Anonymous wrote:

Now for the mast, I can only get 6063T6 aluminium, for 12 m conical poles as well as cylindrical 6 m pieces...Unfortunately, the yield strength of 6063T6 is only 170 MPa compared to 6082T6 with 250 MPa (thus, 6082T6 is about 50% stronger than 6063T6).

According to this Gabrian Chart the 170MPa is for T5 (AKA T52 etc.) and T6 is 214MPa. In my case, I can only get 6063 T5 but I can 6061 T6 at 275MPa. I will note that these numbers seem to vary a bit from supplier to supplier's documentation but generally in the last digit. It seems T52 is similar in strength to T5 but age hardened.

However, lucky me, I thought, as I am offered two conical masts:

• 222 mm/ 5 mm tapered to 90 mm/ 7,5 mm, with the lowest 5 m being cylindrical
  
• 177 mm/ 4 mm tapered to 76 mm/ 4 mm, with the lowest 4 m being cylindrical

Did the manufacture say what they thought the yield strength of the alloy was? 222 (over 8inches) does seem high. I have the funny thing that I live in Canada, a metrified country and all the aluminum tube I can find is in inches. If I look in the US... where they use the kings units (well sort of, the gallon seems to be the taxed size not 10 pounds) are in sway, the tube sizes are all in mm. Go figure.

From a geometric view, the 177/4 profile would be even stronger than the 150/5 profile you suggested: Its section modulus is about 15% higher. Unfortunately, the strongly differing material properties crash it: The 177/4@6063T6 is 22% weaker than your suggested 150/5@6082T6.

Retry with the 214KPa for your T6. It would be interesting to know why they choose 6063 and not 6062 or the more common 6061. Like what the engineer's reasoning is for material choice. Maybe we could learn something. It may be that they use a cold forming process starting from tube and going to tapered and 6063 works better for that.

I will note that Poppy, a 31 foot, 4300kg longbow, with a ballast to displacement ratio of 44.68 has a 150mm mast (6inch). When I do the calculation for my smaller 3500kg Cal29 using 6061 T6 at 275KPa, I seem to need a full half inch in a 6inch tube (13mm thick) which means the bottom 20 feet alone would weigh 91kg. But a move to 8inchx.25 tube at 21ft only weighs 68Kg and is stronger too. So I wonder how thick the tube wall on Poppy is and what alloy too. I wonder how much the flex in the pole has to do with things.

One of the things Arne's calculations do not account for is mast mass. Another is mast flexibility. All of these calculations go back to wooden masts with generally much thicker walls, greater overall mass and a less uniform strength. I would also note that the original JR would have had wood battens and canvas sails which would also increase the effective mass of the mast. In many ways, looking at what has worked for other people is a good measure too.

The 222 mm would weigh 72 kg at the length I aim at. The 177 mm one would weigh 55 kg.

When I calculate the ratio of M_b,yield/M_r,max, the following (rounded) figures appear:

• 222ish mast: Mb/Mr = 2,6
  
• 177ish mast: Mb/Mr = 1,5
  

Now, given only these two options, if I want to stick to the guidelines from your chapter 6b, the decision appears to be pretty clear, doesn't it?

However, a 222 mm Alu mast appears really oversized to me, considering a 25 ft yacht. But this is merely a gut feeling, according to the numbers it would fit just fine. Any thoughts on this?

Even for my 29ft yacht. I keep wondering if I am doing something wrong in my calculations  :) I am beginning to think this is all guesses.

Hi Arne,

it has been some busy weeks, but things do progress. However, I am getting the feeling that availability of mast material is one of the main bottlenecks of my (or any?) junk conversion project.

I did the maths: Ilvy (Maxi 77) and Ingeborg should have quite the same righting arms (1050 kpm to 1032 kpm).This, although Ilvy's ballast ratio is at 40% and Ingeborg's at 58%. Sure, it is due to Ilvy being quite wide (you can see this clearly in practice: directly comparing a BM IF with a BM Maxi77, e.g. during a yardstick regatta, it can be observed that the IF is heeling quite a lot more).

Now for the mast, I can only get 6063T6 aluminium, for 12 m conical poles as well as cylindrical 6 m pieces...Unfortunately, the yield strength of 6063T6 is only 170 MPa compared to 6082T6 with 250 MPa (thus, 6082T6 is about 50% stronger than 6063T6).

However, lucky me, I thought, as I am offered two conical masts:

• 222 mm/ 5 mm tapered to 90 mm/ 7,5 mm, with the lowest 5 m being cylindrical
  
• 177 mm/ 4 mm tapered to 76 mm/ 4 mm, with the lowest 4 m being cylindrical

It is quite advantageous that both profiles are cylindrical at about their lowest third: this ensures that the diameter stays at max at the partners.

From a geometric view, the 177/4 profile would be even stronger than the 150/5 profile you suggested: Its section modulus is about 15% higher. Unfortunately, the strongly differing material properties crash it: The 177/4@6063T6 is 22% weaker than your suggested 150/5@6082T6.

