Page updated: 5 February 2009
Why Go Electric?
Some of the authorities that control rivers and lakes ban the use of fossil-fueled engines. One such site is Rutland Water. Here owners who want power use an electric outboard as the only practical option. Others, such as the Broads Authority, offer discounts on tolls to those who use electric power.
This page describes two brands of engine used by SeaHawk owners. The Boat Safety Scheme page provides some additional information as it affects those whose navigation authority requires certification under the scheme.
Minn Kota
Two owners report using Minn Kota Endura engines. One an Endura 50, with a 36" shaft, the other an Endura 46. The tiller of these engines is telescopic, extending by about nine inches from the length seen in the photograph and incorporate a twisting "throttle" that offers five forward speeds and three reverse. Both of these engines can be supplied with a 42" shaft, but the 36" model is appropriate for the depth of the SeaHawk transom plus the 13" that the company recommends for the propeller to operate without cavitation. (Cavitation occurs when air gets sucked down to the propeller blades, so losing efficiency.)
Like a conventional petrol engine, the weight of these units is concentrated in the motor, but here that is at the bottom of the shaft forward of the propeller. The units are light enough to be easily carried in one hand as all major components other than the motor body are made of various plastics.
Almost any 12 volt battery an be used to power the engine. The Endura 50 owner uses an 85a/h deep cycle leisure battery with a solar panel for charging it. The installation is described in some detail on the Boat Safety Scheme page. The Manufacturer's Data suggests that this combination should be capable of running at full power for 1.72hrs. In fact it has never been tested to exhaustion, but it was once used for a continuous three hour run on the third of the five power settings with no noticeable reduction in speed. A fortnight later, in June, the battery was showing full charge. In contrast, after a very short trip in October, and when the solar panel had not been in direct sun, the battery had not fully recharged after four days.
The other owner, with the Endura 46, reports:
"This was recommended to me for the size of boat and is fine. One interesting point. Because I looked at ways of avoiding the use of a large (and heavy) battery, I took a chance and bought two second hand (wheel chair) dry batteries (sealed) of 30ah. I only use one at a time, keeping the other in a locker below for backup. I didn't know what was going to happen but in fact they work perfectly well.
Because these batteries are less than half the size of a normal car battery and are sealed, it's possible to insert it on its side into one of the lockers, use it and take it home after the weekend and re-charge. I don't know how long one battery lasts before re-charge but think it must be half an hour or so.
The only use I have for the motor is to sail on or off the pontoon and out of the little bay, assuming head-on wind. So I think this to be quite a useful tip as most batteries have to be kept vertical and are fairly heavy."
Manufacturer's Engine Data
Minn Kota do not provide a horse-power rating for the Endura series of engines, which are promoted only as "trolling" engines for fishing boats. Instead they are rated by thrust. Unsurprisingly, the "Endura 50" provides 50lbs. The Minn Kota site offers some useful further background on their electric outboard engines. They point out, for example, that the prop that is fitted is designed for maximum acceleration rather than top speed and that direct comparisons with horsepower ratings are difficult as conventional engines also usually come with propellors designed for much higher speeds than the typical maximum of around 5mph provided by those on a Minn Kota engine.
The Minn Kota FAQ once provided a formula for calculating what battery consumption is likely to be, which it gave as:
0.85 x (AHR) / (MAD) = (HRT)
where:
AHR = Amp/Hr Rating of the battery
MAD = Motor Amp Draw
HRT = Hours Running Time
Torqeedo
Torqeedo, a German company, is a relative newcomer to the market. Their nearest equivalent to Minn Kota's Endura range is the "Base Travel", though the one here is the folding version, called simply the "Travel".
The most obvious difference from the Enduras is that battery and motor are a single unit. The dedicated battery locks onto the head of the motor shaft so it has the advantage that there is no trailing cable to get in the way.
Depending on the model of motor, the battery will either be a 14v 20a/h or 29v 10a/h type. Their general appearance is identical. As can be imagined, at these ratings the Manufacturer does not claim a particularly long running time. The higher voltage model at, maximum power, will discharge its battery in as little as 15 minutes. While at slow speed all models are claimed to provide six hours running time.
For a lake sailer who simply uses a motor when launching from a lee shore or to manoeuvre out of a narrow dyke, these figures are no discouragement. A conventional engine run for such short times will suffer badly and need special maintenance routines, and comes with all the disadvantages of hazardous and smelly fuel. However, if you do need longer running times than the integral battery will provide, an extension cable is available to allow you to hook up to a conventional lead/acid battery.
When aboard, the owner of this motor keeps a spare battery in a wooden box that he constructed. This sits on the cockpit floor, kept above cockpit drain holes by legs and rubber doorstops and is securely attached to the transom beneath the tiller. The motor was in situ when his BSS inspection took place and these arrangement satisfied the examiner.
This engine replaced a conventional outboard on a boat based on Hickling Broad. The owner says of his new motor, "The Torqeedo engine is superb".