Engineering design for mast step loads

Hey all,

Very rusty nav arch/boatbuilder here working on a personal project to convert a keel stepped mast to deck stepped.

I need to calculate the scantlings for a compression post and any other structural components effected, and Im thinking to use rules of thumb rather than engineered calculations of rig loads etc.

What:
50' steel cruising yacht. Launched in the 1980s currently undergoing major rebuild. It's been entirely stripped back to bare metal / sandblasting etc. AFAIK all structural loads are carried by the hull (ringframes, stringers, hull plating). All bulkheads etc were plywood bolted to ringframes -I don't _think_ they were structural. The original builder is doing the re-build and said "don't think so" - they just made them up... Anyway the boat is now a shell with no interior.

There are various engineering calculations to estimate compression load at mast step, and there are rules of thumb -both of which are valid, imho. especiallyon a project like this (not grandprix racing yo!)

I dont know RM, I dont have a hull model nor rig specs etc, and Im a long way from crunching numbers (both because I dont easily have numbers, and I havent crunched numbers for a while!). I could get those things, and re-learn but there's a lot of time in that alone. tbf I should re-learn :/

So... my approach atm is to simply specify a compression post that is 'stronger' by some safety factor than the mast section itself. Transferring the loads from the compression post into surrounding structure is another question.

Questions:
1. Specify compression post to be 'stronger' than mast section. ie make sure the mast itself fails before the hull structure fails - that's reasonable, right?

2. Im assuming compression loads at mast step are basically the same whether its deck or keel stepped?

3. I recall the actual max compression load at mast step will be in order of around 4(?) times vessel displacement (although that was TP52). maybe more? I vaguely recall that research around mast step loads and slamming...

4. Are there particular structural considerations Im missing here? especially around the new deck mast step and immediate hull structure / compression post.

Im interested in any guidance and/or papers etc that can help assure me Im on an adequate track...

I'm going to need to refresh myself on column buckling and moments of inertia and other lovely words from the mists of time...

 

4 times

It seems like a lot to me; but I don't know, my world revolves around 1-2 Tons and this may be another unknown planet for me; but it sounds 2 times as familiar to me:

"double the boat's displacement" (?)

 

The change also increases the lateral loads on the deck. You need to recalculate the mast too, because it changes from a pinned end to a hinged end. It will buckle at a lower applied force.