Vector vs. Raster
Your editorial on bloatware (December 2001) could lead someone to believe that Maptech is a superior product because it copies the government charts. I disagree with that conclusion. I have both the Maptech charts and the Nobeltec vector charts for the entire East Coast and for me the Nobeltec software and charts are markedly superior. The vector charts are more readable at a glance and the quilting feature of the software makes the whole system easier to use for those of us who are computer-challenged.
In aviation, the government puts out instrument charts but all the airlines and most of general aviation uses Jeppesen because they are better. Guess who bought out Nobeltec about a year ago?
I have a boat on order that comes with a Raytheon package, to which I will add from my old boat a 12" daylight bright color monitor fed by my laptop (situated in a waterproof location). I probally will add a Northstar 952 unless the Raytheon GPS has gotten a lot easier to use. I cruise from the Bahamas to the Northeast and love to go exploring unfamiliar waters in a relaxed way, so the easy situational awareness of these electronic charts coupled with GPS and a little common sense makes life easy. Hope you test these systems soon.
Thank you, thank you, thank you for your editorial about vector chart accuracy in the December issue!
Over the past decade I have read numerous articles about electronic charts written by so-called technical experts (including some well-known names). In every case these "experts" swallowed the propaganda from vector manufacturers (layering, "seamlessness," smaller memory requirements) without a single one of them raising the issue of accuracy.
The accuracy issue was driven home for me seven or eight years ago at a visit to the Newport Boat Show. GPSs with integrated chart displays were just coming out and several high-end GPS vendors were proudly displaying their new units. One vendor had a unit visitors could play with. Since redraw rate is an issue for chart displays, I decided to pan and zoom around a bit. I soon found myself looking at the Cuttyhunk Harbor area. None of the entrance buoys was shown! (For those unfamiliar with southern New England waters, the approach to Cuttyhunk is straightforward, but one must steer clear of a couple of well-buoyed reefs standing out from Cuttyhunk and another pair projecting from adjacent Penikese Island.)
I looked for the most senior person I could find in the booth and pointed out to him that someone could rip the bottom out of their boat if they used his company's nice new product to navigate to a popular nearby harbor. He shrugged his shoulders and said "we buy the cartridge so we're not responsible for inaccuracies."
Hand-digitizing a chart is a labor-intensive process. I'll guess it's done by vendors selected for labor costs and not expertise in reading American nautical charts. If vector chart manufacturers can miss buoys, how about all those little plus signs and asterisks that indicate rocks?A vector chart will never get near my boat until and unless vector companies provide statements of accuracy with their charts.
Tim, your wish is our command. See this month's "Navigation Software" story. Luther, we've had experiences similar to yours, both at boat shows and afloat, and our look at the "charting" capabilities of handheld GPS receivers in the December issue did nothing to soften our hearts on the issue of potential inaccuracies in vector charts. Those receivers would be safer, in our our opinion, with no charts at all, but perhaps with better plotting functions and other data collected specifically for mariners.
That said, it had been a couple of years since we took a close look at the big vector charting packages for use on full-powered PCs, and the fact is, their accuracy and operational reliability have improved a lot—so much that we have no strong beef with Tim Palmer's assertions.
In any case, no electronic chart, either raster or vector, should take the place of a paper chart—what we could call "hard copy"—on the water. A paperless office is one thing; a paperless nav station isn't too bright.
Electric Power Feedback
I don't see the long-term problems with using 120- or 144-VDC systems ("Electric Motor Propulsion," December 2001). The reduced losses over 12- and 24-VDC systems will make up for more 12-volt batteries, and with cars going to 36- and 48-VDC systems, batteries in high-voltage packages will appear.
As for inverters running off the higher-voltage systems, hurray— it's about time we get rid of the giant wire problems with 12-VDC inverters. Voltage drop in wire and components reduces the operating efficiency.
The only major problem I see with a 120- or 144-VDC system on board is electrical shock. The wiring codes on boats have been poor for a long time; they're getting better, but still, the wiring practices of some boat owners—well...
Using 12-V for running lights and lights in general may well stay the standard for on the water. But with so many 120-VAC appliances on boats these days, I do not see how having 120-VDC would be any more dangerous. The conversion of 120 to 12 for lighting is a loss, but the conversion of 12- VDC to 120-VAC is even more of a loss compared to conversion of 120-VDC to 120-VAC and transmission around the boat.
As for replacement of batteries versus generating power with a generator or buying power dockside, I vote for replacing batteries ($900 every five years vs. fuel costs plus engine repairs, etc.).
