Define what a Surface-Piercing propeller is

Thank you. That is concise and understood. I have spoken to two major propeller manufacturers in Europe as well as a respected major prop shop and they have stated the foil side generates more propulsive force .

It was counter- intuitive but makes sense based on my experience in aircraft.

 

I assume that by "foil side" you intend the suction or dorsal side of the airfoil. That is actually the side which carries the most of the lift at positive angles of attack which are not close to zero.
You can visualize and comprehend it better by looking at a distribution of the pressure coefficient over an airfoil. This page shows it very well: http://www.mh-aerotools.de/airfoils/velocitydistributions.htm

As the AoA increases, the Cp curve of the upper foil side (suction side) keeps growing and swelling upwards, while the curve of the bottom side (pressure side) remains pretty much constant. It very clearly shows which side does most of the work.

But, if there is a limit to how much the curve on the suction side can grow, then the curve gradually flattens as it approaches that limit. It means that its contribution to the overall lift stops growing too. It happens when the suction side starts to cavitate, for example (pressure approaches a constant vapour-pressure value) or when it starts to ventilate (pressure equals the atmospheric pressure). In both cases the upper side reaches its limit and the bottom side starts to carry a progressively bigger portion of the total lift, as the speed or the AoA increase.

So, imo, you are right - a surface-piercing prop has the suction side (front side) of the blade completely aerated most of the time it is in water, and hence exposed to the constant atmospheric pressure. Which is its limiting pressure. Hence, it is the pressure side (rear side) which has an increased importance in creating the thrust, contrary to conventional submerged props.

 

The surface propeller forces are simpler to ascertain from an observation of its operation.

It was only recently that I had learned that the submerged propeller generates at least as much, if not more of it's propulsive forces from the effects of the foil as opposed to AOA.

That you for the explanation and additional resources.

 

dont forget that you can boil the water on the suction side and if those gas bubbles arrive on the blade on the pressure side they will eat through the surface of the prop no matter what it is made from.

 

Powerabout, I don't want to sound negative or arrogant, but I truly don't see what can someone do with that claim without forther arguments or examples...

 

No problem if I say something, i'm happy to find evidence to help, I have learnt plenty on this site.
you mean you need a photo of cavitation burns on a prop?
shouldn't take 5 minutes, its very common "cavitation burns" 2000 hits in google....
here's one that has just started, any longer and it would have deeper pits
If it was aluminium it would have major pits

 

I know that cavitation causes erosion of prop surfaces, as well as of the paint on the hull plating above the prop (seen it many times).

What I don't get is this phrase:
"you can boil the water on the suction side and if those gas bubbles arrive on the blade on the pressure side they will eat through the surface of the prop"
I cannot imagine a situation in which bubbles make their way from the suction side to the pressure side. Do you have some info I am ignoring about that? I suspect it is just a matter of wrong use of the foil terminology, though.

A foil has two sides, divided by the chord line: a pressure side (the bottom or ventral side for a foil giving positive lift) and the suction side (the top or dorsal side). I don't see how can cavitation bubble arrive from the top to the bottom side of the foil.

 

Sorry that didnt come out very well what I really meant was once they arrive where there is not the low pressure

Dont forget blades are cambered and have rake ( planning props) so you can get leading edge damage or damage in front of the prop that will create a bubble and then somewhere along the thrust face also create the burn but your right not likely to get from one side to the other

 

That's a good and true point.

 

I have many images of propeller blades and I can not see anything like an aerofoil resemblance among them even in fully immersed props but what I do see is blades designed to scoop the inflow water and accelerate that water rearwards and when in surface piercing attitude into the low atmospheric air (like a water jet).

You will get collapsing pressures and many other effects taking place in that working environment.

 

It is the same thing. It just depends on whether you want to make your considerations in terms of pressures or via the rate of change of momentum (of the waterflow).
Thrust = Area * dP
Thrust = mass_flow * dV

 

Why did YOU go to my profile? What if I tell you that I also have a degree in engineering and used to fly F16's...what does that have to do with anything?

