Stern Extension to Help Rowing

Discussion in 'Boat Design' started by Russ Kaiser, Jul 15, 2014.

  1. upchurchmr
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    upchurchmr Senior Member

    Russ,

    Don't use epoxy on the complete bonding surface, it will be a mess.
    The wire might go thru but it will be delayed and that will eat out the foam next to the epoxy seam.
    Just bond the foam with epoxy away from where you will be cutting. The foam really offers almost no strength, it just makes something solid to lay the fiberglass on.

    Looks good, but you will need big clamps or bolts thru the transom since the fairing will be lifted up by its flotation.

    IMHO, the skeg is not needed, you have a slab sided boat that will resist a lot of turning. At the least round the front end and taper off the back end or you will get a lot of water turbulence.
     
  2. Skyak
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    Skyak Senior Member

    Russ,
    when you get done you need to paint a cartoon fish head on the back of the aluminum hull like it swallowed a nice wooden boat. Then you can say "I was just rowing my nice wooden boat and this aluminum monster swallowed most of it, choked and died!"
     
  3. Russ Kaiser
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    Russ Kaiser Exuberant Amateur

    Since everyone thinks I shouldn't hot wire through epoxy I will probably grab a can of that 3M Spray glue to laminate the foam pieces to each other and just use epoxy to attach the foam block to the wood.
     
  4. Russ Kaiser
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    Russ Kaiser Exuberant Amateur

    What! And ruin the custom paint job I have now ;)
     
  5. Skyak
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    Skyak Senior Member

    Think of it as the only sensible way to transition from the brutish bruised patina of the aluminum to the smooth paint of your extension. Only comedy can bridge that gap.
     
  6. upchurchmr
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    upchurchmr Senior Member

    Russ,

    Spray the glue on a piece of scrap as a test.
    The really light weight store bought foam will melt with almost anything.

    It is really pretty easy to shape the foam with a sanding disk spinning pretty fast.
    Just brace your hands so if you bite too far into the disk it does not get pulled away thru all your good work.

    My wife used wood glue (titebond 2) to build a table from foam. I just can't remember if she hot wired thru the glue line.

    Titebond 2 will bond the foam to the plywood just fine. No way the foam will resist enough to require epoxy. Of course the epoxy works if you already have extra.

    I can't imagine what Skyak is talking about on that fine looking boat. ;)
     
  7. Russ Kaiser
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    Russ Kaiser Exuberant Amateur

    Foam Update

    I purchased a sheet of 3/4 inch blue foam insulation from Lowes and laminated 7 strips, 4 feet long to form a long block. Cut in half it was going to fill in the corner contour of my spoiler on each side. I used a Locktite spray adhesive spraying both sides of each mating surface and then I let it sit overnight.

    Observations:

    1. The Locktite Spray Adhesive was a bust, I bought the lowest grip version because it gave a bit of time for alignment. Even after setting over night, it started to delaminate when I handled it. I should have listened to you guys and used something else. :mad:

    2. Considering the size of block that I needed, Russ the cheapskate should have bought the thicker sheet to minimize the number of lams. Lesson learned. :!:

    I still have over half a sheet of foam so I am going to redo the foam block. I do have a sizeable jug of Titebond III. If someone KNOWS that it can be used to laminate blue foam sheeting I will use that. If not, I have plenty of epoxy which I know from a previous project works well but takes over a day to cure.

    The good news is that I took the botched block and figured out how I was going to shape it. I forgot I own a band saw, Duh! There it sets in the corner of my shop, perfectly capable of slicing through up to 11 inches of foam like butter. :idea:

    I did some test cuts on the botched foam block and the band saw worked well. I just set the block in the corner of my frame and traced the two edges that needed to be cut with a sharpie. Then I took it over to the band saw and cut an eighth of an inch beyond the line. The rest of the contouring I can do with a sander after the pre-shaped block is glued in. I'm kind of elated over figured that out. :D

    The plus side is that I won't have to deal with a hot knife and worry about how it will go through the glue seams, no matter what glue I end up using.

    I will post a couple more photos in a couple of days when the foam is done and before I start skinning with fiberglass.

    Russ
     
  8. Squidly-Diddly
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    Squidly-Diddly Senior Member


    First off, you don't have a Jon Boat, that is a rectangle boat for very shallow water.

    I've thought of taking two boats like you got and joining them at transom for large canoe shaped boat or mini fake Viking Long Boat.
     
  9. Manfred.pech
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    Manfred.pech Senior Member

    To get lift from the new stern section it should be slightly concave (about one inch) and it does not need to be so big (drag). Think you will have to do some tests.

    [​IMG]
     
  10. upchurchmr
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    upchurchmr Senior Member

    Russ,

    Titebond 2 will work, I haven't tried Titebond 3.

    Manfred,

    He doesn't want dynamic lift, he wants smoothed flow lines at low speed.
     
  11. Russ Kaiser
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    Russ Kaiser Exuberant Amateur


    Manfred,

    I have to start with a contour that matches my boat transom for a smooth transition. I'm kind of locked into the shape because it is so short. As it stands and considering my current water line, I only expect about half of my new spoiler to be in the water. That said, it is built and at this point it would be hard to change. If it doesn't work, it will be my only attempt. Any further efforts will go toward a new boat.

    I just got back from rowing with my wife seated on the stern thwart. I rowed just as hard as I normally do, but my top speed and average speed were reduced by over 30%. She's not that heavy, but seated where she was she probably caused the transom edge to submerge double what it normally does with just me. The next time we go out together I think I will fill a five gallon bucket with water and put it in front of the front thwart.


    Thanks Upchruchmr. They are very similar so I have confidence that it will work as well. I did however just glue two pieces of scrap together as a test, just to make sure ;) Titebond III is cheaper than epoxy and less messy.
     
  12. Manfred.pech
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    Manfred.pech Senior Member

    Thank you for your kind help upchurchmr. When you lift the stern a bit you will get a better flow for the whole boat. The mod on the pic has an optimized flow from bow to stern for a speed range from fn 0.4 up to fn 1.2.. I think Russ will make his own way and does not need my exp.. Regards, Manfred
     
  13. upchurchmr
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    upchurchmr Senior Member

    Manfred,

    Thanks for the explanation.
    Please don't get upset when I try to understand.

    So with Russ's curved up extension you don't think the stern will rise and the flow lines will be smoothed?

    Seems to me that the dropped lip will add a little volume raising the stern somewhat, but will still leave a submerged transom sucking up a lots of water.
     
  14. Russ Kaiser
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    Russ Kaiser Exuberant Amateur

    Manfred,

    It seems like the modifications you're suggesting would only work for a hull at planing speed. The studies you linked to were for hulls operating above displacement speeds, i.e. fast military vessels.

    I am rowing below displacement speed (my top speed to date has been 3.7 knots) in a boat that was designed for planing. It is 14 feet long, but it has a waterline less than 12 feet in neutral trim. I'm merely trying to clean up how the water departs my hull.

    My spoiler lengthens my hull and adds rocker. Right now, when I am really pulling hard, I'm generating sets of 2 inch wake waves. That has to be taking a lot of power away from my stroke.

    Russ
     

  15. Manfred.pech
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    Manfred.pech Senior Member

    I`m not upset. No problem upchurchmr. Think not to be helpful enough for Russ because I do not know the underwater shape of his rowing boat. But I think the modifications can be small.
    A friend has just tested his latest hull (20ft) for a sailboat. It was clocked 6,5 knots in Beaufort 1 - unbelieveable. And here is the hull: [​IMG]

    http://web.de/sync/afp_animationen/wind_beaufort/index.html
    Please click on the upper red arrow.
     
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