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Thread: Do immersed transoms create less drag?

  1. #61
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    Quote Originally Posted by Nordic View Post
    300 kgs for a bimini sounds like an awful lot, don't you mean 300 lbs?

    regards

    Alan
    I'm considering a hard GRP bimini which will be 10 feet wide by approximately 8 feet long, and be designed to be structurely rigid to stand on. I think I meant to type lbs... not kgs. Pardon the error.

    The transom extensions are a major undertaking and I would hate to 'get it wrong'.

  2. #62

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    Even 300 lbs is incredibly heavy for a roof that size.

    Build it using cored glass and it will weigh less than half that. The cabin roof on my boat is made of the same material as the decks, 16mm Duflex, with 800gsm laminates. It's bigger than the size you're talking about and would weigh less than 35 kg. Two big guys (100+kg) can walk on it right up to the end of the cockpit roof.

    In fact my entire cabin turret (without the windows) would weigh a heck of a lot less than 300lbs. Two people could lift it easily. If it wasn't so big and such an awkward shape I could lift it myself.

  3. #63
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    That's interesting. I had pulled my numbers from this thread on the boat building forum. It's the only chart of it's kind I came across. I calculated weight based on two 1/4 inch glass skins surrounding 1/2 balsa core. The link is here:

    http://www.boatdesign.net/forums/fib...ness-8686.html

  4. #64

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    2x 1/4 inch skins would be much more than you'd need. You can gain some stiffness by going up in the core thickness and use less glass. My decks and roof are 16mm balsa with 800gsm laminates. Very stiff and strong.

  5. #65
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    Rick,

    I agree with 44C concerning the skins and core. You might consider PP honeycomb from Plascore in Michigan. $55/sheet for 1", which would provide a lot of stiffness as well as more insulation. If you use 1708 biax I would think your total weight would be somewhere around 50lbs.

    Mike
    Last edited by mikereed100; 12th November 2008 at 04:54 PM.

  6. #66
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    Default Transom extensions

    Rick: At speed under sail, does the water flow straight back from your transoms for any distance before rising? If so, You won't benefit greatly from extensions. If on the otherhand it looks like the water behind an over-zealous paddle, you might.
    Lofting a hard bimini:
    Acompany here in Annapolis builds light weight hard biminis for monohulls using the boats preexisting steel or aluminum tubing frame. If you have a frame it probably won't be strong enough to stand on, but it will give you an idea of how tall it should be in certain places.
    I think that the airodynamic advantages of an arched bimini would far outweigh the easy of construction of a flat bimini, even counting easier mounting of solar panels. And a flat bimini on a cat is just plain ugly. You might crawl around http://www.boatdesign.net/forums/multihulls/ for some ideas how to lay out and lay up a strong, light bimini. Having watched the development of Maine Cats hard tops, I suspect the best way to support a top is with hefty metal legs, probably mounted where your soft bimini frame was mounted.

  7. #67

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    Quote Originally Posted by Sandy Daugherty View Post
    Rick: I think that the airodynamic advantages of an arched bimini would far outweigh the easy of construction of a flat bimini, even counting easier mounting of solar panels. And a flat bimini on a cat is just plain ugly. .
    I thought the Anatares 44i looked pretty good. It allows for a fixed or removable dodger.

    Pat
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  8. #68
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    Quote Originally Posted by Sandy Daugherty View Post
    Rick: At speed under sail, does the water flow straight back from your transoms for any distance before rising? If so, You won't benefit greatly from extensions. If on the otherhand it looks like the water behind an over-zealous paddle, you might..
    I have a pronounced stern wave under sail. At speed these waves meet about 8 to 10 feet behind the boat. The interaction of these waves creates a mini "rooster tail".


    Quote Originally Posted by Sandy Daugherty View Post
    Lofting a hard bimini:
    Acompany here in Annapolis builds light weight hard biminis for monohulls using the boats preexisting steel or aluminum tubing frame. If you have a frame it probably won't be strong enough to stand on, but it will give you an idea of how tall it should be in certain places.
    I think that the airodynamic advantages of an arched bimini would far outweigh the easy of construction of a flat bimini, even counting easier mounting of solar panels. And a flat bimini on a cat is just plain ugly. You might crawl around http://www.boatdesign.net/forums/multihulls/ for some ideas how to lay out and lay up a strong, light bimini. Having watched the development of Maine Cats hard tops, I suspect the best way to support a top is with hefty metal legs, probably mounted where your soft bimini frame was mounted.
    I agree that metal legs supporting a hard bimini would be the best solution. Sailing in Florida, a bimini is really a requirement, however I've struggled with the soft vs hard bimini question for serveral years and it has immobilized me. The issue has nothing to do with asthetics. It has to do with hurricanes.

    A soft bimini can be removed and easily stored, reducing wind resistance when it needs to be reduced. A hard bimini, aerodynamic or not, significantly increases wind resistance. Yes, a properly designed hard bimini can theoretically be removed.... but then what? It would be too large to transport (I don't own a truck) and I'd lose it in a hurricane anyway.

  9. #69
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    Quote Originally Posted by Pat Ross View Post
    I thought the Anatares 44i looked pretty good. It allows for a fixed or removable dodger.
    Hiya Pat. Good to see you posting here.

  10. #70

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    Hi Rick,

    I am debating the same issue regarding a bimini on the boat I am building. Here in Florida you do need to have the ability to get out of the sun for sure. Frankly I had not realized I would lose a hard bimini during a hurricane, guess I better plug that into the equation.

