handfulz28
04-13-2007, 06:05 PM
OK, I searched Wikipedia and since it's not in there I'll be the first to try and give it a stab.
When someone describes a boat as porpoising, they're describing the rythmic rise and fall of the bow not because of waves, but because of boat trim. While it's obviously most observable on calm & flat water, it's possible to get a boat to porpoise even in a chop or small sea. Typically a boat will porpoise when the drive (or outboard) is trimmed too high for that given power level to sustain the angle of attack.
That's part of the rub. Say at 50% throttle and "neutral" trim a boat might porpoise. Increase throttle and porpoising goes away. Of course trimming the drive in it goes away also.
I believe most conventional theory proposes that maximum speed for a given throttle setting is obtained by trimming "out" until the boat porpoises and then trimming in a hair to stop porpoising.
Now to increase complexity, you throw in trim tabs. Here's where I defer to someone with more experience using tabs to control porpoising instead of drive trim.
Also, the CG of a boat affects the handling, including it's tendency to porpoise. Consider the 22C that had over time at least 4 different powertrain options: big block/trans/TRS, big block/King Cobra, big block/Bravo I, small block/Alpha. I'd bet that all else being equal, the weight and placement of the different setups each cause a unique "porpoising profile".
OK, there's a start. How do the different setups handle?
Thanks,
Michael
When someone describes a boat as porpoising, they're describing the rythmic rise and fall of the bow not because of waves, but because of boat trim. While it's obviously most observable on calm & flat water, it's possible to get a boat to porpoise even in a chop or small sea. Typically a boat will porpoise when the drive (or outboard) is trimmed too high for that given power level to sustain the angle of attack.
That's part of the rub. Say at 50% throttle and "neutral" trim a boat might porpoise. Increase throttle and porpoising goes away. Of course trimming the drive in it goes away also.
I believe most conventional theory proposes that maximum speed for a given throttle setting is obtained by trimming "out" until the boat porpoises and then trimming in a hair to stop porpoising.
Now to increase complexity, you throw in trim tabs. Here's where I defer to someone with more experience using tabs to control porpoising instead of drive trim.
Also, the CG of a boat affects the handling, including it's tendency to porpoise. Consider the 22C that had over time at least 4 different powertrain options: big block/trans/TRS, big block/King Cobra, big block/Bravo I, small block/Alpha. I'd bet that all else being equal, the weight and placement of the different setups each cause a unique "porpoising profile".
OK, there's a start. How do the different setups handle?
Thanks,
Michael