Is this marine vessel an icebreaker ?
I would suggest to use the stainless steel for the hull structure, i.e. framing including longitudinal and transversal stiffeners and shell plating which would boost the boat resistance to sea corrosion.
The value of still water bending moment (SWBM) is at its peak within the highest deck. Therefore, in order to reduce the boat overall weight, as well as to enhance the boat stability by reducing the deck weight and thus increasing the boat metacentric height GM, I would suggest to apply the high tensile steel (HT) for the deck structure including deck plating and supporting longitudinals.
In order to design any floating object one needs to define the following parameters:
The hull dimensions:
Loa - length overall
Lbp - length between perpendiculars
B - beam
T - draught
delta - block coefficient
In addition to the above, I guess you would be interested to know how fast is your boat going to sail. Therefore, you need to determine the shaft power available. Otherwise, you may find out what power is necessary in order for the boat to develop the predetermined required velocity.
In reply to your particular questions I would like to advise as follows.
1. I would assume that any 20+Cr-18+Ni-Mo+3 alloy steel be ok for sea applications.
2. I would not envisage any special problems in joining stainless steel with HT steel, provided that the HT side is buttered properly (definitely higher than the one of the SS material, perhaps 309); with an electrode of a slightly higher Ni content and Cr +6% for SS joints since Cr sublimates The joints should be avoided within the stressed members of the structure, i.e. the entire deck should be made of homogenous material, such as HT.
3. The joining method would be as mentioned above, SAW is the traditional welding method.
4. As long as your propeller is made of similar stainless steel material, I would not recommend any cathodic protection, although you may wish to consult professional corrosion engineers in this matter.
5. The HT material has a fine grain microstructure. Nevertheless, it has quite impressive elongation properties despite its elevated carbon content of usually 0.20%.
6. If this marine vessel is an icebreaker, notch toughness is very critical.
The above is merely some insights that require verification. I am not a Marine Engineer.
Cheers
