Thread: Mast Flex question for PDR Lug rig?

I don't know if you want to look at other cases Ian, but in case you do ...

The original masts for the two OZ PDRs we built in queenland were 55mm square - same 12mm wall.

They were too flexible. One broke at a way too short scarf joint and knot. So I did a recalc based on how much flex I wanted the masts to have.

They might have been strong enough ... but the rig was depowering too much all the time ... it was really hard to get the boats going on a reach. So I made two new ones and we decided to make two new sails to try and find a good match.

[ame="http://www.youtube.com/watch?v=5PVkPHGE2c8"]Crash and Burn - YouTube[/ame]

Also recently Alex made a 62 square mast out of Paulownia. We decided to glass it in discussions. A layer of 6oz all over doubled at the partner.

Hi, Mik,

I can run a few other specs through and get stiffness and strength data if you like. I have a spreadsheet sorted that lets me put the section dimensions and material specs in and outputs stiffness and strength data. It will cover round and square sections, both solid or hollow.

What specific sections do you want numbers for?

Regarding the Paulownia mast covered in glass cloth, I recall running some numbers through for this sort of idea some time ago, I don't think it makes any sense. If you work out the amount of glass you have to add to the Paulownia to get the same stiffness as one of your hollow all wood masts, the overall weight ends up greater than the all wood mast. Some of the problem is glass cloth has only half its fibers going along the spar, so half of it is just along for the ride. Things would look better with linear glass sleeve material, but I don't think you will get much improvement on your hollow square sections in good quality wood. However, glass cloth around the mast immediately where it goes through the partner does make sense for surface hardness and hoop strength.

I think the only way to beat those square/hollow wooden sections for strength and stiffness for weight is to go to prepreg glass on a tapered mandrel, but maybe that is not allowed under the PDR rules?

The brave might look at increasing the size of the square section to say 65 mm and take the staves down to 9 mm on the thicknesser, then take another mm off the stave thickness for the top half of each stave. All dependent of course on having high quality wood, good scarves and no knots!

Woodeneye did his GIS mast in hoop pine, I think outside dimensions of 82 mm square as per the plans, but his staves were down around 10 mm thick, his mast weighed quite a bit less than anyone else's wooden one and he has had no problems in quite hard racing use. (His GIS mast was just over 16 lb, most have been around 20 lb)

Ian

Ok, here are the numbers on the 62 mm square Paulownia section with a couple of layers of glass on it.

Attachment 195252

It doesn't look good. I've used an elastic modulus (E) for the Paulownia of 5 GPa. For the glass cloth, I used an elastic modulus (E) of 15 GPa. Pre-preg linear glass has a stiffness of about 31 GPa, in comparison, glass cloth has only half the fibres running along the spar, so we can halve this E value. This is probably still an optimistic value as the hand laid glass cloth won't have as good a fibre/resin ratio as the pre-preg stuff.

The square section Paulownia has an EI of 6.2 kNm^2. Adding 1.0 mm of glass cloth on top of this only adds 2.5 kNm^2 of stiffness, so total stiffness is 8.7 kNm^2. 1.0 mm of glass cloth laminate takes about three layers of 6 oz cloth, if I remember correctly, but the resulting 8.7 kNm^2 stiffness still isn't stiff enough.

Weights per metre for these sections:
The Paulownia has a density of about 275 kg per cubic metre so this square section will weigh about 0.83 kg per metre of spar length. The glass cloth weighs about 1800 kg per cubic metre so 1.0 mm around the outside of the Paulownia will weigh 0.36 kg per metre of spar length. Total weight per metre is now 1.2 kg per metre. In comparison, we can get a stiffness of 13 kNm^2 for a weight per metre of just over 0.9 kg with the 62 mm square hollow timber section with 12 mm Oregon staves. (Oregon E of 12 GPa, density of 480 kg per cubic metre)

Ian

PS. Does anyone have an actual weight in use for a square section wooden mast? I have Mark's 2.5 inch OD alu mast weighing 9 lb or so, which is very close to my calculation, but when I work out the weight of a hollow wooden one, I'm not taking into account the glue or the blocking or the epoxy/varnish on the outside.


Edit:
OOPs, there are errors my weight calculations for some of the square sections, I'll go through and double check/correct as necessary, I think the rest is OK.

Originally Posted by markmilam

Man, THANKS Ian..

Thats good to know....a lot better than..."Well, it looks about Right"

Actual numbers are way better!!.....Even a normal guy can understand that

So the 3 knM^2 thingy difference between the wood hollow and Al. is not a lot to worry about I gather.....Whats impressive is how good the Wood mast is, I would have thought the Al. mast would have been significantly lighter than the Hollow Wood mast.

So even if the Al. tube flexes a little more, when going upwind, Which would loosen your Downhaul some,....then all you do is reach over and Tighten the downhaul some more....as the wind builds......up to gale force,.......... then its time to switch to the wooden mast........ Thanks again
Mark M.

Yes, I think your alu mast will be OK, but at the 'adequate' end of OK, rather than the 'bombproof' end of OK. Forces you can exert on the mast in a boat of this size depend mainly on how heavy you are, how hard you hike. I'd be prepared to go out and fool around with two in the boat in a gale with the square wooden mast in Oregon, but perhaps not with your alu one.

One more thing - your 18 inch overlap when you're joined the two tubes by sleeving one inside the other. The rule of thumb is an overlap of three times the outside diameter of the bigger tube, so you should only need to overlap 7.5 -8 inches. On the other hand, your tubes aren't that good a fit inside each other, there's a gap of about 1.5 mm all around. I think you could get away with an 8 inch overlap if you wrapped the overlap area of the smaller tube with glass cloth, perhaps four times around with 6 oz cloth, let it go partly off so it's stuck on nicely but not fully cured, then put some more glue on it and inside the larger diameter tube before sliding it inside. That way the gap would be nicely filled and concentric.

