I am exchanging the telescoping beams of a Supercat 20 with solid ones. After long sourcing on the local market, only two sections seem available options:

(6063 T5)

rectangle 'tube' 100 x 120 x 2 mm wall

round tube 100 dia x 2 mm wall

round tube 100 dia x 7 mm wall

I intend to use the 100 / 2 round tube as frontbeam as the round 7mm section seems far to heavy.

I am puzzling with the rearbeam:

I would appreciate to raise the rearbeam more from the water. Using the rectangular section (green) and a 5-10 mm thick aluminium plate (grey) as well as a half section of the previous round section (yellow), this could be achieved quite easily. The question would be if the welding (red) would be strong enough (a suitable method in that area) and if the platform would increase in stiffness or actually become weaker as a round section could deal better with torsion? Any comments? Thanks!

Hm, no comments? Is that related to the question or the bad timing to place it just before Christmas? <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

Wouter, are you 'here'? I remember there was a similar discussion about tubes for the F16 some time ago?

I am a little scared if welding would work, be strong enough? I think the old Dart 18 had her main hound fitting also attached to something welded on the mast? With welding, you change the characteristic of the aluminium to become more stiffer loosing flexibility and easier to get cracks... Any comments on the welding part?

I'm here, but ask me again after new-year. I got heaps to do in my private life at the moment.

Wouter

For a given size, a square shape (or rectangular shape) has a higher torsional stiffness, but also weighs more. Looking at charts of tube properties, if you reduce the wall thickness of the square shape so that it matches the weight of the round shape, the torsional stiffness is very similar, with the square shape having a slight advantage.

I agree with your thinking of the round tube for the front beam, but would double check the wall thickness with the stock size to make sure you are not using a weaker section. I like the idea of the rectangular rear beam, but am a bit leary of trying to weld to it. I work in the metals industry and we try to avoid doing structural welds on aluminum. It is very likely that when doing the weld, you will heat the aluminum to the point where you will change the temper of it making it a bit more brittle than the rest of the tube. This could result in a stress concentration and ultimately a failure point.

How about using the rectanular tube and have some Stainless steel brackets or straps made for the bolt attachments (thru bolted to the tube). You could then re-shape the beam socket around the tube using epoxy and cab-o-sil or some other high density filler. I'm in no way an expert, but this is my 2 cents....

Good luck!!!

TB

A rectangular tube has better torsional stiffness? I didn't think that was the case....why aren't drive shafts square?

Dirk,
I understand your desire to raise the rear beam, I get a lot of waves crashing into the rear beam, slowing the boat noticably when it's blowing over 15 to 18 knots. Could probably be reduced by moving crew weight fwd, but those are the conditions where I constantly have an eye on the leeward bow (I have very gusty conditions usually), and I feel most secure being on the wire, fully aft with a foot in the footstrap. So I'm not moving fwd, if I have skilled crew (not that often), I'll have them moving fore and aft, as well as in and out, trying to balance keeping the rear beam away from the chop, with the strong desire to keep the lee bow visible.
It seems like your design really only raises the rear beam 2", and I wonder how much difference that will make. But your design seems sound to me, what about doing all the welding on the half round section only? Weld a flat rectangle to the half round piece, and attach that piece to the square tubing with bolts and a backing plate inside the tube? That would obviate in need to do welding on the square tube itself. And have "U" brackets made of ss to hold the assembly firmly in the cradles, similar to the original clamping brackets. Seems like this would minimize any weakening of the square tubing from the welding.
How will you attach the tramp to the rear beam? Will you use the original main traveller set up, or a new conventional traveller track? What will the material for the front and rear beams cost you? Where are you?
I hope you'll keep us posted on how this shapes up. I'd like to do it to my boat someday, and I'm sure there are others out there.

Dave

SC20 and Flight Risk

Dirk,

I understand your desire to raise the rear beam, I get a lot of waves crashing into the rear beam, slowing the boat noticably when it's blowing over 15 to 18 knots. Could probably be reduced by moving crew weight fwd, but those are the conditions where I constantly have an eye on the leeward bow (I have very gusty conditions usually), and I feel most secure being on the wire, fully aft with a foot in the footstrap. So I'm not moving fwd, if I have skilled crew (not that often), I'll have them moving fore and aft, as well as in and out, trying to balance keeping the rear beam away from the chop, with the strong desire to keep the lee bow visible.
It seems like your design really only raises the rear beam 2", and I wonder how much difference that will make. But your design seems sound to me, what about doing all the welding on the half round section only? Weld a flat rectangle to the half round piece, and attach that piece to the square tubing with bolts and a backing plate inside the tube? That would obviate in need to do welding on the square tube itself. And have "U" brackets made of ss to hold the assembly firmly in the cradles, similar to the original clamping brackets. Seems like this would minimize any weakening of the square tubing from the welding.
How will you attach the tramp to the rear beam? Will you use the original main traveller set up, or a new conventional traveller track? What will the material for the front and rear beams cost you? Where are you?
I hope you'll keep us posted on how this shapes up. I'd like to do it to my boat someday, and I'm sure there are others out there.

