One of my motivations for this site is that while there are a great many sites that talk about installing a marine electric drive - very few owners talk much about their experience using an electric drive. So this is one of the few sites with real-world post-installation experiences - some good, some...not so much.

First off , I am not an engineer, have fairly average mechanical and design skills (note "fine adjustment" tool in picture at right) and basically just enjoy "messing about in boats". And while I have some experience, having done the conversion of my own 30' Hunter almost 2 years ago, there are far better examples of conversions than my own.

. Most of the information here has been gleaned from the Electricboats Yahoo! group and various different websites. I try to give credit where possible, but if I plagiarize it is

Also - because my experience is with converting a sailboat with inboard drive, and because in many ways a sailboat is ideal for conversion, you'll see the site is biased in that direction. How ever, most of the guidelines are applicable to other types of vessels.

  Much of what is on here is opinion, not fact - I will try to be clear which is which. This is also a DIY (do it yourself) guide - some of the "learning" experiences I mention will not be faced by those willing to pay for the expertise of others and buying "turnkey" systems. DIY for the sake of saving money is a risky proposition - mistakes can be quite costly. However, even turnkey systems can still have problems.

I don't have a lot of specifics for exactly how to do your installation - there are other
resources I'll point you to for examples - this is more about the experience of creating and using an electric-powered vessel and all the things you might not think of ahead of time (my pain, your gain)

  I strongly encourage you to participate in the Electricboats forum once you've ready these basic guidelines. A variety of hobbyist and professional members are happy to answer questions (and will appreciate you've done your basic homework)

Safety is key! While typically using low voltage DC (< 50V), these systems are still powerful enough to injure and kill. Fire is probable if you use improper wiring. In addition, flooded lead-acid batteries, which are the most commonly used, contain highly corrosive sulfuric acid and can explode(!) if not handled properly. Please keep your own and others safety in mind as you progress. Consult with a qualified marine electrician to ensure your safety and the safety of those on board.

Start with your goals

There is something inherently compelling about operating a boat with electric power. Perhaps it is just an aesthetic notion of silent, powerful propulsion. But it can be difficult to seperate some of the hype from practical goals.

You should make sure that your goals are aligned with practical results. For many people the main goal is quiet. Old 1-lung diesels are noisy! And a battery-electric will certainly be quieter. Other considerations:

  • Perhaps it's the smell of diesel, which many people find objectionable.
  • Removing the inherent dangers of gasoline on board.
  • Simplifying maintenance (caution...), winterizing, etc.
  • You might just be trying to revive an old boat with an un-reliable or non-working ICE.
  • You want to support a more "green" recreation. Just getting the hydrocarbons off the boat, whether you burn them or not, might be considered a worthy goal. 
  • You want to experiment

These are all reasonable goals if your boating conditions are appropriate.

If you think you will get more power or speed from your electric installation I bet you will be disappointed. There is unfortunately a lot of hype surrounding the comparison of horsepower from electric motors to ICE's
(internal combustion engines). Yes electric motors have flat torque curves - but this does not mean that 1 electric horsepower is somehow different or better than 1 ICE hp. There are transmissive losses in both systems. What it does mean is that all of the power is available throughout the full range of the throttle in the form of torque. Not true with an ICE and they are therefore generally much larger than they otherwise need to be.

Similarly for expectations of range. I have a 12NM theoretical range - but that will vary by 50% or more depending on whether I read the tide tables correctly that day. Or if the wind happens to come up on the nose at the wrong time.

A great resource (originally on the Electricboats group in the Files section) - a spreadsheet which can, given a few simple parameters, give you an estimate of the range and speed you are likely to obtain with a given set of batteries. This model has proven remarkably accurate for most installations. But it can't take into account the conditions you'll encounter. So I would give a wide margin of safety and drop the estimates by 1/3 or more to account for conditions.

