# Propeller design!!!! PROBLEM

Discussion in 'Props' started by Salvatore84, May 12, 2012.

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### Salvatore84Junior Member

HI guys! this is my problem at the moment:
I have a motorboat of 18.7 m Loa, B = 5.2, Draft = 1.5 m, weight=36 t
I have changed the engines, now I have 2 x 730 hp @ 2300 rpm, gear ratio 2.037.
So I tried to make a calculation to modify my propellers:
Diameter = 820 mm
Pitch = 890 mm I used ( propexpert )
I don't know if my propeller is a B-series or Gawn, how can I see it?

At the moment at 25 kts my engines can't go over 2100 rpm, I think that my pitch is too big.

How can I modify my pitch?!

thank you!

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### MikeJohnsSenior Member

You either obtain new props or you get the existing props re-pitched. Depending on the material they can be bent to a new pitch.

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### Salvatore84Junior Member

Hi, thank you for answering me,

I "re-pitched" it with a pitch of 890 mm. and my engine that is rated for 2300 rpm make just 2100 rpm, so I think that my propeller has too much pitch, but now I don't know how to re-pitch it.

I'm sorry if I was not so clear in the first post.

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Salvatore

In calculating the prop size and pitch, is a straight forward calculation. However, many assumptions are required, for someone else to do the same, and can lead to very different results.

In order for any to check your result, a layout showing the shaft & prop and where/how it sits on the hull...and the hull lines aft, to see the water flow into the prop.

Since you need to calculate the cavitation number suitable for your installation and then ascertain which is the best BAR to match your 'fixed' gear box ratio.

When calculating/designing a prop, you should have a series of curves. The vertical axis being the efficiency of the prop (from the charts) the horizontal axis is the gear box ratio, and then a series of curves (calculated from the charts) of different BARs.

You select the lowest BAR for the highest efficiency to match the gear box ratio.

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### Salvatore84Junior Member

thank you!

It seems very hard, I have no curves as you talk about.
I used the software propexpert. Can you suggest me a way to solve my problem without using this curves?
I need to fix it as soon as possible and I don't know how much to re-picth my propeller.

6. Joined: Oct 2008
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Salvatore

I have no knowledge of propexpert, so I can't comment. I always use the paper copy charts and calculate manually.

But as I noted above, you still need to calculate the cavitation number, and estimate the speed of water flow into the prop, from the lines and arrangement of your installation. The software wont give you that!

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### keysdiseaseSenior Member

Sounds to me like you are very close, 200 RPM off is excellent for "out of the box."

What is your tip clearance from tip of prop to boat bottom? Rule of thumb for typical installations is 10-15% of prop diameter. You may not be able to reduce pitch any more than you already have, some props can't tolerate more than an inch of change, but reducing diameter is easy and appropriate if you are too close to the bottom already.

How about ship loading? Are you extremely light in these initial runs? It is sometimes customary to let the engines over rev a couple of hundred RPM to account for typical loading conditions like fuel, beverages and peeps.

Steve

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### Salvatore84Junior Member

I can't change tip clearance and diameter, I can just move the pitch because I have to use this propeller I can't buy new one

9. ### FrostyPrevious Member

Reducing diam will only increase slip and not necessarily speed --200 RPM low is perfect IMO.

I am assuming from those RPM that it is a deisel.

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### daiquiriEngineering and Design

Another option is to twist the trailing edge of the prop blades few degrees towards upwards (towards suction side). It is a reverse-cupping of the blade. The result of this reverse-cupping will be a decrease of the effective angle of attack of the flow seen by the prop blades (which is equivalent to a down-pitching), a decrease of the torque, a decrease of cavitation level and an increase of RPMs. I suggest you to do it in several steps. On each step you should bend each blade by the same no. of degrees (say, around 5°-10°), mount the props and see what happens with the engine RPM. Since you are off by just 200 RPMs, you could as well end up with the correct RPMs even after just the first step.

I'd suggest you to read this document: http://www.hydrocompinc.com/knowledge/publications/PropellerCup.pdf and apply the described cupping towards the opposite side of what is depicted in the figures.

Cheers

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### MikeJohnsSenior Member

Slavi
Surely the opposite bend would significantly encourage cavitation at the leading edge of the blade ?
One of the effects of "cupping" the blade is the reduction of cavitation.

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### MikeJohnsSenior Member

If you have software then work backwards for a start. Look at the power/torque figures for your old engines and match that with the old pitch dia and DAR etc for your original installation. When You get your figures matching, then introduce the new power- torque RPM figures and play with the pitch.

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### daiquiriEngineering and Design

Mike, a cup is equivalent to the trailing edge flap on an airfoil. The effect of a positive (downwards) deflection of a flap is an increase of lift (for a fixed foil AoA) and a decrease of pressure over the leading-edge dorsal surface, while the negative (upwards) deflection decreases the lift and increases the pressure around the leading-edge area.

Same happens with a cupped prop blade. A positive cup alone (with no change in blade pitch) will increase the lift produced by the blade and also the negative gauge pressure over the ventral side of the leading edge. A lower pressure at the L.E. means more possibility of inception of cavitation in that area. But, if a positive cup is accompanied by a correspondent decrease in pitch (in order to keep the blade lift constant), then the benefit of decreased cavitation may be achieved. See the following three pressure distributions around a foil:
1) a clean airfoil at 5° AoA:

2) airfoil with a 10° flap at 5° AoA:

3) airfoil with a 10° flap at -1° AoA:

Hope these graphs will explain my point in a more clear manner. The first and the last cases have a nearly same lift coefficient. Please note the difference in peak pressures around the leading edge. Also, see the page 2 of the document in my previous post - it shows the reduction of peak pressure achieved by both cupping and de-pitching of the blade (my pic n.3). A cupping alone would have a negative effect on the cavitation, as explained above.

What I am proposing to salvatore is this:

Please note also that the above pics show 30% flaps, much more than is used for blade cupping, hence they serve for exclusive purpose of more clear visualization of various results.

Cheers

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### Salvatore84Junior Member

Thank you guys to everyone!!

So, I read carefully the paper about the "cup" it's a good choice. What I can do at the moment is to bent the blade having a new pitch, or trying to cup the blade of 10°.

From some calculation that I made with my software, playing with rpm and power, I have to reduce the pitch of 70-80 mm, having a new pitch of 810 mm instead of 890 mm.

I will talk with the guy that can modify the propeller, and I will see if to have a new pitch or use a 10° cup!
I will let you know !

have a good day!

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### MikeJohnsSenior Member

Slavi
Yes thanks for your effort but I do know that. I just don’t think it’s a very good way of reducing effective pitch.

I’m not sure how much value your infinite foil, pressure distribution software really is in this case. I think you need to look in detail at the pressure distribution not just a cross section of an ideal foil.
You should be looking in detail at the free ends of the foils and the total pressure distribution pattern on the back of the blade. It might help to look at typical pics of cavitaion damage.

Positive cupping always reduces cavitation at high loads, it’s a valid way of reducing cavitation even at small cupping angles that have little effect on effective pitch.

I really think that negative cupping will have the opposite effect and the cavitation bubble will be trapped to some extent rather than being “swept off the back of the blade”.

I’d be very careful suggesting negative cupping on a ship/boat prop.

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