Trimarans has three hulls: the main hull and the two outrigger hulls.
For a putt-putt steam engine driven model a narrow main hull can’t carry the engine when the hub is high, so you need outriggers.
Fort he outriggers you can choose an optimal hull. You can take as basis the simpler definition of „optimal”. The main attributes are: the stem is the same as the stern. For convenience the geomerty between the midship and the ends on waterline, and of the keel is a bow. In practice it could be something other, e.g. three straight lines with bow transitions, a straight line at midship with bows to stem and stern, or a parabola…
On the other side at low speed the friction of hull is significant, so you can choose the hull form with minimum surface, a semicircle. At least for getting the greatest speed you can take the mass for a minimum, for having less displacement, so you have to move a minimum body of water. The wave resistance at waterline is virtually unimportant.
The scales of practical hullforms suits the requirements, e.g. racingkayaks, that means a length/beam ratio of L/B=11 (at a draft with semicircle D=B/2).
Step 1: Pieces and Tools
Some pieces can be made from usual modelling material, as pinewood, beech veneer, quick epoxide adhesive, and we can make tools for fixing like gages, or flexible sticks.
Step 2: Carry Out the Frame
To carry out a semicircle hullform makes some difficulties, with traditional wooden planks you have to turn off from the semicircle. You can make the approach, that you take two vertical sideplanks, and two underplanks with 120 degree bevel at keel. In that case the intersection of vertical and 120 degree plank is at half draft, and the intersection of two 120 degree planks under the keel is under the semicircle at about 0,043D. The fillet is R=0,31D between the planks.
Forr assembling the outriggers you can use the baseline method. The deck is here a horizontal plane, we put this on a horizontal plate, and build the frame: at first the transverse ribs, then the longitudinal frames, and the planks.
The length is divided in 4 pieces, we put one rip in the middle, 2 smaller rips ont he ¼ length, and at least 2 horizontal triangle-formed rips. The height of rips is the height of semicircle, plus the difference 0,043D, and the freeboard. The thickness of the triangle rips is only the height of the freebord.
Because the sections have 3 fillets, it is subservient to make 3 longitudinal frames, wich have enough width and thickness for making the fillets (width about 0,6D, thickness 0,18D). The longitudinal frames abuts at the ends in a distance from stem and stern of 0,19L. From this point we can cut them straight to the ends.
Step 3: The Gage
For easier assembling you can make a gage.
You draw on a board the outline of waterline. In the middle, at ¼ length and some centimeters from the ends you can put in the centerline a nail (only the slaps). Around them symmetric on both sides you can put obliquely screws (with 45 degree). We place the center-bored rips on the nails. Than we put on them the longitudinal frames, and tie them to the screws on both sides with rubberbands. We stick the frame with quick one component epoxyde adhesive. It stiffens in about 2 hours (even the 5 minute adhesive), becouse in some places there is more adhesive, with light pressing you can see if it is even liquid. When it is stiffed, you can polish to the true form, mainly at the ends, and longways at the 120 degree cut longitudinal frames, because thay have to give longways a continual cylindrical surface for lapping the planks.
When it is stiffed you can tie the Lx0,11D tailor-made thin (0,6 mm ) vertical veneer planks at the two sides (e.g. with twinged pins), than you can draw near the longitudinal frames the outlines, and cut to size (max. width about 0,78D at the sides). Then we slush the planks with a slower adhesive, and fix it. The same way we tailor and stick the two 120 degree planks (max. width about 0,11D), for fixing the screw tied rubberbands are suitable. The joints can be made with successive polishing and cutting. The sticking lasts with a one component 15 minutes adhesive about 3 hours (the adhesive is not film-sized thin, but on some places thick).
We form the stiffed sticked hull to the true form with polishing, and polish up the joint edges to fillets.
Step 4: The Deck
Then we turn the hull down with the deck, and draw the outline of the deck on a veneer, cut it to size, and stick with quick epoxyde adhesive, and polish.
Step 5: Assembled Outriggers
The trimaran outriggers has lateral beams, wich can be sized to the known level of main hull and waterline, and sticked to the outriggers. You can make a gage for this, but the main hull can also fit for adjusting the true levels and perpendiculaires.
Before painting you can smooth out the outrigger surfaces, polish, and ground with 2 streaks slurry achromatic varnish, and then with colours appropriate to the main hull.
At last we can control the empty and loaded weight (displacement).
In case of the carried out model the waterline length L=365 mm, empty weight 70g, loaded weight (displacement =2V) 160g.