I recently ran a simple test: On a windy day - about 15kts steady, I set the boat downwind (no sails up) and set the throttle to 40 amps draw. I reached a speed of 4.2 knots. I then turned the boat into the wind. Now my speed was 2.2kts, and the amp draw increased to 47amps. There was very little chop. The dramatic difference in speed in what amounts to fairly mild conditions is a good example of how you should pick your battles. If I had to fight my way upwind very long - the batteries would quickly be exhausted. A good time to either keep sailing, or drop anchor and pop a beer.  This test also shows how a larger, more efficient prop would help - I would be able to sustain a higher boat speed in inclement conditions (but with higher amp draw as well).

Where do you boat?

This is one of the most important considerations when to consider in an e-boat conversion. If you are going to be encountering fierce winter storms, high current, crashing waves and a lee shore - electric might not be the best choice - or at least not battery electric. But in lakes, rivers, and protected bays it can be an ideal setup. The simplest and cheapest electric boating project of all is a dinghy with a battery and a trolling motor - you would be surprised how far and how fast one of these can go!

You should expect to change your boating habits.

A good way to visualize the changes that converting to electric entails is to think back on all your recent trips using your ICE. Now imagine that you started that trip with only 1 gallon of fuel*. How would the trip have changed? Would you have been in any danger if you ran out of fuel? Would you have spent the night anchored out unexpectedly? How would "the admiral" feel about that? Would you have even started the trip? (*this is about the same energy storage you'll have in batteries...)

The most common use of an ICE is to make a scheduled arrival. The most important use is to get out of trouble. Save your battery juice for the latter!

There isn't really a "gas gauge"
. You can tell how much you've used (approximately) - but don't really know how much is left. This is kinda important.
(photo at right is me and the Admiral limping home after mostly draining the batteries the day before - good thing we have sails...and yes sloppy to have the fender down - I was pre-occupied with the batteries)

So the same "rule of thirds" you use with an ICE applies here - allow 1/3 of your capacity for getting to your destination, 1/3 for the return, and 1/3 for emergencies. In my case, that really drops my range to 2/3*16NM = ~11NM (round trip).

Amps? Watts?

I'm not going to try to explain electrical basics here - I'm likely to get it wrong anyway. There are a ton of resources on the Net to learn all this.
E-Motion Marine has some pretty detailed FAQ-type information (sprinkled with a little self-promotion)


Many people see the relatively low pricing on individual components and think converting to electric is the way to "go cheap" in replacing an ICE. However, by the time you add everything up, you might spend about the same as what you would spend for a quality used diesel installation - at a minimum figure about $3000 for a 30' sailboat, and not including the (ahem) "learning experiences" you are sure to experience (I am going on my 4th controller now...). The drop-in turnkey systems are going to be closer to $5-6K by the time you factor in batteries.

Another cost which can be easy to overlook is battery replacement. Conventional lead-acid battery life can vary greatly, but 5 years is probably a good average. That means somewhere in the neighborhood of $1000 on my boat if I stick with cheap 12V deep-cycle as I am using now. So add a couple hundred/year to the cost of maintaining the boat.

Design considerations for different sizes and types of boats

As mentioned, sailboats - with their displacement hulls, and "hybrid" power (wind) are ideal targets for electric propulsion. But trawlers, cruisers, and other boats with semi-displacement or displacement hulls can also be used.

[Note that it's generally not yet feasible to use primary electric propulsion for planing hulls - such as on a typical ski-boat. Those hull forms are fast when planing, but very inefficient at slower speeds.  Displacement, or semi-displacement hulls are best suited to electric propulsion. Plug a few numbers into the
Electric Boat Calculator to see why]

The ICE "Engine Credit"

Boats that already have either an outboard or inboard motor will realize a "credit" for the weight of the existing drivetrain when converting to electric. Because diesel and gas engines don't increase in weight proportional to their horsepower, the credit is greatest (proportional to the displacement of the vessel) on smaller boats.

