Trailer Sailor Articles & Reviews
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Electric Trolling Motors for Auxilliary Propulsion on Sailboats
Posted By: Charles Brennan and Jobst Vandrey
Date: 10/3/99 6:14a.m.
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Synopsis for those of you that like to cut to the chase: If you are a hull speed fanatic, read no further, Electric is not for you. If you need to maintain a tight schedule when sailing, (regardless of the wind) read no further, Electric is not for you. If you need to motor long distances (several miles or more), read no further, Electric is not for you. If you need to motor against strong winds, read no further, Electric is not for you. If you need to motor against strong currents, read no further, Electric is not for you. However, if you sail on lakes, reservoirs or somewhat sheltered areas, and need a kicker motor to clear the marina, dock, mooring, or ramp before raising sails; read on - Electric might be just the ticket! ____________________________________________ Electric trolling motors vs. gas motors? This is a question that keeps coming up on the Trailer Sailor BBS, so Jobst Vandrey and Charles Brennan decided it needs a definitive article. We compiled this article from numerous T/S contributors, and posts from the archives, (even fixed some of the spelling!) and synthesized it to these points. They are by no means exhaustive, and will likely spark some debate as to finer points, but for an introductory primer, it should do the job. Using an electric trolling motor as auxilliary propulsion for a sailboat breaks down into a couple of categories or groups based on application requirements. Areas of usage: The most successful use of electric motors occurs in sheltered areas, where there is a cove, or a lake well protected by trees, a reservoir that only allows electric motors, a river with marginal current. Almost all the warnings against electric motors revolve around wind (won't go against it), current (won't go against it) and distance (won't go for long). Boat size: This is a Catch-22 of sorts. Small boats (14'-16') have the best results with electric motors, but are the most adversely affected by the additional weight of the batteries. Larger boats, that have no problems carrying a couple of Group 27 batteries, have marginal results with the thrust available from most trolling motors. WARNING-MATH FOLLOWS take your favorite headache remedy and read on or skip this section completely Most modern 12-volt trolling motors draw one amp of current for every pound of thrust. At an assumed electrical efficiency of 95%, the Motorguide T47 motor (rated at 47 pounds of thrust) will consume (47 * 12.8) = 601.6 watts and deliver 601.6 * .95 = 571.5 watts of useful work. Since one horsepower is equivalent to about 768 watts, this amounts to approximately 3/4 horsepower at the propeller shaft. The "Weed Free" propeller designs may sacrifice some of this efficiency. It is nice to remember that a well-trained athlete generally can produce about 1/3 horsepower on a steady basis (like when rowing). This means that the T47 motor is about the same as having two strong rowers on board. The following calculations assume a Com-Pac 19 with a loaded displacement of 2,500 pounds. To calculate how fast this power input will move a boat, we must first calculate the Speed to Length ratio for the vessel: S/L = 10.655 divided by the cube root of (Displacement/Shaft Horsepower) For the Com-Pac 19 with it's 2,500 pounds of displacement (boat, crew, and equipment) with about 3/4 horsepower, this means an S/L of 0.73. Knowing the S/L ratio, we can calculate the projected speed: Boat Speed = (S/L) times the square root of the water line length = 0.73 * 4.06 = 2.96 knots Of course, this assumes a clean hull without wind or current resistance. You can use the calculations to make a few estimates about your own boat. However, the real world experiences outlined below will suggest that if you go electric, you are still better off getting the largest electric motor possible to give you a safety margin when needed. MATH SECTION ENDS -- resume normal reading To translate the preceding math, into usable experience, here are some excerpts from our contributors about thrust: Tim Allen mentions the "Outboard Boater's Handbook" suggests that a good rule of thumb for electric motors is 1 pound of thrust for every 100 pounds of boat to be moved. Mike: I use a trolling motor with my Capri 18 for docking maneuvers and it seems to work very well on the lakes I sail on. It is a T52 Motor Guide ordered from West Marine with 52lbs. of thrust and 5 forward and 2 reverse speeds. Bob Pr: I use a 55lb thrust MinnKota on my S2 6.9 (displacement. 