mast design-how do you go upon calculating the effect of the sail on the mast???

As others have posted in this thread, the dynamic loading is way to complex to tackle in an engineering sense. If you are looking for design purposes, then using a "standard" that has historically worked is the only real approach. You can try to cut safety factors down if you are prepared to deal with the risks.
If you look at static plots of mainsail load, almost all of the load runs from clue to head in the leech with the small top portion (a few feet at most) along the luff transmitting load to the mast and halyard. The rest of the luff for the bulk of the middle of the sail carries almost no load until you get very close to the tack.

Again- what is your aim here? Are you doing more in-depth analysis for academic purposes or highly refined design? If you're just doing a design, and don't need to optimize to save 8 or 9 pounds, then use a standard approach.

 

Here is what I use for engineering a stayed mast. This is a modification of the process used in Skene's Elements of Yacht Design by Francis Kinney. Francis Kinney worked at Sparkman and Stephens (S&S), and they used this method for a long, long time. Since the advanced designs of the America's Cup came along in 1992 (IACC class), the process for a first pass at rig loads was changed slightly. Finite Element analysis is used now, which is beyond most designers' capability (mine included), requiring special talent in engineering and the FEA science. However, the following has worked well for me:

Take the boat's maximum righting moment load, which occurs at about 45-60 degrees of heel, and proportion that moment between the sails according to their respective areas. The jib moment will by a point load where the headstay attaches to the mast, times the moment arm from that attachment on the mast to the base of the mast. Some would argue that the moment should be balanced to the center of the hull underwater profile lateral plane, but then you end up with load assumed going through the hull and into the water. By figuring the moment to the base of the mast, you necessarily have a higher point load on the mast, which is more conservative.

For the mainsail, I distribute the load as an increasing distributed load up to the mid-height of the mast, and then a uniform (constant) distributed load from the mid-height to the mast head. Skene's uses a uniform load all along the mast. The method that I use places more load up high, which accounts for some of the leach tension in the mainsail pulling on the mast. The moment of the sail, equal and opposite to its portion of the hull righting moment, is the sum of the moments of the two distributed loads = load of the lower part x 2/3 of its length to the deck + load of the lower part x its center to the deck.

I hope you can understand the engineering terms I used, and can picture what I am talking about. The idea of using the hull righting moment greatly simplifies the problem and completely steers away from having to figure out the sail aerodynamics. You know that whatever the sail aerodynamic load, it has to be equal and opposite to the hull righting moment. Sail aerodynamics are extremely difficult to calculate, the hull righting moment is very easy to calculate with the hull design software. It is very tedious to calculate by hand. But software is very common now for the last 20 years, so we use the computer to calculate the whole righting moment curve. Therefore, we know where the maximum righting moment occurs, and its value.

I also use a factor of safety between 2 and 4 (2 for more aggressive owners who race and are concerned with weight up top, 4 for conservative cruisers who want the extra strength). This factor of safety is applied to the yield stress of the standing rigging and its various parts. That is, the loads are real, live loads, that are very likely to occur any day of the week. So when you calculate stress in the rigging and its parts, this is live load stress. You want the actual stress to be less than the live stress by your chosen factor of safety, so you size the parts to that lower stress.

I hope that's clear.

Eric

 

Eric
FEA is great for working out what the forces are on elements of a model given applied load(s). But the problem with rigs is that applied load.

I'd be lost without FEA now but it has an inherant danger in that the engineer may believe the output without putting in the correct numbers.

Like using the static forces from the sail loads but neglecting or not guessing correctly the dynamic loads; as happened to the mast in the NZ boat in the last Americas cup.

FEA is great for giving the lower natural frequencies of the rig and this can be really usefull when predicting some modes of failure.

So use FEA sparingly, embrace a simple rule that historically works. And use a good factor of safety, 4 is good, usually lets the rig dip in the water if you can wear the extra mass.

 

Mike,

FEA is like anything else in engineering--garbage in, garbage out. I have found that a good understanding of engineering mechanics and simple calculation tools (good engineering handbook, spreadsheets, etc.) go a long way toward defining where the high loads are. In the case of the America's Cup, the factors of safety are so low that you have to be very exhaustive in determining the loads--the hardest part of boat engineering.

I think that you can ignore FEA in the most common instances and rely easily on traditional engineering. All is forgiven if you have an appropriate factor of safety. This method has worked well for a few hundred years, so if it works, you don't need to fix it.

I will caution some designers that it is my belief that if you have to use a factor of safety of 5 or more, you don't know enough about the problem to solve it reliably. Do some more research to determine the loads. Figuring out the stresses after that is relatively easy. A FS of 4 max should be sufficient in most instances.

Eric

 

Sorry for a newbie question, since i am a not a designer of any sort.

But what do you exactly mean by "dynamic loads"? does dynamic mean the exponential increase in strain on the mast as weather conditions increase e.g. as wind,waves increase the rate of change in pressure on the mast?

or am i completely out of track?

 

It's the constant change in load (magnitude and direction) due to the constant change in wind direction and speed, waves etc.

