Now in terms of the custom wings we built, here were the key parameters of our design:
- Seat style: we went broadly with a Hobie Comfort seat style
- Beam/hull supports: we went with M32-style supports, as revintage pointed out. These are similar to Hobie 18SX wing supports in terms of overall load distribution (a compression/downwards load on the beam end + a tension/upwards load 6-12'' inwards from the beam end). The M32 style seemed easier to make, and sturdier overall vs. the slim support strut tabs and attachments on the Hobie 18SX wings (though admittedly the later are battle-proven). A critical consideration in favor of both styles was the ability to swing the wings up (at dock or if beating upwind in heavy airs where we wouldn't want the leeward wing dragging).
- Mast wire support: to distribute loads further, we reused old trapeze wires to support the wings from the top in two places (near the front beam, and and about 2/3 between the front beam and the aft beam). We tried to distribute the loads about 50:50 between the beam/hull supports and mast wire support. Perhaps we should have put more load on the mast, because we still broke one of the rear beam connection points... (but were able to jury rig it and continue using the wing, though more gingerly)
- Fore-aft support: we had none. That is not uncommon (see M32 wings, and early H18SX wings for example), and felt ok for our use case (one time race). But I would agree that for long duration uses/cycles it would help to support the wings fore-aft. Hobie later added this to the H18SX with a small strut going from about 6''-12'' behind the rear beam and then going forward to the primary wing support strut (see picture here)
- Primary support struts: these are the large posts that go from the beam to the wing itself, and incur most of the loads. The main decision here is whether to go straight (as we did, also mostly on the M32) or with a curved beam (as on Hobies). A curved beam enables a fully-flat seat, while the straight struts mean the seat is always inclined (and more so if you're lifting a hull). While flat may be best for beer can sailing, the inclined seat felts very safe (little change of you falling backwards out of the seat!).
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SL
Nacra Inter 20 (sold)
2017 Race to Alaska "Team Ketch me if u can"
- Facebook: https://www.facebook.com/TeamKetch/
- Race video highlights: https://www.youtube.com/watch?v=YTWp4DP0VcA
Sausalito CA
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Hiking Rack / Wing for Nacra Inter 20
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- Rank: Lubber
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Posted: Nov 15, 2017 - 04:23 AM
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- Rank: Lubber
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Posted: Nov 15, 2017 - 05:47 AM
Finally, I'll go through a few of the materials decisions we made. Probably more to detail I can share later if people are interested, since the devil is in the details (as often). Keep in mind these wings were not battle tested over time, so better material choices could probably be made with more testing.
You can see more pictures of our wings on our FB photos album here, and in this YouTube video here.
- Main wing outer tubes and primary support struts: aluminum 6061 T6, 1.75'' OD, 1.50'' ID (0.125'' wall thickness), weight 7.5lbs/10ft, estimated section modulus 0.242 inches^3 (higher is better). That was what we could get from a local metal wholesaler.
We used 2x 12ft for the outer tubes, and 4x 3ft (approx) for the primary support struts.
Note: Hobie wings are same alloy, but 1.875'' OD, 1.71'' ID (0.083'' wall thickness) (according to this thread), weight 5.5lbs/10ft, estimated section modulus 0.200 inches^3. So ours were heavier but sturdier than Hobie wings, which felt reasonable given the I20 beam to beam distance is longer so the wing tubes need to carry more load (8.6ft vs. 7.1ft on a Hobie 18).
- Wing inner tube (inboard seat tube): a bit smaller/thiner than outer tube to save on weight, 1.50'' OD, 1.37'' ID (0.065'' wall thickness), weight 3.5lbs/10ft, estimated section modulus 0.101 inches^3. This doesn't take a lot of point loads (the netting distributes the loads nicely, and most of our weight was on the outer tubes anyways), so maybe could have even been downsized further. In contrast, Hobie 18SX wings have the same size for inner/outer tubes.
We used 2x 10ft for the inner tubes, cut down to size (approx 8.5ft each).
- Wing fittings: we used off-the-shelf "Hollaender" (aka SpeedRail) aluminum railing fittings wherever possible.
Specifically:
*4x model 41-8 flanges (here) for connecting each primary support strut to its beam. The flange swivels, allowing the wing to be raised.