2600lbs) and it moves it quite adequately for my purposes. I was extremely satisfied using mine on my Ensign (3klb displacement) but my slip was then in a somewhat sheltered cove and it was mainly needed just to get out past the breakwater to where the wind was. On my S2 6.9 (2.6k displacement) it doesn't work as well because the wind comes right across the lake to the trailer ramp where I'm launching now and on windy days, it can be difficult to get out. Stephen Jensen: I hope a 36lb thrust will be enough for my 14' C lark. George Norkus: A West coast person used only one electric motor for his Santana 2023A. The 23 footer had no problems even with six people and in the ocean. It was mainly used in the harbor area only. Harvey Wilson: I used a 36 lb. thrust electric on my 1600 lb. M-17 when the 'Rude was in the shop. Pushed her fairly well out of the ramp to the sailing area on Lake Lanier. No great shakes for speed...probably 2-2.5 knots max. Steve Haines: I've got a 55lb thrust Minn-Kota that I have used on a Seaward Fox(17x8,1300lbs) and a Sea Pearl 21 (21x5, about 1300lbs). Craig Cook: I have used Motorguide trolling motors with great success in the past and find them reasonably priced, plenty of power, with parts such as different props and shear pins easy to find. You can go from a relatively inexpensive unit up to a "Great White" used in salt water. My last boat was around 2,000lbs and my 48lb thrust moved it nicely. The trolling motor had 5 speeds forward and 2 reverse. John Weigandt: I can run my 42lb thrust Minn Kota on full for over 2 hours on a 115 ah battery and not completely discharge. On my boat, 16 foot hull it will push along at 2 1/2 mph or so. Mine is a light boat with no ballast. Kim Wright: I used a 55lb thrust electric trolling motor on my 24' Windrose for a weekend. It worked fine for over 4 running hours. I ran at half power to conserve power and the boat made decent speed. We were hooked up to a group 30 deep cycle battery. I could live with this system if I had to. Jobst Vandrey: I use a 47 pound thrust trolling motor for most purposes on my Com-Pac 19 (2,000 pounds). Usually, only the lowest setting is needed to get me in and out of the marina Jerry Kricks: I've used a little 15 pound thrust electric on a small jon boat for pond fishing for years... love it. So when I bought a Precision 14 this spring, decided to try it. It's okay, but marginal in power, I'd say, since the P-14 has more freeboard for the wind to get onto... plus I'm taking it onto more unprotected waters. For that size boat (wish I knew what it weighed!) I think that a 30 pound thrust motor would be perfect, and I plan to get one soon. Riley Smith: I use a 36lb thrust Great White on my catboat with good results. Don't know how much the boat weighs but the motor gives me excellent results and is built to take the saltwater I sail in. I only use it for getting away from the dock, but once I had to motor about 1/2 mile and had no problems. Charles Brennan: I used a Sears 28 pound thrust trolling motor on my 18 foot Windrose for about 3 years. Mike Q: Get at least a 26lb thrust and don't expect anything more than docking; you can troll for about 3 hours slowly. Dan Krammer: I think it depends on your needs...I am using a 45lb. thrust "Great White" on my Ranger 20. At full thrust, it powers at 3-3.5kts using about 45 amps. This gives me easily 90 minutes endurance with a typical group. 27 deep cycle, and still leave about 30-40% charge in the battery...keeps it lasting longer. In general, people with boats 18' and over, almost to a man, assert that 50 pounds thrust is a minimum, whereas the 14' C-larks say that 36 pounds is more than adequate. Several of the posts mentioned the desirability of multiple speed settings to improve duration of battery, and control over varying conditions. As verified by a few posters, the variable speed models are substantially more expensive but do improve battery utilization at low speed settings. Due to the way this type of speed control operates, it may create some electrical noise that could interfere with radio or other sensitive devices. Make sure to route your motor power cables as far as possible from an antenna or power cables to your radios. Don't forget to twist the power cables, so as to break up stray electro-magnetic fields, for those antiques out there, still using wet compasses! Battery type: Most contributors to the TS BB are in agreement that heavy-duty deep cycle marine batteries are the way to go. Mike: I use a 73 Ah Group 24 gel battery mounted in a covered battery box. Harvey Wilson: I'd go to BassPro, or some such place, and get a good deep discharge marine battery. Trojans are well thought of, as are Optima. Optima is pricey, but no water is required, and they'll hold a good charge for a year (so I'm told); I'll be getting one soon. Craig Cook: I suggest a single, group 27, trolling motor battery (deep cycle), in a battery box. I always use traditional lead/acid batteries and find them relatively cheap to buy, they seem to last for years with proper care. Kim Wright: We were hooked up to a group 30, deep cycle battery. Riley Smith: I have a heavy duty deep cycle battery and keep it well charged. Your power and length of running time really depends a lot on a good quality battery. Charles Brennan: I found that the conventional boat battery wears down pretty quickly, compared to deep cycle discharge types. Dan Krammer: At full thrust, it powers at 3-3.5kts using about 45 amps. This gives me easily 90 minutes endurance with a typical group 27 deep cycle, and still leave about 30-40% charge in the battery...keeps it lasting longer. Craig Norrell: Rule of thumb for power draw is 1 amp per pound of thrust. Don't run your battery all the way down often. You'll kill it early. Shore Power Charging Bob Pr: I'd add and STRONGLY RECOMMEND: Get a marine multistage battery charger. I used my automotive type early on and burned up batteries. The slip was 45' from home and I'd either undercharge or overcharge; neither is good, but overcharge becomes expensive and deadly to the battery. After I got the marine charger ($100 or less from Boat/US or West for one bank, not