Guys I have a question for some input….I have a new 07″ optimax 115 ho and when breaking in I was regularly hitting speeds of 45 mph plus or minus a few witht he fastest being on Chisago lake at 49..Now I struggle to hit 41 to 42 mph for some reason….any input on what the loss of speed might be due too? 
Just curious if maybe it was just due to break in period or what. I run at peak about 5000 RPM’s
Did you hit anything and bend a prop blade? This will cause a slow down. One time when i had an aluminum prop i bent a blade pushing into the garage and did not know until the next time out i lost 4-5 mph. Also remember the boat will run faster in colder water since there is less resistance, sounds crazy but it can mean a loss in speed.
Also although i run my 115 opti at 5,000 rpms with a stainless prop and I like not working the motor any harder i believe this motor can work easily in the 5,500 rpm range which would correlate to better top end speed if you played with different props.
Never thought of the cold water issue before but makes sence to me actually 
What speed does your 115 top out at by chance T-Ellis? The prop is in MINT condition redneck Jr was even nice enough to be able to rip on it for how clean it was still a few weeks ago 
Erick,
A 115 Opti has a max RPM of 5750. You are 750 Rpm short. I would play around with some props. You really should run as close to the top of the RPM range as possible.
Troy
I run a 1850 Crestliner Fishhawk and with two guys and gear i pull 40 mph consistently. Earlier in the year it pulled closer to 43-44 mph with the cold water. Fast enough for me.
The colder weather will provide a more dense charge of air thus producing a greater compression ratio and furthermore more Hp. Now, in this case, we need to determine if the RPMs remained constant between then and now. From the crankshaft down to the prop is “locked” together through gears which means at a set motor RPM, the prop shaft will be at a set RPM also. If there has been a drop in RPMs, that could very well be due to the hotter, more humid air charge, if the RPMs are exactly the same now as they were there could be a few variances. The first of course is the speedometer. Are you using GPS or a paddle wheel style or a pito tube style speedo? What is the water temp difference? Colder water=more dense=less slippage from prop. Simply going up stream vs. down stream can make a difference as heading up stream will actually force more water towards the prop for more bite.(I can’t remember if you said river or lake testing) And lastly, any variance in the prop itself will make a difference. Add in wind/water conditions can also account for small changes. Just a few things to think about.
I run a 1750 fish hawk and earlier was average 43-45 mph not it is around 41 mph and have always run around 5000 rpm’s…I go off the GPS for all speeds…..also how would a guy go about raising my RPM’s than it runs at 5000 hammered all the way down for me.
Colder water is more dense thus the boat plains on the water with less drag probably is the biggest variable if the speed difference is based on water temp from cooler water. Not sure what times of the year you are comparing the difference in speeds.
Remember this rule of thumb: For every 2″ of propeller pitch, rpm’s will change approximately 400 rpm’s. As you drop in pitch, rpm’s increase, and as you go up in pitch, rpm’s decrease. So if we want to raise our rpm’s we should choose a lower pitch prop. Following the rule, based on a 21 pitch going down to a 19 pitch should raise our rpm’s to about 5300. That would help the hole shot and also gain us some more speed.
As you follow the rule of thumb, remember that sometimes rules are meant to be broken. This is not an iron clad rule, sometimes varying 2″ of pitch will only change rpm’s by 200. Switching prop brands or manufacturer, it can sometimes throw off the rule too.
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Colder water is more dense thus the boat plains on the water with less drag probably is the biggest variable if the speed difference is based on water temp from cooler water. Not sure what times of the year you are comparing the difference in speeds.
I am comparing from early to mid May compared to mid June till last week where I lost quite a bit of speed.
Erick, what pitch prop are you running??? My speed often depends on how it is loaded, how much fuel, the weather etc.
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Erick, what pitch prop are you running??? My speed often depends on how it is loaded
Well it seems to be right side heavy when I am driving 
the prop is the factory prop that came with it no help there
I have also been told it is not good for the motor to run WOT without hitting the Maximum RPM’s. Not sure if this is true or not???
Anyone want to comment.
Climate Control
A little foggy about how weather affects horsepower? Allow us to explain
Bruce W. Smith
Bass and Walleye Boats
August 1, 2003
Email this article to a friend!
