Dingy Horsepower, need minimum weight!

[SOLAR SUPPORT]

But it does not matter! 1000 watts in an electric motor can't "be more powerful than a 4-5 hp ICE". More convenient, sure. Quieter? Yep! Less polluting (at least at the actual engine)? Most likely! Simpler mechanically? You bet! But more powerful... can't happen.

Propeller mismatching can bog either type of engine and cripple it's performance, put the power head under water and both suffer, and both rely upon a sort of limited "fuel", either petrol or amp hours, that can run out and leave you stranded, so neither is perfect. But the relative power is constrained by the well known relationship: 1 hp=745.7 watts and no advertising can alter that.

Advertising does not bring anything to me because I am not someone who does business in this field. The topic of discussion is about the need for sufficient power for medium boat speed, low engine weight and the ability to place the engine back on the boat without using any leverage. Electric motors used for Dinghy propulsion have the advantage of lightness.

Many people who are not engineers know the Watt - Hp theoretical conversion coefficient. IC engines can only deliver the "Hp" label power written on them at full throttle and maximum speed. The IC engine installed on the boat is generally not used at full throttle all the time. Half or slightly more of the IC power available at maximum speed is sufficient to provide the necessary thrust to accelerate the dinghy to the target cruising speed.

The overheating problem, which is common in air-cooled IC motors in hot weather, does not occur in electric motors operating in water. It is possible to use electric motors continuously at maximum power. Although there are no overheating problems in water-cooled IC engines with salt water circulating inside them, metal oxidation is inevitable.

You should not neglect periodic maintenance and always carry highly flammable gasoline with you. You do not need to mount the electric motor in the stern pit of the boat. You can fold the engine arm and lift it into the deck port. No maintenance is required other than washing with fresh water. Fuel? With solar energy, that problem is over.

[rslifkin]

The electric outboards often claim that, say, a 6 KW motor gives the same low speed thrust as a 10 HP gas outboard. That may be the case depending on gearing, prop selection, etc. But the 6 KW electric IS NOT 10 HP and will NEVER be capable of pushing a dinghy as fast as a 10 HP gas outboard (because HP is a measure of work done over time and 6 KW is only 8 HP with perfect efficiency).

No matter how anyone slices it up or how things are used in the real world, claiming that a lower powered electric outboard is equivalent to a higher powered gas outboard (without further qualifying that statement) is nothing more than a blatant lie.

There are plenty of advantages to electric outboards. For a non-planing dinghy, you don't need much power, so a lower powered electric with good low speed thrust will do just fine (compared to using a bigger gas outboard to get enough low speed thrust because the outboard isn't optimized for maximum low speed thrust). But for a planing dinghy, more HP = more speed and there's no way around that.

[Kettlewell]

There are plenty of advantages to electric outboards.

I find them intriguing, but a lot of users seem to fail calculating range based on my casual observations of numerous people rowing slowly with the electric outboard kicked up. Nice to be able to just add more gas when needed and be back in business within minutes.

[Mickeyrouse]

Originally Posted by Dockhead

This.


In my experience, there's not much to gain over 3 or 4hp until you get to whatever level of power is needed to get the dink on a plane with a reasonable load. If your dink is not a RIB then you won't be able to plane it period, so you are not likely to have any use at all for any more power than 3 or 4 HP..

I beg to differ. Our soft bottom dinghies( with floor boards) have been planning with a 5 HP 2 stroke toting 2 people for years.

[tomtriad]

Your 2 hp 2 stroke will be plenty. I've been using a 3.3 2 stroke Mercury ((Tohotsu) for years with a 9' Achilles with either plywood or air floor. It planes with me and my wife and our diving gear. 28 lbs. You can buy new in the Bahamas but why bother?

[Dockhead]

Originally Posted by rslifkin

. . . because HP is a measure of work done over time and 6 KW is only 8 HP with perfect efficiency. . .

Sorry, this is not correct. BOTH HP AND kW are measures of the exact same thing -- Power, which is the amount of work done (or energy transferred) per unit of time. See: https://www.engineeringtoolbox.com/power-d_1289.html

One kW is ALWAYS 1.3596216173 HP (metric; an English HP is slightly different). Efficiency has nothing to do with it.

Don't confuse input power with output power (or shaft power). Electrical motors nameplate rating is shaft power, which makes it comparable to the rating of IC engines. They consume somewhat more electrical power than their shaft power rating. Either IC or electrical motors can be rated in kW or HP interchangeably.


But your main point is correct -- there is no magic electrical HP. Electrical motors can be somewhat more efficient than IC engines when driving propellers because their torque is the same almost from 0 RPM, so they can -- theoretically -- drive a larger and slower prop. But thrust is itself a measure of Work just like watts and HP, so whatever difference in thrust vs. HP or kW is down to prop efficiency.

[rslifkin]

Originally Posted by Dockhead

Sorry, this is not correct. BOTH HP AND kW are measures of the exact same thing -- Power, which is the amount of work done (or energy transferred) per unit of time. See: https://www.engineeringtoolbox.com/power-d_1289.html


One kW is ALWAYS 1.3596216173 HP (metric). Efficiency has nothing to do with it.

Agreed. The efficiency reference comes in if the electric is quoting input power in KW rather than output power at the shaft. If it's input power, then there will be some amount of loss as heat in the motor controller and motor.

[Dockhead]

Originally Posted by Mickeyrouse

I beg to differ. Our soft bottom dinghies( with floor boards) have been planning with a 5 HP 2 stroke toting 2 people for years.

You've had better luck than me! Thanks for the correction.