Computer Tuned Propellers •  Marine Hardware



We hear this question more than any other, and hear it many times a day.  The answer is really pretty simple . . . "The right propeller for your boat is one that has the right diameter and pitch to properly load your engine throughout its’ entire power output curve”.

In a perfect world, this would be accomplished with props that varied in pitch depending upon engine rpm.  While these propellers do exist, they are generally too expensive for pleasure boat application, and we have to make do with fixed pitch propellers.  So, given this restraint, where do we start to find the right prop for your boat?

First, let’s clear up a couple of common misconceptions:

  1. A book that lists the proper propeller for all production boat and engine combinations does not exist.  If you’ve ever been to a boat building plant you know that each boat is pretty much hand made, and even identical models come out quite a bit different in weight and bearing.  When you add the variety of options available, differing engine outputs, and the personal gear that each boat owner brings aboard, it’s easy to see why it would be impossible to compile this kind of data.
  2. Computer simulation programs, while very helpful in getting “in the ballpark” for sizing props, are estimates that are based on averages compiled across thousands of boat samplings.  They can be right on the money, but can also be off by a mile, especially as speed increases.

But don’t despair, the best place to start in your search for the right prop is on your own boat with engine manual in hand.  You see, all engines are rated at “X” horsepower at “X” rpm’s (Specs section of your manual), and simply stated, the more rpm’s you turn, the more horsepower your engine makes.  A fixed pitch prop should be sized to allow the engine to achieve its’ rated rpm’s at wide open throttle (WOT).  Every engine and propeller manufacturer on the planet endorses this principle, so if you don’t agree with it, you’re on your own.  If the engine turns more than rated rpm’s at WOT it’s not being properly loaded at cruise rpm’s where you’re losing hp, speed, and performance.  If the engine turns less than rated, it’s not developing hp, is being overloaded, and will suffer internal component damage.

A WOT test is the first step in determining the right prop for your boat, and is fairly straightforward:

  1. Take your boat out with its’ normal load of fuel & people on a calm day.
  2. After the engine(s) warms up, accelerate to WOT.  Stay at WOT only long enough to record rpm’s achieved.  (Don’t trust your installed tachs here, most accurate readings come from an electronic tach or phototach used right at the engine).

Late Spring/Early Summer is probably the best time of year to perform this test because your bottom is still nice and clean and the weather is not too hot (it’s not uncommon to lose 10 to 20% of engine horsepower in the hot and muggy “dog days” of July & August according to engine manufacturers).

Once you know your WOT rpm’s, we have formulas to work with that are very accurate in evaluating adjustments that may be made to pitch, cup or diameter if you’re over or under rated rpm’s.  We also have a precision measuring system to determine the true pitch and diameter of the propellers you ran when making the test.  This is important because you won’t get good results when making adjustments unless you know exactly where you’re starting, and the markings on the prop are usually off substantially even on brand new propellers.

So take the time to do a WOT test.  You’ll know in short order if you’re propped right or if changes can be made to improve performance.

The honest answer to this question is that, "you can't, unless you've got a precision device made to measure propellers like the Hale MRI system that we use at Digital Prop Shop".   While you may think that this answer is a bit on the "self-serving" side, let's step back for a minute and talk about some propeller realities.

Yes, it's true that almost all propellers are stamped in some way to designate diameter, pitch, and rotation.  For inboard props these stampings are generally found on the side and/or end of the hub, and will look like this:  24 RH 26.  This designates a propeller that is 24" in diameter (the diameter always comes first) by 26 inches in pitch with a Right Hand Rotation.  Some I/O or outboard props are stamped for diameter and pitch, but many only carry a part number which must be cross-referenced to a manufacturer listing to determine the design dimensions.

So, "I can determine the size of my propeller just by looking at the stampings". Sorry, even on brand new propellers we've found the pitch to be off by up to 1" overall and up to 1" between blades from the markings, so your propeller may not have ever been the diameter and pitch it was marked.  The only stamping that's likely to be accurate is the rotation.  To complicate matters even further consider the following questions:

Has my propeller ever been reconditioned?  If it was done on pitch blocks, how accurate (generally not very) were they?   In the finishing process did I lose diameter when nicks and dents were ground out?

Has my propeller ever been re-pitched or had the diameter changed?  If so, was it done accurately?  Was the new size re-stamped on the prop or does it still carry its' original markings?

Is my propeller cupped?  If so, what size cup (we use twelve levels of cup)?  And is the cup even on each blade?

It's easy to see why the stampings on your prop may not accurately reflect its' true size, and this can be a real "knock out punch" when you're trying to improve performance or get to target rpm's for your engine . . .  You just can't get to where you want to be if you don't know where you're starting!

For these reasons we offer a NO CHARGE computer analysis for your inboard propeller in our shop.  We'll measure it on our computer system and provide you with a detailed size and condition report.

" How Can I Tell the Size of My Propeller? "


The simplest way to tell the rotation of your propeller is to look at what's stamped on the hub.  Inboard propellers will generally have LH or RH stamped somewhere on the side of the hub.  This may not be the case with I/O or Outboard props, but don't worry, as there is a foolproof way to tell the rotation on any propeller:

Sit the propeller on a flat surface and place one hand on the trailing edge of a blade and the other hand on the leading edge of the same blade.  Now note which of your hands is higher.  If your right hand is higher you've got a right hand prop.  If your left hand is higher, your prop rotates left.

