Changing a full Keel on my boat

Discussion in 'Boat Design' started by discovery, Aug 25, 2013.

  1. discovery
    Joined: Aug 2013
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    discovery Junior Member

    Its been a while, but I have been thinking a bit about my skeg issue, but thinking more about the chines. It has occurred to me that part of the reason it gets driven faster is the spray, at slower speeds, the spray comes straight back toward the cab. It has also occurred to me that the boat used to have chine logs and that these may have assisted with the spray issue. In the boats history, someone has removed these logs from the chine ( I know this as he left the ground off bolts in the hull without anything but the antifoul to stop the leak) . I intend to remove the current wooden spray rail and rubrail (they are half rotted anyway) but was thinking how effective some 3 inch stainless steel angle would be as a spray chine if added to the chine right on the lowest part of the side. The slight angle of the sides of the boat should mean the angle of the water deflects slightly downwards off the stainless.

    The other question I have is about lifting strakes. Would their addition assist in getting the hull out of the water, or more to the point, would they be noticeable? If I were to add them, would I be better off doing it all at once, or do my keel mods and chine spray rails, and see if I need more lift.
     
  2. midnitmike
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    midnitmike Senior Member

    I have been a persistent advocate of spray rails and will remain so as long as my personal experience continues to validate the benefits derived from their use. Not only will they address the spray issue you have but if properly designed they can and will provide dynamic lift. They also function as roll stabilizers when the boat is at rest so the benefits one gets from there installation is noticeable in all conditions.

    With that said I would not recommend using 3" stainless steel angle. Not only would it be difficult to conform to the hull, the simple "L" shape is not the most efficient design in terms of generating lift nor for acting as a roll stabilizer. A more satisfactory result can be obtained by the use of a wedge shaped foam core which is easily contoured and then glassed to the hull. This eliminates the need for through bolts and gives superior performance in every regard because it allows you to tailor the spray rails characteristics specifically to your hull.

    With regards to your keel I wonder why you haven't mentioned using fiberglass as a material option. Personally I would hesitate to construct the entire skeg out of steel especially if one considers the weight penalty and corrosion potential inheriant in such a design. If my choice were Wood vs Steel vs Fiberglass I know which one I'd chose everytime.

    MM
     
  3. discovery
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    discovery Junior Member

    There have been many suggestions about what and how to do my spray chines and keel both from you guys here and the armchair experts down at the wharf. Im used to doing near everything I do in metal, hence my initial thoughts about steel. Properly coated and constructed a much smaller skeg to protect the prop and support the rudder wouldnt be any heavier than what I have now.
    With regards to my spray chines I hadn't thought of foam and glass mainly due to my skilset.Im used to working with metals so this is my first point of call. Another consideration is beaching. It is still fairly common here to beach my boat to cleanthe bottom. This adds to the mix as a large percentage of the weight of the boat will be resting on the underside chine when beached.
     
  4. midnitmike
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    midnitmike Senior Member

    Agreed, it is often best to stay with the material you're most accustomed to using. In the case of the spray rails however the benefits of a properly shaped and contoured rail far outweigh the difficulties in working with this medium. While you may not feel comfortable tackling such a job yourself any competent glass man should be up to the task.

    Over the years I've built 20 or 25 spray rails on commercial fishing boats and have yet to have one fail. In one instance I built a set of 4" spray rails on a 37' Wegley and almost before the glass was cured the boat was strapped and set back in the water. If done properly they are extremely tough and can withstand almost any abuse you're likely to subject your boat to, and if by chance they are damaged they can be easily repaired.

    While I may not have the vast experience of those armchair experts down at the dock when it comes to spray rails I've done more then just think about 'em.

    MM
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    A boat that is running in the mid-teens (knots) is not going to get much help from strakes, especially if it is a heavy beast. The keel set-up on this boat is something of a monstrosity, and that is where I'd be concentrating first.
     
  6. PAR
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    PAR Yacht Designer/Builder

    Agreed, the only thing strake will likely do is knock down spray, which isn't a bad thing from what I hear of this boat's running behavior. Tack on something just for this.
     
  7. discovery
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    discovery Junior Member

    I agree totally with your comments, and yes, I know the keel is the main demon.