Now, if you cruise a lot with a motor this might or might not be the way to go, but even at California's inflated power prices you still cannot generate power (using fossil fuel) cheaper than you can buy it here in the US in small amounts (under a megawatt).
I found the writeup on electric drives extremely interesting. You probably know that electric propulsion has been used for some years on large ships. What Solomon Technologies has done is scale the most modern version down to a size and price consistent with pleasure boats.
I have a few comments to make concerning the motors and drives:
1. The motors which ST have called "brushless DC" are, from the diagram in your article, actually synchronous motors which require a variable frequency and voltage AC supply, hence the need for the "controller."
2. If two synchronous motors are solidly coupled to the same shaft and fed from the same AC supply they will not, unless you are very lucky, share load. The load division may be 10%-90%, 0%-100%, 25%-75% or any other division.
The planetary gearbox looks to me like an awkward way to get around this and to make a virtue of a necessity by getting a variable gear ratio; however, the variable ratio is of little utility on a boat as ST have found. In the ST74 they have found some other means to force load sharing, possibly very careful assembly to control the relative rotational position of one motor with respect to the other, or a dedicated inverter for each motor plus a load balance regulator. In my mind, if you really need the 12 hp, then 2 6-hp units would be simpler and greatly increase reliability for $2800.
3. The quoted figure of 1.1 ft-lb torque per amp seems low to me, but my experience is with much larger motors. On a DC motor this is the torque constant or "Kt" and holds right down to zero speed as long as the field is kept constant. On a synchronous motor AC drive it should be near constant in the working speed and load range.
4. To get variable speed on a synchronous motor you must use variable frequency AC as you pointed out in your description of the controller, but you must also use variable voltage, maintaining constant "volts per hertz" or in other words make both frequency and voltage proportional to commanded speed. The controller also does this function. This must be fudged a bit at low or zero speed, otherwise you have the problem of starting from zero speed with zero volts, hence zero current and zero torque. This may be the source of your statement that the 1.1 ft-lb per amp figure is voltage dependent.
5. The "low buzzing" sound you mentioned is caused by magnetostriction vibration due to the chopped voltage waveform generated by the controller. Compared to an engine it is certainly an improvement, but this chopped waveform can also cause serious interference with other electronics on the boat, particularly the VHF and other sound systems. To prevent this the drive wiring should be well shielded and grounded and other wiring on the boat must be designed for low noise pick-up, something I have never seen on a boat, even professional jobs.
6. How does this electric drive handle a jammed propeller? If something is not done about it, a jammed propeller forces zero speed and the controller and motor will do everything they can to maintain the commanded speed, destroying themselves in the process. I would hope that this has been addressed.
I do not own an electrically powered boat but have much experience with variable speed electric drives in many industries including a bit on ship propulsion.
David Tether of Solomon Technologies responds:
1. As the article said, the motors are, in fact, 3-phase brushless DC and are controlled by a digital 3-phase controller.
2.(a) Because the motors and controls are continually load sensing they share the load equally and 50%-50%—inherent to the operation of this new motor technology.
2.(b) Not so. The Electric Wheel is of significant utility, especially when it makes an entirely redundant propulsion system. The Electric Wheel allows each motor to operate at better than 80% efficiency all the time, and typically at 90% efficiency. The ST37 and ST74 will operate at the efficiency of the rpm that they are used at. Around the dock at less than 300 rpm the 37 and 74 would be at 40-50% efficiency whereas the EW will be at 80%efficiency. You also have the inverse of the gear ratio for regeneration—one turn of the propeller in regeneration causes one turn of the rotor in the 37 and 74, but the EW sun gear would have 2.6 rotations of the rotor—more regeneration.
3. On brushless DC motors (even 3- phase) Kt is the same at even 1 rpm.
4. This is a DC version of the AC variable frequency drive. We have married together the best of both worlds—3-phase, but DC with digitally produced waveform, which can control pulse width and pulse repetition frequency by using digitally controlled switches—it's more efficient.
5. Everything is shielded and EMC tested, and with CE certification.
6. For a jammed propeller it senses the overcurrent and shuts down electronically.
Editor's note: This discussion is heady stuff. We could spend a lot of time sorting out the finer points, but the central questions remain: On any given boat, will there be enough oomph? (This is a technical term we use in order to avoid having to differentiate ad nauseam between horsepower and torque.) Will the oomph last long enough? How much will speed suffer during regeneration? What will the real costs be over time? It is safe?
The proof will be, as usual, in the pudding. However, this discussion is interesting. We'll continue it on the PS website. Look in Mailport Online under "Electric Auxiliary Power."