Aluminium structure

I am currently putting together my dissertation at university and I am designing a 55ft classic looking yawl in Aluminium. I keep coming into difficulty deciding on best ways to construct regions. I have done much of the deck and cockpit structure but im now focusing on the bottom structure. Do you break up the span of the bottom girders using bulkheads like with the deck? I've heard different but dont know what is done in practise. Do you also know of any good book on keel structural arrangement? Im thinking of incorporating the keel into the hull structure instead of having a bolt on fin keel. The profile would still be that of a fin keel but incorporated. Do you have any suggestions?


Will Fox

 

Will,

Bulkheads (of aluminum) are not generally used in smaller sailing yachts, except perhaps a collision bulkhead and an aluminum enclosure for the engine space. The usual thing is to use widely spaced "web frames" which are extra deep and heavily flanged transverse frames.

If you have done work on the deck structure you should be using a scantling rule. ISO 12215 will be the most up-to-date, though I believe they are currently re-working the keel attachment part. Bureau Veritas Rules for Yachts are available on this site, also see Lloyd's Rules for Special Service Craft, Germanischer Lloyd rules, and the old ABS Guide for Building and Classing Offshore Yachts (out of date).

Heavy deep floors that extend down into the keel are quite normal, make the top of the keel (above Ballast) into integral tankage. The ballast can be an external bolted on bulb or internal inside the keel skin.

 

what I normally do, and I do all of my own keel structure is this
I use close frame type construction,400mm centers
use a deep floor abt the keel area and carry the floor to the keel sole It is not normal for majority of al al craft to have bolt on keel structures
A thin section can be tricky to weld into, so you must always consider the weld and plate arrangement If the chord is deep you must allow for plug(slot ) welding A very good idea is the scheel keel, keeps the draft not too deep and the ballast low down
As tad said after the ballast is in you can use the remaining space if any for sumps or tankage But it is absolutely essential to pressure test the keel using a manometer Any salt ingress later can not be fixed
It is not common practice to use girders as such in sailing boats of this size, but a good strong backbone keel and stem bar say 130 x 16 plate and deep floors 6-8 mm on a boat this size using the floors as tankage boundarys makes for a very rigid bottom structure Use 8pl on the bottom, many of the early IOR race boats where built very lightly as far as plate went
I favour the close frame construction as do the Germans and Dutch, it keeps the boat clean and aids drainage If you look at my gallery there should be some stuff there

 

this one does have a fully stiffened B/H she was built under Lloyds for John Holling of Beca Carter Furneaux and Hollings the biggest eng co in NZ,
I built her before CAD cam into use hence I have not cut the sheer in, which I did with a dumpy level and table offsets last of all The frames are 60x40 x6 T6
I still prefer this method over cut frames Can press 4 pairs a day The frames overlap by 2 and a half web as I seem remember this is Lloyd s and ABS reqm
the piece of strip is just there to space the frames
12m older design, we built two very quiet running , very round sections
some may ask why are there no forming marks, that si because I used a molded urethane wheel, bonded to steel and veed up in the lathe only with CLIPPER whould the marks show, cos then the block is on the inside and the metal driven wheel on outer

 

Whoosh,

Is it common practice to flow-coat the interior of the keel with some type of paint after pressure testing?

 

no Joe, what I normally do, is pour the lead in , so that'd just blow paint away
In saying that I saw lead on bandsaw into big chunks, and pour the molten around it, that keeps the structure cool
once it is sealed tight, no corrosion can occur and anyways I have found that lead and al al usually sit ok together
But i have seen some expensive builds where they just looked for leaks without the manometer test, and then they really paid for it later
and if you coated then the seal welding would also burn it up
In palces where water and crud could lie, I usually pour in epoxy resin, al al hates water and crud that can not breathe, so usually I paint inside the bilges, easier to keep clean ,one of the reasons I shy from stringer builds . TOO many pockets
What do you design Joe and where are you?