titanium?

[Jerkizoid]

Just purchased a RH titanium prop, anyone run this material before? It's half the weight of steel at 2 Lbs instead of 4! I would think it is more brittle and susseptable to cracking ? Are they workable? New they are around 2000, i hear.

[Yellowjacket]

Most typical Ti alloys are relatively strong (140ksi yield), stronger than typical stainless and of course almost half as light (density of steel is around .298 and Ti has a density of .17).

The workability of Ti is highly dependent on the heat treat and alloy, but generally it does not have a lot of elongation. Annealed has only 14% elongation and some of the higher strength heat treatments are even lower. Elongation for 304 stainless is 55%, so yes it isn't formable like steel is.

Bottom line is that I wouldn't go trying to put more cup in the blade by beating it over a trailer hitch ball. You are likely to end up cracking a blade and that wouldn't be a good thing... If you had forming tools and knew how to hot form it you might be fine, but this something best left to pros...

[raceright]

Just purchased a RH titanium prop, anyone run this material before? It's half the weight of steel at 2 Lbs instead of 4! I would think it is more brittle and susseptable to cracking ? Are they workable? New they are around 2000, i hear.

Used in the 80's did not work as well as other materials guess was to stiff

[Yellowjacket]

Titanium is actually less stiff than stainless steel, but stronger. If you made blades thinner out of titanium (which you can get away with because of the strength) it won't hold its shape as well as a stainless steel blade because it is more flexible (lower elastic modulus).

Bottom line is that a titanium prop blade is going to have a different shape than a steel blade. If you tried to make the same shape from Ti or steel, it most likely won't work the same. To take advantage of the higher strength of titanium you'd like to make it thinner and then it will flex even more.

[raceright]

Titanium is actually less stiff than stainless steel, but stronger. If you made blades thinner out of titanium (which you can get away with because of the strength) it won't hold its shape as well as a stainless steel blade because it is more flexible (lower elastic modulus).

Bottom line is that a titanium prop blade is going to have a different shape than a steel blade. If you tried to make the same shape from Ti or steel, it most likely won't work the same. To take advantage of the higher strength of titanium you'd like to make it thinner and then it will flex even more.

As with all things boatracin put in on the boat and try it...only sure way.

[Yellowjacket]

As with all things boatracin put in on the boat and try it...only sure way.

With the capability of computers nowadays, you could actually model a conventional blade and a Ti blade and figure out what each would do in response to the forces crated by the water. It wouldn't be cheap, but that is exactly what we do to design turbines (using air of course).

Hard to tell what is less costly, doing it on the computer or doing it in the water. If you had a huge budget or are doing it as a business, you would for sure do it on the computer first because you could do a bunch of iterations and get a lot closer to what you at least thought was really good. If you already have all the very expensive tools for the CFD analysis and the CAD analysis you could do it pretty easily, and probably get some good results right out of the box.

After that, of course you have to test, but doing the analysis first can get you a lot closer to where you wanted to be, and it could reduce the amount of testing considerably.

We are designing a small waterjet with a major waterjet company and before we test anything we are doing CFD analysis on the design and then we will test.

[raceright]

That's nice all the computer ideas. And yes these props are costly so call Harry of Craig or Ron and use there props. Just sayinglololo

[LarryR]

Agree with both. The best use of CAD is to predict what is going to happen, find out what happens and then adjust the computer model to agree (after making sure that the test conditions, are truly represented by the model.) After all that, you can change the model incrementally to predict what changes will do, pretty accurately. Riddle me this: why do steel, aluminum, magnesium and titanium all have about the same ratio for modulus to density, while other metals generally do not?

[Yellowjacket]

Agree with both. The best use of CAD is to predict what is going to happen, find out what happens and then adjust the computer model to agree (after making sure that the test conditions, are truly represented by the model.) After all that, you can change the model incrementally to predict what changes will do, pretty accurately. Riddle me this: why do steel, aluminum, magnesium and titanium all have about the same ratio for modulus to density, while other metals generally do not?

Those materials all have the same density to modulus ratio because they all have the same kind of crystalline structure (body centered cubic IIRC, but it's be more than 44 years since I looked that stuff up so it might not be but that's what I remember)....

Actually that's wrong....and they don't have the same crystalline structure...

I don't know why they have the same modulus to density ratio...

[Jerkizoid]

Interesting

WOW, thanks for that info! Kinda figured the strength characteristic but was shy of the workability factor but still a profile cut is an easy option for future tuning. Diameter and pitch is exactly what I ran with some success.... it is certainly going to be tested and i am thrilled its profile and design resembles the Italian Cary or Record brand configuration. Interesting also is the ID numbers are S/N (Ski class Nostalgia?) 057, my boat number! Nothing appears in the usual hub area but on the front flange. I'll get a picture.
Thanks again, I figured the Outboards had Ti experience/knowledge.

[LarryR]

Neither has anyone else that I have asked in the last 45-years, but thanks for thinkin' about it. -LarryR

[Yellowjacket]

Neither has anyone else that I have asked in the last 45-years, but thanks for thinkin' about it. -LarryR

That's one of those things that you live with every day and don't think about, it just is... We used to use design the aerodynamics of stages in the compressor and do the stress and vibration analysis and then pick the material (Ti, SS or Ni) later. Didn't make much difference which you used, all the vibratory modes stayed in the same place since the modulus to density stayed the same..

I was thinking it had to do with the fundamental crystalline structure of metals, which it still could. I recall something about it my materials course but that was about the same 45 years ago and in the fog of time....

I was hoping you would give me the answer. I looked for a bit on the web and didn't see anything.

[LarryR]

I've tried a few time myself. no luck.

[Jerkizoid]

Little know fact, the famous Lavey and Champion in its day ran a Titanium prop!! It doesnt flex like steel so the bite is always consistant with pitch and rake constant. When the boat was sold and later distroyed, the prop was returned to the manufacture and owner who paid the bill!!! just one of the tricks no one hears about!! Big bucks or contacts, has advantages!

[ZUL8TR]

specific modulus (SM)

Agree with both. The best use of CAD is to predict what is going to happen, find out what happens and then adjust the computer model to agree (after making sure that the test conditions, are truly represented by the model.) After all that, you can change the model incrementally to predict what changes will do, pretty accurately. Riddle me this: why do steel, aluminum, magnesium and titanium all have about the same ratio for modulus to density, while other metals generally do not?

Don't recall the answer why the metals you list have the about the same SM but Osmium and Wrought (hand worked) Iron also have similar SM's. In the wood department dry Sitka Spruce has a similar SM to those metals. Those square rigged clipper builders knew what they were doing with the use of Sitka for selected parts as well as the wood/fabric plane designer/builders and the race boat builders.