    At this stage, the spray chines and the keel removal/ heavy modification will be on the cards. This all requires a bit of planning as slip time isn't cheap. This is the main reason I am talking to people on this forum and trying to glean the relevant information to use in my calculations and plans. The main reason I was asking about strakes is that if I'm under the hull glassing, for a little more prep and planning these could be in the plan too, if they were deemed to be of real help. I've since looked at a hull that is similar to mine except the keel, with 200 mm spray chines (8") without any running strakes and made a decision that they probably aren't needed
     
  8. discovery
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    discovery Junior Member

    Potentially a short strake for the forward 1/3 of the hull 1/2 way between the breakwater and the chine???
     
  9. PAR
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    PAR Yacht Designer/Builder

    I'd run a 75 mm strake from the cutwater to the static chine location. This will knock down spray and not alter the running surfaces or get in the way when you flop her over for bottom painting.
     
  10. midnitmike
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    midnitmike Senior Member

    Gentlemen,
    One thing I think we can all agree on at this point is that Discoverys' main concern should be the refit of an improved skeg. But I have to disagree with the general concensus I see developing on this thread regarding the use of spray rails. A boat capable of speed in the mid teens will in fact gain enormously from the addition of Spray rails / Lift strakes. My experience has shown me that boats of this type are often capable of a 1 to 3 knot speed increase depending on their previous trim attributes. These numbers are based on what I've seen first hand when working with truly "over burdened" commercial fishing boats that have additional deck gear, nets, fish holds, and fuel capacity not in evidence on this boat.

    Over the years I've seen several copies of my designs implemented in one form or another, most often as partial spray rail or lift strake segments. And while these attempt to address a very particular problem they fail (and this is just my personal opinion) to see the overall performance improvements that are common with a fully intergrated system. My approach has always been to look at the boat using a more holistic view since it's impossible to separate the various drag inducing portions from one another. There are gains to be made along every inch of a hull when it comes to reducing wetted surface area and or providing dynamic lift. To suggest that Discovery use an abbreviated version of a spray rail is I think rather short sighted in terms of providing him with the performance characteristics he's previously stated as his goal and with the potential his boat has for improvement.

    MM
     
  11. PAR
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    PAR Yacht Designer/Builder

    Agreed Mike, but this isn't going to be the approach of the OP. He isn't going to develop well defined strakes, suited for the hull form (which require a fair bit of skill) and other mitigating factors. He's going to do something, which will likely be a mutli stage thing, first some sort of wave/spray knocker, then something about the skeg at a later date. The easiest and least impactive thing he can do, is make the boat more comfortable to drive, so the spray knocker is up first (IMO), then when he gets a chance and with some research, refining up the skeg, possible in concert with a real set of strakes, etc. (don't hold your breath) are a distant possibility. This is what I've gathered from his posts thus far . . .
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    This boat is quite flat bottomed, I don't see much scope to reduce wetted area or get extra dynamic lift. Get rid of that ridiculous skeg/keel first, and take it from there.
     
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    Don't forget too that this relatively narrow boat is carrying a lot of iron in the shape of a 250hp diesel, which seems to be over the odds anyway for the type of vessel it is.
     
  14. PAR
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    PAR Yacht Designer/Builder

    I don't think the boat is particularly narrow for it's length (3.3:1 beam/length), nor especially flat bottomed. She looks to be a typical warped bottom with a huge skeg. You could have lots of prop protection with 1/3 the size of the current skeg. Strakes wouldn't bee as effective as you might think with regard to improving her plane efficiency, though certainly some improvement if well shaped.

    You just need enough skeg to protect the shaft and prop, likely with a bridge to support the lower end of the rudder post. This would follow the same angle as the shaft and would have more then enough area to help tracking handily. It should be stout to protect against bottom strikes, but it doesn't have to be massive, nor especially wide. An elliptical leading edge would help with flow penetration and tapered aft to get clean flow to the prop.

    Currently she's running at 3.3 S/L so she's up on plane, meaning the reduction of wetted area in the skeg will make a significant gain, likely as previously noted a few knots on her top end. At this S/L strakes will help somewhat, but mostly offering a fraction of a knot at the top end and more importantly spray control and getting up on plane sooner, which will positively affect fuel efficiency.

    Without hull lines, basic volumetric and hydro figures, it would be difficult to offer more then this.
     

  15. midnitmike
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    midnitmike Senior Member

    Agreed Par, the owner has a number of ways to address these issues and more then enough advice to help him decide on a course of action

    MM
     
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