Boat speed VS Temperature

[imported_JD__]

Hypothetical: If a boat would run lets say, 55 mph in cool Spring weather around 45-50 degrees, how much speed loss would occur if the temperature rose to 75-80 in late Spring/early Summer? My rig has lost a little of its pep and top end, around 2 mph. Everything is the same other than the weather/temperature. What do you think?

 

[cobra 3.0]

Re: Boat speed VS Temperature

My boat is 2 mph slower now than in the summer. I think the colder, harder water and a less warm engine slows it down. I suppose there is an ideal temp of not too cold and not too hot that gets the best speed.

 

[JB]

Re: Boat speed VS Temperature

Barometric pressure will make a lot more difference than ambient air temp.

 

[ZmOz]

Re: Boat speed VS Temperature

Yeah, cooler water should slow you down. It will keep the oil in your lower unit much more stiff.

 

[Hooty]

Re: Boat speed VS Temperature

Lets not forget relative humidity. Cool, damp air is like supercharging.<br /><br />c/6<br />Hooty

 

[kd6nem]

Re: Boat speed VS Temperature

Interesting thoughts. In the world of light aircraft we are taught religiously that warm air and humid air spoil lift AND engine power. Air density is very much affected by temperature and elevation as well as barometer. I'm still thinking on JB's comment about barometer being more significant than temp; I'm not sure whether I will end up agreeing on that point. Cool air is more dense and gives an engine more to work with. I'm thinking that water is affected to a much lesser degree compared to air so long as we stay above 32 degrees. Your hypoid oil, that is a different story. If your engine never gets up to full operating temp it may not be able to realize the full benefit. Aerodynamic drag increases with higher densities to somewhat offset the gain, to what degree depends on how "clean" the boat's design is and just how fast you're moving. Of course drag like that increases rapidly with speed, so the faster you go the more drag becomes a limiting factor which gets increasingly more difficult to overcome. Enough of that. Cool dry air = more horsepower.

 

[1986mariner150]

Re: Boat speed VS Temperature

Cold is faster. Not only from an air temprature standpoint but also water tempuature. Cavatation is actually caused by boiling water. Cold water is much less likely to boil. I pick up a couple of mph in the winter but don't use it as much as the wind chill that comes with in is unbearable.

 

[cobra 3.0]

Re: Boat speed VS Temperature

The key here is the water temperature. Up here in the cold white north, ice-out happened not too long ago. The water is freaken cold and I'm sure it slows everything down. I've been out three times so far(With not another boat in sight on the water!) It just depends how cold is "cold" to your area.<br /><br />Same thing with cars. Although it needs to be quite a bit colder for a car to be affected. When it's really cold, your car puts out less power, not more.

 

[MajBach]

Re: Boat speed VS Temperature

Cobra: What are you saying? That cold air robs power or that cold increases power but only up to a point?<br /><br />I don't think water temp makes a difference whatsoever. Changes in density from temp are minimal and whatever gain you have from an increase/decrease in density in one area, you lose in another, i.e. increase density say at 4° C gives your prop more bite but causes more drag on the boat, even though it sits higher. Again, all this is neglibible. If I were have to explain that water temp DOES makes a measurable difference I would GUESS that it's becasue it prevents engine from reaching oper. temp (something a thermostat should take care of though) but more likely becasue it keeps the gearcase oil much thicker.<br /><br />Anyway, for some effects on temp/humidity on your engine, JD,try these:<br /><br /> http://csgnetwork.com/relativehpcalc.html <br /> http://csgnetwork.com/automotiveconverters.html <br /> http://wahiduddin.net/calc/calc_hp_abs.htm

 

[imported_JD__]

Re: Boat speed VS Temperature

Thanks to all who responded. MajBach I appreciate the links. For what I can determine, ambient temperature definitely has an affect, along with pressure, dewpoint & altitude. I think it clearly shows me where my 2-3 mph went.<br />JD

 

[BF]

