Engine size for Farmers' Piute

The Propeller Handbook by Dave Gerr is a good guide to selecting a propeller size for the technically inclined. PROPELLER HANDBOOK https://www.gerrmarine.com/PROPELLER_HANDBOOK.html It includes a simplified method for estimating power vs speed.

 

I think 20 mph just isn't realistic as a cruise speed with the sort of power mentioned, regardless of propellor optimization, 15mph as a cruise speed would be about the best you could expect, the problem is that frictional drag starts to kill in a boat that has little ability to reduce wetted area at those speeds. And look at that deep skeg, dragging that through the water at speed just worsens that situation, this idea that the relatively low power engine can be throttled back and the thing lopes along at 20 mph, is a fantasy.

 

Out of curiosity I looked up what Duckworks had on their site about Piute.This is the relevant part:

Any motor of about 4-inch bore by 4-inch stroke delivering 20 to 25 hp at 1000 to 1200 rpm and weighing in the neighborhood of 650 pounds will be ideal.
Duckworks http://wwew.duckworksmagazine.com/05/designs/farmer/piute/free.htm

The engine is cited as weighing something like 650lbs.So I don't see why a modern diesel like this Beta,which has more power and around half the weight wouldn't work at least as well,or even better. https://betamarine.co.uk/portfolio/beta-35/

I would think that the better fuel efficiency would allow less fuel to be carried for the same amount of running and further enhance the performance.The real challenge is likely to be selecting a propellor to give the desired performance but an experienced supplier would take all the factors into account before making recommendtions.It might even need a different reduction ratio in the gearbox.

 

The VolvoPenta is about 360 lbs including gears.

 

This is probably the best bet. Treat the hull like a blank slat save the 650 hp expectation for engine weight.

The boat was designed by a guy who died 40 years ago, some of his suggested engines were discontinued 60 years ago. Not much point in trying to laterally figure out equivalents....

Solve what's actually needed from the data given from the hulk and desired speed. Then go engine hunting.