Here's some comments on the problem with using composites for highly loaded safety
critical items like your primary means of steering
. I've provided them to assist you in being able to understand the technical aspects as you work to get your vessel repaired. Note that I'm not criticising the manufacturer of the yacht.
Some of the comments on the composite failure are interesting. The rudder
has obviously been in service
for a while so comments about gelcoat
and voids is somewhat spurious. I would not be making professional judgements on the actual failure mechanism without careful inspection
at both the macro and micro level.
Using a composite post is great for the manufacturer but a less than ideal option for the boat user. Here's why. A composite post is cheap
and easy to make and meet the manufacturer's build quality control. Composites have high specific stiffness (stiff and light) but poor abrasion resistance (low hardness) and very low toughness compared to a structural steel
option. They make sense for well maintained performance and racing
yachts where they are inspected frequently or parts
are replaced on a strict schedule.
As an engineer
i would consider this design (foam filled composite post) to be non optimum when offshore
cruising and as a yacht owner i would consider the design to be insufficiently robust necessitating frequent inspection
and / or replacement. A risk based rationale for the manufacturer using this design would likely pass a spreadsheet cost benefit analysis due to the low probability of occurrence. I always demand risk based assessment to include probability, severity and detectability. Often detectability is ignored because it will discount many cheaper options for the manufacturer.
A solid steel
rudder post, either heat treated high strength martensitic stainless (4xx series) or a lower strength corrosion
resistant alloy such as 316L is tougher than a composite option and therefore will typically fail gracefully giving you feedback that something is wrong. A tubular option could easily meet the strength requirements but also introduces buckling as a failure mechanism. This is why the older blue water yachts typically have a solid rudder post which is way over designed from a static strength perspective.
Note that the primary failure mechanism for a rudder post, that i would use during design will be low cycle fatigue after a high point stress loading from an impact. This is why rudder posts typically didn't fail from an actual impact but generally some time after at a stress loading that can be less than the design stress level. Pros and cons
Option 1 - replace rudder with a manufacturer supplied item
- Potentially a relatively simple fitment providing no other structural damage
- potentially some form of guarantee although in practice this will be limited and will favour the manufacturer and not you
- Foam filled composite rudder post will always fail catastrophically and will be not survive any impact.
- a prudent risk based approach will necessitate that you have a secondary rudder
Option 2 - design and fabricate a steel rudder post and bearing mounts
- design can both functional and survivable by design and material selection
- can build in fail safes rather than just shear off
- can be intentionally designed to fail gracefully
- your location may not give you access to a competent designer
- you will need to ensure the new design doesn't introduce any new emergent problems. The manufacturer may counter that the rudder is designed to be sacrificial to avoid impacting the integrity of the hull
during a rudder impact. This is an entirely plausible design decision and is the reason why yachts are classed as either coastal or offshore
Personally, in your current
situation cruising offshore, Option 1 plus a secondary rudder is probably the best you can manage. I would place lots of pressure on the manufacturer to support you because their choice of design is a significant factor in your current
dilemma and many of their yachts travel the globe. Their reputation is important and it is in their best interests to see you back sailing asap.