How do propellers propell a boat.

I appreciate the information given many thank`s.
The propeller in post 55 was designed using Blade Element Theory. A gas powered quad copter.
Any comments relating that design to a marine environment for a power boat recreation use
would be interesting.

 

While it might make some sense to manufacture such a prop for flying drones with particular requirements, it IMO wouldn't make much sense as a propeller for a recreational powerboat, because there is already a big number of commercially available props of all types and sizes for that purpose.

From the hydrodynamic point of view, the large squared-off chord at the tip would very likely create both cavitation and vibration at high speeds or at high disc loading. I don't see it as a practical solution for full-size powerboats. But it might make sense for low-rpm, low-disc loading human-powered boats, for example.

In any case, both in case of R/C drones and human-powered boats, the design should be done having in mind the fact that the flow over blades will be fully laminar - hence very unstable. It looks like you have used the Clark Y or a NACA 2XXX section for the blade. You should IMO try to spend some more time and make a little research to see what would happen if you used a curved-plate section with a radiused leading edge and a sharp trailing edge instead of the current airfoil. Depending on the size and target RPM of the propeller, you could remain pleasantly surprised by the findings.

Cheers

 

The answer is, propellors don't propell boats, they propel them !!!

 

Thank`s for the suggestions diaquiri, toys (models) have the habit of morphing into full scale machines.The on line experimental project using the propellers shown call for four
two or four blade 3 foot diameter props.

Even if we had access to the wide range of props now available 40 years ago they would have been no use to us because they could not have coped with the conditions which still remain today that is why I have retained interest in the ones we made so cheaply. Some 5 blades. The boats we used were not toys and developed good HP even by todays standards. It probably would be easy to design them today possibly solving the possibly problems you point out. If we had wanted to we could easily swap to a popular commercial props to propell propel our boats.

I have not found much on Google about constant pitch propellers,constant speed yes and variable pitch. Here be dragons.

 

As I explained earlier there are two differences for water and air propellers working environment: Cavitation and speed of sound. In this case you are not anywhere near the the speed of sound, so the only difference is cavitation.

You could use a propeller like that in a very low drag pedal or electric powered boat. Then you can use a big slow rotating propeller maybe even with a typical airfoil profile.

But any more usual recreational boat will need way too much thrust for that. Propeller loading will be higher, tip speed will be higher and that kind of propeller will cavitate wildly and lose all its efficiency and probably also break apart.

Marine propeller design is very much limited by cavitation, which is the reason for totally diffrent foil profiles, much higher blade areas and lower efficiencies.

Calculate that propeller using water and some typical thrust needed. Look at the pressures you get on the suction side of the propeller.

 

While the Wright Brothers propeller design achievements were profound and a principal reason for their flight success, let’s not make more if it than there is. It is unfair and inappropriate to compare the efficiency of their propeller to a recreational marine propeller.

Let me again call on particle momentum and pressure. There exists a “low pressure” zone in front of the propeller (simplification alert!). This zone acts on the propeller, pulling it forward. It also acts on the particles of water in front of the propeller, causing them to move. (This is part of the reason for the induced velocity inflow into the propeller.) Now, any movement of mass from its static position or its momentum trajectory requires energy. This energy is put into the system by the rotating propeller. Looking at it another way, any pressure used to move water mass ahead of the propeller is not used to pull the propeller.

There is a quantitative measure of this effect that is called ideal efficiency. It represents the maximum potential efficiency for a “loss-less” system considering this effect of particle induction. Ideal efficiency is a function of the propeller’s pressure zone, as measured by “thrust loading” (which is basically thrust divided by diameter squared; or a “pressure”). As the low pressure zone get really big (as in a bollard pull, for example), the particles are pulled in from farther away, and not only from ahead but from outside the diameter of the propeller. (In a bollard case, water particle flow lines can curve in from almost perpendicular to the axis.) Again, any movement of mass requires energy, so in these cases of large low pressure zones (i.e., high propeller thrust loading), there will be a proportionally a smaller part of the total pressure that can be applied to the propeller. This means that the potential ideal efficiency decreases as you increase thrust loading. It also tells us that increasing propeller efficiency is largely about reducing thrust loading, such as by increasing diameter or splitting the thrust requirement over multiple blade sets. (Myth buster: this is one of the principal reasons why contra-rotating propellers are so much better than a single propeller of same diameter. The recovery of rotational losses is part of it, of course, but reduction in blade set thrust loading actually is often the greater contributor).

So, back to the propeller image earlier in this thread and the Wright Brother’s propeller. This type of propeller with large diameter and low thrust has substantially greater potential ideal efficiency than a recreational boat propeller with high thrust and a very limited diameter. The Wright Brothers grabbed the low hanging fruit by exploiting what they new about airfoils, recognizing the helical framework, and demonstrating it with their own testing facilities (that were arguably the best in the world at the time). It is no surprise that their propeller efficiencies are on par some boat propellers. Given the very low thrust loading, it would be more surprising if they were not.

We can never reach ideal efficiency. This is a limit line for a “loss-less” system. By loss-less, I mean without consideration of rotational or frictional losses. Then bring in the effects of tip losses and cavitation mentioned in a post above, and marine propellers (with little or no cavitation) can be at best some 90% of the ideal efficiency. This metric can also be a good measure of marketing claims. There are companies flogging propellers that they claim are 80% efficient. I am skeptical. No test data is ever revealed, nor how the tests were conducted. Laminar effects have been raised here before, and this is a challenge in any hydrodynamic testing to insure that flow is properly turbulent to model the real-world expected behavior. They may be using a different definition of efficiency (although I’m not sure how), or simply comparing apples-to-oranges (as has been done in the past with certain design “improvements”). It also is a good check of calculation methods. Any that offer predictions approaching ideal efficiency should be used with care (or avoided altogether as not offering a model of real deliverable performance).

