Whacky stringer concept

Discussion in 'Boat Design' started by Saqa, Nov 10, 2013.

  1. Saqa
    Joined: Oct 2013
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    Saqa Senior Member

    Just trying to separate discussion on the stringer I touched on in the build thread. I downloaded an open source alternate to corel draw called Inkscape, seems fully functional progie, and did this rough sketch
    [​IMG]

    I have a piece of marine rated semi hardwood called damanu, plank size 20' 10x1

    Wondering if I can cut a pair of stringers from that and use as a H frame as is used in a truck to mount the drivetrain and body

    The solid black lines are the stringers running out through slots in the transom. Height extended outside the transom and a piece fitted as a back plate for outboard mount. That should form an outside the hull splashwell

    You can see the how the side panels of the hull extends beyond the transom at a reverse angle and together with the bottom panels forms steps at the transom for entering the boat. The stringer ends sticking outside the transom would form an outboard bracket between the steps

    The pink bits represents the front and rear decks/storage. The front being two step with the foremost an anchorwell. The blue be the console slotting through the sole and fixed to the stringers

    My reasoning to do this is allowing a much bigger motor in the future and the solid stringers taking the brunt of force from the outboard and not the transom. The stringers then distributing that to the transom, frames and hull panels. This will work out lighter then beefing up the transom and passing the force onto more complex framework of ply stringers and frames.....that is if such a design is feasible!
     
  2. Saqa
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    Saqa Senior Member

    I should add that each wall of the decks would be ply frames spanning the width of the hull and height relevant to the deck as well as one or two under the sole around the middle of the boat
     
  3. Saqa
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    Saqa Senior Member

    I should also add that the green represents the hull and I have already glued it up ready for glassing, it has the reverse slope and boarding steps

    Motor should also be mentioned. I currently have an 8hp and have the option to run a 15, 25 and 40hp
     
  4. PAR
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    PAR Yacht Designer/Builder

    Your twin stringer idea is interesting but has a major flaw. The "stair step" notches will be classic stress riser locations and prone to cracking. You can solve this by making very generous radiuses, instead of 90 degree direction changes in the rise up to the engine mount area. Generally we don't do this, because it's just redundant triangulation, which means overly heavy for the loads. Considering how little HP you'll have available, you should make every effort to keep it light.
     
  5. Saqa
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    Saqa Senior Member

    I think I understand what you saying Par

    In a conventional design, ply knees epoxied to stringers would handle that stress in that step stair under the rear deck better due to bidirectional grain?

    If that is the case, would using ply knees epoxied to the damanu stringers and triangles cut from damanu plank bolted and epoxied to the stringer ends sticking out of the transom slots eliminate the flaw? The grain on the triangles running vertical

    I find this idea really interesting too and keen to investigate it. I haven't yet attached the transom to my hull yet. There is a 25hp and a 40hp on offer by a couple of relatives that I have my eye on incase the 8hp doesnt give enough performance to travel and fish the way I want to. I also like the idea of having the splashwell outside the boat and aesthetic **** factor if I can pull it off without a significant weight penality. Twin damanu stringers should weigh in around 4 ply ones which are also damanu and 9mm
     
  6. Saqa
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    Saqa Senior Member

    Came across this boat online in google image search. Was an auction site so a couldnt find a background story on it. I wonder if it uses similar idea, would love to see under sole pics
    [​IMG]
    [​IMG]
    [​IMG]
     
  7. Yellowjacket
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    Yellowjacket Senior Member

    The concept isn't as wacky as it seems, many racing runabouts and hydros use the cockpit sides to take the motor loads and feed those loads into the hull. As Par noted you really need to get rid of the notches that you see in the side view, as any normal wood is going to have a problem with stress concentration in those notches.

    Actually instead of a piece of wood with the grain running in the longitudinal direction, the hydros an runabouts use plywood for the cockpit sides and they are typically about a half an inch thick at the back by laminating two 1/4 inch pieces together and further forward they get thinner. If you use wood with the grain running along the hull you will need to taper the taller parts in the opposite direction. That is as you go further down into the hull, the upright needs to have more longitudinal length, not less as you have it now.

    Most Runabouts and hydros don't cut the transom all the way down to the bottom, they notch the cockpit sides and slip the transom up into the notch. They also generally put a bock of wood aft of the motor to make the sides stable and spread the load of the transom to the sides. This approach can use a relatively thin plywood piece and they generally add a doubler to the plywood to make it 1/2 of an inch thick for a short distance forward and then thin it back to 1/4 of an inch. Obviously if you have a bigger motor, you will need to make it thicker, but if you notch the transom part way down and notch the motor board support pieces, use a backing block on the back side you could do something very similar, and it could work as well as be relatively light. Trying to make the stringer go all the way to the top of the transom isn't going to have the grain oriented in the right direction at one end or the other, and that is why plywood is used. You could use the plywood as a doubler and bond it to the stringers, and so long as it is fairly long in the fore/aft direction it could work.
     

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  8. Saqa
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    Saqa Senior Member

    Yellowjacket thank you!

    I am going to read your info a few times over so it makes total sense to me, I think I grasp the concept as you laid it out but I am dont know if I understand correctly as even though I can visualize a lot I am unfamiliar with many of the boat component terminology

    This should sort out my understanding a bit. Do I change from wooden to ply and cut out a pair of long knife blade shaped stringers that will poke out of a three piece transom. Attach wooden tabs to the stringers where it meets the transom notches inside and out and glue and bolt through?
     
