View Full Version : ThrustHP

01-04-2009, 08:07 PM
"What is the relationship between Thrust and Horse Power, ie. Equivalent of a 30 trust trolling motor in HP..."

01-04-2009, 10:58 PM
"I've asked this question a little differently, and got no direct responses. Good Luck, Mike"

01-04-2009, 11:06 PM

In the case of your trolling motor, you can get a crude estimate. Use a fish scale with a mid range of 30 lbs. Attach it between a fixed object and your boat. Crank up the troller and measure the force on the scale while measuring the volts and amps to the motor. Volts times amps gives Watts and there are roughtly 750 Watts per horsepower"

01-04-2009, 11:25 PM
MMM sounds complicated....I wonder if there is a formula to just translate thrust to HP...

01-05-2009, 11:17 AM

Mike's "watts" technique will get you a coarse estimate which will high. The scales measurement will be much closer.

There's no simple relation between HP and thrust as there are three big variables: drag, gear losses and propeller losses (slip).

the only things i've seen with thrust ratings are trolling motors and bow thrusters. Both of these are 'self contained' in that the manufacturer does control most of these added variables."

01-06-2009, 01:26 AM
"There is a formula (more than one) but they are so convoluted that with anything less than an engineering degree you can't understand them.

When you filter through all the numbers, what is most important to us as boat owners is in the case of a electric trolling motor, how many pounds of thrust equals 1 shaft horsepower (the way all modern outboards are rated) and a term "we" can understand for comparison.

That answer appears to be 58 pounds of thrust = 1 shaft horsepower.

To be fair to the equasion however, thrust does not actually convert to shaft horsepower, instead it is measured in electric horsepower (ehp) which then is factored for cavitation, velocity through fluid (water), length of the wetted surface of the vessel being moved (plus a few greek looking symbols that I don't understand) which is then compared to a table of what a "similar" shp rated motor would achieve under the same conditions.

So depending on the length/width of the boat, the exact shape/style of the prop and those unknown greek factors, the conversion will be a little different on each boat - but guess the 58=1 is close enough for most of us....

I can dig out the formula if anyone is really interested (amongst my files of semi-useless info)."

01-06-2009, 01:40 AM
"I should add, for anyone well versed in math since I'm not trying to start an arguement, that there actually is no direct conversion formula.

You basically work backwards through a "how many shp" is required for "x" speed and then work through a second formula to determine how much thrust is required to achieve the same distance over time result which is then back converted to horsepower through it's purest definition..."

01-06-2009, 03:23 AM
Got it!!

01-06-2009, 10:37 AM
"when I made my request for thrust, I was making transom repairs and wanted to know the loads and moments to expect to better know how to beef up the transom. I was hoping that for my 115 hp there would be a range of thrusts known for the engine or used by industry. Boat mfg's have to have some estimated forces to design transoms and hulls."

01-06-2009, 10:48 AM

what was the 58:1 figured derived from: outboards, inboards, etc?"

01-06-2009, 11:26 AM
"Well, you could get an idea by working backward again. If 1 horsepower is equal to 58 pounds of thrust, then to get the equavalent of 115 shp you would require 6670 pounds of thrust to move the object forward at the same speed over the specified length.

But that is in no way accurate, because it doesn't go from 0 to 115 horse instantly and the thrust calculation factors in the "speed over distance" with "x" pounds of force (energy) required to push the object in the direction of travel for a given distance in a specific amount of time.

Plus, initally, the torque band of the motor is significantly higher than the horsepower band, so I would suspect that "more" thrust is actually applied at lower rpms/lower horsepower (with greater stresses on the transom while it is trying to get to boat on plane) and it will drop off as momentum assists in moving the boat forward and the hull lifts from displacement, through semi-displacement up to full plane with minimal drag compared to the other conditions - which is where the torque curve which had been moving upwards starts falling and crosses the horsepower curve on it's way up to maximum.

To accurately determine "how much thrust is being applied against the transom", you would need to take numerous test measurements, figuring the time and distance covered from a dead stop in the water, over various fixed distances both into and away from the wind, waves etc. Going with the wind less horsepower would result in greater thrust and vice versa.

