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Trolling motor sizing

Trolling motor thrust calculator

Last updated Researched from published figures and manufacturer specifications, not tested in person

Quick answer

The convention is two pounds of thrust per hundred pounds of fully loaded boat, and five per hundred for wind and current. A 2,500 lb boat therefore wants 50 to 125 lb of thrust. The number that matters more is what that commits you to: above 55 lb means 24 volts, and above 80 lb means 36.

Thrust sizing is one rule of thumb and one expensive consequence. The rule is two pounds of thrust per hundred pounds of fully loaded boat, and no standards body publishes it. It assumes flat water and no wind, which is not the day anybody needs a trolling motor, so treat it as a floor.

The consequence is the part worth thinking about first. A 12 volt motor tops out at 55 lb of thrust. Seventy and eighty pound motors need 24 volts, and anything above that needs 36, so the thrust decision is really a decision about how many batteries live in the boat permanently. Enter a fully loaded weight rather than a hull weight.

Run your own numbers

Off your own boat Hull, outboard, batteries, fuel, gear and people. Not a hull weight from a brochure.
Rule of thumb

A 2,500 lb boat points at a 80 lb motor on 24 volts. That is a starting point, not a clearance.

Your result
Figure Value
Thrust needed 75 lb
Smallest stocked motor 80 lb
System voltage 24 V
Batteries required 2
Published maximum draw 56 A
Sheltered water minimum 50 lb
Wind and current figure 125 lb

Shown with the example figures above. Change any field to run your own numbers.

The working

Rule of thumb

thrust needed = ( loaded boat weight / 100 ) x pounds per hundred pounds per hundred = 2 sheltered, 3 normal, 5 wind and current then round up to the smallest stocked motor that covers it

The pounds per hundred figures are boating convention rather than a published standard. Boating and fishing convention rather than a published standard. Taught everywhere, useful, and not a specification. Nothing enforces it and no body publishes it.

Motors that cover this thrust

Ranked by published thrust rating against the figure you entered, so nothing shown here contradicts the arithmetic above. Every rating is the manufacturer own.

Every figure shown is the manufacturer own published specification. The rating on the unit you receive is what governs, and the capacity plate on your own boat governs everything above it.

What fully loaded actually means

Hull, outboard, every battery, a full fuel tank, all the gear, and everybody aboard. A boat that weighs 1,500 lb empty on a trailer routinely reaches 2,400 lb by the time it leaves the ramp, and the thrust calculation wants the second figure.

If you have never weighed the rig, a trailer scale reading minus the trailer weight is the honest way to get it. Everything else is an estimate, and this calculation is only as good as the number you feed it.

What each thrust rating commits you to

The thrust decision is a battery decision. Above 55 lb means two batteries and above 80 lb means three, permanently, at roughly 60 lb each in lead acid.

Published figure A published engineering, regulatory or manufacturer figure. It does not change because somebody disagrees with it.

System voltage and published maximum draw by thrust
ThrustSystem voltageBatteriesPublished maximum drawBreaker at 1.25x
30 lb12 V130 A40 A
40 lb12 V142 A60 A
45 lb12 V142 A60 A
50 lb12 V142 A60 A
55 lb12 V150 A70 A
70 lb24 V242 A60 A
80 lb24 V256 A70 A
101 lb36 V346 A60 A
112 lb36 V352 A70 A

Maximum draw figures are the commonly published manufacturer values at maximum thrust. Check the figure in the manual for your own motor before sizing a breaker or a cable from it, because a specific model can differ. Where the maker publishes a breaker size, that figure governs rather than the calculated one.

Why you should size above the minimum

Current draw follows how hard you are working the motor rather than its rating. A larger motor loafing at a low setting frequently draws less than a small motor held near full power all afternoon, so the bigger motor is both quieter and easier on the battery through a normal day.

That is the real argument for buying above the two per hundred figure. Nobody has ever regretted slightly more thrust, and plenty of people have spent a windy afternoon at full power wishing they had it.

Thrust is not horsepower

A thrust rating is a static bollard pull figure measured at full power in a tank. It tells you how hard the motor pulls against a rope, not how fast it will move your hull, and it does not convert to horsepower. Two motors of equal thrust can move the same boat differently depending on prop, shaft length and how the hull sits.

What thrust is genuinely good for is comparing motors to each other and applying the convention above. This calculator will not tell you a speed, because that depends on a hull it cannot see.

Frequently asked questions

How much thrust do I need for my boat?

At the usual two pounds of thrust per hundred pounds of fully loaded boat, a 2,000 lb boat needs about 40 lb of thrust and a 3,000 lb boat about 60 lb. Those figures assume flat water and no wind, so they are a floor. For wind, current or open water the figure people actually use is five pounds per hundred, which roughly doubles the answer.

Is the two pounds per hundred rule a real standard?

No. It is convention, repeated by every retailer and every forum, and published by no standards body at all. It is useful as a starting point and it is not a specification. That is why this site labels it as a rule of thumb wherever it appears, alongside published figures like the manufacturer maximum current draw, which is a genuinely different kind of number.

Do I use the empty weight or the loaded weight?

Loaded, and the difference is large enough to change the answer. Fully loaded means the hull, the outboard, every battery, fuel, gear and people. A boat weighing 1,500 lb empty on a trailer routinely reaches 2,400 lb leaving the ramp, and sizing a motor from the first number gives you one that struggles on the day you need it.

Why does thrust decide how many batteries I carry?

Because system voltage is tied to thrust. Twelve volt motors stop at 55 lb, 24 volt motors cover 70 and 80 lb, and 36 volts starts at 101 lb. Each step adds a battery permanently, at roughly 60 lb each in lead acid, so a 112 lb motor means about 180 lb of lead in the boat every trip unless you move to lithium.

Will a bigger motor flatten my battery faster?

Only if you use the extra power. Draw follows how hard the motor is working rather than its rating, so a large motor at a low setting often draws less than a small motor held near full power. The published maximum draw figure applies at maximum thrust, which is a condition almost nobody sustains for a whole day of fishing.

Does this calculator tell me how fast I will go?

No, deliberately. Thrust is a static pull figure measured in a tank, and speed depends on hull shape, waterline length, how the boat sits and what the prop is doing. Any calculator that converts thrust to a speed is inventing a relationship that does not hold across hulls, so this one reports thrust, voltage, batteries and protection instead.

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Researched, not professional advice. This page is compiled from published engineering and regulatory figures, manufacturer specifications and owner-review consensus, not hands-on testing. Figures described as a rule of thumb are boating convention rather than published standards, and they are labelled that way wherever they appear. Marine electrical work is not house wiring. Use tinned, finely stranded marine cable, size it for voltage drop and for the ABYC E-11 ampacity table rather than one or the other, and put overcurrent protection within seven inches of the battery positive terminal, because everything between the post and the fuse is unprotected. A lithium battery needs a charger with a lithium profile. Never load a boat past its capacity plate, and remember that canoes and kayaks carry no federal capacity plate at all, so their stated capacity is the manufacturer own figure. Wear the life jacket, and treat early season water as the hazard it is: cold water immersion takes your breath and then your hands long before it takes your core.