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Discussion Starter #1
I know that the summer heat slows our boats down.. I am curious how much difference in top speed numbers you have experience in winter (say 70 to 75 degrees) to summer temperatures ( say 110 +). I know that winter gets alot colder than that but there are not alot of boats out on Havasu until the temps reach at least 70. Is there a certain formula for mph lost to increased temperature?? All I know is that my boat is slower in the summer.


What is your top speed differential?
 

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The HMFIC
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just remember cooler air causes better combustion. your probley talking 1/4's of MPH's at a time. max performance would probley be 1-2 mph diff if i had to guess. im not a mech but i know a little about a lot of stuff:)sphss
 

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The reality, even in the dead of winter, you're probably not drawing 60 degree air inside the motor compartment.

When engine manufactures do their testing, you can bet your ass it's perfect conditions.

Adjusted Altitude (AA)
This one number, expressed as a footage, is a relative performance altitude compared to STP (Standard Temperature and Pressure which is 60 oF, 0% RH, and 29.924 "Hg). We have found this one number, which relates directly to observed engine performance, to be the most accurate value for horsepower correction and predicting vehicle performance, and our software for ET and TS prediction uses AA. All Altalab weatherstations calculate AA.
 

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All I know is that my boat is slower in the summer.
What kind of power do you have?
If it's a carb'd 2 stroke outboard the power difference is huge between 70-110˚F .... unless you re-jet leaner. (and so is the fuel consumption)
I'd estimate your HP output would be down close to 10%
 

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Only a minor difference between cooler weather and 110+, around 1-2 mph.

Where I notice the biggest difference is between Havasu and Powell, around 20% loss in power.
 

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E-7 Sheepdog (ret)
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There is about a 1% change in engine output per 10 degree F. change in intake air temperature.

70F to 110F is worth about a 4% change in power, all other things remain equal.

Not enough to notice any apreciable performance on a vehicle with as much drag as a boat.

Come to Ks and compare 105F to 30F sometime. :) 7.5% change.

I'll watch you from shore.
 

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Both of my boats are 2-stroke outboards. My little boat is 2.5l/260hp EFI. 112mph in the 70 and below temps, 108 is the best I've seen when over 100*.

The Scarab has twin carbureted 4.0l V8 2-strokes. Best is 72mph in 70ish temps, over 100* (last weekend) could only muster 69.
 

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I'm baaaaack...
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Lots of math here, I didn't do any of it. We swap props winter to summer to take advantage of the higher power with higher pitch. After the swap, we gain about 5 MPH. Running the shorter prop we hit the rev limiter at top speeds, and we don't with the taller prop.
 

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The Pool Bitch
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My speeds always went down in the summer. 4% from heat and low air density, and 4% from a heavier ice chest. :D
 
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Seems 3 to 5 MPH is the norm for HO powered boats between summer and winter in AZ
 

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Discussion Starter #11
My speeds always went down in the summer. 4% from heat and low air density, and 4% from a heavier ice chest. :D

You are welcome on my boat with your heavy ice chest anytime. Perhaps we can do some field testing.

I notice somewhere between 4-6 mph difference in the summer.

600 supercharged Merc
 

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The Pool Bitch
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I am surprised you see that big of a difference with the supercharged Merc. I thought they were computer controlled to vary boost depending on air density. I was told that they make about the same power at sea level and at some amount of altitude (almost the conditions you would experience with hot less dense air).
 

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29812
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What kind of power do you have?
If it's a carb'd 2 stroke outboard the power difference is huge between 70-110˚F .... unless you re-jet leaner. (and so is the fuel consumption)
I'd estimate your HP output would be down close to 10%
106/108 in the winter
98/100 in the summer
 

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Discussion Starter #18
I am surprised you see that big of a difference with the supercharged Merc. I thought they were computer controlled to vary boost depending on air density. I was told that they make about the same power at sea level and at some amount of altitude (almost the conditions you would experience with hot less dense air).
Actually this is a very rough, non scientific estimate. My boat was slower when I went to Lake Powell also. In the summer there is also the added weight of multiple people, your ice chest, bimini, wakeboards, snacks, toys, alot more fuel.. etc... I will try to get a reasonable similar condition next time I go out and see what the real difference is. It may be larger than my estimate or smaller. My boat is definately slower in the heat. Just wondering what the average loss in speed was.
 

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106/108 in the winter
98/100 in the summer
I got some good news and bad news for ya.......

The good - your hull isn't hitting the top speed vs. HP wall yet (adding a little bit of power is yielding a good speed increase)

The bad - dude.... you need more HP!!:)
 
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