much more for two banks), my batteries were always full and I never replaced again. If you're going electric, do it right. Currently, (editors note: no pun intended!) I use a single Optima deep cycle charged by a Siemens 55w photo-voltaic going through a Morningstar charge controller (functions like the 3 stage 110v charger). Keeps it up fine. Charles Brennan: Make sure that your charging system matches your battery technology. Don't try to use an automotive charger on a gel-cell for example. Over-charging will boil electrolyte out of your battery and prematurely age it. Under-charging will leave you WAYYYYY out there with a weak battery. Trickle chargers are a good way to go, but even better, are float chargers, which can be left in place until the battery is needed. They can be left on a battery stored for the winter, for example. The down side is that float chargers sufficient for deep-cycle batteries are not cheap. One work-around: Do the bulk of the charge with a regular charger, and top off with the float charger. Solar Powered Charging: The sad fact is that the most efficient 40 pound thrust electric trolling motor (with the best variable speed controller) operating at one half speed for 15 minutes, will use a minimum of 5 amp hours from your group 27 deep cycle marine battery. This might be just the amount of time needed to get clear of the marina to a point where you can raise your sails. Getting back in again at the end of the day will use up the same amount of electricity, so you need to replace at least 10 amp hours of electricity for each day of operation under these conditions. If you have the space to mount one, a solar panel might just be the ticket to keep up with this drain. However, this is not a small investment since a 40-watt panel can cost $220 or more. During the summer in the central mid-west, a 40-watt solar panel will provide between 10 and 12 amp hours of usable electricity to your battery per day. The further north you are, the less electricity you will get; however, if you are sailing southern Florida waters, you will get 12 to 15 amp hours per day from the same panel. This is due to the more optimum sun angle in the south. Other things to consider are that batteries lose charge just sitting idle, so the solar panel also needs to make up for that loss. Many boats also have other electrical appliances on board, in addition to the mandatory running lights. For these reasons, a solar panel of less than 20 watts is rarely of much use to replace the demands made by occasional use of a trolling motor. Regulators and Solar Charging: The manuals suggest that a small solar panel feeding a big battery bank will not require a regulator. In practice, if your solar panel has a maximum output of 1 amp or less and your battery has a 100 amp hour capacity, then a regulator is not really required. As long as you keep your battery well maintained and filled with water, this seems OK - but if you have enough solar power to meet the needs of a trolling motor, you will probably want to install a regulator. Pricing: There are a number of trolling motor manufacturers, but Motor Guide and MinnKota are most often mentioned. These motors are available from a number of sources both marine related, and local general merchandise discounters. Once you settle on a model, shop around for the best price since the price range can be quite wide. For those of you that intend to venture out into the ocean or brackish water routinely, the salt water versions of trolling motors will be a better buy, since they are much better designed to resist the corrosive effects of sea water. One thing to consider is the shaft length of the motor. Gasoline driven outboard motors come in three shaft lengths - standard (15 inch), long (20 inch), and extra long (25 inch). Electric trolling motors come in a wider variety of lengths - but as a rule, the 36-inch shaft length is about equal to the long shaft gas outboard. Some sailboat outboard motor brackets use a very thick board for the motor clamps - this may be too thick for the clamp on some electric trolling motors. Implementation and Performance Tuning: Props: Charles Brennan: Do not be afraid to play with props. These motors are originally designed for bass boats, not sailboats. Displacement sailboat hulls are not anything these motors were ever optimized for. A different pitch and diameter can be a world of difference, in terms of speed, maneuverability, and battery longevity. My Sears motor is using a prop from a MinnKota with a completely different thrust range. I got about a 25% boost in performance from the original prop, which is nothing to sneeze at, when the motor's going slower, and the current's getting faster at the end of a long weekend. They now have multiple blade props for improving performance. If possible, try to borrow a neighbors' prop, rather than buying something that might disappoint you. Jobst Vandrey: Stay away from the 3 bladed props - the manufacturers have told me that the most thrust is always derived from their two bladed units. Some manufacturers offer a "Power Prop" as an option which is a two bladed prop that is not compromised for "weed free" characteristics. (Ed. Note: See how easy it is for two sailors to disagree, and yet still remain civil?) A glimpse into the future: Hopefully, someone will eventually offer a Kort Nozzle design of electric trolling motor - or at least a kit that allows an existing motor to be