Weather and its effect on performance

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TomMoorehouse

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So last time I was out I got everything running well and had a speed of 62mph. The weather was about 70 and average humidity. I made a few setup changes to look for more speed today and went out. today it's about 85 and 75% humidity and my boat was much slower! about 52mph. Now I had to richen my needle to get it running right about 3/4 turn. I finally lenghtened my pipe back out (one of the changes i made from previous outing) and leaned it in maybe 1/2 turn. I was able to get to about 57mph. Is the speed reduction about right for the weather change? 5 mph less with changes that should have made it faster? I did not think I would be that large but it's not everyday I get to have my radar gun to really tell.

what ya think? What are the effects and what changes should be made to your setup to reduce them?
 
TomMoorehouse said:
So last time I was out I got everything running well and had a speed of 62mph. The weather was about 70 and average humidity. I made a few setup changes to look for more speed today and went out. today it's about 85 and 75% humidity and my boat was much slower! about 52mph. Now I had to richen my needle to get it running right about 3/4 turn. I finally lenghtened my pipe back out (one of the changes i made from previous outing) and leaned it in maybe 1/2 turn. I was able to get to about 57mph. Is the speed reduction about right for the weather change? 5 mph less with changes that should have made it faster? I did not think I would be that large but it's not everyday I get to have my radar gun to really tell.

what ya think? What are the effects and what changes should be made to your setup to reduce them?
Click to expand...
It is all about "AIR DENSITY". Temperature does not really play a row in a boat running well but it can make a differents in air density. With cooler temperatures the air density can be highter thus make more power in a motor. But this is not true in all areas of the country. You can have high temperatures at close to sea level with good air density and make great horsepower. Humidity does play a row but it is very small percent and you may not see it.

A motor is a air pump and with the more oxygen in the air which is "AIR DENSITY" you have more compressable volume thus needing more fuel to run right making more horsepower. Another note is that "AIR DENSITY " will change your db noise level also at the boat. The higher the air density the higher db level. But I do not know if the higher db level will travel in the heavier air at distance.

In the past I use a "AIR DENSITY METER" for keeping up with my needles when I to set them years ago. But these days with third channel needles has made me lazy. Hope this helps.
 
Last edited by a moderator:
Tom,

I use 3 primary atmospheric measurements by meters:

1. Temperature

2. Humidity

3. Air Density (MOST Important) as noted by Mark.

Air density is a measurement of the quantity of O2 molecules in the air. The cooler it is, the more dense the air. Traditionally the more dense the air, the richer you go on the needle, and the greater engine power can be generated. When it is warmer, like the 15 degrees you had, the less denser air (less fuel too) and the resultant less power will come. That results in slower speeds - just nature.

Humidity is a measurement of the amount of moisture in the air. It is the least important variable but does affect operations. Temp can very much affect operations. Then throw in height above sea level, which is connected to air density. Easy huh!!!

That is why a flowmeter is vital (keeping records) and a 3rd channel is also.

With the exception of the internats, you see almost all of the time trials in the cooler months of the year. More dense air means potentially more speed.
 
Tom,

I'm sorry but if you came to a day where the only change is the increased humidity then it is not possible to have to richen your motor up.

air density goes down by increased humidity (humidity=percentage of water vapor contained in the air over the maximum capable water vapor possible in the air at the current temperature) this would mean the engine would run to rich and you have to lean out, again if nothing else changed.

so for air density you need air temperature, air pressure and humidity(where the humidity is also related to the temperature again as the maximum vapor pressure is determined by temperature).

changing from a random day from 30% to 80% humidity would cause a RAD drop of about 1.2% this would cause a 1.15% to 1.5% power correction factor (depending on which temperature and pressure correction factors you believe in)

So I suspect other factors might have caused the drop in speed (boat, motor, waterconditions

finally as John stated keeping track of the air density by either a RAD gauge or measuring temperature, pressure and humidity is vital. last season i kept a constant log of these factors and it increased my consistancy in races tremendously.
 
A side note to this.......Humidity,temperature play into this with horses?

Ask BIG BROWN as he ran out of gas,or was not there cause of the heat..

Or,upon further review.......Shoot the jockey...

On with your topic,as I digress....
 
What were the changes? If I had a nickle for all the changes that I was SURE was going to make my boat faster! :rolleyes:
 
Bdonkeydonk said:
Hi Tom,
Who knows where I hail from today ;)

In the larger engines, we speak in terms of Density Altitude. It has a signifigant effect on equipment performance that cannot be ignored...ever imho.

Temperature Barometric Pressure and humidity all are players…though the biggest player is barometric pressure. My personal babydoll is a normalized 540 cubic inch Textron Lycoming that that's De-rated with shorter rods and it has a normalized turbo that will let it blow 33 inches of water in manifold pressure at low altitudes all the way up to the flight levels where the waste-gate shuts then it loses about an inch of pressure for every 1000ft thereafter. It's great for my apps because it will make power to higher alts where drag falls off bigtime whereas most engines loose an inch every thousand from the field.

Anyway…as usual....Mr. Bullard is correct. The best way to think of an engine is as an Air Pump that makes it's best power as it blows the largest total Mass of air possible. Lower pressure at higher altitudes is lower mass as is higher temps and higher humidity.

Here's a link a quick calculator you can use to see what density altitude is doing for testing and planning:

<a href="http://wahiduddin.net/calc/calc_da.htm" target="_blank">http://wahiduddin.net/calc/calc_da.htm</a>

You can get the Kollsman (pressure setting) number by googling and calling your local airports AWOS (automated weather observation) that will tell you the temps and dewpoint too. Any airport within 50 miles should make the calcs plenty accurate unless a stormfront is moving through. Sometimes they'll do you the favor of telling you the density altitude when it's unusually high.

All the best,
Click to expand...


Thanks Kevin B., good detail as usual for your posts. However, way over the heads for most of the racers we have out there.
 
humidity might play a bigger part than expected, imho. the moisture in the air is water. water does not compress. so high humidity would cause higher compression pressures. the higher amount of uncompressible 'solids' in the intake charge would have the same effect as reducing the chamber volume. for sure would affect a smaller motor more. this past weekend i ran at the delmarva race. went from running at my local pond (1173 ft. altitude, 74 degrees temp, & 62% humidity) to sea level, 100 degrees temp, 90 +% humidity. flowed my motor at home, got a # of 34. went to delmarva, flowed it before running, got 34, no needle changes! funny how it works...... trusted the meter, threw it in, ran good. not what i expected to do, considering the difference in altitude. but, the good air, bad air thing pretty much equaled things out.
 

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