The Naiad Voyages

Mark Austen

2021.03.30 - New Task

One of the things that I find irritating when sailing Naiad is her rudder. It works fine and looks good, but getting it down is a pain as the blade is so buoyant. Now, the standard way to deal with this is to weight the rudder blade so that it is either neutrally buoyant or negatively buoyant. In the first case the rudder blade does not sink or float in the water whereas in the second case the rudder blade sinks.

The negatively buoyant rudder blade tends to be used for self-setting rudders in waters where the rudder is in danger of snagging in shallow water. If, for example, the boat sails close to a sandbank, the the rudder pivots up as it slides over the sandbank and when the boat is back in shallow water, the blade sinks back down to the working position. For this to work, the blade is often made quite a lot heavier than water so there is always sufficient force downwards to set the rudder blade.

In the first case, that of neutral buoyancy, the rudder is raised and lowered by means of lines but the lines are not cleated or tied off so that if the rudder snags, it can lift. Then, when back in deeper water, the downhaul line is used to lower the rudder blade back down.

It is the second of these that I want for Naiad. The rudder is currently so buoyant that I cannot lower it using the downhaul and have to use the paddle to push it down and this is the bit that is really irritating. If the rudder snags and lifts for any reason whilst we are sailing, it is a right pain in the neck to use the paddle to lower the blade whist still moving.

So, the new task is to add a sink-weight to Naiad's rudder blade so that the blade may be easily raised and lowered using the up and down hauls.

Now it the right time to do this since the next task on my list is to sand and paint the rudder blade and stock, so adding a sink-weight should be done before that.

But how much weight should be used?

One method is to put the boat into the water and mount the rudder. Then weights are tied to the rudder until the desired buoyancy is achieved. The rudder can be dismounted, brought back to the workshop and the correct weight mounted permanently on the blade.

However, that is not the way I am going to do this. Instead I shall remove the rudder blade and take it to the water trough in the field that the horses use for drinking water and put the blade in the water. I can then add weights on the blade until it just sinks. Or, even better, I can use a digital luggage scale and a stick to push the rudder under the water and the reading on the scale will shown me exactly what weight is required.

Having sorted that out I then need to decide how to fix the weight to the blade. There are three main ways to do this. The first is to encapsulate the weight inside the blade. You'd do this if you were making the rudder blade from scratch. The second method is to cut a hole in the lower part of the blade the same shape as the weight and fix it in the hole. You can also melt lead and pour it into the hole. The third was is to bolt the weight on the outside of the blade and this will be the method I'll use, at least initially. I shall divide the weight in two and bolt the weights to the lower part of the blade with bolts passing through the weights and the blade. 

The reason for using this method is that I can experiment with different weights by adding or subtracting metal during the season to achieve the best result for this rudder. I don't want it so weighted at the tip that the up haul cannot pull the blade up. Once I have the weight set correctly, I can set the weight into the blade permanently the next time Naiad is out of the water.

The rudder blade didn't quite fit into the water trough but it did float once the trough was filled up a bit more. The scale read 480g (17 oz) of weight required to sink the blade, but this is only is only very approximate due to the lack of space in the trough. The solution is to take the rudder blade down to the mooring with a number of weights, I used all my spanners and two vice grips, put them into a net bag and tied that to the blade. Then put the blade into deep water and see what happens. With a rope tied to the blade, of course.

The river test shows that 1.68 kg (59 oz) just allows the blade to float and 2.25 kg (79 oz) allows it to just sink. This also shows that there wan't enough water in the trough for the first test to work.

So, I shall just an initial weight of 1.5 kg (53 oz) including the bolts holding the weights to the blade and see how that works when Naiad is back in the water.