Sunday, September 14, 2014

Aft Mast Design

While the forward mast will be waiting till next Spring to be created, we have pretty much finalized our aft mast design:

The mast will be stepped on deck, just aft of the wheelhouse and will serve multiple functions: electronics mounting, paravane stabilizing rig support structure, 1000lb boom lift to aft deck (enough capacity to lift our Cummins 6BT main propulsion engine in and out of the boat from a dock alongside), lookout platform 16ft above deck, and about 120 square feet of auxiliary propulsion/steadying sail. In order to eliminate deck interference from wire stays and shrouds, we are using fixed diagonal pipe supports leading to padeyes on the wheelhouse roof. These will work in both compression and tension to support the rig (wire only works in tension). I also will need to add two wires from the mast head to the front of the wheelhouse roof to add necessary support for heavy lifting with the boom. We had the rig modeled, then plugged in different pipe sizes, gussets, etc. until we found a combination that wasn't too crazy heavy but would also not come apart under heavy loading.

After the modeling exercise, these are the scantlings we arrived at. The only changes I made was a switch to sch 80 (heavier wall) for the 2" diagonal mast support pipes, and changing the crows nest railings to 1" to reduce windage and weight. This is view from overhead looking from slightly forward on the stbd side. The wheelhouse and hull are omitted so you have to use your imagination a bit here. The triangular structure on the far (port) side is the paravane rig with hinge pints on the side shell, port side. The stbd paravane pole rig is omitted too.


Here's some pics of the model under load. Blue is low stress (good), red is highly stressed (bad).

First set shows boom lifting 1000 lbs under the following conditions:
straight aft, 45 degrees off center line, and 90 degrees/athwartship

This next set shows the Paravane loading, which puts the most load on the rig (upwards of 5000 lbs) on the paravane down rigger pole anchor points about 16 ft up the mast:

So, the calcs say we're good. I rounded a few things up strength wise to agree with my own "armchair engineering" and now all we have to do is build and install the thing. I started this weekend with the pipe bender at work forming up the upper and lower rails of the crows nest platform. I also came up with materials and design for the paravane downrigger pole hinges. Will be spending some more time in the shop after work this week so that we can start welding stuff together next weekend.


  1. Impressive Peter. I admire how you approach and design your work.


  2. Thanks Conall. Ditto on your build.

    Are you still planning on building a mast/paravane rig for your boat?


  3. I'm planning on copying your work once I haul out for the winter, so make sure you get all the figuring out done for us :-)

  4. It is hard to find good ideas to steal for this kind of hybrid sail/lifting/paravane type rig. I was hoping to copy you on the details. Guess I'll be the guinea pig on this one...