Ian

Hi Ian,
The glassing of the paulownia mast was partially to resist denting and partially a way of reducing stress concentrations from unseen defects in the wood.

The paulownia stick was not planned but once it existed it seemed the best response to counter the most likely problems and see if the mast survives.

With thin wall timber masts there's always a problem with getting enough glue area in the joins. That's why the traditional rule of thumb is for 20% with "racing boats" going down to 15% with some added risk.

I think the paulownia stick would be an interesting comparison with the 55mm radiata pine sticks we made for the first two OZ Ducks we made.

Cheers
MIK

The weight calculations for the square sections above had errors in them, I'd reported weights as if they were round. I've gone through the spreadsheet and corrected them.

Firstly:
The Square 62 mm Paulownia with glass cloth around it. Corrected data is here: Attachment 195259

Conclusion remains the same, this comes out quite heavy and not particularly stiff. Strength is a bit harder to calculate, you can't just add them the way you can stiffness values. I could have a go, but I'm not sure it's worth it.

Secondly:
A comparison between the original 62 mm square hollow Oregon section, 12 mm stave thickness: Attachment 195256

I used a stiffness of 12 GPa, perhaps better reflecting the timber quality that is likely to go into a PDR mast. The 2.5 inch OD/0.065 wall Alu is 25% lighter than the wood, stiffness is not too bad (10.6 kNm^2 for the alu vs 12.7 kNm^2 for the wood). Quite a difference in strength though - the wood is twice as strong as the alu. In practice the difference may not be quite as much as the wood is more likely to have a flaw that would drop ultimate strength.

Hope this all helps.

Ian

Hi, Mik, yes I'm curious to see how the Paulownia plus glass combination works, it would be great to get some data on how stiff it ends up and at what weight and see how it compares to what my spreadsheet shows.

AS Regards to Splicing the Alum. mast....I Did use 3 sections of 4in. wide Fiberglass Tape over the 18" splice.....Each section was wrapped Twice and the mast sections had a perfect fit....(I'd rather be lucky than smart any day ) I then sanded the inside of the big Al. tube and mixed up some thickened G 10 epoxy (or G-6, I can't remember) and Slide them together.

Here are the pics. I used pool noodle, and stuck about 2 in. into the top mast section to seal it so it will float....I'll seal the top after I finish .

I want to remind everyone how I handled the mast on my GIS. I wanted to avoid the expense of mast-quality wood, the fabrication labor as well as the ongoing maintenance of a wood mast so I purchased a tapered aluminum flagpole. The one I chose was rated for hurricane strength winds while flying a flag so I figured it would be fine with 105 sf of sail offset by a 200 pound human, in 10-20 knot winds.

It has a 3 inch base diameter, .125 inch wall thickness, and it tapers to 1 7/8 at the top. It's all one piece spun-extruded from aluminum tube.

I sealed and capped the mast at both ends and tested its bouyancy--it floats with the full length at the surface. IIRC it weighed 19 pounds fully rigged and painted. That compares favorably with the original solid round mast but is slightly heavier than the 4 inch square and hollow masts that are more common today. At 3 inches and round, I'm happy to accept the extra weight since it presents less windage.

I started thinking along these lines due to my experience with a much larger ballasted boat of 42 foot length and 20,000 pound displacement. I used two 8 inch diameter flagpoles for masts on that vessel, each carrying a 400 sf junk sail. Those poles saw some serious use and abuse and never showed any sign of being overstressed. The wall thickness was only .187 inches!

Aluminum flagpoles are widely available and inexpensive in the US.

Great reminder MAM. Have you got some pics of the junk rigged boat?

Michael

Thanks MAM,
Are the Flag poles 6061 T3 Aluminum???
Mark

Originally Posted by Boatmik

Great reminder MAM. Have you got some pics of the junk rigged boat?

Michael

I have an entire album but have never scanned the prints. I'll see if I can get my yf to scan a few of them for me.

Not 6061. I believe 6063 is the most common alloy for flagpoles. They can be purchased anodized or bare--I got bare and painted them.

Are the Flag poles 6061 T3 Aluminum???
Mark[/QUOTE]
The alu tubing available in Sweden is now 6063-T6. I just bought some.
See Wikipedia for the difference. Probably not much for us.
6061 aluminium alloy - Wikipedia, the free encyclopedia

• 6061 is widely used for construction of aircraft structures, such as wings and fuselages, more commonly in homebuilt aircraft than commercial or military aircraft.[5]

• 6061 is used for yacht construction, including small utility boats.[6]

• 6061 is commonly used in the construction of bicycle frames and components.
• 6061 T6 along with 7075 aluminium alloy is frequently used in the secondary chambers and baffle systems in firearm sound suppressors (primarily pistol suppressors for reduced weight and functionality), while the primary expansion chambers usually require 17-4PH or 303 stainless steel or titanium.[7] [8]

6063 aluminium alloy - Wikipedia, the free encyclopedia
6063 is widely used in the production of extrusions - long constant-cross-section structural shapes produced by pushing metal through a shaped die. These include "L" and "U" shaped channels and angles.
Peter

Varieties of Aluminum alloy:

6061T6 vs 6063T6

There's not much difference as far as we are concerned. Both have the same stiffness. 6061T6 has a bit (about 20%) higher tensile strength.

If I remember correctly, 6063 is easier to extrude/has a smoother surface/is easier to anodize.


More useful data is in these links below, of relevance is the Modulus of Elasticity (How stiff the material is), and the Tensile Yield Strength (how much stress you can put on it before it starts to give way).

6061T6

6063T6

Ian