Calculating USCG capacities for pre-74 boats

[Fr. Frank]

I learned a cool way to calculate the weight capacities for an older boat. You need a crane hoist and a swimming pool. I just watched a friend put his '71 20' CC into his swimming pool - no kidding. He measured the rise of the water in the pool, then calculated the displacement in gallons, and then in pounds. He calc'd his with the motor mounted, so he has to deduct the weight of the motor.
Total displacement for his 20' was 10,800 lbs. From that, you subtract the hull weight, (actual 1630 lbs), then subtract his motor weight (427 lbs), divide the remainder by 5, giving a legal capacity of 1748 lbs. for persons, motor, and gear, and a maximum persons capacity of 12 persons [(1748 +32)/141 rounded down).

So I then looked up the USCG formula for max recommended hp. FTR, the formula is similar for sterndrive and inboard power.

The formula for recommended maximum horsepower is simple: for a 20" transom, LxWX2-90=rounded to the next higher 5hp increment max recommended hp . Length and width are LOA and WOA, and are calculated to nearest foot. The 20' SF is actually 19'8" , and the beam (including rub rail) is 7'9". So the calculation is 20x8x2-90=230 hp maximum.

According to the USCG, LOA includes permanent fixtures such as bow pulpit, dive platform, anchor rollers, engine drives, etcetera.

Therefore my Seafari 20' with platform and anchor pulpit becomes a 22' boat suddenly, with a legal recommended max hp rating of 265 hp.

For boats with higher than 20" transoms, the calculation is different, but I quit looking after I found the above.

Why the emphasis on recommended max hp? For boats over 13', there is no actual legal maximum horsepower. Just recommendations. The NMMA has their own standards, but they won't tell me what they currently are.

[htillman]

Great info.

[Islandtrader]

I am only missing two things...

Swimming Pool

Crane

[Fr. Frank]

I wonder if there is a way to calculate actual displacement without getting the hull wet?

[gofastsandman]

Yes Fr Frank there is. My other brother in law is an engineer at Bath Iron Works and heads up weights and measures. PM me if curious.
GFS