Dave

SC20 and Flight Risk

I can't really comment on the drive shaft question, but I can say that I am basing these statements on catalog I have of different steel shapes and their material properties - weight, area, Iy or Ix, S, including torsional stiffness. I looked up a 6x6 tube versus a 6" pipe section to draw a conclusion. Perhaps, I am missing something relating to the application of the stiffness coefficient that could prove the pipe section stiffer. I suppose I could take out my old school books and figure it out, but i'll save that for a rainy day.

happy holidays.

TB

Round are considerably better than square. Square tend to “localise” any flex at their corners and unless very well supported internally, will fracture along those corners. Always try not to weld aluminium; particularly if it is going to be subjected to continuous flex and twist, it will fracture along the weld in a relatively short period of time. Always fasten beams by drilling and bolting or pop riveting, always use compression spacers with the bolts so that the load is not localised just where the hull meets the bottom of the beam or else the bolt can tear through the beam. There is a very good reason why the aluminium skin of an aeroplane is fastened with pop rivets and not welded (although now some do use resin adhesives instead)
Oh and by the way there are many agricultural vehicles that DO use square drive shafts.

To avoid the welding you could fill the slot for the old beams then mount the square beams with bolts thru the center and into the newly reinforced hulls. .. <img src="http://www.catsailor.com/forums/images/graemlins/cool.gif" alt="" />

Aircraft dont use pop rivets in structural applications, you cant guarantee how much load they will take before they fail for various reasons. Aircraft mostly use solid aluminium rivets or Cherrys http://www.textronfasteningsystems.com/aerospace/html/product/rivets.htm
Araldite make some of the structural adhesives I use in addition to rivets on aircraft, they have good range for marine as well.
Personally I like a reinforced box section beam, due to ease of fastening and simple load paths.
Darryn
Mosquito 1704

My point was not "pop" rivets as in "pop rivets" that was simply an (as it turns out) oversimplification Darryn as you have noted, the main point being that it is unwise to weld aluminium when it is to be used under flex and stress loads. The Mozzie uses rectangular beams, as you are obviously aware, and although your points of ease of use etc are valid, they have, and still regularly do get more than their fair share of failures when compared to cats with round sections (or mast "pear" shaped)
Any way all the best and good luck next week at Christies Beach

Can you be a bit more specific, as I have had little success with these aluminium glues, which particular glues do you use.

Thanks

Gareth
www.fourhulls.com

Link below to typical adhesive used, good info on how to use it. There is a product finder on the Huntsman website.
http://ww1.huntsmanservice.com/tds/Huntsman%20Advanced%20Materials/English/Long/Aral-2015%20_GB_.pdf

Darryn

Grob

An excellent aluminum and carbon structural adhesive for catamarans is Araldite 2013 made by Ciba. This is what Marstrom uses and what many of the Tornado sailors call Marstrom "grey glue". Marstroms price may seem high but I think he is selling it for about his cost.

Ciba glues are very expensive in the US because of the shipping. I normally use 3M DP-110 or DP-120 which is similiar to Araldite 2013.


Keep in mind that these glues are very expensive if purchased in small quanities from the maker ($15-20/50mL). There are internet sites that sell them but the prices are still high. Try to find local companies that uses large quanities and is willing to resell some. For example when I worked for Lockheed, I could buy 3M adhesives at the Lockheed price. I remeber paying $20 a case or $2/50mL.

All of these are good adhesives and there is a lot of experience in using them so you know how they will age.

ok try, tube service in fremont california. then use a piston honing tool and a 5 gal bucket full of tranmission fluid to hone out the internal. then have the diameter to fit the shims. then have the "stubbs" heli-arced in. i belive the dolphin stiker should be at 8 degrees. not easy but been done, works perfect. PM me for complete details. been done, have the materials for anther set. this wont raise the rear beam. but they will be round and solid. sc20 tr

Thanks all for the supportive feedback!

Coming back to my initial question and the different statements given, I summarize that even if a suitable solution fo fix a square section as rearbeam to the hulls is found, a platform using rectangle 'tube' 100 x 120 x 2 mm wall as rearbeam will be LESS STIFF than the same platform using round tube 100 dia x 2 mm wall as rearbeam?

I thought that the 120 mm height of the sqaure section aginst the 100 mm height of the round tube would REDUCE bending of that beam and therefore would reduce the flexing of the platform. But following Darryl, this assumption is wrong?

I too would like to see that claim explained as I too come to different conclusions.

So Darryl ?

Wouter