Replating a steel hull and deck/superstructure.

[john connell]

Hi,

I would sincerely appreciate some advice/ views on reconstruction replating and associated areas.
I have just cut out the first section ( 3m x 1.5 m ) of hull bottom on a 40ft steel yacht ( heritage/pedigree unknown at this stage but it is well built ). I am just about to replate in new 5mm plate.In all there will be approx 6 sections of same. As I am also replacing some frames/stringers which have thinned out at the same time.I am doing this ( frames/stringers ) more as a precautionary measure and for peace of mind also.See general photo of frames.

Existing motor was an ( old hand crank ) 20 hp x 2 cyl Yanmar which I am upsizing to a 30 hpx 3cyl Lister,elec start.Weight difference might be 100 or so kgs additional weight
Existing framing is 6mmx 50 mm flat bar.
Existing hull/sides plating is 4mm.
Existing deck/superstructure plating is 3mm-but some appears to be 4mm in some sections mainly at bow and stern areas.?

Question

As far as added weight goes -could I use 75mm x 10 mm flat bar instead of 50 x 6 for reframing where needed and as this is also being added to 5mm hull plating and new motor set up am I going to penalise the boat with the added weight or is it not going to hurt in the big picture ?
Is the 10mm thick too much.?

As I have access to quite a few 3mx 1.5m x 5mm sheets of plate am I able to use the 5mm plate on the decks( but not cabin area ) or should I stick with 3-4 mm.?
The common sense side says the extra weight should not be a real problem in the big scale of things and I will know my boat is really strong and able to withstand some pain if it needs to which is very important to me.
The undecided side says watch out for the weight.

The yacht has a long fin keel( see photo ).This will probably open up a can of worms but to get the correct final keel weight I need I am going to consider using railway line welded to the fin keel in long sections.The rail I can utilise weighs 60 kgs per metre and is what they call head hardened i.e. bloody strong. I know this is in contrast to boxing in the fin keel and placing ballast inside the keel and really outside the square but the flow around the rail section affixed to either side of the fin keel if anyone knows rail cross section may be better than flow around a boxed in keel section.To my way of thinking it will be somewhat like a winged keel but not as fancy.At least being affixed outside the hull and able to be seen/treated as needed it gives me maintenance access.
Note.I accept that my idea of using railway line may be seen as a bit too far outside the square and maybe even seem a bit loopy for all of the more experienced people out there but hey I'm a learner so feel free to knock me around a bit.I got plenty of that as an apprentice.!

The superstructure is being redesigned/rebuilt to take advantage of better layout but will be in keeping with existing design parameters.
Should the cabin/top section be kept to a 3mm plate thickness to keep the weight on top low or can 4mm be used.?

Steel angle bar for flooring/framing/structure building was/is 25-32 mm x 3mm thick in general throughout the boat overall.
Any problem in me using 50mm x 4mm/5mm angle bar down in the low section in the boat.
Again- adds weight but will it cause me problems.?

In all,if say an added weight of 250-300 kgs is added with different motor and steel plate/angle bar being used plus the keel weight will this make it sit all that lower and cause stability problems.?
Or am I actually just using better materials for the right reasons.
The flat bar and angle bar makes a difference also as I have it available to use for free and I can't argue with that first up without seeking advice.

Do I need to seek professional advice from a Naval Architect/Engineer for working out the correct weight required in the keel/lower hull area etc or is there a formula for this someone can point me towards ?

Thanks in advance and hope you can possibly point me in the right direction.

Regards
John.