MILD THUNDER

iTrader: (3)

Two engine combos, identical boat.

First engine combo makes 700HP at 6000, and 625FT lbs at 4000RPM.

Second combo, makes 650HP at 5500, and 675FT lbs at 4000RPM.

Boat with first engine combo, turns a pair of 28 pitch props, to 6000RPM. Second combo, turns a set of 31 pitch props, to 5500RPM.

Which boat will accelerate better ? The one with 100 less foot lbs of midrange torque, and 28 pitch props, or , the one with 100 more ft lbs of torque, turning 31 pitch props?

Which boat will cruise more efficiently at 3500RPM? The 31 pitch propped setup, seeing a higher engine load/map scenerio, or the 28P setup, running a lighter engine load/map scenerio?

IF RPM dictates fuel economy, why does my gas dually get 8mpg towing at 65mph, and 13 mpg not towing? RPM at 65mph doesnt change . The load on the engine certainly does though.

Has anyone done any dyno testing of a marine engine, at 3500RPM, with a specific MAP reading, to compare fuel consumption, and power output, at partial throttle openings?

Baja Rooster

iTrader: (1)

I'm calling BS on 13mpg in a gas dually, lol.

SB

iTrader: (1)

Said very basically:

RPM dictates frictional loss amongst other things (it takes more power to spin things faster)
Load dictates how much power you have to use to maintain that speed

Both effect fuel economy

MILD THUNDER

iTrader: (3)

Hand calculated. 13.2 MPG from Kentucky to Chicago 65mph. Towing 38 Fountain to loto last weekend from Chicago, first tank, 8.1MPG, second tank, 7.8MPG. 454 Vortec with a custom tune, 4.10 rear.

SB

iTrader: (1)

MT - without really sitting down and crunching numbers + thinking alot about those ecamples, I would say it really depends on the type of boat and what you want out of it performance wise,,,ie: sacrifice some acceleration vs top speed.

A 28ft twin engine cat will surely give up some 4500rpm power for more peak power even if this falls at 6000rpm or a liitle higher

With same engines A heavy flat bottom deep v may not be able to accelerate to that same peak power rpm.

endeavor1

iTrader: (3)

That's about identical to my V10s numbers with the exception she can dip into the 6s with the BT on her back

4:30 gears

getrdunn

iTrader: (7)

Well this is where full forces issues comes to play to some degree. From what I understood the engine with "more of a load" that was down on rpm needed less 2 to 4 less jet sizes to optimize AFR's and achieve same rpm's as sister engine. Does that mean working an engine harder is more efficient? In a nutshell it does pertain to thread.

MILD THUNDER

iTrader: (3)

Right on. i'd consider my boat to be, well, on the heavier side of things. Its no 41 apache, but Non stepped, 10-11k lbs vee, power robbing SSM drives...I am handicapped, as I have little engines. Making 900ft lbs stump pullers out of 454's, isn't happening. At least on pump gas. So, they are kinda built more along the lines of a small block, than a typical big ci bbc marine build. Meaning, I need to rev those little fukers to make a decent HP number.

With that being said, I'm not sure what more torque , and less peak power would do for me. Just this week at loto, I dedicated an afternoon to simply beat on it, and see what she'll do. Getting on plane, pretty much requires me to feather the throttles, or I'll blow the props out. Cruising along at 3500RPM, I jammed the throttles to the dash, with the drives a hair about level to the bottom. Port side prop broke loose, port engine banged the limiter at 6500. Tucked drives down a bit, repeated, boat accelerated from 55mph to 80mph very quickly.

Then, looped around, and slowly worked my way up in rpm, as I was monitoring afr's, IATs, and intercooler psi's at various rpm. At 5000rpm, I raised the drives a little, and pushed the throttles to the mat. What happened was, the boat kept accelerating, rpm's kept climbing, pulling and pulling, hitting 90mph, and then backed off as I felt the afr's were too lean for my liking.

My last engine combo, had a full point more static, smaller heads, and did make a bit more midrange torque. The new combo, the upper rpm power, keeps climbing better than the old combo. In identical conditions during my last visit to loto, on a much cooler day (this week it was 101 heat index), the best I could wring out of it on calm flat water, was 86mph.

If one was to look at my dyno sheets of the combos, the old setup would look like the winner, because it made more low end torque, and peaked at 6000, where the new combo kept climbing above 6000. However, on the water, the new combo, simply kicks the old combo's ass.

But, like you said, I think its all what someone wants. I personally, don't drag race my boat. I can care less if it took me 8 seconds to get from 55 mph to 80mph, or 10 seconds to get it done. I do however, appreciate a 4+mph gain in top speed, with less compression, less timing, same amount of boost, a few hundred pounds of weight added on the stern, and another water pickup mounted on the transom.

I enjoy watching some of the cockpit footage on some of the raceboats. Watching the 750 supercat engines rev. Those things were certainly not stump pullers, big duration cams, revving to 7k or higher with around 500 inches. But dam, those things have no problem zipping from 4000 to 7000rpm on the course, coming out of corners and what not.

Pulling my boat up a grade coming back from loto, had me thinking. My trucks engine makes its peak torque below 3000rpm. However, If I held 3rd gear, and climbed that grade near peak torque, my mph simply kept decreasing. Downshifting to 2nd gear, the rpms climbed, and the mph maintained, actually started increasing. While maybe the power output at the flywheel was less, the power output thru gear multiplication, simply won. In essence, a propeller is like a gear?

Baja Rooster

iTrader: (1)

Using a block and tackle method, every pulley that you reverse a loop of rope around effectively cuts the work needed to move an object in half, but it also cuts the speed of rope travel in half as well. To lift a 1000# over one pulley you'd need a 1000# of force, but reverse it over two pulleys then you'd only need 500# of force. Add another two pulleys and you'd need only 250# of force to move the same 1000#. Your gear case acts like a set of pulleys, so by downshifting you added more pulley ratios needing less power at the expense of speed, but you increase the speed of the engine to compensate for distance covered. The prop is more like the final gearing in your rear axle.

SB

iTrader: (1)

Yes, but It's more like a tire
They provide the 'traction'
And as you are saying they both effect Torque multiplication to the ground/water by Tire diameter and prop pitch

So, yes, they effect final gearing...but they do a lot of other things that effect performance too.

There are more properties in prop design vs tires that makes performance much harder to equate on paper than actual testing










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