How do propellers propell a boat.

Not sure why you think partial tunnels or pods are so poor, it's probably just a case of operating well away from the design point. [FONT=&quot]Props in tunnels can be detrimental to hullform for manoeuvring in some small craft, but there seem to be some quite successful designs now[/FONT].

Certainly Jets are not good allrounders, they are high speed thrusters and suffer the abysmal bollard pull and low speed thrust efficiency of all high speed propulsors.
[FONT=&quot]The only prop that comes close to a compromise for smaller high speed craft that I'm aware of is the transcavitating, That will give better low speed manouvering and good high speed performance for a small craft that aren't looking for very-high speed.[/FONT]

Thinking of different props have you seen the Axion props made in NZ? they are interesting departures from the traditional.

 

I have looked at Axion props I will look again I think I have book marked it.
Post # 82 is interesting and others.
We had a Marine Engineer living local who among many things designed and built a pocket tunnel for power boats which sold well in the USA. Hydrodrive an image is on this site. Some with outboards built into the tunnel
I had the thrill of traveling in a ocean racer up a local river two gallons a minute at about 60MPH.Friends tried them in their boats and that is where their impressions came from and I have tried other models myself but found turning a boat caused problems and boats did not want to turn,like being on rails. I do not think there is any advantage to put a prop in a tunnel they are no advantage in shallows and like jets you need to have a deep v hull to get the prop into denser water.

 

How do propellers propel a boat

I should not have added "pods" that was not intended just "pockets" and "tunnels" in particular short ones and not racing tunnel hulls.

 

How do propellers propel a boat

Axiom props look interesting and good testing equipment to help.

Trying to get away from dangerous props threshing around and doing away with rudders to find a better way to pilot a boat all sorts of ideas and propellers come to mind. Turbodrive single or multi blade design trimmable for operating depth and thrust but not for breaking records.

 

The Axiom is nothing special IMO. While it has good reverse, it gets it by having a poor forward. Nothing new there.

http://www.flexofold.com/upload_dir/docs/Test_YachtingMonthly_low.pdf

 

Dennis

The Axiom is interesting because it's a significant departure from conventional.

But you have fallen into a trap with that article if you think it's a good comparison. Matching a fixed prop to a hull is not trivial. A useful study would need a lot more test data and matching of props to the boat. A plot of the prop curves would be informative too.

There are many variables not explored, importantly there is no fuel metering. So no attempt at even a single operating point relative efficiency ( for the one prop tested). And no variation of thrust and efficiency over the range of RPM. Neither Bollard pull nor maximum speed are very useful comparisons of a prop design without more data.

I'd expect to find quite different results with some care matching prop pitch and diameter to the hullform and it's propulsion machinery. Also with different types of craft.

 

You can not trust anyone to give the full picture I guess that is sales maneuvering and avoiding any real comparisons between products.
If you are aware of any misleading or lack of information let`s hope others have the same
diligence in making their choices. I make my own judgments of info supplied by any seller of any product. The Axiom props do not seem unconventional to me in Marine, rotary Homogenizers or aircraft or any other usage. Like autos now each is unique in some way and boats are the same.

 

Of course its not going to be 100% exact. It did appear the methods they used were decent to the degree that large discrepancies would probably only have been produced by intentional misrepresentation. Assuming there was no foul play (who can be sure?), the prop manufacturers were the ones who specified which prop to use, and the prop axiom supplied done poorly in F. So if the issue is that the prop was wrong, what hope does the buyer have of selecting the right one if the manufacturer cant get it correct? Neither of us can prove if the prop tested was the best out of the Axiom range for this application, but its safe to assume the maker would have the best shot at getting it correct if they only had one try. BTW I am not saying its a bad prop, its good at reverse, and has amazing stopping power. These 2 features may be very important to some users even if they know F is not as good as others.

But lets ignore that test totally. We still have facts which we can consider.

The simplest reason with the least assumptions a prop like the axiom might do poorly in F, is already known. Props that do equally well in F and R have a poor F compared to props which are optimized in F. Props are just flow turning devices (see link in post 82 by NASA). The most efficient flow turning devices are foils of sort, and all the most efficient foils only work well in one direction. Foils which have equal performance in both directions compromise the ability to excel at either. Attempts to make such a foil are of great interest to proa sailors and it seems they are a long way off finding anything which is not a compromise of some degree.

 

It'd be nice if it were that predictive. Wake fraction for example would have been a simple estimate based on crude hull parameters. On a smaller craft it's usual to test a range of props around a starting point.Whether you get a best match with one initial attempt is something of a lottery without comprehensive data.
But I note from that article that the only thing they were actually comparing was drag. Nothing else and that's probbaly what they told the suppliers.

Shame they published the article as though it were a meaningful comparison.

The Axiom picture posted will be identical fore and aft in free stream. So we will have to wait for a decent test to see how well it really performs but it does fit the requirements for Tom's equally good astern query earlier.

Regarding models of lift, be careful adopting a model and dismissing the others. Newtonian theory is just one model that explains lift, Newtonian models are simplest and easiest to teach to non math students. However Bernoulli explains the fundamental flow field around a foil that results in that Newtonian models validity. It's a matter of where you draw the box for your analysis not whether one is correct and the other incorrect.

I think the efficiency might be low from what I can find, in the 0.5 to 0.6 range compared to 0.7 to 0.8 range for standard props.

 

The prop in the image is SIMILAR to the ones we used to make, flat blade ground to get SIMILAR blade shape simple and easy to make.
Diameter was easy to change just cut tip off. Some were 5 blade wide at base tapering to the tip and rounded.

 

Year ago I raced on Davison 55 yacht, it would do 11.5 in forward and reverse, quite impressive

 

Of course all propellers should be forward facing yes?
especially if you are in the business of fixing them

 

"Forward facing" how? Direction or rotation? Lots of variation in how you define that. Common terms are "tractor" or "pusher". "Pusher" being preferred for hydromechanical reasons.

 

Volvo ips style
typical outboard pushes the gearbox, aft facing
ips is pulling the gearbox as they face forward

 

Meehhh, mostly hype.

There are some reasons for a tractor in exposed mounts, but you have to understand what you comparing. For planeing craft with no tuck, drive legs are more efficient because you don't have shaft drag. Tractors have better inflow but more leg drag so overall they are typically a wash. What you are really looking at there is counter-rotating lowering blade loading and recovering rotative energy. Why not go with Cummins Zeus drives and avoid having to groom you props all the time.