Modifications for lift and spray reduction

Discussion in 'Boat Design' started by majorm, Feb 9, 2013.

  1. majorm
    Joined: Apr 2012
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    Location: South Carolina, USA

    majorm Junior Member

    I was looking at some of the products like the Smart Rails and was curious how beneficial these would be in reality. Its pretty simple to see how they would effect spray at the bow but they also advertise more lift. The added lift leads to more speed by their explination.

    How effective in reality would it be to add to the chine with maybe a 2" extension with a very slight cup? Our boat is a 28 bertram which has a deep V and heavy. The extension would be from the transom to the point the chine exits the water. In theory it would help but is it enough to be worth the effort. To be worth it I would like a 2 or 3 knot gain in speed while on plane.

    For the second half about spray... I live in SC and see the carolina style boats with the flared hulls. If I raised the front of the 28's hull 2 to 3" and added more flare to the bow using that extra room do you see any obvious downsides? If not do you have any suggestions to tie it into the existing fiberglass. The bow rail will be tied into the existing material so it wont put stress there since i know it will be a weaker point.
     
  2. keysdisease
    Joined: Mar 2006
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    keysdisease Senior Member

    Spray rails have been around almost as long as powerboats. They work very well at knocking down spray and providing some dynamic lift, and there may be others that will support additional speed, but except under extremely narrow circumstances in an extremely narrow range of hull types I seriously doubt the more speed claim.

    Taco makes an excellent spray rail at a reasonable price:

    http://tacomarine.com/item--2-1-16-x-1-7-16-Rigid-Spray-Rail--V21-0874.html

    Steve
     
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  3. Eric Sponberg
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    Eric Sponberg Senior Member

    Majorm,

    I have specified and/or recommended Smart Rail a number of times, and they do seem to work as advertised. I would recommend installing them for a few feet forward of where the chine exits the water because at slow speed as you are accelerating up to plane, the front end of the chine is shedding water, and you want the added width and shape of the Smart Rail to help do that. It will get you on plane a little faster.

    For your interest, have a look at the story of Blue Bill on my website where I designed spray rails for a Wilbur 34. This was a very successful endeavor:

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

    As to your second matter--it sounds like you want to add "Carolina Flare" to your hull. To do so could add a considerable amount of weight forward, and that could be a bad thing, adversely affecting the trim, pulling the bow down. I think if you put the Smart Rails on properly, you'll lift the boat out more naturally and will have a smaller problem with spray anyway, so you may not need the Carolina flare.

    Another really good thing to do is to execute a weight reduction program for the boat--get rid of all the extra weight that you don't really need to carry, all the "stuff" that so easily accummulates on boat. Be brutal, get rid of whatever you can; your boat will love you for it.

    Eric
     
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  4. messabout
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    messabout Senior Member

    Eric is one of our most respected forum gurus. Do what he says. Put the boat on a Weight Watcher diet and be vigilant about it forever more.

    Every pound of boat must displace, or generate dynamic lift enough, to offset that pound of weight. We are talking dollars here, not just performance improvement. Adding the Smart Rail thingys will probably help a little, but reducing overall displacement is a much more economical and practical method for making better performance for a given power input.
     
  5. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Adding these spray rails will knock down spray, though I do question their effectiveness in adding much lift. There may be a measurable amount, but nothing compared to the area involved in the plane patch, under the hull. A large, heavy craft like yours, is at a greater disadvantage, mostly because the lift strake area is relatively small, so the lift they provide is also quite small. A 2 - 3 knot gain is probably wishful thinking, but they will knock down spray. A light 18' bow rider might see some speed increases, as the additional strake area is proportionately larger, which is the probable target market for these advertising suggestions.

    As to adding some freeboard to your boat's forward sections, well anything can be done, but a wise builder/owner might want to consider a cost/reward study first, as this would be major surgery.
     
