Busted Prop

[ced]

I was towing my wife on her monoski, when suddenly there was a big bang........stopped the engine, dived under the boat and felt that the prop was in bad shape. We managed to go to the port, put the boat on the trailer and could see the hole mess. I think that someone has dumped a stolen car in the river, don't know anything else can do such damage. Even the finn of the ski is broken, I've never seen anything like that before....
On the way to the port, the boat was shaking, didn't quite run like before........as you can imagine.
Hope nothing else is torn, the boat is now at our local dealer.
What a day, should have stayed at home and painted the garage.
(By the way, I've told the local police station, they are actually looking for the "definetely hard thing" in the river)

[richard]

RE: Busted Prop

I tried one time hit the stone under water about 2 feet,exactly my prop was damaged like your one,also the prop shaft got blended, my friend tow me back to dock, I changed the prop and the shaft ,take a care the strut,little change the shape will make the shaft tight, also I had complain the marine department why don't remove dangerous stone on the frequent sea way. hope you fix it soon.

[simplysanj]

I feel for you man, here's what I did last year smacking a rock, sheared the rudder off and bent the strut too! It cost me a new rudder & prop and I had the shaft and strut reconditioned.

This year I dinged another prop but not so bad. I find that even if you know your area, you can still get nailed when ice flows move things around and the water level changes. Stolen cars underwater definately don't help either. My insurance guy told me about a yacht that nailed something underwater 3 times before they sent divers down and found a submerged stolen jeep. The guy kept hitting the roll bar and ripping out his drive shaft. Ouch!

[OldFart]

Sanj, u boat in the mountains? Might want to try water

[Jesse1]

These pictures are good examples of why you don't want to run a stainless steel prop on an inboard! At least the nibral props will give on impact.

[Rick]

I sent a prop like yours back to OJ for repair. 90 bucks and 2 monthe later it looked new. You might want to do that and keep it as a spare. Good luck

[ST]

These pictures are good examples of why you don't want to run a stainless steel prop on an inboard! At least the nibral props will give on impact.

It seems nibral is actually stronger than stainless steel (95,000psi vs. 80,000 psi tensile):

http://www.baypropeller.com/materals.html
http://www.simplicity-marine.com/propeller.htm
http://www.midwestpropeller.biz/
http://www.marineplus-online.com/boatyard/props.cfm

I also found a reference that said NiBrAl had a higher Brinell hardness than stainless.

Will a Nibral prop really bend more easily than a stainless prop?

[Jesse1]

I have always been told by the people who service my Nautiques that you want the prop to bend whenever it hits an object. Supposedly that is the reason they equip Nautiques with a nibral prop and not SS. It's odd that SS is the performance prop of choice for outboards and nibral is used for inboards, although SS inboard props can be found.

I've only experienced one hard strike in over 20 years of owning Nautiques. In this instance, we hit a submerged bridge when the lake was low. It bent two blades of the nibral prop and slightly bent the shaft. No other damage was found. A friend who owned a ski shop hit the same object with his Ski Centurian which was equipped with a SS prop. The impact drove the strut up into the hull of the boat and nearly caused it to sink.

[bkhallpass]

"It seems nibral is actually stronger than stainless steel (95,000psi vs. 80,000 psi tensile"

Long time since I took materials science in college, but if I recall . . .

Shear strength would be the relative measurement as far as
a prop hitting something. I suspect that SS has a higher shear
strength than nibral.

Tensile strength probably has more relevance with respect to
bending without breaking. If I recall correctly, you measure tensile by
stretching out the metal until it breaks.

BKH

[DavidF]

Yes nibral would have a higher tensile than stainless because nibral will stretch and stainless just snaps. So the stainless will break during the test and the nibral will most likely stretch a bit then yield as the tensile falls off quickly before it breaks. Stainless is hard and brittle, I find it hard to believe that Nibral is harder than stainless.

Nibral = NIckel/BRass/ALuminum.
Stainless = Steel, tin, ?...not sure, to lazy to look up.

Anyway, Nibral is made up of relatively soft metals, so it gives.

I believe outboards run SS because the props can be raised out of harms way, and the outdrive can give a bit when striking something hard. Also, I believe outboards have a shear pin that will let go before lower unit damage is done (most of the time). Just my thoughts, others may disagree and be correct.

One last thing: Nibral is not used on I/O's or outboards, because it will react badly (electrolosis) with the lower unit (aluminum). Again, I think this is true.

Sorry for rambling....

[ag4ever]

Most lower units have a skeg that will tend to protect them, but that is not the real protecton for the prop and transmission.

The real reason outboards and i/o's can run stainless is due to the hubs in the prop. The prop has a hub that attaches to the transmission output shaft. This hub then has a ring of rubber around it, and then the rubber is then surrounded by the ring that has the blades attached to it. The exhaust exits through the ring of rubber most of the time, but some props are designed where the exhaust exits at the root of the blades. It is this ring of rubber that is designed to absorb the impact if the prop strikes anything underwater.

Inboards don't have the luxury of having a ring of rubber in the construction of the prop, hence the need to use a softer material.

If you are going for PURE performance, and don't mind paying bigtime if you strike something under water, then stainless is the way to go. You can use much thinner blades to get the same strength making the prop much more efficient. The NiBrAl props must use a thicker blade to keep it from bending under load causing turbulance as the blade enters "clean" water. This turbulance decreases it's efficiency. Before the CNC NiBrAl props there were the Brass props that required even thicker blades, so the NiBrAl has made a great increase in efficiency, so much so that they are closing in on the stainless, therefore you just run a greater risk of damage to drivetrain with very little gain in performance.

Now that was rambling ... Sorry

[Jesse1]

My 2000 Sport Nautique has the original OJ 4 blade prop. Would it benefit from the CNC Acme props now being used? Top speed with the OJ is about 43mph and it pulls good, although I have considered having the prop balanced or getting a new Acme.

[ag4ever]

jesse, yes you would see some improvement. mostly better hole shot and about the same top speed if going with the 644 from acme.

But if you are happy, and don't have vibration, then why spend the extra cash for a little improvement. I would only get one if you were going to get a spare anyways.

[ST]

Here are more complete specs for Nibral and stainless:

Nibral:
====

Tensile: 95 ksi
Yield: 50 ksi
Elongation: 10%
Brinell Hardness: 190
Modulus of elasticity: 17,500 ksi
Modulus of rigidity: 6400 ksi

T-316 Stainless:
==========

Tensile: 85 ksi
Yield: 35 ksi
Elongation: 30%
Brinell Hardness: 200
Modulus of elasticity: 30,000 ksi
Modulus of rigidity: 11,000 ksi

Nibral is stronger and more brittle than stainless but it seems that it is more "springy." Notice the difference in the moduli of elasticity and rigidity. Stainless is almost twice as hard to bend or stretch as Nibral.

[DougPhillips]

I clipped what was the only fist sized rock in the sand at the rental cottage we used for the last two weeks. It happened at the end of our stay. What are you guys paying for a new and rebuilt Acme prop? I assume the prop on a 216 and 210 may be the same.

For a rebuild do you send it diretly to Acme or another location? What is the current fee for rebuilding?