humidity and outboard performance

Heat and humidity can affect an outboard. Need to know a little bit more about your set up though. What size engine what size prop, and how much of a load are you carrying? All of these take into account as the weather warms you may have to change pitch in props or other methods to achieve the same performance. Hope this helps.96 Eyra/12 Mercury 175 Pro XS with 2015 single axle trailer2017 Victory Gunner

Take it from an old school mechanic , air temperature and moisture content ( I.E. DENSITY / HUMIDITY ) have a big effect on an engines ability to produce power . — In the summer months here in the south , your 200 horse motor becomes a 175 real quick . — The newer fuel injected engines arent affected as much as the older carbureted models were , but they are nonetheless affected . — Water gets thinner too as it warms . — I run a 23 pitch most of the year , but switch to a modified 25 around the first of June . — Seems to work for me . — Hoss

Theres plenty of math available to predict exact horsepower loss given a baseline air temp / humidity reading you want to use for comparison (say February). Your engine can ingest a specific volume of “gas” from the atmosphere. This gas is normally about 21% oxygen, and 79% nitrogen. That 21% oxygen is what you are looking for to combine with gasoline to produce combustion. In the summer, you have two things going against you. First, the air is warmer, which means less oxygen per cubic foot of atmosphere as hotter air is less dense. Second, part of that oxygen and nitrogen is displaced by water vapor (humidity). You end up ingesting less oxygen per unit of time, which reduces combustion pressures and horsepower.There is not a motor on the planet that doesnt lose power in the summer, proportional to temp and humidity rise. Even supercharged engines lose power because they STILL ingest less oxygen per rotation. But some can dial back the boost rate for cold air, and increase it for hot/damp air, to compensate. Not aware of one that does this on an outboard however. I barely touched 72 yesterday with a nearly full tank of gas + two adults and tackle, where back in Feb/March we were bumping right up against 75. Guy with big ranger with a 350 v-rod had tried to pass us and failed by a pretty significant amount. At the ramp he brought up this very topic. Said he was off about 4mph from what he was seeing in the winter… Said he had been told that v-rods didnt suffer. Back in my early drag racing days (cars) we had to change the carb jets multiple times during the day as temp and humidity climbed during the summer. Not so much in the winter. The advantage fuel injection motors have is that they wont run overly rich or lean, since they dont have a fixed jet orifice that dictates fuel flow. They lose power (significant power) due to the density issue, but the mixture doesnt go way off like a carbed motor can/will.Last edited by oldtimer57 on May 22nd, 2017, 4:21 pm, edited 1 time in total.2008 Pantera Classic2014 Mercury Pro XS 200