The only toy that gets better as it gets less efficient...

[DanielC]

"driving a heavier car in to a head wind."

More like driving a truck, with a camper, and a trailer, up a 6% grade.

My Ford Aerostar, witha 4.0 liter V-6 can gat about 20 MPG, on the freeway. In a hour, I can go about 60 Miles. That is three gallons per hour.
My Ski Nautique uses a little over 3 GPH. I am going to guess the load on the engine in my boat is pretty close to the load on the engine in my Aerostar.

[1955 classic]

there is a marine 2 speed auto trans available for high power boats, just cant remember who makes them. basically starts in a first gear type ratio till about 10-15mph then slips into second effortlessly. Im guessing its not a cheap unit though

[tdc_worm]

there is a marine 2 speed auto trans available for high power boats, just cant remember who makes them. basically starts in a first gear type ratio till about 10-15mph then slips into second effortlessly. Im guessing its not a cheap unit though

MB used to run a 2 spd transmission mated to their PCM engines....

[nyryan2001]

Tdc- you are killing me.

the exact same forces are put on the engine and drive train at 4000 Rpm whether at 11mph, 23mph or 37mph. There is no thing such as an easy 4000rpms. Next thing you'll say is 4000rpms at 23mph with 3k ballast uses less gas than 4000rpms at 12 with 5k ballast?

Yes at 11mph it's mainly hull friction, at 35mph it becomes wind drag . Thrust will be the same at a given rpm, then the variables are weight, prop slip and prop shaft angle.

Gph's are solely a function of RPMs. Load on the engine is defined by RPMs. When the "load" gets heavy, you induce prop slip.




Now, as to your question on the 550s and their OEM 15x14 props? They are not ideal because the 17x17s are too big.... And 16x15ish to 17x15ish are not yet available by Acme or Oj.

[DanielC]

"Next thing you'll say is 4000rpms at 23mph with 3k ballast uses less gas than 4000rpms at 12 with 5k ballast?"
I could almost promise you that 23 MPH is a cleaner plane. The boat has moved on top of the water more, and takes less throttle to maintain 4,000 RPM, than at 12 MPH, where the hull is still at displacement speed, and making a big wake. That wake comes from the hole your boat is constantly trying to climb out of. The energy for those wakes that go out to the sides of your boat, and the rollers you leave behind your boat at 12 MPH has to come from somewhere.

Another issue is at 12 MPH the propeller is slipping more. As the boat speeds up, the propeller slip becomes less. If you do not believe me, go and make a chart of speed VS. RPM on your boat. The resulting curve is not linear.

I would even think 4,000 RPM, 23 MPH, and 5K ballast will still use less fuel than than running at 4,000 at 12 MPH, and 5K ballast. You will need a different prop to match the RPM.

Yes at 11mph it's mainly hull friction, at 35mph it becomes wind drag.
No. It is water drag all the way up to top speed. Other wise you would need an engine as big in your tow vehicle to move the boat at 35 MPH, as is in your boat, and probably some supercharged engine burning nitromethane, to do freeway speed towing a G-boat.

"Gph's are solely a function of RPMs."
No, again. Throttle opening or absolute manifold pressure has as much, or a greater effect on the GPH use of any engine.

[aZ`]

The only toy that gets better as it gets less efficient...

^^ this is correct. I tried but it didn't sink in.

Let's also add the prop angle adding to the inefficiency which you (nyryan) have conceded in another thread.

[beach]

If you look up most graphs of an engine's fuel consumption, you'll see it rated as Brake Specific Fuel Consumption(BSFC) vs rpm. BFSC is the amount of fuel it takes to produce a certain horsepower (usually given per hour). It doesn't always increase linearly with rpm. If a wakeboat is planned out when wakeboarding, it might be producing less horsepower (due to the reduction in wetted surface when the boat is on plane) than a surf boat that is plowing through the water. It would depend on the amount of decrease in wetted area. This would have to be significant since the drag increases as a sqaure of the velocity. Depending on the graph of BSFC vs rpm and on the difference in horsepower required at 23 mph vs 10 mph, a wakeboat may burn less gas than a surf boat. There really isn't any way to know this unless the engines have an obd ii port.

It would be hard to figure this out otherwise. I'm not really sure if there is more prop slip at surf speeds or at wakeboat speeds.

Also cars are a whole different story, since they have multi speed transmissions and torque converters. An automatic car will always be at a higher rpm when under a higher load, so it will be burning more fuel.

[nyryan2001]

Of course there is more prop slip at surf speeds, mainly due to the increased weights used and the prop shaft angle in more inefficient.

last bit here and I will stop.

A wakeboat running at 4000rpms delivers XXX amount of thrust, assuming it's not cavitating.

At 4000rpm, it will deliver that same XXX thrust whether at 11 with 5k ballast or at 35mph with none. The same forces are are applied to the shaft at either speed, assuming the RPMs are a constant 4000.... And no cavitating.


The 4000rpm boat hits the "wall" at 11mph from the insane amount of ballast, bad prop shaft angle and water friction.

The 4000rpm boat hits the "wall" at 35mph unloaded due to lesser water friction, aero drag, and drag from the running gear in the water.

