Dinghy Motors and Fuel

I’ve thought about this a lot.  I keep looking for the “easy button” on dinghies – the most efficient, common-sense way to stow, haul, tow, and power a dinghy.  I can’t seem to make it any better (for me) than I have it right now, without a lot of expense or inconvenience.  I have a plywood pram that my boys and I built 15 years ago.  It’s a bit beat up at this point, but a fresh coat of paint and varnish make it presentable.  I’ve replaced/reinstalled fire hose rub rails, epoxied in new pieces of wood as required and generally maintained it in serviceable condition.  This hasn’t taken a lot of effort.

What set me off on my most recent exploration of “improving ideas” was an article from a blog I read (onboardwithmarkcorke.com) on powering dinghies with a portable power pack – like a jump-start battery pack – and a trolling motor.  Now I have to admit, this idea appealed to me a lot.  Most of the reason I won’t use a trolling motor is the inconvenience of loading and unloading the battery – a proper trolling battery will weigh close to 50 pounds – I may as well use a gas outboard for that kind of weight.  However, if a trolling motor could be powered by a battery pack that weighed less than 15 pounds and included a molded-in carrying handle(!) I could manage that easily.

Peak 450 Amp Jump Starter.  This is one has more capacity than most, but it's designed for a big jolt delivered quickly.  Using it regularly like a deep-cycle would ruin it in short order, though it may provide suitable service in the short term.

Peak 450 Amp Jump Starter. This is one has more capacity than most, but it’s designed for a big jolt delivered quickly. Using it regularly like a deep-cycle would ruin it in short order, though it may provide suitable service in the short-term.

Alas, this is not a workable solution, for the simple reason that these power packs are designed to deliver high amperage quickly.  They are not designed to deliver constant power for a long time.  Most of them don’t have much capacity in the way of amp-hours, so they would run out of juice fairly fast, and that sort of use would quickly ruin them.  There would also be the recharge underway problem to solve.  It’s not impossible – but would just need puzzling through to find the right answer.  Obviously, this option works for Mark Cork, and I’m not criticizing his choice or idea.  I’m just fairly confident that for the sort of use I have in mind, this set-up would not work very well.  So, I’ll keep my set of oars and my sailing rig, and be satisfied.

However, here’s another thought – $250 for the motor (give or take), $80 for the jump-starter – that’s not a lot of cash for an experiment that could turn out well.  And if it didn’t work out practically, the motor could be sold off to recoup losses.  Hmmm. . .  Probably worth the risk, when we’re ready to cast off and constantly rely on the tender for transport.

Here are my thoughts on dinghies from a previous post entitled:  Simple Conveniences, Simple Systems (10 May 2012).

“From all the opinions that I’ve read concerning dinghies (in itself, a subject worth several posts), it is apparent that which sort you choose is very personal.  Perhaps this is true of all cruising gear, in the long run.  Years ago, my 3 sons and I built an 8 foot plywood sailing dinghy/tender (stitch and glue) and bought a set of oars.  I think we finished it in 1997, and we’ve been using it ever since. I’ve been tempted to motorize it with a trolling motor and battery, but the idea thwarts the simplicity of this dinghy.  Somehow I would have to figure out how to stow the battery on board our yacht, how to get the battery into the dinghy, spend time attaching the motor and connecting the battery, etc..  Some sort of motor would be helpful when rowing against wind, chop and current.  As it stands, though, any kind of motor is more of an inconvenience.  How can you improve on the simplicity of oars for a launch-and-go application?  The dinghy is not difficult to sail, either, so that is certainly an option for longer distances than you might want to row.  It’s not a great sailer – a situation that could be improved with a bit more sail area – but it will get you there.  Boarding the dinghy from the water could be difficult.  Unlike a RIB (which is very stable when boarding from in the water), if you pull yourself up on the side from in the water to get aboard, it will likely swamp, so for a snorkeling platform, it’s probably not great.  So here are up sides and downsides of our hard dinghy as I see it:

  • Pluses:  I don’t carry an outboard or gasoline; it rows well; it sails okay; it’s easy to launch and retrieve from on deck; it’s easy to repair and maintain; sun won’t destroy it; it won’t puncture; a new coat of paint makes it look fresh again.
  • Minuses:  Can’t board from in the water; is not fast; is range-limited by practicality (you can’t reasonably go five miles away for a snorkeling trip and return in a short time period); short chop, contrary winds and current make for a difficult trip (usually in only one direction, though).

The building cost was about $800 in materials and gear.  A RIB with no outboard will set you back a good bit more than that.  With an outboard, even more.  My plywood dinghy was fun to build – a RIB won’t give you that satisfaction.”

  1. Rick –

    Here’s the problem… a gallon of gasoline has the energy equivalent of 33.4 kWH. A deep discharge group 31 battery will deliver approximately 110 amp-hours when new. But you should only draw a battery down to 50% for longevity purposes. That’s about 55 x 12.6 / 1000 = 0.667 kWH. You’d need to carry 50 of those group 31’s to give you the equivalent energy content of a single gallon of gasoline. Now of course you need to figure in efficiencies for both the internal combustion engine and the electric motor. But even if they differ by a factor of 10, you’d still need to load 5 of those group 31s into your dinghy.

