Dry Strip Planking

Hi there,

I'm going to be starting my first build of a Farrier F32AX trimaran shortly in Normandy, France. It's going to be more of a cruiser than a racer. I'm thinking of using Paulownia for the strip planks in 14 x 25mm, to finish at about 12.5mm thickness. After fairing the hulls are covered with 200g unidirectional glass inside and out.

Initially, after reading the Gugeon Brothers book and having talked with a guy in Canada who built a red cedar F9AX back in the late 90's, I was considering using cove and bead. He actually built his using regular rectangular strips glued together, but said that if he were to do it again he would use cove and bead as he thought that this would make the lightest wooden boat.

However, having read numerous posts on the forum here, it seems that the general concensus was that this took lots of time, wasted lots of wood and at times the strips wouldn't necessarily even line up. So dry strip planking as a build technique had been put forward as a significantly quicker build - sounds great! Ive been reading as much as I can from the few posts on the forum and on the Mahi Mahi catamaran construction blog: Construction strip planking du Mahi-Mahi 2, cata open de 9×6m, modèle : One Off https://one-off-design.blogspot.com/ where he cuts trapezoidal strips at an angle of about 5degrees leaving a space to the outside that he fills in with epoxy.

Anyway I've got some questions that I hope you guys might be able to help me with.

The Farrier plans use a female mold with the forms spaced at about 750 mms apart. This is a bit wide according to the Gugeon Brothers who suggest 400mm for 14mm thick planks. So, I will check it out and consider making intermediate forms if necessary.

What I'm wondering is how small a spacing I can go too? Would 1mm spacing work okay?

One idea I have to maintain spaces is by using Raptor composite nails either side of a tile spacer at regular intervals.

Although this leads to my second question; if the planks are nailed together is there any need to leave a gap for epoxy? Rather, just go over with epoxy, after fairing, to fill in where there are spaces in the wood? Or, yes the glue is important, so leave a space for epoxy at the exterior surface by cutting trapezoidal strips, as in the MahiMahi blog?

So many choices! Any recomendations ?

My other questions are regarding the epoxy.

If the glue does not need to be stronger than the wood, does this mean that it can be mixed up to a similar density as the wood? In the case of Paulownia about 300kgs/m3. So cutting the epoxy down by a factor of three? Or does it need to be stronger?

Of course, my concern with using the dry strip planking technique is that it will use more epoxy and end up being a considerably heavier boat. As, I said I'm not looking to make a super light racing machine, but obviously I want to keep weight down to a minimum without compromising the structure.

Also working in the cooler end of the spectrum, at about 16degrees C, if the epoxy needs to be thinned to flow better and to easier penetrate the interstitial spaces, what would you recommend?

Hope you can give me some pointers to consider!

Best wishes, Mike

 

Since you are using square edge strips there will normally be some gap on the outside, particularly at the turn of the bilge. I'd possibly taper some strips that are in flat areas. Probably make a jig to hold a strip solidly and just bevel one edge.

Do not thin your epoxy, any additive that I'm aware of will seriously degrade the product. The regular method to increase the workability is to warm the epoxy but it is a fine line to walk, easy to have it a little bit too warm and too large a mass and have an exotherm event.

 

Most of your questions can be answered by what the plans call for.
If you are not confident with the plans, then pick a different proven plan.
Second guessing the plan negates the advantage of building to a proven plan.

Stitch and glue would be a much faster ( and lighter ) build.

 

Thanks for your advice about not thinning the epoxy, Skip.

Bluebell - there are no details in the plans for dry strip, just regular glueing of rectangular strips which is why I ask the questions! Info on dry strip I have found only here and in the blog I mentioned.

 

Strip planking is one of the best ways to get a compound, complex , efficient hull shape. There are plenty of Youtube videos on strip plank multihulls, including a recent one, "Sailing SV Lynx", which has two hulls partly strip planked with composite core. After my personal strip plank project, I would be totally happy to do it again, just for the more aesthetic appeal, and the skills to do it are soon picked up, especially if you study a few online stories. It isn't as easy as stitch and glue, but it is very satisfying.

The trade off between difficulty and time will be the value of the hull afterwards. Modern compound hulls will always bring a premium on resale , as well as better overall performance benefits over their life.
There are plenty of plans that use slab sided stitch and glue, and they work fine, and are probably as good at performance in general. Its a case of personal values.
Lots of comments here Wide Gap Strip Plank - what problems could this building method cause? https://www.boatdesign.net/threads/wide-gap-strip-plank-what-problems-could-this-building-method-cause.24170/

The Raptor nails appears to be a good idea, but funnily enough, the most recent project that I know of, went and pulled them all out before glassing. Maybe you can ask them why.
https://www.youtube.com/@Building_a_Boat_by_the_River

Your idea of spacing planks with a wide gap, and filling in with say, thickened epoxy, has been used on several designs over the years. Funnily enough, they used Paulownia as well.
Subsequent comments say that it makes the prospect of laying the planks easier, but the amount of epoxy worked out to be substantially more than expected, and a nuisance to fill and stop from running out of the gaps, and a heavier hull.

My opinion on cove and bead is, that half the planks will need to have the cove planed off to fit the moulds where they taper into each other, so its a waste of time and money. You say your are not after a racing machine, so I suggest that it is better to leave the edges square.

Finally, never think about thinning Epoxy for flow properties. It ruins both the water exclusion and strength properties. Even the thickest epoxy can be made to run into gaps with a modest amount of warming.
My experience with Paulownia is totally positive. One of the better timber for this type of boat building, if you can get some decent clear stock. Bear in mind, it doesn't sand and shape as easily as say Western Red Cedar, but it is more durable.