Either speed, the same XXX thrust is applied and pushes the boat till XXX thrust can't push it faster. Either way the 4000 rpm engine is working the same. At 11 at 4000rpm, where you guys think it works harder... Prop slip takes over, same XXX amount of thrust you see at 35. There is no uphill on a wakeboat perse...like the truck uphill analogy ... As prop slip takes over, whether at 11 or 35. Yes, def a ton more at 11. But again , same XXX thrust.

Same net resultant forces on the drive train, just differnt speeds due to the weights , different drags and shaft angle. Yes there are some efficiencies gained at higher speeds...

But I will stick to my claim that GPHs are 95% of RPMs.... Regardless of activity.

[NautiqueJeff]

An analogy that comes to mind for me is this:

Let's say you're driving a heavy truck down the highway at a speed of 65 MPH. In order to maintain that speed on flat land, the throttle must be open, say, 35%. As you come to a hill, if you don't push on the accelerator more, the RPM's will drop and the truck will slow down. In order to maintain the same RPM's and the same speed, more throttle is needed, obviously using more fuel to achieve the same RPM's and speed.

Doesn't the same theory apply to boats? The heavier the boat or the more it is plowing through water, the more throttle is needed to achieve the same RPM level, correct? This discounts prop slip, but I don't think that totally invalidates my comparison.

In fact, now that I type this, I remember a car I had in college. It had an analog fuel efficiency gauge that displayed real-time MPG info. With the cruise set, you could watch it change (sometimes pretty drastically) as the grade of the road changed, even as RPM's stayed relatively constant.

Another comparison is this:

Put your boat in neutral so that the engine can be revved without the boat in gear. Give it throttle to reach a pre-determined RPM point, and note how far the throttle had to open to get there. It won't be very far.

Now fill the ballast and put your boat in gear. Accelerate to the same RPM and note the throttle position now. I bet it is open significantly more.

[tdc_worm]

Tdc- you are killing me.

At any point that it takes more more fuel and air to maintain a certain RPM due to any external factor (weight, friction, resistance, grade, etc) then you are stressing the engine more. the displacement of the cylinder is fixed. the combustion is variable. increase the combustion enough and the increase in force generated stresses things like head bolts and piston angles and the resultant heat generated will eventually outrun the cooling system. the drive train feed back/resistance from the prop fights the combustion of the cylinders. connecting rods rotate on crankshafts and dont have linear force on pistons....they only time they are aligned is TDC and BDC. any other time of the stroke this is causing side loading of the pistons in cylinder. even at TDC a larger explosion is going to cause increases pressure on the connecting rod. the more resistance felt ultimately drops the RPMs, thus requiring more fuel and a larger explosion to maintain the same rpm and also causes increased side loading of the piston in cylinder walls and lincreased loading on the connecting rods and crank. this is what causes cylinders to wear in an oval fashion...the greater the load, the quicker, more severe the wear.

in a vacuum, your argument has merit. engines dont operate in a vacuum.

[DanielC]

I took an engines class at a local community college a few years ago.
One thing that was mentioned. At what point is the engine, specifically the crankshaft, connecting rod and piston under the highest stress?

It occurs at the end of the exhaust stroke, as the crankshaft, and connecting rod are slowing the piston down, as it approaches TDC, after the piston has reached maximun linear velocity, about 75 degrees before TDC.

And the stress is highest, internal parts come to closest to hitting each other when the throttle is closed.

[aZ`]

All correct Daniel. And is the reason bearing and failure is almost always due to rpm not hp.

However my initial statement I should clarify was (although not specified) more about piston ring and bore wear, this is because this is what is usually noticed first due to a lack of power, poor starting, oil useage etc etc.
I agree with nyryan that rpm is a cause of wear, but affects different components to high load, and would be the most common culprit for catastrophic failure.
But my point still remains that low rpm and high load causes wear, potentially more than rpm, of course depending on how much rpm and how much load we are comparing.

[nyryan2001]

So we need someone to conduct an experiment.

run 60 mins at 4000rpms 5k ballast.

run 60 mins 4000RPMs no ballast.

Case of beer says they suck the same amount of fuel.

[beach]

Actually, don't nautiques have a run time remaining readout based on the amount of gas being used? I thought that was one of the readouts on my 03. We could just see if the boat gives the same time remaining for ballasted vs un ballasted. It wouldn't be exact, but it should be close enough to show there is a difference in fuel usage based on load. Or read this graph. Link:

http://ecomodder.com/wiki/index.php/...FC_cleaned.png

That is a graph of fuel consumption per hp produced. 25% load is producing 25% of the horsepower of 100% load. At no point is the fuel consumed at 25 or 50% load equal to the fuel consumed at 100% load. You will notice that the higher loads produce more HP per lb of fuel, making the engines more efficient in terms of fuel per power produced, to run at a higher load. But the overall fuel consumption(lb, not lb per hp) is still higher at higher loads.

[Whitetail15]

here is a perfect example that can clear this up..
i run a generator everyday at work that is gasoline powered. it is fixed speed , that maintains 3600 rpm all the time no matter what the load.
if you just let it run without drawing any amps it will run for 8 hours on a tank of fuel.
if you hook up an electric motor that draws 4000 watts, it runs for half of that on a tank of fuel.

at the same rpm all the time.

at no load 3600 rpm the throttle plate is closed and requires almost no fuel and air
at max load 3600 rpm the throttle plate is wide open and requires max fuel and air to maintain rpm