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Multi-use skin fittings

5 November, 2017

Using a skin fitting for multiple uses might seem like a good idea to start with as it reduces the number of holes in the hull. This can however reduce the effectiveness of the items sharing the skin fitting.

In the vessel shown above, the seacocks’ primary use was for the cockpit drain but this one has had a manual bilge pump and then an electric bilge pump added using ’T’ pieces. Each pipe has its flow rate determined but its diameter and the pressure you can apply to it. The total flow will be restricted by the diameter of the single outlet.

The cockpit drain will be using gravity to allow water to flow out. If you now start pumping with the electric bilge pump the cockpit will not be able to drain as fast. Similarly if you have to start using the manual bilge pump because of an emergency you will have to pump even harder to force the water through the seacock to reach the full capacity of the pump. In this setup you may also start pumping water back into the cockpit.

In the example shown, the manual pump would also be restricted by the smaller diameter ’T’ piece fitted for the electric bilge pump. This could easily halve the capacity of the manual bilge pump.

An improved installation would be to fit two new above the water line skin fittings for the bilge pumps and have the seacock dedicated for the cockpit drain. Keep all pipes and skin fittings the same diameter to ensure maximum flow.

Marine diesel heaters

9 October, 2017

Not all heaters are created equal.

When looking to buy a diesel heater to extend the summer sailing season some people are understandably tempted to try and save money by buying a cheaper road version of the Eberspächer or Webasto diesel heaters. This has become even easier to do when many are offered on sites such as Ebay or Gumtree describing them as marine units. Don't be fooled into thinking the manufacturers are selling the same unit but higher priced just because it has the word ‘marine’ in front of it. There are differences and they are important ones when installing in a boat.

The marine version of these units use stainless steel fittings instead of galvanised ones. The fuel hose must be copper not plastic and no inline plastic fuel filters should be fitted. The stainless steel exhaust hose must be lagged and comes with a longer length and most importantly, it comes with a skin fitting to take it safely outside of the hull. There are other differences but this is not intended as an installation instruction only a cautionary note.

In the photo above the owner had installed a road going version of the heater in his motor boat. The exhaust was not lagged - even though it can reach temperatures up to 200∘C near the heater. There was no skin fitting, just the hose poked though an open hole with no clamping. The hose also had a corrosion split that was leaking exhaust fumes directly into the living space.

Whilst it is possible to fit your own units you must do your research to find the correct way in order to make the installation safe and if you aren't sure, please consult a professional.

Crevice corrosion

26 September, 2017

Cavity corrosion is another form of corrosion that can occur where you often don't inspect regularly.

Stainless steel is only stainless in the presence of oxygen and if you take the oxygen away, the stainless will corrode pretty easily. Water can become oxygen depleted in areas where it sits without moving. This normally happens in areas that are left hidden in small crevices and cracks such as rigging swages but can equally happen in cutless bearings and stern tubes if the vessel is not used or the prop shaft not turned regularly.

In the example above for some unknown reason, there was a tiny groove cut into the prop shaft taper just under the boss of the propeller. Over time the water that collected in it became stagnant and oxygen depleted causing crevice corrosion to develop unseen. This one was only found when the anode was being changed during a survey and the engineer noticed the propeller move very slightly on the shaft. This shows the shaft taper when the prop was removed, after cleaning it was found that nearly half the shaft had been eroded!

Normally crevice corrosion is only found when taking apart components but obviously things like standing rigging swages can’t be dismantled hence its one of the reasons for changing it every 10 years even though it looks alright on the surface.

To help prevent it, don't use stainless steel in areas that are likely to trap moisture where it can’t move unless it is the best material for other reasons. Crevice corrosion is also one reason why stainless steel seacocks are generally not the best option for boats.

Galvanic corrosion

23 April, 2017

Galvanic corrosion does not always occur below the waterline and can often be found on aluminium masts and booms that have stainless steel fittings attached to them.

In the example above a stainless steel kicker slider has been riveted to the boom but the insulation between the dissimilar metals has failed or was never there in the first place. This becomes far worse in a humid, salty environment and the damage can often be underneath the component where you can’t see the extent of the damage.

When galvanic corrosion occurs the aluminium becomes powdery and crumbles in the hand. This makes the material weak and under load the area can fail resulting in the components pulling out from the mast.

To prevent or at least reduce this the metals should be insulated from each other with an insulating compound or the component can be lacquered before being mounted. This includes the monel rivets going through the aluminium.

Always check your mast fittings for signs of corrosion and if in any doubt get a rigger to remove the components to check the condition of the aluminium underneath

Hose clip corrosion

7 April, 2017

Stainless steel hose clips on hose tails are fit and forget, right? Sadly not.

The problem with stainless steel is that it is only stainless in the presence of oxygen. This is because an oxide layer forms on the surface of the steel and this is what protects it from corrosion. If you remove the oxygen then the oxide layer cannot form and the stainless steel is about as corrosion resistant as mild steel.

When hose clips are clamped tight onto hose tails the underside is no longer in the presence of oxygen and the stainless steel can start to corrode. The trouble is, its not always obvious like the picture above. Normally the only way you can tell is by removing the hose clip and looking at the underside. In severe cases, gently tapping the clip can cause it break open.

Regularly inspect your hose clips and slacken one off at a time to have look at the underside.

As an aside, sometimes I find hose clips neatly lined up with the screw heads next to each other as in the above picture. This is not the correct way, the heads should be opposite each other to get a better seal on the tail.