Marine Shafts - Pacific Expedition Power Catamarans

Hi all,

So we promised some content on marine shafts and here it is! We know you were waiting! ;)

Seriously though, if you have ever had to endure the time and expense of shaft repair and replacement due to inferior materials or installation you will appreciate the importance of proper selection and set up of one of the hardest working components on your boat.

John and I, at Pacific Expedition Yachts, make it our job to stay up to speed on current and future developments in the marine industry in order to make your life on the water in a Pacific Expedition power cat that much more carefree and enjoyable. We custom build our boats to order so please contact us so we can discuss the possibility of building one for you! First here is a quick primer on materials from our friends at Western Branch Metals - They know their shafts! We have been using Aqualoy and Aquamet shafts with great success over the years.

More here if you want to look deeper http://www.wbmetals.com

Comparison of Marine Grade Materials with Standard ASTM Materials
The question is often asked what makes marine grade materials such as Aqualoy 19 or Aqualoy 22 better than a standard 304 or 316 stainless steels. The cost difference favors the standard materials so why would a user want the marine grade materials. Is this simply a case of marketing efforts creating a buzz to allow for higher pricing?
In order to understand the differences in the materials we need to first understand the environment in which the materials are expected to perform. Aqualoy 19 and 22 were created as specific upgrades to their standard counterparts in both strength and corrosion resistance. Table 1 shows the chemical composition of Aqualoy 19, 304,

Aqualoy 22 and 316

Grade	Carbon	Manganese	Chromium	Nickel	Molybdenum	Nitrogen
304	.06	1.5	18.5	8.5		.04
Aq 19	.06	1.8	19.5	8.5		.20
316	.06	1.5	16.5	12	2.1	.04
Aq 22	.03	5.0	22.0	13	2.2	.04

As you can see, the chemical compositions of the alloy pairs are different with Aq 19 having higher Mn, Cr and N than standard 304. Aq 22 also has higher Mn, Cr, N as well as higher Ni and Mo.

These chemical differences serve to improve alloy performance in two ways. First the increase in Mn and Cr increase the solubility of nitrogen which is important as nitrogen helps the alloys to attain higher strength. Secondly, the increase in Cr, Mo and N help to increase the pitting resistance equivalent number which is calculated as follows:

Two of the most common forms of corrosion of stainless steel in marine environments are pitting and crevice corrosion. Typically alloys which are more resistant to one mechanism are more resistant to both. This equation has been used for many years to rank austenitic stainless steels and their relative resistance to pitting in chloride type environments such as sea water. Studies have shown that higher PREN’s correspond to increased resistance to pitting. If we are to calculate the PREN of our 4 alloys above one can easily see the superiority of the marine grades.

Grade	Calculated PREN
304	19.14
Aqualoy 19	22.7
316	24.07
Aqualoy 22	34.06

From the chemistry modifications one would expect and indeed one does see improved corrosion resistance with the marine grades in marine environments.

As far as mechanical properties are concerned, the differences are also dramatic. In fact the original design criteria for Aqualoy 22 were to double the strength of 316 at the same time as the corrosion resistance was improved. As was mentioned earlier, the primary modifications to the chemistry was aimed at increased nitrogen content and when one looks at the annealed yield strength of the 4 alloys the improvements are obvious.

Grade	Annealed Yield Strength (psi)
304	30,000
Aqualoy 19	50,000
316	30,000
Aqualoy 22	55,000

One further modification found on the marine grades is that in addition to the chemical modifications, there area also processing modifications which are designed to increase the strength dramatically above the annealed properties listed above. Depending on the size it is possible to increase the yield strength of both AQ 19 and AQ 22 in excess of 105,000 psi. This will allow more power to be transferred through marine grades of the same size when compared to their standard counterparts. - Western Branch Metals

John adds "Aq22 was always the most resistant to this common issue in yachts. That is shafts sitting for long periods as in yachts with little regular use. You do not see this much in commercial boats based on more use, less periods of in activity.

Crevice Corrosion (oxygen starved) - As its name implies, crevice corrosion involves water, metals and crevices. The crevice is any space that will trap and hold water, while at the same time reduces or eliminates air exposure to the water/metal interface.

It is also called oxygen starved corrosion by virtue of the fact that little or no air is allowed to get to it. The water/metal interface results in oxidation of the metal which concentrates the hydrogen content of water, and turns the water into an acid. This changes the electrical make up of the affected materials, generating an electrical current that "dissolves" the metal involved. These crevices or closed cells can become dynamic, meaning that the process can perpetuate itself for a long time -- either until the acidic water is exhausted or an oxygen source is created that lowers the acidity of the water and stops the corrosion. If no oxygen source is introduced, the corrosion process continues until the metal is completely gone.

So the bottom line (literally this is the bottom line of the story) - know your shaft maker material and installer or find someone who does!"

Thanks

John and Pat

Pacific Expedition Yachts

www.pacificexpedition.com

http://www.wbmetals.com