Its a great project. Just make sure you do it in an easily heatable/coolable , indoor space, both for comfort and quality .

Good luck.

 

There are many questions here and not all answers are straightforward.
1. Paulownia. You can answer this one by looking at the difference between the lamination schedule for wood and foam. No difference means the wood was considered a core and paulownia is ok, big difference means the strength of cedar was considered and changing the wood mechanical properties must be compensated.
2. Raptor nails. They have no shear strength so there must be enough epoxy between strips. The epoxy mix must be strong enough for that.
3. Gap or trapezoidal strip. Since this is a female mold lying horizontal you must do something to ensure gravity doesn't affect the epoxy filling before hardening. You can do a very stiff mix wich will be hard to press in correctly, tape the gap on the underside, relieve enough material from the strips so that the outside touches and you still have room to fill.

My advice: make some test panels with cheap spruce/pine/fir and see what you can do. Set up two frames and plank up, a third conventional square battens glued one at the time, a third with a gap, a third with trapezes. Sand and fair as you would for glassing. Stop the time for each method (including preparing the strips and final sanding), cut them up to see glue penetration, weigh them, break them, etc.
You will quickly find out what you consider to be better in the weight/ease/speed compromise. Keep in mind, having a helper to mix epoxy or using cartidge based premixes significantly alters the equation.

 

Paulownia

"Paulownia Elongata": 28 Newtons / mm^2

Okumé Plywood 10 mm: 30 Newtons / mm^2

 

Paulownia Elongata

 

What surprises me about multihulls is the fiber

For a monohull of 6 meters in length (19.64 feet) and 800 kg, 1250 kg at maximum maximum load, I am thinking of 800-900 g / sqm of fiber

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not because of the pressure of the sea but because of the amount of Epoxy resin to insulate from humidity and for abrasion, friction in port and when landing on the sand

I am in a port that is very moved by waves in autumn and winter and I am very concerned with friction and therefore my case is not translatable to other cases.

 

I glue each plank when strip planking. The chances of forcing epoxy all the way through a thin gap are pretty small and the mess left on the outside would make fairing harder. I've been doing quite a lot of this lately and I love the process. Easy to fair, thin skins that don't require vacuum bagging, and strong for the weight.

 

I never understood the reason for working on a female mold. You continuosly get in and out of the boat, have to build hanging scaffolding for planking and laminating, and is much harder to see if the hull is fair. What are the claimed advantages?

 

I didn't use epoxy when strip planking my hull. The shear forces between planks are less than the strength of the wood anyway, so adhesive that gap fills and excedes the wood strength is appropriate. I used a polyurethane called PL Premium from LePage. All of my test panels failed through the strips not the glue line, and strength beyond that is wasted. It cost about $10 usd per liter, caulked from a gun, expanded slightly on cure to fill gaps, and epoxy bonded to it. The excess was easily removed with a chisel before sanding and fairing, before epoxy work began.

 

Don't rely on E Glass and Epoxy if you want high wear protection. Its not going to happen.

You may want a final covering of something like Dynel.

More discussion here,
Fabric for abrasion resistance for hull https://www.boatdesign.net/threads/fabric-for-abrasion-resistance-for-hull.14005/

 

The hull does not lose integrity or flop because all the floors/bulkheads are done. If you have a male jig; the removal of the hull causes deformation.

If the bulkheads are used to make the form of the boat; boats built this way tend to not come out as fair because of the precision involved, and also can end up with hardpoints.

Of course, typical s&g designs are built on male jigs. But. To build them well requires a willingness to actually allow the dev surfaces to not absolutely touch the jig, but be close.

In order for a boat built on a male form that stays in place; a significant degree of precision is needed, but even still this can result in hard points.



to the OP

I am not of a fan of the plan. There are lots of issues. First of all, clear epoxy is very runny. It will all run out of vertical joints, so clear epoxy is not an option.

And thus, you need to make epoxy putty. This also will be lighter than clear epoxy. Epoxy is way more dense without fillers.

However.

Epoxy fillers do not work super well in dry seam layups. There are three problems. The first is dry joint phenomena. This occurs when the epoxy is pulled into raw wood leaving only fillers behind. The second problem is shrinkage. Most epoxy putties shrink a bit and so after you slather the epoxy putty all over the place; you’ll discover the seams are not fair. The third problem is the putty will not want to stay in the seam and will go out beyond to the other side and fall out, a light pass will require tons of effort filling the backside of the mould. All in all a terrible idea.

I have done strip planking for a 16’ Gilpatrick Laker canoe. It is a tedious process, but a gallon of Titebond 3 goes a long way. And what you’ll find is the process goes pretty fast. The strips start on say the middle of the hull. You setup a strip station and you can go up and down and side to side and never run out of work. When strips require customizing; you flip a 3x21” belt sander upside down and form them.

Anyhow, skip that epoxy mess. It sounds easy, but it ain’t.

The key to planking fast is reducing steps. So, you need to design efficiencies into the process on the fly. Keeping strips near the hull. Keeping trim tools near, eliminatinf walking, etc.

Once you get the planking done; you’ll have plenty of epoxy fun as you are gonna laminate I assume.

Pay attention to the author Russell Brown who you were fortunate to have post for you.

For a big job; you may also find a way to roll the glue onto the edge with some glue rolling dispensing tool.?

 

What he said. Eccept that epoxy doesn't make a good bond to titebond. Chemistry eh?