R 22

Rhodes 22



The problem here is that all outboard motors have housings made from aluminum alloys in order to save weight & cost. The outboard manufacturers try various coatings such as hard anodizing, baked on powder coatings, etc. But, in salt water, nothing works very well & the aluminum alloys corrode. The outboards which are rated for use in salt water all have a sacrificial zinc mounted somewhere on the lower unit. You need to keep an eye on this zinc, don't coat it with bottom paint, & replace it as often as necessary. It also helps to raise the outboard up out of the water & do a fresh water flush on the cooling system after every sailing session. This will extend the life of your engine by 10-20X over an outboard that sits in salt water all the time as well as prevent any banacles from getting started.

You don't need to do a corrosion test. Corrosion of the common aluminum alloys in salt or brackish water is well documented in the literature. Assume you have corrosion going on & take the above steps to minimise it.

Roger Pihlaja
S/V Dynamic Equilibrium
15 Jan 2001

Electrolysis is caused by metals of differing valence or nobility being immersed in a conductive solution,i.e. water. Salt water is extremely corrosive because of its higher conductivity coupled with high positive valence due to mineral content. Its called electrolysis because the chemical reactions that occur generate electrical current. Stray electrical currents from badly grounded boats can accelerate the process, but the presence of an electrical source is not necessary. An outboard motor typically has a lower housing of aluminum alloy, but the gears and prop shaft are steel. Props may be bronze, steel, aluminum, or plastic. The presence of these different metals in close proximity in salt water, and in direct contact with each other, is all that is required for electrolytic corrosion to occur. Metals which have a higher number of electrons in their moleculular structure, negative valence, will have the excess electrons stripped away by metals or solutions which have fewer electrons or positive valence. The reaction changes the molecular structure of the metal to somthing else. Zinc anodes are used to protect other under water metals on boats because they will corrode first. That is why they are called "sacrificial anodes". Virtually all outboards have a zinc anode bolted to the prop shaft or the lower housing somewhere for this reason. They should be replaced when they have lost about 1/3rd of their original mass.

15 JAN 2001

Shaft lengths:

  • 1) The very first Rhodes22 used an outboard bracket or a well.
  • 2) When Stan first came out with his system it would lift 15-inch shafts clear, Long shafts tilt and clear.
  • 3) Then 20 inch long shaft will clear (most Rhodes22), extra Longs tilt and clear,
  • 4) The last year or two extra Longs will clear, using the latest motor lift.
Why they need to clear:
  • 1) Drag when sailing.
  • 2) Growth on the lower, unless treated with tin based paint (copper antifouling will destroy it).
  • 3) Electrolysis of the lower portion depending on the motor [the skid plate and prop can be in the water but not the zinc.]

A long shaft will hold in about 3-4 waves on a Rhodes 22, the extra long will hold in about 4-5-foot waves. A sailboat makes a lousy powerboat. You can play games quartering the wave to stretch this number, and a trolling motor will not help you. A trolling motor needs to draw 615 amps at 12 volts to equal a 9.9 hp motor (1 hp = 746 watts, (746 watts * 9.9 hp)/12volts= 615 amps, the motor are rated at the shaft so the power head is about 20% higher), that why you see the trolling motors rated in pounds of force in the 30-50 range (that is a peak number and can only be sustained for a few minutes before burn out). My 9.9 Yamaha is will produce 240+ pounds of force; the trolling motor might have a longer shaft but it is not going to be of any help in those conditions, because they do not have the power or the battery range.

Electrolysis is a fact of life: take a glass pan, put some water in it, us a volt ohmmeter, hook a 1 oz gold coin to one lead and place it in that water, take a 1 oz magnesium plate (they us them in place of zinc's in fresh water), hook it to the other lead and place it in the water, should read about 3 volts on the meter(add a little salt if having trouble). The only metal in the water on a Rhodes22 is the motor and bracket, on larger boats you read in the specs about the bonding system, what this is doing is tying it everything to the zinc's for protection. It also protects from lightning.

16 JAN 2001

[... the first part of this email concerns trailer maintenance and can be seen at Trailer Maintenance...]

Besides the outboard motor, the other common place on a sailboat where we encounter aluminum vs stainless steel corrosion is when we attach things to the mast. The aluminum mast extrusion is less noble than the stainless steel & thus gets to be the anode & go into solution. It doesn't take very long for sufficient corrosion to occur to weaken a threaded connection dangerously - like about one summer's worth of exposure! Any place you have stainless steel in contact with aluminum, look for pitting in the aluminum along with a granular white corrosion product in the pits. Let's consider an example of good practice:

Assume we want to attach a 36 inch long section of black anodized aluminum T-track vertically to the front of the mast with stainless steel machine screws.

Why? This type of T-track is used to provide an adjustable spinnaker pole car for supporting the in-board end of a spinnaker pole or whisker pole. The T-track will be heavily loaded & failure of the mast-to-track connection could ruin your whole day!

The T-track is attached to the mast with 1/4 inch diameter flat head machine screws. The mast extrusion is only about 3/6 inch thick in this area, so it's stronger to use the 1/4-28 UNF fine threaded machine screws instead of the more common 1/4-20 UNC coarse threaded machine screws. After you've drilled & tapped the mast for all the machine screws, you need to decide if you will EVER want to remove this T-track. On Dynamic Equilibrium, my answer was ,"No, I will never take this thing apart." To isolate everything, make the installation stronger, & preclude the machine screws coming loose; I coated the backside of the T-track as well as the threads & the countersunk holes in the T-track with epoxy during final assembly. So far, this installation has shown no sign of corrosion (or weakness) for 12 seasons & counting. If you wish to be able to take an aluminum/stainless steel connection apart in the future without having it corrode in the meantime, things get a little more complicated. You should make some sort of isolator to go in between the stainless steel & the aluminum. This could be a coat of epoxy, or a thin sheet of an electrical insulator like an engineering plastic, hard rubber, or ceramic. In some cases, you can make the necessity of an isolator serve double duty. For instance, if you wanted to mount some flat piece of harware on a curved surface, make the curved-to-flat adaptor also be the isolator. Loc-Tite Corp & many other companies make a thread anti-sieze compound which should be coated on the machine screw threads during final assembly. Coating the threads is probably the single most important step. The stainless male threads have a lot of surface area & are forced into intimate contact with the aluminum female threads. Without the anti-sieze, corrosion in the threads is almost a foregone conclusion.

Roger Pihlaja
S/V Dynamic Equilibrium
17 Jan 2001

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