Hi Dave,
Propshaft angle talk;
For a model to run properly on the plane, the angle of the propshaft should not exceed 7 degrees.
Attached is a picture showing the angle on my M.A.S., given that the hull is 110 cm long and the propshaft is 50 cm long, the shaft is roughly half the length of the hull, this gives me the opportunity to have a 5-6° angle and still have room for a Ø60 mm prop.
If the Riva should arrive at a similar angle, the shafts should be about 40 cm long.
You measured only 24 cm, which is dramatically short if the motors will be mounted in direct drive, as shown on the plans.
Sure, the boat will run, but the steep propshaft angle will lift the transom and push down the bow, once you get over a certain speed, making the boat plow through the water like a tug.
More power will eventually get it to plane out, but the running attitude will not be good.
Either build the boat according to the plan and accept the fact it'll run disappointingly, or go for (much) longer propshafts, or use belt drives to reduce the propshaft angle as much as possible, using 24 cm long shafts.
Longer propshafts with the motors under the front deck, in order to not compromise the interior, is a major change and should be done while you still have access to the interior (prior to planking).
Using toothed belts, so the motors sit next (or on top) of the propshafts, requires very little change, but should also be done prior to planking, to have good access.
As an example how toothed belt drives look:
https://www.rapidonline.com/reely-toothed-drive-belts-iso5296-521179In the Riva I would use small pulleys and short belts, to make things fit under the motor hatch.
Here's an example of a forummember (Red181), he tested various combinations to arrive at the optimal rpm for the prop, I'd go with a 1:1 gearing, using two of the small belt wheels, so the shaft will be as close to the bottom as possible:
https://youtu.be/e0vJGoCM8vMLet me know what you think.
Regards, Jan.