Help designing a knockabout 3.4m cat dinghy with FREESHIP

Discussion in 'Boat Design' started by catdinghy, Sep 11, 2013.

  1. catdinghy
    Joined: Sep 2013
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    catdinghy Junior Member

    Heres the file - hope it works
    Transom cutout I would have done on the day, I wasnt going to worry about drawings - same for seats, splashwell foredeck etc

    Transo line has to be shown for a half model doesnt it?
    Obviously I would cut it as one, but it hasn't even shown up on the pattern sheet.
     

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  2. catdinghy
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    catdinghy Junior Member

    Gotcha
    The transom shows as a curved section on the bottom
    That's the odd bits.
    Fiddling with now

    And_They're_Gone
     
  3. lewisboats
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    lewisboats Obsessed Member

    The transom is easy... get everything lined up... really close in and it should straighten out. The other problem is as I suspected. You have a part that should be on another layer stuck on to the side layer. In this case it is the bottom of your transom... which also could be it's problem. attached is a picture of the offending chunk. you need to highlight this and assign it to the transom layer. Do this by unchecking everything but the side panels, turning on interior edges and highlighting the part of the transom that is assigned to the side panel. Go to the drop box for the layers and select transom and it will reassign that part to the transom where it belongs.

    ETA: Try as I might I can't seem to get the transom to unify. Probably because I have been up since 4pm yesterday.
     

    Attached Files:

  4. catdinghy
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    catdinghy Junior Member

    Nup, I've given it a go as well, zoomed right in and checked everything and get the same.
    But at least those extra plates have vanished.

    Now to figure out how to get a frame for stations
    180
    1080
    2160
    3240
     
  5. philSweet
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    philSweet Senior Member

    frames-

    click calculations >> intersections

    A window with the stations will appear. You can access waterlines and buttocks here as well.

    Click trashcan to delete all the current stations or you can select individual stations to delete or modify. Add the locations you want.

    You can now print a set of offsets from the file >> export >> offsets menu.
     
  6. lewisboats
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    lewisboats Obsessed Member

    Clear all your stations and then place stations at these points. Export the offsets and where it says "Knuckle" is where each panel joins another at that point. Remember that the surface has no thickness so you will have to decide whether what you see is the inside of the skin surface or the outside. If it is the outside then you must subtract the planned skin thickness (eg: 6mm for 6mm ply) from each side and bottom of the station offsets. Better to draw it out according to the offsets then subtract the thickness on the drawing... less chance of error.
     
  7. catdinghy
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    catdinghy Junior Member

    Thanks for the tips above Phil and Steve, got that sorted now.

    Tell me, does anyone know if this sort of hull would make it ride/handle better than the earlier version?

    The flat section where the arrow points has a slight kick down in the thought that it may aid with getting on the plane a bit faster, a bit like trim tabs or the alloy foil I had on the outboard of the last dinghy.
     

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  8. lewisboats
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    lewisboats Obsessed Member

    It's called a "hook". I expect that there isn't enough planing surface area to increase lift but there is a lot more drag surface area. I would expect this to adversely affect your performance. It would also add to the tendency to trip which the Vee reduces from a flat bottom. You can add a bit of hook to your original hull: 3/8"-1/2" or so over the last 3-4' would be sufficient.
     
  9. catdinghy
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    catdinghy Junior Member

    On the 4m tinny with a 20hp motor I just sold it had an alloy "permatrim" style foil that I made for the motor.
    [​IMG]
    Surface area was similar to what is shown in the drawing above and it made a HUGE difference to performance allowing the boat to get on the plane faster, remain on the plane at lower speeds and it also increased top end speed

    Trip?
    What do you mean trip? Like pitchpole?
     
  10. lewisboats
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    lewisboats Obsessed Member

    There is no way that the surface area of that little fin, which works mostly to help keep the prop from aerating is the same as the surface area of those things along the chines of the boat. Remember that you have to drag all that extra along for the ride until you get up on maximum plane. Tripping is a feature of flat bottomed hard chine boats that happens when you turn too quickly and the edge of the chine digs into the water and the boat barrel rolls... overpowered Jon boats do it on a regular basis. What your chine modifications effectively do is create flat bottom as seen from the edge... the same edge that the water sees in a turn. Instead of leaning into the turn like a Vee bottom it will want to dig and trip.
     
  11. catdinghy
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    catdinghy Junior Member

    That little fin was about 300 long and 300 wide
    The "Hook" on those strakes is about 300 x 150 x 2 (one each side)

    The motor on the boat never had an aeration problem, but it did have an issue maintaining plane at low speed with one person and 50L of fuel onboard.
    The fin did sort this problem out and did it very well considering it was a bit of scrap plate and an hours work.
    My 8x4 flat bottomed punt doesnt do this but it also only has a 2hp engine

    OK, so if it's not overpowered or if not turning at full noise (what sort of idiot would) its shouldn't be a problem.
     
  12. lewisboats
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    lewisboats Obsessed Member

    I'm talking about the whole strake... not just the hook. The whole thing is in the water and creates drag.
     

  13. Mr Efficiency
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    Mr Efficiency Senior Member

    kiss !!!!!!
     
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