Lifting rail alternative

Discussion in 'Powerboats' started by Toolate, Sep 30, 2013.

  1. Toolate
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    Toolate Junior Member

    QL 450's. Just one pair. They project down about 1" and are 450 mm long so about 18".

    I think even the pros might be guessing a bit on lifting rail size/placement no?
     
  2. Toolate
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    Toolate Junior Member

    You said strakes- you mean the spray/lift rail in the pic? Just making sure.

    This is a fast boat though and is probably somewhat (re)designed to cope with higher speeds than say 16-18kt. I had a hard chine 21 Seaway and it was great but it would run almost 40 mph. Lot of these older smaller downeasters have been re-invented for outboard use at higher speeds.

    That said, I would think that at 15kts this boat would be more stable than the older design and maybe slightly more stable at anchor- probably not much.
     
  3. DCockey
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    DCockey Senior Member

    What you see on the side.

    What do you mean by strakes?
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    15 knots is not fast enough to be getting appreciable lift from dynamic "devices", unless you are in a lighter, smaller boat. I suspect you will only get marginal improvements at best, and the boat may stiffen a little in pitch underway.
     
  5. PAR
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    PAR Yacht Designer/Builder

    Some might, but with experience comes a level of understanding that can only be appreciated by those with it.

    Bingo, lift strakes (rails, whatever you want to call them), particularly on this hull, at the speeds you can muster, are really just spray knockers. Some interaction will occur, but not as much as you might expect, certainly not enough to pull several degrees of bow rise out of her.

    At these speeds a hook would be ideal, though this too has to be carefully designed, so you can hit a target. Too much and she'll be too slow, too little and you'll have what you have now, maybe a degree less. Several of the old school warped bottom designers, where quite famous at designing a hook, so the boat would run at a target speed, often on surprising little power. But, they couldn't be pushed much past this speed, as the exponential increase in drag, just sucks the life out these hopes.

    Next up without the design issues would be wedges. I'd start with a 12" deep wedge, 3/4" thick at the transom, maybe 24" long (across each side of the transom). I'd use a hot glue gun and some cedar shingles. Glue on several, take it for a spin and note the trim. Add more, remove some, make it wider, maybe deeper, etc., whatever she wants, until you get 4 degrees and call it an afternoon. With the newly created and finalized hook dimensions, you can make a more long term solution with foam and 'glass. On small boats I've used duct tape and shingles, but on this one, I'd probably opt for a heavier duty, temporary attachment.
     
  6. Toolate
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    Toolate Junior Member

    I think of strakes as the smaller lines/ridges under go-fast hulls or maybe just smaller sized steps- I was just making sure we were talking about the same thing.

    To me that eastern has a "rail" be it spray or lifting rail. Semantics...
     
  7. Toolate
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    Toolate Junior Member

    You think that would do much more than the volvo tabs then. Interesting. I might just have to try that esp because its the cheapest to just try.
     
  8. DCockey
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    DCockey Senior Member

    My understanding is longitudinal steps on the bottom primarily function to provide clean separation inboard of the chines and improve stability when the boat is going fast enough that the immersed chine length is small. They don't directly provide significant additional lift.

    Any estimate of the fore/aft location of the CG on your boat? How much is the transom immersed at rest. Do you have any photos of the boat floating at rest?
     
  9. PAR
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    PAR Yacht Designer/Builder

    Yes, the steps you've described David are simply for clean(er) separation, but a hook (formed with shingles or built in) changes the shape of the bottom, though much like a step, usually more substantial in regard to patch angle change and pressure wave.

    I too think it's more of a CG issue and would also look at trim, before making major hull bottom revisions. Some static profile shots (photos) would be helpful.

    [​IMG]

    A classic "designed" hook. Even the chine has a hook in it. The target speed is probably around 20 MPH on a 100 HP inboard. A 200 HP inboard might get her to 23 - 24 MPH. Closer inspection will show a difference in the hook on the centerline and the chine, probably because of the inboard install, but not unusual. Nailing down these sets of shapes will require a fair bit of design, considerable experience or simply an afternoon of experimenting with shingles.
     
  10. Toolate
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    Toolate Junior Member

    Agreed although I think we need to be careful with the terminology- you say "steps" here and I am thinking of stepped hulls like on go fast boats today with steps more or less permendicular to the centerline of the hull. You mean strakes though- not meaning to nit pick but still want to make sure we are on the same page.

    Agree on the lack of lift from them.

    Here it is at rest- lot has happened since this but boat floats on its original water line- maybe 1/2" of bottom paint showing. New engine is maybe 200lbs heavier and sits 2" forward of old engine so I don think is contributing to the attitude of the boat. No idea on where the CG of the boat is but I suspect it hasnt changed much. Transom is just maybe 5" in the water at rest. Same as always and others I have seen.
     

    Attached Files:

  11. Toolate
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    Toolate Junior Member

    PAR I understand the line drawings and profiles at different stations but the term hook is lost on me. Can you clarify?

    Also, my boat has round chines not hard like that line drawing aside from that though very similar esp in the keel sections. Thanks for your time on this- not looking to be a pain in the *** just learn what I can.
     
  12. DCockey
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    DCockey Senior Member

    Not the solution to the current problem but may be relevant:

    Article by Peter Brown on the history of the Sisu 22 which had spray rails added in 1982 when new molds were made: http://compactyachts.com/boats/the-...ocket-lobsterboat-for-your-next-fishing-trip/ From the article:
    Sisu quickly discovered that outboard power over 120 hp could make the boat unstable at high speed and limited power to 115 hp.

    In 1982, Sisu made new molds adding a full length spray rail that ended at the transom about 2” above the waterline. This corrected the stability issue and power limits went up to 150 hp.​
     
  13. Toolate
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    Toolate Junior Member

    Its a good point. I have seen a couple Sisu's that ride with similar attitude to mine and maybe what I should do is just focus on adding a little stability under way and be happy with whatever lift/speed I might get from adding rails.

    Lots to consider. The wedges would be easy and cheap to try so will probably do so. Then the rails if I still want more.

    I am fine with the way she runs now but would like to improve if possible like anyone.

    I can say that if I was to do rails they would be glassed on rather than pvc/screws. Would love to see someone who did this- maybe shaped in foam then glassed. Another thread maybe.
     

  14. gonzo
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    gonzo Senior Member

    Hook is a reverse curve in the buttock lines at the aft sections. It works like trim tabs.
     
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