Why the keel radius?

[Mark Gudschinsky]

We're finally back in the boat shed working on Laura's Skipjack. We're nearing the point of putting in the radiused fillet at the joint at the keel and the bottom. A few questions have arisen:

1) Why is it there? structural? / astatic? / Boat Speed? / Because the Plans Say?

And if the conventional wisdom convince us that it should be there what are material options?

bondo is out because of prior longevity experience.

So? Epoxy putty? what kind or mix of formulas? Fillers?

[Barry Pyeatt]

The radiused fillet serves a couple important functions. Smooths waterflow across the bottom/keel junction which makes the hull more efficient. Water tends to get stalled in sharp crevices and create drag.

Adds a measure of strength to the junction of the two surfaces which is important when under stress of normal sailing conditions.

The fillet can be made up of a number of different combinations of materials. A coved piece of wood can be placed/epoxied into the junction which will fill the space nicely and only require a small amount of filler to make a really smooth radius. That can then be epoxy and a suitable filler such as wood flour or microballons. Both are easily shaped and sanded to smooth profiles. Wood flour is the least expensive and readily available. It can be as cheap as sanding dust from the project or purchased for consistency in whatever quantities you might want. Microballons are easier to use and sand leaving a nice ready to prime and paint surface when sanded. Easily sanded with normal finishing papers like Bondo is.

Is it critical, no, but it will give much better performance with the fillet than without. A short section of 2" PVC pipe will give it a nice shape.

You could also wrap the glass cloth up over the fillet onto the keel a bit and make it a continuous piece. Then fair it in if you so choose.

[PAR]

There are two types of fillets, cosmetic and structural. Both usually have some cosmetic issues associated with it (you don't want an ugly fillet, for example) In the Weekender's bottom panel to keel joint, you'd be wise to treat this as a structural joint.

Microballons and other very light filler materials mixed with epoxy are for cosmetic fillets only, though a structural fillet may have a "skim" coat of light weight filler covering it, to smooth it out. Microballons impart very little strength to an epoxy fillet and the keel to hull joint is usually highly loaded.

Structural fillets are made with high density fillers (there are a number available), most of these should be coupled with 'glass tape set in epoxy to complete the "composite flange" element of the engineering, which all it truly is.

The cosmetic fillet is as it sounds, a decorative radius added to smooth an area. This may be for water drainage in the cockpit, an easily cleaned locker bottom, or to "fair" a thru hull fitting into the underwater areas to make the water flow past it easier. These types of fillets are usually completely of light weight fillers and epoxy, such as microballons. The cove piece of lumber Barry mentioned, is an application tip, used in non-structural fillets, to lessen the amount of epoxy needed to make the transition smooth. Typically these fillets don't need the added reinforcement of 'glass tape.

All the things mentioned, for putting into epoxy; microballons, wood flour, chopped strands, 'glass tape, etc. are reinforcements. Epoxy is a brittle and relatively weak material if it wasn't for the addition of the many materials we put into the goo. These materials add specific qualities to the "matrix" of cured goo and reinforcement. This combination of elements are engineered structures and can be relied on for repeatable performance, but the procedures, methods and techniques should be adhered to, if the desired results are what you will expect.

Log onto www.WestSystem.com and look up their methods and applications guides. These will explain the different fillers and their applications. These guides can been down loaded (very good idea) or they'll mail them to you free (also a very good idea) They will help your understanding of these materials a great deal and make your learning curve shallower.

[Mark Gudschinsky]

Well I was hoping someone was going to tell me it was just for looks and save me some time. Oh well.

On our Weekender there were a few spots were we had some minor problems that I attributed to using bondo as filler. And the thought of that much epoxy to form the radiuses made me pause. So I'm leaning toward the idea of forming a coved piece of wood as a basis for this joint and glassing over it for strength and finish.

[Mike Pennington]

Mark and Family,

It is good to hear that the gang is back in the boat shed. I need to do the same, but I think I would have a serious problem, especially if Joan had to give up her fancy interior parking space. :wink:

Happy Holidays, :x-mas:

Mike

[Mark Gudschinsky]

Hey Mike, good to hear from you.

But now that you live way down south. You shouldn't even need an indoor boat shed. I hear it hardly ever gets colder than 20 or 30 below in your part of Alaska. You're not getting soft on us are you?

Happy holidays

MARK

[Greg Luckett]

Par,

Is wood flour considered a "high density filler"?

Thanks.

[Charlie Jones]

I consider it so Greg.

