How accurate is the calculator? Its power curve checked against Nordhavn's published figures

Every figure the calculator gives — motor size, current, cable, battery — starts from one estimate: how much shaft power your boat needs at the speed you want. If that estimate is off, everything downstream is off by the same proportion. So here is the estimate set against every published figure we could find to test it with, including the ones it gets wrong.

→ Run your own boat through the calculator →

The estimate

The calculator works out shaft power per tonne of displacement from your speed as a fraction of hull speed, F (speed ÷ 1.34 × √waterline in feet):

kW per tonne = 1.233 × F² + 1.238 × F⁵

Multihulls get 0.8 × that, and a saildrive adds 4%. The curve was fitted to one boat: the Nordhavn 40 that Nordhavn sent around the world, whose fuel burn Nordhavn publishes at 6, 7 and 8 knots. The About page has the background, and how many kW a sailboat needs applies it.

The shaft doesn't care what turns it, so diesel fuel burn is a fair test of a power estimate. To turn fuel into shaft power we use 0.27 litres of diesel per kWh — a typical figure for a marine diesel at cruise, and our assumption rather than Nordhavn's.

The fit: Nordhavn 40

These are the points the curve was fitted to, so it agrees by construction. They're here so you can see the starting point.

Nordhavn 40: 10.8 m waterline, 22.68 t, stabilized, "relatively calm conditions".

The test: Nordhavn range statements

Nordhavn's model pages give a range at a stated speed, a fuel capacity, a waterline length and a displacement. Range ÷ speed gives hours, fuel ÷ hours gives litres per hour, and 0.27 L/kWh gives shaft power. None of these boats was used to fit the curve, except that the first row is the same Nordhavn 40 with a separate range claim.

Where Nordhavn says "more than" or "in excess of", the boat goes further than stated, so the implied power is an upper limit. The Nordhavn 55's page gives the 1,500 nm range in the same sentence as its 9.5-knot hull speed; we read it as the range at that speed.

What it shows

At cruising speed, mostly within 15%. Across the four statements at 80–90% of hull speed, the calculator ranges from 14% below the implied power to at least 10% above it. Three are within 15%; for the Nordhavn 62 we only know the calculator is at least 10% high, because Nordhavn's range is a "more than" figure. The three known differences sit inside the ±25% the calculator already states for any one boat, which is about what you'd expect from boats with different hull forms, loading and propellers.

At hull speed, it can fall well short. The Nordhavn 55's hull-speed figure implies 37% more power than the calculator gives. Power climbs so steeply near hull speed that small differences in hull form or loading — Nordhavn quotes that range "under full load" — make a large difference. The curve was fitted to one boat; at 100% of hull speed, another boat can need much more.

Two assumptions push every row the same way. We assume all the fuel is burned, and that 0.27 L/kWh holds. If Nordhavn's ranges keep a reserve, the implied power is lower than shown, by the size of the reserve. If the engines burn less per kWh, it's lower again. Either would move the calculator's differences upward: the −14% and −9% towards zero, the +10% further above.

What it means for your boat

• Size for cruising speed, not hull speed. Below about 90% of hull speed, every difference we could pin down was within the calculator's ±25%. At hull speed, the one statement we could test was 37% out.
• Keep the reserve. The calculator's default 15% reserve, and its 25% headroom on current, exist for exactly this kind of spread.
• Treat a hull-speed target as a warning sign. If your plan only works at hull speed, it depends on the part of the curve that's least reliable — and on the part where power, current and battery size rise fastest. The range planning guide shows how much range slowing down buys.

What we couldn't test

Every point here is a heavy, full-displacement trawler. We found no published measurements of power at speed for sailboats, electric or diesel — makers publish motor ratings and boat-size guidance, not speed and power at the shaft. So for sailboats, the curve is an extrapolation from trawler data, cross-checked only against makers' sizing guidance (see how many kW a sailboat needs).

If you've logged battery power or current at a steady speed on your own boat, those are exactly the measurements that would test the curve where it most needs it. Owner repower reports collects them and publishes each one next to the calculator's prediction.

Sources

Checked on 8 November 2026:

• Nordhavn — Nordhavn 40 around-the-world performance: fuel burn of 1.6 / 2.6 / 4.0 US gal/h at 6 / 7 / 8 knots, stabilized, in relatively calm conditions
• Nordhavn — Nordhavn 40: waterline, displacement, 920 US gal fuel, "more than 2,400 nautical miles at 7 knots"
• Nordhavn — Nordhavn 475: waterline, displacement, 1,200 US gal fuel, "3,000 nm range at about 7 knots"
• Nordhavn — Nordhavn 55: waterline, displacement, 2,250 US gal fuel, range at 8.25 knots and at 9.5 knots
• Nordhavn — Nordhavn 62: waterline, displacement, 2,652 US gal fuel, "in excess of 3,000 nautical miles at 9 knots"
• The calculator's power model: the curve, the multihull and saildrive factors, and the calculated column. The 0.27 L/kWh conversion is our assumption, as used to fit the curve.