What types of electric motor are best suited to powering each of the three types of electric boat?

There are broadly three 'sizes' of motor that can be successfully used with each of the three boat types.

- The first is the electric trolling motor - usually in the 0.5 - 1 hp range - suitable for craft carrying between 1 & 4 people.

- Then there are the 1 - 3 hp motors, either in the form of an electric outboard or for inboard mounting.These would be fitted to boats in the 25 to 30 foot region

- Finally, there are the more powerful inboard motors, taking anywhere up to 120v and producing 8 - 10 hp, powering boats from 30 foot upwards

What is the significance for electric boats of the different types of motor construction?

There are only two types to consider, AC or DC motors, with two main sub-types of DC motor. Traditionally DC motors have been the natural choice for home EV projects, since in spite of being heavier and more expensive than AC motors, they require a much simpler, less expensive, controller and they don't need an inverter. At the present time a DC motor total installation cost is less than that of a comparable AC one.
Within the DC group, there is then a choice between series-wound and permanent-magnet (although shunt and other winding types are manufactured). Whilst the harsher, more varied conditions experienced by electric cars have resulted in the more flexible series-wound DC motor being favoured for automotive use, the lower power, lower speed envelope of electric boat (EB) usage means that modern permanent magnet motors, such as the LEMCO and E-TEK, can be a superior alternative in this application.

What voltage is best? What power output do I need?

The choice is between 12, 24, 36 or 48 volts for home EB usage. Automotive and commercial applications often use higher voltage motors but any benefits they might offer in terms of efficiency or increased power output would be wasted in the typical day-boat EB application.
The voltage generally recommended as the best all-rounder is 24 volts. It avoids the susceptibility to voltage losses of the 12 volt option, and is more flexible in battery configuration than the other two.

The answer to power really depends on the specific application - the hull efficiency, the cruising speed, the maximum speed, load capacity etc., etc. However, typical hobby EB requirements (2 to 6 passengers) in non-tidal waters would generally be satisfied with a continuous motor output power between 1/2 and 2 hp (horsepower).

If you want to follow this strand, here are some links (all electric vehicle sites) that provide supporting data to what has been said on this page; they also provide links to manufacturers and suppliers of suitable motors:

One good idea that I spotted at this year's (2004) U.K. Wooden Boat Show is the assembly of a Minn Kota motor into a combined rudder and tiller as shown in these photos:

This particular example was made by Bossoms Boatyard, a UK manufacturer of electric boats, but any electric-experienced boatyard could produce a similar assembly. Note that this might invalidate the warranty of a new motor!