Example Data Table
| LWL |
Loaded Weight |
Speed |
Wind |
Current |
Suggested Size |
| 20 ft |
3,500 lb |
5.0 kn |
10 kn |
0.5 kn |
6 hp |
| 24 ft |
7,450 lb |
5.5 kn |
15 kn |
0.7 kn |
15 hp |
| 30 ft |
11,000 lb |
6.2 kn |
18 kn |
1.0 kn |
25 hp |
Formula Used
Loaded displacement: dry displacement + crew and gear + stores.
Hull speed: 1.34 × square root of LWL.
Speed through water: desired speed over ground + adverse current.
Speed length ratio: speed through water ÷ square root of LWL.
Base horsepower: loaded tons × 4.5 × (speed length ratio ÷ 1.34)3.
Adjusted horsepower: base horsepower × wind factor × sea factor × role factor.
Rated horsepower: adjusted horsepower × efficiency correction ÷ derate correction × reserve correction.
Electrical equivalent: horsepower × 0.746 = shaft kilowatts.
How to Use This Calculator
Enter the sailboat length at the waterline. Add the dry displacement from the boat data plate or manual. Include crew, gear, fuel, water, and stores. Enter the speed you want over ground. Add adverse current and head wind. Pick the sea state and operating role. Use a reserve value for safety. Press the calculate button. Review the suggested horsepower, shaft length, fuel estimate, and electrical power equivalent.
Why outboard size matters
A sailboat outboard does more than move the hull. It gives control in marinas, rivers, chop, and tight fairways. Too little power can make docking stressful. Too much power adds weight, cost, and stern drag. The best size sits between those limits. It should push the loaded boat near useful hull speed. It should also keep a reserve for wind and current.
Load and resistance
Displacement is the first sizing clue. A heavier boat needs more thrust. Length at the waterline also matters. It sets the natural speed range for a displacement hull. When speed rises near hull speed, resistance climbs quickly. The calculator uses this idea. It raises the speed length ratio to a power. That helps show why a small speed increase can demand much more power.
Wind, current, and reserve
Real conditions change the answer. A light harbor boat may need modest power. A cruiser that fights tide needs more. Windage from topsides, dodgers, and gear can raise the load. An adverse current is handled by increasing the speed through water. Sea state also adds drag. Reserve power is then added. This protects the estimate from fouled bottoms, old fuel, poor propeller match, and engine wear.
Electrical planning angle
Many owners compare fuel and electric options. Horsepower can be converted to kilowatts. One horsepower equals 0.746 kilowatts. Electric drives also need allowance for controller, cable, and propeller losses. Battery planning should use energy, not only peak power. A motor that draws four kilowatts for one hour needs more than four kilowatt hours from the battery bank.
Propeller choice
Propeller choice matters too. A high thrust prop can improve stopping force. It can reduce slip at low speeds. This often helps in crowded slips.
Practical setup tips
Check the transom rating before buying. Match shaft length to the mounting height. Keep the propeller deep enough in waves. Select a propeller made for thrust, not speed. Wide, low pitch props suit sailboats well. Test the boat after installation. Record rpm, speed, fuel use, and handling. If the motor cannot reach rated rpm, the propeller may be wrong. If the stern squats, weight may be excessive. Use the result as planning guidance, not a surveyor approval.
FAQs
What size outboard is best for a small sailboat?
Many small sailboats work with 4 to 8 hp. The right size depends on displacement, waterline length, wind, current, and load. A high thrust prop can matter as much as raw horsepower.
Does a heavier sailboat need more horsepower?
Yes. A heavier boat needs more thrust to accelerate and hold speed. Added crew, fuel, water, batteries, and cruising gear should be included in the loaded displacement.
Why does the calculator use hull speed?
Most sailboats are displacement hulls under power. Resistance rises fast near hull speed. This makes large speed gains expensive in horsepower, fuel, weight, and noise.
Should I choose the next larger motor?
Choose the next larger size only if your transom allows it. Extra power can help in wind and current. Extra weight can hurt trim and handling.
What shaft length should I use?
Measure the outboard mounting height. Short shafts suit lower mounts. Long and extra long shafts keep the propeller deeper, which helps in waves and reverse thrust.
Can this calculator size an electric outboard?
Yes. The calculator gives a kilowatt equivalent. Use that value for motor comparison. Battery sizing also needs run time, voltage, current limits, and reserve energy.
Why is propeller choice important?
Sailboats need thrust at low speed. A low pitch, high thrust prop often improves stopping, backing, and control. A speed prop may slip too much.
Is this result a final safety approval?
No. Use it for planning. Always check the boat manual, transom rating, local rules, propeller match, and a qualified marine technician when needed.