Building a stitch & glue boat in Fiji

IIRC, as a general rule, you need a minimum of 2X the material thickness overlapped each side for taped joints. So if using 9mm ply, you need a minimum of about 18mm overlap each side of the joint. As the material thickness increases, so does the overlap.

On the Mcgill Corp wesbite (they are manufacturers of aerospace industry sandwich panels approved for commercial aviation requirements) they specify this double thickness lap for most taped joints, irrespective of butt joint, tounge n groove, Z-joints, T-joints etc etc.... I also beleive the tape thickness needs to be ~150% of the std panel laminate to reach same strength of a panel without taped joint.

This makes sense in that at 2X the thicnkess of the material the stresses cannot be transmitted at a lesser than 45deg angle from panel edge, over the width of at least 1* material thickness.

Of course, anything wider than this perfectly fine, its just extra weight and cost for little structural value above whats required...

 

Will doing that mean I would be getting the benefits of taping and sheathing in one step? Cos if I use tape 2 feet wide then it covers 100% of the hull surface and I save resin and cloth....that is assuming that 450gsm is plenty for the seams on my boat and and 450gsm is also plenty for the sheathing on my boat. That's all I have anyway, 10m of 450gsm fabric 1.27m wide

I have tried to find out about this previously but I think everyone replying thought I meant taping and sheathing wet on wet rather then just one length of fabric acting as the tape and sheath

 

its because the sheath in 450gsm is overkill... you could sheath in 200gsm and be done with it, but this is nit picking as the difference is only 500grams per meter (fibre + resin) or a total of maybe 3 kgs in the whole boat - so who cares...

Means less fairing for you also, so yes- i would sheath the entire outer hull with 450gsm dbias and you can probably forget any additional tape along the outside joins - depends on the required strength of the chines and gunnels etc - are you using chine logs etc - i dont know....

On the inside, id probably just tape the joins and simply epoxy coat the ply rather than sheathing it, but this is up to you and how much toughness you think you need - consider dropping fishing or dive gear and puncturing the epoxy coating allowing water into the ply etc... maybe just sheath the floor? Again its only a dingy so its up to you if you arnt following any plans...

 

Correct, no commercial plans. I only have 450gsm double bias fabric 1.27m wide 10m of as well as 600gsm woven roving 1.2m wide and 10m of. Dont have any 200gsm fabric

No chine logs. But will be using the 600gsm roving as inside tape as well as sheath for the transom (maybe) and sheath for the deck and sole. Planning to do inside hull panels just epoxy coated. Wide fillets on the inside seams. I can cut the 450 double bias into tapes for the inside seams too but the roving sits nice and flat too

 

If it were me, id sheath the outside panels fully in 450gsm with addional layer of 450gsm tape along the chines and keel. Id also add another tape of 600gsm WR along the keel as a bit of a wear surface for dragging up the beach or bouncing across the coral etc... the keel and chines always wear out when it sits on the beach and rocks back and forth on the sand and in the shallows... broken coral beaches, shallow reefs etc... itll take the paint off in these areas too if you let it sit or moor it like this... i also live in the tropics and do lots of reef fishing

Inside, id just tape the seams with the 450gsm dbias cut into 100mm wide tapes and simply treat the boat with a bit of care and respect...

I cant think of another use for the woven roving other than perhaps 1 layer each side of the transom, along with 2X450dbias layers both sides to get a quadraxial laminate on it with a decent knee for bracing, woven fabrics dont really have much use structurally other than building up thickness and toughness, or light wovens for cosmetic reasons. Stitched, non woven fabrics are used for best structural efficiency.

 

hmmmm With 10m of 1.27m wide 450 I can get sheaths on the outside and just enough offcuts to use as tape on the inside. Which means a ending up with a single layer of 450 double bias on either side of each seam. If I tape inside and outside I will not have enough to sheath with

Glassing scheduled for tomoz, lol I thought I had it all worked out, back to the drawing board!

 

Well just use the wr as added toughness or wear thickness... it won't wrap as tight around the chines tho. You'll need to round them off to wrap them without air bubbles, a bigger radius than the 450dbias woukd require. Then afterward, build them back up sharp again for planing performance with thickened epoxy.

 

Here is a thought that just popped into me head. Would the roving be any good to make strakes with? I am pretty handy with a jack plane. Maybe I can layer it on the chines without folding over the seam and plane off the excess for sharp clean chines?

