Thread: Mast Flex question for PDR Lug rig?

Well.....Here is my results with two 45 lb dumbbells for weights.....

90 lb in the middle will flex my Alum mast 1.75" or 44.45 mm.

Using a string stretched at each mast end and supported between two workmate benches....with the bottom of mast being supported about 10 in. (250mm) up from bottom and the top about 2 in. from top of mast.

45 lbs (20.4 kilos) only deflected it 7/8" .

Hi Mark,
The mast of a balance lug rig should not have any bend at all. The yard should bend when you tighten the downhaul to flatten the sail. The round at the top of the sail should be cut to match the initial bend of the yard when you put a moderate tension on the downhaul to tighten the luff, which should be tight like a jib luff. If the round is not enough or the yard too bendy, the sail will be flat in moderate winds and as you tighten the downhaul to keep the luff straight in strong winds the sail will get vertical wrinkles. I know, I had to make a new, stiffer yard because of this. If the mast bends, it upsets the tension in the luff and the shape the sail, as the distance top to bottom decreases with the bend. If it bends, you could try to stiffen it with glued battens down the front and back and sides but you will probably need more diameter.
Peter

Mast bend with the balanced lug rig is of little concern--stiff or flexible, it just needs to be strong enough. Since the sail isn't secured to the mast by a track the mast can do what it wants with no affect to the set of the sail.

As was stated above, however, bending of the yard is critical as it is for the boom if the sail is lashed to it.

Umm...Thats Clear..Two different answers ......I was planning on using a open foot sail on the boom and using a 2 in. dia. Alum. boom and a 1.5 in. dia yard, both in .065" wall thickness..

Thats the set up that the winner of the World's used in OK....He was using a Polytarp sail though......I'm planning on going Dacron with a spectra luff rope.

If I should, or need to take the flex out of the mast, would a Schedule 40 PVC pipe slipped inside the top 5 ft of my mast, make it much stiffer....or would it be minimal??? just wondering if anyone has tried that??

Try the mast you have and see how it goes, maybe it is OK. The place where it is likely to break is at the partners, where the mast is supported by the deck. This could be reinforced by short piece of another slightly smaller or large diameter alu tube or a glass or carbon sleeve epoxied in the area. PVC is too soft. For the yard, as tubing does not change diameter, and you want the ends to flex more than the middle, and the greatest load is where the halyard is fastened, you could stiffen the middle where the halyard is fastened by glueing on a tapered wood strip, as was done with sailing canoes in the good old days. The boom of a loose footed sail has to be stiff or bending will effectively shorten it and put more draft in the sail just when you don't want it (strong wind).

Howdy...
For the lug rig ... the main thing is not too much mast bend. So you are OK. Main thing is reasonable stiffness without tooo much weight.

I would recommend that you speak to Brad Hickman if you would like further details. He has done the most development on the Alloy spar rig.

The PVC pipe solution never worked and never willl. PVC is way to flexible and it creeps as well - taking on a permanent deformation over time.

In the early days of the Puddleducks there was a lot of talk of curtain rod mast with PVC tube over the top to make it "stiffer". I am pretty sure that the now deleted old PDR forum ( had one person who actually checked the difference with and without the PVC tube and found little difference.

This was also a solution for very short spars back when PDRs had 40 or 50 square feet of sail as opposed to sails up in the 80s which were introduced by the OZ concept boats.

It's not the sail area that makes the spar bend because the boat will always capsize with the same load ... but the length of the spars that defines the bend.

Best wishes
Michael

Thanks MIK,
I did speak with Brad and that is the size of Al. tubing I'm using...I just couldn't get a 12' section shipped..they only would do 8 ft, so I spliced.

The yard is Brads size's, so thats good to know he has done a lot of research already .....that's the hard part .

Mark

Originally Posted by markmilam

Does anyone know "ABOUT" how much weight one would hang in the middle of the mast to see if its to Flexible or not for the Lug Rig 89 sq ft.??

I have a Alum. mast I made, but only could get 2.5" diam (.065" wall thickness) that was 8 ft long......I spliced another section of 2.25" diam.(.065 wall thickness) to it.....to give me a total lenght of 13'6".........the 2.25" dia. mast sticks out of the 2.5" about 5 ft.

The splice is 18" long where the 2.25 goes into the 2.5.

I need to make another mast for another PDR, was wondering if it would be better to try to secure some 2.5" that is 13 feet.....OR

to use some 2" with thicker wall (.12"), which would be twice as thick and that mast would weigh 11.68 lbs....... The mast I have now Is about 9 Lbs.

