What size bow thruster do I need?

It depends on how much wind your topsides catch, not on your boat’s length alone. Both Sleipner and VETUS say outright that sizing on length is invalid. A 40-foot trawler with a flybridge needs far more thrust than a 40-foot sailboat, because it presents far more area to the wind.

Why do manufacturer sizing charts disagree with each other?

Because they make different assumptions about where the wind pushes relative to where the boat pivots. Published charts differ by roughly two times for the same boat. That is why this calculator gives a range rather than a single number.

How much wind should a bow thruster be able to hold?

Owners consistently report that a thruster which cannot hold the bow in 20 to 25 knots is the one they regret buying. Sizing for a calm day is the most common and most expensive mistake.

Some links here are affiliate or referral links, and we may earn a commission if you buy through one. It never changes what we recommend, and it has never moved a number in a table on this site.

How much thrust does your boat actually need?

Most sizing charts ask for your boat's length and nothing else. Both Sleipner and VETUS say in their own documentation that length alone is not a valid way to size a thruster — and published charts disagree with each other by roughly two times for the same boat.

So this one works from the thing that actually matters: how much wind your topsides catch, and how hard the thruster has to work to win that argument. It gives you a range, and it shows you the reasoning.

What this is doing, so you can argue with it

The wind load on your topsides is standard fluid drag — half the air density, times wind speed squared, times a drag coefficient, times the area the wind sees. Nothing proprietary about it.

The thruster doesn't have to overcome that whole force. It has to win a leverage argument about the point your boat pivots around. Because the thruster sits near the bow and the pivot sits near the middle, the thruster's lever arm is long — which is why the number it needs to produce is a fraction of the total wind load.

thrust × long arm = wind load × short arm

The wind's push acts near the middle, so it is working with a short lever and most of its force goes into pressing the boat sideways rather than turning it.
The thruster sits in the bow, as far from the pivot as it can get. A long lever is why a few hundred pounds of thrust answers a few hundred pounds of wind — and why sizing off length alone misses the point entirely.

The thruster wins on leverage, not on brute force. It sits as far from the pivot as the boat allows, while the wind's push acts close to it. Schematic. Where the pivot actually sits varies with hull form and trim, and that variation is why published sizing charts disagree with each other by about two to one.

The reason we give you a range rather than a number is that the gap between where the wind pushes and where your boat pivots varies from boat to boat, and it's precisely that gap which makes the published charts disagree. Anyone quoting you a single confident figure from your length alone is guessing with more confidence than the physics supports.

Size up, not down. The most common regret in owner reports is a thruster that can't move the bow in a real breeze. As one owner put it: a thruster that can't hold you against 20–30 knots isn't worth having at all.