Thread: Developing a WIKI for storerboat performance - lugs, sprits, foils &c

Hi Mik, can you PM or email me the details of how to get on and help editing the wiki?

Given that we have such a good oportunity to collect some useful data on things relevant to performance of these boats, could we add another couple of data columns to the spars and mast data tables:

For the mast it would be good to know the balance point as a distance from its base. Important as weight up high/weight reduced up high has much more influence than the same weight changes down low.

For the yards, it would be good to have a circumference measurement as well as the diameter.

Whenever I do theoretical stiffness for round spars from section and material information, I get stiffnesses quite a bit lower than those that come from the measured deflection with a weight calculation. On the other hand, the same calculations done on square/box sections agree remarkably well.

I presume this is mainly due to the round section being a bit bigger than round as its been rounded by hand, not on a lathe.
A circumference measurement would sort out if the round spars are actually bigger than round.

Thanks,

Ian

Hi Ian,

I have sent you an email from Googledocs I don't know if you have to start a google account or not.

The anomaly between the calc for square and round is likely to be a manufacturing difference .. good point about the diameter being possibly a bit different. Can you give a rundown on the calc - ie verbal. Are you allowing for the taper? Just looking at deflections from the longitudinal strains or looking at the shear as well - not that I would expect differences from round to square - just so I know your methodology.

See how you get on cribbing in some more columns ... can't go any wider than this or it won't fit on the webpage. I have saved a copy just now so if you make an accidental mess I can recover. You can probably save a copy too before you start.

If you can't fit the columns then make a separate table.

MIK

Originally Posted by Boatmik

The anomaly between the calc for square and round is likely to be a manufacturing difference .. good point about the diameter being possibly a bit different. Can you give a rundown on the calc - ie verbal. Are you allowing for the taper? Just looking at deflections from the longitudinal strains or looking at the shear as well - not that I would expect differences from round to square - just so I know your methodology.
MIK

Hi Mik - assumptions/methedology I'm using:

I'm making no allowance for taper, I haven't found a reference to this and haven't the motivation/time at the moment to try and derive the beam bending equations for a beam where stiffness varies along the length. I have a feeling that the kind of tapers we use on spars doesn't affect the deflection by much as they're taking material out of the part of the spar that is under less load anyway. I have the opportunity to check this as my carbon yard is tapered at one end only, so I'll try and hang the same weight off either end and see how much more the tapered end bends.

I'm just using the standard beam deflection with weight and stiffness formulae, here is a reference for a beam that is supported either end with a weight in the middle:
Structural Beam Bending Equations / Calculation Supported on Both Ends Single Load at Center## - Engineers Edge

These don't take account of shear forces, but in a beam much narrower than it's length and where bend is also small in relation to length, these are not significant.

The formula for max deflection (d=WL^3/(48EI) has an EI (stiffness) value in it, I'm calclulating I (second moment of inertia) from the formulae for the different cross sections:
List of area moments of inertia - Wikipedia, the free encyclopedia

I'm using what I think are plausible E values for wood - about 9 GPa for Radiata pine, 10 or 11GPa for spar grade Oregon or Hoop Pine.

For carbon, I'm using E values quoted for the different laminates by my spar manufacturer - between 87 GPa and 95 GPa depending on the laminate composition.

Numbers agree very well with reported deflections with weights hung off the middle for Woodeneye's 60mm by 40mm by10mm wall thickness rectangular section boom, and for my round carbon yard, but overestimate the deflection of everyone's round solid section yards by quite a bit.

From memory a 40mm dia solid round section of 11GPa timber 3.6 metres long should bend about 70mm with 10kg hung off the middle, but measured deflections are about 50mm. I'll re-check calculations and post a table, but I'm thinking 40mm round is actually a bit bigger in average diameter because it starts from square stock and you finish rounding when it looks round. Other variation is perhaps due to how close to the ends the yard is supported, a small reduction in the span makes a bigger difference to the deflection due to the "span cubed" term in the formula.

Ian