So a McCauley Borer propeller was originally for 125hp but now most common on 150hp. It is 82 inches in dia. and came in the past from 39 inch pitch and now advertised 41, 42, 43 and 44 inch pitch. It has an area of (41 x 41) x pie = 5281 square inches or 36 2/3 square feet or 3.4 square meters. It can turn up to 2600 rpm continuous or 2800 rpm for five minutes on a O-290 D of 125hp or a tip speed about 680mph and aluminum propeller (the older 39 inch pitch).

Take a B&S, size dependent largely on what you choose to believe but say between 30hp and 45hp and 3400 rpm to 4100 rpm. Figure a wooden propeller so tip speed below 580mph (51000 feet per minute or 15,545 meters per minute) or more conservatively 550mph (48000 feet per minute or 14,630 meters per minute).

A 54 inch dia. propeller turned around 3600rpm (580mph) or 3400rpm (550mph) 15.9sq ft or 1.48sq m.

A 48 inch dia. propeller turned around 3900rpm " " or 3700rpm " " 12.5sq ft or 1.17sq m.

A 45 inch dia. propeller turned around 4300rpm " " or 4075rpm " " . 11sq ft or 1.02sq m.

When you divide the hp of the 125hp engine by the hp of the B&S engines you will depending get a number(s) around 3 to 4 and when you divide the disc area of the Borer prop by 3 or 4 you will get a disc area similar to those shown above for the B&S engines. Conversely if you multiply the disc area's of the B&S's by 3 or 4 you will get parity with the 125hp/Borer prop combination.

While not the whole story it gives me pause because longer propellers will be thinner due to the hp available to drive them and at some point there must be diminishing returns?

Considering clearance issues, weight, and complexity I wonder is there a real advantage of longer propellers and redrives or at what point the diminishing returns kick in?

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All Pusher, twin boom. And still iterating, but definitely 3 engines.