Cavitation Plates

[BrendellaJet]

Been talking with Jeff Bennett about a blueprint job on my boat. Sent him some pics, and he noted that the bottom of the boat looks to be a Southwind copy. The top is a bubbledeck-looks just like a rogers. He said that at high speeds these boats are known to porpoise, and that this has only been solved successfully using cav. plates. Anyone out there know anything about southwinds? Just looking to confirm what I was told...
THanks.

[UBFJ #454]

Jeff Benett is one of the most knowledgable and experienced hull men in the business ... He definately knows what he is talking about.

[Squirtcha?]

Brendella,
Did Jeff happen to mention what he considers to be higher speeds?
I'm just curious because I've got a Kachina which is supposed to be a splash of the Southwind too. I'm currently running just a bit over 80 right now with the normal gofaster goodies (setback, shoe, rideplate, loader etc.) and don't have any porpoise problems. I have a couple buddies with the same hulls (Kachinas) that are running in the low to mid 70's and they're o.k. too.
I've heard they have some weird handling characteristics after 85 but I can't go that fast yet.
One guy I know (FoMoCo) that's run his to 85+ (also a Kachina) has reported porpoise problems and I don't know if he's ironed it out yet or not.
I was comtemplating running a bigger nitrous shot which would push me to the 85mph mark (or thereabouts), but now I'm wondering if I should even mess with it.
Oh yeah, mine's got a hook. It's pretty bad, but only on one side and I'm not sure how much it's affecting top speed or ride (porpoise/no porpoise). It's on the second strake out from the center, right side about 24" forward of the transom.

[sanger mike]

brendella could you post a pic of the back of your boat for us.

[BrendellaJet]

Squirtcha- He mentioned mid 80s as the point at which handling begins to suffer. Im building a 496 with "dyno proven power", so Im thinking after all of the mods I should run no less than mid 80s.
Also, check out the realjetboat forums, Jack posted some interesting notes on the southwind bottoms. I read that with some work to the 2nd strake, they were able to get this hull into the mid-high 90s. The only hook my bottom has looks like it is built in, both sides are identical, AND the running surface is straight as an arrow. For pump work, I had jack rebuild the 12WJ Jacuzzi to his stage 3 specs, including shoe, ride plate and a one of a kind loader.
River Dave- Idont know, I thought those things were for prop boats. I need to talk with Mr Bennett some more to see what he thinks. I thought a diverter would solve the problem...?
Sanger Mike-Your boat looks incredible. Ive seen it at MPD several times-very nice!. Here are some various shots of the rear-let me know what your thoughts are? If you need a different shot, let me know.
http://www.***boat.com/image_center/data/520/212100_0063-med.JPG
http://www.***boat.com/image_center/data/500/212100_0055-med.JPG
http://www.***boat.com/image_center/data/500/212100_0078-med.JPG
http://www.***boat.com/image_center/data/500/212100_0091a-med.jpg
Thanks for the replies!http://www.***boat.com/image_center/data/500/212100_0080-med.JPG

[superdave013]

a true foot controlled cav plate set up is gonna cost ya. :eek: :eek:
But it would have to be bitchen to be able to give it a lil plate in some of the rough stuff.

[Rexone]

I've seen several jets with plates, both fixed and adjustable like vdrives, over the years. As a general statement they work good. Any time you effectively extend the bottom at the rear corners you're going to stabilize the boat from chine walk and usually control porpoising. If those extensions are adjustable all the better because you then can fine tune the ride attitude and the subsequent effect on speed and proper trim. Usually plates will increase speed regardless over a porpoising boat without plates if the porpoising is eliminated. Porpoising scrubbs major speed off. Every hull is different though and your best bet is to talk with someone whos very familiar with the particular hull you have before you consider going to the expense of adding adjustable plates. It may also affect where the nozzle (point of thrust) needs to be since with plates the boat bottom effectively becomes longer.
Usually it's a split shaft setup as the pump is in the way of a full transom shaft like a flatbottom would have. Usually the cross-shaft has to be inside the boat, 2 short control shafts externally with 2 thru hulls. Very similar to the late model setups they're running on some of the sterndrive ski race boats rigged by GT and Prime Marine where they use 2 short shafts because of the sterndrive in the way. And this is all one-off stuff for the most part. It won't be cheap no matter who does it.

[BrendellaJet]

You mean I have to spend more money on my boat?! Damnit!:D
The wife is not going to want to hear that! Thanks for the comments.
Anyone got any pics of a jetboat with this setup?

[Mohavekid]

I've been thinking of putting some plates on my boat as well.
It's got a nasty porpoise at mid range speeds which is a pain in the ass.
Here's a pic of a jet with some killer adjustable plates.
http://www.***boat.com/image_center/data/521/1177SkiRaceJet-med.jpg

[fourspeednup]

Hey brendellajet, what year is that hull? Mine's a '76. :cool:
Not the greatest pic, but it's all I have:
http://www.***boat.com/image_center/data/500/1154my_boat-med.jpg

[BrendellaJet]

fourspeed- its a 77 model. What make is yours? Does your hull look like mine from the bottom? Whats your top speed?

[fourspeednup]

It's a brendella, sorry a little unclear in the first post.
Looks the exact same top and bottom
ran easy 62 mph running next to a boat w/ gps
-first motor I built myself, still had a little pedal left--olds 455

[superdave013]

Originally posted by Mohavekid
I've been thinking of putting some plates on my boat as well.
It's got a nasty porpoise at mid range speeds which is a pain in the ass.
Here's a pic of a jet with some killer adjustable plates.
http://www.***boat.com/image_center/data/521/1177SkiRaceJet-med.jpg
Photo by yours truly :D

[BrendellaJet]

Okay, well I think I will wait until I have the boat running again. I guess Ill just have to keep an eye on it initially given the # of changes that are being made. If the cav plates are necesary then I'll do it then. Would it be possible to salvage parts from another system to save cost? How much to build this set up with quality parts?

[Rexone]

Most jet porpoising can be solved with a Place Diverter or wedges and proper setup. I'd look at cav plates, especially full adjustable ones as a last resort if the normal methods fail to control it. It would have to be extreme I would think to not respond to the above.
:)

[Mohavekid]

Another option is to add some weight for ballast in the bow of your boat to keep the porpoising in check.
I currently use a combination of extra weight and the place diverter to keep the porpoises away.

[Kindsvater Flat]

This Rogers dragstar was setup with cav plates back in the day. Check out Rick's pictures. There are some detailed ones of it. I also believe the nitrous kit thats on it will be for sale, not sure yet. He has since switched to running nitro in it.
www.do-it-yourself-dragboat.com/ (http://www.do-it-yourself-dragboat.com/)

[AZKC]

Can you get the same effect from hydraulic ride plates that a cav plate gives? Me being Panther impaired:D I have fewer options on jet mods.
KC

[PLACECRAFT20]

I dont know how plates will work on a smaller boat but, I have a 20' Placecraft that had a terrible porpoise. I installed the plates and it cured the problem as well as added about 3mph. The plates are very sinsative to the setting on the turnbuckles. i keep them set for me, my wife and a thirty pack. if one person extra gets in or out of the boat it wants to porpoise slightly. These are so far the best investment i have made on the boat.http://www.***boat.com/image_center/data/521/252tabs-med.JPG

[Bense468]

Thats good to hear. Hey place I just got back from Glamis. The weekend was bitchen and the banshee was screamin.