Length of rudder post

Hi
I am building a high aspect spade rudder in foam/GRP for my 33 ft sailing boat. The blade is around 1.6m long (5 feet 2 in). Is there any rule of thumb for how long long the rudder post should be inside the blade (like precentage of blade lenght)?

Thanks

Mikael

 

Welcome to the forum.

There's no "rule of thumb" or percentage you can follow. Rudder stocks are made to suit a strength and stiffness requirement, not designed to a rule. How much load do you anticipate on this rudder? When in doubt or doing retrofits, the usual practice is make it as deep into the blade as practical.

 

If weight is not critical, all the way to the bottom is the safe way to go. Otherwise you will have to do some engineering calculations.

 

All the way to the bottom if practical. Most spade blades tend to taper quite a bit at the tip, so given a specific blade with and shaft diameter, you'll need to stop short at some point.

 

Do you think there is any advantage to a sacrificial tip? How would you go about constructing it?

 

On dayboats in normally shoal waters, yes a hardened sacrificial tip is a good idea, but on a shoal cruiser, not so much, as you just don't bang bigger craft around as much. Sacrificial tips serve to protect wooden boards, more so than inert materials, such as the proposed foam composite. Most of my boats do have a hardened leading edge and sacrificial tip, but I'm in notoriously shoal waters. It's a semi annual ritual to repair dinged edges and tips. So, the question is, how often do you bang into stuff with your appendages and is the frequency enough, to warrant this approach. Lastly, on foam core composites, where the sheathing bears the brunt of the load, the shaft is typically longer, so you can absorb some of the load with the rudder stock, instead of fabric bulk.

 

I was thinking here of protecting the rudder shaft at the hinge point of the hull.
Recently I was looking at a spade rudder where the lower third had been broken off in what had obviously been a grounding (the keel was scarred too) the rudder was still functional with an apparently straight shaft so the boat would have been normally usable after the incident albeit with reduced steerage.
It seemed like a good idea but how would you construct this to be sufficiently strong for normal use but sufficiently weak to protect the shaft?

 

This can be accomplished, but there's always going to be a chance you'll bend or break something unanticipated. Generally the sacrificial aspect it to save the blade from rot, not necessarily the shaft. You can experiment with adhesive application techniques to create a natural break line. This can also be done with fasteners, likely much easier I'd suspect, as the yield point would be more constant and more importantly known without experimenting.

 

Here is a video from one of the largest manufacturers or rudders Jefa in Denmark. Watch from about 2 min 30 sec in to the film.


Looks like a fair bit is unsupported in the bottom