Strange performance improvement

[Moody Blue]

40HP 4cyl mariner on a 14ft composite f/g fishing boat. This boat launches out of the hole (2 seconds)and typically ran 30-33 MPH all summer with 2 adults at WOT. Went out yesterday for a final day of boating/fishing before the snow comes and had something interesting happen. Was able to hit 44 MPH at WOT with the same 2 adults on board. The ONLY differences were 1) added fuel stabil to the gas 2)water temp was 43F not 70F 3)air temp was 48F not 70F. At first I thought maybe the speedo was wrong but it really felt that much faster. What is going on here ?? I don't have a GPS but was relying on the same spedo I have been using all summer. I don't mind the extra speed but now I'm not sure if it was real or imagined. Thoughts anyone.

 

[wilde1j]

Re: Strange performance improvement

Motor will produce more power when air is colder (more dense and more O2), but not that much.

 

[jimalsk]

Re: Strange performance improvement

Does stabil boost the octane in fuel? You know regular to high test?

 

[MrBigStuff]

Re: Strange performance improvement

Do you also know the engine rpm reading for each instance?<br /><br />If so, you might be able to estimate the ratio of engine to prop performance that contibuted to the overall gain.<br /><br />I agree that the engine will produce more power in denser air and optimal humidity conditions. The water will also be denser and this affects prop efficiency as well.<br /><br />Here's a decent page describing prop performance. It talks to water density also playing a role. http://www.psubs.org/designguide/prop.html <br /><br />I think the prop performance had a big impact as well as engine performance. Given your great results, perhaps your prop is not optimized for the normal load in warmer waters. Maybe you could realize at least some of the gains during the summer by changing prop diameter??

 

[Moody Blue]

Re: Strange performance improvement

Thanks for the feedback. I suspected just the same but was not certain. Could such temperature differences really account for such an improvement in performance ? Is it possible that the fuel stabil boosted the octane ? Is it possible that a prop could be affected to such a degree by water temp ? I would really like to know for sure what happened. Any further thoughts or ideas ?

 

[rodbolt]

Re: Strange performance improvement

hello<br /> internal combustion engines love cool air. the power can vary as much as 15% or so due to air temp and humidity. stabil wont boost octane. all things being equal in your 40 you will find a wot power loss with high octane fuel. we can control the intake charge, the time we light the fire, the amount of static combustion pressure, but the fire has to go out all by itself. so a low octane faster burning fuel actually makes more power. remember there is only 1 1/2 inch or so of travel before the ports are exposed. one of the big problems we had with yamahas in the 89-95 range v6's was high octane fuel would knock off 500 RPM of the top end.<br /> so keep it using stabil and the grade of fuel the owners manual suggests and go play

 

[ddaigle]

Re: Strange performance improvement

Your engine may be jetted too rich. I had a similar occurence with a 4 stroke lawnmower. Ran old premix in it to get rid of it and it ran better than it ever had with noticeably more power. It seems the oil in the gas leaned out the fuel mixture of a too rich running motor. The stabil coupled with all of the above cold weather density issues could explain it.

 

[jim dozier]

Re: Strange performance improvement

I dunno 10 mph is a helluva speed increase. On that kind of boat it would be so obvious and so dramatic as to be spectacular. Maybe the engine was running hot before. Cooling the air temp of the intake charge is good for ~1% horsepower increase for every 10 degrees F decrease in temp. Drop it 40 deg F and thats about ~4% of 40hp or about 1.6 hp. at best which would be lucky to even make a difference much less 10 mph. If the engine was overheating before and now due to the cooler water and air it isn't that might make a big difference.<br /><br />Personally I suspect the speedometer and the beer cooler are in cahoots .

 

[ebubeck]

Re: Strange performance improvement

Could have nothing to do with the temperature. Might have just unclogged/freed something - carb, fuel line, throttle cable, etc. In a normal running outboard that type of improvement will not happen at that temp change - in fact you probably won't even notice the difference. Just my 2 cents.

