Best Horsepower for Planing at Low Speed in Rough Water

  ContinuousWave
  Whaler
  Moderated Discussion Areas
  ContinuousWave: Whaler Performance
  Best Horsepower for Planing at Low Speed in Rough Water

search | FAQ | profile | register | author help

Author Topic:   Best Horsepower for Planing at Low Speed in Rough Water

Smithsm posted 06-05-2009 08:44 AM ET (US)
Here is a variation of my question about the best propeller for planing at low speed in rough water.
All other things the same--boat, weight, propeller, etc.--would an Outrage 20 be better planing at low speed in rough water with a 225-HP or 150-HP engine of the same make and design?
I would imagine the 225-HP would be better because it would have more torque at low RPM, enough additional torque to offset the extra 100-lbs on the transom. Once again, comments from the collective brain trust are appreciated.
Oe would the 150 be better for planing at low speed in rough water because it would be outfitted with a much less aggressive propeller, giving up top end but being able to run in higher RPM range, therefore more torque at the lower speed. This high-RPM, high torque, moderate to low speed is what I seemed to have on my 1988 Outrage 20 with twin 60's, and it works well so long as you didn't want top speed over 30-MPH.
Tom W Clark posted 06-05-2009 10:58 PM ET (US)
Skip,
It all depends. All motors have an RPM at which they (or you) are most "happy". If a more powerful motor has to be run at less than this "ideal RPM" to maintain the minimum planing speed of the boat in question than the less powerful motor, then the less powerful motor will be the preferred choice.
Conversely, if the less powerful motor has to be run at higher RPMs to maintain the minimum planing speed ten the more powerful motor will be the better choice...for this application.
Peter posted 06-06-2009 07:35 AM ET (US)
Following on Tom's remarks, the Ficht outboard motors, like the 225 HP model you have, runs in one of two injection modes, stratified charging (low RPM) and homogenous charging (high RPM). There is an RPM zone where it switches between these modes (the transition doesn't completely occur precisely at one RPM level and suspect it may vary based on factors other than RPM as well).
On a particular boat, if the low planing speed happened to coincide with this transition zone on the 225 HP (225 is operating in a combination of stratified and homogeneous modes) but a low plane speed would be clearly above the transition zone RPM on the 150 HP motor (150 is operating in homogenous mode), then the 150 might be a better choice although it has to turn higher RPM to maintain plane.
jimh posted 06-06-2009 08:28 AM ET (US)
It has been my experience that when horsepower is limited, it may not be possible to maintain a low-speed plane. A motor with limited horsepower may not be developing enough power at the lower engine speed to sustain the boat on plane.
If operating the boat at the very slowest planing speed was a primary goal, you might have to adjust the propeller pitch to favor this mode. The propeller pitch would likely have to be reduced.
I suspect that in many cases the engine speed which gives the helmsman the feeling of being in the sweet spot of the engine's power curve is probably coincident with the peak in the engine torque curve. Here is a plot of engine torque as a function of engine speed for a direct-injection two-cycle motor which might be considered somewhat typical for engines of this type:
http://continuouswave.com/whaler/reference/images/ETEC250HO/ torque640x449.png
You can see from the plot that if the low-speed plane for your boat and propeller combination required the engine to run at 2,500-RPM, the engine would not have nearly the available torque that it does when running at 3,000-RPM. To over come this problem, you would have to change the propeller pitch to a lower pitch, and then the engine speed could increase into a region where there is more power available.
Tom W Clark posted 06-06-2009 11:14 AM ET (US)
Based on Peter and Jim's arguments, it would probably be best if a 1998 Outrage 20 had a 150 HP motor instead of a 225 for running at minimum planing speed. What you want is low boat speed but higher engine speed and that will occur with less horsepower, not more.
My own intuition tells me this is true base on recent experience with my own boat.
On Tuesday, my wife and I played hooky and took our Revenge 25 with twin Mercury 150s from Seattle to Port Townsend to celebrate her birthday. It was lovely weather with near ideal water conditions. About half way through the one hour trip, the port motor crapped out forcing me to run the rest of the way on only one motor.
I normally cruise at 35 MPH because at 4200 RPM that is where the motors produces the best fuel economy and seems to be happiest. But running on only one motor, boat speed was reduced to 27 MPH.
Admittedly, the single motor was over-propped for pushing the boat by itself, but I was still able to get the motor to 4200 RPM with only a little more throttle by trimming it allow the prop to ventilate slightly and thus slip a bit. The Mercury MIRAGEplus propeller will allow this because if its significant cupping.
While the remainder of the trip took longer than planned, there were two benefits:
The fuel economy improved from 2.3 MPG to 3.0 MPG.
The ride was MUCH smoother. If you can slow this boat down, it will push through anything but I rarely ever run it this was because with 300 HP, it just does not like to go that slow.
Smithsm posted 06-10-2009 09:01 AM ET (US)
Tom
theory and your experience both suggest your answer is correct. your mpg delta between one and two engines is significant - and impressive to get 3 mpg with that boat and weight
I will take my time and run at least the Mirageplus prop and perhaps a 4 blade 18" pitch prop to see how that works out.
I am in no rush to do this - but one option down the road is to sell my low hour 225 to someone with a larger boat who really needs the power, while on my boat so folks can sea trial it, then replace it with either a 150 or 175 and a less aggressive prop so it can be run at higher speeds. I wouldn't mind taking 100 pounds off my transom either.
What I have now works for lots of situations but is apparently not optimal for some conditions and situations I will encounter - and apparently is not my best mpg option.
thanks again.

