Limber hole size rules and frame strength?

I have limber holes about 1/2" radius half round (12mm). After many coats of paint, it is barely 1/4". The slightest debris plugs them up.

I read somewhere that limber hole size can be 25% of the frame height without significant strength loss.

For me that equals a radius of just under 3/4" (17.5mm). I would sure like them to be bigger. Can anybody weigh in on the structural effects of enlarging the holes further? Would some kind of doubler on the frame be needed?

 

Are you talking wood or metal frames? The strength in your frames will have a number of factors to consider, including what the hull is made of and how the frames are connected to the hull. You should be able to calculate the strength of your frames with the hull structure included, if the frames are attached to the hull in a permanent and complete fashion around the limber holes. The strength above the holes will experience mostly compression and tension forces with lateral sheer or bending forces introduced at the point of failure. The attachment to the hull turns the frame into a 'T' beam and that becomes part of the frame's strength below the limber hole.

If you are talking about a metal hull, how are the hull plates attached to the frame? Are they just tacked to the frame here and there, or is there a full weld making the hull plate and frame one piece? If it is wood, are your frames fully glued to the hull, nailed, is the hull plywood, or planking? What does the skin contribute to the strength?

Drilling out your limber holes will be like putting holes in a beam or a bulkhead. The overall width of the frame is important to calculating stiffness, but material will be needed to have strength.

The other things to consider, of course, are, how close together are your frames, what size boat do you have and what is it used for? What types of stresses are possible to apply to your boat?

Tell us more. I don't think it is that easy, that a 'rule of thumb' can be relied upon here.

My initial thought is, it will be fine to make your limber holes bigger, but I am not an engineer nor an architect. I'm just a boat lover.

Good luck and I can't wait until someone who knows what they're talking about chimes in.

- Just an artist

 

The answer depends on if the join (of the frame to the keel) should be considered "fixed" or "pin".
In the "fixed" case there will be a lot a reinforcement (gussets etc) to prevent the join from "flexing".
I would not enlarge a hole if the join is "fixed" as this is where the maximum bending stress will be.

In the "pin" join, it has practically no bending stress (yes there are still shear stress and other stresses).
You would probably could get away with it here.

Learn how to do the calcs and then you can have more confidence in your decisions.

AlanX

 

Many people, more than one, should study how to analyze a problem and then, only then, study how to do the calculations.
Yes @Steelboat, larger holes can be made as long as it is compensated for the material removed.

 

That is reasonable but to really zero in on the remaining "meat" on the web after the hole, we need to know how much load the beam is carrying. Can you provide the length of the beam and the profile dimension of the frame? And the material used.

Looking at your drawing, it seems it is an "end" connection where the shear load is greatest. It is usually bracketed.

 

You can have up to 50%.
In your case an R25mm will suffice.

You can also add thickness to the member that has the mouse hole, if you're concerned about any loss of shear strength.

 

Thanks for the excellent replies. In this situation, accurate engineering calculation is very difficult, due to lack of needed data.

The material is all A36 mild steel. As I do not have the original design drawings, I would need to carefully measure and produce them. Not an easy process, as most of the inner hull above the waterline has a thick layer of sprayed foam, obscuring exactly where the welds are, and the weld dimensions.

The full frames in this build are 2 3/4 by 3/8 steel, floating from the hull plating, except in some bilge sections, where they are welded. Frames are mostly spaced 26" apart. Naturally, frames are thoroughly welded to longitudinal stringers. Longs are welded to the plating intermittently after the first couple inches out from the frames from what I see so far. The boat has a very heavy stem bar 3" by 1/2" running from stem to stern. The first chine hull plating in the bow is 1/4". By my eye the steep bow sections with that big stem bar are massively strong, likely way overdone by any calculations.

So with all that, I think experienced eyes may be the best way to guide such details. I am not sure how to execute a frame strengthener in this situation, but made a guess below.

The frame to hull gap is about 3/8". I am thinking to not weld the doubler plate to the hull, just frame and stem bar.

 

Adding the doubling plate to the frame will work fine for steel.

Is your boat power or sail?
If you provide the LWL, Tc (the midsection/maximum draft of the "canoe body") and the maximum speed of your boat (if it is a power boat),
I can calculate the maximum design pressure (based on AS4132.1).

What is the span of the frame between the keel and the first major longitudinal member (usually the chine)?
And displacement in tonnes?
Based on the frame spacing (26") I can calculate the required section modulus of the midsection frame (worst case).
As I have the frame section, the notch size, the frame spacing and span, and material type, I can calculate the worst case required section modulus and the frame section modulus.

Note: I am not a NA so you need to make your own assessment as to the suitability of my estimates for your purpose. As they say, this is for educational purposes!

Regards AlanX

 

Great. Another definition that I can add to my vocabulary. I can't seem to pin the definition of "limber hole, drain hole, air hole" in the illustration the OP has posted. I know it is supposed to be at 20% of the span.

You did post an image of that end connection here a long time ago. Please post it again. A picture is worth a thousand words.

 

Hi Alan
An interesting calculation, thanks for your efforts. Here are those data.
Sail type: displacement hull, maximum speed about 7.8kn
LWL: about 36 ft
Canoe body draft: about 4 ft
Stem bar (centerline) to first longitudinal stringer 9"
Stem bar to first chine: 19"
* at the area of repair
Yes, heavily built.

 

Hmm. 20% of the height of the frame? Any source on that rule?

 

I don't need displacement for a sail-boat.

The span I need is from the keel to the first main (serious) stringer.
I am going with 19" as it makes a square panel.

Results:
Basically there is a large reserve/excess safety factor for a clean frame member (x5).
With a 17.5 mm radius limber hole reserve safety drops to 2.8.
With a 25 mm radius limber hole reserve safety drops to 2.1.
So I see no problem making the limber hole larger.

AlanX

 

Thanks Alan. That is a whole lot of amazing calculations. Perhaps it is confirmation bias that I like your results? 2:1 sounds good, but I might just weld in a doubler, and keep that safety factor of 5

Are you on the study path to becoming a NA? It seems you must be close. Great work understanding those concepts.

 

No, I am a retired mining engineer.
I did a structures unit 40 years ago.
I have been an amateur boat designer/builder for perhaps 18 years.
Regards AlanX