[PAR]

There are three basic types of fillers (reinforcements) used in epoxy for fillets, thixotropic, thickeners, and fibrous. Thixotropic is to control flow, prevent sagging, cling to vertical surfaces, etc. A liquid that has thixotropic properties will flow nicely under a putty knife or squeegee, but will stay in place once you leave it alone. These liquids also have good gap filling qualities, so it easy to understand why we like these materials in our epoxy.

Silica types, such as Cab-O-Sil, Aerosil, West 406, plus mini or micro fibers, and wood flour are the usual suspects for thixotropic reinforcement additives in epoxy.

Quartz microspheres, phenolic microballons, West 410 and wood flour are typical thickening fillers. These add bulk to the epoxy mixture. Some will also provide some thixotropic properties (like wood flour), but others will not and if used on vertical surfaces or in less then perfectly fitting joints will run out or slide down. This is a common issue with beginning epoxy users. Using microballons, for example, as the thickener in a fillet, just to find it wouldn't stay in shape, but preferred to become a pool of goo, having little trouble finding that gravity always pulls in the same direction.

Milled fibers, plastic mini or micro fibers, West 403, chopped strands, etc., are types of fibrous reinforcement. These materials add strength to the fillet, particularly tensile.

Most every fillet will need more then one type of reinforcement material added to the epoxy, to take advantage of the properties they provide. You'll need some thixotropic material in the mix to keep it in place and some other material to provide the needed qualities, be that a light weight, easy to sand fairing compound or a heavy, well cross linked material to insure maximum strength.

Of the materials used in fillets, these can be further broken down into two groups, adhesive and fairing. These can be high density (adhesive) to low density (fairing) in nature. As a general rule the adhesive mixtures will form a strong, hard to sand plastic, typically used in structural applications (like keel fillets). The light weight stuff is of course easy to sand, shape and is for smoothing and comedics.

There are a number of materials that can be added to epoxy, each having it's own qualities incorporated into the mixture when cured. Wood flour isn't what I would use for a structural fillet on the keel. It's at the lower end of the scale for the adhesive type of reinforcements. It's moderately easy to sand, which gives a clue how strong it is.

Some adhesives, like West 403 or micro fibers seem to increase the wetness of the epoxy, making for a good stick. This is particularly true in laminations. I use Colloidal silica with a touch of micro fibers (to increase the wetness thingie) for a keel fillet or other structural fillet, I don't want to have to worry about later.

A friend just called me last night about a new stem he made last winter. He laminated it up and used microballons and some wood flour (heavy on the balloons, because his flour came from the belt sander) After only a year of use, the stem is opening up, showing the glue lines on the laminations and letting in water. A lamination requires a strong, adhesive reinforcement, not the very light weight, thickening filler of microballons. He'll be redoing his stem again this winter.

In the end it takes lots of practice and the willingness to test your work to destruction every so often. NO, don't bash your boat into something hard and pointy, but make up text pieces, when you have a batch of epoxy mixed up and glue some scrap chunks of lumber or plywood together. Then wait a few weeks and test the bond. This will give you a good idea of what your repair or new construction will tolerate. You may (as I have more then once) find the glue wasn't up to the task or wasn't applied correctly, etc. It's a lot easier to fix when you know about it, then when you're trying to figure out what's wrong and the boat is leaking.

[Greg Luckett]

Par,

Did he use epoxy resin with fillers on that stem? I have built rudders, furniture, boats, etc. and never had anything like that happen. I have only used West Systems epoxy and fillers, the 406 colloidal silica and 404 mostly. My boats are out in the weather year round, sometime even under a tarp, and for 5 years now. My suspician is that your friend may have been stingy with the epoxy, or mixed it wrong, or maybe did not spread it well? It could also be that I am not understanding the problem too.

[capt jake]

Wood flour isn't what I would use for a structural fillet on the keel. It's at the lower end of the scale for the adhesive type of reinforcements.

This is contrary to what Devlin states in his book. Wood flour is known to be harder to sand and used in conjunction with colloidal silica, forms a strong fillet. I recently started using more flour and like the thixotropic properties as well as the bulking and ease of mixing. I hope Joel (boatbuilder.org) will see this thread and give his thoughts.

[boatbuilder.org]

I do not know how your boats are built as far as glassing goes, but when I need a structural joint, I treat the fillet as nothing more than a way to get the glass to lay nicely. In our construction the glass is the structure and after multiple layers of 12 oz biax the fillet doesn't really mean anything. If you are filleting and just covering with a couple of layers of 6 oz then I guess that is a different story, but I don't think I would use anything but a Cab-O-Sil wood flour mix. I always overbuild and try to keep is simple so I never get bogged down with this type of thing. If I were get trying to keep thing very light and cut down on materials I guess I would have to re-think my approach to construction.