Current design, as it relates to efficiency, is just stepping up toward ideal efficiency in little chunks. Larger chunks are found not so much by improving basic efficiency but by eliminating large losses in the system. Better cavitation control is one example, as are reduction of tip losses (like winglets have done for aircraft).

Don MacPherson
HydroComp

 

Don,

Many thanks for your input to the discussion. As to the bit on the Wright brothers and their propeller, I did not have any intent to compare their results to marine propellers, only to illustrate that their research program was extremely successful for the time and the same process has yielded the better marine propellers that we have seen over the years. Based on previous propeller work that they had to work from, I do think they made a significant step forward beyond what others were using. While it may seem obvious, such advances do require considerable insight. I've not built or designed any marine propellers but have built a couple light aircraft wooden props..

 

Tom, I was not really pointing the post to your discussion, but rather to some of the other posts suggesting that an airplane-style propeller would be a potential design choice for a marine propeller.

 

I see Don. Being wood, the props I built were necessarily much thicker to take the loads that a 74" diameter imposed. I have seen a couple props on human powered submarines that appeared similar to aircraft props. They were carbon and were probably too thin near the root because they bent a lot under load. Unfortunately, did not get to see any followup designs to correct that bad behavior. I am out of my depth on the details of prop behavior (electrical engineer) but have been dealing with outboard props for over 60 years. Greatest limitation of those seems to be the small diameter imposed by a lower unit design intended for general use.

Has there been any transfer of constant speed air prop technology to marine props? Is there a need for this in the marine field?

 

The outboard propeller diameters are optimal for the shaft rpm and typical speed. They also have rather good efficiency at the typical speed. Using an outboard in a displacement vessel is a different story. Then a bigger diameter and lower rpm would be much better. But you can't just pick one. There is no point with a bigger diameter unless you can use lower rpm as well. Otherwise you end up with an extremely low pitch to diameter ratio, which will give poor efficiency.

 

It seems that typical speed for many is WOT. Current concern is with with speeds from displacement to a max of 25mph or so and optimum range between 10 and 18 mph. Thus the interest in larger diameter and lesser RPM. The best that can be done with off the shelf outboards are the high thrust versions with largest dia prop available. Anything else readily available?

 

It is not about WOT vs. half load it is about typical boat and OB combination. I have no idea are you talking about 6 hp or 300 hp OB nor what size and type is the boat. 10-18 mph sounds like semidisplacement unless the boat is extremely small or big.

OB are not optimized for displacement boats nor semidisplacement. 90+% of outboards are used in planing boats and typical top speeds depend on the size of OB ranging from maybe 20-25 knots for 20-50 hp to 30-45 knots for the biggest OB. If you have a typical combination, the propeller diameter will be quite optimal at any planing speed.

The high thrust models are somewhat better for displacement and semidisplacement boats, but are still very much less efficient than IB with much lower shaft rpm and bigger diameter.

The standard OB propeller diameter is also too big, if your boat is much faster than typical.

Allowing a bigger diameter increases the drag of the drive unit. Also higher gear ratio will lead to higher losses.

E.g. a 50 hp OB might have 1.8 kN thrust at 25 knots WOT. A typical propeller might be something like 10x13 or 12x11 providing about 70/72% open water efficiency according to B-Series at typical 3000 rpm shaft rpm. OB propellers aren't B-Series, but you get close enough values for this purpose.

If you could use a 14x10 propeller (still about 3000 rpm), the open water efficiency would drop to 62%. So there is no point using a bigger diameter with this gear ratio.

A 14x20 propeller would get close to 78% open water efficiency, but then you would need about 1800 shaft rpm.

At half load, say 18 knots and 1.5 kN 10x13 and 12x11 are still providing 66/71% open water efficiencies. A 14x10 would be 66% so clearly worse than 12x11, but equal to 10x13. The 14x20 with different gear ratio would be 75%, so not much better than 12x11.

The problems start when your top speed is much lower than typical. E.g. that 50 hp at 18 knots top speed. Say at 2.5 kN thrust the open water efficiency is just 64% for 12x10 and 60% for 14*8. Now you can't get above 70% with any 14" propeller (free gear ratio) and to get to 78% you would need something like 20x28 with over 6:1 gear ratio.

 

How do propellers propel a boat

Our main worry was having any sort of propeller at all that would do the work we needed in bar filled harbors and costing little as possible.

The image shows a similar propeller we could make by the bucket full and out of any material at hand such as high strength steel. The blades some times were just flat strap steel and no foil shaping and the aluminum props were ground in many configurations even hollow ground from root to tip.

Composite wood and steel came into use as in the post war period there was nothing available without overseas funds and special import license.
Even to go boating fuel was rationed and wood alcohol and used refined oil was used in outboard motors and tractors. Power kerosene ran many motor cycles a mixture of kero and low grade petrol. We have some wonderful machines and tools now to help in making things in our sheds and it is amazing what is being done now. Some of our outboard and inboard power boats could reach 60 MPH. Thank`s for all of your info and suggestions.

Added images show a 60`s New Zealand built Riptide O/B by a local manufacturer that I had, It look`s like a Merc I suppose.

 

Looks somewhat like those vintage British Seagull's props

 

Very similar but different.