hp v lbs of thrust

[gazelle]

As far as I know there is not a conversion for hp to lbs of thrust. They measure two different things. I am looking for a new mount for my trolling motor and some of the ones I have looked at have lbs of thrust maximum. I have an older Minn Kota 4hp 24 volt trolling motor. <br /><br />The mount that I am looking at is a max of 36 lbs. Anyone know if my 4hp is more than 36lbs of thrust?

 

[kraky]

Re: hp v lbs of thrust

From my limitted studying I'd say anything with 24 volts is probably a minimum of 48-60 lbs of thrust. Check out the minn kota website and see the thrust ratings of their motors. Also, they now have a new electic "outboard" and uses 4 batteries and has 48 volts--they "say" that's a 2 hp so, it seems even the experts don't know how to convert thrust and hp.

 

[DJ]

Re: hp v lbs of thrust

gazelle,<br /><br />I would have to say that it is definetely more than 36. Probably closer to 60+.

 

[Larry Carnahan]

Re: hp v lbs of thrust

I had an old Minn Kota 3hp saltwater trolling motor that was the largest 12v. When I replaced it with the riptide the largest 12v was 55 lb. One could infer that 4 hp would be 55lb / 3 hp * 4hp = 73 lb: perhaps 72 lb thrust?

 

[n0ukf]

Re: hp v lbs of thrust

Where is the MinnKota web site? I've tried www.minnkota.com, but that only gets me to MinnKota Power Co-op. At least three boating web sites I've tried also didn't have a link to MinnKota's web site.

 

[neumanns]

Re: hp v lbs of thrust

www.minnkotamotors.com should lead you there. Pequot Lakes here where you hail from.

 

[n0ukf]

Re: hp v lbs of thrust

Lake of the Woods

 

[Larry Carnahan]

Re: hp v lbs of thrust

You can also try www.johnsonoutdoors.com and click on the motors link.

 

[NathanY]

Re: hp v lbs of thrust

I was told once by a MinnKota Rep at a boat show that it was 18lbs=1hp. I dont know how accurate that is though.

 

[Paul Moir]

Re: hp v lbs of thrust

It doesn't seem too accurate. By that method a 74lb thrust Genisis 74 trolling motor should take the equivilant of 4hp to drive. Yet it only consumes 1200W, or about 1.6hp worth of electricity. And that would be with 100% efficency.

 

[MajBach]

Re: hp v lbs of thrust

LOL. I knew eventually somebody would meake that comparison, i.e. 746 watts = 1hp.<br />Paul, don't forget that you are looking at only about 30-40% rated power of a combustion engine to make that comparison. In other words, a 4hp motor = about 1.3hp or so.

 

[Paul Moir]

Re: hp v lbs of thrust

Ok, why derate a combustion engine by 60%? If it's turning out 4hp, why not move 4hp's worth of water? Or do you mean the engine out to be operated at 1/3 throttle?

 

[Luna Sea]

Re: hp v lbs of thrust

Quote from MinnKota's website........<br /><br />"How does thrust compare to horsepower?<br /><br />Horsepower is a measurement of "work" being performed. <br />One horsepower is a unit of measurement equal to 550 foot pounds of "work" per second. <br />There is no direct correlation of thrust to horsepower. Contrary to what you may have been told, fifteen pounds of thrust DOES NOT equal one horsepower. <br />As noted in the previous definition thrust is simply a static measurement of force. "

 

[MajBach]

Re: hp v lbs of thrust

So true. But there is a general correlation betweeen an engines' rated 'power' and the static force that they can apply. As you can dervive from the statement '[550 foot* lbs of] work per second', it is a measurement of energy expended per unit of time; just like voltage * amperage = watts = power. Providing the engines creating the the force run at the same efficiency, a comparison can be made. However, gas engines are not as efficient as electric. That's why one is rated in hp and the other in watts (or for the laymen, lbs of thrust).

 

[SlowlySinking]

Re: hp v lbs of thrust

Hi, go to your web browser search line and type in horsepower vs thrust, hit enter, many sites with great reading.

 

[Paul Moir]

Re: hp v lbs of thrust

I'm still confused. True, an electric motor will run somewhere in the neighborhood of 80% efficient, while a gasoline combustion runs something like 25%. But gas engines are not rated by the theoretical power available by the gas it consumes. Electric motors are also rated primarily by their output and still often in hp, although their nameplates normally also state consumption.<br />I understand the problem with comparing HP or KW to lbs of thrust since you need to consider the propeller. That's why I didn't make a direct comparison between HP (or KW) and lbs of thrust, more than the Rep. <br />Both HP and KW are instantaneous measurements and are directly translatable. I think what you're forgeting is that torque has a time component that must be removed to convert it into HP, hence the /sec. That is to say, ft-lbs translate to Kw-hrs in metric.<br />You will also notice by studying the specs that there is no precise correlation between current consumption (watts) and lbs of thrust. It seems to be somewhere between 9W-13W per lb of thrust. They also have a 2 hp model there. As expected, it consumes 1800W while a 101lb thrust trolling motor consumes 1300W. This suggests a hp produces somewhere over 50 lbs for them, rather than 15.<br />Or there's the much simpler example of a Coleman generator that produces 5000W via a Techumseh 10hp engine. <br />So why derate the gas engine 30 to 40%?

 

[NathanY]

Re: hp v lbs of thrust

huh?

 

[n0ukf]

Re: hp v lbs of thrust

my 2 cents...<br />>If it's turning out 4hp, why not move 4hp's worth of water?<br /><br />I just heard the other day that older outboards were rated at the engine shaft, but newer outboards are rated at the prop shaft. Don't take my word for it, it's just what I heard.