What are the implications of reverse chines that are rounded rather than sharp?

Discussion in 'Boat Design' started by XJ9, Mar 22, 2012.

  1. XJ9
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    XJ9 Junior Member

    The boat that I am currently building has reverse chines. It is constructed from fibreglass over a male mold. The chines as "molded" have about 100mm flats with the edges rounded with about a 10mm radius. I have read though a heap of posts here and googled a fair bit looking for some information on chine shapes/shaping, but I haven't yet found what I'm looking for. How much water is likely to ride up the sides of the boat if the chines are left like they are? Is the drag and spray likely to be significant? Are there any advantages to reverse chines with gently rounded edges rather than sharp ones?
     

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  2. daiquiri
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    daiquiri Engineering and Design

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  3. XJ9
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    XJ9 Junior Member

    Thanks Daiquiri, I will have a read of that thread. I'm sure that I have read parts of it before, but probably with another question in my mind. The trailing edges of the transom have already been sharpened up a bit to about a 2mm radius curve. I kept a small curve to combat chipping and to aid painting. I will have to read your linked thread a few more times to see if I can come to some conclusions.

    The picture is of the starboard chine taken from the rear (facing the transom). As you can see, the boat has fairly plumb sides at the rear largely due to the fact that the hull has been extended at the transom. The flare increases considerably trowards the bow. There are some more photos from different angles on this thread: http://www.boatdesign.net/forums/fi...-extension-transom-stringer-design-42129.html
     
  4. XJ9
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    XJ9 Junior Member

    Daiquiri, I read and re-read the discussion you linked in your post. I still haven't been able to determine the net effect of leaving the chines on my boat rounded. PAR suggested in this thread http://www.boatdesign.net/forums/hydrodynamics-aerodynamics/curved-chines-transom-edge-37521.html that rounded chines may have minimal effect on the speed of a 12' boat. How does this translate to a 21' boat? I am mainly concerned with good handling rather than absolute top speed. The designer of the boat was not too concerned about the shape of the chines as is, but having said that all of his other designs that I have seen have sharp chines.
     
  5. daiquiri
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    daiquiri Engineering and Design

    Naaah, that's an overkill. :)
    Unless you are seeking a racing performance for your boat, 6-8 mm would have been more than enough, and that's roughly a radius which majority of production boats have at chine edges (a couple of mm more or less).

    You have to consider this issue from different conflicting aspects:
    - hydrodynamics wants an ideal chine to be razor-sharp in order to have a clear line of separation between air and water phase, in order to get a minimum drag and maximum lift from the chines, strakes, etc. and to minimize the possibility of water rising up the hull walls.
    - building process wants to round the chine as much as possible, because glass fibers cannot follow too-tight curvatures, so fairing a chine into a small radius becomes more labor-intensive.
    - painting process prefers the well-rounded chines too, to minimize the possibility of paint peeling off from too-tight edges
    - a structure engineer would like to see rounded chines in order to avoid stress-concentration points, but not too round because angles do help creating rigid structures.
    - the owner wants a rugged boat, which can be beached from time to time, and too thin chine edges are rather vulnerable to damage.

    If you take a look at the existing boats, you'll find out that the industry has arrived to something around 6-8 mm (finished, faired and painted) as an acceptable compromise between these requirements. The radius to be applied to your boat will depend on which of the above requirements are most important to you.

    Cheers
     
  6. XJ9
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    XJ9 Junior Member

    Nope, just want to end up with a nice centre console fishing boat that doesn't spit out the crew or give them a shower at every opportunity. The boat is a bit experimental, especially with the built in hook in the last 5 feet, so anything could happen!

    It's hard to know when to stop with obsessive fairing. The chines are already thickened and would satisfy most engineers in regard to spreading out the stress points. The main criteria, as you suggest, is to minimise the chance of damage to the chines and to keep the water where it should be. Some posts I have read suggested that nice sharp reverse chines might increase spray problems. I would rather go a bit slower than have to wear wet weather gear on every outing.

    I have seen other Solocraft boats with chines that would be closer to 6mm diameter (rather than radius), say something like diameter of a pencil or less, but this may well be overkill for an open fishing boat. I suspect that with a bit of cleaning up, the chines probably are fine as is even though they may be more than 6-8mm.
     
  7. XJ9
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    XJ9 Junior Member

    I have attached as picture for reference purposes to get an idea of the roundness of the chines in question. The rail at the lower right is 3/4 inch diameter pipe.
     

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    Last edited: Mar 28, 2012
  8. tom28571
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    tom28571 Senior Member

    Sharp chines cause increased spray problems? No way. Well sharp chines will kick spray further outboard and a cross wind could blow that back in the boat. That is much better than having the water climb the sides and deposit itself in the boat, which the chines you have now will do. Many boats have spray rails well above the aft chines to prevent this problem.

    I see no reason other than building convenience to have such rounded chines. It is not difficult to work glass on a chine to the diameter of a pencil. 6mm is the geatest radius I would have on a planing boat and have never used that much.
     

  9. XJ9
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    XJ9 Junior Member

    Thanks Tom. Since I have no formal engineering/nautical background and limited experience with planing hulls, that's the sort of information I wanted. My feeling has always been to want to add some points to the chines as has been done wiith the transom. Water doesn't always act like air though, so I will defer to those with more practical experience. The transom has been faired to a sharp edge and then rounded over ever so slightly. I don't want to fix something that isn't broke, given that I'm not after a record breaker, but now is the time to "fix" these potential problems before the boat is turned over. So... the current plan will be to apply some high density epoxy filler to the chines to achieve a better shape that will deflect the water away.

    I also thought to do a similar thing to the box keel and put a sharpish termination on that too. Currently it has been sort of faired into the hull extension that may be a source of drag. It's not quite a stepped hull though and Henry didn't seem to care too much. Who knows, we can't ask him now.

    By the way, Tom, I have admired your designs since I first saw them a few years ago. They are very pretty and obviously very practical :)
     
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