Q:  For the last six months, I’ve been getting conflicting answers to the question of whether or not owners of diesel engine boats need to take any special measures to make up for the ultra low sulfur diesel now being pumped into our tanks. Some say it definitely requires an additive to lubricate engine parts; others say it’s not needed; and still others (notably the manufacturer of my engine) say that they don’t know! I’d appreciate any advice on this issue.

A: Petroleum companies are always bringing new products to the marketplace for various economic and environmental reasons, and sometimes it can lead to confusion for consumers. Low sulfur diesel fuel has been distributed for a few years now to reduce emissions, especially in the truck transport industry. 

New formulations of diesel still contain lubricants, which provide protection for engine bearings, seals and rings. The only potential fly in the ointment is that some fuel lift pumps may require an additive. It would be best to check with your engine manufacturer to make sure, although the majority of engines would not require this treatment.

If diesel oil required more lubrication for running in engines we would have a major problem—and not just in the marine industry. Every truck driver in the world would be up in arms having to pay an additional expense to put additives in their fuel tanks. The short answer is diesel fuel in its present form is fine for marine and land-based diesel engines.

Q: I’m confused about caulking compounds. I get conflicting advice on material type and application and am no closer to understanding what to use where and why. Please help!

A: There are three basic applications for materials generally referred to as caulking compounds. Modern caulking compounds are the gooey stuff sold in tubes in
chandleries. Use only materials identified for marine application and follow instructions to the letter. A lot of testing has gone into each tube, so follow the directions for the advertised results.

Adhesives are used to bond parts together forever and are characterized by very high adhesion so they stick to other stuff well. Their very high cohesion means they stick to themselves really well. Polyurethane compounds such as Sikaflex (most formulations) and 3M’s 5200 are common examples. These stick so well that they can and often do damage substrates when parts bonded with them are separated. For example, gelcoat may lift when removing deck hardware bonded in place with high-tensile polyurethane. Adhesives are moderately resistant to UV and petroleum attack (some are specially formulated to increase UV stability), cure within 24 hours (usually) and can be painted. Polyurethanes are usually not compatible with plastics such as acrylic and polycarbonate. The adhesive attacks the plastic, causing cracking and cloudiness.

Sealants, used to seal parts that are mechanically fastened and that will be disassembled for service or replacement (such as deck hardware and thru-hulls), are characterized by high adhesion and moderate cohesion. Polysulphide materials most commonly have these characteristics. They are highly resistant to UV radiation and petroleum attack. Polysulphides cure slowly, are sandable once cured and can be painted. They are not suitable for use with acrylic or polycarbonate windows.

Gasket material is a category limited to silicone materials. Gaskets are formed between surfaces that are mechanically fastened together. Silicone has relatively low adhesion and high cohesion, so it peels away from whatever it is applied to fairly easily (think of all the failed caulking jobs we’ve had to deal with around bathtubs), but it resists being pulled to pieces quite well. The secret to the success of silicone is that it must remain under pressure between the items it is gasketing, but it must never be relied on to sustain adhesion. The seal formed by the silicone gasket relies on the mechanical fastenings to maintain pressure on the gasket, thus maintaining the seal. Silicone is very stable under UV rays and cures within 24 hours (usually). Chemically, it is relatively benign and is safe to use with acrylic and polycarbonate, making this material the number one choice for sealing plastic ports. However, be careful! It cannot be painted over and will prevent all other materials, including paint, from sticking to any surface that it has touched, regardless of how well it’s been cleaned up. Contaminated surfaces must be scraped or ground to remove residue.

Hybrid caulking materials are also available. These formulations are mixtures of two or more of the above materials. They have blended service characteristics, such as polyurethane/silicone caulks that are less tenacious, more UV-resistant, faster curing and non-corrosive, making them more versatile than some of those listed above. Some have a very short skin-over time. Once the caulking has skinned over, it will not bond well to the mating surfaces, so assembly must be finished before the material has skinned to the point of having no tack to the surface.

Application  Now, a word about general application guidelines: All surfaces must be clean, dry, free of oily contamination and stable. While adhesives are very strong, if parts are sliding past each other, bond failure is not far off. With all these materials, film thickness is essential to their proper function. Adhesives work best when applied in a relatively thick layer. This allows the adhesive to stretch with moderate movement in the joint and remain bonded to parts and to itself. All parts must remain stationary while the materials cure.

Sealants rely on the mechanical fasteners through the part to keep it in place. The primary function of a sealant is to stay bonded to the assembled parts and to keep water out.
Gaskets also rely on mechanical fasteners to keep joints together. The gasket can only function while under pressure. Gasket beads should be wider than they are thick, and thickness should be around four millimetres to ensure sufficient material to respond to expansion/contraction of the joint.