stevemid
19-02-2014, 04:38 PM
Thanks to Grand_Marlin who posted the Yamaha performance data sheets http://www.ausfish.com.au/vforum/showthread.php?197262-Boat-amp-Motor-Performance-Sheets I was able to pick a boat similar to the one I'm building with the Yammie 70's that I am buying and do a little dreaming about what kind of performance I am going to get.
The boat I chose to compare was the Sailfish Reef Runner 5.8M which is the same displacement as me, same twin Yammie 70's etc etc. but 1.2M shorter. http://www.yamaha-motor.com.au/sites/yamaha-motor/files/Sailfish_5.8%20Reef%20Runner_F70AETX.pdf
HOWEVER, when I shared the above with my friend, George, the communist mathematician, he vomited all over the top speed claims for this boat. He said, "This is simple arithmetic:
6300 rev/2.33gear = 2703 RPM at the prop
Now, theoretically, a 14" prop produces 14" of forward motion for each revolution. So....
Multiply 2703 * 14" * 60 minutes and you get 2703*14*60=2,270,520 inches traveled per hour
Divide by 12 and that's 189,210 feet per hour. Now there's 6000 feet in a nautical mile so 189,210/12=31.54 NM/hr
They say they got 31 knots and the absolute best that could be gotten with that setup is 31.54 leaving only a figure of 1.7% to account for prop slip (the inefficiency of the prop.)
From my research, 10% slip is very efficient, 15% is probably more average and less than 10% is rare. Given that, the boat in question, with average 15% slip was probably actually doing 26.8 knots (.85 * 31.54= 26.8)
The moral of the story? All fishermen are liars.
Steve
Boatbuilder, not yet a fisherman)
PS George's way of accounting for this is comparing it to his trusty Trabant (all communists compare everything mechanical to their experience with the Trabant.) "Back in Hungary I used have a very good mechanic who serviced my trusty Trabant. I complained to him that my Trabant uses too much petrol, and my friends say their Trabants use significantly less. His answer was, "Why don't I claim better economy too!"
The boat I chose to compare was the Sailfish Reef Runner 5.8M which is the same displacement as me, same twin Yammie 70's etc etc. but 1.2M shorter. http://www.yamaha-motor.com.au/sites/yamaha-motor/files/Sailfish_5.8%20Reef%20Runner_F70AETX.pdf
HOWEVER, when I shared the above with my friend, George, the communist mathematician, he vomited all over the top speed claims for this boat. He said, "This is simple arithmetic:
6300 rev/2.33gear = 2703 RPM at the prop
Now, theoretically, a 14" prop produces 14" of forward motion for each revolution. So....
Multiply 2703 * 14" * 60 minutes and you get 2703*14*60=2,270,520 inches traveled per hour
Divide by 12 and that's 189,210 feet per hour. Now there's 6000 feet in a nautical mile so 189,210/12=31.54 NM/hr
They say they got 31 knots and the absolute best that could be gotten with that setup is 31.54 leaving only a figure of 1.7% to account for prop slip (the inefficiency of the prop.)
From my research, 10% slip is very efficient, 15% is probably more average and less than 10% is rare. Given that, the boat in question, with average 15% slip was probably actually doing 26.8 knots (.85 * 31.54= 26.8)
The moral of the story? All fishermen are liars.
Steve
Boatbuilder, not yet a fisherman)
PS George's way of accounting for this is comparing it to his trusty Trabant (all communists compare everything mechanical to their experience with the Trabant.) "Back in Hungary I used have a very good mechanic who serviced my trusty Trabant. I complained to him that my Trabant uses too much petrol, and my friends say their Trabants use significantly less. His answer was, "Why don't I claim better economy too!"