Aluminium Construction - ISO vs Gerr

I'm continuing my education about alloy construction. Not so much with a view to designing a boat myself - I leave that to you guys!! More just so I have a better understanding of it all.

So, for my theoretical 25ft planing boat, I did the scantling calculations using Gerr's Elements of Boat Strength rules. This gave a frame spacing of approximately 750mm, with longitudinals (flat bar 30 x 6mm) spaced 215mm apart. Now, Gerr's are widely considered to be very conservative rules, so, when I checked them against the ISO 12215 rules, I expected to find that I could make things considerably lighter.
For a category C craft, with a top speed of 30 knots, however, I find that the required section modulus for the longitudinals is at least 3 times that provided by the ones specced by Gerr.
I can only assume that I've made an error somewhere, as I can't imagine anyone installing closely space longitudinals that are 60+mm deep in a boat of this size....

So...where's my error....?

 

You'll need to post your calculations if you want someone to find the error (if there is an error). Does Gerr consider slamming pressures at 30 knots?

 

The Error is in ISO calculation. Check design areas, they are usually wrong (with my students at least). With your numbers they shoud be for panel 0.11 and for longitudinal 0.19.

For more help I need more information.

 

Yes
Daniel

 

Gerr's method is not really suitable for planning craft...

 

According to the author, Gerr's rules are suitable for vessels between 10 & 120 feet and with a maximum speed of up to 45 knots, so I would have thought they'd certainly cover planing boats.
He typically accounts for slamming pressures by increasing the result of a particular scantling calculation 'by 1% for every knot over 15 knots', or similar.

I think I may have erred in my calculations in that Gerr results in a frame spacing of 660mm (not 750 as I previously posted) with 30 x 6mm longitudinals 215mm apart.
I'm still surprised, however, to find that the ISO rules considers this to insufficient - at a minimum the longitudinals need to 35 x 6, and towards the bow they need to be considerably more.
But is all this internal structure really necessary? I'm not suggesting the BS approach, where there are no internals at all, but your average 25 foot alloy boat doesn't really have frames every 660mm and longitudinals every 215, does it? The ones I've looked at don't, anyway...
So how do you go about avoiding so much structure?

 

But that's nonsense - for 10-footer 'over 15kts' will be planning speed, but for 120-footer still displacement speed... So the load pattern should be completely different.

 

Hmmm...point taken ... but is the slamming pressure related to SL, or the actual velocity that a given panel impacts the water? I would have thought it would be the latter, in which case whether the vessel is actually planing would be imaterial...?

 

Once the boat is planning, it is experiencing hydrodynamic lift on bottom, and impact loads due to vertical/pitching accelerations (3...8g). This does not happen on displacement boat, where hydrostatic forces dominate.

 

You can design less longitudinals and floors (ISO), but you need to make them bigger and perhaps increase bottom thickness. Use wide hat longitudinals ( two/ side) to keep structure simple.

Terho

 

Sorry - what do you mean by two / side?
You mean two top-hat type extrusions each side of the CVK (keel)?
At the moment, for my imaginary 25 footer, I need about 5 flat bar longitudinals for the bottom, about 40 x 6mm....

So, I guess it depends a great deal on how you set everything up, but is it generaklly lighter to use more internal structure and a thinner skin, or visa versa?

 

Yes and yes. But check the minimum panel thickness, you can't go under that.

T-bars are more efficient than flat bars.

 

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For the same scantling number you didn't mention clearly.
One scantling number (meaning one set of parameters giving a scantling number) at what speed? that is what Gerr said.

 

Daniel,
The calculation for the "Scantling Number" in Elements of Boat Strength is a simple
Length x Beam x Depth of Hull / Constant (depending on metric or imperial)

Resulting scantlings are then modified if the service falls outside ecrtain boundaries - speed, workboat etc.

 

Ok.... now, if I wind up with two bottom stiffeners per side, one of these would be the engine beds - say for a sterndrive boat, so at the transom. Where they cross a transverse frame or bulkhead, is it ok to change their size (height / shape / thickness etc)?
Should this be done gradually from one frame to the next? ie if the engine beds are say 150mm high at the engines, but only 50mm high for the rest of the length... do you just butt the engine beds up to one side of the bulkhead and the smaller longitudinals up to the other, or should they slope down from 150 to 50mm high over the span of the next frame?

And another question... yes sorry I do have a lot! .... is it ok to run much taller, but vertical longitudinals, that span between the bottom and cockpit deck panels? And then is there a risk of web shear if these vertical stringers are too tall / thin?
I've certainly seen this done, with the stringers (are they correctly called girders?) just plug welded to the cockpit sole. I was under the impression though, that the longitudinals should always be run with the web at right angles to the bottom panel...