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kubark42

4" Carbon Fiber Tube as Boom

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I have the opportunity to get a 4" x 15' CF tube and I'm contemplating replacing my stock F27 boom with it. For really no good reason, other than that it will be a fun project. I can sell it as worthwhile because it will save some weight on the trimaran, but really I just want to have some fun and learn something in the process.

I know that a 4" tube would absolutely ruin the airflow if it were used in a vertical part, e.g. the mast, but does it make a big difference if used for the boom? Since its horizontal it would only marginally increase the wetted area for airflow in a part of the boat which sees relatively low wind velocities. I can't think of any big reason why this would be a bad idea, but maybe I'm missing something?

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I suppose it's not a bad idea if you don't mind it blowing up on you.

A few points:

You haven't given much info other than 4" dia..  The sectional inertia is important, and a round section won't have much in the desired vertical section Ixx .  EI is even more important.

What grade is the carbon and how is it laid up?

Where is the mainsheet (I'm not familar with the F27) - mid boom, end boom?

This is just a small list I put together without too much thought..  I'm sure other posters will come up with a much larger one.

Tough to come up with any conclusion since you've provided such little info.

But if your okay with it possibly blowing up - knock yourself out.

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Fair questions. I guess in the interest of brevity, I didn't go into the full details. My background is in mechanical engineering, and I work in drones/aviation so am used to CF and its failure modes. But making CF sailboat parts is new to me and I want to be alert to the things that one just don't think about when starting something new. I think the unresolved question in my mind is aerodynamics and windage.

Answering your questions for completeness:

  • The CF tube is the good section from a broken research boat. I don't know much about the CF tube's construction, and I haven't measured the wall thickness. But you can look at it and tell right away it's "waaaay too strong". If I were doing racing parts, or even marginally optimized parts, I would go for much smaller. Glowboat, an F24, is very happy with a reinforced windsurfing mast as a boom. And the extra sail area on the F27 is very small in comparison to the extra CF vs windsurfing masts, so the anecdotal experience points toward likelihood of success.
  • How are the coordinate axes defined? Is x forward (aviation) or athwarships? I suspect it's athwartships, but it's good to confirm.
  • The mainsheet on an F27 is at the end of the boom.
  • Totally okay with possibly blowing up. I've still got a jib, right? B)

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Sounds like a good project. I would guess that the boom weighs at least 50lbs so there is certainly some room to make improvements. Airflow for the 4" boom should not be a major concern for you, I would have to look at my boom, but it is probably near 4" tall anyway. I would be more concerned with functionality. You will need to build a new gooseneck, or perhaps make an adapter to fit. You will also need to make a similar adapter for for the end boom fitting. Do you intend to use the out-haul track as per OEM, or will you create your own system? Similarly you will need to plan to re-install the out-haul and associated sheaves. All of this should be pretty easy to work out. The only true concerns that I would have are compression strength which you will need to engineer, and the fact that you will no longer be class legal (which most people dont seem to care about anymore.)  The attached file might help. It is from the F-boat yahoo group I suggest you join as it has a huge wealth of information. https://groups.yahoo.com/neo/groups/F-boat/info

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@Mizzmo, thanks for the input. Since you've got an F-boat, you probably have the same tear-drop shaped boom as I do, which could be that shape because of aerodynamics, or simply because that's the right way to make a groove for a bolt-footed sail. I suspect it's the latter, and I'm happy to see that you agree it's not the former. 

Class legality isn't a particular motivator. PHRF seems to be the rule up here with NEMA, and I prefer to use my boat for weekend destination sails.

Regarding the outhaul, I was very impressed with the simple outhaul system in the RS400. The sail is held to the boom with velcro (for the F27 I might have to up the spec to a dyneema strop). The outhaul line comes out of a sheeve on the end of the boom, goes through the clew, and then goes to a notch on the opposite side of the boom. A button knot keeps the end of the outhaul wedged firmly in the notch, giving a nice 2:1 ratio before starting with the internals. Which is good, because I always wished I had more leverage on my outhaul, I can only ever tighten it when directly head to wind.

I'll build a cascaded purchase system out of aluminum climbing rings and dyneema. This worked incredibly well on my sidestay tensioners, which I increased to 16:1 (I know that friction takes a chunk, but it still works out well) and can now adjust from the cockpit with a simple pull, instead of needing beefy ropes leading up to the winch.

I'll hold the tack to the main in a similar way to the outhaul, with a velcro (or dyneema) strap. I might use an additional velcro strap to hold the tack to the mast as well.

For the gooseneck, I will visit a local boat scrapyard to get some donor parts so that I don't have to take apart my main boom (hey, maybe the next guy after me *does* want the boat to be class legal!). If it works easily, I'll just stick with it. And if it doesn't, there are several 3D printing companies around here where I can go to get the right part made. But I think the gooseneck will be pretty easy.

As you point out, the boom end-fitting is similar. In fact, that one should be even easier, because it'll be enough to lathe up a shouldered UHMWPE and then plate it with a bit of CF for toughness.

