Re: Merc 305 Vs. 350 - Input Please!
So, SA, explain to me why a smaller capacity engine making huge power (like a fully revved up 350 making 400hp @ 6,000rpm) is crap in a boat where as lumbering big displacement diesel making 250 horsepower @ 3,000rpm is so much better in the same boat. If that's not a statement about torque vs horsepower, nothing is!
Really not sure why I'm called "SA", but if it's what I think it means it's uncalled for. If you have a problem with my posts, you can feel free to ignore them if you wish. I come to forums to exchange information, and by doing this I learn a LOT. You clearly know more about boats than I do, but this is simple physics.
You call a high-spinning 400 HP SBC "Crap" in a boat, but I would challenge this argument. Why do we have high-revving, lightweight, high-power outboards hanging off of the fastest bass boats? Nearly every one of these engines spins to 6K+, and they do a GREAT job performing their task. What about the jet boats of the 70s and 80s, with their 3-500 HP high-revving V8s? Would you call those "crap" as well?
I can think of a couple reasons why you'd want the diesel in a large planing hull though. First of all would be fuel economy. An engine making it's rated power at 3,000 RPMs will be FAR more efficient than one at 6,000 RPMs, assuming that they are operated near their rated power RPM fairly often. Furthermore diesel fuel has ~30% more energy per unit volume, further increasing fuel efficiency. A small-displacement V8 may not have the torque to get a large planing hull on plane, and therefore it may never be able to rev to it's HP peak. Might not But if it were, and were correctly propped, it would be moving MUCH faster at 6,000 RPMs than the diesel could ever attain.
People love to talk about torque, but they rarely recognize that because torque is an integral part of HP an engine that makes more torque at a given RPM is also making more
horsepower. Meaning that the 250 HP diesel is making more
horsepower at 3,000 RPMs than that high-revving gas v8 could. As RPMs climb, diesel torque comes down and the gasser is just coming into it's domain.
It's all about the right engine for the application. My Mustang makes 412 HP @ 6,500 RPMs. The engine is a nice, light, 450-ish lbs, and therefore it does a GREAT job powering my Mustang. My Caterpillar C15 powering the truck that I used to drive weighed 3,200 lbs, and made 435 HP. Obviously this would be "crap" in a Mustang because of weight, and my 5.0 would be "crap" in the truck because of longevity. That's right, I said longevity! With reduction gearing I can make my Mustang engine output 1,750 ft-lbs like the truck motor can, but to make enough horsepower to move the truck up a hill at highway speed it would be screaming at redline! Certainly wouldn't last the ~1,000,000 miles that the C15 would!
