Surveyor’s Opinion to Strapped Full Rudderblade on a Taiwan built Force 50 Ketch:
“At my inspection the GRP rudder blade had been dismantled for a while and was placed under the vessel. About 10 bores of about 40 mm diameter had been drilled into the blades sides/ cheeks with a punch drill. Most of the bores had reached the foamcore of the rudder blade, some had reached the opposite laminate. At the time of my inspection I could confirm the uniform and very high moisture readings reported before. The foam core was partially darkened (as it happens with moisture ingress) but looked mostly in honey colour. From the one core sample I could inspect, I concluded that the adhesion between the core and the GRP cheeks of the rudder was not bad.
The rudder blade is attached to the rudder stock with by metal fasteners in sheet metal.
In normal use, rudder blades go through a lot of stress. With every turn, the blade held in place by the forks formed by the metal bands and the stock is subject to a cycle of compression and tension. Years of sailing can accumulate a lot of these fatigue cycles. Shock loads, groundings, competitive sailing all contribute to the rudder blade working against the metal forks and their fasteners. Eventually, even if perfectly sealed in the beginning, water will find its way into the rudder blade’s sandwich. The problem is much more with spade rudders, but also a full rudder blade is not at all immune.
The foam core is usually of the so called closed cell type and should keep the water from penetrating deep into the structure. However, not all closed cells are perfectly closed, the lamination is not always perfect and has voids and gaps (see image – although somewhat blurred you can see them) and over years water will find its way into any creek and cranny of the structure.
This condition is not desired as the unprotected GRP is sensitive to exposure to water, will take up water and the material will lose its property, eventually also its physical resilience.
However: I can tell you safely that about nine out of ten rudder blades we inspect have very high moisture readings. Often, when we drill a hole into a rudder blade, litres of water spurt out.
I see three options for you:
1) Split the rudder blade into the two shells, remove the foam core, dry the two shells entirely, thereafter refit them, treat the areas around the fasteners generously with epoxy , refill the hollow blade with two pot closed cell polyurethane foam. Finish with epoxy paint. This way you will end up with a dry blade, however, within one year or two it will pick up moisture again, even if the work has been carried out carefully and properly.
2) Have a rudder blade manufactured in marine plywood (RINA classed marine plywood is locally available) and cover it with a few layers of epoxy and glass and finish with epoxy paint. This rudder blade will survive the vessel, but has one disadvantage: That rudder blade will no longer be a “sacrificial” part and the next time you touch the concrete pier with the trailing edge of the rudder blade it is likely that you will develop an expensive repair to rudder bearings, heal and stock, justifying immediate drydocking.
3) Similar as in (1) improve the fastening holes with epoxy, let the blade dry, as is, as much as possible through the few bores (I do not think that significant drying will take place) then reinforce the blade with a few layers of epoxy glass, seal it with a good epoxy paint and keep it under observation on a yearly basis. In the past I have attempted to suck the moisture out with a vacuum pump and had some success with it.
(2) is a long lasting solution, but you have to be very careful about impact complications, (1) will be a good solution as well. (3) is the most dicey solution, but may turn out at the end not worse than the others!
As you can imagine, (2) is expensive, (1) is somewhat (but not much) less expensive and (3) can be done within a moderate amount.
I hope to have been able to help you to decide.”
Dr. Yusuf Civelekoglu, CMI