SUBJECT: # 13110: Horsepower to RPM relationship, what's the formula?

Submitted by Bayoudog (209.213.130.61) from TEXAS on 12/14/1999 8:54:00 PM

If a boat motor has a power rating of, say, 300 hp at 3000 RPM, how much hp would it put out at 1500 RPM if propped correctly? Square? Cube? Any info appreciated.

1. 12/14/1999 10:05:00 PM Submitted by Sal (205.188.197.22) from CALIFORNIA says Off the top of my head.
   Less then 100 hp. Don't know the formula but the engine dosen't even start to develop it's hp until the last 1,500 - 2,000 rpms. Sal

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2. 12/14/1999 10:08:00 PM Submitted by Sal (205.188.197.22) from CALIFORNIA says Sounds like a 3208T.
   Is that the engine your thinking of? Sal

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3. 12/15/1999 6:42:00 AM Submitted by cptntony (207.172.196.239) from NEW JERSEY says HP vs. RPM
   B'dog!....I know you threw in the prop issue just to bust!. What kind prop did the manufacturer put on your car to measure horsepower?. Lets concentrate on the engine since the propping is only the way to best harness its throughput depending on what hull you are "pushing".

   Each type engine is different. Also different .- same engine with different induction (carb. size vs EFI), valve sizes, number of valves per cylinder, aspiration, timing, etc. Each type engine will have its own Torque & RPM curve relationship. A general formula would be a toughie because you don't have a TRUE straight line along the rpm range of the specific engine. Luckily, the horsepower line is close enough to being "straight" that you could get a very, very close approximation by using the simple formula for a straight line where Y= mX+b . You would be plotting horsepower (Y) vs. RPM (X). m= slope of the line and b = your intercept (zero from data- and can be ignored). But please note that we are actually working your question "backward", that is, we are getting the formula empirically (running the data first for that specific engine AND THEN calculating IT'S formula from a graph). Why backward?, because of the many type engines / aspirations, etc. So first we need to get "the numbers" for that type engine. Here are some actual numbers: Please note that engine performance graphs also include torque. What you have are two vertical axes, one for torque and another for horsepower. vs. the horizontal axis (RPMs).

   These data are for a turbo-charged, 4-valve / cyl., EFI, hemi-head....get the picture....many factors defining a measly 6 cyl. automobile engine (with no prop.) - just kidding!. I'm reading from the graph, so numbers are close approximations.

   1000 rpm. torque = 185, horsepower = near ZERO

   2000 rpm torque = 240, HP = about 50

   3000 rpm torque = 250, HP = 142

   4000 rpm torque = 245 (going down) HP = 190

   5000 rpm torque = 235, HP = 220

   5500 rpm (redline) torque = 230, HP = 230

   6000 rpm torque = 220, HP = 220 - both drop suddenly within 500 rpm

   I guess the best way I can answer your question as to whether the HP formula is a function of square or cube, the answer is neither. The resulting plot is pretty much a straight line up to red-line. If it were a square function we would see a parabola. If a cube, a hyperbola. Sorry!

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4. 12/15/1999 10:13:00 AM Submitted by Doug (207.30.27.2) from FLORIDA says HP/Torque formula.....
   Here's the formula for calculating HP. If you have an accurate HP curve for the engine, you can calculate the Torque for a given RPM (Or vice/versa).

   Horsepower = Torque (ft*lbs) x RPM / 5252

   or

   Torque (ft*lbs) = HP x 5252 / RPM

   Doug

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5. 12/15/1999 7:17:00 PM Submitted by Bayoudog (209.213.130.67) from TEXAS says Yes, it is a 3208
   This guy Sal can read your mind. I'm preparing for the inevitable - one day having to repower and an engine that has always intrigued me is the 3208T from Cat. The only problem is that it puts out more power than my "displacement" hull can use efficiently. If I went with the Cat, I'd have to run it at probably no more than 1500 RPM. The problem with manufacturers curves that Tony mentioned is that they all provide HP & torque curves based on motor running wide open throttle. On a boat, when running at less than WOT, I must assume that the engine is NOT putting out the full HP for the RPM level in the performance chart because it is running at part-throttle, not full throttle. Makes sense? Hence my inquiry as to what the relationship between HP and RPM - on a boat, not a car! Thanks for the info, it's gonna take my little brain a few days to digest all the data you provided, Tony.

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6. 12/15/1999 8:35:00 PM Submitted by Leprechaun (12.79.53.88) from NEW YORK says SO you want a big fat Cat, eh?
   Doggie - would it help if you had a real big reduction gear? Say 2.5:1 or more to get the engine rpms up and the prop speed down? Then you could run that big Cat up closer to its torque peak rpm-wise and still get the power down. I am aware that your boat has a finite hull speed, but you DO want a 12 Liter v-8, right? Leprechaun

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7. 12/15/1999 10:31:00 PM Submitted by Sal (152.163.197.198) from CALIFORNIA says Bayoudog.
   I'm trying to figue why your boat can only handle an engine that turns 1,500 rpms. The cat is rated at 2,800 rpms. The turboed 3208T gets it's poer the last 600 rpms. If your set on a Cat 3208 but don't need the extra horses of the turbo 3208T, why not think the natural asperated 3208, I think they develop 225 hp [ also painted yellow, the turboed is white ] Either way you can swing a larger prop or smaller prop. allowing you to run wot. I'v owned 3 - 3208Ts twins in one boat & a single in the other, absolutly love those engines. They are quite not like a jimmy or detroit where you can't hear yourself think. They are simple to work on & dependable as hell just keep clean air filters on em. How big is the boat your thinking of putting it in. Sal

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8. 12/16/1999 7:53:00 AM Submitted by trouty (203.38.14.209) from Australia says BD - Heres a tip for you!
   Like Sal Says - keeping clean filters in her will keep her alive forever. Last year I worked a lot of days on a lot of commercial vessells with lotsa different diesel powerplants - and as inevitably ends up happening, ended up in the bilge working on em as well. I've seen 500Hp Iveco turbo Twins with allminium corrosion probs in the Header tank - water pump connector sleeve at 12 months from new. I also saw Twin Fiats 750 Hp Turbos that ran sweet as a nut - but they were both new, and interestingly, got to change oil on a GM Detroit V8 1000HP Twin Turbo. It's the GM I learned something from...and the owner swears by this setup.

