We’ve had a four hour hang in the slings in Palma to replace our damaged stabilizer fin. We are not happy with what we found.
The leading edge of the keel, which hit just before the port stabilizer. Note that the microballoon fairing on the keel is dinged, but the undercoat is still intact. This is a good indicator of the probable loads, which were light.
Before we get into what we think of the fin engineering, let us say that we have been very pleased with the NAIAD performance and service. There have been some hydraulic QC issues on all the boats so far, but the system, once operating, has been excellent. We have discussed the comments which follow with them and they have their reasons – right or wrong – for doing things a certain way. They are not going to change what they say works in fin construction for one small customer.
Our logic in cruising yacht design is that mistakes happen. People run aground, hit logs, bang into ice. This is part of cruising. We want to be able to make modest mistakes – such as the one which caused this damage – without having to visit a boat yard. Hence our keels are designed to four times the ABS rule, rudders to twice.
Two things immediately draw your attention to this stainless insert formerly molded into the fin. The first is the lack of foam adhesion to the stainless.
The shearing on the foam surface on the other side is what we would expect everywhere. The bond line between foam and its substrate, properly done, is normally the strongest point in the foam.
The second item is the size of the stainless insert relative to the fin. This is maybe 1/12th the fin’s area. Even if the foam had not released it would have quickly sheared off this small area.
Now some logic. The way we want to engineer a risk element like a stabilizer is so that the damage occurs outside of the boat. Ideally:
- The stabilizer tip is frangible, absorbs energy on impact, and fails leaving the majority of the stabilizer behind.
- The balance of the stabilizer foil fails before it transfers damaging load to the stabilizer shaft.
- The shaft fails before it damages the bearing carriers and attachment points to the hull.
If this were a steering rudder – it acts just like one – you would have a framework of spar and ribs (wing style), extending down from the shaft insert. The bottom third would be weaker than the upper portions. The foam would be flexible, like Airex, and the laminate tapered in reinforcement, with a resin used that had elongation properties that matched the reinforcing fabric.
This design might add 50% to the cost of the stabilizers, maybe even double the price, but if done this way we would have just avoided the haul out.
One of the mechanics, removing the hydraulic decoupling tool from the insert, gives some scale to things.
Installing a big stabilizer, weighing close to 400 pounds/185kg, requires a fork lift, three strong men, and care.
The final step, using a huge torque wrench to apply the appropriate pressure to the bolt which holds the fin onto the shaft.
You could say that having covered in excess of 45,000 miles without incident, including debris strewn waters in British Columbia and Alaska, and then dealing with ice in Greenland and Svalbard, would qualify NAIAD for a pass on this. And it may be that their average client’s boat is so weak as to make this type of failure mode mandatory. Then to, it was an error on our part that lead to this discussion.
But we think they can do better. And we’d much prefer to have them supply us fins engineered on the principles outlined above rather than having to design and build our own.