Engineer’s Angle: Tow, tow, tow your boat gently ’cross the (Gulf) stream

Jan 15, 2020 by JD Anson

Engineer’s Angle: by JD Anson

The recent arms race in tender size has begun to present some interesting challenges. For all but the very largest of yachts, gone are the days of lifting the tender on board and running off to the next exotic destination. Tenders of 40-plus feet are now regularly towed behind the mother craft from port to port. For short jaunts, it may be simpler to run the tender on its own ahead of the yacht, but for multi-day passages it is not practical.

Weather conditions that may be annoying to those comfortably on board a yacht can become downright dangerous when traveling in a small craft. In poor weather, tenders frequently cannot maintain the same speed as the yacht. Navigation at night is much more difficult, and tenders simply do not have the range of a large power yacht. Also, manning needs require a minimum number of people on board both vessels, but many yachts do not have qualified spare crew to operate two vessels over long range.

Some yachts have forward-shipped the tenders to where they will be cruising. This is inconvenient since the tenders now cannot enter customs as part of the yacht. Transit time for the yacht and the ship are different, thus requiring logistics help on both ends. And it is costly, as a transport cradle usually has to be custom built and shipping costs are high. The cradle will then need to be stored until its next use.

For the above reasons and more, towing these large tenders has become popular. When this first became common a few decades ago, so many tenders were lost at sea that insurance companies refused to cover them. Techniques were as varied as the tenders themselves, with some more successful than others.

Low-tech polypropylene towing hawsers have given way to ultra-high-molecular-weight polyethylene (UHMWPE, UHMW) commonly known as Spectra and Dyneema. This exceptionally strong line reduces storage room, is lighter and is highly abrasion-resistant. With a specific gravity of 0.97, it floats on water to help avoid fouling propellers when retrieving the tender. However, it has very high lubricity, which makes for poor knot-holding capabilities. These towlines and bridles lines are best professionally made up with splicing and sewing.

The advent of AIS (automated identification system) transponders now allows real-time location of the tenders should they become detached from their towing hawsers. Each transponder has a unique identifier that is programmed into it and displays vessel name and position, making relocation of lost tenders quicker. Beware a common mistake when commissioning AIS: using the yacht’s identifying number. Each vessel has to have a unique number, easily obtainable through the flag state. Using the yacht’s AIS number with an “a” at the end is acceptable, but only for tenders carried on board – not for towed tenders.

The biggest complaint we get about towed tenders is keeping the AIS, nav lights and bilge pump powered over several days. Though AIS and LED nav lights are low-draw items, days of constant use coupled with frequent bilge pump activation in rough weather can easily drain the batteries. Despite larger tenders, builders are still installing the same small batteries in 12V configuration that they used when boats were 25 feet. There simply is not enough juice to last.

The best solution is a dedicated house battery just for the loads necessary for towing configuration. Coupled with a properly sized solar panel mounted on the hard top, this will keep the lights on happily over many days. If there is not room for this dedicated battery bank, a switched “tow” circuit can be designed that removes all loads except the necessary ones from the house batteries. This will extend the power life of the batteries to hopefully reach port. Some tenders have included a spreader light that will activate with a high-water alarm to alert those on board the mother ship that there is an issue.

Many panels we have seen are woefully undersized. One must consider not only the load, but factor in cushion for cloudy, rainy days when production is nil. Remember, stated wattage ratings are for perfect conditions and the testing procedures can be a bit dubious. Take all declared specifications with a grain of salt

With the proper equipment and careful deployment, that not-so-little boat will still be there at the end of a passage.

JD Anson has over 20 years of experience as a chief engineer on megayachts. He is currently project manager at Fine Line Marine Electric ( in Fort Lauderdale. Comments are welcome below.