The OGT MkII, wind-vane steering

Now, I hastily typed together this little write-up, called

«20190104 'Otto', a wind-vane steering gear

(type OGT MkII designed by Bill Belcher)»

found under Letters in ‘my’ page:

https://junkrigassociation.org/arne/

Since it was inspired by the needs for a vane gear for Jami Jokinen’s Gallion 22, I suggest any comments on this is written on the "Galion 22 conversion" thread

Arne

Hear, hear,

a supporting vote from one summer's experience (and no theoretical understanding). I have an aluminium/aluminium mast of 6+3,2 meters and 600-700mm bury, glued using grp belts and epoxy. 

I have not been kind to my mast on this first season, but I have had zero doubt about the integrity of the joint.

James,

An engineer’s job is to make use of the scientists’ basic data and formulas, and try to make useful rule of thumbs out of their information  -  plus from practical experience. This lets them later design things much faster than if they had to go into 'deep science mode' all the time. I can imagine, when real new ground has to be gained (like when building the Concord SST) that scientists and engineers had to work hand in hand. We are not there with our junkrigs...

Engineers often get it right, but it has happened that bridges, houses, aeroplanes and even sailboats’ masts have fallen down because the formulas and rules have been stretched too far.

Still, I feel confident that my home-made mast joint rule is conservative enough to be safe. Thanks to Graeme K, who did that testing of epoxy on aluminium, I am sure the mast will not suddenly ‘go telescopic’, even with no screws or bolts added.

Cheers, (..and cheers and good luck to Mark...)
Arne

Thanks, that makes sense.  I wasn't expecting "high science" but really what gave you a feel that this was the right amount.  Unfortunately I'm no engineer, so I was quite nervous designing my mast. 

I'm trying to think what the failure modes are.  What I can come up with is:

(1) topmast breaking/deforming at the top of the main mast

(2) topmast trying to pivot around the top of the main, and deforming the main in the area of your lower GRP belt (heel of the topmast pushing out the alloy tube of the main)

(3) topmast trying to pivot around its base, and deforming the top rim of the main mast, effectively ripping apart the alloy tube

(4) differential flexing between top and main, leading to failure of the glued joint

Any other failure modes? 

I think the PJR formula is against (1), as the partners are assumed to be solid and immovable.  I think I was more worried about (3) and (4).  One reason I chose to have a close-fitting topmast, glued the entire length of the overlap, was to try to make them as much as possible behave as a single, solid unit.  (Another reason was that I didn't think of using GRP belts!).  I agree with you that the longer the overlap, the greater protection against all of those failures.

Sorry if I seem to be thinking aloud at you - I'm finding it very helpful to understand what's going on with a hybrid mast!

Thanks, Arne, for another excellent technical guide.  I don't know about armchair reading, but it's a good breakfast table read!

The mast for River Rat is very similar to what you describe, but I made the topmast from aluminium tube half the diameter of the main part, sheathed in wood:  

I didn't know how much overlap to allow. My first thought was that if you regard the topmast as a mast, and the main part as a tabernacle, then I should use the PJR suggestion of at least 10%.  The length of the top part above the main part is 3m, so should be 30cm.  But I got nervous, as I wasn't sure how the forces would affect the 100mm dia tube of the main part, so went for a full 1 metre.  In hindsight, this is far too much, creating a lot of extra weight aloft.

Your rule of thumb suggests a bury of 4-5 times the inner diameter of the lower tube.  This would give 40-50cm.  This feels about right to me.  But I'm wondering whether there's any science behind your suggestion? 

(I should warn you I have some particularly mad mast ideas for my next conversion - but they're best kept for another thread!)

James