How to Select the Best Propeller

[Locozodiac]

Hello to all boaters...

There has been some threads, inquiries and one’s self answers about propeller pitches on Ribnet. Found this nice information about this misunderstood issue, here’s the condensed article important facts to be aware of :

Best all around performance is achieved when wide open throttle (WOT) engine operation occurs near the top (but within) the wide open throttle RPM operating range design by the manufacturer for that specific engine. Improperly propping an engine can not only reduce best performance, but in fact damage and ruin the engine.

An engine that does not reach the rated RPM at Wot is in an over propped condition resulting in lugging. This high torque operation puts a tremendous load on the pistons, crankshaft and bearings. The engine runs much hotter and may overheat from having over advanced spark timming for the reduced amount of fuel entering the engine.

The mechanical strain on an over propped marine engine is like starting an automobile in third gear from a dead stop at the bottom of a hill. The resulting severe strain can lead to detonation, piston seizure and severe engine damage.

On the other hand, an engine that revs past the recommended RPM at Wot will have higher than normal wear and can also be damaged by fatigued parts breaking and passing through the engine. This is why is so critical to be sure your engine is correct propped for your boat/engine combination and the type of boating you want to do. To make this selection propeller charts are published as general guidelines for general applications, they are not intended however to be the absolute recommendation as boats and operation conditions vary . Remember the best propeller selections for your particular boating needs can be determined only by experimentation.

You should have more than one prop if you use your boat/engine for more than one type of activity, such as cruising, fishing, skiing, you may well need different props for the best performance in each type of activity. In any event you should keep on board at all times, along with a nut wrench , pliers, spare nut and tab washer. Propeller lines normally are designed so that the next size pitch will change rpm by 300 to 500. So if the engine rpm falls too low on your first prop selection, try a lower pitched prop to bring the rpm. Higher pitched props reduces the engine rpm, lower pitched props increases engine rpm.

Pitch Change Calculations : Just follow these steps : A tacho will be a must.

1-Chack in the manual your engine recommended operating RPM range at Wot

2-With one person, 2 gallons of gas, run the boat/engine combination at Wot to determine the max rpm achieved. Adjust engine trim angle for optimum performance.

3-If the Wot rpm is below the recommended rpm range of the engine, note the reading of the tacho. Take that reading and subtract it from the top end of the operating range.

A Practical Example :

Engine Operating Range : 5000-6000 rpm
Top End Operating Range : 5600 rpm
Tacho Max Reading : 4800 rpm
RPM Difference : 800 rpm

Fine Tunning Adjustment: For every 1” of pitch change, the effect will be aprox 200 rpm. Knowing this, take the difference in the above example at 800 and divided by 200. The result is 4”. So, the next prop to try will be 4” in pitch less than the prop which was first used. You should now either have the right prop or be only one size off.

Hope this article helps solve may pitch prop technical doubts…

Happy Boating
Locozodiac

[memo]

Lozo,
What about swapping from Alu to Stainless in props. Do you know a rule of thumb for them?

[Locozodiac]

Originally Posted by memo

Lozo,
What about swapping from Alu to Stainless in props. Do you know a rule of thumb for them?

Hola, sorry no info available will have to ask the prop guru! Think that if you mantain the same size & pitch for both there shouldn't be any difference in performance...

Happy Boating

[Locozodiac]

Originally Posted by Locozodiac

Hello to all boaters...

Propeller lines normally are designed so that the next size pitch will change rpm by 300 to 500. So if the engine rpm falls too low on your first prop selection, try a lower pitched prop to bring the rpm higher Higher pitched props reduces the engine rpm, lower pitched props increases engine rpm.

Pitch Change Calculations : Just follow these steps : A tacho will be a must.

1-Chack in the manual your engine recommended operating RPM range at Wot

2-With one person, 2 gallons of gas, run the boat/engine combination at Wot to determine the max rpm achieved. Adjust engine trim angle for optimum performance.

3-If the Wot rpm is below the recommended rpm range of the engine, note the reading of the tacho. Take that reading and subtract it from the top end of the operating range.

A Practical Example :

Engine Operating Range : 5000-5600 rpm
Top End Operating Range : 5600 rpm
Tacho Max Reading : 4800 rpm
RPM Difference : 800 rpm

Fine Tunning Adjustment: For every 1” of pitch change, the effect will be aprox 200 rpm. Knowing this, take the difference in the above example at 800 and divided by 200. The result is 4”. So, the next prop to try will be 4” in pitch less than the prop which was first used. You should now either have the right prop or be only one size off.

