GOP Home Build

Discussion in 'Boat Design' started by schultzfactor, Oct 25, 2013.

  1. Mr Efficiency
    Joined: Oct 2010
    Posts: 6,037
    Likes: 121, Points: 63, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    Pictures of the boat out of the water, particularly showing the engine area, might help. What kind of props are you running, stainless or aluminium ? The latter will give a lot of cavitation and/or ventilation problems on cats. The smoothness of the hull is not a factor for this. Even if your cav plate is running level with the bottom, and there is no appendage causing turbulence, alloy props usually let go, particularly in turns.

    EDIT: I just looked at the pics in post #173. Seems to be aluminium props. Unfortunately testing with stainless won't be cheap. Some people have resorted to five-blade stainless on some cats. Three-bladed worked well for me though. Check you are getting the revs into the recommended max rpm range before choosing any props, you may need to come down in pitch.
     
  2. schultzfactor
    Joined: Oct 2013
    Posts: 106
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Rotorua New Zealand

    schultzfactor Senior Member

    They are Aluminium Props. I want to go to four or five blades, stainless would be good, but as you say expensive. Ill get some pics next time I take her down. I don't really care for speed, just want a reasonable cruise, and lots of bite. Can the ventilation be that noticeable with rubbish props?
     
  3. Mr Efficiency
    Joined: Oct 2010
    Posts: 6,037
    Likes: 121, Points: 63, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    Yes, aluminium is far from ideal where there is a tendency for the prop to lose grip. Is it happening in turns only ? If it is, I'd say stainless will be the answer. Don't spend money on more alloy. Stainless gives you better turning, more trim range. The difference is surprising.
     
  4. schultzfactor
    Joined: Oct 2013
    Posts: 106
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Rotorua New Zealand

    schultzfactor Senior Member

    Ok will do. I'll need to start calculating pitch/diameter vs revs, and that's going to be hard if I keep getting Ventilation.
     
  5. Mr Efficiency
    Joined: Oct 2010
    Posts: 6,037
    Likes: 121, Points: 63, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    To re-iterate, the main thing is when this problem occurs, is it happening in straight-line running, once on plane, or only in turns, no matter if they are gentle turns ?
     
  6. schultzfactor
    Joined: Oct 2013
    Posts: 106
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Rotorua New Zealand

    schultzfactor Senior Member

    Straight line when I throttle up too much. At low speeds no issue, as soon as it comes on the plane seems to lose bite and then start to over rev and lose speed. On one motor no cavitation. In turns cavitates at the same rate as throttle up.
     
  7. Mr Efficiency
    Joined: Oct 2010
    Posts: 6,037
    Likes: 121, Points: 63, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    You mean only one of the engines does this ? If that is so, you may have a slipping prop hub, which is easily repaired by a marine shop. To test that, clearly mark both the rubber hub and the propellor boss at the same spot on the circumference, if after use the marks don't line up, it is slipping under load. Alternatively, that prop may have blade damage. Inspect for that.
     
  8. schultzfactor
    Joined: Oct 2013
    Posts: 106
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Rotorua New Zealand

    schultzfactor Senior Member

    Nah, I mean it does not cavitate when under power with one motor only.
     
  9. Mr Efficiency
    Joined: Oct 2010
    Posts: 6,037
    Likes: 121, Points: 63, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    I'd have thought it would not plane on one motor.
     

  10. schultzfactor
    Joined: Oct 2013
    Posts: 106
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Rotorua New Zealand

    schultzfactor Senior Member

    Correct, it's not planing on one motor.
     
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.