 

Something that I find very instructive in all types of stress analysis wether it be hand calcs or FEA, is to reverse engineer known examples of the type of boat you are trying to build.

By reverse engineer, I mean obtain or measure the dimensions of known boats and do the calcs using this.

This is a good way to build up a database of appropriate safety factors, you can build on the experience of others.

 

The rules of the American Bureau of Shipping (and virtually all other classification society rules) for all kinds of vessels, are all based on reverse engineering, otherwise known as empirical data--i.e. data from the field or by experiment. The result is that they are fairly reliable, even if they are somewhat conservative (that's what you get from empirical data). If you want to engineer something with more refinement, then you have to go back to first principles or more complicated methods of engineering.

Eric

 

Hi,
I'm trying to analyze the sail loading on the mast when the boat is going in stern, equipped with the mainsail and the spinnaker.
I don't know how can I distribute the total spinnaker force between the mast-head and the outrigger boom..anyone could help me?
bye.

 

This one is difficult. There are a couple of ways. First, one method for the strength of the mast and rigging is to calculate the drag on the sail, and assume that the total of that load is borne by the halyard pulling at 90 degrees forward of the mast. Another method is to calculate the load and divide it by 3, so that 1/3rd is at the masthead pulling forward as before, 1/3rd at the outrigger pole pulling in compression (compression is worst, and sometimes it pulls in tension, sometimes it pulls sideways and the pole lays against the headstay. You have to check all conditions. Tension won't be critical, but compression in the pole and bending of the pole across the headstay are the two most critical loading situations), and 1/3rd is in the spinnaker sheet.

The load can be calculated as: Load = 0.5 x rho x A x V^2 x Cd, where, rho is the mass density of air, A is the spinnaker area, V is the wind speed in feet/sec or Meters/sec, and Cd is the drag coefficient (dimensionless) which should be some value between 1.0 and 1.5. Be sure to keep consistent units.

For the mainsail, you can use the distributed load as I described above, exactly as used in transverse loading, but orienting it fore and aft. The rationale for this is that although the relative wind speed on the mainsail is less when running downwind, the boat nonetheless experiences surging and pitching which can put unusually high momentary loads on the rig.

I hope that helps.

Eric

 

thank you very much!
I'll try both methods and I'll tell you the results.
bye

 

Hey All.

I appreciate all the input. Water addicted asked the question:

"Again- what is your aim here? Are you doing more in-depth analysis for academic purposes or highly refined design? If you're just doing a design, and don't need to optimize to save 8 or 9 pounds, then use a standard approach."

Well, I'm designing a free-standing mast as part of my 4-th year mechanical engineering thesis project. I'll be generating a CAD model using Catia, and doing the FEA on the mast using Catia. As i started out, I had the notion that boat design, and more particullarly mast design was more defined, and say not as empirical as it is. I could of guessed otherwise from taking several machine design courses as well several fluid mechanics courses/thermo courses, which are highly empirical, that mast design was going to have this similar feature. In any case, I guess the method I'm leaning towards is through the calculation of the maximum righting moment, and the distribution of the forces accordingly. I do have a question...in larsson and eliassons PYD, they say the following: "The starting point when dimensioning the rig is to calculate the righting moment. It is commonly agreed that a heel angle of 30 degrees is a good design angle. This corresponds to a reasonably high wind strength with the sails still generating high loads and the boat making good speed through the water. Letting the boat heel over more (ie using a higher righting moment), in reality means a slower boat owing to increased resistance, with a correspondingly smaller dynamic force."

So I'm wondering...should I use the maximum righting moment, or the moment developed at 30 degrees?

 

Sail & Rigging Loads

Just thought those persons on this thread subject should be aware of a couple of other interesting postings on this subject:

Translation of Section 1, of Chapter VI, of Pierre Gutelle's work from "Design of Sailing Yachts". Most of this work is common sense, but he does give a little insight into how the sails work in loading the mast and rigging, something that is missing from most other works.
http://www.boatdesign.net/forums/showpost.php?p=34686&postcount=33

Please find the remainder of Pierre Gutelle's Chapter VI posted with this message.
http://www.boatdesign.net/forums/showpost.php?p=35427&postcount=41

 

To Airturb,

I always use the maximum righting moment because there are times when the boat will heel to 60 degrees or so, not necessarily by wind action but by wave action or a combination of wind and wave, and therefore it is an expected live load. It would be interesting to see the results of your FEA analysis, if you care to present it, because of the off-axis loads that should appear. That is, as the mast bends, how much strain is there in the laminate in directions NOT parallel to the mast axis. How much fiber and in what orientations do you have to include in the laminate to carry those strains? It would be interesting to compare your results with my typical design practices.

Eric

 

To Mr. Brian Eiland

Thanks for the excerpts from Pierre Gutelle's, "Design of Sailing Yachts". It's a funny thing...I have a copy of this book, but it doesn't have most of the info. that You've posted. Maybe it's a different version??In any case, I'll be reading over those sections in a week or so in my well deserved reading week

Cheers.