Lengthening Nacra beams :: Catamaran Sailboats at TheBeachcats.com

Lengthening Nacra beams

revintage

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Posted: Feb 06, 2018 - 07:57 AM

Got a little carried away after building the new 10.5ft beams/1st image) for the 6.0 hulls I am waiting for. As I have a complete 5.5 with hulls and beams I got the idea of lengthening the beams instead of fabricating new from scratch. Want to remake the 5.5 to a single hander with 11ft width.

As 100x5mm and 100x6mm alu tubing is available at reasonable cost here, the ca 1200mm needed costs ca 50USD and the stainless 40x3mm needed for a new DS-strap is ca 25USD. So total will not be over 100USD.

Not much work needed either. When turning down 100x5mm to the inside diameter of the beam there will still be 3,8mm thickness left of the tube, with 100x6mm 4,8mm.

Two M8 bolts to hold it together. The strikerband will take care of lengthwise compression.

Compared to the front beam I built(first image), weight penalty will be 1kg with 5mm or 1,8kg with 6mm. I don´t mind this as no racing is involved.

As the Nacra beams has a trampoline groove my idea is to remove the first cm of it and weld the beam to a round tube. After that, the lengthening tube should be slotted for the groove. Don´t know if this will make things better?

Maybe a crazy idea, please try to find the weakest spot! Sketch in the 2nd image.

Edited by revintage on Feb 06, 2018 - 11:17 AM.

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Brgds
Lars

Frankentri 5.8/5.5/Inter20
Aerow trimaran foiler

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Andinista

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Posted: Feb 06, 2018 - 10:07 AM

Looks like you will build an 18 square, I would check with someone with experience on that boat, not so much for the building details but to clarify if your expectations will be fulfilled. You might find the boat underpowered if you are not thinking to add more sail area or sail it singlehanded, which is what 18sq is meant for. If you intend to put more sail or sail it in stronger wind, I would check if the hulls can take it, I'm thinking on weight balance and bows wanting to sink, for instance. Just thinking out loud, I know my 5.5, no experience with the 18 sq.
I like my 5.5, I wouldn't make it wider because I sail it mostly with two up, occasionally 3, and like to fly the hull. If I was just myself 80% of the time, then maybe, not sure if that much though.
Andinista

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Posted: Feb 06, 2018 - 10:07 AM

double post...

Edited by Andinista on Feb 06, 2018 - 10:09 AM.
revintage

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Posted: Feb 06, 2018 - 10:32 AM

Hi Andinista,
I actually made the math before designing the beams . A std 5.5 with two men trapezing will have slightly better stability, than singlehanding this heavier 5.5 with 11ft beams using only the F18 mains together with my 2.8ft folding wings. So this bird will surely fly. Going up to 11.5ft would make them even wrt stability.

Edited by revintage on Feb 06, 2018 - 10:48 AM.
Dogboy

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Posted: Feb 06, 2018 - 10:58 AM

At some point, torsional stiffness of the platform will start to become a factor as the beams get longer and longer. Maybe this won't be an issue, but as you continue to extend the beams, one hull will want to twist independently of the other (i.e., one bow goes up, the other dips down). The only way to correct that would be to increase the diameter and/or wall thickness of your tube or switch to a stiffer material (carbon fiber). The load on the beam bolts would also need to be taken into consideration since you don't want to shear them off.

Cool project otherwise.

sm
revintage

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Posted: Feb 06, 2018 - 11:38 AM

Hi sm,

Actually torsion is an issue, although a Nacra 18SQ had no reinforcements at all in their 11ft 101,6x2mm beams.

Have never sailed one, though.

My thought was that the M8(M10?) bolts with inner distances should take care of the torsion between the original and the extended part of the beam. No epoxy, no rivets.

My guesstimate is it will work OK as it is a singlehander and the sail area is even less than the 18SQ.

As I can restore the beams close to original, all I can loose is ca 100USD and some work at the lathe. But work is free when you are retired .

Did a weight calculation using 100x6 front and 100x5 rear and its not a low weight mod at 5kg extra, compared to making a pair of 11ft beams exactly like 5.5.

But is it really necessary to care about 5kg/2,5% weight addition in a nonracing situation?

Lars

Edited by revintage on Feb 06, 2018 - 11:40 AM.

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Brgds
Lars

Frankentri 5.8/5.5/Inter20
Aerow trimaran foiler

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samc99us

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Posted: Feb 06, 2018 - 04:02 PM

revintage,

Your drawing looks a bit backwards to me as I would expect the through bolts and sleeving to be sitting over the hulls. A couple other notes:

1) I would make a pretty beefy sleeve that actually goes into the socket for the bolts and extends out at least 6".

2) I would carry the dolphin striker strap into the hull socket, a modification on later Nacra wide boats to help strengthen that entire area.

