Just as fast as a boat with a diesel or petrol engine
Basically: just as fast as a boat with a diesel or petrol engine. The acceleration with Propel motors will be faster for sure. Like electric cars, our motors have instant torque.
But please bear in mind the laws of boating. Simply said: there are two types of hulls. Planing hulls and displacement hulls.
For non-planing boats, the maximum speed is equal to the so-called hull speed. That is the speed at which the wavelength of the bow wave is as long as the length of the waterline of your boat. The closer you get to that hull speed, the more energy is consumed (or wasted) in making waves.
And while you can sail a little faster, that requires even more power, and therefore increasing the rate at which you consume energy. A more economical speed, the cruising speed, is somewhat lower.
We have created a lengthy article that may help to answer even more questions related to electric boating. You can find the article here: The ABC of Electric Boating — The most important questions answered.