Propeller Pitch - what it means and choosing the right one

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Propeller Pitch - what it means and choosing the right one

Post by Maltelec » Tue Dec 22, 2009 12:11 am

Propeller pitch is quite a simple one really.

Say you had a 26" x 20" propeller. This means it is 26" diameter and 20" pitch.

Just like a thread on a bolt, the pitch tells you how far it moves in 1 revolution. E.g. a thread with a 1mm pitch will move into a hole by 1mm in 1 revolution of the bolt.

So with a propeller, given there is no resistance, as the propeller turns, it will push its self through the water, 1 revolution giving 20" of forward motion in this example.

Prop Slip

But as we know, there is resistance. Its called a boat. This causes propeller slip, where although a propeller does 1 complete turn, instead of travelling 20" it only travels 15".

Working out the propeller slip is very straight forward. You need to know your propeller speed and hull speed.

For example, the hull is doing 6mph and the 26" x 20" propeller is doing 400 rpm.

400 rpm x 20" = 8000" a minute.

6mph = 6336" a minute.

6336 / 8000 x 100 = 79.2% -> This is the % of used thrust.

Thus 100% - 79.2% = 20.8% slip.

In the ideal world, you want 0% slip. In reality, anywhere between 10% and 20% would be ok, but on the smaller steam launches you can often find the slip is as much as 25% or more.

Choosing the correct prop

Again this is really much simpler than just having to guess.

The important thing is to know your engine speed. This will have been designed into the engine, so if it recommends running at 450 rpm, best aim for that. Higher speeds also means more power.

You should also work out your hull speed.

Hull speed (knots) = 1.34 x Square Root of (Length of Water Line in feet)

1 Knot = 1.150779 mph

E.g. for a 15 foot boat, the hull speed is:

Hull speed = 1.34 x SqRt(15 foot)

Hull Speed = 1.34 x 3.742

Hull Speed = 5.19 knots = 5.97 mph.

Say you calculated a running speed of 6mph, and you want to try for 14% prop slip, all you do is calculate the correct pitch.

Adding this into the calculation:

14% is the slip, meaning that there is 86% of useful thrust.

450 rpm x pitch = "pitch distance" a minute.

6mph = 6336" a minute.

6336 / "pitch distance" x 100 = 86%

"pitch distance" = 6336 / 86 x 100

"pitch distance" = 7367" a minute -> This is the distance the propeller would move without slip.

So 450 rpm x pitch = 7367

Meaning the pitch required is 16.4".

You may want to go for an 18" pitch in this case, it is usually better to have the engine slightly slower with the larger pitch than slightly faster with the smaller pitch.

Propeller Diameter

In this case, bigger the better. The larger the propeller the better their efficiency. The difference between a 14" and a 16" will decide who gets the good jetty spot if the boats are otherwise identical.

Steam launch propellers generally have 3 large blades. This also adds to their efficiency as we are not running them at high speeds, surface piercing or anything other than stuck firmly under the water and generally not stressed. There is nothing wrong with 5 blades, its the total blade area which is the key to their efficiency. As long as the pitch is suited for the engine speed and hull speed, you shouldn't go far wrong.

Brass and Bronze are the traditional metals, though a few stainless ones have appeared lately, the odd one being home made.
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Re: Propeller Pitch - what it means and choosing the right one

Post by artemis » Wed Dec 23, 2009 5:48 pm

Although the above is "sort of right", there are some glaring errors, like if you don't have slip, the propeller won't work. And while 1.34 as a multiplier of LWL to "guesstimate" speed (in knots, not mph) was considered as good average, that is not so anymore. And diameter is the main item - much more important than pitch. The propeller determination article in Steamboats & Modern Steam Launches, pp 18 - 20, Sept/Oct 1962, available from Elliott Bay Steam Launch, is probably the best for a quick initial determination.

Probably most hobby steamboats don't have a best propeller fitted. A really good book on the subject, which anyone can work his/her way through and apply, is Dave Geer's Propeller Handbook, available online from Amazon. Follow the information and formulas therein and you may be amazed at how fast that 2HP engine will move a 20'+ hull if you select the correct prop!
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Re: Propeller Pitch - what it means and choosing the right one

Post by Maltelec » Wed Dec 23, 2009 7:35 pm

How does a propeller not work if there was no slip?
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Re: Propeller Pitch - what it means and choosing the right one

Post by artemis » Thu Dec 24, 2009 3:10 am

Maltelec wrote:How does a propeller not work if there was no slip?
A propeller must develop thrust to move the boat forward. From Gerr's Propeller Handbook, page 50:
People frequently mistake slip for efficiency and thus try to eliminate it altogether... Slip, in fact, is actually required to produce thrust... You cannot efliminate slip and would not want to if you could, for then you would have no thrust at all.
For those not aware, the book is copyrighted 2001 and Mr Gerr is Director of the Westlawn Institute of Marine Technology and owner of Gerr Marine, designing commercial and pleasure vessels since 1985. He and his designs and books are known and praised internationally.
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Re: Propeller Pitch - what it means and choosing the right one

Post by Maltelec » Thu Dec 24, 2009 11:52 am

Ah I think what Mr Gerr is pointing out is that because water is a liquid, you will always get slip (which was pointed out in my 1st post). He doesn't mean that slip is required, just that it is inevitable. Mr Gerr will be making allowances for all types of props and boats, we are mostly interested in ones for the average steam launch. If you take a land propeller, these do not slip because the contact material is solid (up to the point where the ground gives way).