    The problem (or advantage, depending on how you choose to look at it) is that gasoline contains a *lot* of energy. And batteries not so much.

    s/v Eolian

    • Bob,

      Right – gasoline is much more practical for constant use and long distances. Motorizing a dinghy is another “arms race,” similar to committing to refrigeration (plus more battery capacity, more charging capacity, more robust electrical system, etc.). It’s not as simple as *just* getting an outboard for the dink. It’s making arrangements for a stowage place, fuel (and stowage), weight to move on/off the dink, licensing the (now) motorized dink, securing the motor. So the trolling motor was/is attractive, but limited in capacity. How far can one go with a trolling motor and a booster jump pack? Not that far, of course. Not that far even with a big, heavy deep-cycle (which would be a pain in the neck to handle). But would the jump pack/tolling motor be appropriate and practical for harbor hops as a “liberty launch” to and from the yacht? The answer is, I don’t know. It might work out okay. It would never be as effective as a gas outboard, but then it wouldn’t be as inconvenient (for me) either. Where is the trade-off regarding comfort and convenience? Every person has to answer that for themselves. The more I think about it, the more satisfied I am with oars and sails. I think I read that Eric and Susan Hiscock only used an 8-foot dingy with oars for propulsion on at least one entire circumnavigation, so it’s certainly possible. It seems to me as though convenience (where a boat is concerned) often costs a great deal in terms of money and special arrangements. Perhaps the costs average out over time.

      We lived in Jacksonville, FL for four years, and I kept the boat on a mooring, using the dink almost exclusively to travel the 200 or so yards. It was easy and simple, but occasionally difficult due to contrary winds and steep chop. A motor would have been welcome on those few occasions! Again, a motor would be welcome for touring an anchorage, or going on a snorkeling trip. None of these things are impossible with oars and sails, just slower and less convenient.

  2. Anonymous said:

    I have a 7′ fattyknees w/ oars and sail..rows well..tows well..stows on the bow of my W27 if needed..great exercise..don’t see a motor is in my immediate future!!

    • I know that works well. It’s low-impact, low tech, and reliable.

  3. Hello again Rick. Well you know we own the W33 and so it has more storage volumes plus more spots on deck to put things. But I have to balance the luxuries with the expense and power demand just like you said to Bob above. To us living here in glacier country we will really value a heating system onboard. We already have a small portable freezer which can run off shore power or 12 volt plus it is well insulated so it’s power draw on 12 volt can be low. My wife still wants that microwave and using an inverter for one of those energy gobblers is just not a place I want to go to. So it will run off shore power. We also plan to install our little Honda 2000 generator/inverter under a corner rail seat in the cockpit. It will not run as much as a larger unit but it only weighs 55 pounds with fuel and can run 8 hours on less than a gallon. Our dingy at the current time is really a child’s 8 foot sailboat. Since it has a dagger board well in the center it’s weight carrying capacity is limited. 2 adults might very well swamp it. It have thought of ways to correct that but it may be simpler to buy a small 10 or 12 foot RIB. We already have the davits for a boat as well as the place to store an outboard and the crane to help one get it onto the RIB. So my thinking is that we will get a very one shaft 6 hp outboard with integral tank. I can then use this on the RIB if I don’t want to row(I actually enjoy rowing) and at least carry it on the RIB to help tired people returning to the anchored boat with a bunch of groceries(plus the dog). This small engine can also act as the kicker when placed on a mount at the very rear of our swim platform. It’s thrust would be about 7 to 8 feet behind the rudder and could rally help us in and out of a slip. Most marinas in this area that cater to sailboats are prepared for 27 to 30 footers and a boat like ours is really quite big to maneuver in the lanes of the marina. Having the little engine will be a great help with that maneuvering. It would be nearly impossible to use it in any waves since the swim platform is higher off the water than most due to the way the W33 stern is built at around 16-18 inches but that is not high enough to ensure that it would not get swamped in a seaway. I envision mounting it on the platform after sails are dropped in the channel that leads into the marina. I think it was the Yamaha that I liked the most but I am not sure. One of them can be ordered with a 25 inch shaft and a “high thrust” propeller and still weigh in around 70 some pounds with it’s .9 of a gallon. I just hope I see another Zodiac or similar quality RIB for sale cheaper than new on Craigslist when I want to buy. Many people start with the smallest one and then quickly move up selling the small one which is what would fit our boat. These are my plans but I still want to say that I would consider the motor as auxiliary to the oars.

  4. Sounds like you’ve got it sorted out the way that will work best for you. It’s such an individual thing – charging batteries and electrical complexity; dinghy and motor choice – what works for one doesn’t work for all, that’s certain.

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