Hope this article helps solve many pitch prop technical doubts…

Happy Boating
Locozodiac

The previous article had a erroneous data reffered to the top operating range, read 6000 rpm instead of the max 5600 rpm used on this example, this might have lead to confussion, sorry.

[Nos4r2]

It usually works out that you have to drop the pitch by 1" (on most props with 15" pitch and below) or 2" on most props of 17" and above when going to stainless.

For example:-
Correct ally prop=13". Correct stainless should be 12".

Correct ally prop=19", correct stainless should be 17".

It's only a rule of thumb though and depends on the stainless prop and ally prop you had on there before...

[jyasaki]

Aluminum to stainless (identical in all other ways) should see a 100 to 200 rpm difference (drop) due to the stiffer material. Hardly ever applicable directly, as things like cupping and blade shape play a large role in the actual rpm change as well. Nos's rule of thumb seems reasonable.

A change in prop diameter of 1/4" should give see a change of about 100 rpm (smaller diameter will increase rpm)

A pitch change of 1" should see about a 200 to 250 rpm difference (Larger pitch numbers dropping rpm.)

Of course, these are guidelines, not "rules", as every boat and every prop reacts differently.

I personally feel that WOT prop selection should be done with a "normal" load (as in, how the boat will be run most of the time.), rather than with the light load that Loco suggested. The slight over-rev when running light will be offset by having the capability of revving to max rpms most of the time.

My $.02;

jky

[Locozodiac]

[QUOTE Jyasaki;234043]

"I personally feel that WOT prop selection should be done with a "normal" load (as in, how the boat will be run most of the time.), rather than with the light load that Loco suggested. The slight over-rev when running light will be offset by having the capability of revving to max rpms most of the time".

This is not my suggestion, it's the article's, the reason of wotting light is because if you ever need to full wot this way (a very rare occasion) you'll be at the max engine rpm that the manufacturer recommends. A sort or over rev potection when running light. Can be applied for racing.

On the other hand many small to medium users (tiller mostly) don't use a tacho ever, generally use a mid pitched propeller that most of the time is standard factory delivered and perhaps will never wot their boat/engine to obtain the best performance that can be achieved. I've seen engines with prop changed, thinking that will perform better, but running under or over rev, a short path to premature engine disaster. Most customers don't know anything about correct prop tacho wotting, unless a well trainned dealer offers them this precise info including test props to top their engine/boat performance.

Have wotted my engine this way and can full throttle knowing that my engine is at the max rev safely. Just as a experiment and to take the nail out, wot light & normal and see how much rpm & speed difference between both. Would be nice to write any outboard factory and ask them which of the stated two possibilities will be the correct or applies better. Anyway...

Happy Boating

[BassBoy]

As I understand it, if 1 revolution of prop produces forward travel of 22" then this is the pitch of the prop...22"
Therefore, why do you say that equivalent of 23" alloy is a 22" stainless? Surely a 22" is a 22" and a 23" is a 23" ?

Someone told me that alloy props are made in odd no. pitch , e.g. 19" 21" 23" and stainless in even no. e.g. 20" 22" 24" etc etc.

Anyone know if this is correct?

[9D280]

In theory, yes.......

However the entire force pushing your boat along is supported by the blades. The pressure is quite large, and so the stiffness of the material comes into play. The prop blades will deform under the pressure, altering the pitch, the ally ones more than the stainless. The mechanics of it will be down to the design of the blades, the force they are trying to withstand etc etc.

As for the odd / even thing it certainly doesnlt apply on small Suzis - My 25 had a choice of 11,12,13,14 & 15"...... Maybe that rule holds higer up the HP scale?

[Nos4r2]

Originally Posted by BassBoy

As I understand it, if 1 revolution of prop produces forward travel of 22" then this is the pitch of the prop...22"
Therefore, why do you say that equivalent of 23" alloy is a 22" stainless? Surely a 22" is a 22" and a 23" is a 23" ?

As 9D280 says, deformation comes into play. Also the thinner the blades (ie stainless) the less slip you get.Therefore a 21" or 22" stainless prop will produce as much forward movement as a 23" ally one.

Originally Posted by BassBoy

Someone told me that alloy props are made in odd no. pitch , e.g. 19" 21" 23" and stainless in even no. e.g. 20" 22" 24" etc etc.

Anyone know if this is correct?

Nah, s'carp. Different manufacturers make different pitch props. Some do odds, some do evens, some do all. As in most things, you get what you pay for.

[BassBoy]

I am about to change/test 22" Stainless from 23" alloy. The stainless prop must weigh at least twice that of the alloy if not a it more.

My questions are about inertia.

