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building 21ft on aluminium TITAN ,tug boat.,i am in doubt which type of material sould use to build the ruder and shaft.,ss/or aluminium, i am no to kind of desimilar metals but ss it is more strong than aluminium , open for ani advice will be welcome ....carlos bairo

Stainless, definitely. The bearings/bushings will isolate the shaft from the aluminum.
The biggest danger to an aluminum boat comes from copper and brass. There are also sacrificial zincs made of a slightly different alloy for aluminum boats.

Here's a handy chart, from here. The higher metals on the chart will be corroded more readily than those further down.

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-Brian

"Do or do not. There is no try."
- Yoda

Brian, Carlos,
I agree that SS would be stronger than aluminum, pound for pound, and a stout rudder is critical to seaworthiness, but since the boat is small and the speed is displacement [<12knts] I'm not completely sure I'd use SS over aluminum just for the strength considerations.

At slower speeds and a relatively low displacement the forces on the rudder shaft and blade and bearings is not as much as would be with a 40' aluminum power boat, loaded in a following sea; in that case I'd agree that SS was the premium material because the needed strength in aluminum may require a 3" diameter shaft.

Aluminum is easier to cut, weld and machine and since it could be designed in a somewhat larger diameter shaft to handle this steering, the ease of working the material may be worth using it over SS? It goes without saying that the SS shaft and rudder could be thinner to achieve the same strength and that means smaller bearings, and less expensive parts like the quadrant and pintle bearing materials.

A 1" diameter aluminum shaft would be adequate for the rudder top shaft and I would recommend it be coupled by a flange to the rudder body/blade at the top of the rudder just below the hull line. Often these flanges can be recessed into the top of the rudder blade to avoid a flange obstructing water flow by incorporating the coupling flange into a plate mounted on top of the rudder blade. The same technique works at the lower edge of the rudder for the pintle bearing stub shaft and this arrangement allows the rudder to be disassembled easily without lifting the hull high enough to remove an integral top shaft above the rudder.

My making the rudder pintle shoe bolt-on to the keel and the rudder top shaft, stub shaft and blade all removable; you achieve the easiest assembly and repair by design. When you use this arrangement you could also easily use welded aluminum pipes, bars, and plates drilled and tapped to make all the parts.

I think SS would be the best if the size to strength of a higher speed or heavier displacement boat were being built; but here, the size and speed will allow you to build in appropriately sized aluminum without penalty. Welding each of these assemblies together should be done with TIG if possible, so that you avoid any weld-end craters. But, if you only have MIG available (?) preheating carefully [280-300 def F] would allow good fusion and then you could give extra care to doubling back on the weld ends to avoid MIG craters and future cracking of those welds under vibration.

I have also combined SS top shafts and stub shafts with aluminum rudder plate/blades and that has worked well too. Most of the boats I've built with fixed running gear were in the 28'-40' class and all were commercial fishing boats used/in use in South Central Alaskan coastal waters. As Brian has noted all the bearings are plastic and diaelectric so direct galvanic corrosion is minimized or avoided so mixing 316L and 5086/5083/5052/6061/6063 is low risk.

One of the advantages of SS shafting flanged to an alloy rudder is the presence of bearings, stuffing boxes and other hardware already in the market place for standard shafting. This avoids having to machine these parts specifically for stock aluminum shafts of a heavier size, and would help make a combination of SS shafts and aluminum rudder plate more cost effective by using smaller diameter machine fit seals, shafts and bearings.

cheers,

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Kevin Morin

Kevin,

ypur design of the rudder pintless shoe bolt on the Keel and ridder removable arrangement is a great design , i will use this arrangement on Titan, the displacemtns will be 6.5 Knots Maximum witha veta engine of 25 HP 18 " by 10 " Pitch prop + 1 1/4 " shaft.

I agree that 1 " or 1 1/4 " Diameter shaft wouldbe adequate.

Would it be possible for me to please have a sketch of your design to do it right .

Thank you for yur constant support

Cheers

Carlos

Carlos, I'll be happy to post some images but current work load isn't going to make that possible for a few days, as I can take time I'll try to post some images of the methods we've used in the past and I'll trust you'll edit them to your own version.

Cheers,

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Kevin Morin

kevin ..thanks for your suport , i have some idea of your design,but i will be more confident to follow your printing or sketch ,the ruder plate is a 10mm 5083 h112 ,the area of the rudder 4sq.ft, displacement@dwl/5850lbs engine 25 hp/blade/18inchx10''pitch,power rage sugested 15 -40 hp. ani sugestion of you will be welcome cheers carlos