Aboard our 42-foot gaff-rigged cutter BalÃ¦na, we particularly enjoy voyaging to less visited places, such as the fjords of Greenland or Patagonia. Poorer countries and more remote areas of developed lands often lack charts with the detail and accuracy that we expect when sailing in the U.S. or Europe. Sailing in those waters carries a risk of collision with uncharted dangers. In addition, choosing a secure anchoring place often involves a preliminary survey to ensure that we won’t be bumping into a rock if the tide goes out or the boat swings with a change of wind. Even quite detailed charts can miss out on features required for this task, and who’s to say that someone has not decided to scuttle an old barge in the place that we chose? Such surveys have often resulted in a bump to our keel, and over the years we have talked about how fine it would be to have the ability to look forward and see what lay ahead.
Five years back in Patagonia, where many useful anchorages are just white blanks on the chart, we even experimented with a standard fish finder mounted in the bow and looking forward at an angle of about 20°. This did give us some forewarning of underwater obstructions but the display was so difficult to interpret that we abandoned the idea. Since then we have been watching the development of forward-scanning sonar and wondering when we would be able to afford one.
Sailing in the muddy waters of the Intracoastal Waterway we often touched bottom where markers were far apart and the dredged channel narrow. We wished for a forward scanner that gave a horizontal picture and allowed us to see where the channel was. Uncharted dangers were not a problem on the North American East Coast until we reached the north of Labrador and once again had to use charts that had large blank spaces marked “many uncharted dangers are believed to exist in these waters.” One time pack-ice blocked the only route where, in 1927, Dr. Wilfred Grenfell’s survey ship had made one line of soundings. BalÃ¦na was forced to sail for hours over the blank part of the chart in almost total ignorance of what lay under the waters ahead. How we wished for a forward-scanning sonar that would show a rock rising precipitously up from the seabed.
In Greenland we saw ice every day, huge icebergs or broken pack-ice but occasionally, in what appeared to be clear water, we would suddenly see a small (remember you only see one-tenth of a piece of ice; the rest is below the surface) growler right ahead. Mostly we avoided them but we still had half a dozen hard knocks on the hull during our three months in those waters. How nice it would be to have a sonar scanning ahead just below the surface to warn us of lurking dangers.
We could never decide whether it would be better to have a horizontal or vertical scan. The first such device we ever saw was an EchoPilot unit aboard a yacht in harbor. The display clearly showed the harbor wall lying some distance ahead. We were impressed.
Finally this year it looked as though we could afford a unit and we started to assess the options. We found two instruments suitable for a yacht using solid-state electronic transducers as opposed to those that can be mechanically steered.
€. The most economical was the EchoPilot that we had already seen and liked. This had a clear display and simple controls, but range and depth were very limited – not so important in an anchorage but important if trying to spot hidden rocks or ice ahead when steaming. There was no horizontal scan option. The transducer is plastic and requires a 1 3/4-inch hole to be cut in the boat, no small issue for a skipper who refuses to have more than one underwater seacock on the whole boat.
€. Interphase has a reputation for quality and service. The company’s word of mouth on the voyagers’ grapevine – probably one of the most reliable sources – was very high, and it had started to sell units that combined both functions in one set. The transducer was very large but was mostly bronze and only required a 1-inch hole in the hull, something that I found much easier to live with. Plus, Interphase offered factory reconditioned units at a substantial discount, which made the price just attainable. I was also greatly impressed by the immediate, detailed and helpful replies Interphase gave to my e-mail inquires. We decided to take the leap and buy an Interphase Color Twinscope.
A normal sonar or fish finder can be mounted almost anywhere, though it is best to stay away from turbulent areas, such as around the bow. Conventional voyaging boats mainly have wineglass-shaped hulls so they will require fairing blocks to give a level surface. Transducers with a shaft length of four to five inches will be suitable for fiberglass or wooden hulls. (A modern fiberglass boat with an almost flat bottom requires much less length.) Most people mount the transducer just below the turn of the bilge to one side of the keel, and this will be fine for downward-looking units. But if the sonar is also going to scan horizontally the keel will get in the way. Again, modern boats with flat sections can probably mount a transducer just to one side and slightly back from the forward edge of the keel. However, our experience is that around 50 percent of the voyagers that sail to the sort of places where forward-scanning sonar will be most useful have more conventional hull forms. The only option then is to mount in the stem or just to one side. Interphase recommends that, in order to avoid reduced performance due to turbulent water and bubbles dragged under the boat, the mounting should not be closer than one-third of waterline length from the bow. This is just not possible on many voyaging hulls.
Interphase offers a two-transducer option, which would solve most of the above problems, but I was not willing to drill two holes in the hull and have the associated extra drag.
