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.)
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.
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.