The following sections represent viable options for electric powering. (Consider Outboards, Consider Outboards as Inboard Power, Consider Commercial Inboard Motor packages/Installation, Consider "Do It Yourself" Inboard Electric Motor Conversion). Each path has there own advantages and disadvantages as will be pointed out. While not a complete "how to", you will be guided to the information needed to accomplish these installations.
Outboard electric motors and submersible electric motors are often overlooked for primary power of electric boats. This may be because people are unaware of the range of motors available and the many appropriate types of boats for such motors. Outboard electric, as was the case for outboard gasoline engines, is probably the best first entry point into electric boating for most people. Cost and installation complexity are substantially lower than for inboard powered electric boats with the potential for similar performance in many cases.
Outboard electric motors are now available in a broad range of prices and sizes. Prices range from about $100 to about $5000 (without Batteries). Power ratings range from less than 1/4 hp to about 6 hp. There is now a primary propulsion outboard made by Minn Kota which provides a true 2 hp of continuous power (E-Drive Electric Outboard) and sells for less than most primary propulsion outboards. Toqeedo brand outboards are available with self contained Lithium Magnesium batteries
There are a number of excellent boat types for electric outboards. The following represents a good start but should not be considered complete.
A quick look through the electric boat builders web page will reveal that many of the electric boat models offered, have trolling type submersible motors built into the hull. Its not as elegant as a true inboard installation and not nearly as efficient, but the price for such boats has been dramatically lower than the price of similar inboards.
The same is true for "do it yourself" installations. A 24 volt 65 lb thrust submersible motor can be installed into a glassed in tube in the hull of a boat. Such a motor can drive an easily driven hull with a full up weight of under 1000 pounds at about 5 knots for a cost of about $600 including batteries. A 36 volt 107 lb thrust motor can and has been used for up to about a 1600 lb launch/sailboat hull about 5 knots speed. One could install two such motors in the hull of a boat either fixed or steerable (both have been done) and get a respectable 2 hp or 3 hp which is generally enough for a good size launch (18 -24 ft long). There are fixed mount type motors now available which can be installed on a hull with remote controllers.
I advise using the salt water versions of the motors where available because the electronics are heavily sealed in epoxy making it more weather tolerant where exposed. Its also important to heavily glass the motor tube into the boat at a frame or bulkhead. If you are not knowlegable in making changes which effects the structural integity of the boat, you must engage the services of someone who is before glassing a tube into the boat. One alternative is to purchase plans from Glen L for this installation.Glen L ETM Mount and Bracket I suggest a skeg to protect the motor and prop. It's important to note that 12 volt motors generally can move a boat no faster that about 3.5 knots. The 24 volt and 36 volt motors use the same prop but drive it at much higher speeds allowing speeds in excess of 6 knots in some cases. Higher thrust 12 volt motors don't always increase the speed of easily driven boats. (I've actually tested this on a variety of different boats).
Some of the negative aspects of submersible motor installation include the following and should be considered:
Since the Etek Motor became available at very reasonable cost(see Evparts below), There have been many reports of Etec motor based outboards being built with used or new lower units intended for gas outboards. The motor is capable of up to 8 hp in continuous use and 15 hp peak. It is said to be a little noisier than other motors, however, it has wide appeal to those building their own outboards. These motors may be in short supply since manufacturing has stopped.Try the Etek Electric Outboard Project for some insite into such a conversion
It's a somewhat challenging task to determine the right motor, reduction ratio, prop and battery bank to obtain required speed and range for a given boat, let alone select all the other ancillary components and set it up properly. Most vendors of marine electric drives will work all of this out for you and sell you the components or perform/supervise the conversion (preference varies with vendor). This approach gives you the best chance of achieving your design goals with a robust long lasting well designed system. It is, however, an expensive option, especially if the vendor requires factory or agent installation.
This is now a totaly viable option for those so inclined. There are a number of good, proven, reasonably priced industrial motors and controllers for marine applications. There are also ample sources for other components needed (props, thrust bearings, batteries, etc). But most important of all, the information, help and examples of successful installations are now readily available. Even the "Do it Yourselfers" who buy all new parts seem to save substantial money compared to the packaged commercial installations. Used motors, Controllers and other parts are often employed for somewhat greater savings.
Most of the information you need is rather neatly contained in "Electric Boats" by Douglas Little (Out of print but available on the internet as a used book) . A book on marine wiring is also highly recommended such as "Boatowners Illustrated Handbook of Wiring" by Charlie Wing. Although Doug Little's book is pretty thorough on prop selection, some will want to delve deeper for which I would suggest Dave Gerr's "Propeller Handbook". I would also get the electric boat VCR tapes from Glen-L Glen-L and obtain at least one of the electric vehicle books from EVparts EV Parts. A great deal of practical experience has been generously shared in the electric boat group Electric Boat Group on Yahoo I would recommend that you look over and consider joining this group.
Since the availability of low cost hand held GPS units, a great deal of useful performance data has become available for more electric boats, often consisting of amps drawn paired with knots for various operating levels together with other specifications. You will find such information in the electic boat club messages linked above and in web pages of most of the electric boat suppliers.
The components page Electric Boat Components has a full list of sources for motors, props and other resources. Glen L provides plans for installing an inboard electric drive using an outboard motor lower unit. Glen L Electric Drive
So, How does one put all of this "stuff" together towards a successful electric boat installation? Well, there are so many examples to pick from (with specifications and performance information), that you can select a power, reduction, prop, and battery combination for a boat of similar length weight and hull shape. From there you could get the advice of people in the electric boat club (or from other sources), who may have had experience with a similar boat and perhaps perform some calculations.
I would go through the calculations in Doug Littles book. There is a series of steps rather than a single formula for doing this. Some cautions: The hp requirement in most tables is based on internal combustion engines and needs to be based on empirical information from real electric boat installations. Tables will show 500 lbs per hp to approach hull speed, however I can show you performance figures for electric boats which achieve hull speed with 1000 lbs per hp. Calculations often don't consider prop area. this is vital for low speed low power boats (such as electric and small diesel powered craft) which require low area props such as sailboat props. Try to compare boats with similar prop diameters to the size you are planning for as this can make a tremendous difference.
Many of the "do it yourself school of electric boat conversion" have been using Advanced DC series wound motors and Curtis controllers. Some have also utilized used golf cart and scooter motors and controllers. Timing belt, and pulleys have also been a common means of gear reduction. Golf Cart Batteries have been popular as have Absorbed Glass Mat Batteries. Be sure the batteries are rated for the current you expect to draw. These should be batteries rated for electric vehicles. Granger Thrust bearings are often used. Prop diameter should be as large as you have room for (up to a calculated maximum) and your tolerance for boat draft allows, as the efficiency (thus range) increases rapidly with increases with propeller diameter. Common sizes are 14 and 16 inches in diameter.