Compressive strength of TPI masts

Compressive strength of TPI masts

Postby ordosays » Tue Sep 11, 2018 9:01 pm

Hello forum minds. I've been actively looking at purchasing a Freedom Cat Ketch/Schooner for a long haul cruiser and have been paying a lot of attention to the rig. I've read just about every thread on this forum about the good, the bad, and the ugly trying to weigh the potential weaknesses of the rigs.

Honestly, the biggest concern to me is their age. I understand that the scantlings were excellent for the time (they seem pulltruded maybe?), but seeing as how these weren't designed as column type masts and looking at what accidents have occurred (it seems that running a spin/kite is a pretty good recipe for a broken mast), I'm left with a pretty singular question - what kind of compressive strength do these masts have? Could "babying" the masts with a diligent set of running stays work well or create a problem?

Example: If I ran running stays/shrouds out of something like dyneema with quite a bit of tension (like 2-3000lbs), treating the mast as a column, could/should I? I'm thinking about this in terms of a safety factor in an aging system, when dealing with gusty/fluky conditions, and running "aftermarket" sails like spinnaker or a fisherman to get that last erg out. If I did run stays, where should it be secured from? It's a tapered mast so maybe not exactly at the mast head for shrouds? I know that laterally, these suckers were designed to take the weight of the boat, now what about in column? I raced on a triple spreader, rod rigged racer with multiple hydraulic stay adjusters and two sets of running backs, so hopefully understand how stays can effect both the shape (and make sure you don't lose a mast when running oversized head sails).

I'm hoping Eric Sponberg can help with this, but I wanted to turn this into a public post so that it gets archived. I'm very specifically looking at the TPI built F39 express/PH masts. Also I've been thinking about the fairly common practice of getting a sailboat off a sand bar by hauling on a halyard and how this is considered deadly to these masts. Would my "running shrouds" be sufficient to counter this?

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Re: Compressive strength of TPI masts

Postby ewsponberg » Tue Sep 11, 2018 10:27 pm

Hi Ordosays,
Although I designed the F39 masts, WAY back when, I could not tell you what the strength properties are. You'd also need the diameters and wall thicknesses to determine all the structural stresses, and I don't have any record of that. I want to say about 60,000 psi for the compression strength, but I cannot guarantee it.

If you want to fly a spinnaker, then I always recommend running backstays to keep the mast from pumping too much. Pumping puts a lot of stress cycles on the mast, and you'd like to avoid that if possible. The attachment of the running backstays should always be directly opposite the spinnaker halyard. That may require a modification to the mast to add appropriate tangs to attache the running backs.

Cantilever bending, as they were designed, is a much severer loading that pure column loading. However, you do want to pay attention to the mast staying in column. I would hesitate to careen a boat without using some kind of back-up stay to keep the mast in column when careening or pulling a boat off a reef.

Over the years, the masts have held up well. I don't know of any Freedom mast failure that was due to general fatigue or aging. There have been a number of explained failures, such as hitting a bridge, getting run over by a fork lift (you'd be surprised how often that has happened!), or by doing an unauthorized modification--saw one of those not too long ago.

I hope that helps.

SY Corroboree
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Eric W. Sponberg
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SY Corroboree
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Re: Compressive strength of TPI masts

Postby ordosays » Fri Sep 21, 2018 3:19 am

Thanks for the reply. Food for thought.
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Re: Compressive strength of TPI masts

Postby mike cunningham » Fri Sep 21, 2018 1:58 pm

Eric, thanks very much for your contributions to this forum.

I was wondering if you could expand upon "unauthorized mods" what kinds of mods are you referring to?
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