There is a small 13' long x 6' wide, 1,000 lbs (total weight of everything) all-fiberglass white pontoon boat with dual flat-bottom fiberglass pontoons. (Each flat pontoon bottom is 2' in width, with an empty 2' space between the two, running the length of the pontoon boat). The boat is in a lake and sits with 2.5" of it being below the waterline. There is a 2 HP electric motor mounted on the back of the boat. It is tiltable and extends beyond the 13' length of the boat itself. Its propeller is on shaft that places it nearly two feet below the water line when submerged.

The boat needs to be driven on to a foam-filled drive-on/drive-off floating dock. The floating dock is secured to a bulkhead at one end and tapers/slopes downward for about a foot at its loading end, where the two pontoon noses of the boat would commence movement on to the dock. The dock has a flat top surface area of 13' x 8' and has 3" of freeboard (i.e., height of dock that is above the water line) without the boat on it. The rigid top surface area of the floating dock has a permanent friction-reducing industrial coating or surface. (The co-efficient of friction would approximate that of outdoor carpeting.)

QUESTION: Will the 2 HP electric motor be sufficient to power/drive the 1,000 lbs, 13' boat on to the floating dock? If so, how much of the computed capacity margin to do so will be needed to do the work?


(1) The 2 HP motor must drive at least 12' of the 13' long boat on to the floating dock. Power is controlled with a throttle handle.

(2) Although the boat could approach the dock with some kinetic energy, moving at 2 or 3 MPH, the worst case scenario is that the boat is static in the water, with its nose only several feet from the sloped end of the dock, before the 2 HP motor starts its attempt to drive the boat on to the dock.

(3) The expanded polystyrene (EPS) foam filled dock is rated to keep up to a 1,500 lbs load entirely above the water line. With a 1,000 lbs boat on it, the dock would still have freeboard of 1".

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    $\begingroup$ The fundamental quantity you need to know is not the power of the motor, but its force. If the motor is connected to a large propeller, it will allow for greater force at a given motor power. Without this knowledge, one can only guess, and my guess is that you will eventually have to step out, call few friends and pull the boat manually with a rope. But this is not much about physics anymore... $\endgroup$
    – dominecf
    Apr 19 '16 at 20:21
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    $\begingroup$ Would Engineering be a better home for this question? $\endgroup$
    – Qmechanic
    Apr 19 '16 at 20:35
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    $\begingroup$ Get a spring scale capable of measuring several hundred pounds of force. Measure the force of the motor by having the boat drive away from the dock, pulling on the scale, which is secured to the bulkhead. Next, use the same scale to measure the force required to pull the boat up onto the dock with a come-along or other pulley arrangement. Compare. $\endgroup$
    – Dave Tweed
    Apr 19 '16 at 22:04
  • $\begingroup$ horsepower is only one of several critical variables. Examples: what's the top speed in water you could achieve? What is the actual coeff of friction? But in any case, given that the front of the dock is above water, you're just asking for significant damage to the dock and/or prow of the pontoons if you drive onto the dock. I'd recommend ropes,and also a couple people standing on the end of the dock to force it below pontoon depth before attempting to beach the craft. $\endgroup$ Apr 20 '16 at 12:16

For a winch a 2HP would probably be adequate for dragging a 1000lb boat across a smooth surface and this is certainly in the normal range of winch motors.

However the difference between a winch and a propellor is that a winch can be geared to provide the torque required whereas propellers generally operate at much higher RPM and in a fairly narrow range at that.

If you already have an electrical power supply on the boat it may be easier just to have a separate electric winch which won't be especially heavy or expensive.

  • $\begingroup$ @Jon Thanks for the help! Without more data, I think the answer should assume to be "No". The pushing force won't be sufficient to do the work. So, the pulling arrangement (winch) is needed. $\endgroup$
    – Bill D.
    Apr 21 '16 at 19:01

What you need to know:

  1. What is the coefficient of friction between the boat and the dock? (Both static and dynamic)
  2. What is the total surface area of the contact patch between boat and dock? (So you can calculate the drag force)
  3. What is the efficiency of the propeller into the water? (So you can calculate the push force)

And, the coefficient of friction may change based on the exact pressure the boat (weight over suface area) puts on the dock. Also, depending on lubrication -- is there barnacles or slick seaweed in the way? If it's frozen, is there slick ice? Is it always homogenous water?

You haven't provided any of this information, and thus we can't answer the question from a mathematical point of view.

From a practical point of view, why does it matter? Could you have a steel cable and a winch on the dock, that allows you to hand crank it up when the motor can't do it? Could you electrically power that winch if needed? (With enough gearing and low-loss gearbox, a small motor might be sufficient.)


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