Well, back of the envelope calcs for the quantities Ian Farrier specified for my Command 10 (33' tri) plans are as follows. Prices are taken from the Bote-Cote website, a well reputed Australian supplier and are in 2015 AUS$$.
Plywood (Heavy build – original):
1/4" (6mm) 15 sheets @$95 = $1425
3/8" (9mm) 61 sheets @$123 = $7503
1/2" (12mm) 25 sheets @$165 = $4125
1" (25mm) 1 sheet @$225 = $225
Plywood (Lightweight build – modern):
1/4" (6mm) 76 sheets @$95 = $7220
3/8" (9mm)25 sheets @$123 = $3075
1" (25mm) 1 sheet @$225 = $225
912' of timber (approx 273 lineal metres). [NB: He doesn't specify these, so ther'll be 2"x1", 3"x1", 4"x1", 3"x2", 4"x2" etc, so an average is probably 4"x1" @$8/m, so approx $2184
50kg epoxy glue; 50kg epoxy resin for glassing = 100kg @ $722/30kg = $2160
e-glass cloth approx 75sq m (60 lin m off 1.27m wide rolls). Based on a quick calc, this is not enough to cover the whole boat, so was probably only for the wetted areas of the hulls as a waterproof membrane and abrasion resistance. The rest of the hulls would have relied on epoxy coating only for waterproofing and the strength of the specified timber.
I suspect many would have been done in epoxy only, so the spec probably relies on the strength in the timber and plywood rather than in the glass.
Also, many low-cost builders in the seventies used matt glass, rather than woven fabric, which has no strength at all, so Ian probably took that into consideration in the spec as well.
These days, builders might choose to use thinner ply, less scantlings, and stiffer biaxial (double-diagonal) fabric at a higher rate, such as 8oz or more, and taped joins rather than timber scantlings to reduce weight.
This depends on builder
If you chose to do the 'original method' and only use 6oz cloth, or less, as a waterproof membrane only, then use 60 lin m of 60z @ $6.50/sq m = $390
So if you chose to use a lighter ply for the hull
skins, like 4mm instead of 9mm, and substitute 9mm for the 12mm in the specs, and used 100mm double-bias tapes on the joints and then 8oz cloth over the entire hulls, you'd need to recalculate the above.
You’d probably use half as much again in fabric, stiffening stress points and using double layers for additional load-bearing, so say 90sq m rather than 75sq m, or around 70 lin m off 1.27m wide rolls.
Using 8oz (225gsm) double-bias fabric (biaxial, double-diagonal, whatever it's called in your locale) 45deg x 45deg woven fibre @ $14.50/sq m x 70lm = $1015.
You’d also need to reduce the amount for timber scantlings, say by half, and replace with double-bias fabric tapes. So half the amount for timber: $2184/2=$1092
Approx half the scantlings in fabric tape would be approx 420 lin m of 8oz double-bias tape, 100m wide @ $1.20/m = $504
Plus you’d probably need at least as much again for stiffening bulkhead joins and anywhere the lighter-weight sheets join, so double that and add another $504
Plus you’d need extra epoxy to wet out the heavier fabric and tapes, say a third as much again as a guess, so 120kg (up from 90) = $2888.
So building in the traditional ‘ply/epoxy’ method with heavy scantlings, light fabric and mainly epoxy waterproofing cost would be around = $18,012.00
Going with lighter ply, heavier fabric, taped joins, approx = $16,523.00
Then on top of the hull
costs you have:
Hatches and ports
For a total of approx = $17.5K
So total of $35,512 for the traditional method, or $34,023 for the ‘modern’ method.
Not much change out of $40K by the time you add instruments and incidentals, screws, fastenings etc.
Add in say 4000 thousand hours of labour, (roughly 2 years full-time work) at say $20/hr, and this adds $80K to that.
So you’d need to charge $129K just to break even, straight out of the shed, which is why any ply-built trimaran
under $120K is ‘good value’, and anything les than $50K is a bargain.
I’ve actually seen a Command 10 for sale
, here in Australia
, just last year, asking $89K.
If I had the cash I would have bought it and saved myself the back-breaking labour, and just gone sailing!
So even a ‘fixer-upper’ that has no hull rot
, just in the topsides and wings, is probably still a bargain at anything less than US$40K.