Why are my rudders so ineffective ? (pwc converted to propeller)

Discussion in 'Boat Design' started by avi8r, Feb 18, 2008.

  1. avi8r
    Joined: Jan 2008
    Posts: 56
    Likes: 5, Points: 0, Legacy Rep: 77
    Location: Greenville,nc

    avi8r Junior Member

    I am moderately happy with the setup right now, and on smooth water it handles perfectly well, but 3 areas that need to be improved upon ( and as always... suggestions welcome)

    1 When leaving water and re-entering (boat wakes or rough inlet waves) the re-entry is unsettling and darty. I am thinking that the rudder is applying force and being this ski is so small, that it being on left side is causing the weird feeling. THis may not bother me as much after more riding time, but having come off this ski at 96mph, I am apprehensive

    2 At idle speeds, It makes large circles when turning. Not bad, its acceptable, but certainly doesnt spin on itself like it did when was a jet. Would like to have more rudder authority at idle

    3 At high speeds , the rudder is too sensitive. The feeling is "twitchy" any little movement has the ski changing direction abruptly. This is worst issue of the 3 to me.

    Heres the effect I think a center rudder behind the prop address my issues.

    1 I've given it some thought and think a rear rudder may help all 3 areas?
    certainly it seems a rear rudder would make the ski more balanced entering and exiting the water as long as it didnt cause nose to stuff :eek:

    2 at idle speeds it seems that a rudder mounted 3-4 inches behind prop would direct thrust and give it more authority at idle/ slow speeds

    3 at high speeds, this surface drive would be grabbing large amounts of air and stuffing it into the water just infront of the rudder. I am thinking the arearated water would cause the rudder to loose much of its effectiveness and reduce drag.


    Do you guys agree or disagree with my reasoning on this ? What potential problems could I be creating or benefits reaped? How far back from prop should leading edge of rudder be? 4inches?

    Yesterday I got into PRODUCTION MODE and was in the shop from 630 until 11pm last night working on the ski ( stopped for dinner only)

    I've addded a tracking skeg under bottom where pump inlet was and further up right behind keel added additional piece to legnthen for additional directional stability. I am not versed in boat design, just try what intuitively sounds good to my head.
    I pulled out the welder and started working on this making a bracket to move rudder behind the propeller. It came out really well with only minor cutting on my ski needed.

    [​IMG]

    [​IMG]

    [​IMG]
     
  2. powerabout
    Joined: Nov 2007
    Posts: 2,944
    Likes: 67, Points: 48, Legacy Rep: 719
    Location: Melbourne/Singapore/Italy

    powerabout Senior Member

    Avi8tr
    That middle strake that stops foward either needs to be removed or run the full length.
    The water will never follow the fairing behind it but it will probably try and go from side to side causing a strange feeling especially if it is just where the hull is coming out of the water.
    Also any appendage hanging down like the fin and the rudder will be creating lift at high speed. (They need to be as thin as possible but when the bronze is as thin as it could be it may not be strong enough)
    These could also be causing strange handling.
    The rudder you need for high sped is not going to work at low speed so you might be on the right track by trying to get something in the water at low speed that is out of the water at high speed.
    Try to get a video of you running straight at the camera to see how much crabbing the boat is doing, i.e. going through the water sideways due to the prop torque.
    Now think of that little tab going through the water sideways and think of that as a centerboard in a dinghy giving lift sideways!
    Hence we get back to moving the drive around to get the hull to track straight -ish without having to resort to adding anything else that could have a negative effect on handling.
    Moving the rudder further out ( to the edge of the hull) also help to counter the prop torque.
    Keep up the great work
     
  3. avi8r
    Joined: Jan 2008
    Posts: 56
    Likes: 5, Points: 0, Legacy Rep: 77
    Location: Greenville,nc

    avi8r Junior Member


    Great constructive criticism post. I was reading it thinking.. dern, all that hard work and I did it all wrong. So much for my intuitive thinking. :eek:

    The way you explained why it wouldnt be as effective as I hoped made perfect sense to me :idea: (thanks) Especially about the keel addition. ( the water wouldnt follow around to it)

    I will get the video of it running straight at camera (for tracking) and post. Ive never seen how it runs other then side view videos.

    One problem I was having yesterday.. I changed my bottom to feed more water to prop, and now my motor isnt torquey enough to spin it. ( wont climb on plane ) My engine has 200hp at 7300rpm but below 5000 the torque is so weak, prop selection has been a nightmare.

    Today i am trying it out with nitrous oxide assist where I am adding 30ft torque at low end. That should allow me to jump on plane and do more testing of different prop designs.

    below are some that I have to test

    [​IMG]
     
  4. powerabout
    Joined: Nov 2007
    Posts: 2,944
    Likes: 67, Points: 48, Legacy Rep: 719
    Location: Melbourne/Singapore/Italy

    powerabout Senior Member

    You mentioned you had 6% prop slip, that sounds fine to me?

