Changing Prop or gear ratio
04-07-2005, 10:29 AM
#22
Registered
Re: Changing Prop or gear ratio
tcelano: Because guys are usually adding power to a hull with known "before" speed numbers, and because you don't always have accurate weights, hull conditions, drive heights, C of G etc., models are usually being calibrated with before numbers. As long as the power increase and associated speed gains are relatively small, this seems to work. If something else has changed, like a shorter drive, then it may not work as well.
I like your model; the prop advance and wetted length numbers are very informative. Do you use prop efficiency curves? Have you considered a graphical output? This is useful for seeing where the curves cross, and can give the user a better feel for how choosing prop pitch for absolute top speed will affect acceleration.
Welcome to the board, get used to having your brain picked!
I like your model; the prop advance and wetted length numbers are very informative. Do you use prop efficiency curves? Have you considered a graphical output? This is useful for seeing where the curves cross, and can give the user a better feel for how choosing prop pitch for absolute top speed will affect acceleration.
Welcome to the board, get used to having your brain picked!
04-07-2005, 01:13 PM
#23
Re: Changing Prop or gear ratio
Tomcat,
Thanks for the welcome! My model is based on Savitsky-based mathematics, with lots of extra hydrodynamics and aerodynamics added in to account for aerodynamic lift and drag on the hull as well as gearcase drag.
The basic problem with most available methods is that you have to choose from an infinite combination of running trim angles and keel depths. My research yielded a method for doing so, and is based on longitudinal/vertical stability (porpoising) limits. Essentially, my code is showing you what you would get if you could balance your boat right at the threshold of porpoising for every speed.
I also developed a propeller algorithm based on everything I could find on supercavitating prop tests. Being an engineer, I know that no matter how complex I make it, there will always be factors that can't universally be explained, and the marine environment has more than its share of those.
I agree with your assessment on my output needing to be more graphical and user-friendly. It is hard to articulate to people just how much is behind it when all you get is a lousy string of html numbers..
I have been spending my time studying for the PE exam, so when I get that off my plate, there will be more upgrades to this code. I seem to have it dialed in very well for boats with #6's, and pretty good with other SSM's. Bravo drive boats seem to have the widest variability in results, and I think it is simply the fact that the hulls those drives are on just aren't built and maintained with the same care.
The code will make "blind" predictions for many boats quite well. Most that are off a bit predict a bit faster than what users actually see, which makes sense. How many boats out there do we really think are actually set up perfectly? How many props have pitch distributions which are dead-on? Heck, its even hard to get the same boat to repeat a performance exactly. Also, there are always variations and errors in the way parameters are measured. If it was an exact science, it wouldn't be any fun!
Thanks for the welcome! My model is based on Savitsky-based mathematics, with lots of extra hydrodynamics and aerodynamics added in to account for aerodynamic lift and drag on the hull as well as gearcase drag.
The basic problem with most available methods is that you have to choose from an infinite combination of running trim angles and keel depths. My research yielded a method for doing so, and is based on longitudinal/vertical stability (porpoising) limits. Essentially, my code is showing you what you would get if you could balance your boat right at the threshold of porpoising for every speed.
I also developed a propeller algorithm based on everything I could find on supercavitating prop tests. Being an engineer, I know that no matter how complex I make it, there will always be factors that can't universally be explained, and the marine environment has more than its share of those.
I agree with your assessment on my output needing to be more graphical and user-friendly. It is hard to articulate to people just how much is behind it when all you get is a lousy string of html numbers..
I have been spending my time studying for the PE exam, so when I get that off my plate, there will be more upgrades to this code. I seem to have it dialed in very well for boats with #6's, and pretty good with other SSM's. Bravo drive boats seem to have the widest variability in results, and I think it is simply the fact that the hulls those drives are on just aren't built and maintained with the same care.
The code will make "blind" predictions for many boats quite well. Most that are off a bit predict a bit faster than what users actually see, which makes sense. How many boats out there do we really think are actually set up perfectly? How many props have pitch distributions which are dead-on? Heck, its even hard to get the same boat to repeat a performance exactly. Also, there are always variations and errors in the way parameters are measured. If it was an exact science, it wouldn't be any fun!