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Sorry, I guess the attachment didn't go through. I like your ideas for securing the clew and tack. As for the gooseneck fitting, remember that because the boom has to rotate, it is completely different from anything you will find on other boats, and that the forces that go through there are huge. Same for the boom end fitting, it translates all of the mainsheet force into the boom. I think both are certainly do-able, just be sure you understand the forces involved and properly engineer it. How you engineer for a windy accidental gybe, in 40kts, I have no idea :-)

I pretty much always race under a rating system, but keeping my boat compliant with the F-27F rules means that it is very easy to get a rating when I travel to venues outside of my local area. If you don't comply, then the rating process will be much more of a headache.

Boom Details from F-27Rig Checklist.pdf

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@mowgli, thanks for the dimensions. I'll assume that as long as I meet or exceed, it will be okay for an F-27, even if the weave isn't optimal.

I really liked the detailed pics, http://members.ziggo.nl/arno.molenaar/thrillseeker/fotos/giek14.jpg gives me an idea for how to do the end with a dyneema strop, instead of installing a rotating joint. I think it would be cool if I could just put a dyneema loop around the boom and let it slide as the boom spins. Maybe have it in some kind of built up "groove", so it can't slide forwards or backwards. Or maybe instead of a loop, I could just use a really long soft-shackle.

@tomtriad, I already have an aluminum bowsprit, as well as a handy supply of smaller CF tubes which I think could be (mis)appropriated for a replacement bowsprit. In a similar vein to the boom, not for speed so much as for fun.

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9 minutes ago, kubark42 said:

@mowgli, thanks for the dimensions. I'll assume that as long as I meet or exceed, it will be okay for an F-27, even if the weave isn't optimal.

I really liked the detailed pics, http://members.ziggo.nl/arno.molenaar/thrillseeker/fotos/giek14.jpg gives me an idea for how to do the end with a dyneema strop, instead of installing a rotating joint. I think it would be cool if I could just put a dyneema loop around the boom and let it slide as the boom spins. Maybe have it in some kind of built up "groove", so it can't slide forwards or backwards. Or maybe instead of a loop, I could just use a really long soft-shackle.

@tomtriad, I already have an aluminum bowsprit, as well as a handy supply of smaller CF tubes which I think could be (mis)appropriated for a replacement bowsprit. In a similar vein to the boom, not for speed so much as for fun.

maybe you can make a dubble groove, I made one for my genaker boom from a piece of rope and put some carbon over it

boom10.JPG

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@mowgli, that's a really neat solution. I'd want to find something with more squarish edges in order to properly contain the rope as the mast rolls. A triangular shape which could give me a V-groove would be even better. Short of trying to lasercut laminations out of balsa, I'm not sure how to get a profile made which will slide onto the boom.

P.S. I was onboard my boat today and indeed, the boom is 4" tall. It's a teardrop profile so not as wide, though. It's fair to say that there is very little aerodynamic difference between the two. Jackpot!

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On 22-9-2017 at 1:20 PM, kubark42 said:

@mowgli, that's a really neat solution. I'd want to find something with more squarish edges in order to properly contain the rope as the mast rolls. A triangular shape which could give me a V-groove would be even better. Short of trying to lasercut laminations out of balsa, I'm not sure how to get a profile made which will slide onto the boom.

P.S. I was onboard my boat today and indeed, the boom is 4" tall. It's a teardrop profile so not as wide, though. It's fair to say that there is very little aerodynamic difference between the two. Jackpot!

You can laminated a small tube on the big one and cut it open so that a bold rope fit in it for your lasybag

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I built the boom, gonna give it a go tomorrow.

The clew will be held on by a strap, and the mainsheet and topping lift will also be held on by a strap. Straps were sourced as dyneema climbing slings at REI, as I feel that will do a better job distributing the load than a dyneema rope.

Gooseneck was done by adding G10 to reinforce the CF. An extra-long pin was made by carving up a solid 316 SS rod from a pulpit. (Big shout out to Mass Marine salvage yard in Quincy, MA!).

I still am figuring out the outhaul purchase system, and have to solve how to keep the mainsheet and topping lift from sliding fore and aft. I've got some natural rope which will hopefully shrink enough when wet that it holds on tight in a nice ring, preventing the mainsheet sling from sliding around. And tomorrow is a 5kt downwind run to Boston, so no worries about being able to  pull the outhaul tight enough without a purchase system.

IMG_2699.jpg

IMG_2696.JPG

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Took it sailing yesterday, worked out great! I even hit my head on the boom while loading it onto the car roof rack and I can report it's much softer than the aluminum boom.

One issue is that it's a little more annoying to use the roller-furling part of the boom. I need to polish that part a bit, and figure out how to keep the topping lift from walking forward.

Another issue I noticed is that without the bolt groove on the top the boom is a very slick place to put fingers and hands.  That's a problem when you're trying to stabilize yourself by reaching out to the boom, so I need to do something to make the surface less slippery. Another thought is to wrap the boom in a single helix of fibrous rope, which will give plenty of purchase as well. Lastly, could try painting non-stick. Does anyone have a favorite approach?

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