Sometimes … Stronger is not better.

Stronger is not better, getting the right strength for the challenge ahead is a key. Or how I got bit using schedule 80 pvc pipe (much thicker walled, stronger, and wear resistant than schedule 40 – that what you typically find in house construction)

Chain Locker Design

One of the weaknesses of my Saturna is the design of the chain locker. The chain locker on this boat was designed with a rope based anchor rode. Some of the places I want to go have all sorts of sharp stuff on the bottom, and I want to sleep nights, need to sleep nights.

There was space available forward of the water tank, under the V-berth. On the plus side it was low down in the boat and nearly under the windlass (winch for anchor handling – probably from windless which is often the reason for putting the anchor down). It is not the ideal shape for an anchor locker, broad instead of tall but my calculations say that there is room for 150 to 200 feet of 5/16 high tensile chain.

The challenge was to get the chain from the windlass to the locker with a minimum impact on the usability of the V-birth. My first design (that I implemented, not the first on paper) was a straight drop from the windlass to the locker. The run blocked the door to the old chain locker so I put a removable section in the pipe run. I used schedule 80 pvc pipe because I liked the heavy walls of the pipe. However the chain pipe came in at the shallow end of the locker.

This design moved the chain well but when we loaded the full length of chain (getting the chain was a epic in itself, perhaps the subject of another post) we had problems with the chain mounding up under the chain pipe and not sliding down to the deeper part of the locker. (There must be a little gecko in zinc plated chain – it really stuck to a fairly steep slant. Looking back maybe some UHMW plastic sheets would have solved that problem – slick and tough)

(My son Shane spent hours trying to figure out the problem and ways to fix it without starting over – thank you Shane)

Problems keep mounting

We did partially fix the problem by adding a section of pipe in the locker to drop the chain in the deep part of the locker. That addressed the worst of the mounding problem, or at at least put the mound where it needed to be.

However when we started using the system out in the real and muddy world it started jamming all the time, rendering the windlass effectively useless and forcing us to rely on our backup anchors. This lead to one spectacular drag on the first day out, fortunately in an empty anchorage and along the length of the anchorage. (It also cost a mile of hard one distance.)

But what is the problem?

Clearly this is not an acceptable condition. So on the first of December I took the anchor off and started to repeat the tests Shane had done, focusing just on the jamming problem. I started to remove one section of pipe at a time. After much fuss and bother I was down to just a single elbow, and the chain was still jamming with great regularity.

I finally took the elbow off, and the chain started running smoothly. I put an unused schedule 80 elbow back on (maybe I had a defective elbow?) and the jamming returned.

Much head scratching ensued. Finally, under close examination with a bright light and a dental mirror I spotted the problem. The inside diameter of the pipe was not the same as the inside diameter of the fitting. There was a distinct step up from the elbow to the inside of the pipe.

I ran some experiments with a clean piece of chain. It ran smoothly, just like it had in the bench tests. I ran a chain with just a little bit of mud on it, the very finest of coatings. It jammed, repeatably and often, but only if the elbow wasn’t perfectly aligned with gravity. The links we had marked with wire ties jammed more often.

Back to the hardware store – Stronger is not better

So I walked back to the hardware store, Patricks Hardware in Hampton, VA. (Patricks is nice old style hardware store, right out of my child hood.) And spent a lot of time looking at PVC fittings and how well they fit with the pipe.

My discovery, stronger is not better. The schedule 40 pipe and fittings don’t have that step up from the fitting but in particular the 45 degree fittings have a nice clean sweep and no step to catch.

So I designed a new pipe, bought the parts, and installed. Sitting in the slip it moves the chain smoothly for the first time since we got the chain dirty. The design is a little more obtrusive, but it doesn’t block the hatch and I can remove it if needful with a minimum of pain and suffering.

In conclusion, there is always a price for greater strength, make sure you understand what the trade-off is and make sure you can afford it. And watch out for the grit in the gears, the difference between lab and real world. (Now out into the real world to test it)