Analysis of stepping a Tanzer T22 mast with shrouds attached

Every spring and fall the Tanzer T22 mail group that I subscribe to has a conversation about stepping the mast when inevitably someone brings up doing it with the shrouds attached. At first glance this appears desirable to stabilize the mast against side-to-side movement. And inevitably someone else says “don’t do it, you’ll bend the chainplates” with the usual explanation that “the chainplates are not in line with the mast pivot”. I decided to finally have an analytical look at the situation, to attempt a definitive statement on why it is a bad idea, and explore a couple of options for stabilization with the shrouds.

On a T22 the chainplates are offset from the mast pivot (the bolt in the step) by 4″ in the forward direction and 3 1/8″ in the downward direction. That is, the chainplates are forward and below the mast pivot. If a 90° toggle is used between the chainplate and the turnbuckle then the downward offset is reduced to 2″ (if your toggle has a center-to-center spacing of 1 1/8″ like mine).

The critical metric to consider is the change in the distance between the chainplate and the shroud-attachment point on the mast as the mast rotates from vertical to horizontal. If the distance decreases then the shroud will go slack and no damage will be done. If the distance increases then the shroud will be stressed, applying a force to the chainplate, likely bending it or worse.

If the chainplate offset was only vertical, that is, it was in line horizontally with the mast pivot but below it, then the chainplate to shroud-attachment distance would decrease monotonically and the mast could be safely lowered without stressing the chainplates.

If the chainplate offset was only horizontal, that is, it was in line vertically with the mast pivot but forward of it, then the chainplate to shroud-attachment distance would increase monotonically. The chainplate would be stressed more and more as the mast was lowered.

But the chainplate offset is both horizontal and vertical – the vertical offset is helping, the horizontal one is hurting – and the net effect depends on the relative displacements of the chainplate and angular position of the mast. As the mast rotates from vertical the chainplate to shroud-attachment distance increases up to a maximum and then decreases, though not all the way to zero.

The link below points to a .pdf image of the Mathematica notebook that I used for the analysis. The graph in image shows this point. (Note that the X axis of the graph is angle above horizontal . The mast is horizontal at the left of the graph, vertical on the right.)

Tanzer shroud stretch

The maximum increase in the chainplate to shroud-attachment distance is 1.9″ and it occurs at 38° from horizontal. If there is a toggle at the chainplate then the maximum increase is 2.42″ and it occurs at 26.5° from horizontal.

This means that if you add an extension to the shroud of the above lengths you should be able to lower the mast without any stress on the chainplates. Of course, the extension must be free to rotate about the hole in the chainplate or toggle.

But not so fast. If you do add an extension of 1.9″ to the shroud (upper or lower), with the mast vertical the top of the mast will be free to move 20″ in either direction from center (25″ with the 2.42″ extension), a movement of about 3.5° either side of center. That’s exactly what we wanted to avoid by keeping the shrouds on. If your boat is not subject to rocking (no wind, waves, or weight shifts) then this issue is moot. The notebook for this computation follows.

Tanzer mast movement with shroud extension

Is 3.5° movement too much? I don’t know and the answer likely is: “it depends”. If you have play between the mast and the casting at the base of the mast (as I did until I replaced the pop rivets) then it is probably fine – the 3.5° rotation will be absorbed by the play. However, if your mast to mast base to mast step is all tight then the 3.5° movement will be transmitted to the step-to-deck connection deforming the deck and beam.

Also of concern with this 3.5° movement is the force of impact of the mast on the shroud as the mast reaches its limits of travel. That could be a lot of pounding if a boat happens to leave a good-size wake just as you’re about to get started.

For these reason I prefer the approach recommended by a member sometime ago: to use a hobby-horse spring instead of a rigid shroud extension. The spring is stiff enough to hold the mast from violent side-to-side movement, but flexible enough to accommodate the change in distance between the chainplate and shroud attachment point as the mast rotates down.

Disclaimer – I have checked my numbers thoroughly and reviewed it with my mechanical-engineer son. But it has been many years since I last used trig so take all of the above with a grain of salt. I welcome independent confirmation of the results.

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About wrwetzel

Retired EE / software developer. Hacker Maker Photographer Musician
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9 Responses to Analysis of stepping a Tanzer T22 mast with shrouds attached

  1. Jeff Bishop says:

    Bill,

    Yours is going to be a very helpful analysis for me. I’m going into my second sailing season and aim to avoid the difficulties of last year – no bent fittings or strained backs this time! I’ve been planning to add a lenght of chain between the outer shrouds to allow some lateral movement but also to provide security while the mast is raised. If the thing were to go sideways while we raise it, I’m not sure we’d be able to prevent it from going sideways all the way…

    Two questions:

    1. Does your analysis apply to the inner shroud or the outer one, or both?

    2. I’m not sure what is meant by a 90º tooggle (though my turnbuckles do have a ‘flap’ that rotates at the attachment point) but, in any case, I take it that a length of chain or a spring that would extend the shroud’s length by three to four inches would be about right. Is this what your analysis shows?

