qc said:
While I agree that higher peak torque will help plane a heavy boat, the reason it does is because it determines the horsepower at that RPM. Horsepower is ALWAYS what you care about. It takes torque, adds RPM, and voila you have horsepower. No way to calculate speed without it.
I can't say I agree with that.... If one wants to calculate speed all you need to know is rpm of prop, pitch, and slip. It doesn't matter if it's 140hp engine in a 17 foot boat or a 260 horsepower engine in a 25 foot boat. If I assume that either engine is able to spin up to full rpm/power - whatever that rpm/power factor is, and make full power, and if I know the gear ratio, with those two bits I can figure out prop rpm. And from there all I need to know is pitch, and what kind of theoretical slip I am going to have. From there you can get your inches per revolution, and convert that to miles per hour.
Anybody can read up on the history of horsepower and the old comparison between a steam engine and an actual horse. But in todays world we still use horsepower instead of torque for 2 reasons. 1. tradition. 2. It's simply a sales technique. Average people who are not engine savvy are groomed from the time they start looking at their first go kart that "mo horsepower must be mo betta" when that is definitely not the case. But the majority of all adults, young and old, still believe that today.
I won't get into math equations or anything like that. I will make just very 1 simple point with the word "disappointment"
Over the last 7 or 8 years how many people have bought mid size boats in the 20's with 5.0 MPI engines rated at 260/270 hp paired up with either B3's or DP's, they go out and run the boat with 1 person on it and it's not that bad. But the instant you put a lot of people on the boat, fight a current or wind, or try and tow a tube with it. The engine completely dogs, and does not have the power to do what the owner wants it to do. And more importantly, it doesn't have the power that the salesman promised the owner. Those owners all start by looking at other older boats, they ask around the lake/marine before calling in the mechanic. And what they all learn on their own is they learn their buddys boat that has a 5.7 carb with 260 hp and usually a single prop drive out performs their new "state of the art" boat with the same amount of horsepower. At first they think something is wrong with their engine, that it must be broken or have something wrong with it. Only for somebody who is not the salesman to have to give them the whole torque vs horsepower explanation. And explain to them that a 5.0 liter simply does not have the torque of the 5.7. In the end, what you are left with, aside from confusion.... is disappointment. Unfortunately most people end up learning the hard way as to why torque is more important than horsepower.
I don't know... When it comes to marine engines, I think the OEMs need to print torque and horsepower curve charts on their sales literature. The problem with caveat emptor/buyer beware is somebody can not truly really be aware unless they have all of the information and facts in front of them to study, which is what they don't get when they buy an average pleasure boat. Imagine if you were buying a truck for towing that boat. And Ford, Chevy, Dodge, Toyo ect all came back and said yup the truck will tow X amount of pounds, and the engines all make Y amount of horsepower.... But we are not going to give you a torque number because we don't think you need to know that.... How could you buy the best truck to meet your needs if you didn't know that.... the simple answer is you can not.