Hop to:

Contact Us | RETURN to ContinuousWave Top Page

Powered by: Ultimate Bulletin Board, Freeware Version 2000
Purchase our Licensed Version- which adds many more features!
© Infopop Corporation (formerly Madrona Park, Inc.), 1998 - 2000.

Author	Topic: Best Horsepower for Planing at Low Speed in Rough Water
Smithsm	posted 06-05-2009 08:44 AM ET (US) Here is a variation of my question about the best propeller for planing at low speed in rough water. All other things the same--boat, weight, propeller, etc.--would an Outrage 20 be better planing at low speed in rough water with a 225-HP or 150-HP engine of the same make and design? I would imagine the 225-HP would be better because it would have more torque at low RPM, enough additional torque to offset the extra 100-lbs on the transom. Once again, comments from the collective brain trust are appreciated. Oe would the 150 be better for planing at low speed in rough water because it would be outfitted with a much less aggressive propeller, giving up top end but being able to run in higher RPM range, therefore more torque at the lower speed. This high-RPM, high torque, moderate to low speed is what I seemed to have on my 1988 Outrage 20 with twin 60's, and it works well so long as you didn't want top speed over 30-MPH.
Tom W Clark	posted 06-05-2009 10:58 PM ET (US) Skip, It all depends. All motors have an RPM at which they (or you) are most "happy". If a more powerful motor has to be run at less than this "ideal RPM" to maintain the minimum planing speed of the boat in question than the less powerful motor, then the less powerful motor will be the preferred choice. Conversely, if the less powerful motor has to be run at higher RPMs to maintain the minimum planing speed ten the more powerful motor will be the better choice...for this application.
Peter	posted 06-06-2009 07:35 AM ET (US) Following on Tom's remarks, the Ficht outboard motors, like the 225 HP model you have, runs in one of two injection modes, stratified charging (low RPM) and homogenous charging (high RPM). There is an RPM zone where it switches between these modes (the transition doesn't completely occur precisely at one RPM level and suspect it may vary based on factors other than RPM as well). On a particular boat, if the low planing speed happened to coincide with this transition zone on the 225 HP (225 is operating in a combination of stratified and homogeneous modes) but a low plane speed would be clearly above the transition zone RPM on the 150 HP motor (150 is operating in homogenous mode), then the 150 might be a better choice although it has to turn higher RPM to maintain plane.
jimh	posted 06-06-2009 08:28 AM ET (US) It has been my experience that when horsepower is limited, it may not be possible to maintain a low-speed plane. A motor with limited horsepower may not be developing enough power at the lower engine speed to sustain the boat on plane. If operating the boat at the very slowest planing speed was a primary goal, you might have to adjust the propeller pitch to favor this mode. The propeller pitch would likely have to be reduced. I suspect that in many cases the engine speed which gives the helmsman the feeling of being in the sweet spot of the engine's power curve is probably coincident with the peak in the engine torque curve. Here is a plot of engine torque as a function of engine speed for a direct-injection two-cycle motor which might be considered somewhat typical for engines of this type: http://continuouswave.com/whaler/reference/images/ETEC250HO/ torque640x449.png You can see from the plot that if the low-speed plane for your boat and propeller combination required the engine to run at 2,500-RPM, the engine would not have nearly the available torque that it does when running at 3,000-RPM. To over come this problem, you would have to change the propeller pitch to a lower pitch, and then the engine speed could increase into a region where there is more power available.
Tom W Clark	posted 06-06-2009 11:14 AM ET (US) Based on Peter and Jim's arguments, it would probably be best if a 1998 Outrage 20 had a 150 HP motor instead of a 225 for running at minimum planing speed. What you want is low boat speed but higher engine speed and that will occur with less horsepower, not more. My own intuition tells me this is true base on recent experience with my own boat. On Tuesday, my wife and I played hooky and took our Revenge 25 with twin Mercury 150s from Seattle to Port Townsend to celebrate her birthday. It was lovely weather with near ideal water conditions. About half way through the one hour trip, the port motor crapped out forcing me to run the rest of the way on only one motor. I normally cruise at 35 MPH because at 4200 RPM that is where the motors produces the best fuel economy and seems to be happiest. But running on only one motor, boat speed was reduced to 27 MPH. Admittedly, the single motor was over-propped for pushing the boat by itself, but I was still able to get the motor to 4200 RPM with only a little more throttle by trimming it allow the prop to ventilate slightly and thus slip a bit. The Mercury MIRAGEplus propeller will allow this because if its significant cupping. While the remainder of the trip took longer than planned, there were two benefits: The fuel economy improved from 2.3 MPG to 3.0 MPG. The ride was MUCH smoother. If you can slow this boat down, it will push through anything but I rarely ever run it this was because with 300 HP, it just does not like to go that slow.
Smithsm	posted 06-10-2009 09:01 AM ET (US) Tom theory and your experience both suggest your answer is correct. your mpg delta between one and two engines is significant - and impressive to get 3 mpg with that boat and weight I will take my time and run at least the Mirageplus prop and perhaps a 4 blade 18" pitch prop to see how that works out. I am in no rush to do this - but one option down the road is to sell my low hour 225 to someone with a larger boat who really needs the power, while on my boat so folks can sea trial it, then replace it with either a 150 or 175 and a less aggressive prop so it can be run at higher speeds. I wouldn't mind taking 100 pounds off my transom either. What I have now works for lots of situations but is apparently not optimal for some conditions and situations I will encounter - and apparently is not my best mpg option. thanks again.