  9. Saqa
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    Saqa Senior Member

    Just did a new sketch in Inkspace, this is how I visualize the boat. Picking up what I could understand from Yellowjackets post. The sole will fit on top of the stringers whose raised bits will poke out from the sole near the legroom and rear deck. I am going to make the sole self bailing
    [​IMG]
     
  10. Yellowjacket
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    Yellowjacket Senior Member

    Just a couple of points. You want the plywood pieces to go further aft than the transom. If you stop the plywood at the transom it won't be strong enough to hold the motor. The idea is to put a block of some kind behind the transom where the motor bolts on. This block stiffens the plywood and basically transfers the aft load at the top of the motor mount to the plywood with the block in compression and the loads going out to the plywood with a larger shear area behind the transom that then puts the plywood into tension. Both the transom and the cockpit sides of the race boats are notched and the transom slides up into the notch in the sides. Most builders also put a 1x2 flat against the sides just ahead of the transom and use long screws through the transom into the 1x2 to help spread the load to the plywood. I don't know how much motor you are planning to use, but these racing boats are between 20 and 50 hp rigs, so if you are thinking of using more power everything needs to be scaled up, both in thickness and size accordingly.

    The transom needs to be the full width of the boat, and generally a solid piece of wood is used with the grain running across the boat, with plywood on the front and back of the solid piece. This tends to prevent cracking of the transom, but the beam running across the boat in your case is likely to be about a foot high or higher and that will provide you with a solid tie between the sides of the boat. Race boats use a 1 inch core of spruce, and if you are going to be a bigger boat you might go thicker. Then plywood is added, front and back to increase the strength and stiffness.

    You want the notch in the transom to be, at a minimum 8 inches or more so that you have a good piece of plywood as a carry through structure and can hold the motor on the boat. Remember that this piece of plywood on the sides is what is mostly holding on the motor and it is in tension. If you make to too light you could tear it all off. The objective here is to give you a good bit of solid wood to take the aft motor loads. On race boats the wood along side the motor mount area of the transom is a half an inch thick. Again, if you are holding a bigger motor you need more thickness.

    Here is a picture of a typical transom joint on a race boat. The whole thing would be glued together with epoxy. This joint is a very efficient way to transmit the transom loads to the plywood sides efficiently and lets those loads come out in tension in the plywood.

    I don't know how much power you are using or if the use is in the ocean or a calm lake, so you need to be responsible for your particular design. What I'm trying to do is show you how it is commonly done on race boats, not design your particular attachment. A joint designed like this is commonly used in applications were up to 50 hp is present, but these are race boats and may not be as robust as you might want if your use puts more loads into the boat.

    In racing boats the cockpit sides are generally a bit less than 2 feet apart, so this might give you some guidance on how to space your stringers.

    Hope this helps
     

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  11. Saqa
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    Saqa Senior Member

    Ok thanks for explaining that. I think I am on the right track. The stringers I have in mind doesnt stop at the transom but pokes through it so it looks like a mounting bracket. And the parts of the stringer outside the hull be laminated to solid semi hardwood

    My transom is 9mm ply laminated with a layer of 1" marine grade semi hardwood. Max motor I am looking at is 40hp and to be used in lagoon conditions. 50-60cm is what I am looking at as distance between stringers too. My beam at waterline is 1.2m
     
  12. Yellowjacket
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    Yellowjacket Senior Member

    1.2 meter seems a bit narrow in terms of total beam. I realize there may be a desire to use a single sheet of plywood or 48 inch wide pieces for cross members, but unless there is some overriding need to be narrow I'd be thinking about more beam.

    You should also look at designs like the Diablo, since it basically has the bottom shape of a Class D racing runabout, and it is a very easy boat to build. A slightly narrower planing surface (like 24" wide) coupled with a chine that opens up from there will handle better since it will roll into a turn (and not need a fin on the bottom) like a pure flat bottom will. And it will likely be a better riding (not pound as much) and be faster than a pure flat bottom design

    Bottom thickness is dependent on the rib spacing and there may be an advantage in a thicker bottom and fewer ribs..

    Just a thought.
     
  13. SamSam
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    SamSam Senior Member

    The forward slanted transom, the green part, will go forward alright, reverse will have problems. If it doesn't dig in and sink you, I would think it would slop waves right into the boat.
     

  14. Saqa
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    Saqa Senior Member

    Yeah trying for a narrow boat that can row for on the water exercise as a big feature of the boat, working on a compromise between plane-ability and row-ability Its 4.2m long. That skinny jetski type steering column will be hinged to drop down allowing middle seat rowing with 9' oars. Drop a couple of lures back and should make for a good day on the water dont you think? Drive under power to the reef edge and row around the breaks in the reef doing a bit of trolling. Build progress here....
    http://www.boatdesign.net/forums/boat-building/building-stitch-glue-boat-fiji-48653.html

    SamSam, the transom is straight, might even angle it back some degrees. The transom is not attached yet as still working on the design trying to get these stringers right. The reverse rake on the side panels are design to accommodate boarding steps which is another feature of this boat as a major use will also be snorkeling....Lol you nailed it in your slogan :)
     
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