3) I would seriously consider pricing out carbon tubes, they will be substantially stiffer especially if you have them bias wrapped.

4) If you can, I would bond the beams in. Bolted connections are very flexible. On the F20c I can feel the beam moving in the socket when going upwind even in milder conditions.

5) New tramp wrapped around the front beam or bonded on (latest a-cat tramps).

Edited by samc99us on Feb 06, 2018 - 04:22 PM.
P.M.

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Posted: Feb 06, 2018 - 04:18 PM

Sam,
Regarding item 1. I believe he said that he has a beam sleeve made from a beam extrusion. As long as it is extends from the end inward about 2'. Standard practice on the later 6.0NA's which were beasts.

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Philip
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GilleyNM

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Posted: Feb 06, 2018 - 04:33 PM

Revintage:
I am a 255 lb, grey hair, Nacra 18 sq sailor and have 25+ years of 5.5m/11 ft. wide experience. I suggest that if you make the sleeves sufficiently long and both epoxy and mechanically fix them in place, you will have no problems. I even have a 2 foot extension on my mast allowing for an over sized mainsail. All without problems. I sail in the Gulf of Mexico in all conditions and have yet to experience any crossbar or mast failures. I do perform routine checks on the entire boat and maintain it without question. The only failure is the stripping of a crossbar / hull strap bolt due to not paying attention to over applying torque not the 'sailing." I say 'do it the right way' and be prepared to have to sheet out to talk with fellow cat sailors. NMG "Fast Attack"
revintage

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Posted: Feb 06, 2018 - 05:12 PM

Seems like there are quite a few misunderstandings here. To make it easier for you to read, I have color coded the parts.

Edited by revintage on Feb 07, 2018 - 09:12 AM.
samc99us

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Posted: Feb 06, 2018 - 05:33 PM

revintage,

I'm doing my best to interpret the drawing, but where exactly the hull is in that photo remains a mystery. FYI the internal castings on the new Nacra beams are simply held in place with the beam bolts and possibly the striker bolts, along with 2 carefully located rivets. I would avoid adding more through holes to the beam on the inside of the hull than necessary, as that will weaken the beam. Epoxying the sleeve in is one decent option-I would be a little bit concerned about thermal expansion/contraction weakening that bond over time. Another simple one is a couple of monel rivets placed for and aft along the beam to hold the sleeve. I would say 6 front and 6 back 3/16" plus the through bolts in the socket would get the job done.

Of course, looking at those beam attachments I believe you are relying on the straps? Those are poor due to lack of torsional load transfer. Through bolts are much better (as are hexagonal beams like Cirrus use but that is another headache). Anyway, putting two rivets at 45 degrees of the top dead center into the stainless steel strap and through the sleeve will do a pretty good job, but I would also advise a countersunk rivet or fastener on the bottom underneath each eye strap.
revintage

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Posted: Feb 06, 2018 - 06:35 PM

The sleeves are sufficiently long with common industry practice of two times tube diameter.

Sam, hope this gets you oriented, check the red rectangle below. Have also redrawn the sketch above to avoid confusion.

There is actually not much need for neither bolts nor rivets in the front beam, except for the strikerband. Why, you have to figure out yourselves, think compression and think slot for the tramp groove .

The rear beam is another problem due to the lack of striker band and tramp groove. I think epoxy and riveting are really needed there both to keeping it together and for avoiding torsion.

Sam, you are absolutely right about the weaknesses of the beam straps, but the system is used by Nacra Australia still producing the 5.8 today. All they have done is exchanging the bolt and alu casting inside the beam for a "through the outer strap, through the beam bolt", though. Must point out, I am only lengthening the beams and have no intention of rebuilding the hulls. It´s not worth the effort on a 500USD boat.

Anyway will give it a try IRL when back from my skiing vacation in a few weeks.

Edited by revintage on Feb 06, 2018 - 06:56 PM.

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Brgds
Lars

Frankentri 5.8/5.5/Inter20
Aerow trimaran foiler

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texastuma

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Posted: Feb 06, 2018 - 06:57 PM

Revintage,

I pieced together a cat, "FrankenKitty", out of parts of 7 or so different boats. Basically, it is a 10' wide Prindle 18-2 that I race singlehanded. To sum it up, I have done 26.4 knots. years ago, I had thought about making the 6.0 10' and it would be a beast. One thing you will notice, is the reaching stability. Think about the distance from leeward bow to windward stern.. this has increased more than the boat width. This is the additional leverage you are gaining on a reach. My issue is that it is still a Prindle; once the bow goes under - you stop very quickly. The Nacras will have a lot more forgiveness. As for the beam; I would just fabricate it as we do masts: In internal sleeve with recessed fasteners. Many masts use stainless rivets. I'm not a fan of rivets. The Nacra dolphin striker design may help. The thing we ran across with the older Tornados is the beam would load and twist at the hull connection. This would be my hesitation of the factory diameter beams. Look at what the "new" Tornado beam cross-section is. On my P-18-2 mod, I am using Tornado beams. The internal sleeves may help with this problem, but having minimal tolerances between the hull and beam is crucial. One of the biggest issues is trailering at that width.