Re: Boat speed VS Temperature

interesting Q... I'm sure temp makes a difference, but how much is the issue. For engine HP issue, colder is better (providing engine can get to and hold normal operating temp). Snowmobiles will run strongest in the coldest temps, but you have to make sure you jet them correctly (lest you run 'em too lean with the dense air and blow 'em up). The reason cars may make less power in extremely cold weather is because they may not reach normal operating temp and the ECU runs the fuel injection in "rich" startup mode continually. (that's why the mileage may plummet with the temp)<br /><br />I have a difficult time believing that water density has much effect on speed. I think the density changes with temp aren't big... enough to allow 4 degree water to sink to the bottom (?). That's one thing what keeps our lakes from freezing solid if I remember correctly. But enough to increase the resistance of the prop/boat ? Doubt it.<br /><br />As for gear lube, after a good run in a cool fall lake, the gear lube is nice and warm (good for draining), so I'd guess that the thrashing it takes in the gearcase is enough to warm it up even in cold water (just a guess).<br /><br />Anyway, those are my thoughts... anyone ever toasted an OB by running it too lean in cold (near freezing) conditions? Like I said, happens on snowmobiles all the time, especially if you've optimized the jetting for warmer conditions and a cold snap hits.<br /><br />Brent

 

[Drowned Rat]

Re: Boat speed VS Temperature

I think this is splitting hairs in as far as the marine application is concerned. I think Bearcat is right on though. Cold air in denser which means more oxygen molecules per sq. inch of air. More O2 means more complete and more rapid combustion. Also like JB said, the closer you are to sea level, the better the performance for the same reason, the air is denser.

 

[cobra 3.0]

Re: Boat speed VS Temperature

Majbach, what I was trying to say is that there is an ideal temperature range for best performance. Neither too hot nor too cold.

 

[DJ]

Re: Boat speed VS Temperature

Denser air is a plus until you reach speeds in which aerodynamics come into play.<br /><br />Airplanes perform better (higher speeds) at higher altitudes (thinner air) as long as the engine can produce the HP needed (boosted).

 

[JasonJ]

Re: Boat speed VS Temperature

The last few weekends I have ran my boat in 32 degree air temps (in the morning), and water is in high 40s. Boats seems to perform like it did last summer in 90 degree weather. I am still hitting my near 6000 rpm redline, fuel consumtpion is normal. I don't think there is a huge difference in performance on cold vs warm water. Cooler air can make a difference, but I am guessing that the air temp in the cowl of a fully warmed up outboard is fairly close whether its 30 or 90 degrees outside. Now, if you could duct pure cold air directly into the intake from the outside, and rejet accordingly, maybe. Of course, you could maybe melt a piston or two also messing with jets, so who knows...

 

[RJS]

Re: Boat speed VS Temperature

Cold temp + low humidity + low altitude = more HP

 

[DJ]

Re: Boat speed VS Temperature

Cavitation IS boiling water.<br /><br />The water effectively "boils" during cavitation. <br /><br />The reason being is that a super low pressure exists and thus the boiling point of water drops drastically.

 

[SingleShot]

Re: Boat speed VS Temperature

Here's an intreasting one, this year my boat is under proped with both my stainless and stock aluminum. It's like my 125 added a few ponies over the winter. Stock prop is 12/3/4 x 21p no cupping @~4800 rpm for 45 mph and the stainless 13 3/8 x 19p @ ~5150 w/cupping for 41 mph. Now I need to keep an eye on the tach and keep the revs at 5000, but my speed has dropped 7 and 5 mph respectivly. I'm considering on trying a 13 1/4 x 21p stainless or 4blade 13 1/4 x 21p aluminum. I don't mind spending the cash if I new which one would perform the best. No one around here will let you demo a prop.<br /><br />Singleshot

 

[RJS]

Re: Boat speed VS Temperature

Singleshot,<br />Go to the Michigan Wheel web site and use their prop selector. You have to provide a lot of information, but the recommendation that I got from them worked perfect for my boat. It doesn't cost anything and is worth a shot.

 

[RJS]

Re: Boat speed VS Temperature

Here is a definition/explanation of Cavitation that I found on the Web<br /><br />Cavitation, (which is often confused with ventilation), is a phenoma of water vaporizing or "boiling" due to the extreme reduction of pressure on the back of the propeller blade. Many propellers partially cavitate during normal operation, but excessive cavitation can result in physical damage to the propeller's blade surface due to the collapse of microscopic bubbles on the blade. There may be numerous causes of cavitation such as incorrect matching of propeller style to application, incorrect pitch, physical damage to the blade edges, etc...