Lifting Strake Construction

Discussion in 'Boat Design' started by WoodenPontoon, Nov 19, 2009.

  1. WoodenPontoon
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    WoodenPontoon Junior Member

    I want to add lifting strakes to my wooden tri-hulled pontoon boat. I am looking for some different ideas.

    My initial thoughts are to construct an approximate hull-length, "light weight" plywood "V" with reinforced edges and point. I thought I could fill the ceter with 8 or 16 pound pour-in-place foam. The foam would then be cut to match the curvature of the hulls. I would attach the strakes along their length with screws and plugs into the hull (both edges). I would then fiberglass the strakes to the hulls to seal and strengthen them.

    Should I include strakes on both sides of the outboard hulls? What is the best shape for these; sharp or rounded edges? What would be a reasonable dimension for these?

    Ideas are strongly requested. Thanks in advance!!!!
     
  2. gonzo
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    gonzo Senior Member

    What are you trying to accomplish?
     
  3. WoodenPontoon
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    WoodenPontoon Junior Member

    I'm trying to gain the ability to come up on plane. Originally the boat was designed for a smaller motor, but now I have a 90 HP on it. The boat never gets on a true plane. I'm sure lifting strakes would give me more flat surfaces to ride up on. Many of the larger commercial pontoons now have lifting strakes for the same reasons.
     
  4. gonzo
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    gonzo Senior Member

    Can you post photos of the bottom? Are you sure the structure can take the stress?
     
  5. PAR
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    PAR Yacht Designer/Builder

    I'll assume the bottoms of your pontoons are flat. Given this, you probably have enough efficient planning surface, so the question is a matter of power versus weight (literally). Though you may have a 90 HP outboard, you have to be fairly light to get a typically pontoon boat up on plane. Just looking at the few photos you've provided of your wooden boat, she's considerably heavier then typical and therefor needs more horse power. Without more design specific information, such as dry weight, approximate planing surface area, pontoon length, WL beam, pontoon beam, etc. then how much you need is pure speculation.

    Lift strakes can alter when you plane and how you plane to a degree, but they will not help an underpowered hull get up on plane.
     
  6. Yellowjacket
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    Yellowjacket Senior Member

    Pictures of what you have now would help, along with the size and weight
     
  7. WoodenPontoon
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    WoodenPontoon Junior Member

    My hulls are not flat at all. They are 24 1/2" diameter U-shaped logs. There is a slight taper into a V-hull towards the aft end. The 2 outboard hulls are 21 1/2' long, while the center hull is 14 1/2' in length. All three hulls are aligned with the aft end. The bow sections on all three hulls are pointed and taper out to the U-shaped mid-sections. The overall beam is 8 1/2'.

    The boat weighs about 3000 pounds (wet weight).

    Here are some pictures:

    AFT END
    [​IMG]

    Center Hull AFT
    [​IMG]

    Center Hull FWD
    [​IMG]

    Outboard Hull AFT
    [​IMG]
     
  8. gonzo
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    gonzo Senior Member

    That is really heavy. You have a narrow displacement type hull. To make them plane it's going to take a complete redesign of the bottom. The easiest and lightest way is to get your skillsaw and cut them at the waterline. Attach new bottoms with a proper shape for planing.
     
  9. PAR
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    PAR Yacht Designer/Builder

    You might be able to bond some "wings" to her from say about midship aft. These would be some big 'ol flats, the width of the pontoon. Maybe start with some deadrise and work them flat as they move aft. Think of them as horizontal lifting strakes, one on each side of the centerline and extending out the full width of the pontoon. Also make them dead nuts straight on the bottom, no curve. This may offer enough surface to plane on, but only time will tell. Of course, these strakes will have to literally support the weight of the boat, bouncing along on plane.
     
  10. WoodenPontoon
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    WoodenPontoon Junior Member

    Thanks for the thoughts guys. I am understanding straight, flat and strong.

    Would a plywood V-shaped strake be strong enough? I would install stiffeners periodically along the length of the strake and then fill between them with high density foam. These would then be screwed to the hulls and plugged. I would then add fiberglass & epoxy to seal them to the hulls.
     
  11. PAR
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    PAR Yacht Designer/Builder

    At the entry of the new plane surfaces a V shape would be fine, but you want to improve the planning efficiency as quickly as possible, by flattening out the V as it moves aft. It takes more energy to get a V shape to plane then a flat one. This is one reason Jon boats get up and scoot with little power. Your static trim suggested a slight stern down attitude anyway, so maybe it'll be enough with pontoon width flats, to get her up on plane.

    Will it be strong enough, who knows. It depends on the plywood thickness, the spacers, attachments, how much additional weight the boat must endure and laminate thickness. Don't over build, you're plenty heavy as it is. Think about making the aft part of each hull basically square in cross section. The bottom can have some V shape, but don't go crazy (you already know what happens with way too much V shape), say 5 degrees at most to keep plane efficiency up (flat is the most efficient planing shape).
     
  12. Commuter Boats
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    Commuter Boats Commuter Boats

    No need for plugs, plywood, epoxy, and fiberglass. High density foam would be a plus.
     
  13. PAR
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    PAR Yacht Designer/Builder

    He's built a wooden boat and now you want him to make a cored 'glass laminate? I think it would be best if he stuck with what he's familiar with and likely has tools, supplies and materials for.
     
  14. WoodenPontoon
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    WoodenPontoon Junior Member

    Par - Actually, my entire boat is of fiberglassed core construction. It just happens that the core is very visible wood. All of my hulls are glassed inside and out. There is an estimated 32 gallons of epoxy used for her construction. Having to glass some plywood lifting strakes would be very routine.
     

  15. Yellowjacket
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    Yellowjacket Senior Member

    Before you go trying modifications perhaps some ROM analysis might be a good idea.

    I'd suggest that you first make an estimate of the beam of of the center hull and the two side hulls that you think the planing surface is going to have near the transom and then apportion the weight on a per hull basis in relation to those widths. If for instance the middle hull is twice the width of the outer hulls, then the middle hull will be supporting about half the weight and each of the outer hulls will be supporting 1/4 of the weight.

    Then I would run Dingo Tweedies excel spreadsheet for the inner hull and the outer hulls, with the power apportioned in the same way as the weight.

    The idea here is to look a deadrise in increments of five degrees and try to get an idea of how deep the hull will run when planing at what speed and how fast it should go.

    This isn't going to be super accurate, since hull width effects will cause some inaccuracy, but it would be helpful to have some idea what to expect and what kind of deadrise you can get away with, or if it is going to plane at all.

    Also you should think about some type of fences or edge turndown effect at the outer edges of the surfaces. This type of edge control really improves the efficiency of narrow planing surfaces at relatively low speeds.
     
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