Speed at a glance
There is no single answer to “how fast is a boat with X hp”. Two boats with the same engine can differ in speed by a factor of three. Hull shape, length and the weight on board make the difference.
A note on units: the SeaStorm and Mercury figures in this article are metric horsepower (PS). We give kW alongside where it matters.
How fast is a 15 hp boat?
A light, flat hull of around 4 m gets onto the plane with 15 metric hp (PS), which is 11.03 kW. It then runs at about 30 km/h. That is the catalogue figure for every SeaStorm model with 15 hp. A heavy displacement boat, such as a houseboat, goes much slower with the same power. It stays near its hull speed [2]. Horsepower cannot be converted into km/h: the speed depends on hull, length and weight.
One metric horsepower equals 735.4988 W [4]. The imperial horsepower (550 ft·lbf/s) is slightly larger at 745.6999 W [4]. When you compare engine ratings in kW, check which of the two a brochure uses; when in doubt, the kW rating on the engine plate or in the manual counts. The Mercury 15 hp manual, for example, states the rating as 11.0 kW (15 hp) [1].
Planing or displacement: why horsepower is not enough
A displacement hull pushes water aside. At a certain speed the bow wave is as long as the boat, and wave drag becomes large. That speed is the hull speed. By the usual approximation it depends only on waterline length, not on engine power [2]:
Hull speed in knots ≈ 1.34 × √(waterline in feet). In metric terms: about 2.43 × √(waterline in m) in knots, or 4.5 × √(waterline in m) in km/h [2].
| Waterline | Hull speed (approx.) | Calculation |
|---|---|---|
| 4 m | approx. 9 km/h (4.9 kn) | 4.5 × 2.0 |
| 6 m | approx. 11 km/h (6.0 kn) | 4.5 × 2.45 |
| 9 m | approx. 13.5 km/h (7.3 kn) | 4.5 × 3.0 |
A heavy boat generally cannot exceed its hull speed unless it planes [2]. A planing hull rises onto the water as it speeds up. Resistance drops, and the formula no longer applies. That is why a short boat with enough power can run at several times its hull speed.
An example from the catalogue: the SeaStorm 12 is 3.66 m long. Its waterline is shorter, so its hull speed is below 4.5 × √3.66 ≈ 8.6 km/h. With 15 hp it reaches about 30 km/h according to the catalogue, more than three times that. Only a planing hull can do this.
How fast is each SeaStorm?
The table uses the catalogue figures that also appear in the configurator. Pick an engine power to see only the matching variants.
Speed by engine power
How fast is each SeaStorm?
15 metric hp equals 11.0 kW. Options in the catalogue: 4, approx. 20 to 30 km/h (11 to 16 kn).
| Model | Engine | approx. km/h | approx. kn |
|---|---|---|---|
| SeaStorm 12 | 15 hp (11.0 kW) | 30 | 16 |
| SeaStorm 12 | 20 hp (14.7 kW) | 33 | 18 |
| SeaStorm 12 | 25 hp (18.4 kW) | 37 | 20 |
| SeaStorm 12 | 30 hp (22.1 kW) | 40 | 22 |
| SeaStorm 13s | 15 hp (11.0 kW) | 30 | 16 |
| SeaStorm 13s | 30 hp (22.1 kW) | 40 | 22 |
| SeaStorm 13s | 40 hp (29.4 kW) | 50 | 27 |
| SeaStorm 13s | 50 hp (36.8 kW) | 55 | 30 |
| SeaStorm 13s | 60 hp (44.1 kW) | 60 | 32 |
| SeaStorm 14 | 15 hp (11.0 kW) | 30 | 16 |
| SeaStorm 14 | 25 hp (18.4 kW) | 38 | 21 |
| SeaStorm 14 | 30 hp (22.1 kW) | 42 | 23 |
| SeaStorm 14 | 40 hp (29.4 kW) | 45 | 24 |
| SeaStorm 17 | 15 hp (11.0 kW) | 20 | 11 |
| SeaStorm 17 | 40 hp (29.4 kW) | 40 | 22 |
| SeaStorm 17 | 50 hp (36.8 kW) | 45 | 24 |
| SeaStorm 17 | 60 hp (44.1 kW) | 50 | 27 |
| SeaStorm 17 | 70 hp (51.5 kW) | 60 | 32 |
| SeaStorm 17 | 80 hp (58.8 kW) | 65 | 35 |
Approximate top speed from our catalogue. Load, propeller, weather and fouling lower the figure. 1 kn = 1.852 km/h, 1 metric hp = 0.7355 kW.
