how about an engineering forum?

Dionysis, assume the sail is rectangular, with an area equal to the real area, and the same luff length. This becomes a uniformly distributed load on the mast using whatever wind pressure will take the boat to max RM. Then apply a safety factor to the moments obtained and design to that moment curve.
Steve

 

Saildesign, bending moments not in the free version NPS program
but thanks for the great database, here a sample from it but what relation is there between diameter and wall thickness in bending strenght? and
is the sample not a bit overdone for a 300 kg load on 2 pipes 4 meter long and supported on both ends? its not so easy, the shop guy started looking like he was getting a toothache when i asked

NPS v2.2
========
Material - Aluminum Pipe

Outside diameter - 8.89000 cm
Wall thickness - 0.7619999 cm
Pipe length - 1.00000 m
Material density - 2.71277 g/cm^3
Flow area - 0.0042614 m^2
ID surface area - 0.2314097 m^2
OD surface area - 0.2792875 m^2
Linear ID volume - 0.0042622 m^3
Linear weight - 5.28313 kg

Fluid velocity - 1.00000 m/s
Fluid density - 1.0000000 g/cm^3
Volume flow - 0.0042622 m^3/s
Mass flow - 15354.86 kg/hr
Static weight -
fluid only - 4.26524 fluid+pipe - 9.54837 kg

 

seems a good idea

Steve.

A safety factor of 2-3 say. for an ocean going vessel? and then adjust the section inertia up the mast so it stays within allowable stress and deflection. Is that the idea?

thanks, dionysis

 

found a fairly good materials math site (gives f.e. bending loads on pipes i think) http://www.matweb.com that needs -free- registering for -not very active- forum. my respects for the pro NA / YD only rises, still wishing -easy- things were a bit simpler

 

A good web site that I use often is www.efunda.com. It requires membership, but you can try it out for free for a certain amount of time.

Michael

 

Eigenvalues vs pitch

Note that a free standing mast will have first mode eigenvalues (natural frequencies of bending) at a much lower frequency than a stayed mast. If the eigenvalues match the pitch / slam natural frequency, there could be severe amplification of stress, and potentially fatigue failure.

There is an article on use of FEA for vibration problems in the December 2002 issue of Desktop Engineering, www.deskeng.com.

 

thanks for the info

You have made a good point.

I would have thought though, that if the mast is stiff enough the first mode frequency would be attenuated to a marked degree.

Together with this, slamming frequencies will not be constant, so any resonance would be naturaly damped.

Nevertheless, I take your point, and will look further into this aspect of mast design.

Thanks for the pointer.

Cheers, dionysis.

 

Pitch RAO

You are correct that the boat will see a wide range of frequencies due to pitch, but it will have a significant peak in its pitch RAO. If this is in a frequency range close to those of typical wave spectra, and both are close to mast eigen value, you could have an interesting situation.

You can do probability based rainfall fatigue analysis for something like a mast pretty easily, because the frequency-stress RAo is pretty simple, and then see if the probability of survival for the lifecycle is acceptable. It's kind of an interestingf analysis too.

 

interesting

We are talking about amplification of stress due to pitch resonance here. Not the so called normal inertias experienced by any rig.

Would you agree that the range of wave spectra capable of driving the first mode of a mast, depend on the mast's length, and this in turn on the, (all things being equal) length of the boat.

So if the boat is large, say over 60 ft, and not necessarily that heavy, the pitch frequency will be quite low. Higher frequencies would be damped, together with the added damping due to hull form.

Add to this the damping effect of stressed sails, and the angles of attck etc.

Again, this is not to say that this aspect of mast design should not be considered. Far from it, the more I think about it the more important it seems to be. So thanks for that.

Another aspect is that, if the mast is tapered as it would be, it would introduce higher natural bending frequencies. This would also have an attenuating effect.

I must say I will need to study some eigenvalue/frequency mathematics to give some numbers to this argument.

Yes, it is an interesting kind of analysis.

cheers, dionysis.

 

by the way

Do you know of any readily available, specific and mathematical treatments on this subject, either on the net or in a library say. This kind of fatigue analysis has to apply to hull strength design in general as well.

Fatigue is one aspect of design that up untill now I have neglected.

thanks, dionysis.

 

Spectral fatigue

Here is a guidance document for floating offshore oil production systems that might be useful:

http://www.eagle.org/rules/downloads/104-FPSOSFA.pdf

 

thanks guest, and cheers again. dionysis