Acceleration and efficiency
 

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?

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

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

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.

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.

That's about identical to my V10s numbers with the exception she can dip into the 6s with the BT on her back:angry-smiley-038:

4:30 gears

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.

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?

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.

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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I'm jealous. My 2013 Z71 doesn't do any better even with its puny 5.3 because that hamster is spinning its heart out.

that is decent mileage considering the weight and the hills in missouri.i pulled a 43 ft enclosed trailor with the drag boat&support equipment to arazona with a totally stock 97 1 ton chevy dually[454] and averaged 5.9 mpg round trip.reg gas was around 2.25 per gallon that year.

Flat fronted steel trailer ? Been there !

aluminum,even the floor and yes flat nose with a box covering the generator that was mounted up high in the front.

When looking for a purpose tow vehicle for our 38' Powerquest, I was torn between a late 90's 454 Chevy ot late 90's 7.3 Powerstroke. Very happy with the 7.3 as we get 10-12 mpg towing to and from LOTO and Omaha.

Yep, I made the mistake of buying a brand new F350 Lariat with a 5.4 Gas engine. Felt fine around town. Hooked the boat to it, towed to loto, omg, what a dog. Dam thing spent most of the trip revving its azz off and got horrid mpg. Quickly traded that in for the 6.0L diesel. Power was way better, and mpg was better, then the engine problems began shortly after.

So my first question is....what is the prop slip on both props ?? Without knowing what the slip numbers may be, you can do all of the calculations you want. My switch from 4 blades to 5 blades made a huge acceleration change, but cost me 2 mph up top. Obviously slip went down.

With what you have provided us, my pick would be the higher torque motor, but it will have less top end.

More pitch, better fuel economy. Boat runs 50 At 3000. 60 at 3500.00 with the cruising locking props. With the go fast props the speed at the same RPMs considerably less and burns more fuel to get to the same speed as the locking props.

More Rpm more fuel.

Also a heavy load consumes more fuel. Boats are on heavy load only planing and WOT imo


My truck 3/4 to 6.0 gets 13-15 until I put the boat on then its. 4 and sometimes 8 .

AS far as the RPM argument on a diesel . My 22,000 pound motor home gets about 7 in the mountains and hills on the way to havasu. When you add about 10,000 pounds of boat behind it it gets just about 7.. RPMs equal fuel consumption

High Torque , Low Rpm Combo any Day for a Boat. Now on the other hand if you had the ability to shift , Low Torque, High Rpm would accelerate you faster if you can keep the revs up.

If I ever get around to putting my engines in I should be able to test this pretty well and see what difference it makes. My old torque peak was at 4600 so when accelerating from a 4600 rpm cruise to wide open torque was decreasing. My new torque peak is 5200 so torque will still be increasing as I accelerate from 4600 to as far as 5200. When torque does start falling off the increase in hp on these engines is kicking in. That's how it works in my head anyway. I'm hoping to get them installed in the next week or so.

This is the curve from the old engines. Note torque is read from right side, hp from left. Second pic is the final combo going back in. I ended up with the 1 in. spacer. 2 in. made a little more power but peakier/ not as broad.

What would all that TQ be good for in a low RPM motor? acceleration? I don`t use my boat for tubing or drag racing... I want rpm since there`s no gears and the faster the prop spins the faster the boat goes.

My 6.7 diesel gets 11.3 mpg at WOT pulling a 43' steel trailer with an almost 35' Fountain on it. Not sure of top speed cause I got cheated on the speedometer only goes up to 100. Truck really doesn't know the boat is behind it unless your running under 40 and lay into it because it starts wheel hopping real bad.

Wait what? You measure mpg at WOT? 11.3mpg @ WOT ?? I get like 4mpg pulling away at the light.:confused: so thats pretty impossible.
and who the hell goes over 100mph with a trailer in tow?? You trying to kill yourself or just everyone else on the road??
Jesus man., maybe lay off crack

Sometimes you have to let it stretch its legs a little bit.

I was thinking of putting a supercharger on my 5.3 to help towing. Anyone here have experience with that?

Its ok He has his boat strapped to the trailer

Ha- I was driving home to Chicago 6 years ago from Corpis Christi TX, and some guy flew buy me around Dallas with a Ford F-350 towing a 35-38' Fountain. I thought I would catch up to it and check it out. I was doing 100+ and could barely catch the guy. Maybe it was Black Baja!

Lots of people do that (with their GPS on) so people think they have a 100 mph boat :kiss::kiss:

Just make sure it's not a Livorsi... They don't work and you can't show everyone the tattle tale :-(