  6. Eric Sponberg
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    Eric Sponberg Senior Member

    I should clarify that I would look at adding the Smart Rails from a few feet forward of there the chine enters the water and running them back to the transom. The reason they work and do create a bit of lift is because of their bottom shape, which is concave. As the water crosses the Smart Rail, the concave shape necessarily turns the water flow downward. Very act of doing that is a change of momentum for the water. Any change in momentum, as described by Newton's Second Law of motion, results in a force. That is, momentum is mass times speed = m x V. Speed, V, is a vector, and a vector has propterties of both value and direction. The speed of the water in this case is constant, but the direction changes. Therefore, there is a change in the vector, which means a change in momentum, and this results in a net force upward against the Smart Rail and the hull. The Smart Rail causes the water flow to lift the hull.

    Thanks for the compliment, Messabout.

    Eric
     
  7. midnitmike
    Joined: Apr 2012
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    midnitmike Senior Member

    Majorm,
    In my experience a properly designed spray rail will have a significant affect on the speed, trim and fuel efficiency of a vessel. With that being said the rather generic versions like the Smart Rail, Fast Rail, or Taco Marines' spray rail won't have the same effect due to their smaller relative size. They're also pretty expensive for what you're getting..with the larger Fast Rail costing $2400 for a pair of 8' rails...Wow!

    Personally I think you'd be money ahead to build your own and leave the bow modification idea in the dust bin of bad ideas.

    MM
     
  8. keysdisease
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    keysdisease Senior Member

    Hence my link to the Taco rails, more like a couple of hundred dollars than a couple of thousand.

    Steve

     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    I'd have thought superimposing a Carolina flare onto a Bertram 28 should be a crime ! Total waste of effort, imo. And those add-on rails might be rated a minor misdemeanour !
     
  10. boatbuilder41
    Joined: Feb 2013
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    boatbuilder41 Senior Member

    I just built and installned custom spray rails,lift rails, and fin stabilizers built into 1. Similar to 25-45 foot jefferson lafitte hulls. Check out there design. I even got pics of the ones I just put on my lafitte skiff. The lift was absolutely incredible. And a truly superb spray rail.also at trolling speeds act as roll chock stabilizers. They are 4 1/2 " wide . I got pics in action if you need to see them. So much lift I increased speed 6.5 kts...for a total of 31.3 knots. With only 300 hp. A 30 foot jefferson laffite.with full cabin.
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    The way I see it, adding these rails won't make a massive difference to a boat that already has wide chine flats, but if there isn't any to start with it certainly would. I suppose in extreme conditions a boat with these tacked on could roll over more easily, but that would be survival mode type conditions, where a breaking sea hit on the beam, and the rail increased the tendency to trip, rather than slide.
     

  12. majorm
    Joined: Apr 2012
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    majorm Junior Member

    Thanks for all the great replies. As to the Carolina Flare... Dont worry Ill put that idea in the trash. Everything looks easy on paper until you decide how to build it. Yall had some great thoughts on the side effects of the modification as well.

    For the spray rails. I honestly dont like the way they look up by the bow but back by the waterline I can deal with. I would consider something small up front if it would have some function. you make an interesting point about the dded lip giving the boat something to catch on in a bad situation. The 28 does rock and we do plan to keep the flybridge but probably wont ever use a hard top. That could however be negated with the weight of a radar and its arch. Quantifying the lift created is like you said the hard part to see if its worth it on my hull.

    boatbuilder41: I just looked up those boats and do see the lift rails. If you have the picture handy I would mind seeing it though. It looks like the ones I saw had a flat bottom. Did you use a concave bottom or even just angle it down to create more of a pocket?

    Im not going to sidetrack this much but the boat is a little underpowered with diesels that are between 170 and maybe 200 hp. Other 28's with similar power diesels get ~19 knot cruise and maybe 23 knot max speed. So thats why I have been looking for any gain possible. As was said a diet could really help the boat maybe more than anything except engines.
     
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