Need Input On Adding Lifting Rails To A Lobster Boat Hull

Discussion in 'Boat Design' started by cbob11361, Aug 25, 2010.

  1. cbob11361
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    cbob11361 New Member

    I have a 35' lobster boat that i converted to recreational fishing. I am considering the addition of lifting rails and need some input from people in the know. the boat is a 35' Nauset with soft/round chines. the deadrise midship is approx 13 degrees, flattening out to approx 3 degrees aft. Any suggestions on placement and angles will be greatly appreciated.
     
  2. frank smith
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    frank smith Senior Member

    I would take pictures of her while running at speed for a start , to see where
    the natural line where separation takes place.
     
  3. Eric Sponberg
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    Eric Sponberg Senior Member

    You can go to my website and see what I designed for Blue Bill a while back, a Wilbur 34. These lifting strakes worked excellently.

    Link: http://www.sponbergyachtdesign.com/Bluebill.htm

    Eric
     
  4. cbob11361
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    cbob11361 New Member

    Thanks for your help. Spoke with a couple of builders in Maine and think i have a good idea for placement. Thanks again for your help
     
  5. Easy Rider
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    Easy Rider Senior Member

    Eric,
    That was a really excellent response to the thread question.
    While your'e on a roll I have an idea and question about about spray rails.
    I think a huge part of the problem w spray rails is the velocity of the water the spray rails are called on to deflect. So much velocity the end result can only be fine spray and that get's blown everywhere. My idea is to install one or two "rails" that would be 1/4 to 1/3 of a round piece of stock the idea being that the water would be slowed down greatly allowing it to roll out from the upper spray rail as heavy slop and not fine spray. the "pre-rails" would be like speed bumps and act as multiple stages of a system. The heavy slop would drop down readily and not be easily blown everywhere. What do you think?

    Easy Rider
     
  6. Eric Sponberg
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    Eric Sponberg Senior Member

    Easy R,

    If I picture this correctly, your 1/4 or 1/3 rounds would be like sections of a pipe or tube attached to the outboard edge of the spray rail? The water coming up from the bottom and across the strake encounters the round and turns downward instead of shooting straight outboard and/or up. It makes sense and certainly could be tried.

    In fact, a properly designed spray rail would not need these. Properly designed, the inboard corner of the stray rail should be radiused so that there is a smooth transition between the bottom proper and the spray rail. In the forward half of the boat, the strake is slanted downward a few degrees, generally not more than 5 deg or so, so that the water is directed downward and away from the boat. A fault of some stray rail designs is that the downward angle it too large, and the water is directed downward too close to the boat such that when it makes contact with the sea, it creates a secondary splash and spray that bounces back up onto the boat. So, you can have too much of a good thing. Aft of amidships, the strake should be horizontal with no downward angle.

    Therefore, if you wanted to add rounds as you suggest, a 1/4 or 1/3 round might be too much. Maybe an 1/8th round or less.

    Alternatively, you can use the Smart Rail system which is a glued-on strake that does essentially as you suggest and is shaped similarly. Link: http://www.thesmartrail.com/.

    I recommended the Smart Rail to a client once, and he was so grateful at the way the boat performed with them that he sent me a very nice gift that I have hanging on the wall. All I did was recommend it--I don't have any connection with the company. They're pretty neat.

    Eric
     
  7. Easy Rider
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    Easy Rider Senior Member

    Eric,
    What I have in mind is 2 or 3 spray rail strakes. The first (upper) is shaped normally and preferably large like your lobsterboat example. The lower strake or strakes is/are shaped like a rounded speed bump and run fore and aft like lifting strakes parallel to and probably equal distance from the regular SR above. It's like a multi-stage system. The first "pre-rail" is rounded and slows the water down a bit. The 2nd pre-rail is higher and a bit more abrupt in it's curvature and slows the ascending water down more before the upper spray rail rolls the water out from the hull. If the pre-rails have done their job the discharge water from the spray rail should be of a low enough velocity to drop heavily to the sea surface causing little concern for boaters.

    Easy Rider
     
  8. Eric Sponberg
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    Eric Sponberg Senior Member

    I see what you are getting at now. I am not sure that would work. Rather than slowing the water down, I think your intermediate rounded strakes will more likely break up the water into spray even before it gets to the upper most strake, effectively creating more spray and turbulence.

    Eric
     
  9. Easy Rider
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    Easy Rider Senior Member

    Eric,
    Of course one would need to experiment with how big and what radius to make the pre-rails and of course how many would be needed. Every boat and application would be different just like regular spray rails, keels ect. It's just an idea I have and it may be unique as I've not seen anything like it.

    Easy
     
  10. frank smith
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    frank smith Senior Member

    How about adding sponsons to the hull ?
     
  11. Eric Sponberg
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    Eric Sponberg Senior Member

    That's right, that is the best way to find out what will happen.

    Eric
     
  12. mydauphin
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    mydauphin Senior Member

    Would any of these add drag or lift to hull?
     
  13. WickedGood

    WickedGood Guest

    Is that Nauset Raytheons old boat "PATHFINDER"? It had a Chevy 454 in a long time ago. If so thats your problem.

    I ran it out from Portsmouth to Isle of sholes one day and back and it was a DOG.

    Replace the engine with a little 500 HP Turbo Diesil and yo wont need the lifting strakes.

    You are referaring to building out the soft chine into a reverse concave to eliminate the roll and get more lift on plane. correct?

    Add 12x24 core hudraulic trim tabs to it and yo will be fine.

    The boat also needed a nice dive platform. I would go something like 12 ft wide and 4 ft to make a real nice boat .

    Capt Walt
     
  14. Eric Sponberg
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    Eric Sponberg Senior Member

    They would probably add drag but not certain as to how much; not certain about lift. Another reason to do tests.

    Eric
     

  15. Easy Rider
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    Easy Rider Senior Member

    Any drag increase would be won back several times over by reducing wetted surface above. Your'e probably a very busy guy and I appreciate your time very much. I think controlling spray is worth serious effort and it seems like the issue gets only "passing glance" attention.

    Easy Rider
     
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