*4x model 5Ext-8 tees (here) for connecting each primary support strut to the main wing outer tube. In hindsight, we could have tried the similar but lighter weight 5-8 or 5E-8 tee models (we chose the EXT model because it would better distribute the point loads at the end of the support strut)
*4x model 17-8 adjustable tees (here) for connecting each secondary support strut to the primary support strut
*4x model 5-7 tees (here) for connecting each wing inner tube end to the primary support strut
In hindsight, we could have gone with -7 models (Schedule 40 1.25'' pipe, i.e 1.72'' OD) rather than -8 models (Sched 40 1.5'' pipe, i.e. 1.94'' OD), to save on weight. With the -08 models we had to custom machine some metal rings to bridge the gap between our 1.75'' OD tubes and the 1.94'' fitting sizes, whereas with the -07 models we just needed to shave off a bit from the fittings in the workshop to get the 1.75'' OD tube to fit the 1.72'' fitting sizes.
- Custom beam plates: to attach the primary support strut to each beam, we custom-made and riveted 1x flat metal plates into the beams about 5-6'' from the beam end to distribute the loads, and then screwed in the 41-8 flange into each plate. This would allow us to remove the wings, while keeping the riveted piece in place. Because the rear beam's main track creates a protuberance, we had to work around that (see here). The rivets on one of the rear beam plates did let go, so we clearly didn't get everything right (it seems one of the secondary support strut slipped, transferring all the load onto the primary).
- Secondary support strut "feet": we made some rubberized pads to help the secondary support struts distribute the loads on the beam ends (and to prevent them from moving). We probably should have found a way to pin these struts in a fixed position to avoid them slipping at all.
- Wing seat nets: we used some old Corsair F31 trimaran nets and cut them down to size by hand, lashed with dyneema.
- Additional "safety" nets (lifeline netting): we added these to make sure nothing could be dropped overboard between the hull and wing.
Total cost: $500-600
- $190 for aluminum tubes
- $210 for aluminum railing fittings (those add up fast, $12-15 a piece)
- $40 for lifeline netting
- Some miscellaneous parts amounts for custom beam plates, screws, metal rings, dyneema, etc
- Wing nets were free
- Labor was free (access to metal workshop)
Total weight: approx 50-54 lbs (vs. 58lbs for Hobie 18SX?)
- 33lbs for aluminum tubes (cut to size)
- 13lbs for aluminum railing fittings
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SL
Nacra Inter 20 (sold)
2017 Race to Alaska "Team Ketch me if u can"
- Facebook: https://www.facebook.com/TeamKetch/
- Race video highlights: https://www.youtube.com/watch?v=YTWp4DP0VcA
Sausalito CA
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Posted: Nov 15, 2017 - 02:23 PM
Thanks much for posting up the details - the 1.72/1.75 fit and off-the shelf brackets position this well for home-brew. a flapper wheel inside the bracket or emery cloth outside the tube would clear .03 quickly and still leave a tight fit.
Randii -
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Posted: Nov 22, 2017 - 07:06 AM
Hi Southstars2012,
Nice and ingenious build, together with great info. This is how we did it this summer, for us retired guys to get rid of the trapezes. This winter we are shortening the wings a little, moving the fittings to the inner of the hull and widening the beams. Will also go M32 at the rear(great minds think alike) keeping the lines at the front. Not as wide as yours, but we are not heading for Alaska
. We are very pleased with the system lifting the leeward wing automatically when sitting on the windward. Will take my time and read all your info, maybe I could loan some of your ideas. You can follow this winters build and our other Frankencat/boat builds at: https://www.facebook.com/groups/1792963567605832/ . Would appreciate you as a group member.
https://www.thebeachcats.…opic/topic/16946/start/0
Edited by revintage on Nov 22, 2017 - 07:14 AM.
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Brgds
Lars
Frankentri 5.8/5.5/Inter20
Aerow trimaran foiler
https://www.facebook.com/groups/1192604934176635
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Posted: Nov 22, 2017 - 10:14 PM
Revintage -- I liked the pictures in your BeachCats thread. Thanks for sharing those -- it helps to see the details.
Looks like a great setup, I actually think it's as wide as ours...? (based on how you are sitting -- when we sat our feet touched the hull a bit) I definitely don't think ours replicated a full double-trapeze, which is why we kept the ability to trapeze further out.
The PVC tee fittings you used to connect the different pieces of the wing seem interesting -- might shave a bit of weight (ours were ~0.6 LBS). The fitting weights add-up...!
