Aboard Creeky: November 2014

Where was I? Mast step. Right. As I may have mentioned in prior posts Creeky came with a lot of important parts and loads of potential, but very little in the way of instructions. Most of the time this is great because I get to use my imagination and come up with creative engineering solutions while double-checking my work with some trusted references. However, designing and building a mast step is one of the handful of key components of boatbuilding that didn't really come up in the textbooks I have. There weren't even many online arguments about the best way to build one in the forums I usually poke around in. It's either assumed that this was already built into whatever structure sits below the floor or that the clever reader has the requisite knowledge to scratch something out on the back of an envelope without giving it much thought.

I took it to be slightly more complicated than that though, and decided to over-engineer it to a sufficient level of mental comfort. Even now as I look at the mast sitting on top of the step I have that little itching question in the back of my mind. "Could I have built it just a little bit stronger?" What I think some people take for granted about the mast step is the amount of force it must withstand. Even at rest the downward force on the mast step is tremendous. Consider the following:

Approximate extruded aluminum mast section:
Dimensions: ~12in x 6in
Wall thickness: ~.3 in
Weight/ft: 15+ lbs (taking internal roller furling structure into account)
Length: 62 ft
Overall weight = 15 x 62 = 930 lbs

Now add in all nine 3/8in 1x19 stainless steel wire rope stays, each with a breaking strength of ~17,600lbs, driving the mast into the step like several giant compound bows all at once. Assuming they are tensioned at 1/10 of their breaking strength (total guess), that would make:

9 x 17,600 = 158,400/10 = 15,840 lbs on top of the mast weight.

So at rest we may be looking at almost 17,000 lbs of force directly onto the mast step! That doesn't even begin to take into account the myriad variables that can't be calculated or planned for, so there needs to be a considerable safety factor on top of that. No problem...

The basic premise of a mast step is that it is a structure that distributes the load imparted by the mast to a larger section of the hull and keel. I decided to build a step that would be strong enough to withstand the necessary loads but also allow for fore/aft adjustment of the mast foot on top of it. This was primarily to give me a little room for error when cutting the hole in the deck for the mast, but could theoretically allow me to give the mast a little bit of rake if I wanted.

Lining up height of mast step

The basic design of the step was to have four 2 x 6 in pieces of mahogany, each ~3 feet long going from port to starboard, with a 1/2 in thick, 6 in wide, 18 in long piece of 304L stainless steel going fore/aft. This would be secured at either end and underneath to 1/4 in angle steel for extra sturdiness, and thoroughly bonded and fiberglassed to the top of the keel.

Bonding

I used a WWII era Bridgeport mill to fabricate the stainless pieces. I could write about that machine for days, and I barely know how to use it...

Drilling

Milling

And more!

Below you can see how the midsection of the stainless plate is attached to the mahogany bulkheads (I wanted to call these stringers, but my friend Nick corrected me and said that stringers go fore/aft and bulkheads go starrboard/port).