Just starting a scratch build of a 20th Scale ELCO 80ft PT boat most likely PT 109 at just over 48ins long and a beam just over 12in.
Will try to construct with a combination of chine and planking construction (planking towards the bow) still need to finish of frames

Finish of frame cutting, some left as solid bulkheads.
Added frames to base board
Cut extra patterns for bow shape both side and deck profiles (minus 6mm stringers)
transom set at 12 degrees.

Have been soaking the 6mm square sectioned stringers in the bath for a length of time, over night in cold water or 2 hrs in hot water seems to do the trick.

Put stringers in place held by plastic ties or string until dry (24hrs) then remove and refit with glue and the plastic ties to hold in place.

Please note, don't glue a wet/damp timber as it will shrink as it dries and pull the boat out of alignment.

The blocks at aft are for the three propeller shafts, will be shaped (one has in a later photo) to hull profile. The wood is a very light like Obechi.

Please note. The Models main priority apart from cheapness is to produce a light weight hull that is large enough not to disappear on a pond but light enough for me to carry to a pond/lake as my back is not what it used to be.

Deck
From the construction hand book:-
Decking shall be single layer of mahogany plywood, approximately 9/16 inch thick, installed in general accordance with plan, BuShips No. PT486-S1106-411193, subject to development of satisfactory material.
Note. I have seen photos of some perhaps later ELCO 80s with planked decks.

Planking
62 degrees hull planking angle not 45 degrees as many have used.
stern transom at 12 degree angle approximately.

Prop shafts of real boat.
All three propellers turned in the same direction clockwise looking from rear, not the greatest configuration for a model boat.
propshaft angles are around 10 degrees.
The centre shaft is at a larger angle to the side ones.
The centre prop shaft angle is 11 degrees and the wing prop shafts are 9 degrees.
Using these angles may restrict your propeller selection.

The centre shaft appears from hull further aft than the side shafts but the propellers are all at the same distance from the stern or transom.

Finished gluing in stringers a total of 8 in all, started profiling stringers and rudder and stern tube (prop shaft ) supports.

An important piece of advice from my Dad is to "make sure every thing is fair to the eye." Meaning it must look straight or the curves must flow, no kinks or unsightly lines.

Thus some of my frames had to be adjusted by perhaps padding out or moving the position of the stringer in the frame.

Added gluing supports around the bulkheads and other frames this is to support the 0.7mm ply joints until I get to the Bow when I intend to use small strips of ply or wood.

Added some strength to the Bow also act as glue points for the bow strips of ply or planking.

Created a template of the first section of ply to go on, thought I would mark out the rudders, prop-shaft supports and shaft exit points.

Unfortunately due to cost restrictions I cannot afford to buy or have the facilities to make the correct layout of brackets and supports for the shafts, which I feel may not be up to the job when coping with the high outputs of brushless motor, (still that my excuse)

So to use my poor-mans prop-shafts (with integral support skeg) I have had to reconfigure the layout slightly but will keep the original 3 prop-shafts and 3 non-scale rudders, but may shape them as per the originals.

Yes, jumped the gun and brought the motors.
Already have 3 light weight Aluminium prop-shafts that I intend to use:
Tube Length:25CM/9.84inch
Tube diameter:8.5mm/0.33inch
Shaft diameter:4mm/0.15inch Stainless steel
Weight:67 g

The Motors will run on 3s -11.1v

Three of these (one as spare):
High Performance 2808 1400KV 14 Poles Brushless Motor
Max Watt: 350W
Voltage range: 7-15V
Max amps: 35A
No load: 1.2A

One of these as centre motor or my swap for spare of above:
High Performance 2826 900KV 14 Poles Brushless Motor
KV(RPM/Volt): 900KV
Max Watt: 1000W
Voltage range: 7-18V
Max amps: 60A
no load : 2.2A

After some calculations I find I need to extend the reinforcing for the port & starboard shafts forward towards the bow.
Created some wooden guides when drilling the prop-shaft holes
and glued them in position temporarily, as you make expect the drill needs to be rather long, so I made one out of an old brass shaft basically sharpening it so it could cut wood.

Next job will be to cut out for integrated shaft support brackets (2mm wide and minimum of 35mm long slot) and the tube shafts are 8.5mm in diameter.

Dry fitting pro-shafts (Note, all shafts parallel with the keel)
Drilled with an old prop-shaft which I modified to a cutting tool then finished with a round file and cut slots for propeller-shaft support bracket with a hacksaw, file and knife.
Note these shafts are lightweight aluminium tube with steel liners and steel shafts. props at currently 40mm but as with all things scale things are not always practical in scale size such as propellers and rudders and in this case the direction of rotation (not all going round clockwise).

Have added reinforcing to the frame supporting the end of the shaft tubes, 3 pictures show dry fit.
and once dry glue in shafts with epoxy resin, 3 pictures of fixed shaft tubes.

Now looking at an extra part frame to support the three motor brushless out-runners.