I also really like the metric 50 OD x2 ID mm tubes, that's very close to the Hobie sizes (50x2mm is approx 1.97'' x 0.079'', vs. Hobie tubes are supposed to be 1.875'' x 0.083'' -- by my math they're about the same weight, and the metric section would be a bit stronger than the Hobie one).
Our wings seem marginally lighter (58-62lbs vs. ~65lbs with yours if I take your 15kg/wing measurement), despite being a bit longer (12ft on the outside). Note I revised upwards our wing weight as I realized I had underestimated the weight of our smaller sections (1.5'' x 0.125'' not 1.5'' x 0.065''). I think you may actually have overestimated your weights a bit:
- The extra tube you use by your feet definitely adds weight (~9-10lbs by my guess, assuming ~9ft per side). Since that's not that structural, I could envision eliminating it or downsizing it to a much smaller tube...
- However our wings would have been ~9 lbs lighter if we had used Hobie or 50x2mm metric sections like yours, instead of the heavy 0.125'' ID sections we had locally.
So net-net they should weigh about the same!
I could see getting the wings down to ~45-49lbs in our design if we used your sections and also removed the forward M32 style supports so the wing was supported up-front by the trapeze wires (using M32 supports only on the back). That would be a very nice weight gain!
Edited by southstars2012 on Nov 22, 2017 - 10:27 PM.
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SL
Nacra Inter 20 (sold)
2017 Race to Alaska "Team Ketch me if u can"
- Facebook: https://www.facebook.com/TeamKetch/
- Race video highlights: https://www.youtube.com/watch?v=YTWp4DP0VcA
Sausalito CA
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- Posts: 40
Posted: Nov 22, 2017 - 10:23 PM
Correcting some mistakes in my prior post containing the technical details of the R2AK I20 wings. I quoted our wing inner tube sections wrong, and overall weight wrong as a result.
Edited by southstars2012 on Nov 22, 2017 - 10:25 PM.
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SL
Nacra Inter 20 (sold)
2017 Race to Alaska "Team Ketch me if u can"
- Facebook: https://www.facebook.com/TeamKetch/
- Race video highlights: https://www.youtube.com/watch?v=YTWp4DP0VcA
Sausalito CA
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- Rank: Lubber
- Registered: Oct 09, 2016
- Last visit: Mar 08, 2018
- Posts: 40
Posted: Nov 22, 2017 - 10:39 PM
Revintage -- One more thing: while I do like your automatic lifting setup (we had considered something similar, again great minds think alike!), in the end we decided we didn't want to force lifting of the downwind side, and found it quite useful to be able keep the downwind wing down. For example, we could split our weights sitting on the downwind wing and upwind wings in light airs, to help the windward hull skim. On the other hand, we lost out on the advantage in heavy air of moving the leeward wing weight further inboard to increase righting moment... (we could manually clip the leeward wing up if we really wanted it, as we did while docking, but this would be hard to do quickly in heavy airs tacking).
I think the ideal solution would be a 'best of both worlds' approach -- using your lifting setup, one could add a secondary adjustment that allows shortening or lengthening the dyneema line between the two wings. In 'short' mode (medium/heavy airs), the weight on the windward wing then lifts up the leeward wing automatically (as in your current setup), while in 'long' mode (light airs) the line is long enough for both wings effectively stay down (as in ours)...
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SL
Nacra Inter 20 (sold)
2017 Race to Alaska "Team Ketch me if u can"
- Facebook: https://www.facebook.com/TeamKetch/
- Race video highlights: https://www.youtube.com/watch?v=YTWp4DP0VcA
Sausalito CA
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- Last visit: Nov 29, 2023
- Posts: 280
Posted: Nov 23, 2017 - 05:59 PM
Southstars,
Will just give a short reply now, will come back when I have consider all your valuable input.
1 You designed your setup for open sea, I designed for lake sailing. This might explain some of the differences on our basically alike designs.
2 You mention weight and this is something to really look into. Have calculated the weight of PVC-tubing to use in some places and it is 0,66kg/m while the alu tube is 0,81kg/m. Not very much to gain there. But the T-fittings are really something, will compare weight with the alu pieces.
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Brgds
Lars
Frankentri 5.8/5.5/Inter20
Aerow trimaran foiler
https://www.facebook.com/groups/1192604934176635
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