If my picture in front of the aircraft looked photoshopped, I hope you wouldn't comment on it - after all you would be "turning" on a member through his profile.

As far as your comment that a profile is to share; I only want to share information and learn about boats. not share my personal history and background with the general public. I am not looking for a date.

I did not turn on the "professor" ; he very quickly asserted his idiosyncratic behavior thinking tolerance of the same by strangers would validate his distorted self impression.

In the end, telling a member he is "wrong" then never providing any clarifications is just pathetic. It is also where he is mistaken regarding human interaction.

People outside of his family do not have to tolerate his abrasive idiosyncrasies. It is not endearing or cute. Being a cranky old guy with an attitude is pathetic. - if he still thinks I or anyone cares that he proclaims himself a teacher and in a position to hand out "F"'s? Physically, he is long in the tooth, but emotionally he has a long way to go. I hope when I am in the twilight of my life I have more self security, dignity and personal accomplishment to behave with maturity and act as a mentor; not as child.




You will see that in these forums, well-adjusted people generally follow a tact of giving evidence that might refute another members assertions and let their evidence speak to the validity of another members position -not just run around telling people they are wrong (as in his case). Even worse, never elaborate, just go off into tangents.

As far as my "education"- internet credentials are a fools errand. You want to know who I am? Send me your phone number and I will call you and we can chat.


I hope you understand.

I am done with this thread. I need to finish my current boat before months end.

 

I went to look at your profile to see who you are. Since you are curious to see other members profiles. You didn't remember?
Yes I know the forum quite well, probably better than you.
Thank you for your telephonic offers, an other time perhaps? I am quite shy.
That said the only thing I understand is that you want to finish your boat, and for that I am genuinely happy for you.
Building a boat is the most rewarding thing to create in life
But I am quite biased.
Good luck, and post some pictures of your boat. (not you, I don't want to date either )

P.S. I deleted my post, it was not necessary.

 

I enjoy building boats actually more than using them. Only in boat building do you build something that you then submerge into an acid bath. I have been working on this 339 Intrepid for almost 4 years. I personally rebuilt the engines, transmissions and performed the majority of the fiberglass and all the painting.

There are cosmetic personal touches such as carbon fiber transom and dash lamination, Custom Awlgrip SE paint, HID Underwater lights that were taken from aircraft taxi lights (50 watts). Bi Color

Technical implementations include:
CV joints to connect the Transmissions to surface drives
USB programmable Power distribution modules
Monitoring of all drive line functions to include non-contact temp sensors "looking" at the CV joint temps, Intercooler in/out, Transmission temp/pressure, drive temps and fluid levels
Transom water pickups are the only source of raw water for all components via a manifold and dump to regulate water pressure when on plane for items adversely affected by high pressure (like the genset when not operating).
Fuel system boost, priming and purging of water separators if water alarm sounds
Bilge pump overrun alarm as well as high water alarms.
Deck fills with LED displays, Fuel system with cleaning, priming and Water separator purging capability).
Overtemp alarms on all exhaust systems (engines and genset); Remote battery switches, Electronic keyless ignition modified for diesel engines. Bilge pump overrun (90 seconds) alarms.
Security system includes ir deck beams to send my cell phone sms if someone crosses the beams, or enters the cabin, but it also send information about battery levels, temperature and high water alarms.
It has a GPS system that give location geo-fencing and the capability to shut down the engines and closes a remotely located ball valve to stop all fuel via cell phone sms.
Cameras located over drives, engine hatch, anchor locker and under the pulpit.

I spent a lot of time while at work thinking about things to do, then spend the time designing and implementing systems that I hope are robust and will add to the operational capabilities and usability of the boat.

I always look for innovative and creative things people do with their boats. I enjoy discussions regarding engineering aspects and accessories.

I threw this together impromptu but need to take time to create a timeline and history of the rebuild.

Thank you for your remarks. I prefer to talk about boat than the people who talk about boats.

 

Very impressive, thank you.
You seams to know all about up to date technologies, and how to put them at work. Always the most complicate part.
Almost avionic of course!
Keep us posted