Re: Boat Performance Secrets - How Speed, Acceleration and Efficiency Works!
Hi Mark,
I added the torque curve to Chart 1, the engine power in the original post to make it easier to discuss (and easier on QC so he doesn't have to calculate the torque values!
) Just make sure you use the numbers on the right side when you're looking at the torque curve! As QC said, horsepower is calculated from: HP = Torque times RPM / 5252. The 5252 comes from a string of constants required to get the units (feet, minutes, pounds, etc) to work out.
As QC says, you’re absolutely correct on your main point Mark, your second point needs a little more clarification. I’ll go through both of them
From TexasMark:
Hence the torque would be different and at the higher rpm's associated with the 19, at the low speed of 30, and assuming the torque curve peaks out around 3000, torque is what would provide his rate of acceleration....in short, how fast can I get my rpms up and to me torque is what does it.
As QC said, at 30 MPH with the 27 pitch, you're turning 2000 RPM and need 210 lb/ft out of the available 400 lb/ft to make the 80 HP required. With the 19, you're turning 3000 RPM and using 140 lb/ft to make the 80 HP required out of the 440 lb/ft the engine is capable of producing at 3000 RPM. You're using the throttle in both cases to restrict the airflow into the engine and limit the boat to the speed (30 MPH) that you want.
If you open the throttle all the way, the engine at 2000 RPM with the 27 pitch puts an extra 190 lb/ft (400 available minus 210 required) into the prop shaft, which gets used to accelerate the prop above the RPM required to maintain 30 MPH. The engine at 3000 RPM with the 19 pitch puts an extra 300 lb/ft (440 available minus 140 required) into the prop shaft. As Mark said in his quote above, more torque gets the RPMs up faster. So the 19 pitch will accelerate (gain RPM) approximately 50% faster than the 27 since we're putting about 50% more torque into the 19 pitch (the 50% comes from 300 lb/ft is about 50% more than 190 lb/ft). If these two boats were side by side the boat with the 19 pitch would theoretically accelerate about 50% faster than the boat with the 27 pitch.
This is why a 310 HP Mercruiser 454 will out accelerate a 300 HP Mercruiser 350. They both have almost exactly the same torque at 4500 RPM (have to since they have the same HP at that RPM (almost anyway - 310 vs 300)). But the bigger motor will have a much flatter torque curve (higher numbers) lower in the RPM range. It works the same way with comparable 150 HP O/B’s, they both make the same max power (and will therefore have the same max speed) but the bigger (more cubic inches) engine will have more torque and get there faster. This is obviously a general rule, since a smaller displacement modern engine could have more torque (thus power) throughout the RPM range than a bigger but older motor. I’m assuming we’re comparing engines of the same generation here!
From TexasMark:
I realize that if it were at the same rpm's the shaft torque would be equal on both props.
This ones a little trickier to look at, you have to back into the torque values. If the ENGINES or SHAFTS were turning the same speed then the 27 pitch boat would be going faster than the 19 pitch boat. This puts them at different points on the “POWER REQUIRED” curve.
Say both ENGINES were turning 3000 RPM (With a 1.5 reduction lower this means that both shafts would be turning 2000). From Chart #2, the 27 pitch is going 46 MPH and the 19 pitch is going 32 MPH. Now look these speeds up on the POWER REQUIRED curve. To go 46 MPH the 27 pitch (or any pitch) needs 150 HP. To go 32 MPH you need 80 HP. At 3000 RPM, it takes 150*5252/3000 = 262 lb/ft of torque to make 150 HP. So the 27 pitch shaft has 262 lb/ft of torque on it at 46 MPH.
With the 19 pitch shaft at 3000, you only need to make 80 HP. So that shaft has 80*5252/3000 = 140 lb/ft of torque on it.
So if the ENGINE (or SHAFT) RPMs were the same, the 27 pitch would have more torque on the shaft. You would be using more throttle, to allow the engine to use a bigger percentage of its available torque to make more power to maintain the higher speed!
Hope this makes sense. I know it can be kind of confusing when you can't ask questions to clarify stuff as you go through it.