converted. The Kort Nozzle is a shroud that fits around a customized multiblade propeller and will increase the thrust of a unit by about 25% without any increase in electrical consumption. There is no free lunch, this increase comes at a cost of poorer performance in reverse and only operates at low speed (under 10 knots). However, most tug boats use Kort Nozzle drives to get increased low speed thrust and this seems like a logical improvement for sailboats that want to use an electric motor as an auxilliary propulsion system. Maybe one of you readers will take on this challenge as a business opportunity. While we are on the topic of the future, there is also the near term (next two years) possibility of direct methanol fuel cells being available as a power source. A small methanol powered fuel cell has the potential to keep your battery charged as well as providing enough power to use a trolling motor continuously. Stay tuned to see if this option ever materializes. Power / Gauge: Charles Brennan: Getting the energy from your battery to your motor in the most efficient manner is often overlooked. Minimize the connections (point to point is electrically ideal, but logistically difficult), and maximize the gauge. To give you an analogy for current flow, you can't suck a milkshake through a cocktail straw. Due to economies of manufacturing, the cables from the motor itself are usually inadequate. Then people see that size and assume that the supply cable can be just as small. Wrong! If you can get yours apart, and beef up the gauge directly to the motor connection, do so. The greater the cross section of copper you can get directly to the motor contacts, the better. As a rule of thumb, anything less than the size of your little finger is suspect. Also, do not scrimp on marine quality electrical connectors. Think of every eye terminal (you don't use fork terminals, do you?), crimp, and splice as another bottleneck in the current flow from the battery to the motor. Jobst Vandrey: As a rule, most modern trolling motors require one amp for every pound of thrust. Based on the wire size tables found in most marine sources, a 30-pound thrust motor with a battery mounted 12 feet from the motor should use size 6awg marine wire. For a 50-pound thrust motor, the size goes up to a number 2awg marine wire. (The wire gets bigger, as the numbers get smaller.) This is similar to very heavy-duty automotive jumper cables and should be composed of many small strands to maintain flexibility. Multiple motor speed switch settings: Charles Brennan: It takes more energy to overcome inertia, than to maintain it. Start with your maximum thrust, then back off to whatever extent will still keep your speed reasonable and constant. Too many people ignore the middle thrust ranges, at the cost of battery charge longevity (and the ability to get all the way back home!). Pluses, Minuses, and Equals (Charles Brennan and Jobst Vandrey): Pluses: 1) Always there at the touch of a switch (versus stalling an outboard motor right at the worst possible time). 2) Instant Reverse. 3) Goes slowly, which is nice when docking. 4) Quiet (no screaming over a motor). 5) No gas exhaust fumes in the cockpit when running before the wind. 6) Price can be less than gasoline engine. 7) No volatile fuels to deal with. 8) Opens up travel on some nice local lakes that are electric auxiliary only. 9) Trolling motor can be easily shifted to a dinghy to serve both needs. 10) No cooling water pump to clog or fail at a critical time Minuses: 1) Limited ability to fight against wind. 2) Limited ability to go against any current. 3) Usually needed most when there is least battery current available (like after a week-ender when the battery is nearly depleted, and you're trying to make that last 500 yards to the dock, and the tide is moving out against you quicker and quicker). 4) What you gain in less motor weight, you lose in more battery weight. However, the weight is better distributed since the battery weight is not hanging aft of the transom like an outboard motor. 5) Hydrogen gas from battery can be fire hazard, if improperly installed (like no battery box, etc.). Equals: 1) Snags seaweed and clogs just as good as a regular motor (both types claim weed cutters; neither really works in all cases). 2) Props get nicks as good over shallow bottom as a regular motor prop. However, the props are plastic and MUCH cheaper to replace when necessary. Hint: They're pretty easy to smooth out the nylon ones with a good emery board or nail file. 3) Just as prone to theft as a regular motor (some say easier, as they are lighter), and harder to secure. 4) While not nearly as much maintenance as a gas motor is required, some minor electrical maintenance may still be necessary. Check items such as: good motor to battery connections (if you unplug your motor for stowage), check that corrosion does not build up on motor connector, or battery terminals, check electrolyte fluid levels in the battery. It could be argued that this kind of electrical maintenance is required on a boat anyway, however having an electric motor attached to your electrical system, increases the normal maintenance requirements markedly. 5) Can leave you just as far from home as a gas motor. There you have it, more than you ever wanted to know about electrical trolling motors on sailboats! Now it's time for you to decide what will work for YOUR application. Charles Brennan and Jobst Vandrey |