CONTINUOUSWAVE

I looked long and hard about fitting an electric trolling motor. I came to the conclusion they are not a viable alternative to a small gasoline-powered engine.

A general rule of thumb is [for an auxiliary engine is to use] 1-HP for every 3-feet of boat. For example an Outrage 17 would need a 5-HP or 6-HP auxiliary engine for trolling.

One-horsepower=745.7 watts. Assuming [the electric motor is 100-percent efficient, then] in a 12-Volt system, 62-Amperes are needed to produce 1-HP of thrust from an electric motor.

Minnkota tells us the Amperes drawn by their 12-Volt motors of 55-lbs-thrust or smaller thrust is roughly 1-Ampere per 1-lbs thrust. Thus a motor drawing 55-Amperes is producing 55-lbs thrust. An electric motor drawing 55-Amperes at 12-Volts cannot produce the equivalent of 1-HP of power--even if 100-percent efficient

I review the relationships mentioned in the post above.

In terms of a rate of performing work, the units Horsepower and Watt are related by

745.7-Watts = 1-Horsepower

In terms of electrical power consumption at 12.00-Volts, to consume 745.7-Watts requires a current of 62.14-Amperes.

A relationship between electrical power consumption and propeller thrust is mentioned as being approximately given by manufacturer MinnKota (without citation) as:

1-lbs Thrust = 1-Ampere (at 12-Volts) consumed by electric propulsion motor

With the assumption that an electric propulsion motor might be capable of perfect efficiency in converting input power to output power, then a calculation reveals that a 55-lbs thrust motor presumed to be consuming 55-Amperes at 12.00-Volts would have an input power of 660-Watts, or 0.88-HP, and would produce an output power 0.88-HP at the propeller shaft.

In terms of how much horsepower is needed to propel an Outrage 17 to a speed suitable for "trolling" (as suggested by the description "trolling motor"), a value is put forth that 6-HP to 7-HP is required on the basis of 1-HP per 3-feet hull length. This premise seems to me to be on the most shaky ground, that is, I don't see any basis for it.

The first problem is to define "trolling" speed. Without knowing what actual speed in miles-per-hour constitutes trolling speed, the assumption that 7-HP may be needed seems mostly a presumption that has not been established as a fact.

Since the notion that 6-HP to 7-HP would be required to propel an OUTRAGE 17 to "trolling" speed is not backed by any evidence or theory, to then conclude that there is no possibility that an electrically powered trolling motor could ever be used seems to me to be quite a stretch of logic or proof.

Further, since there are many articles describing the use of an electric trolling motor on Boston Whaler boats of 17-foot length, there seems to be a substantial amount of evidence that 17-foot boats can be propeller by electric trolling motor at speeds suitable for trolling, which is in complete contradiction to the conclusion presented above by LIMECC in his post.

ASIDE: regarding the power needed to propel a hull to a speed below the hull's displacement speed, we can estimate the maximum displacement mode speed (also called "hull speed") from the hull waterline length with a relationship of

Speed (in nautical-miles-per-hour) = 1.34 × (Length of Waterline in Feet)^0.5

If we allow for a 17-foot hull to have a 14-foot waterline length then its hull speed will be 5-nautical-miles-per-hour or 5.75-MPH.

On that basis a "trolling" speed would be assumed to be less than 5.75-MPH, and perhaps more like 2.5-MPH to 4-MPH.

jimh--Some great info. Thank you.

My initial requirement was to use an electric motor as backup in event of main engine failure, and not for fishing in still lakes where the main force to overcome is from windage, not hull resistance in water. I totally get an electric solution could work in this instance.

Throw a few rivers and sea into the mix and 0.9-HP is simply not going to work. Tidal or river flow could typically be 4-nautical-mile-per-hur or more. Even if an electric trolling motor could hold its own, the boat would just sit there stationary until the electric motor battery gave up. Lead acid batteries don't have a long service life if depleted below 50% capacity so a 110-Amere-hour battery will give about one hour service in such conditions--and then you are done. With a gasoline engine, the onboard tank can be re-filled or the main outboard fuel connected

If trying to make headway and both wind and current against, then the forces combine and chances of getting home are even more bleak.

