Gussets and strength issue

Discussion in 'Boat Design' started by rasorinc, Jan 6, 2013.

  1. rasorinc
    Joined: Nov 2007
    Posts: 1,854
    Likes: 70, Points: 48, Legacy Rep: 896
    Location: OREGON

    rasorinc Senior Member

    Most of my V hull bottom frames are a 4/12 (18.5*) pitch. They are 1" net by 5.5". The side frames lap the aft side of the bottom frames and are epoxied with 2- 3/8" bolts througth. Would I gain much additional strength
    by placing a 3/4" plywood gusset between the 2 pieces. The plans call for doing this to only 1 frame out of the 8 total frames. Thanks much in advance. Stan
     
    1 person likes this.
  2. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 470, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Stan, gussets work best if they sandwich the frame futtocks. They will not do much if just filling in the triangle between upper and lower futtock. Attached is typical and one of my Coopers, I did several years ago.
     

    Attached Files:

  3. philSweet
    Joined: May 2008
    Posts: 2,172
    Likes: 123, Points: 63, Legacy Rep: 1082
    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    What PAR said is what you almost invariably find historically. But I wonder how much of that is just tradition. I think it harks back to the era of large bolted and spiked timber construction where the cost of the hardware was very large compared to today. Imagine if you had to hand forge and hand tap the bolts and nuts. You would design things a bit differently. With glued joints and with the hull glued to the frames, I'm don't think the situation is the same at all compared to past practice. In boats your size, the gussets would have been nailed and bent over or possibly clinched over a rove. This was hard on the exposed faces and exterior gussets took the brunt of the abuse. It also gave protection to the endgrains of the frames, sealing in the dope and slowing down rot. They weren't attached to the shell, so easier to replace. If the modern gusset is glued all 'round, I don't see why the middle gusset won't work. But as far as strength and rigidity goes, you have to determine each separately to figure out the gusset design and a mixed bolt and glue joint is a bit of a mystery to begin with. So I don't know how you would design the thing in practice.
     
    1 person likes this.
  4. SamSam
    Joined: Feb 2005
    Posts: 3,817
    Likes: 153, Points: 63, Legacy Rep: 971
    Location: Coastal Georgia

    SamSam Senior Member

    I have the book "Boat building with plywood" by Glen L. Witt and Ken Hankinson. They show and discuss 6 types of frames and call the lap joined frame (not half-lapped) simple and commonly used. The lapped frame with gusset like in the op is called rugged and simple. The double gusseted frame is called an excellent way to build a frame.

    So I would say a gusseted lap frame is stronger than a lapped frame. There might be a reason why it wasn't done like that in the plans though, like the gussets might get in the way of the sole or floorboards or something. It's hard to say without seeing the plans or knowing the shape and size of the gussets you plan on installing.

    I kind of figure whoever draws the plans has it all figured out and the only reason to change them is a different or more extreme use than planned for. Then again, there are people that will post their drawn "plans" and they don't have much idea what they're doing.
     
  5. viking north
    Joined: Dec 2010
    Posts: 1,864
    Likes: 87, Points: 48, Legacy Rep: 1146
    Location: Newfoundland & Nova Scotia

    viking north VINLAND

    Follow PARS practise and take it from this old builder--sandwich gusset every frame. If desireable for added strength, ease of fastening internal woodwork, rid dirt catching, cut an additional piece to match and fill in those voids between the gussets where they bridge the frames. I usually did this but then again I tend to overbuild. :)
     
  6. messabout
    Joined: Jan 2006
    Posts: 2,912
    Likes: 168, Points: 63, Legacy Rep: 1279
    Location: Lakeland Fl USA

    messabout Senior Member

    Do it the way that VN says to do it. Yes, be sure to fill the gaps as VN suggested. Make the fillers fit nicely ang epoxy them in place. That method will make as good a joint as you can devise. The joint will now be stronger than the principal parts. So OK this option is a little more labor intensive but this is a boat after all.
     
