The Top 50 Advantages of Junk Rig

Discussion in 'Boat Design' started by David Tyler, Jan 16, 2014.

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David TylerJ. R. A. Committee Member

Maximum GZ is 1.57 ft, 30 degree GZ is 1.18 ft (Autoship), displacement is about 8 tons, so about 28,200 lb ft max, 21,150 at 30 degrees.

If I were buying a mast in the USA today, I'd have to go for either 8" dia x 1/4" wall, or 10" dia x 3/16" wall.

Here's what I wrote for JRA members:

The right way to do it is to match the righting moment of the boat to the bending strength and stiffness of the tube.
If the boat has been designed using one of the modern suites of computer software, there will be a table of hydrostatic calculations available. These will include the righting arm at 30 degrees of heel. Multiplying this by the displacement of the boat will give the righting moment at 30 degrees of heel.
If the righting arm calculation is not available, and an older, bermudan rigged boat is being converted, the righting moment can be measured. The spinnaker halyard is made fast to a load cell or large spring balance, which is made fast to a pontoon or dock. The boat is hauled down to 30 degrees of heel using the spinnaker halyard and a winch, allowing the boat to move away from the pontoon or dock so that the spinnaker halyard is vertical. The load as measured by the load cell or spring balance, multiplied by the horizontal distance to the centre of buoyancy of the boat (approximately at the waterline) is the righting moment of the boat at 30 degrees of heel.
The strength of a tube is calculated like this:
First you must calculate the Section Modulus of the tube; this is given by

Z = 0.098(D4 - d4)/D
where D is the outside diameter, d is the inside diameter.
The stress at deck level is given by:

S = Righting moment/Z

(All of the above quantities must be in a consistent system of units, either imperial or metric)

Aluminium alloy is subject to fatigue. The lower the levels of stress to which it is subjected, the longer it will last. However, it has been found by practical experience that:

if the stress S at 30 degrees of heel is kept below 10,000 lbs/in2 or 69 N/mm2
for aluminium alloy grade 6063 T5 or T6
if the stress S at 30 degrees of heel is kept below 14,000 lbs/in2 or 96.5 N/mm2
for aluminium alloy grade 6061 T5 or T6

then the tube will have a long life.

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David TylerJ. R. A. Committee Member

That's way too big. My 8 5/8" x 3/16" wall mast (with a doubler inside of doubtful effectiveness, made from an offcut of the original too-long tube) shows no signs of falling over, after 80,000 miles of sailing.

A JRA member has just bought an old 40ft alloy boat near here. It has two 10" x 3/16" wall masts, and they seem to be in good shape.

For a 33ft boat with two masts, the biggest diameter you need is 8", with a wall thickness of 1/4", for maximum life, or 3/16" for adequate life. I'd choose the former, for serious cruising, the latter, for a "weekends and holidays" boat.

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bpwSenior Member

Ok, I am doing something wrong,

If I go to the beam deflection calc here:http://www.botlanta.org/converters/dale-calc/bending.html

And put in a round tube, 12 inches long, 12 inch diameter and .25 wall and then put 25,000 pounds of force on it I get a bending stress of 11,296 lbs, or just about 1/3 of the yield strength of 6061-t6

If I do the same with a tube of 8.625 X .1875 wall I get 29,237 bending stress, or just about the failure point of aluminum. So adequate but no safety factor for shock loading.

Where am I going wrong here? Its late and I am probably missing something easy. Probably should go to bed and worry about it in the morning.

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bpwSenior Member

Just re-did the math using your equations and got numbers that make more sense with your spar size.

9780 psi for your mast with 25000 ft pound RM

943 psi for the 12 inch x .25 wall section.

not sure what I am doing wrong with the beam calc.

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David TylerJ. R. A. Committee Member

The formulae as written down in the text attached to that java calculator are correct, so something is wrong with it behind the scenes.

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David TylerJ. R. A. Committee Member

... and this is actually a very good way to get into junk rig, if you're at all unsure of its benefits; and to experiment with new forms of it before you build something big and expensive. Get an old dinghy, of up to about 12ft long, and rig it with a simple straight tube for a mast, a sail made from polytarp or old bed linen, garden bamboos, polyprop rope from the hardware store... You can do all that for next to no money, and if it doesn't work for you, well, you've lost next to no money.

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In the 1960 Observer Singlehanded Race Jester (modified Folkboat with junk rig) placed second (of 5 finishers), taking 48 days to cross the North Atlantic. Cardinal Vertue (25' sloop) took 56 days and Eira (standard sloop Folkboat) took 63 days.

In the 1964 Observer Blondie finished in 37 days, placing 5th out of 14 boats and beating another standard Folkboat, Vanda Caclea, by 12 days.

For the 1968 race Mike Richey took over Jester and she covered the course many times thereafter, always very slowly. I believe this shows the skipper is a bigger handicap than the rig.......

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bpwSenior Member

I think I figured out what I was doing wrong, treating the mast 1 foot above deck as an unsuported beam taking the full righting moment when the mast above the deck is actually a couple feet above the point the boat rotates around.

Need to think some more to actually visualize the forces and what the equations are actually doing.

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David TylerJ. R. A. Committee Member

Yet if you put Jester into a round the buoys race, she'd do abysmally, being very slow to windward. This is one of the points that I'm trying to make, that in real life ocean sailing, whether racing or cruising, windward ability drops some way down the priority list. Sailing reliably, without fuss and dramas, is more important.

I don't think that Mike was ever actually racing, in the sense of trying to cross the Atlantic faster than the other guys. He was happier in the middle of the Atlantic than anywhere else, and didn't see much need to hurry to the other side.

