Like Tree0Likes

Loss of speed in flat vs. choppy water

Reply
Old 06-19-2010, 07:39 PM
  #1
Registered
Thread Starter
 
Join Date: May 2007
Location: Italy
My Boats: Fountain 27 Fever
Posts: 259
Loss of speed in flat vs. choppy water

On my boat I've seen a best of 70mph with light load and choppy water.
In a couple of occasions with similar load, but with completely flat water, I've not been able to go above 66/67mph.
Can such loss of speed make sense for the different water conditions alone, or is that a coincidence and there must be some other reason I'm not aware of?
Thanks for your views!
mapism is offline  
Reply With Quote
Old 06-19-2010, 07:49 PM
  #2
Racer
Racer
 
Wahoo ATV's Avatar
 
Join Date: Jan 2006
Location: Columbus,MI
My Boats: 38 Fountain Canopy, V24 Bat Boat
Posts: 2,573
Default

yes. the chop airates the bottom like steps.
Wahoo ATV is offline  
Reply With Quote
Old 06-19-2010, 07:52 PM
  #3
Platinum Member
Platinum Member
 
Too Stroked's Avatar
 
Join Date: Apr 2010
Location: Rochester, NY
My Boats: 1992 Progression 22
Posts: 1,736
Default

Most boats benefit from "getting some air under them." Generally speaking, air has a lot less drag than water. So, it's not at all unusual to gain some speed when conditions are not glass smooth. You'll see it mentioned in Powerboat Magazine many times where their testers thought a particular hull had more speed in it, but the flat water held it back.

Another thing tends to come with glass flat water many times though - humidity. Your engine will not perform as well in hot, humid air as it will in cool dry air - again - generally speaking. So, you might have actually had two things working against you.
Too Stroked is offline  
Reply With Quote
Old 06-19-2010, 07:52 PM
  #4
Charter Member # 55
Charter Member
 
Griff's Avatar
 
Join Date: Oct 2000
Location: Omaha
My Boats: .......32............. ...............Active Thunder....
Posts: 15,752
Default

Normal. My boat will run about 74mph in a 1ft chop vs 72mph on dead flat water.
Griff is offline  
Reply With Quote
Old 06-19-2010, 08:59 PM
  #5
Registered
 
Join Date: Sep 2008
Location: LOTO
My Boats: Spectre 36 Poker Run Donzi 28ZX
Posts: 240
Default

Same reason they put steps on boats.
Get some air under them.
Donzi Dude is offline  
Reply With Quote
Old 06-19-2010, 09:38 PM
  #6
Neno the mind boggler
VIP Member
Trade Score: (1)
 
glassdave's Avatar
 
Join Date: Sep 2001
Location: toledo oh
My Boats: Batboat, 38 Scarab Thunder, 16 Sidewinder, 27 Magnum
Posts: 12,667
Default

I loose at least three or four mph in dead flat water. Its commonly known as sticky water
__________________
Throttles- Cleveland Construction 377 Talon
08 OPA Class 1 National Champion
08 Class 1 Geico Triple Crown Champion
08 OPA High Points Champion
10 OPA Class 1 National Champion ( happy now Ed! )
glassdave is offline  
Reply With Quote
Old 06-20-2010, 10:40 AM
  #7
Registered
Thread Starter
 
Join Date: May 2007
Location: Italy
My Boats: Fountain 27 Fever
Posts: 259
Default

Thanks folks, probably I wasn't clear enough in my question, I'm already aware of the aeration effect in principle, but it was the amount of it that I was wondering about: 3 or 4 mph out of 70 seemed a lot...
I never thought of humidity, though. Two Stroked raises a good point there.
And I'm also above sea level, btw, though not a lot - 650 feet or so. Does anyone have an idea of how much that can affect the performances?
mapism is offline  
Reply With Quote
Old 06-20-2010, 11:23 AM
  #8
Registered
 
AIR TIME's Avatar
 
Join Date: Dec 2000
Location: EASTON MA.
My Boats: BAJA OL 650HP/ SABER 28 Gellner unlimited with konrad ace
Posts: 6,281
Default

Quote:
Originally Posted by mapism View Post
Thanks folks, probably I wasn't clear enough in my question, I'm already aware of the aeration effect in principle, but it was the amount of it that I was wondering about: 3 or 4 mph out of 70 seemed a lot...
I never thought of humidity, though. Two Stroked raises a good point there.
And I'm also above sea level, btw, though not a lot - 650 feet or so. Does anyone have an idea of how much that can affect the performances?
just like stated above different boats loose different speeds, my baja 24ol would run 80 on the river flat water but a mile out in the bay it would run 3 to 4 mph faster with 1 to 2 1/2 ft chop maybe more but I ran out of prop back then.
AIR TIME is offline  
Reply With Quote
Old 06-20-2010, 02:36 PM
  #9
Platinum Member
Platinum Member
 
Too Stroked's Avatar
 
Join Date: Apr 2010
Location: Rochester, NY
My Boats: 1992 Progression 22
Posts: 1,736
Default

Quote:
Originally Posted by mapism View Post
Thanks folks, probably I wasn't clear enough in my question, I'm already aware of the aeration effect in principle, but it was the amount of it that I was wondering about: 3 or 4 mph out of 70 seemed a lot...
I never thought of humidity, though. Two Stroked raises a good point there.
And I'm also above sea level, btw, though not a lot - 650 feet or so. Does anyone have an idea of how much that can affect the performances?
There are standard correction factors used on engine dynamometers that allow for things like changes in humidity. Maybe one of our other members has that correction data handy?
Too Stroked is offline  
Reply With Quote
Old 06-20-2010, 03:37 PM
  #10
Charter Member
Charter Member
 
bowtie's Avatar
 
Join Date: Jan 2001
Location: New Jersey
Posts: 665
Default

Salt water vs fresh water also can add 1 or 2 mph.
bowtie is offline  
Reply With Quote
Reply



Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On



All times are GMT -5. The time now is 05:49 AM.


Copyright 2011 OffShoreOnly. All rights reserved.