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
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
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
The possible, on-going effects of this contamination if left untreated
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
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
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
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.