Trolling Thrust vs. OB HP

[Donna D]

Hello.... Is there a chart somewhere that one could find what pound thrust equals what horsepower?

I'm sorry if this is a repeat question, but I have looked in the FAQS and could not find it there.

Example: 36 lb. thrust = 1 hp............42 lb. thrust = 2 hp........ 55 lb. thrust - 5 hp.

I tried Google, and there was a person named Bear in July of 2002 that directed someone to this site that had it listed. I looked, but you start from 2003.

Anyhow, I would really appreciate it.

Thank you,
Donna

 

[WillyBWright]

Re: Trolling Thrust vs. OB HP

TM's would be in the hundredths or tenths on one horsepower. Consider your washing machine motor. Most are 1/3HP and they're gigantic compared to a TM.

 

[dingbat]

Re: Trolling Thrust vs. OB HP

There really is no straight answer. Thrust is a time/force calculation so the HP equivalency would vary by the speed of which the object is moving.

 

[jtexas]

Re: Trolling Thrust vs. OB HP

strickly speaking, thrust doesn't exactly translate literally into horsepower - thrust measures the actual pounds of straightline force the motor applies to the boat, whereas horsepower is a measure of engine torque - force going round & round - plus which you have the drive train variables to consider.

But try this: horsepower x 746 = Watts. Watts = voltage x current. Most trolling motors are either 12-volt or 24-volt motors. Battery starts out at 12.7 volts and is pretty much done for the day at, say, 12.4, average of 12.55. Current draw depends on a number of factors, but a 36lb motor probably draws between 10 & 20 amps, call it 15; 15 x 12.55 = 188 watts, x .00134 = about ? horsepower.

Put another way, one horsepower is 33,000 footpounds per minute. If my 1500lb boat displaces 15 cu.ft. of water, that's 930lbs, leaving 570lbs of weight to be moved, and if my 36lb trolling motor can move it at 2MPH, that's 176 feet per minute, or 100,320 ft. lbs. per minute, or about 3 horsepower.

so, 36 lbs of thrust is somewhere between ? and 3 horsepower. Extrapolating, 42 lbs = .29 to 3.5 hp, 55 lb = .38 to 4.6 hp. Any higher and you're talkin bout 24-volt motors, it would require more thought.

It might actually be easier to measure the pounds thrust of a gasoline engine. Tie it to a scale and tie the scale to a tree and try to drive away?

disclaimer: the above analysis was completely unencumbered by the thought process; actual results may vary.

[after further thought: at 12 lbs of thrust per horsepower, my 70 hp outboard has 840 lbs of thrust - if the boat weighs 570 lbs in the water.......my boat is ballistic!! I may have to get a new speedometer, that's marked off in increments of mach!]

 

[dingbat]

Re: Trolling Thrust vs. OB HP

after further thought: at 12 lbs of thrust per horsepower, my 70 hp outboard has 840 lbs of thrust - if the boat weighs 570 lbs in the water.......my boat is ballistic!! I may have to get a new speedometer, that's marked off in increments of mach

Don't run out and get a new speedometer. A "pound of thrust" is equal to a force able to accelerate 1 pound of material 32 feet per second.

With 840 lbs of thrust you can push a 570 lb boat at, very roughly, 32 miles per hour.

 

[tashasdaddy]

Re: Trolling Thrust vs. OB HP

i have it bookmarked at another puter. will try to remember to post it on Friday, when i get home.

 

[Texasmark]

Re: Trolling Thrust vs. OB HP

As I recall acceleration is the second derivative of velocity which is 32 ft per second, per second.
(sorry just had to jump in)

Nice calcs j.

Another variable is prop slippage. When you initially energize the TM, the prop must overcome the the fact that the mass (of the boat) is at rest. The prop turns up to probably 90% of terminal rpm's but there is no forward movement of the boat.

So prop slip starts out at 90% or so and dwindles as the boat starts picking up speed......to what, have no idea....guessing 20%?????

So, as stated it's a complex calculation.

But I'll say this, the 67lb thrust Motorguide Brute (24V) will make my 17' alum bass boat fly....way overpowered for the application......but's that's OK, if I ever need it, it's there. Son has a 56 lb thrust Motorguide (24v) on a pretty heavy ProCraft 18' FG boat with a Merc 150 and it flies too.....if that helps.

This was fun.

Mark