" How Can I Tell the rotation (rh or lh) of my propeller? "

It depends!  There's nothing magic about either a 3 or 4 (or more) blade propeller.  The best number of blades for your application depends on a multitude of factors such as boat weight, distribution of weight, power, reduction gear, clearance, bottom configuration etc..  There is a good chance (but not a certainty) that the manufacturer that built your boat or the Naval Architect that designed it gave some thought to these factors and delivered your boat with the right number of blades based on pre-delivery testing.  On the other hand, we have seen performance improvements achieved just from decreasing or adding blades, so we do look at this question on a case by case basis.

We'll be happy to consult with you on the number of blades issue, and may even have experience that directly correlates to your application.  However, keep in mind that we may be covering ground already trod by your boats' manufacturer in a  "trial and error" and testing phase . . .  A call to them may save effort on your part.

That said, there are some generalities when comparing 3 & 4 blade propellers that can be stated with a fair amount of certainty (assuming that both the 3 or 4 blade propellers being compared are properly tuned and are the proper size to allow engines to reach rated rpm"s).

  • A 4 blade propeller has more blade area than a 3.
  • This increased blade area will provide better maneuverability on the low end.
  • A 4 blade propeller will generally handle weight variations less noticeably than a 3.
  • A 4 blade propeller will generally come "out of the hole" better.
  • A 4 blade will run harmonically quieter than a 3.
  • Because of the extra drag caused by the 4th blade, a 4 bladed propeller will be slower on the top end than a 3.
  • A four blade propeller will have to be smaller in diameter, pitch, or cup to achieve same rpm's as a 3 blade.

" is a 4-blade prop better than a 3-blade? "

Diameter - Diameter can be reduced but not increased.  Theoretically you can cut diameter all the way down to the hub.  Realistically however, a couple of inches would be the maximum reduction you'd want to make in diameter.  The problem is that the blade edges get extremely thick even when reducing diameter as little as an inch.  While this may provide a very tough blade edge; thick, blunt blade edges are tough on performance.

Pitch - The rule of thumb is that you can increase or decrease pitch by 2 inches.  Bear in mind however that large propellers, and NiBrAl propellers especially, are very tough and resilient.  Reducing pitch 2" is no problem, but while it may be possible to achieve an even and well distributed 2 inch pitch increase on the table in the shop, this may not be maintained when your props are put under pressure.  Better to stick with no more than 1 inch when talking pitch increase.

Cup - Cup can be added or removed from a propeller in 12 steps ranging from .000" to .182". (Applicable to some, but not all, I/O and outboard props)

Bore - Bore can be increased up to the maximum allowable dependent on the hub size of the propeller, and decreased through the use of bushings.  Cannot be changed on I/O and outboard propellers.

Rotation - The rotation of a propeller cannot be changed.

" how much can the dimensions of my existing prop be changed? "

Handle your props carefully.  The edges especially are susceptible to bending and dinging if they come into contact with other hard objects.

1.    Smear some valve grinding compound (available at any hardware or auto parts store) around your shaft taper.  Install the prop on the taper without the key and rotate 3 or 4 turns in each direction while pushing prop forward on shaft.  This will remove minor metal imperfections and help assure a proper fit on the taper.  Mark the shaft at the forward end of the prop hub at it's furthest point of advance with a pencil or magic marker.

2.    Remove prop and wipe off all compound from shaft taper, prop bore, and keyway.

3.    Fit key in shaft keyway.  It should fit snugly and “bottom out” in keyway.  Remove from shaft and try for same fit in prop keyway.  If any areas bind or won’t fit properly, dress the key lightly with sandpaper wrapped over a flat surface.  If the key rocks or is loose, replace it, as looseness here can cause vibration and possible shaft damage.

4.    Place key in shaft keyway and after aligning prop keyway, slide prop up onto shaft to the point which you previously marked.  At this point the key should be completely captured by the prop and centered laterally within the prop hub.  Never, ever, hammer on the propeller to make it fit on the shaft.  If the prop doesn't move up to your marked point, the key is binding due to poor fit.

5.    Take your large shaft nut and hand tighten up against hub.  At this point many installers will place a block of wood between the boat bottom and end of propeller to keep the shaft from turning while they tighten the nut with a wrench.  This practice should be avoided if at all possible, as it can cause blade distortion.  Use a plumbers strap wrench to hold the shaft or have a friend hold the prop down close to the hub while you tighten the nut and firmly seat the prop on the taper.

6.    If you have a single "castle" type nut, finish by installing your cotter pin at this point.  If you have two nuts, remove the large shaft nut and replace with the small one.  Tighten small nut using the same procedures as in step 6.  Now, thread on your large shaft nut behind the small one and tighten (this is your lock nut).

7.    Install cotter pin, and bend back prongs.

We do not recommend painting propellers with regular bottom paint as it degrades performance, can cause electrolysis, and generally does little to retard marine growth.  We recommend using a zinc product like Petit's "Barnacle Barrier" if you have a problem with marine growth fouling.

" what is the best way to install my inboard propeller? "

" What's the Right Propeller for my Boat? "