Steel to alloy weight saving – the theoryAssume steel rudder blade weight 100 units
Specific gravity of steel, approx Specific gravity of aluminium, approx Same-sized aluminium blade weight 7.82.72.7/7.8 x 100 = 34.6 units
Correction for increased thickness from 8mm to 10mm ie, the substitute alloy blade should weigh less than half the steel one, representing a weight saving of more than 50%. 10/8 x 34.6 = 43.3 units
In my case, the 56lb of steel was replaced by 24lb of alloy, saving 32lb off the transom – and saving my back when shipping and unshipping the rudder. It all seemed very attractive...SeamanshipThe original rudder of the Seal 22 is a ruggedly-constructed unit comprising a heavy stock laminated from marine ply
boiler plate. As far as I know, these original rudders do not fail but are very heavy to which hangs on the transom, with a pivoting blade of 8mm
ship and unship, especially for trailer-sailing. For this reason, many owners have been tempted – as I was – to build a lighter one. I decided to substitute 10mm
aluminium for 8mm steel for the plate: given that the density of aluminium is only about one-third that of steel, there would still be a substantial weight saving.
A friend who is a consulting engineer reassured me that this should be sound: the increase in thickness should compensate for the lower strength and stiffness of the
aluminium. For this experiment I managed to scrounge the aluminium (of uncertain ancestry) from a friendly scrap dealer.Having cut out the blade with a jigsaw –
a long, slow job – and having profiled the leading and trailing edges with a rasp (more blisters), I was still not convinced of the strength of the finished
part, so I devised a test unknown to British Standards. I rested the blade horizontally on the ground on a house brick at each end and then jumped up and down
on it as hard as possible in its centre. Nothing broke or bent, and there was no noticeable flexing either. Now satisfied with it, I painted and fitted it.
BANG!For three or four years all went well. It was probably just my imagination, but I convinced myself the boat went faster
with less weight on the transom, less dead wood, less drag and all that. However, oneday I was sailing, with an enthusiastic but not very experienced friend, from Menai Bridge up the strait past Puffin Island towards Moelfre. The wind was building all
the time, and we had just decided that the upwind bash to the pub at Moelfre was not worth it. Rowing ashore in my tiny inflatable would have been risky in the
conditions, and we would have been late back for a family barbecue in the evening.Force 5 or 6, and I had already been forced By then the wind was probably about
to drop the main to carry out a running repair with spinnaker tape where the top horizontal seam had started to split.When we turned back somewhere off Red
Wharf Bay we had a quartering sea which was lifting the stern and swinging us off course. I really had to heave on the tiller:unusual for the Seal, which is normally light on the helm. After I did this for a while, a bigger wave lifted and pushed the stern to starboard, so I heaved again
on the helm, and BANG! In effect, the rudder was stalled, and the blade was being shoved sideways through the water until it broke off along the line of
maximum stress where the blade exited the clamping effect of the stock, which also coincided with the waterline.
Steering on the sailsFirst I had the difficult job of securing the broken blade which was crashing about the stern, suspended by the wire
rope strop of the downhaul. Hanging precariously over the transom, I managed to grab the blade and lash it to the old outboard bracket, retained for its
convenience as a boarding step. I could not unscrew the shackle which was out of reach underwater, and the wire strop passed through a fixed fitting and had
a hard eye at both ends, preventing it from being released.says in the books. I had done this before, Next we tried steering on the sails like it
in perfect conditions on the River Fal in a dinghy after its plywood rudder had broken off. With a gentle breeze ahead of the beam that wasn’t difficult, but this
was different: a lumpy quartering sea and a following wind. We were all over the place. I took off all sail, started up the outboardThe jury’s out!
Tony Nield’s Seal 22, dried out at Beaumaris in Anglesey, WalesTony Nield improvised a jury rudder from a headboard after the aluminium rudder on his Seal 22 failed in the Menai Strait