The Naiad Voyages

Mark Austen

2016.08.06 - Rudder Stock - The Sheaves

Today's first task was to complete as much of the rudder stock as I am able without having the correct sheaves.

The second task is to make a decision about in which order the next few tasks should be carried out. If I paint now then I will have to remove some of the paint later on. If I put on the foredeck now then painting is going to be very difficult in the front part of the boat and the construction of the lockers and bunks will be more difficult. If I complete the bunks and stowage in the forward part of the boat then painting is going to be very difficult and I'll have to be ultra careful when fitting the foredeck that I don't stand on the wrong part of the bunk/stowage construction and break it or my ankles.

My decision is that the order will be paint then the foredeck and then the bunks and stowage. So I carefully cleaned and vacuumed the inside of the hull and applied a coat of the Owatrol ESP. That task took most of the afternoon and now I have to wait for the ESP to dry thoroughly, as it says on the tin, before I paint. The specifications say not less that 2 hours and not more than 7 days. I think I'll be able to find some time in that interval to get the fort coat of paint done.

This coming week I'd like to get the gunwales finished. They are still held in place only by the temporary screws. I need to take them both off, sand the hull back to the wood and then use thickened epoxy and the final screws to put them back in place. This is a two person job, at least for putting them back in place, and I'll have to do it in the cooler evenings as there is a lot of epoxy to mix up and spread.

Before starting the foredeck I've decided to construct the afterdeck. It is smaller and easier to get to so it will be not as difficult as the foredeck and will give me an insight as to the type of things I'll need to watch out for when I construct the foredeck. The afterdeck has a horse mounted on it, not the animal but the rail that the mainsheet runs along. You can see what I mean from the photo to the right from Charles Stock's website.

I had the horse made up for me at the end of last year and it will require some stout backing pads on the underside of the deck. These pads will also be used to strengthen the deck where the cleats will be mounted. Nevertheless, the task will still be easier than the foredeck construction.

The first thing to do on the rudder stock is to tidy up the repair I made last thing yesterday. The excess wood was trimmed off but not finished since this will be done when the rudder stock has been constructed and is ready for painting.

Holes for the bolts upon which the sheave will turn were carefully measured and drilled in the cheeks and the sheaves put in place loosely as you can see here. Some small shoe-lace string was used to show his the system will work.

Here you can see the rudder assembled and some baling twine used as the up and down hauls. The rudder blade is in the down position here.

The downhaul is tied to the new anchor point as previously described and runs up the slot between the two cheek pieces.

At the top of the rudder stock the downhaul runs between the two sheaves and exits the stock from the top and going forwards.

Likewise the uphaul runs over its sheave at the aft top of the stock and then exits the stock alongside the downhaul.

Now the blade is in the up position.

The downhaul is still tied to the anchor point but now runs along the groove in the rudder blade before running up the slot in the stock.

There is, however, a problem with this arrangement. Here you can see the rope going around the sheaves as required but it would not stay this way unless there was continuous tension on the hauls preventing them from going slack.

If the hauls were to go slack this would be the result. The rope has come off the sheaves and is now around the bolts instead.

The solution to this is to fill the gap between the sheave and the stock so that there is only a very small gap between the two and this will stop the rope from being able to come off the sheaves. But, until I receive the real sheaves which are wider that the ones I'm using here, I can do no more.

Now those sailors reading this, especially dinghy sailors, will immediately ask why I'm going to all this trouble. Their lifting rudders have up and downhauls and they just go over a smooth pin. What's the point of all the sheaves? And this is a very good question. The short answer is friction and wear. The dinghy sailors unship their rudders after every outing and will quickly notice if there is any wear in the hauls and replace them. I'll not be doing this. The rudder will be shipped and unshipped at the start and end of the season and probably not in between unless I decide to take the boat somewhere on her trailer for a sail in another area. So it is unlikely that I would notice any wear in the hauls until it was too late.

Murphy's law states that if this happens it will do so as to maximise the inconvenience caused. This is not a good idea on a boat. So, the sheaves ensure that there is no friction that would cause the hauls to wear. It also reduces the friction generated when pulling the blade up or down. If the hauls were just over pins as in the dinghy rudders, then the harder you pull on the haul the more friction is generated between the rope and the pin and if I remember my mechanics, the relationship is not linear but increasing. The sheaves will prevent this entirely. And the cost is reasonable. The sheaves cost £12 and the rest is just my time.

Also, I trained as an engineer and friction and wear are a total anathema to any engineer. We will do just about anything to prevent it or, if that is not possible, to reduce it. You wouldn't run the engine in your car without oil, now would you?