03-06-2011, 12:48 PM #1
Hydrodynamics, porpoising, tabs/OPAS blockoffs ... ?
Something I have wondered about for awhile and cannot figure out ... particularly on the RXP. Once we get to 75 mph or more, we have to add good OPAS blockoffs (REs or Rivas) to stop porpoising ... yet when I look at pictures or video of the RXP at speed, it does not seem like much water (if any?) is really hitting the blockoffs. Plus they are angled up and away from the side of the hull and would not seem to be able to have much impact on ride attitude or porpoising.
So my question is, does anyone know how or why the REs/Riva blockoffs ACTUALLY impact the hull performance and eliminate porpoising? I can more understand how the tabs on a GPR hull can affect this, since they are more on the flat part of the hull and much more directly involved. On the RXPs in particular, I can't figure out HOW the blockoffs impact the ride and porpoising ... Anyone know or want to speculate?
03-06-2011, 01:37 PM #2
I did not know that RXP would porpoise at 75+ with opas. My RXPX with opas does not porpoise at 80-81. I thought the hulls where the same except for the intake tunnel.
03-06-2011, 02:01 PM #3
Good point ... most of us toss the OPAS parts very quickly ... the cheapo blockoffs seem OK until you hit 75+ mph, then with nothing in the void, they will porpoise. Not very many keep the OPAS, so there are not many people who have experience with a RXP/X that goes 75 - 80 mph with the stock OPAS. Its interesting that your ski does not porpoise with the stock OPAS at the speeds you are going.
03-06-2011, 02:10 PM #4
Stuffing the tunnel will start the bounce.
03-06-2011, 02:13 PM #5
sorry this is a stupid question.. but what is porpoising?
03-06-2011, 03:13 PM #6
in my findings, the spray of water alone that comes from the bottom of your hull has enough pressure to touch either the blockoffs or sponsons in the back of your hull to help keep the nose of your ski down or help lift the back of your ski keeping it from porpoiseing...... rember as long as your pump tunnel is in the water your ski is always going to spray water out each side of the hull, BUT the faster you go that same spray of water starts to come out the back of the ski and that's where the spray hits the back of the sponson, blockoff's, or hooks in the hull creating lift....... i feel seadoo put the hooks where they are because that's where all the pressure is? TAKE THE HOOKS OUT AND YOU GOT A MESS
the hooks, blockoff's and sponsone all create lift in the back of the ski. when these skis porpoise it's beacuse the front of the ski has more lift then the back of the ski.... so the front of your ski want's to clim higher but the back cant, so that is why i feel these ski's porpoise. also if you put too much angle in a ride plate you will also porpoise because you lose the lift in the back of the ski and add more lift to the front of the ski. pump pressure is not a problem till U hit 90+mph and that's called the HUCKEL BUCK.lol.........
03-06-2011, 04:13 PM #7
vid o609 showes a bad porpus i had inmy new hullli am working on...
it all has to do with lift..and balance ..having enough lift to either get up and stay up or not come up at all..i have been playing with my ride plate to crate more liftso it can get up and stay up..and i am getting it..
i agree the block off aren really in the water at speed..my sponsons are 5 to 6 inches out of the water..
vid 24 is after a few ride plate adjustments dont quite have it out but close..
03-06-2011, 06:06 PM #8
03-06-2011, 06:44 PM #9
Very good points. I do know for a fact that as speed increases the roller wave in front of the intake also increases causing the back of the ski to ride the roller thus causing it to pitch forward. A lot of the hull spray is from the tunnel not processing the amount of water required. You are correct it not in the buck zone but the tunnel is not processing all the water and it is throwing it forward. 75 is common for unmodified hulls and 82 is common for modified hulls. Every thing plays a role in where the craft becomes unsettled from power to prop to sponsons and seat load etc..
03-06-2011, 09:36 PM #10
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