Changing pitch

That is the way it´s done in a professional manner. "Umdrücken" is the term in German, do´nt know how to translate that (English is´nt precise enough).
And the 2´max. range given above is right! The prop root cannot be manipulated. You are better off, I guess with a new pair of wheels.

Regards
Richard

 

Engines should yield 85 hp at 4600 rpm, torque approx 170 nm. The reduction is 2:1.
At full throttle there is 3000 rpm, so 1500 at the prop, which is not enough to get over the hump. This may sound way off target, but I think 500 turns more might do the trick.

 

Sorry, ones and zeroes are not my friends....

 

Ooops what happened here, how do I delete??

 

So, what kind of speed did you get with those props, then? And how much of a tunnel are we talking about, and what kind of boat (planing/displacement, weight, lwl et c.)?

Note that a propeller in a tunnel normally will work in a considerably retarded flow (high wake factor), so the output from "standard" calc procedures will often overestimate pitch.

On the art of heating and beating propellers; considering the labor the producers spend in making the blade profile perfect, within tenths of millimeters, a good propeller is easily spoiled by overeconomical stupidity and a hammer! Those props are eating from your fuel budget for a looooong time, and you easily loose ten percent or more if hitting the wrong spot!!!!

First, give us correct basic data as above. Then we may come up with what you should have had. And finally, sell/swap the ("unhammered") screws you have and get your hands on a correct pair.

It is easier to make a good increase in pitch (mostly done by sensible cupping) than a good reduction! So when ordering a prop for a new project, select pitch on the low side! Best twister I've seen is a guy here in Sweden (Richardson); he is using heavy clamping mechanism for hub and blade sections, allowing him to twist the complete blade to a new pitch, without changing blade profile; nice job! Of course, this generates a slight difference in pitch between root and tip sections, but it is far better than the s-secton resulting from terrorizing leading and trailing edges.

 

Hi baeckmo,
you can delete a post by using the edit function in the lower right corner.
And thanks for your valuable input. I am sure CDK will provide the link to his thread showing the arrangement he has built.
Obviously you know what you´re talking. I have noticed that already on the Jet related thread.

Regards
Richard

 

"terrorizing leading egdes and trailing egdes"

That is all you can do the root can not be altered.

Pressing a blade into a preformed cast would be nice,--if you got that stuff, yet "tweaking" a blade for a small adjustment would not be necessary.

To adjust a blade to advance 1 inch more than it did does not mean the blade is bent 1 inch. Pitch is the average of the blade anyway and not all the blade would be the same pitch.

To advance by means of pitch a 1 foot dia prop the leading and trailing edge would be altered by a very small amount like less than 1/4 of an inch and would be altered as high into the blade as possible.

Depending on the blade only one of the edges may be altered if you have a very aggressive leading edge for instance and you wanted less pitch that would be sufficient to alter just that.

Some props are easier than other due to the thickness of the blades which when you get to 14 inch dia bronze it can be difficult.

No alteration of pitch is as good as the cast pitch but the idea of a 3 stepped blade by the altering of trailing and leading is not so, its a very slight alteration and if a concrete cast is made of the prop before hand as I have already explained the pitching procedure can be reasonably accurate.

 

CKd

"...Engines should yield 85 hp at 4600 rpm, torque approx 170 nm. The reduction is 2:1. At full throttle there is 3000 rpm, so 1500 at the prop, which is not enough to get over the hump..".

Ok, im getting a bit confused now.

You start off by saying you have a 14x17" prop
1) Is this selcted by you, or by the engine supplier or prop supplier?
2) What was the speed with a different prop and its size?
3) What is the expected speed of the boat and what is the actual max speed at max RMP?

Since you're now talking about getting over the 'hump'. This is a totally differnet issue. And must not be confused with you're orginal post of pitch/diameter of a prop.

So I need to be clear what is the "problem" first before wading in.

 

I am with frosty ...we have mended many props here at the Noosa Prop Shop ..welding 2 inches on to the end of the blades of a bashed up 13 inch aluminium prop to restore its diameter ...grinding by eye and thinning with a few bangs on the anvil ....they all worked fine and some better than the fancy equivalents and never had any vibration problems ..mind you cast bronze is not aluminium or red hot stainless .....now that bends easy .....get the oxy out .....