---Joel---

[PAR]

Greg your success can be traced back to using 404 (West's highest density and most adhesive filler) and 406 (West's second highest density and adhesive filler). Joints, fillets, etc. made with high concentrations of these materials will be very strong, quite difficult to sand and long lasting. Steve's stem issues (the guy who did the work) was using microballons and course powder from the belt sander. Wood flour needs be exceptionally fine (usually milled) and the finest belt sander paper will not produce very fine particles. His second mistake was the mixture, mostly (by a lot in his words) microballons with a little wood flour (chips really) from his sander. This resulted in the thickened epoxy having some thixotropic properties (likely not enough) and little adhesive properties (microballons are a bulking agent). I'm sure it was easy to sand, but this isn't what you want in a stem lamination.

My laminations need to be machined (table saw, router, etc.) before I even think about dressing them up. This is because the filler usually is much harder then the material glued together and sanding this usually removes more wood then epoxy. This is why I machine them flat first. After, I can start with a level playing field.

West system fillers listed in hardness (and most adhesive qualities) are 404, 406, 403, 405 in descending order. They offer a couple of fairing fillers 407 and 410, which are the least adhesive and the most light weight.

Sam Devlin pretty much refined the stitch and glue methods seen today. His use of wood flour is to add thickener and thixotropic properties with the same material, which is fine. He also backs up his joints with a strip of tape, which imparts a great deal of strength to the joint. Wood flour in a structural fillet certainly can be used, but there are materials that have better adhesive and strength properties. This is why I recommend silica or higher density fillers in structural fillets in the previous post. When it comes to an immersed joint, I tend to error on the side of over kill, rather then good enough.

I've preformed many tests on products and materials over the years. I remember casting some 1" diameter cylinders of different filler combinations with my brand of epoxy. These were about 5" long and left to dry for a month. Some were clamped in vice and sanded down an inch with a belt sander. This was timed event and how I determined the densities of West's fillers. I also crushed them in a bearing press, placed them in tension and performed sheer and bending load tests too. I've tested all sorts of stuff over the years. This is how I know that Dynel is 3 times and Xynole is 5 times (close to 6 times) more abrasion resistant then regular 'glass cloth, set in epoxy. I currently have a plywood test on going (two years now) on the effects of different surface treatments and the baseline (bare ply).

Cart. Jake, Colloidal silica is a high density (and very adhesive) filler. Most anything mixed with it will be very hard to sand.

Joel's build style is treating the wood substrate as a core, this also true of any fillets, that may be there. Cored (or sandwich) construction relies on very different principles then stitch and glue. The wood only provides distance between the inner and outer skins (like and "I" beam) and this permits light weight construction with high strength. Of course the sheath must stick very well to the core material to take advantage of the engineering built into this method.

[boatbuilder.org]

Joel's build style is treating the wood substrate as a core, this also true of any fillets, that may be there. Cored (or sandwich) construction relies on very different principles then stitch and glue

I guess that is a new one on me...

---Joel---

[capt jake]

I guess that is a new one on me... 

And I guess I am glad you stated that. Uhhhh, all of Sam's designs ARE S&G........

[PAR]

In our construction the glass is the structure and after multiple layers of 12 oz biax the fillet doesn't really mean anything.

This isn't sandwich construction? What did I miss?

[boatbuilder.org]

This is what we do:

http://www.devlinboat.com/constructiongodzilla25.htm

http://www.devlinboat.com/constructionsockeye45.htm

http://www.devlinboat.com/constructionsurfscoter25.htm

Do you consider it sandwich construction? I certainly do not.

---Joel---

[Greg Luckett]

Hey Mark ,

Any pics of Laura's Skipjack being built?

Thanks.

[Mark Gudschinsky]

Hi Greg,

Yes a few, but still in the camera. I intend to have her post a few right after the Christmas bustle subsides. And just to make sure see is lured back into the boat shed after Christmas, I'm pretty sure Santa is bringing her some special wood working tools she has been eyeing. But I can't say more until gifts are open, or Santa won't let me in on future secrets.

[lbrewer]

Joel's build style is treating the wood substrate as a core, this also true of any fillets, that may be there. Cored (or sandwich) construction relies on very different principles then stitch and glue.

Do you consider it sandwich construction? I certainly do not.

---Joel---

Hi everyone,

I think I might be missing something here. Why are the terms "stitch and glue" and "sandwich construction" exclusive of each other? It would seem to me that if the plywood panels are stitched together with wire to temporarily hold them while they are filleted (glued) that pretty much seems like "stitch and glue". If those same panels are glassed on the inside and outside then the plywood would seem to me to be sandwiched between the layers of glass.

Larry