Performance Differences Between MaxProp Sizes and Pitches

[fstbttms]

Originally Posted by zboss

When you say "15% of diameter minimum"...

If I select an 18" prop... that would mean 15% was 2.7" - would that mean 2.7" on each side or 1.35" on each side?

2.7" from the blade tip to the hull. Not sure what you mean by "each side."

[skipgundlach]

Originally Posted by Krilleonnova

I got my propcomments in under the diveboard thread can some computer
guru pls move it

Krilleonnova

Open that thread, copy and paste your comments from there to here, and then delete the misplaced comment...

[GrahamHO]

Originally Posted by zboss

Hi,

I am looking to replace my existing fixed prop with a Maxprop.

The recommended prop by Westerbeke is a 18" x 8P and this is what was originally fitted on my vessel but it was old and needed to be replaced. I replaced it 2 years ago with a 17" x 11P under the recommendation of a prop shop.

However, I want to move to a Max Prop to improve sailing efficiency. Maxprop originally recommended a 3 Blade 17x10 but I have found that my existing 17x11 doesn't seem as efficient as the old 18x8 seemed to be.

Also, when fully laden, our waterline length is higher than the designed waterline length and the boat is much heavier, hence why I think that extra inch helped a lot.

I asked about moving up to an 18" maxprop and he said that best I could get was 18x9.

However, I am still a little confused as to what kind of performance difference I would notice between the two maxprop sizes - 18x9 and the 17x10.

What do you guys think?

The general rule within limits is that take 1" off the diameter of a prop you need to increase the pitch by 1" so I wouldn't expect any difference. A larger diameter may possibly be too close to the hull which from memory ideally it should clear by a minimum 10% of the prop diameter.

[zboss]

Originally Posted by fstbttms

2.7" from the blade tip to the hull. Not sure what you mean by "each side."

If you take the diameter of a circle of 18" and multiply by 1.15 you get a circle with a diameter of 20.7".

2.7" on each side would be 30% of the diameter.

So is it 15% or 30%? Sounds like 30%

[JSSailem]

This 15% is a distance from the hull surface that reduces/ stops hull cavitation - vibration. So if the blade is moving (as in the diagram) less than the 15% at (1) will create cavitation/vibration which you will hear and feel on the boat. Cavitation is the introduction of air bubbles on the prop blade. Spin the prop faster means more air to a point that you go no faster while the prop spins in the air bubbles space created. Not good for blade or engine. ( the created bubbles pop and can pit the prop blade damaging it). Position 2 is a smaller surface. It will contribute less if a cavitation event. None is best. But may not be possible as you try to maximize you prop function. So as in all boat issues you compromise based on desired result and boat construction. You must be the judge as to how close the prop is at position 2 based on the knowledge of cavitation principals.

Sent from my BNTV400 using Cruisers Sailing Forum mobile app

[GrahamHO]

Originally Posted by JSSailem

This 15% is a distance from the hull surface that reduces/ stops hull cavitation - vibration. So if the blade is moving (as in the diagram) less than the 15% at (1) will create cavitation/vibration which you will hear and feel on the boat. Cavitation is the introduction of air bubbles on the prop blade. Spin the prop faster means more air to a point that you go no faster while the prop spins in the air bubbles space created. Not good for blade or engine. ( the created bubbles pop and can pit the prop blade damaging it). Position 2 is a smaller surface. It will contribute less if a cavitation event. None is best. But may not be possible as you try to maximize you prop function. So as in all boat issues you compromise based on desired result and boat construction. You must be the judge as to how close the prop is at position 2 based on the knowledge of cavitation principals.

Sent from my BNTV400 using Cruisers Sailing Forum mobile app

Quite correct about needing to compromise based on desired result. Some years ago I was documenting the construction of a couple of Volvo water ballasted round the world racing yachts. They were fitted with Volvo saildrives. In order to reduce the sailing drag they were fitted so the legs protruded a minimum and with quite small folding propellors. The engineer was shaking his head and quietly saying to me that they won't be able to motor off a lee shore. So that was an extreme example of compromising motoring performance to improve sailing performance.

[fstbttms]

Originally Posted by zboss

If you take the diameter of a circle of 18" and multiply by 1.15 you get a circle with a diameter of 20.7".

2.7" on each side would be 30% of the diameter.

So is it 15% or 30%? Sounds like 30%

You are overthinking this. Your 18" prop should come no closer to the hull than 2.7".