   He's had a supplemental centrifugal oil filter canister fitted to this GM, that I think are a Volvo part! This thing spins at something like 15000 RPM and separates all the carbon build up outta the oil by depositing it in a thick layer around the outside of the removable revolving cannister. When changing oil, you litterally scrape the Carbon off the inside of the cannister with a knife and it comes out like the consistency of peanut butter.

   I've never seen anything like it. For anyone setting up a diesel from new - I reckon you'd be mad not to include one in the iol filtration line. Keep using the standard cartridge filter as well - but for my $ - don't go past a centifuge filter as well! They must be available as a spare part or accessory from Volvo I'd guess. I have the Skippers Mobile number - so could get exact details if anyone gets interested! Ps. the detroits sure make a rackett, but hey do they push!

   Cheers

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9. 12/16/1999 4:06:00 PM Submitted by Michael (208.222.63.106) from ALABAMA says x Caterpillar Engineer in Engine Division
   First of all, I worked for CAT in the engine division for 5 years, most of which in marine division. The 3208 is a good choice if you are not worried about weight. The engine is of old design and has a poor BSFC (Brake Specific Fuel Consumption) compared to newer models. In other words, you would get better fuel economy with a newer style engine, say the 3116. If neither of these concern you the 3208 is a very well proven work horse. Now about the hp issue. Go the CAT website...www.cat.com and get the power curves for the engine you are interested in. They give 2 curves, one for max hp vs. rpm and a PROPELLER curve, which assumes the prop is sized for rated horsepower at rated rpm. There are many formulas for determining gear ratio and props to match the engine, so be sure you talk to someone very knowledgable in this area. I was once certified by CAT as a Marine Analyst and have all the books so I can easily help you with any of this. I am in the aerospace industry now and really miss the marine work. And do NOT use a linear line to estimate hp. This is ok for very low hps but a prop curve is NOT linear, rather closer to cubic. Depending on gear ratio this could make a lot of difference and you would not want to size a prop with this. If you need any help or information feel free to email me direct....good luck

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10. 12/16/1999 6:21:00 PM Submitted by Dave (152.163.213.51) from TEXAS says Cummins engines
    Check out Cummins B series. 150-355 hp. These are real work horse motors. The 150 hp 4cyl would be perfect for a trawler, or maybe the 210 hp 6cyl. Trawlers cant use those big Cats very well.

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11. 12/16/1999 6:21:00 PM Submitted by cptntony (207.172.176.93) from NEW JERSEY says Question, Michael
    I too make my living as a scientist (28 yrs.). But, I'm not an engineer, I'm a chemist with a decent background in physics and mathematics. Judging FROM THE ORIGINAL info from post by B'dog and for the rpm range of 1500 to 3000, I have yet to come across a set of rpm vs. HP curve which I could not get a darn close approximation using the formula for a straight line even without the need of applying the least-mean-square formula. Would you kindly e-mail me a set of ACTUAL data (rpm vs. HP or Torque) so that I can plot it and see the "cubic" relationship for that rpm range. Is it the same for turbocharged engines?. Differnet for diesels? Live & learn I guess!

    Thanks, Tony

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12. 12/17/1999 10:33:00 PM Submitted by Bayoudog (209.213.130.94) from TEXAS says Thanks again everyone
    Appreciate the input. I'm gonna come back with more specific questions when I gather more info on this subject. Sal, it is a 34 Ft trawler with a 12 ft beam, hull type not determined whether displacement or semi-displacement, currently powered by a Ford Industrial (British tractor) engine rated at 120 hp.

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13. 12/18/1999 1:06:00 AM Submitted by Sal (205.188.197.181) from CALIFORNIA says Bayoudog
    I really don't think a 3208 would be right for your trawler. Seriously, i'd look into a John Deere Lugger, 6 cylinder, puts out 300 hp at i think 2,200 rpms, very little fuel needed for these engines. Many of my buddies up north [ Alaska ] are putting them in their gillnetters, 32' by 14' wide, displacement hulls. They all just rave about how quiet they are & 5 gph fuel consumption, not as fat as the Cat [ my favorite ] & very easy to get around, good warranty, about $30,000. I feel this would be a better engine for your type hull & you can run her near her max rpm rating & twist a wheel about 35". Could give you 15 to 18 mph if your semi displacement. Just my .02 worth but worth looking into. Sal

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14. 12/18/1999 1:07:00 PM Submitted by bAYOUDOG (209.213.130.98) from TEXAS says Luggers
    Sal/I've come across those in a publication (Boats & Harbours) and they are distributed by Alaska Diesel out of Seattle I believe. I'm gonna check into it when the time comes. Thanks for the tip. By the way, the new Kadey Krogen Trawler comes with a John Deere and it is designed for trans- oceanic crossings.

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