 

[Moody Blue]

Re: Strange performance improvement

Thanks again for the input. I guess getting an extra 10MPH for nothing is asking for too much. I'll have to wait till next spring now before getting back on the water and experimenting. Sure seemed fast though. I might just keep adding the stabil though.

 

[rodbolt]

Re: Strange performance improvement

beer coolers is almost as good as fuel coolers <br /> but we did find that running fuel through a dry ice cooler made a big differnce in elapsed times. could not use ice as no liquid ballast was allowed.

 

[tibb]

Re: Strange performance improvement

You can take it from a Chemistry teacher and Pilot Laspeckking, it is without a shadow of a doubt the cooler temperatures and all of you are correct. It is both the temperature of the water (x2) and the temperature of the air (x2).<br />The water density makes a huge difference in prop performance just as it does when an airplane prop cuts through cold air. The performance can increase by even more than 5-10%. Also the water temp cools the engine which results in a cooler combustion. Combustion reaches a plateau at a certain optimal temperature. Too hot and the fuel burn is inefficient. <br />The cold air and resulting density increases the number of air molecules per square inch, but does NOT increase the amount of O2 in the air (sorry WildeOne). O2 concentration never really increases in our atmosphere. It always stays relatively the same. The availability of those molecules are greater however because they are compressed closer to the ground. Cold air sinks and the engine is more likely to use up those O2 molecules.

 

[wilde1j]

Re: Strange performance improvement

tibb, YOU are confused. Denser air DOES have more O2 per unit volume trhan less dense air! Otherwise, denser air would make zero performance difference. What do you think is in an air molecule, anyway? The percentage of air's components (N, O, CO2, etc.) doesn't change with density altitude changes, BUT the absolute quantity of those components per unit volume does. If this wasn't so, there would be zero change in performance with air density. That's why unboosted, carburated engines lose almost 3% per 1000 ft of altitude increase from some base, such as MSL. Sorry, tibb.

 

[ebubeck]

Re: Strange performance improvement

Like, I said just my 2 cents, but similar to many of you on this board I've been running outboards in water temps from 40's to 90's and air temps lower, higher than that my whole life and have never experienced more than very minor differences. You are correct in all of your points, however, I think it unlikely to boost performance this significantly. I think RodC should be very skeptical in soley contributing the performance to the temperature. Could be, just not buying it right now. RodC - let us know what you find out.

 

[Moody Blue]

Re: Strange performance improvement

Thanks to all for their 2c. I agree that colder temps could not result in such a dramatic performance increase. We would be talking almost a 40% increase in speed. What additional HP would be required to do that? Perhaps the stabil in the gas "unstuck" something in the motor. What is that stuff anyways ? I'm really beginning to wonder if this was real or imagined. I know the speedo said 42, question is why. If I uncover a "secret" I'll have to be sure to patent it. Next spring I'm going to get someone with a GPS and verify the speedo at different speeds to check for accuracy.

 

[steam_mill]

Re: Strange performance improvement

Rod:<br /><br />Where did you clock 42 MPH. I'm in Toronto too...<br /><br />Just might want to try your bit of water.<br /><br />All kidding aside, with a 40 hp motor utilizing a regular prop, I suspect if you calculate your max speed with no slippage, it may not be 42 MPH.<br /><br />What's your pitch?<br /><br />Joe

 

[Moody Blue]

Re: Strange performance improvement

We have fast water here in York Region. Running on lake simcoe - on a calm day that is. Not sure of the prop. It came with the package. The boat is less than 14ft x 64" wide and only weighs about 600Ibs including the motor. It has pretty much a flat bottom with prominent chines ?? at the stern portion and a V shaped bow. Boat handles extremely well in all waters Ive tested it on. I can trim in and crank the wheel at WOT and it just bites in and does a tight 360. As for the prop it is an aluminum 3 blade cupped style 10 1/2" dia. No numbers of any sort marked on the prop. Looks a bit small for the motor as there is about 1 1/2" clearance to the cavitation plate. Excellent performance out of the hole to planing in about 3 seconds.