One big question at the start of the search was: "How big a hull can I push with my old motor?" I wanted to go with as large a hull as possible, in fiberglass if possible, but on the grand scale of things, 90 horsepower is not a lot of juice.

In the case of Toy Boat 2, I would be trading off speed for size. Here's how I was predicting things would wind up weight-wise:

Would the old 90 HP be enough? Gut feel says "No". However, a couple of reference books ("Fishing for Tuna and Marlin" by Capt. Peter Barrett, and "The Fisherman's Boating Book" by Bob Stearns) offered the following rule-of-thumb formulas for monohulls:

For a modified-V hull (I'm assuming 16 degrees or less):
Total Weight / 30 = HP required to drive the boat at 25 mph, at a 75% throttle setting

For a deep-V hull:
Total Weight / 25 = HP required to drive the boat at 25 mph, at a 75% throttle setting

Using these formulas, Toy Boat 2 would need somewhere between 90 - 136 HP. My experience has been that under most conditions, a cruising speed of 20 mph is actually more realistic offshore, due to the effects of wind chop and swell. So a 90 HP motor looked to be just about right. Remember, these are general rules, and I wasn't expecting them to be more accurate than perhaps +/- 10%.

Incidentally, we have checked these formulas on a number of published boat tests, and they generally seem to hold, when you allow for variations in horsepower (see sidebar).

We've also incorporated this formula into a horsepower calculator. Please take a look and give it a try!