Ever wonder why your boat seems like it runs better some days than others? Rest easy, friend. It’s not your imagination. Just as weather patterns have a direct effect on the feeding habits of fish, changes in air temperature, humidity and barometric pressure affect an engine’s performance — be it an outboard, inboard, two-stroke or four-stroke. Such weather-related performance issues stem from the fact that internal combustion engines are, essentially, air pumps — and in order to run at peak efficiency, they depend on a specific ratio of air mixed with fuel.
“In turn, the horsepower and torque available from a normally aspirated engine depend upon the density of that air,” says Richard Shelquist, an expert in engine mechanics and owner of Shelquist Engineering in Longmont, Colorado. “Higher air density means more oxygen molecules are available for combustion and, hence, more power. Lower air density means less oxygen and less power.”
Change any of the three factors — temperature, humidity or atmospheric pressure — and the amount of oxygen available to “stoke” your outboard changes proportionately. Altitude also plays a major role in how an engine will run. But altitude isn’t weather. We’ll explain more on altitude’s effect in just a bit.
WEATHER OR NOT
In terms of climate, figuring out exactly how much power your outboard gains or loses due to weather changes requires a working knowledge of the engineering formulas used to derive correction factors for engine dynamometers.
Because atmospheric conditions play such a critical role in power, the Society of Automotive Engineers (SAE) established a standard for measuring horsepower and torque (SAE J1349 JUN90) so everyone has the same reference point. The SAE standard is based on 77 degrees Fahrenheit at 29.235 inches-Hg (inches of mercury) with zero percent relative humidity.
To see how these factors change your outboard’s power, log onto Shelquists’s convenient horsepower calculator (http://wahiduddin.net/calc/calc_hp_abs.htm) and plug in a couple of weather-related numbers to get the percentage of gain or loss for those conditions. It’s fun and informative.
And even if you’re not interested in precise numbers, you’ll still benefit from a basic understanding of how each of these elements affects your boat’s performance.
HUMIDITY AND HORSEPOWER
Humidity is an interesting factor in the horsepower equation. And while it doesn’t play as big a role in engine performance as air temperature or altitude, it does make a difference.
Rick Martin, an engineer with Yamaha Marine (800/526-6650; yamaha-motor.com), explains, “Humidity is water vapor. In a given amount of air, it displaces oxygen content, making the air less dense. As a result, the engine ingests less oxygen, and its output is reduced. For example, on a hot day at sea level, high humidity would cause a carbureted outboard to run rich.
“Humidity differences could range from 0 percent humidity at 40 degrees F in winter, to 70 percent at 110 degrees in summer. This would result in a 4 percent change in horsepower output,” Martin says.
Do computer-controlled, electronic-fuel-injected (EFI) outboards fare better dealing with humidity? Not necessarily.
“EFI engines are able to compensate somewhat for reduced amounts of oxygen by leaning the fuel mixture, keeping the air/fuel ratio the same,” explains Martin’s engineering partner, Alec Hoshiba. “But the computer cannot replace the missing oxygen. Horsepower would still be reduced — but it wouldn’t be as noticeable as with a carbureted engine.” This is because most carbureted outboards don’t have the ability to automatically modify the air/fuel mix, so the effects would be more pronounced.
Mercury Racing project engineer Dan Marcellis gives a numeric example of how humidity affects horsepower: “If the humidity went from 0 to 30 percent — and all other factors remained the same — the loss of power on a 200 hp engine would be 1.4 hp. This small a change wouldn’t be noticeable to anyone except a racer tuned right on the edge.”
However, Shelquist’s calculator shows that if humidity jumps to 90 percent (as it does in the South and Midwest during summer), power falls off by 3.6 percent. This equates to a loss of 7 horses on a 200, which would be noticeable to anglers running the faster fishing platforms.
PUT ON THE PRESSURE
Both bass and walleye alter their feeding habits as the barometer rises and falls with each change of weather. Your outboard reacts to changing barometric pressure, as well — in much the same way it reacts to changes in altitude.
“Barometric pressure influences the intake-charge density and flow through an outboard,” notes Marcellis. “So the higher or lower the barometer, the more or less power the engine can make. For example, if the barometer increased from 29.20 in.-Hg to 29.70 in.-Hg, the power of a 200 outboard would increase 4.2 hp, and speed on the average performance hull would go up by roughly 0.4 mph. If the barometer drops by the same amount, the same amount of power [and speed] would be lost.”
Yamaha’s Martin and Hoshiba explain it in simple terms: The higher the barometric or atmospheric pressure, the more oxygen that’s available for combustion, and the better the engine’s power output. If you’re looking for a baseline measurement of barometric pressure, it’s normally 29.92 in.-Hg. at sea level, which is equivalent to one atmosphere, or 14.7 psi.