 

What about transverse frames... have you considered these yet? You'll need stiffeners of some sort and these need to be taped in too... anchor well could use a layer of wr so you can throw the anchor in there without worrying about damaging the ply... just sit on the wr for now, you'll think of something to use it on later...

 

Strakes ? You mean chafe protection skids on the bottom of the boat running fore and aft ?

Best to just glue a strake of wood on. No fasteners, no eglass tape. When they get all buggered up remove and replace. If they are buried under a mountain of glass fiber its a big mess to remove or repair. If you use fasteners they will leak. Cut the shape into the strake don't bend it onto the bottom,

Only use biax on the outside. Roving is so crude that you will need a few kilos of fairing compound to blend it in and make it look good.

Chines of thickened epoxy are durable and easy to repair when they get buggered.


Adding all kinds of extra reinforcement only adds weight.

Remember when a lightweght boat bounces on a rock it goes DING...when a heavyweight monster bounces on a rock it goes CRUNCH and sends people nearby running for cover.


Keep your boat light dont over build

 

Have had little time to spend on the boat in the last few days due to the trailer build. That project is coming along fine. We went back to a slide out dolly design due to ease of build. found some Teflon bushing for cars that should make perfect rollers between the chassis and dolly

This evening draped and cut the 450gsm double bias fabric on the hull. Doesnt drape as neatly as the roving but I cant see any problems wetting it out as is. The fabric is in two lengths, one for each half of the hull length, overlap on the keel, no tape


There is enough fabric left to sheath both sides of the transom and tape the insides as well as some tape for the frames and such. I also have 10m of plain weave tape that I intend to sheath the tops of the gunwales and transom with

Decided to pick up that 10x1 20' damanu plank. It is rated as a marine quality semi hardwood. Intend to use that for the rub rails. Under sole frames/ribs as well as maybe stringers for a more ambitious build if its feasible

I am thinking to cut 10" high slots into the transom from the bottom. With the intention of using two stringers that will stick out through the transom. The stringers to be 10" tall under the rear deck and acting as the knees there. Since I am not building from plans, I think I can then from there make the stringers 3" high and run forward planning the bottoms to follow the floor curve. Fit the ribs to that and sole on top

I now intend to make a short height console just forward of the rear deck. Something I can drive while sitting or standing so about 600mm wheel height. Reason I want to put that in is to incorporate a solid lean section for fish fighting. I know the Teleflex steering system is strong enough to bum rest and thigh lean from past experience as well as for maneuvering while fighting a solid fish. console will have a seat on the forward slope for the passenger and tackle draw in the side. Will make that using s&g using ply and give it an aesthetically pleasing curved front like on some RIBs. Console to be a perm part of the boat slotting in through the sole and fixed to the stringers

My hope is that I will end up with an out of boat splash well with that design, something like on the back of a Mac 570 as well as an ability to run a much bigger outboard at a later date without making the boat much heavier. That the hard wood stringers would take the thrust from the outboard and distribute to the transom and hull panels without the complexity and weight penalty of overbuilding the transom and inside framing if mounted on the transom

 

Remember. Minimum ..two inch offset for every later of cloth. Think of an automobile leaf spring and its layers.

When you have a 4 inch taped seam that equals 2 inch on each side or the joint. When you cover this seam with sheathing fabric your overlap will be the 2 inch overlap that is already applied to the joint, plus 2 inch overlap on the sheathing fabric......total 4 inch.
Your overlap on centerline and at the transom looks too short. Extra overlap in OK, minus overlap is a structural problem

 

Difficult to cut slots, fit a piece of structure, then laminate into a waterproof joint. I prefer two components , each bonded and taped.

 

A transom extension can work..it can add extra buoyancy aft by the motor and it can extend the waterline and deliver better performance.

The devil is in the details

Best sketch it out.

An extension of the bottom hull panel makes this entire panel vulnerable if. you have a stern accident..best if the extension is not part of the hull structure and is replaceable, repairaale

 

You need not extend the sheathing fabric all the way up the side to the rail.

Ther is very little load on the sides of a boat..the sides are there only to keep waves and spray from washing away your beer cooler.

The bottom of the boat, to about five inches above the chine , is the part that is most stressed and must withstand pounding loads and crew weight...not the sides

All that glass on the sides is parasitic weight .