On Board with Mark Corke

Living on board

There was good story in recently in the Seattle Times about living aboard which you can read by clicking here.

But it is a shame that they did not check their facts a little better as according to the otherwise good article the average 40 foot sailboat uses one gallon of fuel every half mile while a power boat of the same length uses only half that.
 

Osmotic blisters

If there is one thing that most owners don't want to find when they haul their boat for an inspection it has got to be the dreaded hull blisters. Osmosis is the most often used term for this malady and is caused by water migrating through the outer layer of gel-coat and into, usually, the first layer of laminate which is often the chopped strand mat. Because water is required for the blisters to form blisters above the waterline although possible are rare and as was the case  with this particular boat the topsides looked great and it was only when the boat came out on the hoist that the extent of the damage became evident. The blisters in this particular case ranged from a half inch in diameter up to about one and a half inches, not the worst i have ever seen but bad enough to warrant a major repair.

In a nutshell water migrates through the surface of the hull where it is trapped in small voids that it finds in the laminate. The water then reacts with the styrene and forms an acidic fluid. When the blisters are pictured a smelly brown fluid is released which is the result of the water sitting in the hull for some time. If left unchecked the blisters will grow progressively bigger and permeate deeper into the laminate until the structural integrity of the boat is compromised. We are not at this stage with the boat shown in the picture above but the boat will have to be peeled and the bottom made good, a new barrier coating applied before finally being anti fouled. The cost to repair the damage to this 70 foot motor yacht was estimated at $25,000, not a small sum. If you have a glass fiber boat then be on the look out for blisters each time the boat is hauled and if you spot a problem occurring look into fixing it early. Small blisters can often be filled fairly easily but waiting until things get to this stage is not a good idea. If ever there was a time to use the old adage ' a stitch in time saves nine' then this is it.

Sea chests

I for the life of me cannot understand why more boats do not utilize sea chests. This has nothing to do with gold doubloons or buried treasure but it has a lot to do with organizing the plumbing on board. Basically a sea chest is a manifold from which all sea water services are drawn. They can take many forms but I do like the one here which has been made I understand for a custom 42 foot sailboat. There are various ways of organizing a sea chest but this one here uses a fiber glass tube into which all the marked sea cocks connect. What makes this one special is that the tube reaches higher than the outside water level and has a clear removable top enabling the owner to easily clear weed and other debris from the intakes all from the comparative comfort of the engine room. If I had to take issue with this one it would be the use of a box section. Easy to make perhaps and connect up the sea cocks but I wonder about the hard corners at the bottom that could lead to stress cracking.

Out gassing

Lest you think that I am suffering from severe flatulence I should explain that out gassing is a phenomenon whereby trapped air within a structure forces its way out through a surface coating and mars the finish. So it was when I made a repair to this oak deadwood recently. Before the wood could be painted I wanted to coat it with epoxy to seal the surface. I applied the epoxy fairly early in the morning but we were all set for a hot day and as the wood heated up the trapped air within it expanded and caused the bubbles to appear on the surface (click the picture to make it larger). Normally this is not a problem here in New England as it does often get that hot but we have been having some very high daytime temperatures lately and as the mercury rose to 94 degrees the bubbles appeared.

It was not too much of a problem and wet sanding the epoxy after it had cured got rid of the bubbles but I'll know next time to do this sort of thing later in the day; with the temperature falling slowly instead of rising the bubbles would not have the chance to form as air is tending to be sucked into the wood instead of driven from it. Finally I should add that coating any substrate that contains air such as foam can also produce this effect so be on the look out for it and check the forecast especially if working outdoors.

Cavitation and other maladies

 

I had the pleasure of surveying a delightful Williams 36 last week and the boat was almost perfect in every way. In fact there was little to find fault with but one of the things that I did spot when the boat was pulled from the water for an under body inspection was this.  What you are seeing in the picture above is erosion caused by the prop wash to the lower rudder support skeg. You can just make out the tip of the prop. The clearance between the tip and the support was little more than half and inch, each time that a blade passes the skeg there is not enough room for the water coming off the blade and it is compressed between blade and skeg. This has a scouring action and causes the damage that you see here.

Here is another shot of the same boat and as you can see there is plenty of tip clearance between the top of the blades and the hull bottom but the clearance at the bottom is very small.  Looking from inside the boat it was clear that there was not any more room to lower the end of the shaft and thus raise the outboard end a bit. It may also be that because these are semi custom boats and they all have different engines the shaft, prop, alignment combination was dictated by the Yanmar 350 hp engine; it's hard to say but my guess is that the turbulence caused by the low tip clearance is having an effect on top speed and fuel economy. If the boat were mine I think that I would do a little experimentation by grinding a little out of the skeg and adding some suitable reinforcement of course, then maybe altering the size and pitch of the prop a little. I bet that if the turbulence could be reduced or eliminated the boat would run a little sweeter.

Wooden Boat Show

Here is a slide show of images from the 2010 Wooden Boat show at Mystic Seaport.

Hope to see you there next year.

Crevice corrosion

My friend Glen and I were working on Mallard's rig the other night when Glen spotted a hairline crack in one of the stainless shackles. The shackle was holding on, but only just and I was very easily able to pull the two bits apart. I would appear that a small crack had at some time started in  the shackle, water had found it's way in and the insidious corrosion started. Luckily we spotted it just in time or something bad might have happened sooner rather than later.

For crevice corrosion to start there has to be certain factors in play. Firstly as it's name implies there has to be a crevice where water can collect, if the crack is too wide then air can get in and crevice corrosion is unlikely. Secondly there has to be moisture to enable the corrosion to occur. We had both with the shackle, it is quite likely that over tightening the shackle at some point in the past has fractured the top of the bow by an imperceptible amount, sea water got in and the corrosion followed shortly after. Sea water is an ideal electrolyte and although crevice corrosion can and does occur with fresh water salt water exacerbates the problems and failure will often be sooner than would be the case with fresh water.

It should be noted that crevice corrosion only occurs under very specific circumstances, namely; there has to be a microscopic crack, this crack then has to fill with moisture which then becomes de oxygenated and finally the part has to go undisturbed allowing the corrosion to take place. All these factors were in place with the shackle it seems. The only cure is a sharp lookout  and to be aware of the factors that allow crevice corrosion to take place.