Prop Manufacture

I have said what I thought,

I will bow and step away backwards.

BUT should'nt the root of the blade be thinner/smaller to cause less drag. At either side of the hub there looks to me to be just power absorbing flat to the rotation blade area.

The pitch may be 23 but thats an average.

What Im saying is that if you cut half off each blade I think you would have similar HP absortion with far less thrust. The whole blade is not working to its max potential.

Your computer programme wont tell you that it just gives you the result of what you ask it.

 

Frosty
It is a high efficiency prop so not quite what you see for most boat props that are designed with a draft constraint. Generally the hub on a boat prop is much larger relative to the diameter. Hence the blade angle at the hub is quite low.

Also you need some meat in the blade near the hub so that it can withstand the bending force. I have attached a picture of an air propeller of similar proportions and you can see how thick the section is near the hub. In my case the milling head does not allow you to get a square corner so it needs to be radiused. This also helps reduce stress concentration. The inside of the blade does not contribute much thrust but it does improve the efficiency of the outer portion of the blade as the pressure gradient is less so the induced drag is lower.

The aim with the prop is for one fellow to travel 260km in 24 hours without wind or current assistance. Everything needs to be light and efficient to do this. The prop is designed right and looks to be made close to my design. There was a little extra meat added to allow for burnishing. Testing will tell if it is right.

Rick W.

 

Using my viscious wake models I don't get numbers as high as yours (~70% at 35N) but then I don't have your section data or pitch distribution. What I do see is that the root needs to be well twisted up (which reduces thrust) to avoid going to negative AOA and I see an increase in efficency of ~3% if I increase hub diameter about to 7-10% D to get rid of that twist up.

 

John
Getting the foil right makes a big difference.

As far as the root twist goes you cannot see a lot in the posted image from this point of view. The section at that point is highly cambered. It has a slight negative AofA but it is still lifting.

I have little understanding of the viscous wake models other than I believe they were developed to better model the performance of water props with low aspect blades because the induced drag is a very large component of overall loss. In my case I am using a relatively high aspect foil at relatively low lift coefficient so the induced drag is quite low.

I have verified the performance of similar props on my boats and am confident the efficiency prediction is close to the money. I have also assessed the performance of a variety of props using JavaProp or my own segmented model and they compare favourably.

Rick W.

 

John
I have attached the JavaProp result for the operating conditions using an E193 foil. It is one of the sections available in JavaProp. It is not the section I have used but it is similar and the results compare. My section starts a little thicker and most of the blade is an 8% section with a 37mm chord.

In my model I continuously adjust for the Re# throughout the blade optimisation whereas JavaFoil only has discrete Re# and limited range so my model is more precise.

I would be interested to know what your method predicts for a prop of this design so it can be compared against JavaProp.

Rick W.

 

The friendly machine shop has been doing a bit more work on the prop installation. I have attached the latest picture of the prop showing the spinners and shaft strut tube.

The tube houses two water lubricated acetal bearings with glass balls so low drag and long lasting.

This image gives a better idea of the pitch angle at the hub.

Rick W.

 

Rick that looks very beautiful. If you and the machine shop ever fancied going into hpb prop design and production, you've got a market here in huddersfield!
I've been totally under the cosh at work, and not been able to work on the pedal powered cat at all, but things should ease off in early June, and its only a couple of days work to put the bits together... (famous last words!)
very interested in your posting on the solar cruiser too, btw., though a windmill version might be more suited to UK weather!

 

The machine shop believe it would be profitable for them to make props at CAD550. With the bearing housing for the prop strut and fore and aft spinners the price would be CAD700. So not in any way low cost by comparison with what can be done by hand but they are made with precision and I expect will perform well. So I think the market is limited.

I am happy to provide the prop CAD design for specific applications. If they need to be bigger than shown then cost to mill might be higher.

The prop shown should be tested next week on a sleek new boat that is a copy of my V11A in shape but full carbon fibre layup.
http://www.adventuresofgreg.com/HPB/HPBmain.html
The 24ft long hull will end up around 8kg.

Rick W.

 

Played JavaProp for an elboat hull 7m long, 500kg load, 12kn speed. Funny: For a 80% efficiency, one would need a 1m dia prop at 250r/min. Only about 70% efficiency is possible, if prop diameter is 0.4m at 1300 r/min. In practice motor efficiency may be up to 90%, and consequently only about 60% efficiency by a total drive system would be realistic?

Question:
In JavaProp it is possible to modify eg. prop blade chord of that optimized one. But how to list or plot modified features (eg. new efficiency)? How to estimate existing propeller features using JavaProp?

confused,
redu

EDIT 11 may:
JavaProp default blade profile = "flat plete blades". As a diletant, I used the default. This is a poor profile for a prop! About 10% better eta is available with a proper profile in blades.

redu

 

I looked at your Michlet data and it shows about 800N drag at 6m/s (say 12kts). I then looked at what JavaProp produces for this using the MH 9.8% foil at 1 degree AofA. I have attached the result. It is much better than you have stated.

I also tried the same foil at 1300rpm and prop diameter of 400mm and it gives 82%.

I expect I could do better than this with a better section foil but I think you should rethink the hull design. It is not very well suited to doing that speed as it is climbing up the wave drag curve.

In terms of efficiency you have to take all factors into consideration between the batteries and pushing the hull. These are prop, appendages, gearbox, motor, controller and batteries. You need to work hard at each to get better than 60% overall. That is why it is best to start with a low drag hull so that the required output is low.

Rick W.

 

Thanks Rick!
Yeees, used "flat plate blades" as was default in JavaProp! This was a mistake! With real wing profiles I now get 10% better etas.

redu