How wide to make fillets

[Arron]

Hi. Imagine you are making a stitch and glue boat, say a canoe or a small rowboat. You're using 4mm ply and BoteCoat epoxy. You have a good reason for wanting the narrowest possible fillets (where the ply planks are stitched together). Further, you wont be laying fibreglass tape across the fillets on the inside, though the outside will be sheathed in 3oz fibreglass. What would be the minimum width fillet that you would use ?

What about if you omitted the fibreglass sheathing, how wide would you make the fillets then (or would that be a bad move) ?

I'm after some genuine non-book advice here from people who actually have experience with fillets of different widths.

cheers and thanks
Arron

[PAR]

Fillets serve a number of purposes; they bridge gaps, serving as a flange, they decrease point loading and stress riser propagation, they provide a radius for reinforcement fabrics to lie neatly against, they seal the end grain of plywood panels, they offer a cosmetic solution that is also easy to clean and naturally sheds water away from the joint, etc.
When I work up scantlings, I use three types of fillets; heavy structural, light structural and cosmetic. Cosmetic is self explanatory and just a radius to make a corner neat, easy to clean, etc. It doesn't have to be there, but does "clean" things up if it is. The fillet radius can be pretty much anything you want for visual appeal.

A light structural fillet is bonding two pieces together, but being not highly loaded, doesn't need a fabric reinforcement over it. The filler mixture will be structural in nature and it's size may be dictated by anticipated load, but the 3:1 rule usually applies to these, if for no other reason then you need the bulk of the fillet to perform some duty.

A heavy structural fillet is just like a light structural fillet, except that it has a fabric reinforcement over it. I just did the scantlings on a 33' custom powerboat and speced the biggest fillets I've ever designed. The loads are huge in a boat this size and the fillets reflect this. The 3:1 rule stills applies, but I also have a fabric reinforcement rule that I use, which is 6:1. Attached is a "snap shot" of a fillet instruction sheet I supply with my plans and it shows these ratios as they apply to a fillet and the fabric reinforcement.

On the powerboat I just did, these ratios where tossed out the window and I also had several layers of fabric on the fillets, but the drawing works for boats up to about 18' (6 m).

To answer your questions, when doing a small, light weight hull, you might be able to get by with no fabric reinforcements and small fillets (less then 3:1) but the fabric is what provides the bulk of the elongation strength, which is especially true on the inside of a corner. On the outside of a seam, the fabric is usually just abrasion protection, though some joints need reinforcement on both sides for strength. In most cases a chine fillet (for example) is used as a substitute for a chine log in conventional plank on frame building. Without the lateral stiffness of a fairly wide fillet and the tension reinforcement of fabric on the inside of the joint, the seam will just "unzip" under load. By this I mean the fillet alone, may just stay stuck to one panel and tear the outer veneer off the piece it's bonded to.

You'll also notice in the drawing I like to back cut the plywood a little. This prevents hard spots, but more importantly surrounds the end grain which is vulnerable. In short you need "fetch" across a joint to increase strength, which is why the 3:1 rule works. Using fabric over the fillet increases this fetch (grip range), adds cross grain strength and improved joint integrity.

In a canoe or kayak, I'd use light weight tape on the chines centerline seam and stem and maybe get by with just fillets on everything else, but the most important place for these fabrics is on the inside of the joint, which prevents the joint from "opening" up under load. If you use 4 oz fabrics for these seams, then they will be transparent once wetted out. 6 oz. is just barely visible. If using regular cloth, instead of biax, cut the cloth on a 45 degree bias. This places all the cloth fibers across the joint which is what you want.

[Arron]

Thanks very much for the detailed response.
Distilling this down to a plan, it sounds like I should do 3:1 (12mm) or 4:1 (16mm) fillets throughout, with tape reinforcement on the chine and stems (inside). I'm trying to build this light, with a committment to being careful in use.


thanks again
Arron

[PAR]

There's no need to go to 4:1 except for cosmetic reasons. If you have sufficient structure, then tape the inside of the seams and hope for the best. By this I mean sufficient structure to prevent the hull from "panting" or other wise working the panels. With tape or fabric on both sides of the joint, this isn't an issue, but with it on only the inside, the outside can crack along the joint if much flexing or movement is experienced.

If you post what design your building or offer some sketches of the boat, I can tell you how much, where and what it'll weigh. On small craft, epoxy and taped seams doesn't weigh all that much, compared to the rest of the hull. In other words, if a couple of pounds of epoxy and tape are busting your weight budget, then you've cut the scantlings way too fine. On the other hand, the powerboat I mentioned above has several hundred pounds of epoxy and fabrics added to the hull structure. In fact, that boat has 97 pounds of paint on it!

[Arron]

Light bulb moment. 'Sufficient structure'. More structure means less flex on the hull means less working of the joints. That makes sense. I had overlooked the relationship between structure (in my case things like bulkheads) and joint strategy till you mentioned it.

again, thanks
Arron

[Boatmik]

Howdy,

The data I have for fillets is from University of Queensland testing. They found that a fillet size of 2.25:1 when ideally applied was sufficient for ply under 9mm (3/8")

So 3:1 is already incorporating a safety factor. After seeing hundreds of structures done in this way there have been almost no failures - I don't think the glass is warranted in the small sizes - same as PAR is suggesting.

With fillets 9mm and larger there is a different equation - and that is the cost of the large volume of epoxy - so it starts to make quick sense to do a much smaller fillet and use tape over the fillet to provide the strength. Dave Gerr's excellent book on construction has a table in it for how much glass for what ply thickness and I did once find the table online - maybe on Google books.

The main thing to note is that good practicing professionals like PAR try to hit things at the right balance of cost to get the required strength, whereas amateurs often add a bit here and add a bit there. Very damaging to prospects of lightweight boatbuilding.

Perhaps the persistent trend to glass the outside and sometimes the inside of plywood kayaks indicates this creep toward heavier weight without good evidence of the benefits. I clearly remember how 3mm and 4mm ply kayaks would last indefinitely epoxy coated only - no glass except for tape on the outside of the joins - fillets on the inside.

But once one person starts saying that glassing the whole thing is "better" then others do tend to follow and a new standard is set. Than later someone comes along and says you have to glass the inside as well.

It might be right if you are surf landing onto a rock shelf ... but most people don't use their boats that way - and a light boat is always much more useful than a heavy one.

It is important to analyse why the weight is being added at each point - great to see that happening here - if you want to build a lightweight boat.

Best wishes
Michael

[Boatmik]

Just for those interested. A 3:1 fillet contains 9 units of epoxy (3 width x 3 height) but a 4:1 has 16 units. Almost doubling the quantity of epoxy. Bad for weight and bad for cost. Thought it would be good to put some numbers there.

Where fillets get too flat to have structural strength - like in the bow of boats where the bottom ply and side panel ply tend to meet edge on - I do put a glass tape over the fillet but just in the ends where needed.

MIK