Two patterns stand out. At 40 metric hp (PS) the catalogue lists the shorter SeaStorm 13s at about 50 km/h and the longer SeaStorm 17 at about 40 km/h; length, weight and hull shape all play a part. And more power does not add speed in a straight line, because resistance grows with speed. How much power your use really needs is covered in the guide How much horsepower does my motorboat need?.
Converting km/h, knots, hp and kW
On the water, speed is often given in knots. One knot is one nautical mile per hour, which is 1,852 m per hour [3].
| Conversion | Factor | Example |
|---|---|---|
| km/h to knots | ÷ 1.852 | 30 km/h ≈ 16.2 kn |
| knots to km/h | × 1.852 | 20 kn ≈ 37 km/h |
| metric hp (PS) to kW | × 0.7355 | 40 hp ≈ 29.4 kW |
| kW to metric hp (PS) | ÷ 0.7355 | 11.03 kW ≈ 15 hp |
What makes a boat slower
Catalogue speeds are approximate. In everyday use you will often be below them. The manual for the Mercury 15/20 EFI FourStroke lists the reasons [1]:
- Weather: heat, low air pressure and high humidity reduce engine power. On a hot, humid day the engine can lose up to 14 %, and the boat 3 to 5 km/h (2 to 3 mph).
- Load: more people on board or a water skier in tow pull the rpm down.
- Hull: growth such as algae or barnacles adds friction. A bottom bent out of shape, for example by poor support on the trailer, also costs speed.
- Propeller: it has to suit the boat. At full throttle with a normal load, the engine should run at the top of its rpm range. For the 15/20 EFI that is 5,700 to 6,200 rpm.
- Weight distribution: weight moved aft usually raises speed and rpm; weight moved forward helps the boat get onto the plane.
Checklist
Boat slower than expected? Check
Speed and boating licences
Speed can also decide whether you need a licence. The rule depends on the country. As the check below shows with its legal source, some countries tie it to engine power, others to top speed: the Netherlands, for example, to more than 20 km/h. Check your model, engine and country:
Licence check
Licence: what applies to your engine?
Pick a country, model and engine. The result comes from our rules database, every rule with its primary source and the date we checked it.
Licence required
- Engine power 11.03 kW, limit 4.4 kW
Führerinnen und Führer von Motorfahrzeugen für Sport- oder Erholungszwecke mit einer Antriebsleistung von weniger als 4,4 kW
The figures come from the primary sources linked above and are not legal advice. What counts is the law of the waters you navigate, on the day you navigate them.
The check applies the rules of the country you select. Local speed limits on your waters apply on top and are set out in the local navigation rules.
Where the SeaStorm fits
The SeaStorm models are HDPE boats between 3.66 m and 5.17 m long, with outboards from 15 to 80 hp (Mercury; Yamaha on the 70 hp). According to the catalogue, their speeds range from about 30 km/h to about 65 km/h, several times their hull speed. How fast you actually go depends on your waters, speed limits and the weather. Judging weather and sea state remains the skipper’s duty.
The overview HDPE motorboat shows all models and engines. The smallest boat is the SeaStorm 12; the strongest engine, 80 metric hp, is offered on the SeaStorm 17. A stronger engine makes the whole rig heavier; what your car may tow is covered in Boat trailer: which driving licence, which towing capacity?. Regular outboard maintenance helps the engine keep its power.