    Pat

  11. #71
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    I'm not worried so much about losing the bimini, as losing the boat. In hurricane prep, rule number one is the reduction of windage by removing everything you can from the boat. Sails, solar panels..etc with the goal to reduce your windage so as to reduce the pull on your anchor rodes. The number one cause of boat lose is ground tackle failure or chafing through lines.

    Obviously, with a soft bimini, this isn't a problem.

    There is a secondary issue as well, however, I'm less concerned about it. Adding a bimini of any type will affect the boat's performance in high wind conditons. this doesn't necessarily mean a hurricane. I've been caught out in squalls where the wind rapidly increased to 50-60 mph sustained. In those conditions I can barely keep the nose into the wind under full engine power. Adding a bimini would make this impossible as my particular boat was not powered with a bimini's wind resistance in the power requirements. I'd have to increase horse power to some extent to offet this.

    Now you can see how I've managed to immobilize myself on this topic.

  12. #72
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    I think the reason for removing sails and covers before a hurricane is to save them from damage rather than to reduce windage. I haven't heard of a hard bimini (from the manufacturer) being blown off. I don't think that the additional drag soft goods represent is going to make any difference in whether your ground tackle holds; In these extreme conditions, if the wind don't git ya another boat will. BUT:

    Not being able to hold the bow into the wind IS A PROBLEM. With a bare pole and a stiff breeze across the beam my boat blows bow down. It may be just the rudders causing this. In a heavy gust I have to cross control the engines to head into the wind, making life interesting. With the bimini up, this problem is minimized, because the bimini is like the feathers on an arrow; the center of wind resistance is aft of the center of lateral resistance of the hull form.

    I like my soft bimini because I can take it down and use the boom to hoist things aboard, like batteries and outboards, or a man overboard. I don't like it because it gets in the way of furling the main, and I have to hang my solar panels somewhere else. But there is no way I'm going to sail in the summer around here without a bimini. In the end, I decided to stay with the soft top because I don't need any more weight, and I can throw the extra money away on frivolous, non-boaty things like food, shelter, and taxes. Not the same as hull extensions.

    If you get a chance Rick, take some pictures of your wakes, from one transom to the other, and looking back from above each transom. You might be able to avoid pouring all that wind energy into wave making.

  13. #73
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    Quote Originally Posted by Tropic Cat View Post
    I'm not worried so much about losing the bimini, as losing the boat. In hurricane prep, rule number one is the reduction of windage ......
    Now you can see how I've managed to immobilize myself on this topic.
    Alan,
    sorry this is going a little away from the original subject, perhaps a new thread could be started.

    Rick,
    I agree, high windage is a menace in strong winds.
    I am going to have a retractable soft bimini as I have drawn, a bit of a compromise.
    The 'catwalk' can house the solar panels and the rollers for the canvas once retracted in case you want a bit of sun or the wind is going to be strong.
    If you get caught in hurricane winds, it will hopefully just rip off and fly away without endangering the boat.
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    Keep Smiling
    Stefano

  14. #74
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    if the cat walk holds the solar pannels where does the cat walk?
    is this a good place to put something worth manny $ as a sacrificial item?
    majika

  15. #75
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    Quote Originally Posted by majika View Post
    is this a good place to put something worth manny $ as a sacrificial item?
    Sorry for not being clear, the soft canvas would rip off, hence reduce windage.
    Keep Smiling
    Stefano

  16. #76
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    Quote Originally Posted by Sandy Daugherty View Post
    If you get a chance Rick, take some pictures of your wakes, from one transom to the other, and looking back from above each transom. You might be able to avoid pouring all that wind energy into wave making.
    I've stuck my head down there the best I could while under sail. I'm might be a little dense but don't understand what I should be looking at. The stern waves begin about 3-4 feet forward of the transom and as I've mentioned meet behind the boat.

    Having said that I haven't clue what to do about it.

  17. #77
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    My bad. I meant to say "take some pictures of the water as it leaves your transoms" from above, looking back behind the boat and from one side looking at the other side. What you have described sounds like the interaction of your bow or quarter waves as they meet under and behind the boat. There's no help for that. And there wouldn't be much help from transom extensions if your transoms are clear of the water at rest. They would allow you to put more weight aft, such as a dinghy and a targa arch for antennas and solar panels. The only Catalac I've had a good look at lately had both, and seriously drug its heels. It should have been a 46 instead of a 42!

    I should warn you that if you are the only 8 or 9 meter with extensions, your resale value will drop.

    In any event, go sailing and tell me about it. It snowed here yesterday.

  18. #78
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    Unfortunately is unseasonably cold here in central Florida. The last two weeks the temperature has been in the high 40's and low 50's. I haven't been to the boat in over a week.

    Sadly, I'm catching up on the admiral's list of projects around the house .

  19. #79
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    Rick,

    I agree with Sandy, it is most probably your bow waves interacting. Your stern wave starts at the trailing edges of your transoms (for the horizontal part) and another part can come from the water under the hull moving up behind the transoms.

    Alan

  20. #80
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    The bow waves are nearly non existant. What there is comes off the bow and meets the other hull under the middle of the bridge deck. What I'm referring to are actually generated near the stern where the hulls rise up from the waterline.

    I'm going to look and see if I have a photo.

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