Hello,

I plan to use my nacra 6.0 for kitesailing. Would you agree that it is a good idea to get rid of the v-brace and DS, because without the mast no pre-bending of the front beam is needed?

Best Regards,
Lukas

Where do you plan to anchor the kite? It seems to me that you could use the striker for some support. Most DS systems are not constructed in an ideal way for loads pulling up on them, but I would think you could modify yours to provide some additional support. A triangle would be better than just a beam IMO.

--
Greenville SC

Offering sails and other go fast parts for A-class catamarans
--

The birth of a new sport Kiteamaran

http://www.youtube.com/watch?v=aTJtYyiVBTU

--
Kenneth Purdy
Hobie 16
Nacra 5.2 (2)
Banshee
First Coast, Florida
--

Has anybody thought of the jib blocks to steer the kite?

A nacra 6.0 could be a neat boat for a kitemaran.

What is the total weight of you and your crew?
What kind of wind speeds do you see? Steady or gusty?
What size kites will you be flying?

I want to try a Hobie 16 kite-catamaran with about 350lbs of crew flying a 12m kite in 15 mph winds at an inland lake. 15mph to us means lulls to 10 and gusts to 20, so I think I need a 12m kite to fly fast enough to keep it in the air in the lulls. I don’t know if a 12m kite will have enough pull. ….When it is 20mph+, I will just kiteboard. I want the smallest/lightest boat I can get.

The jib blocks could be interesting for steering the kite. This would be best in steady, light winds and parking the kite in one position.

You would need to create a system to control 4 lines. Two front power lines to front corners of kite. A left steering line to left rear of kite. A right steering line to right rear of kite.
Full power is all four lines same length. Minumum power is front two lines 12” shorter.
This steering comes from turning the kite control bar which creates a 6”-16” difference in left and right steering line length

In variable winds, the kite control bar needs to be steered quite a bit. Think riding a bike at slow speeds and turning tight/balancing, not just locking one jib sheet in a block and leaving it.

I would leave the v-bar and DS attached. When things change/go wrong, the kite has the potential to jerk and yank the attachment point more than a sail, so I want all the strength I can get. I don’t see the advantage to removal.

Let us know how it works and your setup! Sounds like your summer will be a blast!

Quote

I would leave the v-bar and DS attached. When things change/go wrong, the kite has the potential to jerk and yank the attachment point more than a sail, so I want all the strength I can get. I don’t see the advantage to removal.

Think of the loads from an engineering point of view, & visualize the forces a Cat applies to the V brace, & how it transfers them to the hulls.
It is only strong when dealing with a vertical downward load, applied to the center of the beam. When that occurs, the result is to put the V brace strap under tension. (as the beam bends downwards the resulting curve tries to stretch the V strap. The strap wants to move upwards towards the beam, but is prevented from doing so by the DS rod). You can easily see this by tying a string(v strap) to a stick(beam), then bending it, AKA a bow & arrow. The bending stick creates a greater distance in one direction, stretching the string. The system can be made much stronger by increasing the length of the DS rod, but a practical limit is reached because you cannot have the brace dragging in the water.
If the force is upwards,(kite attached to mast support) the V brace does nothing, ditto if the load were horizontal with the water surface,(having the kite directly out front & low to the water, in a power position.)
The entire design of the cat is to transfer the loads downwards onto the hulls, where it is spread out over quite a few square inches via the beams bedded to the hulls. The shrouds are behind the mast, taking the loads that try to pivot the mast forward, or sideways.
If the front beam is recessed into the hulls, the kites pull will transfer loads to the hull via the sides of the recess.
A beam bolted flat to the hull will try to bend the bolts when the kite is near the horizon, & try to pull the bolts out(shearing the threads) with the kite high.
If you attach the kite to the center of the beam, the limit will be the bolts threads, or what it takes to bend the bare beam (without any brace) to failure. The DS/V brace does nothing.
You need to know which component will take the greatest force before failure, bending the bare beam, or pulling/bending the attach bolts out of their anchor. If the beam is the weakest, stronger would be to attach a line at each end of the beam.
You could also rotate the beam & V brace so it faced upwards, strongest would be to have the DS in line with the kites pull. This could only be done with a system that uses straps to hold the beam, like the early Nacra. The limit would then be reached when you tore the bolts out of the anchors.
I would use a Cat I didn't care much about



Edited by Edchris177 on May 05, 2014 - 11:43 AM.

