Diesel oil in timber

Demise of the thumper
Before the advent of today’s fast running, sophisticated marine diesels there was a general acceptance that private yachts and diesel engines just didn’t mix. Diesels were the source of unwanted vibration and they were dirty, noisy and smelly – worst of all they ran on diesel oil and ‘everybody’ knew that diesel oil and timber could form a long-term and uneasy relationship.
Two revolutionary changes in the pleasure craft industry were to change this line of thought for ever. First - small, high-speed diesels were developed which dealt with the problems of vibration, dirt and smell and could provide all the sophisticated facilities previously confined to the petrol engine. Second - the most popular boat building material changed from timber to GRP (FRP) which is significantly more resistant to diesel fuel contamination.
The following notes are based on the survey of a 48ft twin screw diesel yacht constructed of 11/2” Honduras mahogany planking on oak. She had standard caulking payed with white lead stopping.
Defining the problem
A heavy deposit of diesel oil was found in the bilges. How, why or when this spillage occurred is outside my terms of reference.
In my opinion, because of the possible potential effect that this condition could have on the caulking and the adhesion of coatings, it falls within the scope of any current survey carried out on this vessel. I can confidently state that without remedial action this condition can only worsen.
First it should be stated that diesel oil is an excellent wood preservative and its presence should not detract from the structural integrity or strength of the timber affected. Equally it should have no effect on the fixings, either the copper clenching or the galvanised coach bolts fixing the engine bearers. It is also true that diesel fuel is volatile (proof of this is that it smells) and, provided this problem is aggressively tackled at this stage, any residual contamination should, over time, gradually dissipate of its own accord.
On first viewing, the contamination on the antifouling looked severe and affected about 30% of the below-the-waterline planking surface. However, because of a process called ‘thin layer chromotography’, diesel oil will stain a large surface area of a soft, porous substance such as antifouling, from a comparatively narrow source such as a seam. This type of staining produces an immediately recognisable soft edge. Diesel oil will also blacken the antifoul colour. The combination of these factors can often make the problem appear worse than it eventually turns out to be.
The pattern of the staining suggested that the initial source of the spillage was from the fuel tank area on either side and the spillage had run down the inside of the planking to accumulate in the bilges.
I concluded that, from the pattern of the staining, the diesel oil was probably being drawn through the seams by gravity possibly assisted by the capillary action of the caulking. Nevertheless scraping away small quantities of saturated antifoul over the seams, to reveal the white lead stopping, showed them to be remarkably tight.
External inspection below the waterline showed the staining pattern to be of equal distribution on both sides of the hull. The sole in the forward cabin was lifted to reveal considerable spillage in the bilges (probably up to 10-15 gallons) which would need to be pumped out at the earliest opportunity.
The possible, on-going effects of this contamination if left untreated could be:-
1 continuous smell of fuel oil below decks
2 difficulty in making coatings adhere below the waterline
3 possible degradation or loosening of caulking compounds
This was much as I could ascertain at this stage without carrying out destructive testing.
Recommendations -
The following recommendations are based on my experience with timber vessels over 30 years and the results of conferring with a number of colleagues including technical experts on marine coatings at International Paints, Nigel Clegg of ‘Passion for Paint’ and Paul Stevens, a fellow surveyor with considerable experience in wooden boat surveys. The one thing upon which there was agreement was that there is no perfect, foolproof answer with guaranteed results. There are also a number of undefined factors which could affect the outcome of each stage, for example the length of time that the oil has been laying in the bilges, the soundness of the caulking, how much protection has been given by the internal bilge paint and, possibly, the moisture content and quality of the timber.
The following procedures are recommended -
1 Physically remove as much of the diesel oil as possible from the bilges as soon as possible.
2 The introduction of a slightly diluted liquid detergent into the bilges (Yachtline Supercleaner is recommended by International Paints’ technical advisor but ‘Budget Washing-up Liquid’ may be considered as a less expensive possible alternative!). This should be followed by vigorously flushing through with fresh hot water to emulsify and remove most of the remaining oil from the internal surfaces. It may be necessary to do this three or four times before the flushing water runs clear. There are waterways through the timbers at either side of the hog and the echo sounder transducer is mounted at the deepest part of the bilges. If unscrewed and temporarily removed the remaining skin fitting provides an effective drainage point for cleaning fluids or flushing water. Allow the ‘clean’ bilges to dry out.
3 Steam clean the exterior and allow to dry out. This will probably need to be done two or three times. Examine the planking for signs of diesel oil absorption. If there is any doubt two or three applications of acetone or No.3 thinners could help lift any visible grease residue left by the diesel oil.
4 I suggest that a small section of caulking should removed from the worst affected areas on each side of the hull in order to check the bonding and also from an unaffected area for comparison. (If there is any doubt at this stage it might be considered expedient to send 4/5 samples to a lab. for comparative analysis. This however can be an expensive process and is only suggested as a last resort.)
5 If the results thus far are agreed to be satisfactory the sample areas can be re-caulked and payed and any other areas where stopping is missing refurbished.
6 In my opinion it would be of benefit to sand the planking using a coarse abrasive grit in order to raise the surface grain and provide a key for the coatings. The surface could then be coated with four coats of good quality marine metallic primer (Primocon or equivalent) allowed to rest and, within a month, antifouled in the normal way.
I would stress that these recommendations provide a suggested course of action and, should any unforeseen circumstance arise during the process, the procedures should be reviewed.
NEW, STEEL FUEL TANKS HAVE BEEN FITTED IN THIS VESSEL (ONE TO PORT, ONE TO STARBOARD). EACH OF THESE HAS BEEN PRESSURE TESTED AND EXTERNALLY COATED BEFORE INSTALLATION AND PRECAUTIONS HAVE BEEN TAKEN TO AVOID REOCCURRENCE OF THIS PROBLEM IN THE FUTURE.