Stems should be pretty heavy, even in fiberglass boats. Ours was 12 inches thick at the point where we decided to mount the transducer. We were able to chisel into the stem to make a recess for the transducer, and this helped reduce the distance somewhat, but it still required more than the 6-inch shaft supplied with the transducer. We had to drill a 1 3/4-inch hole from the inside down to the point where the retaining nut could be fitted. (On a fiberglass boat a fairing piece would be needed inside and out, that thickness would have to be added to the stem thickness and might get very close to 6 inches.)
The transducer was 4 inches wide and 2 1/2 inches high. It was not particularly hydrodynamically shaped and needed quite a bit of fairing to minimize turbulence. We sealed the cut edges of our mounting position with heavy glass cloth and epoxy resin and then used pieces of polystyrene foam coated with epoxy to achieve the fairing. It is recommended that the unit is painted with water-based bottom paint. Solvents are said to damage the construction material. No such paint was available at the yard where we hauled out in Norway. We coated the unit with epoxy resin and then used regular bottom paint over that.
There are two cables to lead through the boat. The cable supplied was 30 feet long and just sufficient to reach our navigation station. Here we mounted the display and found the mounting base to be rather oversized, as we were mounting on a bulkhead, so the base had to be turned back behind the unit, which stuck out considerably farther than our other instruments, such as radar and GPS.
After checking for leaks, it was time to see how the unit would perform. We were near Ã…lesund on the west coast of Norway, probably ideal conditions to try out the sonar. The underwater geography of the fjords is every bit as dramatic as above water. Rocks can appear near the surface when surrounding water is hundreds of feet deep, but the charts are excellent so there should be no excuse for collisions while learning.
We spent a lot of time learning to use and interpret the display and were far from competent even after a few months of intensive practice. There is no magic signal that says “watch out, rock ahead;” we just had to learn to interpret the display. So we had more near misses in the first few weeks after installation than before. It would have been hard to do this when sailing single-handed. The best way to practice is to find a shallow channel that is navigable but has known dangers, and steam back and forth trying out different display options.
The Interphase Color Twinscope is a fantastic piece of technology that should be welcome on any serious offshore voyaging boat. There are limitations but they are outweighed by the advantages. This is not a simple piece of equipment and requires careful installation and setup, study of the manual and much practice on the water. A good comparison is with radar, which does a similar job and also requires skill and experience to interpret. Radar, however, works in a range of miles, while the Twinscope works in feet!
When reading the display it is important to keep in mind that different bottom types and the angle at which they lie to the sonar are important factors in interpretation. Soft mud gives a weaker reflection than rock (that is quite convenient as we are much more concerned about rock than mud). An upward sloping bottom ahead returns a strong echo, while a level bottom gives a weaker return and a downward sloping one the weakest or no return. The colors on the display are graduated to indicate strength of return (not necessarily the same as density of the object – a fact that is important to remember) and this makes interpretation very much easier.
–Beam angle: The Twinscope’s horizontal beam is at about 10° from the water’s surface. I suspect that to arrange for a beam with controllable angle would require a large mechanical transducer or a considerable increase in complexity and expense if done electronically. The angle of the beam means that it strikes a level bottom at a distance ahead of approximately five to six times the depth. In effect, this means that the range is limited to five times the depth, whereas in favorable conditions the vertical view may be able to perform better. Because the obstruction is cut by the beam at a certain depth only, it may be difficult to decide how deep it is.
–Scanning time: The scanning time limits the speed with which the display is updated; objects can suddenly change position on the screen when the boat is moving or turning. Faster scanning time is presumably not possible because of the relatively slow speed of sound. The scanning speed automatically increases with shorter range.
–Gain adjustment: Surface clutter and noise are always problems with sonar, with high gain settings and long ranges there is often the appearance of an obstruction just ahead of the boat, and in rough weather it can be difficult to distinguish objects close to the surface. The image can be greatly improved by careful manual adjustment of the gain control.
–Power consumption: The unit’s electrical needs are very reasonable at about 1 amp, but it is about 10 times that of our old depth sounder.
The Interphase Color Twinscope is a true look into the future. I believe that conventional depth sounders will rapidly give way to forward-looking sonar, and this instrument leads the way.
I have been impressed by its performance and abilities, and the criticism I have raised can be seen as mainly a wish list for improvements. Interphase invites user feedback, has been very helpful and speedy in answering my questions, and has promised to look into my suggestions for improvements. It is possible that some of them may be implemented by the time this article is published.
Andy O’Grady grew up in England and Ireland. He sailed to New Zealand in a 26-foot wooden boat built in 1939, where he spent four years building BalÃ¦na while working full time in a medical practice. O’Grady and his partner Ulla Norlander have written on a variety of voyaging topics. They’ve edited the second edition of the RCC Pilotage Foundation Guide to Chile and have co-authored A Guide to Ocean Passages and Landfalls with Rod Heikel.