One horsepower is 550 foot pounds per second (ability to lift 550 pounds straight up in one second). If it was this simple the below would not be true
But there is more to it than that Peterbuilt diesel engine 380 horse.
Dodge Hemi 390 horse.
With the Pete I can pull 80,000 lbs 80 miles an hour all day and you would be lucky if you untracked 80,000 lbs behind the Dodge and there is no way it will pull it 80 miles an hour down the highway.Now the dodge has ten more horses than the pete and will run faster with a load
How do you explain that?
Final post from me on this thread,
Jack I hate you got your drawers in a wad over this,I wasnt looking for an argument and I was sincere in asking if you could explain why the measurements of horse power didnt work out.
I look at thinks a little differently than most folks. There are very few formulas that stand on there own,displacement of a hull is another I will use for a example,most folks take it as a flat set in concrete number when in fact it isnt, temperature effects it,salt and fresh water will give you different readings,amount of air trapped in the water ,etc you get the ideal.
Disagreeing or expressing my views seems to e taken as a personal attack and it isnt ,it is my own thoughts and questions.
I hope every one on this board keeps tight lines ,great looking boats , trips and experiences .
In all this discussion about thrust, horsepower, torque, valve timing, etc., there has been no mention of the prop.
Thrust is not a characteristic of an engine. In a marine application, such as is being discussed here, thrust is the force generated by the total system which includes the engine, gearbox and prop. You can measure low speed thrust by mounting a tension scale (like a big fish scale) in a line and tying one end to the dock and the other end to the transom and letting the boat pull against it.
You can take two identical boats and engines (let's say Detroit Diesel 671 engines) and put a direct drive (1:1 ratio) and the appropriate prop on one boat, then put a 3:1 reduction gear and a prop sized for that setup on the other boat. The 1:1 ratio setup will have higher RPM but less torque at the prop shaft, smaller prop diameter, probably with a pretty fair amount of pitch, and would be better for higher boat speeds. The 3:1 setup would have 1/3 the shaft RPM, more torque at the shaft, a larger diameter prop, probably less pitch, to generate low speed thrust like a tugboat or trawler needs. If properly sized by the right combination of diameter, blade area and pitch, both props will absorb the same horsepower at their optimum boat speeds, yet give very different performance. The smaller, higher speed prop will have a lot more slip and less thrust at low speed. The larger, low speed prop will have less slip and more low speed thrust but not be capable of the higher boat speed of the one with the smaller prop. All this with the engines producing identical horsepower at identical engine RPM.
Assuming (perhaps foolishly) that the engine manufacturers rate their engines honestly, regarding the 2 and 4 stroke engines, both rated at 2 HP, my guess is that the 2 stroke engine has less torque but is turning at higher RPM than the 4 stroke engine to produce the same 2 HP. If they both have the same lower unit ratios, then the 2 stroke one would also have less torque but higher shaft RPM at the prop and would have a prop with some combination of less area, diameter and pitch. The result is a prop that generates less low speed thrust.
Two motors with identical HP but different pitch to the props will have different actions. After all it is what is in the water that pushes the boat , not what is above it ( being equal ) , that just makes the unit in the water work.
My guess that is the reason they make so many different props with all sorts of blade configurations and pitches (angles to the blades ) .
That's it in a nutshell. Determining reduction ratio, prop diameter, blade area, pitch and cup on the blade trailing edge is a critical science in getting the maximum performance out of a boat. On larger boats it is not unusual to try two or three sets of props before settling on the final ones.
Actually you are wrong. Horsepower is exactly that - a true sense of power output.
If the Dodge engine was attached to a ten speed transmission with appropriate gearing, then yes, it could do exactly what the diesel could do. The reason diesels are used for heavy trucking applications is economy (diesels are generally more efficient) and durability. If it was just a matter of power output, you could just as well use a gas engine, though with lower gearing to compensate for the higher RPM / lower torque that gas engines operate at.
Diesel locomotive pull more weight faster than any Peterbuilt, and the engines don't even drive the wheels. They drive generators that power electric motors. You could generate that electricity with a gasoline engine, a gas turbine, a steam turbine, or hamster cages. Torque has nothing to do with it. Diesels are used for economic reasons (fuel efficiency and durability vs. price)
You were right about different engines having different characteristics that make them suitable for different jobs. Jack said the same thing.
But as to a measurement of power output, that is exactly what horsepower is.
Put it this way- If an engine makes 2000 ft/lbs of torque but can only run 100 RPM, you aren't going to get a lot of work done. And if an engine can turn 20,000 rpm but only makes 2ft/lbs of torque, you aren't going to get a lot of work done. You have to consider both, and then adjust gear ratios (or prop size and pitch) to match the kind of work that you are trying to accomplish.
By the way, I realize this thread is ancient. I haven't been on the forum much lately and this subject caught my eye.
Still dont agree with you.
That Thats the reason they put the engines they use in the trucks ,In theory your right, in the real world your wrong, put the gears to the dodge a couple things are going to happen,one you lose HP because of the drag of the parts, so you cant transmit it to the tires, nu 2 the dodge will blow in no time from running at close to 5,000 rpm .
So in the real world saying horse power is same just want fly.
Another instance is a 5 horse electric motor and a five horse gasoline,rated the same ,no comparision on what they produce or life span.