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

[airturb]

Hi All.

I'm playing around with the design of a mast. I'm just wondering how do you go upon calculating how much force is transmitted from the sail to the mast, the distribution of the force on the mast, and things related to this topic. Any help would be appreciated.

Thanks,

Artur

[Stephen Ditmore]

For preliminary calculations I'd assume even distribution from boom to masthead from the mainsail. Then there's both forward and transverse force imparted by the headstay, which is usually countered by shrouds or shrouds and runners, converting it to compression. The total transverse force on the sails times the heeling arm will be limited by the boat's maximum righting moment. Finally, there's compressive force imparted by the halyards and force imparted by the boom from outhaul & vang tension.

[MikeJohns]

The actual forces on the rig vary so much that they are nigh on impossible to predict with anything other that ballpark figures. Sail angle wind strength sea state, boat stability, roll inertia. Thats why the rules are used or we develope our own rules

Dynamic loads are often higher than the sail generated loads. The whole exercise becomes very complex indeed.

[Thunderhead19]

Ahhrrgh! Sail loads! It's Black Magic!....Burn him, he's a witch!

[airturb]

Thanks for the encouraging news)

[water addict]

for basic rig design check "Principles of Yacht Design" by Larsson & Eliasson.

[airturb]

Yup,I've got a copy of "Principles of Yacht Design",but thanks anyway.

[water addict]

PYD should be good enough for basic design. Are you looking for more in-depth analysis?

[airturb]

The thing is, PYD shows you how to calculate point loads at the center of effort of the sails. I'm wondering how would I transfer these loads onto the mast, at the same time converting them into a distributed load. Is it sufficient to replace the point load at the center of effort of the sail with a point load and a moment at the mast??

[tspeer]

Here's an example by Eric Sponberg: http://www.sponbergyachtdesign.com/Articles.htm. He assumes uniform load for the main. This is probably a conservative assumption, intended for the purpose of ensuring the rigging is adequately sized.

If you have a different objective, then you may need to get some actual test data on comparable configuration or go to sophisticated analysis techniques.

[airturb]

I appreciate the input. I came across that particular website. However, I'm toying with the idea of a free-standing mast. I was siffting through the different threads on mast design, and i found a particular response that mentioned a book "skene's yacht design", in which they assume that the load on the mast is a parabolic function. I'm going to try and look into that.

[tspeer]

This spreadsheet, http://www.tspeer.com/DesignTools/vortex95.xls, may be of some help in calculating a more realistic spanloading for a cat rig.

It won't tell you the division of load between main and jib for a sloop rig, however. For that, you will need to go to a vortex lattice code at a minimum. Most panel codes include the same capability. See:
http://www.ae.su.oz.au/aero/vlm/vlm.html
http://www.wb-sails.fi/news/95_11_MacSail/MacSail.html
http://www.curtin.edu.au/curtin/cent.../papers/prc98/
http://www.flyg.kth.se/divisions/aero/software/tornado/
http://www.casde.iitb.ac.in/MDO/modules/
http://www.desktopaero.com/LinAir1.4CatalogPage.html
http://www.aerologic.com/cmarc.html
http://ciurpita.tripod.com/rc/wing/air_db/wing.html

[brian eiland]

Originally Posted by airturb

The thing is, PYD shows you how to calculate point loads at the center of effort of the sails. I'm wondering how would I transfer these loads onto the mast, at the same time converting them into a distributed load. Is it sufficient to replace the point load at the center of effort of the sail with a point load and a moment at the mast??

You've probably seen this posting already, "Sail Loading on the Rig, Rig loading on the Vessel", but in case you haven't here is the reference. I never did get the time to pursue this tread further.....maybe sometime this winter.

[MikeJohns]

This really is not trivial static engineering, without a thorough course in Dynamics and some knowledge of systems higher order differential equations you havn't a hope in predicting the rigging loads in anything other than a flat calm. If you come up with static figures then what factor of safety do you apply to account for the dynamic loads? Suddenly you have a fiddle factor and you are straight back to a rule of thumb.

If we can measure or find similar figures for the dynamic loads then it can be a much more exact science.

To the point of tedium I would like to stress that its the dynamic loads that you should be concentrating on with conventional rigs.

For enginering purposes of mast and other riging components the largest possible load should be used. For some components this might be in the spinnaker or jib only condition; for other components the fully reefed main only needs to be considered.

For enginering a rig multiple loading conditions must therefore be analyzed.