That would most likely lead to conclusions such as - with the wind 1 hp = 45 pounds of thrust - into the wind and waves 1 hp = 75 pounds of thrust.

And technically, a jet sitting on a taxi way with it's brakes locked whose engines are putting out 35,000 pounds of thrust in anticipation of take off, are producing "ZERO" horsepower because there is no movement over time to calculate.

I am currently working on a build of a glass over plywood boat that will have a 140 mounted on it. The best advice that I got for the transom was "build it twice as strong as the original spec, and if you think that's overbuilding it, make it even stronger, just to be sure"

Now my brain hurts so I'm going to leave this thread for a while http://www.marineengine.com/discus/clipart/lol.gif"

01-06-2009, 11:49 AM
"The 58 lbs was derived by comparing a small air-cooled outboard, transom mounted and a MinnKota trolling motor producing 60 pounds max thrust.

Not super scientific, I think just a few guys who had alot of time on their hands and could do some algebra in order to try and settle an arguement.

A comparative formula was produced and posted on a "nerds" website from one of the major universites (which I have now long forgotten). I did cut and paste it with some of the narative which explained the method and stored it away in one of my many "useless information" files which contain such other great debates as effectively coverting bhp to shp in broad terms."

01-06-2009, 02:03 PM
well it is still too complicated... I'm just trying to figure out if it would be better to have an electric emergency motor one of those that mount on the cavitation plate or just go with a small 10hp for my old 21ft Stamas walkaround...

01-06-2009, 04:05 PM
"This has turned out to be a great post. Thanks Nick for coming up with the great question and Graham for the insight and detailed responses. Mark your slip comments were right on too.
Graham and Mark, I am curious of your backgrounds if you don't mind sharing. I have taken some sophmore level Naval Arch. courses but it center on displacement hulls.
Thanks, Mike"

01-06-2009, 07:23 PM

my "profession' started fixing cb radios for truck drivers. then did the auto mechanic thing. went to college and have ee degree with a fair amount of post grad studies.

did mini-bike and lawnmowers in my early youth and have been tinkering with boats ever since. Got a job, part-time, at a mercruiser/mercury master mechanic job,too. learned a bunch about outboards and sterndrives from him. He used to swear i'd pass basic cert if i ever went to NJ but never did. this all came in handy when i started charter fishing over 20 years ago. when something breaks in the middle of tuna season, trying to find a mechanic to come and fix your boat (yes, tonight...so its 11pm) was impossible."

01-07-2009, 12:34 AM
I basically grew up in an autobody shop (my grand-dads). My Dad was a machinist turned Diesel/Small engine mechanic so I had a body file in one hand and wrench in the other from the time I could walk

My professional background - no where near mechanical stuff. Degree in geography and 22 years in the Canadian Airforce (Intelligence).

Retired 7 years ago, now split my time between working on outboards and riggin' during the "season", working on a project boat build (personal) and spend a few hours a week working for a logistics company (for extra spending money)doing customs work for Canada/US shipments.

My mechanical skills (or lack thereof) are self-taught or rubbed off from all the time I spent in shops when I was younger. All the rest of the useless information I can spout out is from reading everything I can get ahold of...

I have extensive collections of service manuals, production lists, service bulletins, technical papers - etc etc - everything marine - to use as reference.

As to the question of electric or gas as a backup or "emergency" motor. Personally, I would go with a gas back-up (and do run them).

Even on my 14 footer which only has a 15 horse, there is a little twin cylinder 2.5 horse Ev hangin' right beside it and have used it on one occasion to bring the boat back to dock when the float valve stuck on the bigger motor and flooded out.

You could easily kill the battery trying to use an electric to bring you back to dock. And in rough conditions, wind, or a long haul back - would take you forever.

And besides, ideally you would at least want your "kicker" to be able to move you along at least hull speed for your rig (about 6 knots or 7 mph for your particular boat). Off the top of my head would figure you would need 5 or 6 horsepower to accomplish that in all but perfect conditions."