boot Practical Boating Guides

Yacht Maintenance & Repair - Part 1: Underwater Hull

Getting your boat shipshape for the new boating season

Yachten im Winterlager - Foto: © TCCC / G. Schneider

While your yacht is still ashore, it makes sense to take a closer look at the underwater hull. After all, it is the basis your boat is built on. And the condition of the hull below the waterline usually gives a reliable impression of the general condition of the boat as well, without even having looked on or below deck.

If numerous old layers of paint are "piling up" down here, it does not look too good. Or even worse: large areas that have simply been painted over after old layers of paint had come off. Obviously, not a lot of effort went into the regular maintenance of the hull; neither down here nor, presumably, above the waterline. Because anyone who takes good care of their yacht will, for good reason, start down here below the waterline. Even if it may be tedious work: the old paint has to go first! At least most of these layers, depending on their condition. Although it does not doe any harm at all to completely rebuild the paint structure including the primer every few years. Only then you can really inspect the hull and only then the coating really gets a good hold on the hull.

When first appearances make all the difference
Just to scurry over the ship with a high pressure cleaner once in autumn and then apply a new layer of antifouling is not sufficient at all. Since for this purpose we are dealing with an antifouling paint which contains substances that cannot really be called health-promoting, you should always use proper equipment and wear complete protective clothing for these tasks. For sanding the underwater hull, always use a tool with dust collection device, wear an overall with a hood, googles and a breathing mask.

Achieving great results step by step

Underwater hull after sanding

Blistering can be a sign of osmosis?

Keep calm and carry on...
...if a few blisters appear at the waterline of a GRP-yacht. There is no reason to panic as it does not necessarily mean that the entire ship is in the process of being destroyed by osmosis. However, you should take a closer look at other areas of the underwater hull and if doubts remain, better consult an expert like a boat builder or yacht engineer who can examine the hull with moisture measuring instruments to determine its actual condition. Small bubbles to be found from time to time at the waterline can be opened up. If there is no smelly liquid running out of them (this would be a sign of osmosis), you can simply scrape them out and fill them up with GRP or epoxy filler.

Keel, rudder & propeller
Other parts to look at below the waterline are the keel and rudder your yacht. Are there any rust marks where hull and keel are connected? If yes, you should carefully inspect the keel bolts from the inside in the bilge. Pulling and replacing keel bolts, on the other hand, is a job to be handed over to a professional shipyard. Also check the rudder blade of your boat by moving it repeatedly to and fro. If there is too much play, the rudder bearings are worn out and need renewal. Then take care of the propeller, rid it of fouling and barnacles, and perhaps protect it with a special propeller paint. Folding propellers may well fail occaisonally if they are used only infrequently and therefore get overgrown by barnacles at the berth in the course of a season, whereas fixed propellers lose at least a good bit of their power. The Dutch paint manufacturer Epifanes or Hempel and International, for example, offer a brush-on silicone coating especially for the protection of propellers.

Keel bolts in good order

Not a good sign: rust and cracks

Foto: barnacles and fouling on a bow thruster

Barnacles on a bowthruster

Rudder and propeller with sacrificial anode

Degraded sacrificial anode

Barnacles in through-hull fittings
Barnacles also like to grow in all kinds of through-hull fittings like in- and outlets of water for engine cooling, sinks or toilets. It is important to take good care to keep them free and they should be thoroughly scraped clean before the boat is returned to the water. The worst case would be if barnacles block the water inlet for engine cooling and reduce the stream of cooling water too much, or obstruct any of the sea cocks so it will not close off properly anymore.

Renewing sacrificial anodes in time
Do not wait to replace the zinc or sacrificial anodes, which protect various metal parts from electrolytic damage under water, until they have already been degraded almost completely. Once they have been used up to about half their original size they should be changed. Electrolytic reactions can be quite different depending on where you are or what is "going on" in the immediate vicinity. Therefore you need enough "leeway" with your anodes in place to be always on the safe side.

Biocide-free antifouling
Finally, you get to the point at where you can apply a new underwater varnish. This is done with a varnish roller, not too thick, on a smooth, dry and clean surface. All underwater paints with an antifouling effect must be registered within Europe and bear an appropriate registration number. Under this number you can theoretically obtain the list of substances contained, but in most cases the manufacturer provides the list on the product data sheet. The major paint manufacturers such as Hempel, Epifanes Or International actually all have biocide-free underwater coatings in their product range that meet today's standards. On some inland lakes and waterways, however, special rules may apply for underwater coatings. Make sure to get all the regional information neccessary for your in advance.

New products on a silicone basis
A relatively new alternative is coating the underwater hull in an antifouling foil which also meets all environmental regulations. At boot Düsseldorf, the "Django 980" was on exhibition as a yacht covered with such a foil coating on the underwater hull. The main component of the foil is silicone, and it reliably protects boats from marine growth for several years. When the yacht is in port, a layer of slime forms on the foil, which is washed off when the boat is in motion. An additional plus: the foil also significantly reduces frictional resistance in the water; more speed for sailboats, less fuel consumption for motor boats come as a free extra, the manufacturer points out. Silicone is also used in the spreadable Silic Seal from Hempel, which complements the new Silic One series and has similar properties to Renolit's foil solution.

Text: Detlef Jens

Read on: Yacht Maintenance & Repair Part 2 - Hull & Deck

boot Practical Boating Guides

Yacht Maintenance & Repair - Part 1: Underwater Hull

Getting your boat shipshape for the new boating season

Achieving great results step by step

Antifouling paint or foil