Question regarding laminated frames?

Hi everyone,

I'm currently working on the construction details for the athwartship frames and longitudinals for my tunnel hull and I have a question.....
The original design specifies the bottom frame pieces as 3/4" net with 3" molded width and 6mm ply double gussets to the side frames. I would like adjust the molded width of one of the bottom frames for clearance because of the height of the tunnels. I'm modifying the design of the gussets to stretch across the bottom frames from side frame to side frame as single pieces, laminating both fore and aft faces of the frame assemblies.
I've been all over the net trying to find any info to calculate the structural strengths of the laminated frames, and comparing this to the strength of the 3/4" as specified, to see what I can do with the bottom frame and maintain the structural integrity, but I haven't had any luck finding it.

Any information to help figure this out would be greatly appreciated!!

Thanks,
Dave

 

It will largely depend on the materials you use. A frame can be treated as beam. However, I don't really understand what you are doing. Can you post some drawings? This looks like a re-engineering of the hull structure, which is probably more than can be explained in a short post.

 

Hi Gonzo,

I did a couple drawings regarding my question. These are just really quick doodles and have no dimensional accuracy, but I think they illustrate what I'm working on....

I misspoke regarding the dimensions also, I apologize. The original 3/4" bottom frame is 3.5" in molded width at the keel centerline and tapers to 1.5" where it attaches to the side frame, with the top of the frame being level running across from side frame to side frame. It is also a "floating" frame design, with the battens sitting proud to the frames.
Frame construction will be using 18mm Khaya with 6mm Okoume ply for the laminations/gussets. Also of note, the center pod and sponson tunnel walls will be load-bearing longitudinals, built from 18mm Khaya with 6mm Okoume ply laminated to the outer faces (planking side). The outer sponson wall is 1.75" and the inner center pod wall is 2.5" molded width at this frame.
The narrowest part of the bottom frame will be above the tunnel roofs and that is where my concern lies, hence, the reason for my question...

Thanks,
Dave

 

I would favor the method that is shown on the lower of the two sketches. That is depending on getting a good bond between the separate pieces. You can treat the frame as a simple beam, although it is not actually so simple because it is tapered.

You can calculate the relative strength of a simple beam with this simple equation. I =( bd^3)/12. I is the mass moment of inertia which is a measure of the stiffness of the beam. b is the breadth or thickness of the beam and d is the depth of the beam raised to the third power with the quantity divided by 12. That is the simple part. If the frame is regarded as a uniformly loaded beam....that is a wild notion..... but let us consider it in that way. The center of a beam that is uniformly loaded will be the point of maximum deflection. By having it tapered, you will have increased the value of I by a useful margin. That will compensate for the mid beam deflection. We could get into a sticky set of calculations that accounted for some of the variables, but let's not bust our skulls unnecessarily. Just build the frames as you have shown on the lower sketch and be careful to glue it up carefully and efficiently.

 

Hi Messabout,

The upper sketch shows the frame and gusset assemblies of the original design that I based my project on and the lower sketch shows one of the modifications that I am incorporating into the design with laminating 6mm ply to both sides of the bottom frame members, for the entire span of the members. The lamination schedule for the frames and other components in the build also includes vacuum-bagging to ensure proper bonding.

I'm looking to gain some understanding of the difference in the strength, or stiffness, of a frame constructed using the 18mm Khaya core laminated with 6mm ply on both sides in comparison to the original using 3/4" Khaya by itself? As an example: If this 30mm laminated beam offers 25% more stiffness than the original design, this results in the laminated beam with 3" depth that offers the same "strength" as a non-laminated 4" beam built using 3/4" Khaya?
Also, from what I understand, the mechanical properties of the 6mm ply / 18mm core / 6mm ply lamination will be greater than a non-laminated Khaya beam of the same thickness (or even a beam laminated using only 3 layers of Khaya....). Please correct me if I'm off on this Messabout.

Would it then be appropriate, for the sake of simplifying the process, to just gather the averaged "I" for each component of the laminated beam and just add the values together ((I1)6mm ply + (I2)18mm Khaya + (I3)6mm ply) to get a decent view of the strength of the laminated beam for comparison purposes?

