Page 1 of 2 1 2 LastLast
Results 1 to 10 of 18
  1. #1
    Vern's Avatar
    Join Date
    May 2005
    Location
    Grand Haven, MI
    Posts
    3,594
    +1
    128

    Question 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?


  2. #2
    Mod less, ride more! troyheb's Avatar
    Join Date
    Jul 2007
    Location
    Nederland, Texas
    Posts
    8,725
    +1
    1,267
    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.

  3. #3
    Vern's Avatar
    Join Date
    May 2005
    Location
    Grand Haven, MI
    Posts
    3,594
    +1
    128
    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.

  4. #4
    Moderator beerdart's Avatar
    Join Date
    Aug 2006
    Location
    CT
    Posts
    25,929
    +1
    1,407
    Stuffing the tunnel will start the bounce.

  5. #5
    MacDaddy_rxpx's Avatar
    Join Date
    Jun 2009
    Location
    Michigan
    Posts
    220
    +1
    1
    sorry this is a stupid question.. but what is porpoising?

  6. #6
    SPEED COMES 1/10 @ A TIME SpeedFreeks Racing's Avatar
    Join Date
    Jun 2007
    Location
    BROOKLYN N.Y.
    Posts
    407
    +1
    1
    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.........

  7. #7
    home of the dislexic fingers breon's Avatar
    Join Date
    Aug 2007
    Location
    port st lucie fl
    Posts
    3,471
    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..



  8. #8
    5.0. alex_baker202's Avatar
    Join Date
    Aug 2007
    Location
    Lebanon, Missouri, United States
    Posts
    1,027
    +1
    24
    Quote Originally Posted by MacDaddy_rxpx View Post
    sorry this is a stupid question.. but what is porpoising?
    It's when at higher speeds the front on your ski seems to bounce up and town. Some can be trimmed out but on flat water it can be hard to
    Control if it's bouncing. Plus you lose speed.

  9. #9
    Moderator beerdart's Avatar
    Join Date
    Aug 2006
    Location
    CT
    Posts
    25,929
    +1
    1,407
    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..

    Quote Originally Posted by SpeedFreeks Racing View Post
    in my findgs, the spray of water alone that comes from the bottom of your hull has enough pressure to touch either the block offs or sponsons of your hull to help keep the nose of your ski down or keep 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 ski, BUT the faster you go the spray that comes out the sides starts to come out the back of the ski and hits the back of the sponson, blockoff's, hooks and opas area....... 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.........

  10. #10
    home of the dislexic fingers breon's Avatar
    Join Date
    Aug 2007
    Location
    port st lucie fl
    Posts
    3,471
    Quote Originally Posted by SpeedFreeks Racing View Post
    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.........
    that is exaclty what i did to start the correction i took lift out of the front and added lift in the rear..still not quite.there but real close...

Page 1 of 2 1 2 LastLast

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Similar Threads

  1. Riva OPAS Blockoff RXT Handling?
    By RXTStephen in forum 4-Tec Performance
    Replies: 5
    Last Post: 12-28-2006, 09:50 AM
  2. riva opas blockoff's installed
    By BlaZinMJ in forum 4-Tec Performance
    Replies: 13
    Last Post: 12-27-2006, 07:45 PM
  3. RXT OPAS blockoffs
    By rxpblast in forum 4-Tec Performance
    Replies: 7
    Last Post: 08-01-2006, 06:31 AM
  4. pro series opas blockoffs
    By PAINKILLER in forum Sea Doo Open Discussion
    Replies: 13
    Last Post: 09-15-2005, 02:12 PM
  5. Homemade OPAS blockoffs
    By WildBill9000 in forum 4-Tec Performance
    Replies: 1
    Last Post: 09-08-2005, 04:43 PM

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •