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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

BigGrizzly
04-13-2007, 06:12 PM
You are mostly correct except the unique part. They are not that unique.

Carl C
04-13-2007, 09:05 PM
From what I've seen most Classics porpoise at low speed. It's no big deal and is part of the uniqueness of these boats. It's easy to control with tabs and trim but most just avoid that speed. It's also fun to get it bouncing good then drop the drive and roll on the gas and it pulls right out of it.:) Awesome boats.:yes:

RedDog
04-13-2007, 10:05 PM
The prop is also another variable according to what I have read. Board consensus has been that the Turbo I eliminates porpoising on the 22. I have not found that to be true on mine though. Mid-range speeds require down tab. Last year I dinged up my Turbo I pretty bad. Over the winter I had it repaired and had some additional cup added. That is supposed to add additional bow lift - less porpoising. Testing to confirm is on hold until the lake waters rise and dogwood winter ends.

p729lws
04-13-2007, 10:57 PM
Some day I'm going to drive my 22 at a low speed just to see what the hell you guys are talking about:wink:

Dan

gcarter
04-14-2007, 06:28 AM
Then there's the issue between the "OLDER" vs. "NEWER" boats (post '92???) and the effect of the lower "X", different engine mounts, moved stringer location that exists.
This first seemed to come to light at Lake Powell last summer when there was a good collection of both.
The older boats exhibited little or no porpoising, the newer boats were dragging tabs most if not all the time.
It caused a lively discussion.

Carl C
04-14-2007, 07:01 AM
I use tabs only to correct listing and when running at speed on 3-4 foot chop. Porpoising is not a problem for me. It's easy to control with trim or a little more throttle. Like I said, sometimes it is fun to get her bouncing at low speed and people ask me to do it. I'd be disappointed if my boat didn't porpoise. But don't listen to me, I'm just a crazy speed demon. 2005 22C w/496 HO 25p Mirage +. 73-75 mph. So remember; sometimes when you see a Donzi bouncing along at low speed it is on "porpoise"!:yes:

Pismo
04-14-2007, 08:49 AM
I wonder how a TRS 22C handles this. No negative trim but engine further forward and more weight. I need full down trim, so negative trim, to stop the bouncing at 20-2500rpm. Stops itself by 30-3500rpm.

3rdDonzi
04-14-2007, 09:47 AM
I had an '87 Z21 with 454 Mag. Alpha that porpoised terribly and was not easy, actually it was impossible to accellerate through like a Classic. I couldn't stand having to plow and lose performance to stop it or almost as bad, inducing drag with the tabs. A local guru put me on to a "High Five" five bladed prop popular on the hot rod bass boats, saying that it would give "lift" to the back of the boat.
Whatever it did it improved greatly and was noticably smoother. Also much quicker to get on plane at the expense of no more loss of top end than tabs or downtrimmed outdrive.
On my 18 Classic, it wil porpoise in a very small range and with an increase of only one or two hundred RPM it stops. Not enough to be an issue.
Corky