Ring frame for mast support

Hi, first post! we are shopping for a yacht to live aboard, (we have previously owned a 38ft steel yacht) and the boat of interest currently is a South Coast 36. It's fairly common in Australia, grp, 36ft with a centre cockpit. The biggest issue I have with the boat layout is that the companionway ladder is steep and dumps you right into the mast support post. Am I crazy to consider a ring beam to take the mast load/ Or even a beam and two posts spread further apart? Here is an example of the interior, the support post is in the post directly in front at the bench end. Not a good photo I know sorry.

 

Welcome to the forum Warby12.

As always it "depends", and upon many factors. However, in general, no is the simple answer. This is because what is holding up the mast and what is supporting it when under load?

If you had the mast terminating on the main deck, this becomes an "encastre" cantilever beam, this has serious implications for the mast and its own performance, let alone the structural integrity.

A simple way to describe this is thus:

If you had a pole say 1.0m long and at the base place one hand and then another hand at say 300mm from that end (where your other hand is). If you applied a load at the free end, your 2 hands can resist this load which is now a bending moment.

If you now slowly moved your hand, which is 300mm higher, closer and closer to the hand that is at the base, it becomes hard and harder to prevent bending and deflection. The load going into your hands gets higher.....the lower hand, is the keel of the boat, your upper hand, is the main deck. If you remove the base, from the keel, all you now have, is one support just like hand on the base of the pole!

Does this make sense?

 

Hi Ad Hoc and thanks for your reply! It's a fantastic forum! I understand what you are saying and agree that side loads would be unsupportable (I googled cantilever beam" but how does this differ from any normal deck stepped mast? (I have read other posts on here and do not want to start any deck stepped vs keel stepped controversy ). Maybe I'm not being clear I'm wondering about support as in the left image attached.

 

As I noted above, it depends.

You can of course do this, if you desire. BUT, and this is the point. If the hull is designed like this from the outset i.e. a blank sheet of paper before the boat is built. You can arrange, structurally speaking that is, what ever you like. You have to runs the numbers to establish the loads being imposed on the structural arrangement you desire. You have to satisfy yourself that there is sufficient strength, load paths and of course fit for purpose and a long fatigue life, not to mention ease of construction and maintenance and of course, basic longevity of the structure. All this will have been done (or should be) with a new design...it is "designed" that way.

What you are now doing is altering a major structure part of the boat, but, what loads are being imposed, what structural load paths do you have are the attachments sufficiently stiff and have sufficient shear area and so on..

Thus, you require major, not minor, major modifications to a boat to do as you suggest. And it requires a significant amount or detailed design to understand the implications...which in effect means, designing the boat as it is (running the numbers for yourself), to find out what the loads are and satisfy yoruself...yup..the mast, as it is, is fine, the hull is fine, the keel is fine and shear and buckling is fine. And from that, using these assumptions and calculation to then "redesign" it for how you want it.

If this is designed-in, from the outset, it is easy...if it is a redesign, not so, and not something i would recommend without seeking proper professional advice.

You may find this of some use too:
http://www.classicmarine.co.uk/Articles/masts.htm

 

Fair enough and I do appreciate your help. The good thing is now I can put this out of my mind as a slightly silly idea. Possible but not advisable. I am not a designer and it might be better for me to either put up with it or look for a different boast. Cheers !

 

anything is possible, the question is the cost worth the benefit. It will add weight and take up more room inside the hull (though in a different place than the current post).

If you have a deck stepped mast, replacing the structure under it to support will not affect the mast or rigging, or the water tight hull envalope. the forces in this location under the mast are very large, but it can be accommodated if you have a competent NA or marine engineer do the design for you.

You might contact a local Navel Architect or engineer to see what it would cost to have the design done, and cost of doing the refit. the NA might be able to give you a reasonable cost estimate to do the change, if it is out of the question in terms of cost, than you have your answer.

Good luck.

 

It's not a crazy idea, but as pointed out, not a casual undertaking. You'll need some engineering, but this isn't that uncommon a request. I think a combination heavy beam and ring frame/bulkhead/partition will work, though the loads will need to be looked at.

 

Thanks so much. I might do some more research on costs. My wife doesn't like the layout but there is always compromise!

 

yeah keep the boat and sell the wife !!