--
Hobie 18 Magnum
Dart 15
Mystere 6.0XL Sold Was a handful solo
Nacra 5.7
Nacra 5.0
Bombardier Invitation (Now officially DEAD)
Various other Dock cluttering WaterCrap
--

Thank you for all your comments.

Quote

If the force is upwards,(kite attached to mast support) the V brace does nothing

That's what I think as well. The V brace does not help if the force is in any other direction than straight downward as from the mast. Doesn't the pretension even make the beam weaker for a force pulling in the wrong direction?

The other benefit of removing the V brace/DS would be that I would just replace the DS with an eye bolt to have a neat anchor point for the kite.

Does someone happen to know the maximum tensile stress the sort of aluminum used for the beams can withstand? I could then measure the wall thickness and calculate the maximum force for the bare beam.

Quote

What is the total weight of you and your crew?
What kind of wind speeds do you see? Steady or gusty?
What size kites will you be flying?

Total crew weight would be between 300 and 550 lbs (2-3 people)
I would wait for a day with at least 12 knots wind with an onshore component. The wind would then be relatively stable.
I have 7, 11.5 and 17m kites.

Using the jib blocks to steer the kite might be interesting in the future, but for the moment steering the kite with the bar is how I will do it, because I am used to it.

check out youtube for people who've done it. someone needs to steer.
http://www.youtube.com/user/schroedelmax/videos

maybe you're over-thinking it.........
just press your foot against the mast ball and hang on.
http://www.youtube.com/watch?v=aTJtYyiVBTU

http://www.kite-boat-syst…oducts2/product-overview
j



Edited by arch on May 05, 2014 - 03:12 PM.

--
Aquacat 12 (sold)...'87 Nacra 5.8 (sold)...'03 Nacra Inter18 (sold)
Venture 15 (sold)....'89 Nacra 5.8 (sold)...'91 Nacra 5.8NA (sold)
'99 Nacra Inter20 (sold)
--

Quote

Does someone happen to know the maximum tensile stress the sort of aluminum used for the beams can withstand? I could then measure the wall thickness and calculate the maximum force for the bare beam.

It's not quite that simple. By simply adding large washers at the beam attach bolt heads, you would engage more material, enough so it will not pull through. Your limit would then be how strong are the anchors holding the bolts, or the threads.
The strength of the beam itself to resist bending is a combination of the material the beam is made of, it's wall diameter, AND the diameter of the actual beam.
For a given mass of metal, the strongest beam is a cylinder with thin walls, of the largest diameter you can form from that mass of metal. Of course there are caveats.
You may have done the time honored engineering experiment of How Much weight can a Beer Can Support?
We know a beer can is a large diameter, very thin walled tube, consisting of very little mass of aluminum.
The answer is, it can hold a surprising amount, BUT, if you so much as plink the side of the can it will instantly collapse.
I don't know where you are located, but maybe Dan Berger has a wrecked Nacra beam/hull you could tie down, then pull to find what load the anchor bolts fail at. Then support the beam & stress the center til failure. you now know the weak link. The beam could be easily beefed up by inserting another tube inside, or lathe a piece of pressure treated 4x4 to a snug with, then drive in with lube.
I'm thinking the anchor bolts are the weak link, they carry little upward load & the design/build reflects that.
Make an album & be sure to post some video of the project.

--
Hobie 18 Magnum
Dart 15
Mystere 6.0XL Sold Was a handful solo
Nacra 5.7
Nacra 5.0
Bombardier Invitation (Now officially DEAD)
Various other Dock cluttering WaterCrap
--