle with enough thermocouples recording the fidelity with which the cycle is followed and it would get expensive for an amateur to pay somebody to do that,even supposing the ideal ramps and dwell times were made available.

Just to raise a point that might be almost heretical,what about staying the mast?

 

[laukejas]

One way to stiffen the mast is with a cable or low stretch rope inside. It will also let you adjust the stiffnes. When the mast bends, the cable touches the side and makes it stiffer. However, it also may cause earlier buckling because of the tension.

That is the weirdest thing I've ever heard lol. I wonder how one would even calculate the strength/stiffness of such a thing. I will definitely try this with a scale model.

The wood acts as a shear web much like the web on an I beam. The printed parts shear characteristics would vary wildly depending on type of filament, amount of infill, type of infill and orientation. I suspect it's an area I'll have to investigate if I build another boat.

Now that you've got some time ;-) and good wood is hard to come by you might look at using foam core for your mast. The aircraft and model airplane guys do composite wings with a hotwired XPS core all the time. A couple of years ago I designed a 48' needle of a hull 15" wide that was built carbon fibre over a hotwired foam core. Thinking back to the foam core mast I made several years ago I think I had to cover the care in stages , was way too bendy with just the foam. I laid a single strand of 12k carbon on opposite sides of the mast as it was held straight and then repeated the process at 90 degrees after the first strands cured. The mast blank was then quite stiff but had no strength to speak of but was easy apply the braided carbon for a very still extremely light small mast.

I think XPS foam would be about as "good" as my 3D printed plastic in terms of being a core, so I assume the carbon would need to be thick enough to carry all of the load. I am pretty sure that using unidirectional, maybe in the way you suggested, or all around the circumference, followed by some 90° hoops would work. However, that is a lot of carbon, and difficult to tell how much exactly I would need to use to achieve desired stiffness. Calculating carbon in FEA is extremely complicated...

I’d venture the mast was NOT fully cured. Needed an oven..

I'd agree that the cure may not have been perfect,but good luck finding post cure recommendations for an amateur using a general purpose resin.Several hours of exposure to 50 deg C would have done something,but it isn't comparable to following a carefully programmed post cure cycle with enough thermocouples recording the fidelity with which the cycle is followed and it would get expensive for an amateur to pay somebody to do that,even supposing the ideal ramps and dwell times were made available.

Just to raise a point that might be almost heretical,what about staying the mast?

I was using EasyComposites IN2 epoxy resin, here is the datasheet: https://media.easycomposites.eu/datasheets/EC-TDS-IN2-Infusion-Resin.pdf. The datasheet clearly states that post-cure in an oven is possible to reduce the cure time, but absolutely not necessary, and the resin will cure to 100% of it's mechanical properties in room temperature in 2 weeks. I didn't wait 2 weeks, but the average ambient temperature was way above 21°C, more like 25-30°C most of the time. Plus the sun. Even if we say that this is not enough, I still have that other oversized mast that has been curing for way more than 2 weeks now - and I checked it after getting back from this launch - and it behaves in EXACTLY the same way (bending and staying bent). So I'm pretty sure the resin cured fully.

As for staying the mast, I could add a forestay, but the hull is way too narrow to add shrouds. Check the first pic of the boat, you'll see what I mean. Maybe with some whisker poles or something, but that would be way more complicated than just building a proper mast...


Anyway, I see 4 options right now:
1. Go with aluminum and bite the bullet about the weight penalty;
2. Build another carbon spar using unidirectional carbon on top of another 3D printed core or foam - probably with some removable steel bar inside to allow me to support the spar on the ends while I lay the uni and the hoops;
3. Build a birdsmouth mast, no reinforcement (same weight penalty as aluminum, so not worth it);
4. Build a birdsmouth mast, and reinforce it with carbon biaxial sleeve (simple and convenient), or unidirectional + hoops (more difficult, but should provide far more stiffness). In both cases, I do need to figure out how much I can reduce the thickness of wood staves to have the same stiffness and strength as option #3. If anyone has any formulas/suggestions on how to estimate this with a mix of wood and carbon in such an arrangement, it would be REALLY helpful...