    Thanks in advance for your reply.

    Regards,

    Jeff

    • wrwetzel says:

      Jeff,

      I neglected to answer the second part of your second question. An extension of two to three inches would be fine according to my analysis. I strongly encourage you to use a hobby-horse spring, a couple of dollars at a big-box hardware store, and not a chain or wire or anything else that won’t give. Insert the hobby-horse spring between the chain plate and the top of the turnbuckle. You may have to use some chain or S-hook to get the length right. The spring should be under a bit of tension with the mast vertical.

  2. wrwetzel says:

    Jeff,

    The analysis applies to the outer shroud, assuming it is fixed at the tip of the spreader and is located directly above the chain plate. The extension required for the inner shroud would be slightly less because it is not parallel to the mast.

    A toggle is a small fitting with a flange on one end and a fork at the other. The fork goes over the chain plate and the turnbuckle goes over the flange. The flange and the fork of a 90° toggle are rotated 90° from each other. I use it to allow for some flexing in two planes, front to back and side to side. I tried unsuccessfully to fine an example of on at a number of marine suppliers. They may be no longer available as modern turnbuckles now include a toggle built in at the end serving the same function. Here is a photo at my Flickr site.

    As long as we are talking about raising the mast, here is a set of photos showing how I do it with a fixed A-frame.

  3. Jeff Bishop says:

    Many thanks, Bill, for the reply and explanations. Very helpful.

    Best Regards

  4. Maggie Langan says:

    Is there a way to determine if a mast that has some damage can be used?
    Tanzer 22 .

    Maggie

    • wrwetzel says:

      Yes, it can be determined but a lot of information is needed, some of which may be hard to describe without photos. Instead of even attempting to address the problem here I suggest two things.

      1) Post your query to the Tanzer Yahoo newsgroup. It is easy to join if you are not a member.

      http://groups.yahoo.com/group/tanzer/

      Include as detailed a description of the damage as possible, possibly including links to a photos. (Photos cannot be attached to Yahoo groups mail so you will either have to host them elsewhere or post them to the group site.) The Tanzer group includes members with far more experience and expertise that I have and are always willing to help.

      2) Ask a marine surveyor in your area. Mast damage could be inconvenient or it could be life threatening. It is nothing to fool around with.

  5. Jeff Bishop says:

    Hi Bill,
    Just thought I’d let you know I stepped my T-22 mast this spring using 5 1/2″ hobby horse springs for safety, attached directly to the chainplates and shrouds. Worked like a charm, although my pop top hatch does make bolt insertion a trick operation when four retired guys are trying to make fine adjustments to the position of the mast on a rocking deck!

    The spring ends fit tightly at the shrouds and the geometry seemed secure at the chainplates, so I didn’t take any precautionary measures. But, once the mast was up, I found that one spring had come precariously close to becoming unhooked. During this sailing season, I’ll be mulling over the possible ways to prevent the springs from inadvertently setting themselves free. The usual trick of using vinyl tape to hold things in place temporarily probably wouldn’t be appropriate in this case due to the forces involved. Any suggestions?
    Cheers,
    Jeff

  6. wrwetzel says:

    Jeff,

    Thanks for your note. It is good to have the length of a spring known to work.

    I have mixed feelings about tape as a solution. On one hand, as you indicated, tape doesn’t have a lot of strength. On the other hand, the tape only has to keep the spring from coming out of the hole in the chain plate or off of the turnbuckle. That requires a lot less strength than the tension on the shrouds.

    As an alternative to tape consider using a shackle with a thickness greater than the opening on the spring. That way you will have to open the shackle to attach/remove it from the spring. Or, consider using a shorter spring so that it is always under tension.

    • Jeff Bishop says:

      Thanks Bill. Just to clarify, the 5 1/2″ spring is between the shroud and the chainplate. The turnbuckles aren’t used on those shrouds, but one end is attached to the other pair of shrouds ready to be attached to the chainplates when the mast is up and before removing the springs.

      There is a lot of tension on the spring as it goes through the extension portion of the cycle. Enough, I think, to rip through tape and cause a potentially catastrophe.

      Cheers

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