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Propeller & repower calculator for every boat

Turn propeller guesswork
into a calculated answer

Diagnose the prop you have, size the one you need and see how it runs before anyone orders it. Free for boat owners. Pro for the shops that fit them.

Check my propFree · no accountabout a minute
Engine and gearbox data
built in for
Volvo PentaYanmarMercuryYamahaBeta MarineZF Marine
Why Propbench

Most boats run the wrong prop.
Almost nobody can say by how much.

Your answer34 ft · 55 hp diesel
2,550 rpmshould reach 2,700–3,000
Right size16–17 × 10–11
Top speed≈ 7.4 kn
Overpropped150 rpm short of rated

An honest answer in a minute

Black smoke, slow to plane, won’t reach full revs: it is usually the prop. Propbench works out where your engine actually lands at full throttle, tells you plainly if the prop is right, and shows the size range that fits. No account, no sales pitch on the answer.

Check my propHow it works
How it works

Three questions.
One honest answer.

What kind of boat?
MotorboatSailboatFishingSpeedboatCruiserWorkboat
01Tell us the boatType, length, engine and the prop you have. Simple drawings show where to find each number.
Where full throttle lands
1,500rated 2,700–3,0003,500
2,550 rpm
02See the verdictWhere full throttle lands against the maker’s range, the size range that fits and the speed to expect.
Rudes Propeller12 km · re-pitch, new props
[Shop name][distance] km
03Get it fixed nearbyOnly if you want: shops near your harbour turn the range into one exact prop and send a price.
What it does

Everything a prop shop works out.
In under a second.

The same engine curves, propeller series and hull maths the specialists use, in a tool a boat owner can run and a shop can sell from.

Race · 19 × 14 vs 18 × 15Expand ↗Speed through water6.2 kn+0.1 kn ahead
Race it before you buy itTwo props, same boat, same water. The simulator shows which one is ahead, at what rpm, and what the owner will feel at the helm.
Top speedPullStoppingAsternEfficiencySailing drag
Fixed 4-bladeFolding 3-blade
Every prop type at its best sizeFixed, folding, feathering and self-pitching, each sized for this boat and compared on what matters to the owner.
band0102030401.5k2k2.5k3k
Where full throttle landsEngine power against prop demand. See the rpm today, the rpm with the new prop and the band the maker asks for.
Full throttle in the rated band3,010 rpm
Back cavitation (Burrill)89 % of limit
Blade areaEAR 0.55 · needs 0.53
Blade loading · tip speed60 kPa · 27.8 m/s
Inside the B-series rangeP/D 0.74 · 4 bl
Tip clearance to hullmeasure: ≈ 72 mm
Six engineering checksCavitation, blade area, loading, tip speed, series range and hull clearance, with the margin for each.
Bollard pull ahead618 kg
Bollard pull astern408 kg
Stopping from 6 kn10.5 m
Pull and stoppingThe numbers workboats, tugs and fishermen ask for.
19 × 16 in
Diameter
−19+
Pitch
−16+
2,750 rpm · 147 rpm under the band
Any size, liveStep diameter or pitch and every number updates in a quarter of a second.
Inboard
Outboard
Sterndrive
Saildrive
Every driveInboard and shaft, outboard, sterndrive and saildrive, with engine and gearbox data built in.
Branded PDF reportProp drawingCopy summaryShortlist of 3Send to customer
Ready to hand overOne click to a report with your logo, a prop drawing, or a summary for the customer.
Accuracy

Honest about
how sure it is

Most calculators give you one confident number. Propbench tests itself against boats that were actually measured, tells you the error for boats like yours, and gets sharper with every sea trial you log.

12.8 %top-speed error, uncalibrated
→
≈ 5 %after one logged sea trial
Model vs measuredtop speed · real boats
Boat · enginemodel slowermodel faster
Island Packet PY 41 · Yanmar 4JH4-HTE+5 %
J/42 · Yanmar 4JH3E+9 %
Hunter 34 · Yanmar 3GM30F−14 %
O’Day 34 · Universal M-30+19 %
Lobster boat · Nanni 65+24 %

127 measured boats, each with its source. Every result shows the expected error for its boat type until a sea trial calibrates it.

Two ways in

Free for the owner.
Pro for the shop that fits it.

For boat ownersFree
Is my prop right?
  • ✓A plain verdict in about a minute
  • ✓Where full throttle lands against the maker’s range
  • ✓The size range that fits, and the speed to expect
  • ✓Drawings that show where to find every number
  • ✓A prop shop near your harbour, only if you want one
Check my prop
For prop shops and yards€99 / month
Size it, prove it, sell it.
  • ✓Exact size, every prop type and every alternative
  • ✓Sim race, six checks, pull and stopping
  • ✓Branded reports, drawings and summaries
  • ✓Leads from owners near you, boat data filled in
  • ✓The check on your own website, with your quote button
  • ✓Calibrated with your own sea trials
Start 13-day trialSee pricing
For prop shops

It pays for itself
with one job.

Win the job with a recommendation you can show, not just a price.

[Your logo]REPORT 1042 · 28 OCT 2026
Propeller recommendation
CURRENT17 × 122,550 rpm · 7.0 kn
RECOMMENDED17 × 10.52,940 rpm · 7.4 kn
1,0003,000 rpmPrepared by [Your shop] using Propbench · inputs and assumptions on page 2
A report with your name on itCurves, the recommendation, alternatives and every input, on one page your customer keeps.
Mette K. · Haderslev34 ft sailboat · D2-75Overpropped
Lars P. · Aabenraa11 m fishing boat · M92BUnderpropped
Jens H. · Sønderborg6 m console · Mercury 150On target
Leads with the boat filled inOwners near your harbour send their check to you. Boat, engine, prop and verdict arrive with the request.
yourshop.dk[Your logo]Is your prop right?Get a quote from [Your shop]
The check on your websiteOne line of code. Owners run it on your site and the quote button sends them to you.
Questions

Straight answers

Is Propbench free?

Yes, for boat owners. The check, the verdict and the size range need no account. Prop shops pay for the Pro tools, branded reports and leads.

Does it push a propeller brand?

No. The maths is cross-brand and gives the same answer whether or not anyone pays. Paid shop listings are labelled Featured.

Why don’t I get one exact size for free?

A wrong prop is expensive. The exact size should come from a shop that checks the boat and stands behind it, so the free check gives the verdict and the range.

How accurate is it?

The prop maths uses the published Wageningen B-series. Hull estimates are tested against 127 measured boats: uncalibrated, top speed is off by about 12.8 % for boats like yours, and one logged sea trial brings that to about 5 %. Every result lists its inputs.

Which drives and engines does it cover?

Inboard and shaft drive, outboard, sterndrive and saildrive, with engine curves and gearbox ratios built in for the common marine engines.

Run it on your boat

Three questions. About a minute. Free.

Check my propFor prop shops