I really appreciate the help Messabout. Thank you!!

Dave

 

Strength and stiffness are two different things. What mechanical properties are you comparing? For example, stiffness is the mechanical property that will affect how much a material deflects when a force is applied. Strength is usually referred as how much force can be applied before the material yields or fails, depending on which strength you are interested in. Toughness is how much energy a material can absorb without breaking. These mechanical properties can tell you that rubber is tougher than steel, but not as stiff. Wood is stronger than steel by weight, but less puncture resistant, etc.

 

Plywood gussets will be weaker (about 25 - 30%) than solid wood gussets, simply because of the nature of the material, compairtivly. Also how things are assembled will make a difference, such as bonding compaired to fastening these gussets to the solid stock. When you make changes to a set of plans, it's important to understand the ramifications of them, such as the aforementioned stress risers on the stringer notches. This usually means understanding material properties and basic engineering principles. For example understanding plywood's inherent weakness in longitudinal stiffness, compaired to similar sized solid timber. It's cross grain strength is better, though in thinner sizes (like 6 mm) it needs to be good 5 ply, not cheap 3 ply or it's actually weaker and more flexible.

The notched image at the bottom will make two huge stress risers in the assembly, dramatically weakening it. When "things" occur in the design process, you have to recognize them and the simple solution to this one, is a deeper "web" on the stringer assembly. In this case, the area fore and aft of notch on the stringer needs to be deeper, to accommodate what was removed by the notch. You can just "bulk" up the area as you've envisioned, but this will require a much heavier assembly then simply adding some depth. Of course, you don't always have this option, because of other stuff or requirements in the design, so maybe it would be more helpful to see what you're working with in a more encompassing drawing.

 

Hi Gonzo and PAR,

In reply to your question Gonzo, I am looking for a way to quantify and compare both strength and stiffness in regards to loads placed on the athwardship frames of a plywood on frame hull so I can lower the top edge of the frame 2 bottom frame member for passenger clearance. Strong enough to take the cycling loads without failure, and stiff enough that the hull stays "square" and doesn't warp under those same loads. I know the scantlings in the original design were appropriate for the design, with it being proven over several builds.

This is the design that I'm working on shown in Linesplan and I also sketched the major components into the Linesplan in the second image to show scale along with the restrictions brought about by the design. The hull is 12' 6" long with a 12" long drive extension pod (which is a continuation of the tunnel center pod) aft of the transom, to properly locate the drivetrain in the design. 5' 0" beam and the hull is 17" deep. The build will be in Joubert BS1088 Okoume ply over Khaya solid stock frames and longitudinals. Many of the frame components and longitudinals (keel and battens) will be laminated with 6mm ply and assembled using silicon bronze fasteners with thickened epoxy and vacuum-bagged.





Frame 2 (located at the dash beam in image 2) is the one I'm working on and you hit the nail on the head PAR, in regards to the restriction on making the bottom frame member deeper. I want to be able to quantify load-capability of the 30mm laminated beam (frame member) in comparison to the 3/4" solid stock of the original design so that I can have a bottom frame that will take the loads generated, and be low enough in height to not be a big issue in the cockpit area. I don't have the same issue with any of the other stations in the design.
I did do a couple more sketches of the frame in question to show what I've done so far.....
Tentatively, I've set the 18mm main beam of the bottom frame member shown is 2" deep and is a single piece from side frame to side frame. The sponson and center pod pieces below the main beam are edge-laminated to the main beam and the whole assembly is laminated on both fore and aft faces with 6mm ply gussets as described above. the sketches also show the inner and outer tunnel walls/battens as load-bearing longitudinals built from 18mm Khaya with 6mm ply lams, also described above, to spread loading to the frames
but nothing is concrete until I know that it will work as required...


Frame 2 assembled minus gussets showing longitudinals and planking


Frame 2 assembly minus planking shown as exploded view


Frame 2 assembly gussets

If necessary, I can do some actual load tests to determine the info that I need, but if I can get this done without the added time and expense, it would be great.