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eyschulmanSenior Member

I have no experience or any significant knowledge of the Junk rig other than reading about them. What does alert me to the fact they may have some significant problem is that Junk rigs are rarely if ever offered as a option on production boats while many other off the beaten track rigs are or have been offered. So if these rigs are simple and cheap all desirable characteristics for a production builder what's up? I don't think it is a matter of selling it to the public since some of the other rigs offered are in the same boat and all the claims of superiority are just what marketing looks for or invents. So I keep having this nagging feeling that there is something wrong with the rig. I admit it could be a culture thing since it is common in Asia, but there has been a fair amount of time for a good thing to overcome that sort of thing.

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David TylerJ. R. A. Committee Member

Newbridge Yachts and Kingfisher Yachts both offered junk rig as an option, and a significant proportion of their customers took up that option. In the UK, there is a sizeable fleet of junk rigged boats as a result. I think you're seeing things from a very American perspective, forgetting that there's a place outside the USA called the Rest Of The World

Both of those went out of business long ago, and Nonesuch Yachts and Freedom Yachts ceased production long ago, too. So which production builders apart from the above would you put on your list as offering an "off the beaten track " rig? I can't think of any, let alone "many". Tanton, perhaps, but would you class them as a production boat-builder? The Aerorig is not currently offered as a production boat option, I believe, though it has been. The Omer wing sail hasn't really got off the ground.

No, the truth is that a builder has a hard enough job to sell his boat, against the competition, without adding the extra work of educating a public that is conservative - "I'm not going to buy anything that I don't understand already" - and herd-following - "I'm going to buy what everyone else does, so that I won't be taking any risks with my resale value". A boatbuilder can't risk going out of business due to offering rigs that he would have to spend too much time explaining to folks who know nothing about them. He goes for the easier option of "hanging up two white triangles", an idea which the boat-buying public is familiar with.

Really? Do you not agree that knowledge of a thing is a prerequisite for judging its worth?

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eyschulmanSenior Member

I am not saying that I know the rig is a problem. What I am saying is the lack of a more universal acceptance raises my suspicion that there might be a problem. And of course those who believe they know otherwise are therefor challenged to show me why my doubts are unfounded as you are attempting. I don't have to be an expert on a subject to ask why it is not more popular when so many advantages are claimed. If a particular product is on the market and does not get wide acceptance there is usually a reason and some times the reason or reasons relates to the nature of the product. Some of the posts defend the junk rig others point out the possible problems and so far the western world sailing market seems to be voting for other than junk rigs. possibly if the junk rigs have 50 aspects better than other rigs things will change. I am 75 years old and am not going to hold my breath to long on that one. I have no vested interest as to the nature of a junk rig, just stating what seems to me obvious. If I sailed one and I liked it I might use it. I used the cat and gaff rigs and liked them when properly applied. I have used fully battened main and jib on a rotating mast and liked that. Unfortunately my use of a junk rig is unlikely since I sail often where there are thousands of boats and I so rarely even see a junk rig under sail.

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motorbikeSenior Member

Junk rigs are not widely used for a reason, they evolved using materials and hull designs that have been superceeded. The modern junk rig is less junk and more lug moreover the cost advantage has shrunk somewhat when large engineered spars and synthetic sailcloth are used. Junks also have a "following" like bolger boxes, they have their place but the "followers" often are blind to the disadvantages, hence silly lists with 50 items. The conventional sloop is currently the best combination of value, performance, ease of maintenance etc. It is not the best in all conditions and its not the cheapest but all other factors being equal I would choose a conventional sloop rig for an up to mid sized boat before a junk.

Now for originality and style, plus the freedom to have a weird hull design with wild colors, I'd go Junk any day!

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rwatsonSenior Member

I am glad to see Peter Wiley noticed this thread, as he is building a Tom Colvin designed boat with a junk rig.

The very important points he made of cost benefit didnt raise much reaction, but it really is a very critical point for many cruising boats.

he has steel round section tubes for the masts, and the price of the good quality sail material, plus the ability to make the sails himself, are all very plus points if you don't want to spend hard earned money on 'luxury items'. It could be said that his savings went on a better performance, larger diesel engine - which for long distance voyaging, is very sensible.

Also, if he spent even triple the cost on a 'conventional' rig, the performance increase in a hard chine, heavy displacement steel boat would be hard to justify.

On the other hand, I remember talking to the owner of a cruising boat with very sailing friendly lines, and a good marconi rig, that placed a lot of importance on fast transit times between ports. As he pointed out, the sailing part was the tedious, dangerous bit of the job ( note - he had little helming weather protection) , and the ports he visited were his main objective. If he could cover another 120 NM ( say 5 knots over 24 hours, with better pointing ability, more efficient and lighter hull, the extra cost of both hull and rig were worth it to him.

Mind you - there are many who would say that sailing is one of the most expensive ways to get anywhere, based on time and cost - and a good diesel does the job even more efficiently.

As Par pointed out - you can only make decisions based on your priorities, not overall performance criteria - otherwise there would be no long distance sailing boats in existence, just cheap jet airfares.

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David TylerJ. R. A. Committee Member

Mr Watson,
Thank you for reminding me of the cost/benefit factor. Not a few authorities have said that if you only have a shoestring budget, or if you only have access to low grade materials, then junk rig will get you the best bangs for your buck.

I would put it like this: Efficiency is a ratio. If you care to define it as "miles sailed per unit of currency spent", or "VMG to windward per unit of currency spent", or "miles sailed per unit of hassle for the crew", or "miles sailed per unit of work done by the crew", then the junk rig comes out ahead. If you care to define it in other ways, then perhaps it won't.

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