After abandoning the idea of going electric, I next turned to the possibility to use a Chinese-made gasoline outboard engines. After reading reviews and watching YouTube I rejected that as well.

If the goal posts have moved from a trolling motor for angling to a get-home motor for use when the main propulsion engine is lost and headway must be made against opposing current and wind, then your assessment that an 55-lbs thrust electrical trolling will not be suitable is quite likely correct.

A better electric propulsion motor will be needed, something along the lines of a TORQEEDO outboard. Be warned the expense will be significant.

A TORQEEDO Model CRUISE 4.0 RS may be suitable. The engine is rated at 4-kW, or 5-HP. Here are some retail prices:

• CRUISE 4.0 RS at $4,500
• Battery model POWER 48-5000 at $5,200
• Top-mounting remote throttle at $1,400
• Charger for POWER 48-5000 at $900
• Or, optional Fast Charger for POWER 48-5000 at $2,200

The total investment would be about $11,100 for the engine, battery, and remote throttle, and then add $900 or $2,200 for the battery charger, giving a total of $12,000 to $13,300.

With the above configuration you could run the motor at full power for 1-hour. The battery weighs 80-lbs. The engine weighs just 37.5-lbs.

I don't think investing $13,300 into an electric propulsion system as a get-home engine is particularly cost-effective. And given the limited running time at full power, don't get too far from home.

TORQEEDO has a good essay on comparison of propulsion power using various methods, such as input power, shaft power, static thrust, and propulsive power. I think the essay is a good corollary read for this topic. See

https://www.torqeedo.com/us/en-us/techn ... iency.html

jimh wrote:I don't think investing $13,300 into an electric propulsion system as a get-home engine is particularly cost-effective. And given the limited running time at full power, don't get too far from home.

Correct Jim. I'd rather invest that money and trade up my BW.
Fyi how it worked out for me was I got an as-new 3.5hp Tohatsu weighing 40lbs for $550 and upgraded it to 5hp with a kit from Ready Set Boats (the kit included a high thrust alloy propellor). Total cost under $750.

P.S. the US version of this engine already has a fuel pump so it was easy to add an external fuel connector.

jimh wrote:TORQEEDO has a good essay on comparison of propulsion power using various methods, such as input power, shaft power, static thrust, and propulsive power. I think the essay is a good corollary read for this topic. See

https://www.torqeedo.com/us/en-us/techn ... iency.html

Interesting article.

Q: are all [unclear acronym—please do not invent unusual acronym and instead just write out the adjective] outboards really as bad as Torqeedo claim? Some engines are not so bad, others are not so good.

Q: did TORQEEDO take a bad example?

The torque curves I've seen so far were quite flat because conventional outboards are in quite a low state of tune compared to what is used for a motorcycle engine with multiple forward gears.

I'm sure a small gasoline engine can be optimized to drive a heavier craft like the Torqeedo, but I'm not sure what that means in terms of propeller design.

I guess prop slip has to be kept to a minimum and be matched to the hull being driven.

Normally these engines come with a prop more suited to a lightweight PVC tender. There must be more to it than simply lowering the pitch, two blades instead of three might be needed. I read the most efficient propeller would have only a single blade.

In regard to the comments from TORQEEDO about their propulsion engine compared to competitors' engines, I am sure they wrote the comparison with the intention of presenting their engine in a very favorable position in order to promote sales of their product.

In regard to electric motors in general, the ability to generate very large torque at low RPM is a characteristic of electric motors. The use of electric motors in railroad locomotives is a good example of the ability of electric motors to produce high torque output at very low speed of rotation. Many large ships use electric motors for propulsion, with conventional engines generating the electrical current needed. Modern electric motor are no longer strictly driven by direct current. That is a complex topic, and not really suitable for discussion here

What is suitable for discussion here is the very high price to be paid if you want to utilize electric motors for propulsion of boats like a 17-foot Boston Whaler, and also clear is the very limited operating time possible due to the still unsolved problem of storing electrical energy in a battery.