[Rhodes22-list] Crane question

[ROGER PIHLAJA]

Hi Allyn,

OK, I’m going to put on my engineer’s hat and do a safe load calculation for you.

Assumptions:

  1.  Model the tube as if we were buckling a theoretical column with the bottom end fixed and the top end free to move out of column
  2.  The column can buckle in any direction
  3.  D = 2.00 inch, t = 0.125 inch, L = 6.00 feet = 72.0 inch, [Pi] = 3.14159
  4.  The MOC is 6061-T6 Al Alloy     [Modulus of Elasticity] = E = 10000 ksi = 10000000 psi
  5.  [Safety Factor] = 3.0
  6.  [Moment Of Inertia] For A Tube = Ix = Iy = ([Pi] / 64) * (([D] ^ 4 – ([D] – (2 * t) ^ 4)) = 0.3250 inch ^ 4
  7.  Mast is moved slowly, so dynamic loads don’t need to be considered

With all these assumptions, the [Critical Axial Compressive Load] = [Pcr] = [Pi] ^ 2 * E * I / (4 * (L ^ 2)) = 1547 lbs

[Psafe] = [Pcr] / [Safety Factor] = 516 lbs in compression

You would need to do a statics calculation to determine the worst case compressive load on the column.  I don’t know your gin pole geometry, so I don’t have enough information to do that calculation.  My guess is, this is probably OK; but, its close enough that, I would do the calculation before I tried it.  Also, note that this is assuming failure by buckling under a pure compressive load.  If the aluminum tube moves out of column even a little; then, things will start to go crocked in a hurry!  I would want to see some sort of guy wires or a sacrificial assistant to hold the tube in column.

Hope this helps, good luck!

Roger Pihlaja
S/V Dynamic Equilibrium

From: Todd Tavares<mailto:tavares0947 at gmail.com>
Sent: Tuesday, March 30, 2021 10:08 AM
To: The Rhodes 22 Email List<mailto:rhodes22-list at rhodes22.org>
Subject: Re: [Rhodes22-list] Crane question

Allyn,

The .125 wall thickness aluminum tubes should be adequate if the diameter
is large enough > 1.5 or so.
Raising from the stern raised cradle using the GB mast raising system pole
is much easier and less risky than raising from the bow using the boom as
the gin pole.
There are plenty of pictures of the GB mast raising system on the web and
this list if you have the skill and desire to make a clone for yourself.
The GB system uses a winch with friction brake, mounted to the top of the
capitol mounted gin/crane pole. I use the trailer winch.
I can post some pics if I can dig them up.

Todd T.


On Tue, Mar 30, 2021, 1:04 AM Graham Stewart <gstewart8 at cogeco.ca> wrote:

> Allyn:
>
> I assume that you have the standard mas. These older boats raise their
> masts using the boom as you mentioned. I do that twice every year.
> I have attached the manual for these older boats that explains, in Stan's
> inimitable style, exactly how it is done.
>
> Graham Stewart
> Agile, Rodes 22, 1976
> Kingston Ontario
>
>
>
>
>
> -----Original Message-----
> From: Rhodes22-list [mailto:rhodes22-list-bounces at rhodes22.org] On Behalf
> Of Allyn Baskerville
> Sent: Monday, March 29, 2021 11:23 PM
> To: rhodes22-list at rhodes22.org
> Subject: [Rhodes22-list] Crane question
>
> My boat, which was a tad over $2,000, didn’t come with a winch and cradle,
> and genuine ones are most unfortunately out of my budget. I have access to
> three 2” diameter x 6’ length x .125” wall thickness aluminum tubes.
>
> I have a question though. Is a 1/8” wall thickness adequate for the crane?
>
> I’ve heard of people using their booms as gin poles to raise their masts,
> so this is definitely thicker.
>
> Thanks
> Allyn
>
> Sent from my iPhone
> -------------- next part --------------
> A non-text attachment was scrubbed...
> Name: Rhodes 22 Manual.pdf
> Type: application/pdf
> Size: 2853572 bytes
> Desc: not available
> URL: <
> http://rhodes22.org/pipermail/rhodes22-list/attachments/20210330/7679325c/attachment.pdf
> >
>