How much extra load will the extra weight of he prop put on the final drive and gearbox? Will the rubber bushing cope with the extra load? What effect will the extra weight have on constant engine speed i.e. will it produce more of a flywheel effect and therefore take less energy to turn it once running at desired revs?

Over to you hydrodynamicists!

[BassBoy]

I am about to change/test 22" Stainless from 23" alloy. The stainless prop must weigh at least twice that of the alloy if not a it more.

My questions are about inertia.

How much extra load will the extra weight of he prop put on the final drive and gearbox? Will the rubber bushing cope with the extra load? What effect will the extra weight have on constant engine speed i.e. will it produce more of a flywheel effect and therefore take less energy to turn it once running at desired revs?

Over to you hydrodynamicists!

[Chris]

Apparently one effect is to reduce vibration.

Originally Posted by BassBoy

I am about to change/test 22" Stainless from 23" alloy. The stainless prop must weigh at least twice that of the alloy if not a it more.

My questions are about inertia.

How much extra load will the extra weight of he prop put on the final drive and gearbox? Will the rubber bushing cope with the extra load? What effect will the extra weight have on constant engine speed i.e. will it produce more of a flywheel effect and therefore take less energy to turn it once running at desired revs?

Over to you hydrodynamicists!

[9D280]

Originally Posted by BassBoy

I am about to change/test 22" Stainless from 23" alloy. The stainless prop must weigh at least twice that of the alloy if not a it more.

My questions are about inertia.

How much extra load will the extra weight of he prop put on the final drive and gearbox? Will the rubber bushing cope with the extra load? What effect will the extra weight have on constant engine speed i.e. will it produce more of a flywheel effect and therefore take less energy to turn it once running at desired revs?

Over to you hydrodynamicists!

Ignoring the hydrodynamics momentarily, Here's a mechanical view:
The extra weight will make next to naff all diference - IF it's properly balnced! The weight (which will reduce in water anyway thanks to Mr Archimedes' principles) will act vertically down - I bet you could lift either prop using a large bent paper clip to support the dead weight. however the shaft on your 140 is sized to take a LOT more force than that - think of the effects of bouncing through waves etc etc, and for that matter keeping the powertrain together as you shove 140Hp through it! So the weight will have no more effect than not taking a 2l bottle of your favouritre fizz with you on board.

The bushing in the prop will have been designed to cope with the forces it will see. The biggest of which is a circular shear between the hub and the body of the prop. You'll know if that's dead 'coz you'll get all the revs, but not a lot of action! Also you'll see a kind of extruded ring of solidified ex molten rubber round the back end of the bush if it's slipping.

The "flywheel" effect on paper will I guess be there, but again, on a 140Horse engine, it's going to be next to negligible difference -the flywheel up under the lid will have a lot more effect on the engine than the spinning mass of the prop. As Nos says, the blades are thinner, so the angular momentum probably won't be too far away from the Ally anyway. But with an engine that size the difference will be pretty negligible compared to the forces created by the drag of moving the blades & the boat through the water.


So yeah, on paper you'd probably see a miniscule fraction of a difference, but nothing you're likely to notice when you're out there. Getting a suitable pitch for the metal of your choice is the best bet.

Over to the Hydrodynamicists?

[BassBoy]

9D
Thanks for giving this so much thought. I'm reassured!
BB

[Dirk Diggler]

Originally Posted by Nos4r2

As 9D280 says, deformation comes into play. Also the thinner the blades (ie stainless) the less slip you get.Therefore a 21" or 22" stainless prop will produce as much forward movement as a 23" ally one.

Not necessarily, your forgetting about diameter.

[Nos4r2]

Originally Posted by Dirk Diggler

Not necessarily, your forgetting about diameter.

For the purpose of the explanation I'm assuming both SS and ally props are roughly equal in diameter and blade size-just a different pitch.

Pedant!

[Dirk Diggler]

How about cupping and rake then?

[Nos4r2]

Originally Posted by Dirk Diggler

How about cupping and rake then?

Yeah Ok, you do have to look at everything-point taken. It's only a rule of thumb after all though and will usually get you somewhere near if you use a close equivalent SS to ally prop.

Whether intentional or not though you do make obvious how much there is to be gained in using a good supplier who'll let you trial/exchange props and then keeping what works best-ie not the 'mail order and hope' method... Been a victim of that one myself-but also did quite well out of it too. AFAIK there isn't a Michigan Wheel dealer in the UK but the best ally prop I've found for my setup is a Michigan Wheel Match which isn't available here.

[Cookee]

Originally Posted by BassBoy

9D
Thanks for giving this so much thought. I'm reassured!
BB

What does an ADV instructor do exactly?