    I would have thought that adding more water at 6% slip will only create more drag, but your prop guy should put you right there.

    When you have something in the water you must be clear that either the water will follow it or it must break away as in between things get very scary.

    PS I can only see 2 props in the photo that would be safe at 100mph and the 3 blade will be faster than the 4 blade BUT the 4 blade will probably be better in the mid range and maybe getting on the plane?
    The 2 bladers are for hydro's and the bronze ones for ski boats
     
  5. avi8r
    Joined: Jan 2008
    Posts: 56
    Likes: 5, Points: 0, Legacy Rep: 77
    Location: Greenville,nc

    avi8r Junior Member

    Wow, you really know your stuff and your 100% correct.

    The 3 and 4 blade props are f1 roundy round boat racing props. They ventilated so bad on takeoff and once on plane would push the nose down ( causing very poor midrange until about 60mph) then the boat would lift nose up and it ran good on top end.
    I was asking in other forums what I needed and was offered the advice that in the 10in diameter range, if I switched my shafts from 15spline merc shaft to 1in taper shaft with keyway, there would be lots more propellers to choose from and I could buy used props rather to dial in rather then new ones.

    My prop shaft is 2inches above the bottom of boat so def have to use surface piercing propellers.


    So I switched over to shaft and contacted Dallas at Menkens propellers as he seemed to be one of the best prop builders for drag boats. Since I needed a prop to get me out of gate, I went with him and paid 800 for him to custom build me a propeller for this application. He was excited about the project and said if I paid for 1 prop, he would rework it or make a new ones until we found one that worked best.

    The first one was the 2 blade 8 3/4 and 16 pitch it ventilated so badly I couldnt get on plane. So I sent it back to him and he put more bite into it and also sent a second one 9 3/4 2 blade 16 pitch. Yesterday I tested both and the 83/4 still ventilated too much to get on blade ( not enough water to prop) and the 93/4 bit so hard I couldnt get the motor in rpm range to make enough hp to get on plane.

    Since round trips to arizona and back (getting props to menkens) take a few weeks, I purchased the 2 bronze ski props off ebay cheap. They were both 12x12 and both would climb on plane ( due to very 12 pitch, didnt need to ventilate) I shorted one to 10inches with hand grinder and it actually climbs on plane better then any other props. I dont know much about shaping them, so only rode once with the bronze props before props came back from menkens a couple days ago.

    I had an adapter made from I can mount my original props on the 1in taper shaft. It was a propeller shaft cut and lathed with drift pin to keep from turning.

    So now all my efforts have been to feed more water to the prop for climbing on plane. Today I reinstalled my nitrous system on it to help me get on plane with any propeller. Its frustrating to drive 45miles to water and not even be able to get on plane. The addition of extra power out of gate should help eliminate that.


    Sooo my question to you.... why do you feel like only the 3 and 4blade f1 are best for 100mph speeds? Why would you feel like the 2blades wouldnt?

    Please continue with any suggestions :D
     
    1 person likes this.
  6. powerabout
    Joined: Nov 2007
    Posts: 2,944
    Likes: 67, Points: 48, Legacy Rep: 719
    Location: Melbourne/Singapore/Italy

    powerabout Senior Member

    avi8r
    The spoon blades will do 100mph but as outboards stopped using this design in the 60's and they continued on inboard hydro's and drag boats ( of which Menkins business is) tells me they are not what you are looking for.
    I dont have experience with hydros and drag boats so I can't really say what properties those propellors are designer for.
    2 blades are technically fast but the vibration wreaks havoc with everything ( assuming there is enough blade area to get on the plane)

    You also have a very small hull for the power hence when at the cutting edge there is no text book ..yet

    When racing outboards in a particular class we knew the prop we should be using and then kept altering the whole setup to get that particular prop to have the correct slip ratio and turn the rpm we were looking for only then would we start to tune the prop.
    Of course that was easy as there was a bench mark to work from.

    How about working on your ski to lighten it with the aim of getting on the plane more easily?

    Regards

    Powerabout
     

  7. SnappingTurtle
    Joined: May 2008
    Posts: 11
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Germany/Texas

    SnappingTurtle Snapping Turtle

    Well I suppose this is as good as any thread, to make my long awaited appearance.

    What a thread. What a project. Just read it all and can only say I wish I was back in the States. I would love to see your baby live. Great work!

    I also have to say that there is as to this point, and I have a lot of admiration for the group of posters here.

    avi8r you have done something, I "thought" about for a long time.

    Congratulations and continued success.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.