[sailcrazy]

We put an Autoprop on our Taswell 43 about 14 years ago. It continually adjusts the blade angle to balance the power applied(rpm, torque) to the resistance (boat speed thru the water). There's nothing to set, the blades are always at the optimum blade angle, regardless of your eng rpm. We still do have some prop walk in reverse, but I wouldn't have anything else. But you do want to pay attention to the prop diameter vs the hull-to- blade tip distance.....it really does make a difference!

[Pauls]

Hi Zboss. That would mean 2.7" clearance on each side, or as fastbottoms said, 2.7" from the prop tip to the hull.

[GrahamHO]

Originally Posted by fstbttms

You are overthinking this. Your 18" prop should come no closer to the hull than 2.7".

Skene's Elements of Yacht Design, on page 144 suggests that a hull clearance of 12% of the prop diameter is good.
There are several pages in the book of graphs and calculations for deciding on propellor sizes and pitches. Maybe it's on Kindle. My own paper copy is perhaps 35 years old and becoming dog eared. It is all equally relevant as it was years ago

[zboss]

Does anyone know of an online calculator that does reverse computations?

In other words... here are the basics of my boat and here is my propeller - tell me what happens when I change my waterline length. Tell me what happens when I add 5000 pounds of cruising equipment.

[Chuck Hawley]

I came across a spreadsheet on my computer from long ago. I don't know if it would be helpful, but it's freeware and I thought I would pass it on. Unfortunately, I don't know how to upload it to the forum. The name of the spreadsheet is propcalc.xls, and it was distributed by Mythical Solutions

Cheers,

Chuck Hawley

[GrahamHO]

Originally Posted by zboss

Does anyone know of an online calculator that does reverse computations?

In other words... here are the basics of my boat and here is my propeller - tell me what happens when I change my waterline length. Tell me what happens when I add 5000 pounds of cruising equipment.

I don't know an on line calculator, though I'm sure designers have their own or available programs. What you can do for example is read Skene's elements of yacht design. Using information from there you could reverse the fuel consumption calculations. That is you might know your fuel consumption figure and from that you can accurately calculate how many horsepower your motor is delivering. You then need to allow for your alternator which can easily absorb 1 HP. Also any freezers etc.
I'm sure most formulae were originally developed from trial and error, then making formulae which fitted practical findings.

[Pauls]

Again, if you want to do this as well as possible, use Gerr. His Propeller Handbook will give you a detailed and thorough approach. You will clearly be able to answer your questions about waterline length, increased displacement, etc. This you'd expect as it's a whole book on propellers.

On the other hand, Skeene is only a couple of pages so it will be quicker and easier to get thru.

This is only my opinion, but I would far rather study up a bit and do the calculation myself as opposed to using an online calculator. The online calculator is dead easy. But you have no idea how the person who composed it put it together. It could be accurate or it could be crap, you will have no visibility into how the answers have been derived. I'll say this - if the calculator does not require as inputs the depth of the prop below the surface and the blade area of the prop then it is not checking for cavitation resistance, which is an important factor in prop selection.

[GrahamHO]

Originally Posted by Pauls

Again, if you want to do this as well as possible, use Gerr. His Propeller Handbook will give you a detailed and thorough approach. You will clearly be able to answer your questions about waterline length, increased displacement, etc. This you'd expect as it's a whole book on propellers.

On the other hand, Skeene is only a couple of pages so it will be quicker and easier to get thru.

This is only my opinion, but I would far rather study up a bit and do the calculation myself as opposed to using an online calculator. The online calculator is dead easy. But you have no idea how the person who composed it put it together. It could be accurate or it could be crap, you will have no visibility into how the answers have been derived. I'll say this - if the calculator does not require as inputs the depth of the prop below the surface and the blade area of the prop then it is not checking for cavitation resistance, which is an important factor in prop selection.

I agree entirely with this except to suggest you use both references and any others you can find. Working the old fashioned way from basic formulae seems to me better than some online calculator. No reason not to use a computer to work through them. Skene's book will provide some aspects that Gerr may not and vice versa. L. Francis Herreshoff's "Common Sense of Yacht Design" is worth reading as well. Old fashioned, but still relevant as he writes common sense and the sea hasn't changed....He also looks into his then future view of design in his chapter on the "Sailing Machine" and talks about such things as bulb keels which have now become quite common in New Zealand at least. I've got one. Everyone hates them until they try them for themselves. But that's another story.