This is the black art of propellers. Water will slide around the propeller, so you must work with a percentage of slip in mind. Trying to calculate a good propeller to reduce slip is detrimental to the efficiency (as Mr Gerr states) because it will be there.

However a lot of the black art of propellers occurs when you have deliberate cavitation and ventilation of the propeller, something which you don't get with the standard steam launch. When you get into high-seed and power-power props, the "classical" prop design as I stated in my 1st post goes out the window.

As for the diameter of the prop, again this depends on your purpose. If you have a 2000 HP high-speed racing boat, a 10 foot prop isn't going to do anything for you.

The general theory is the larger diameter the prop, the more torque is required.

Steam engines have no shortage in torque. The test we did on the Elliot Bay triple gave us 94 ft.lb of torque @ 360 rpm, 6.4 HP.

Thats more torque than a Nissan Micra 99BHP car engine can put out, and the triple wasn't even up to full power, it should do twice that.

However we then get back into the dark art of the propeller design. Although more torque is required for a larger prop, and as we know steam engine are mostly torque (thus the bigger props are better), the amount of torque difference required by a larger prop is quite small to the power difference required by a high-pitched prop.

So given that, for a steamboat, torque isn't an issue, the only issue we have is the speed. The pitch is responsible for the speed of a non-cavitating, surface piercing or ventilating propeller, so the pitch should be calculated for the speed of the steam engine + the slip percentage which you would be likely to get (25% on our boat).

The calculations for which are in my 1st post.

The reason more torque is required by a larger prop is not so much because it has more water to move, but because it is slightly more efficient.

Because of this, a smaller prop would put less load onto the engine, allowing the engine to turn faster. To compensate, you want a smaller pitch (opposite to what you would think) to achieve the hull speed. This is one point which catches people out.

The main point which catches people out, is that you can't go faster than hull seed (on your average steam launch).
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Re: Propeller Pitch - what it means and choosing the right one

Post by barts » Sun Dec 27, 2009 9:07 pm

I have a copy of David Gerr's Propeller Handbook - it is indeed excellent.

Some rules of thumb for steam boat designs from various places (displacement hulls, low power):

1) Use the largest possible propeller diameter that will fit the hull.
2) try for a 1:1 diameter pitch ratio if possible - it's the most efficient; up to 1:1.5 is ok,
past that efficiency drops badly. Pick the pitch to result in the RPM you want
at hull speed using 20% slip as a guesstimate; more if the prop is unusually small
for the boat & engine.
3) fair (smooth the lines and transitions of ) the hull & keel so the prop is working in
smoothly flowing water as much as possible.
4) blade area is needed proportional to power to prevent cavitation; since most steam
boats are low powered, the best prop design would have a long thin blade (more
towards airplane propellers than ski boats props....); this would reduce drag and
wetted surface. This doesn't make a big difference, though...

- Bart
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Re: Propeller Pitch - what it means and choosing the right one

Post by dampfspieler » Tue Dec 29, 2009 6:07 am

Hi,

I've found this little online calculating sheet Propeller Calculator

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Re: Propeller Pitch - what it means and choosing the right one

Post by farmerden » Thu Jan 14, 2010 5:12 am

Dietrich-Alas the site you mentioned only works above 500RPM Den
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Re: Propeller Pitch - what it means and choosing the right one

Post by dampfspieler » Thu Jan 14, 2010 9:26 pm

Hi Den,
... only works above 500RPM
of the machine :shock: :shock: - to get lower Prop-RPM the gear-ratio must set >1.

An good and easy way is to set the gear-ratio to 2 and the machine-rpm to twice of the chosen prop-rpm (steam driven).

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Re: Propeller Pitch - what it means and choosing the right one

Post by farmerden » Sat Jan 16, 2010 2:20 am

Dietrich OK I tried that .I used 820 RPM,Ratio of 2 ,Pitch of 28 ,Speed of 7.1 MPH and that gave me a slip of .35. That is reasonablely close as using my GPS at 410 RPM ,pitch of 28,speed of 7.1 MPH I had calculated slip at 38. What I wanted to do was slow the engine down to 375 go to a 21 in D and a pitch of 24,same speed and the slip came down to .16.The reason for this is my boiler is maxed out,if I slow the engine down I should be able to run at a higher steam pressure which should make my compound run more efficiently and make up for the loss of power because of lower RPM's! Life is just one grand experiment isn't it? :lol: Den
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