Sort of looking for a reference like, " a 30 lb weight hung in middle will deflect mast 3 in.".........or something like that kind of reference...just know if I'm close or not........I know the Downhaul on a lug has to be tight and didn't want the mast to flex to much in a say, a 18 kt wind, to loosen the down haul to much.....If the flex even would do that???

Thanks for any info
Mark M.

We should be able to get a pretty accurate theoretical comparison between the stiffness and strength of the alu sections you mention above, and the original wooden mast specified in the plans. Can someone tell me the dimensions of the wooden mast from the plans, basically just the diameter at the base and whether its solid round, solid square, hollow round or hollow square. I'll work out how stiff that is and give you a comparison to the alu sections you mention.

Ian

Ian,
The Wood mast is 3888 mm long...Im not real good at understanding what the plans mean.....Hence why the Alum. Spar

But I'll try my best: Looks like he recommends to make it Round, but build it Hollow.....and the diameter at bottom is 62.0mm and the top is 49.6 mm....then the spar gauge has stuff like 80.0 mm guide, but I wouldn't know what the top would be .......I could be wrong

.It's all so confusing to someone that never built a mast.....I've seen them already built and admire the workmanship.....does that count .

Ahhh....my old ugly aluminum, always there for me, no knots, easy to visualize, simple dem. Length, wall thickness and diameter...all you need to know ........

This photo was taken by Kevin Hahn at the 2011 Worlds. I've enlarged it to the point it's not very clear but you can get an idea of how much my mast flexes with gusts in the mid 20 mph range. the bottom 12' of the mast is 2.5" OD and the upper 1.5' is 2.25" OD, both are 0.065" wall 6061-T6.

Originally Posted by BradLH

This photo was taken by Kevin Hahn at the 2011 Worlds. I've enlarged it to the point it's not very clear but you can get an idea of how much my mast flexes with gusts in the mid 20 mph range. the bottom 12' of the mast is 2.5" OD and the upper 1.5' is 2.25" OD, both are 0.065" wall 6061-T6.

Hi Brad, thanks for the specs on your alu mast, if I know its 6061-T6 alu alloy I can get an indication of strength as well as stiffness. I assume Mark M's mast is probably the same 6061-T6 alloy.

In the meantime, can anyone confirm the exact specs from the plans of the wooden sections at the base of the PDR mast. I'm assuming they are usually round at 62 mm diameter, and could be done hollow at about this diameter, but I have some recollection of there being a spec for a square hollow section.

I should be able to put the numbers through my spreadsheet this afternoon.

Ian

Hi Ian,

PDR mast is square with 12mm walls. 62 is correct Using something around the density of hoop pine or oregon. Similar taper geometry to the GIS. ie assume the mast goes to the top and then cut off where needed (on paper ... not practice )

MIK

Hi, thanks for that Mik.

OK, some rough numbers:

A solid round wooden mast of Oregon or similar 62 mm diameter at the base will have a stiffness of about 9 kNm^2.

Mik's 62 mm hollow square section with 12 mm wall in wood will have a stiffness of about 13 kNm^2

The 2.5 inch outside diameter 6061T6 aluminium section with 0.065 in wall will have a stiffness of about 10 kNm^2.

Don't worry too much about this kiloNewtonthingimy business, what it tells us is your Alu mast will be a little stiffer than a round wooden stick, but not as stiff as Mik's hollow square mast.

If I compare ultimate bending strengths of the two masts, your alu mast looks a little weak.

If I convert to something easier to understand, if we were to put the PDR on it's side with the mast in place, and compare how much weight we could hang off each mast one metre above the partner:

Your alu mast would likely break at the partner with 136 kg hung one metre from the partner.

Mik's hollow square mast could take about twice as much before it breaks.

So, if you are 220 lb or more with long legs and the abs and the motivation to hike out to the max going to windward in a near gale, you might come close to breaking your aluminum mast, if you are not so heavy, or don't hike quite that hard, you should be OK.

Weight comparison:

The aluminum section is about 40% lighter than the solid wood one, but fairly close in weight to the hollow wood square section.

Hope this helps, I'll run the numbers through for the smaller 2.25 in OD/0.065 in wall alu section when I get the chance.

Ian

PS: Detail for the technically minded:

I got specs on the alu from here. Elastic Modulus of 69 GPa, modulus of rupture of 276 MPa

I based the wood calculations on an E of 13 GPa and a modulus of rupture of 80 MPa

Just added all that to the lug rig wiki
WIKI for setting up and tuning Lug and Sprit Rigs | Michael Storer Wooden Boat Plans

MIK