As always, any help, thoughts and suggestions would be greatly appreciated!!

Thanks,
Dave

 

Again, the solid wood is stiffer longitudinally than plywood and also a bit lighter too. Lets ignore the weight issues, as it's just a few pounds, it's the height of the "web" that is critical in this type of structural element. Simply put, decreasing the stringer height dramatically weakens it longitudinally. Your well radiused gusset in the last image will go a long way to eliminating stress risers in the area.

Maybe you should take a different approuch on this frame and actually laminate it from relatively thin solid timer. Make them a bit thicker in molded dimension and a solid stack of say 1/4" - 3/8" material, bridging the notched out area, to get the extra glue lines and stiffness the laminate stack can provide, without a hugely, thick piece being necessary.

To "quantify" the differences between plywood (I'll assume this is what you're calling a laminated beam) and solid stock, everything being equal dimension wise, then about 30% less in longitudinal stiffness for plywood, with about 45 - 50% better cross grain strength than solid stock, of similar species. There are other attributes to consider as well, such as compression modulus, tension, etc. Without a much better understanding of the design scantlings and hydro's on this puppy, getting more precise loading information will be difficult. This isn't all that unusual and why tests are performed all the time, if only to save the fee of having an engineer workup the load paths. Make two frames, one as the plans called for originally and one notched as you've drawn it up and put a point load in the center of the notch area on both frames, to see which gives first and about how much it takes to do so. This doesn't have to be a very fancy rig, just a floor jack and a bathroom scale will like make a good showing, representative of what's what.

 

Hi PAR, thanks for getting back to me so quickly.

Yeah, the loss of longitudinal stiffness and strength with the reduced bottom frame depth is my concern (especially with the stress risers present with the interruptions in the bottom frame member due to the tunnels).
I also wanted to apologise and clarify what I'm calling the laminated beams in case I misspoke. The laminated beams would not be plywood alone. The 18mm core material in the frames would be solid timber and both sides of the core would be laminated with 6mm ply in single pieces with the gussets to the side frames formed at each end of the ply laminations.
From your previous reply, I started to think that using a laminated stack of solid timber may be a better way to go. Is there any way to calculate the strength difference between the laminated stack of solid timber compared to a single solid timber? Or is load testing still in my future?

Thanks again!
Dave

 

The ultimate strength of the structure is only one of the important properties. If the deflection increases it will affect the behavior of the boat at high speeds. It will create a hook on the bottom and may make you lose control.

 

Hi Gonzo,

I agree completely, and it is one of the reasons that I chose to build the tunnel wall/tunnel roof corner batten assemblies using 18mm timber and ply. This is meant to create load-bearing longitudinals and tie the 3 frames together to disperse the loading as a "boxed" structure, so to speak. This is meant to greatly reduce the chances of the hull doing anything goofy at higher speeds.

Thanks,
Dave

 

The tunnels create an extremely complicated flow modification. They are likely to have an aerodynamic effect too.

 

Yeah Gonzo, the tunnel design can definitely complicate matters and aerodynamics are certainly relevant to this design.
I spent a lot of time with Jim at Aeromarine Research and made very good use of his Tunnel Boat Design Program that I purchased for the project.

The end result, with the Honda PWC powertrain that's slated for the build, is a design that planes before 20 mph, shows a WL of 6.25' and 616 lbs of total drag at 45 mph, transition to aerodynamic lift starts at 48-50 mph and I've been able to tweak the design to the point of good stability past 86 mph with center pod WL of 2.1' with total drag of 348 lbs. At these speeds, the hull form is generating 278 lbs, or 32% of total lift aerodynamically.
I had no idea how involved the tunnel design process was when I started this, but Jim was great to deal with and the learning process was a blast!
So with the design itself pretty well scienced out, I'm finalizing the construction details to make sure the boat is as safe and predictable as possible within it's intended performance envelope.

Thanks Gonzo!
Dave

 

That sounds like a fun project. Congratulations.