A popular goal for electric boat performance is "4x4" - 4 hours at 4 knots. This is a reasonable goal with a reasonable amount of battery weight in a typical 30' sailboat and gives a range of 16nm - suitable for almost any inland waterway. In my experience, 5-ton displacement is about the top limit in practical size for a sailboat conversion without adding a generator, taking a weight penalty or greatly reduced range. (See the Range Optimization page). Larger boats will more often have a generator anyway, and as the price of lighter-weight lithium batteries comes down, the range of practical conversions grows as well.

Basic Components:

Electric Outboards

There are commercially available electric outboards from a variety of sources. I do not have personal experience with any of them. One which looks particularly interesting is the TORQEEDO Travel 401 Electric Outboard.

It's also possible to DIY your own using the lower-unit and head from an existing outboard but the pricing on the Torqueedo is pretty compelling - about $1500 for the unit.

Inboard drives

A basic inboard system is comprised of the following major components:

  1. Motors
  2. Controller
  3. Batteries
  4. Battery Charger
  5. Wiring (both control and propulsion)
  6. Instruments (meters and such)
  7. Throttle
  8. Switches
  9. Drivetrain (gearbox, pulleys,etc
  10. House power
Issues to watch out for:
Corrosion & Chafe
Power Consumption and Range

There are at least a couple of systems (see Resources) designed as direct, drop-in replacements for the Atomic 4 engine that was built into a great many boats in the 70's and 80's. These are ideal candidates for conversion, however you need to look carefully at the balance of prop and motor as the Atomic 4 did not use reduction gears and hence used a fairly small, high RPM prop in most installations.

There are also external "pod" type systems which offer interesting benefits - for example a dual-motor setup for greater maneuverability or the ability to use oversized props to increase the potential for practical regeneration.

The remaining options are "DIY" - building the mounting system yourself. I opted for a very space-efficient design with the motor mounted over the shaft because I had bulkheads well suited to that installation - and I wanted the space the old diesel occupied for batteries.

Where to get "stuff"

See the resources section where I've listed quite a few suppliers and vendors.

Grainger is the best convenient source for all of the various mechanical parts for a DIY driveline. However they are a wholesaler and I'm not sure they'll ship to individuals. I have done will-call with them, but if there isn't one in your area that may prove a problem. I actually got most of my stuff from drillspot.com - a bit odd, but they basically are retail for Grainger.

Here's a sample parts list (stolen from the Electricboats group)

> > Grainger Parts List for driveline only
> > -- NOTE: You might need a longer/shorter gearbelt, kinda depends --
> > bushing split taper 2L270
> > pulley Gearbelt 2L673
> > bushing split taper 3X576
> > pulley gearbelt 2L681
> > Mounted Ball Bearing 4FJ89 these are pillow blocks, need 2
> > Gearbelt L 72 teeth 1DHP4 get 2, have 1 spare
> >
> > Also needed:
> > * bushing for thrust/flex coupling to shaft
> > * 1" shaft stock x 12 in (worked for me) + keyway cut into it at the
> > spot(s) needed I needed 2 key splines cut
> > --> requires machine shop, ideal one located in St Helena (near
> SF)
> > * keys, e-tek shaft is smaller and 1" shaft is larger
> > --> got them at machine shop, guess: 3/16 on e-tek shaft
> > guessing 1/4" key on 1" shaft.

To size my drivetrain, I registered at http://www.emerson-ept.com/ (maker of Browning synchronous drive/pulleys) - they have a little wizard there which takes your shaft dia, motor HP, etc and builds a laundry list. I ordered the parts from Drillspot.com (retail version of Grainger -
Here's my parts-list from Drillspot) - took a little work to match the part #'s from Browning to Drillspots search engine. These parts are beefy, but soft steel - treat gently - I had some trouble with binding during final assembly. Drillspot delivered in 3-4 days, and seems to have good service.

I originally ordered the wrong size pulley ratios because I was looking at the no-load speed on my motor (a Mars) instead of the loaded speed. There is a 1000 RPM difference! So make sure you are looking at the loaded vs no-load speed for these calculations.

All material Copyright 2009 Keith Redfield

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