Our project boat, Toy Boat 2, started out as a hull upgrade. We had a Tracker aluminum bass boat hull, but after a few years of saltwater use, the hull was in pretty bad shape, despite our best efforts at keeping corrosion at bay.
Most of the other critical components were still in good shape, so we decided to see if we could find a replacement hull that would be seaworthy, but small and light enough to be pushed at reasonable speeds with our existing motor (a 90 horsepower Johnson 2-stroke).
Before we could begin our search, the first question we had to answer was: “How big a hull can we push with our old motor?” We wanted to go with as large a fiberglass hull as 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 probably have to trade speed for size. Here’s how I was predicting things would wind up weight-wise:
| Item | Toy Boat 1 | Future Toy Boat 2 |
| Motor | 305 lbs | 305 lbs |
| Hull | 750 lbs | 1,200 – 1,750 lbs |
| Gas tank / Oil reservoir | 13 gallons / 1 gallon | 35 – 50 gallons / 1 gallon |
| Batteries | 2 x Group 27, 1 x Group 24, wet cell | 2 x Group 27, 1 x Group 24, wet cell |
| Boat gear (anchors, etc.) | 50 lbs | 70 lbs |
| Auxilliary power | 41 lb thrust Motorguide 12V | 65 – 80 lb thrust 24V electric |
| Livewell / Bait tank | 18 gallon | 30 – 45 gallons |
| Food & drink, ice | 12 lbs | 35 lbs |
| Passengers & personal gear | 2 @ 200 lb | 2 @ 200 lb |
| Fishing tackle | 35 lbs | 35 – 50 lbs |
| Estimated total | 1,950 lbs | 2,700 – 3,400 lbs |
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. We’ve tabulated the results in our post “Horsepower Estimator Check“.
We’ve also incorporated this formula into a Horsepower Calculator page. Please take a look and give it a try!