BREAK OUT THE THERMOMETER
When you’re talking climate — not altitude — air temperature has the greatest effect on the way your boat performs. The warmer the air, the less dense it is; consequently, the less oxygen it contains. This means poorer combustion, and a proportionate loss in power.
“A good example is if the temperature increased from 70 to 90 degrees during your day on the water (and assuming other factors remained constant), the power loss on that 200 outboard would be 5.8 horses,” says Marcellis. “The average performance hull would see a drop of 0.6 mph.
“The inverse is also true. If the temperature dips from 70 to 50 degrees, power would increase by 5.8 hp and top speed by 0.6 mph.”
Taking this a step further, the difference in performance between a 35-degree winter day and a 105-degree summer day can be as much as 10 percent — if both humidity and barometric pressure remain constant. Change those parameters, and the difference can be even more dramatic.
The key thing to remember, however, is that temperature, humidity and barometric pressure are rarely constant. And though the performance changes might be slight, this explains why your boat will perform at different levels at different times of the hour, day, month and year.
GIVING IT 100 PERCENT
All three engineers gave some weather-related examples using a 200 hp engine as a baseline. On a 77-degree day, with the barometer sitting at 29.53 in.-Hg, and 30 percent humidity, power output is 100 percent.
If humidity remains constant, but air temperature drops to 70 degrees and the barometer to 29.20 in.-Hg, power is down to 198.6 hp — a loss of 1.4 horses.
Run the same engine on a 100-degree day, with the barometer at 29.53 in.-Hg and the humidity a steamy 80 percent, and power is now down to 185 hp — a whopping 15 hp loss.
Conversely, if barometric pressure remains 29.53 in.-Hg, but air temps drop to 50 degrees and humidity plunges to 20 percent, power output jumps to 205.8 hp.
Drop the temperature to an ice-fishing 20 degrees, raise the bar to 29.7 in.-Hg, and cut humidity to 0 percent, and that 200 is now making 211.4 hp. Of course, it’s too cold to do anything with all that horsepower, but it’s nice to know it’s there, all the same.
’TUDES AND TEMPS
Even though altitude is a matter of geography, not weather, elevation plays the biggest single role in the horsepower/performance equation. For every 1000 feet you climb above sea level, there is a reduction in air density of approximately 4.5 percent, according to Marcellis. So, for every 1000-foot gain in altitude, there is a corresponding loss of horsepower, which means our 200 hp outboard gives up 9 ponies.
Water temperature also plays a role — although it’s miniscule in the grand scheme of the performance game. “Water temperature can impact power, depending on the way the cooling system is set up,” Marcellis says. “But the effect is minimal. If you have a closed cooling system with a thermostat, the thermostat will keep the engine’s operating conditions close to constant.
“On open cooling systems — like those found in our Merc 2.5 Drag — the lake water has a direct effect on cylinder-head temperature. The cooler the temperature, the denser the intake charge and the greater the power produced. However, the amount of the power change is difficult to establish, because you don’t know how much temperature change is involved.” Yamaha’s Hoshiba puts it in layman’s terms, saying, “Cool water keeps the engine cool, and allows it to draw in more air.”
CUT TO THE CHASE
What it all boils down to is that if you want to impress your fishing partners with the performance of your boat, take them for a ride across a rippled section of water that’s at sea level, when the air is cold and dry and there’s a big ridge of high pressure parked above. Your outboard will be impressive.
On the other hand, if you find yourself boating at high altitude on a hot, humid day — when the barometric pressure is so low the fish are hiding — go it alone. Your outboard’s performance will be as poor as the fishing. The good news is, so will everyone else’s.
SIDE BAR
Calculating Horsepower
Shelquist Engineering horsepower calculator: http://wahiduddin.net/calc/calc_hp_abs.htm
Computer Support Group Network:
csgnetwork.com/relativehpcalc.html
csgnetwork.com/relhumhpcalc.html
csgnetwork.com/automotiveconverters.html
Moreyes, very cool article and i think it explains it.
Snowmobiles are the worst try going from -30 to + 40
Anyway glad to help
atta boy, Kirt…
Jack.
Thanks for all the help guys 
Moreeyes those sled sure run good when it is below zero. 
Thanks for the article.
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Snowmobiles are the worst try going from -30 to + 40
Anyway glad to help
That’s part of the fun!!
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