Propeller Selection on new toy

[QC]

Re: Propeller Selection on new toy

calkearn,<br /><br />My point is that horsepower is fuel. And if you intend to run at 4800 RPM anyway it will require the same bhp if you stay with the same prop. Just because a modified engine would theoretically make more power and would potentially pull 5000 RPM does not mean that it will get better fuel efficiency at 4800 than it does in current configuration at 4800. In fact, despite all of our feelings to the contrary, WOT is the most fuel efficient point for a throttled engine (I didn't say boat). Don't spend anytime trying to make this right in your head as I am NOT saying that a boat will be most fuel efficient at WOT RPM, I said most fuel efficient at WOT i.e. low RPM at WOT. That is why auto manufacturers continue to run faster rear end ratios. They do this to get the RPM down and the throttle further open.

 

[walleyehed]

Re: Propeller Selection on new toy

They do this to get the RPM down and the throttle further open.

I know you do alot with testing of these types/areas of piston engines, but we're taught (and still pushed by all manufacturers of aircraft engines) to avoid low RPM, high manifold pressure situations due to the extreme cylinder head temps it creates.<br />We have a Pontiac car that has a fuel monitor in it, and it consistantly gets better gas milage at 70MPH @2300RPM, than it does @1900RPM (OD) at the same speed.<br />In boats, outboards in particular, we want the throttle plates open only enough to achieve a certain RPM..If we are cruising at 4500 and have a max WOT of 6000 and it will hit that, the throttle plates are closed down quite a bit at 4500, whereas if we prop for a max of 5000, at 4500 the plates are basically wide open...more fuel for the same RPM. The car is set up to maintain a certain "speed", the boat is set up to maintain a specific "RPM", the speed is a side result of the work being done.<br />In an aircraft, we pull back the throttle (manifold pressure) closing the plates, then we pull the prop back to drop the RPM, but never below square (normaly asperated)to achieve greater speed. fuel mixture is different on the aircraft because we have control of that, but the higher the manifold pressure the less you can lean down, so, I guess I don't understand why they would want to get the plates open MORE at a given RPM, or Speed..???????

 

[QC]

Re: Propeller Selection on new toy

Kenny, <br /><br />It is very hard to get your head around this. I have been working on it for about 10 years. I really don't know the answer concerning aircraft engines and I am a two-cycle idiot. A few things to ponder:<br /><br />1) Diesel engines get most of their fuel efficiency benefit from the absence of a throttle. Compression ratio helps a little. Most people get hung up on the fuel having more energy in it which is true. But when engineers compare efficiency they speak in energy units i.e. BTU's not volumetric units like liters and gallons, so the fuels are equalized.<br />2) Think of a throttle as an intake restriction. We all absolutely know that a clogged air filter causes poor efficiency.<br />3) ALL of the major engine manufacturers have been trying to come up with a gasoline engine that is throttless for efficiency reasons. This is the reason for the old POS Caddy 4, 6, 8 disaster. Get the throttle open at lower speeds. This type of strategy is coming back. I think Mercedes is doing it now.<br />4) CVTs (continuously variable transmissions) are becoming popular to allow engines to operate closer to WOT at more operating points. That is to enhance efficiency.<br />5) A throttled engine's worst efficiency is at idle where the throttle is the most closed.<br /><br />Boats throw a whole bunch of new variables into this equation and the first problem is that the only point (other than acceleration) where the engine is at WOT is high speed (inefficient) and highest RPM (inefficient). If you could take a little conservatively rated 4 banger that makes say 100 bhp at WOT at some reasonable RPM like 3000 and install it in a boat that originally had a 5.7 rated at 300 bhp. Run the boat at lets say 25 mph, and lets assume at 25 MPH the required bhp was 100 bhp. The four banger at WOT would get WAY better efficiency than the original 5.7 at a very closed throttle position. Same would be true if you propped the 5.7 so it would pull only around 1100 RPM and make that 100 bhp at WOT. Yes, it would probably have a major failure down the trail AND it would never get on plane, but if you could drop it from a helicopter running at that speed it would get dramatically better efficiency than that same 5.7 propped for 4800 RPM running at 25 MPH at say 2400 RPM. Weird stuff.<br /><br />Oh, and BTW, don't trust me I am a salesman and just make all of this crap up. Honestly though, it is all well known in automotive circles.

 

[walleyehed]

Re: Propeller Selection on new toy

Oh, and BTW, don't trust me I am a salesman and just make all of this crap up

No, I DO trust you...I just have to make sense of it all...try and understand the parts "I don't".

 

[QC]

Re: Propeller Selection on new toy

Originally posted by walleyehed:<br />I just have to make sense of it all...try and understand the parts "I don't".

Yup, me too. Like I said, I have been trying for 10 years, and I am just about there. Boats are where it goes screwy again

 

[calkearn]

Re: Propeller Selection on new toy

Good Point and all true, I know that more hp needs more fuel, like you said a 5 hp wot will burn less than 500 hp. It takes fuel to make hp. But more power will pull better or easier so longer <br />life is the result. Here is a new twist on my boat, last test was on the lake, went to the river this weekend, could not get the motor over 4500 rpm and top speed was about 48 mph. Driving me nuts, why so slow on river, when on lake 58 at 4800. same load in boat. i didnt find that to happen with the stock prop, rpm and speed same on lake and river. down stream up Stream didnt matter.

 

[danie]

Re: Propeller Selection on new toy

And dont forget the other variables,<br />fresh water<br />salt water<br />day temperature<br />air pressure.