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Scott

Prindle Fleet 2
TCDYC

Prindle 18-2 Mod "FrankenKitty"
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revintage

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Posted: Feb 07, 2018 - 04:43 AM

Hi Scott,

Building Frankencats is great fun!

Referring to the new 10.5ft beams I built for my 6.0 beams last week, I don´t use the Nacra style dolphin striker anymore. Instead a Tornado/Goodall style system with an adjustable alu post and stainless striker strap. See image below.

The build up of the 5.5 beam I am lengthening here, will be far more effective than doing it with an internal separate sleeve. By combining the sleeve and extender in one thick wall piece you can rely to the dolphin striker for pushing the beam and extender together. As you also have to cut a slot in the 4.8mm part of the extender, going into the beam for the trampoline groove, you also take care of torsional stiffness in the joint. Might add one or two bolts to keep it together, though.

Will use the existing Nacra striker post but with a stainless striker band.

Penalty of using a 6x100mm extender tube, turned down to 97,6x4,8 in the 200mm overlap, is you will add 5kg, but this is next to nothing for a leisure boat, built of bits and pieces lying around.

Edited by revintage on Feb 07, 2018 - 04:59 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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revintage

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Posted: Feb 26, 2018 - 04:42 AM

Did some more thinking while on my ski vacation. Found out it would probably be better with the joint between the beam straps. This limits overall beam length to 297cm, adding 21cm on each side, but thats just fine, considering the F18 rig to be used. Also went down to 100x5mm lengthening tubes.

Will not use the corroded original main beam, will instead use the leftover 100x2 tube from the first front beam build, as tubes are sold in 600cm lengths. Will use the original Nacra striker post, striker casting and ball.

Also remember this is a below $100 mod, with material already in my stash. Compared to using Nacra beamtubes of the same lengths, there is a weight penalty of 2.8kg.

Hope these simplified sketches are understandable, please comment if finding any radical errors. Also made a manipulated image from something I borrowed from Mr. Ragan on the internet.

Rear joint, beamstrap in blue:

Main beam joint:

Rear image manipulated top, original bottom:

Edited by revintage on Feb 26, 2018 - 06:00 AM.

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Brgds
Lars

Frankentri 5.8/5.5/Inter20
Aerow trimaran foiler

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revintage

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Posted: Apr 06, 2018 - 09:25 AM

Some pics from my friend Pers workshop, were we produce the inserts for the Super-5.5. These are for the rear beam.

Edited by revintage on Apr 06, 2018 - 09:30 AM.

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Brgds
Lars

Frankentri 5.8/5.5/Inter20
Aerow trimaran foiler

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pnwsailor

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Posted: Jun 08, 2018 - 12:14 AM

Nice looking project revintage. Any updates?

I have been working on my Supercat 20 and have debated going to a solid beam. Slightly cost prohibitive..So nice solution you have.

I couldn't help but notice the aluminum netted wings in the photos. How did they work out? I have been wanting to make some of my own.

Cheers
revintage

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Posted: Jun 10, 2018 - 07:51 AM

Hi,
Actually the boat is ready and rigged, but not sailed. The wings is a new lighter version of those we made last year. The older ones where 115cm wide and worked well. The new are half the weight and 62cm. They are also telescopic for single handing with trapeze at 85cm width to keep the same RM. When sailing two no trapeze is needed.

Edited by revintage on Jun 10, 2018 - 09:22 AM.

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Brgds
Lars

Frankentri 5.8/5.5/Inter20
Aerow trimaran foiler

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pnwsailor

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Posted: Jun 14, 2018 - 10:16 PM

Looks really nice. Will be a blast to get it out on the water!

I just sold my nacra 5.8 a few weeks ago and I was already having second thoughts. This looks like a really cool setup. I really like having the RM with the wider boat.

I need to get to building some wings for my supercat 20. Telescoping is an interesting idea.
revintage

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Posted: Jun 15, 2018 - 10:24 AM

We have a pair of 5.8 hulls lying in the workshop for our next Frankencat build. Have built 10.5 ft beams for them and have planned for 2.8 ft wings. Larger rig.

This is a simple spreadsheet I have made to calculate RM.

http://racerdirekt.com/ourcats.xls

Edited by revintage on Jun 15, 2018 - 10:28 AM.

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Brgds
Lars

Frankentri 5.8/5.5/Inter20
Aerow trimaran foiler

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