Will a 5-meter boat that's 2.4 meters wide cruise OK with 2HP?

Question

I am building an electric commuter boat for 4 people, fully enclosed, for use in a harbor with a climate comparable to Vancouver. I want to power it electrically with a 2HP motor. I hope to get about 5 - 10 knots out of it.
What do you reckon, any chance?
Thanks

Answer 1

Short answer is you'll need more power to insure 5 knots under realistic conditions unless your boat weighs less than the passengers or your passengers are small children. You'll need a lot more power (10x or more) to exceed the hull speed of 5.4 knots.

A 5 meter boat has a displacement hull speed of about 5.4 knots. To move faster than this you have to push the boat up onto it's bow wave which takes considerable power unless you have a planing hull and a very lightweight boat. Small single person sail craft and overpowered speedboats are the only craft that can pull this off.

The formula for displacement hull speed is:

Hull Speed (knots) = 1.34 X (LWL in feet)^(1/2)
where
(LWL in feet)^(1/2) is the square root of the lenght of the water line measured in feet

The horsepower formula for driving a displacement hull at hull speed is:
HP = Displacement in pounds / (22500 / (Hull Speed in knots)^2 )

5 meters is 16.4 feet so hull speed is (16.4)^(1/2)*1.34=5.4 knots
You'll need a planing hull and a lot more than 2HP to move faster than 5.4 knots.

You mention four passengers so I'll assume 800 lbs in people and 800 lbs in boat, which is pretty light for an enclosed boat that holds four but not unreasonable. Your HP requirement for hull speed becomes:

HP = 1600 / (22500 / 5.4^2) = 2.07 HP
note 5.4^2 mean 5.4 squared or 5.4 x 5.4

So you could theoretically limp along at just over 5 knots with a 2 HP motor on a perfectly flat lake with no current and no wind. This assumes you have only 800 lbs of boat and motor and battery and 800 lbs of passengers including the captain. With even a couple of knots of tidal current and a few knots of head wind you'll be lucky to hold position let alone make any headway.

For a boat with an enclosed cabin navigating on open water with four people aboard I think 20-50 HP is closer to a reasonable size although you might be OK with 5-10 HP if you are happy with 5 knots, don't leave a protected harbor area and don't go out in windy conditions. Running such a rig with electric power could be a challenge since the battery weight adds up fast. There are reasons why electric marine drives are rare outside of low speed trolling motors and diesel electric submarines. You don't mention anything about the range you'd like but even a 2 HP electric motor is going to draw over 125 Amps at 12V (1 HP = 746 Watts) and you'll find that with that current draw you'll be lucky to run your 120 Amp hour deep cycle battery for 15 minutes (the amp hour ratings are typically for 10 or 20 hours of constant draw not one hour). If you run four batteries in series at 48V and 30 amps you might get an hour at about 2 HP but that still only moves you 5 nautical miles under ideal conditions and you have to find a 48V DC motor to spin the prop.

I have a 16 ft day sailor that displaces about 600 lbs empty and it can hold 4 people (barely if they are all adults). My 3 HP electric trolling motor moves it along about 1-2 knots (slow walking pace) but I suspect that 3HP is the stalled power not the power output at speed. I wouldn't be surprised if HP ratings on electric boat motors follow the same misleading power rating conventions that electric motors on saws and power tools do.

My 3.3 HP gasoline motor moves the boat along pretty well and probably makes around 5 knots (brisk jogging pace) under most conditions. However, I use this boat in protected waters and it has a sail as the primary propulsion. The motor is emergency backup so I can get home if the winds are light and it starts getting dark. I don't think I'd ever consider using it as primary propulsion across any significant distance of open water.

I hope all this helps you make a more realistic power plant decision and saves you from wasting your time and money or worse yet getting into serious trouble on the water in an underpowered craft.

Max L.

Answer 2

No, you'll need a much more powerful motor, try at least a 10 hp, because you never know if you might need the extra speed to maneuver out of a dangerous situation. Also, you can find pleanty of cheap used motors in your area on the net, I wouldn't use an electric motor if you don't have too.

Answer 3

A 2hp gas motor wouldn't push it that fast, so I doubt an electric would either. You'd need at LEAST 5HP, more likely 10 or 15.

Answer 4

i didnt even get what u r talkin botu man, 2 much cience 4 me, i give up!

this is another stupid answer by:
Saint Jimmy

P.s: bush is a terrorist