The 222 mm would weigh 72 kg at the length I aim at. The 177 mm one would weigh 55 kg.

When I calculate the ratio of M_b,yield/M_r,max, the following (rounded) figures appear:

• 222ish mast: Mb/Mr = 2,6
  
• 177ish mast: Mb/Mr = 1,5
  

Now, given only these two options, if I want to stick to the guidelines from your chapter 6b, the decision appears to be pretty clear, doesn't it?

However, a 222 mm Alu mast appears really oversized to me, considering a 25 ft yacht. But this is merely a gut feeling, according to the numbers it would fit just fine. Any thoughts on this?

To worsen the decision making, there is Atropos, a junk rigged Alu schoner located in Kiel. She wears 35 m² on her main, with exactly this 177 mm mast I described above. Given her weight, scantlings, ballast ratio and so on, I calculate her mast to Mb/Mr = 0,8 (I roughly divided Mr by her sail area ratio of main mast and fock mast). That is quite a low figure, and I know that she sails happily now for almost 20 years, and is not afraid to hit bad weather.

Compared to Atropos the 177 mast on Ilvy appears conservative to me, but compared to your guidelines it appears way too optimistic. If you could share some thoughts and experiences - or anybody else - I would be very grateful!

Cheers,

Paul

Sidenote: I only found two companies in Germany selling 12 m long Aluminium profiles. They offered exactly the same masts to me, both 177 and 222, both manufactured by Nedal. However, the price difference is substantial. After I contacted Nedal directly, they linked me to one of those two - Alfa - the one with a really good price. Thanks to Nedal for their service, and thanks to Alfa for the effort of sending me technical drawings, double-checking alloys for me and just being very helpful.

Hi,

short update from my Maxi 77 conversion: over the last two months I played with some design concepts for the tabernacle, rethought, tested alternative interior ideas and rearranged priorities. Luckily, I found a way to step the mast onto the keel and thus omit the on-deck-tabernacle. Sometimes, limited time helps to straighten a plan...

The solution for us on Ilvy is to rearrange the interior to reduce the social impact of the mast being located right in the V-berth. This means to rip out the two lockers which divide the V-berth from the saloon. However, the aft walls of those lockers will stay in place to act as bulkheads. Due to the removal of the lockers, the V-berth can be lengthened aftwards, creating more space to sleep. Also the head can be shifted aft. Now the mast in the middle got to an acceptable obstacle in that room.

Limitation for the mast position are now:

- aftwarth: the hatch including the angled step

- forward: the forward wall of the former head compartment. This wall is part of the inner shell, and I would like to not cut that away.

This range is about 25 cm. With 35 m² of sail area, I am aming at a 20cm aluminium mast. Plus minus. Let's see if I can find someone to sell me one. However, this should fit snuggly into the given space.

Arne, if you won't mind I would just use the exact same sailplan as on that Kelt 8.50 rig you directed me to. Seems practical to copy a working design. However, do you remember what mast dimensions he used? It appears quite massive to me when comparing to similar junk rigs, but I may be wrong.

So much for now. I append some fotos documenting the mess that we created by now (we already removed the lockers and the Bermuda rig mast step including that bulky table.

Cheers,

Paul

Paul,
I had uploaded that Kelt-8.50 rig onto my JRA-page under Sailplans;
https://bit.ly/3GSvilq

As you can see, there is  a little camber, even in the top panels. 

However, if you want freedom to increase the mast balance to 26% (probably not necessary), then you must adjust the lengths of the batten pockets to fit this high balance.
If you want 30% mast balance then the yard angle must be lowered to 55 degrees...


Arne

Paul

Good to see that you have received some first hand experience with JR. That BB17 probably belonged to Sebastian Hentschel for a while, and of course, these go well to windward with a good sail.

The reason why I am a bit sceptical to the 35sqm rig on a Maxi 77, is that the Maxi is taller and the need for a tabernacle (10% of the LAP), brings the rig quite high up. The total length of Ingeborg’s mast is 9.7m, so the mast top is around 9.6m above the waterline, while the mast of that Maxi will reach over 10.3m up. This will also bring the CE higher up. I notice that when I sail my Ingeborg in a rising wind, the actual top speed when close-hauled or on a beam reach with 6 or 5 panels set is higher than when sailing  ‘7-up’.
Still, I don’t think the 35sqm rig will kill your Maxi. If you find it to be too tender, you can always chop off some of the mast and reduce the sail to 6 panels later. I did that once to my first junk-rigged boat, Malena (in 1992).
On the drawing below, I have shifted the mast further aft (..it could maybe be moved another 50-100mm...), but have not moved the sail. This increases the sail’s mast balance, which will make steering easier downwind, with the CE now sitting closer to the mast. To make the sail accept the increased mast balance, I have picked a modified master sail with a yard angle of only 60°. The sail I show you here is actually a straight 1:1 copy of the one seen on that Kelt 8:50, in the appendix of Junk Rig for Beginners. (http://goo.gl/vzGLzi )

Have a look

Arne

(Full size diagram on Arne's sketches, section 7-24)