    1 person likes this.
  7. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 470, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Gussets, typically need to be thicker than the futtock stock to be effective and secondly the "reach" of the gussets also play a role too. The reach is the distance up each end of each futtock the gusset attaches. Of course the longer the better for strength and stiffness.

    As to their need, well it's tough to tell, without looking at the plans. A single plywood gusset, sandwiched by the futtocks will make the joint weaker, because the plywood is weaker, which is why the gussets usually sandwich the futtocks and their combined thickness is more then the thickness of the futtock stock.

    I agree in that modern adhesives have to some degree removed the necessary of gussets, but gussets do permit lighter framing stock. In the above image I posted, the gussets aren't very big, but do exceed the strength and stiffness of the futtocks. In the fore foot, where pounding will be severe, their reach across the futtock ends is longer than further aft.

    A triangular filler, between the two sandwiching gussets does stiffen things up a tad, but not as much as you'd think (if the gussets are sized properly). Their biggest benefit is to seal up the slot between the gussets, which makes for a neater appearance and eliminates a dust and debris catcher. This filler adds little weight, some fitting complexity, but really cleans things up under paint. It also adds "meat" where chine notches are usually cut too.

    All framed boats will have sole and countertop (or other furniture) fitting issues. It's the nature of the beast, unless you install a ceiling. As to using them, if you really want to save space, make metal gussets and "let" them into the ends of the futtocks, hidden within the joint. This is the bullet proof way with no visual or physical impact on the rest of the build.
     
  8. Petros
    Joined: Oct 2007
    Posts: 2,937
    Likes: 139, Points: 63, Legacy Rep: 1593
    Location: Arlington, WA-USA

    Petros Senior Member

    it would be a waste of time, materials and effort to make the hull stronger than it needs to be, particularly if the joint in question is not the weakest part of the hull. Overbuilding only adds weight and cost, so unless you know that this particular connections is a weak point, or has a history of being problematic, I would not bother with it. If the hull is overloaded and there is some other connections that is likely to fail first, that is where I would install stronger connections. Or if you are intending to load the hull for more than it was designed, than adding strength is not a bad idea, but if that is the case you should get some professional guidance.

    I will often change details to make it easier to build, or to add strength if it simplifies the construction, but more often than not I will build parts lighter and simpler to save weight and cost than what the plans call for (of coarse, I am and engineer, so it is second nature to me to consider the actual loads and construction considerations, and redesign if I feel it warranted). I have found that most plans for amateur construction tend to be over-built already, so there is no reason to add even more strength. But just overbuilding a design for no reason is wasteful.

    If you can post some scans of the plans of the hull structure perhaps we would have a better idea if added stronger guessets are necessary. If you have professionally designed plans for a well proven design, I would be surprised if it was necessary.

    good luck with the build.
     
  9. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 470, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Petros, you and I have a distinct advantage in this regard and I agree, that over building is not only wasteful, but self defeating on a number of issues too.

    It is difficult to tell if, in this instance, Stan's gusset concerns are warranted.
     
  10. Wavewacker
    Joined: Aug 2010
    Posts: 696
    Likes: 21, Points: 18, Legacy Rep: 226
    Location: Springfield, Mo.

    Wavewacker Senior Member

    A thought, what about using metal, seems if you had a break press you could bend plates with a flage at the angle needed to catch two surfaces and as a gusset. Something like joist hangers or hurricane straps in frame construction, screw and glue. ???
     
  11. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 470, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    I've seen some work boats with metal gussets, but there's always going to be corrosion issues, unless you have deep pockets. They can be a lot smaller.
     
  12. rasorinc
    Joined: Nov 2007
    Posts: 1,854
    Likes: 70, Points: 48, Legacy Rep: 896
    Location: OREGON

    rasorinc Senior Member

    Thanks everyone for your comments. I really appreciate your thoughts and suggestions.
    Stan
     

  13. Submarine Tom

    Submarine Tom Previous Member

    Stan,

    I'd follow the plans.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.