 

John he made a tunnel covering the prop, but has limited waterflow at the suction side making issues.

Let us have the link to your thread CDK.

Regards
Richard

 

Here is where I first formulated my idea. It was so weird nobody responded:
http://www.boatdesign.net/forums/surface-drives/sterndrives-jets-surface-drives-18742.html

Then there were the details, written while I was constructing:
http://www.boatdesign.net/forums/surface-drives/diy-tunnel-drive-20433.html

The choice of props. I have studied general engineering, but "general" at the time did not include much hydrodynamics. Lots of aerodynamics, probably because one professor was fascinated by the subject. So I asked the forum for help:
http://www.boatdesign.net/forums/inboards/simple-propeller-question-19421.html

And then the project showed some flaws:
http://www.boatdesign.net/forums/boat-design/tunnel-drive-performance-issue-23715.html

I've skipped several other threads like the marinized VW diesels, the unorthodox drive train and the electronic throttle and clutch controls. These parts went exactly as I intended.

With bated breath,
Cornelis Koger

 

Tunnel drive performance issues has a picture of CDK's tunnels.

Its hard to see really the length of the tunnels which at first glance don't seem to be long enough. I think your vibration and inability to get onto the plane is that the props are drying out due to the tunnels not allowing enough water into the tunnels. I would think that that is the vibration too where the prop blades are leaving the water and when the go back in they recover thrust so your thrust is not a constant flow but thrust in each blade. Surface props are prone to this hence 4 -5-6-blades.

You could try some surface props or get the ones you have cupped which then they may tolerate both situations.

In one of your threads above 632 days ago I did say you would have prop issues and would need to play with many props.

I would say that your tunnels should be about 3 times the dia of the props and the tunnels should have a gentle rounded edge to its entry to encourage water flow if not it will break free like it was the transom.

Please get some more pics of the tunnels ( if its out of the water)

 

CDK

briefly viewed your threads.

Well the basic problem, as far as i can see, is that you doing it back to front. You're now trying to design a boat, that has already been built.

The design procedure will have more than likely prevent much of what you have done, simply because any naval architect would have told you it wont work or if you do decide to continue XXX will happen or this way is better or will cost more etc. Which is not what you probably wanted to hear....(no one likes criticism on personal projects).

So you have a hump issue, that is more problematic than the pitch...Hump issues are major and fiddling about or focusing on the pitch is wrong. You need to address the fundamentals of the design first...and is the design achievable or not. If it wont work from the start, then no amount of pitch change will assist!

What waterline length is the vessel, how much does she displace fully loaded and max HP of engines. This will give some indication of the what the speed "should roughly" be.

 

Ad Hoc, Frosty, I can't blame you for not reading the whole roll of threads, but believe me, there are no problems with the amount of water entering the tunnels and there are no vibrations. I wrote about that at the end of the "tunnel drive issues" thread.

There is almost 2 ft of tunnel between the gently sloped entrance and the prop. Just because there is enough water there is no air pulled in from the sides. Instead of more than 4000 rpm and a lot of vibes at full throttle I can only reach 3000 now with 10 knots readout from the gps. The bow lifts between 1 and 2 ft, but this Norwegian design with 26 ft. waterline weighs 3,5 metric tons and there simply isn't enough torque to increase the prop rpm beyond that point.

I have means to increase the engine output well over 100 hp at near 5000 rpm but the torque at 3000 rpm is all the engine can give. My target however was a maximum speed of 22 knots, which was stable planing speed when the boat was still equipped with twin 140hp Mercs.

At the present I am quit happy coasting along at 6 or 7 knots with a fuel consumption of approx. one tenth (!) compared to the Mercs at that -or any other- speed. So I did really achieve something with this weird concept.But the sea in front of my door is a treacherous one that may turn mean even on a sunny day, so I want the possibility to quickly reach a sheltered bay.

 

Well, based on your rough figures above, your best speed possible is going to be around 16~17knots, max. If you have bought everything expecting 22 knots, that is where you problems have started. Not matching the propulsion train delivery of thrust to the design limit.