Hopefully someone can answer this...

[Leigh77]

I have a 54lb transom mount electric motor which I use for a 2nd motor on my boat. I also use it on my kayak for those long stretches of river that I simply do not want to paddle.

Here are the top speeds recorded on each craft.

BOAT (385 Quintrex Explorer Trophy) : With 2 guys, 3 deep cycle batteries and all associated fishing gear = 4-5km/hr

KAYAK (Perception Acadia 2800) : With 1 guy, 1 deep cycle battery and a small amount of fishing gear = 6-7km/hr.

Now if the motor can push the boat along at 5km/hr...surely given the kayak weighs nowhere near as much the top speed would be quicker...a lot quicker.

Or is there some scientific explaination that escapes me...I am baffled! :-/

[Hydrotherapy]

Are these GPS readings?

[tigermullet]

The answer probably lies with the formula for displacement hull speed, which is the square root of water line length (in feet) multiplied by 1.34 to give theoretical top speed in knots. Usually the square root of the distance will be sufficiently close and if you can achieve the theoretical top speed all of the boating world will want to hear from you.;D

[Leigh77]

Hydrotherapy : Yes, they are GPS readings.

Tiger : The Kayak is 2800mm long, which is 9.2 feet. I worked out that the theoretical top speed would be 3.34km/hr. But I am not sure if I used the formula given correctly.

[Noelm]

it also comes down to the simple fact that a certain pitch prop turning at X rpm will make something go at X KPH less any extra drag (from the bigger Boat)

[tigermullet]

Yep - at 9ft long the square root is 3 and therefore equals 3 knots which, in turn, is equivalent to 5.6 kmh.

The exact theoretical speed would be the square root of 9 multiplied by 1.34 which equals 4.02knots.

Therefore your observed speed as reported in your initial post seems to be about right.

The problem with hull displacement is that it takes an awful increase in horse power to overcome the natural 'hole' in the water that the craft makes. If you want one more knot above the theoretical limit you need to double the horse power or convert to a planing hull.

As an example, our boat can achieve its hull speed of 6knots with 30hp but if we want to get to 7 knots the required horsepower goes to 60. That's why really large yachts always get across the line much sooner than smaller boats.

It really is a drag.