placement and spacing of frames

CaptBill,

Based on the golden ratio you discussed, should the main bulkhead be placed closer to the stem or the stern?

 

What You should read first is Larrson and then ISO12215-5 (Annex A for simplified method of scantlings). Don't dig in complicated issues now

 

In your case you would have your short measure, 11.54, from the stern (the minor division). It will typically be about at the widest beam section. Look at most boats. You see this general 'proportion' to most boats simply because it is what works. You don't see many boats where the load is centered. You see them divided at this general proportion of phi, even if they didn't consciously intend to. A tug boat is set up in the reverse pattern of the longer section (major) in the after section, minor section forward, but it is still the same ratios/proportions at work. It is just what works. Fluid dynamics has been and always will be the prime focus determining this, so we are not talking anything new or unconventional here.

How long is your hull?

Ps. This gives you a CENTER calculation for your bulkhead. The actual walls of the bulkhead need to be added, which will determine how wide the bulkhead should be, which could be viewed as TWO closely spaced bulkheads (measured fore and aft of 11.54). So how wide to go is something to decide as well.

 

alanrockwood,

As mentioned already there are special considerations between mono and multi. But, to me, the real interesting area is one of structural design for high-speed craft.

For a good general introduction read Professional Boatbuilder Number 67 and 68 (Y2000), By Joseph G. Koelbel Jr. After this take a look at some offshore Tunnel Cat designs, to understand how mono and multi high speed power deal with localized accelerations of up to 1G. For visualization purposes relative to sea state, I like to look at similar designs constructed from aluminum at around 55 ft.

In actual random waves, I see the placement of frames (transverse) and longitudinals as forming a support structure to the hull plate, being subjected to an impulse which is also dynamic in its footprint.

I like to try and visualize how the hull slamming loads propagate from point of impact through the structure relative to the hull position. For mono single chine, for example, bottom plate up to chine, as we move from impact on plane to the stern. The thinking is to look at each hull patch, supported by frames and stringers, as being subjected to the wave impulse, but the hull is not flat and the wave not perfect. So to me there is a geometric relationship of the hull patch relative to the net area exposed to the impulse. In other word, the shape of the hull and its expected impact load areas, will dictate the patch rectangle orientation and size.

So then this is another way to visualize required frame and longitudinal spacing.

Mark

 

Captbill,

Well I guess I should ask, based on you touching on hydrodynamics, stiffness, and the golden ratio, how do we determine frame/longitudinal spacing per size of plate?

Keep in mind there are other needs for the number of frames and stringers per the design. For example engine beds and tankage. Also, how do you use your suggestions for CATs, which have some cantilever and torsional considerations, and includes the study of resistance of wave interaction.

More importantly: How does the water know whats bracing the egg?



Mark

 

CaptBill,

The trial design is 20 meter long by 4.25 meter wide. It is roughly based on the concept of a Dutch Barge, but with a V-hull rather than a flat bottom hull.

To reiterate a point I made earlier. This is a trial design for my own amusement and education. I have no plans to build it, though at some point if I were to get far enough along and the design were actually a good one (as validated by a qualified consultant) I suppose the boat could, in principle, get built eventually.

 

Here is the Golden Section values from the seed number 300 (the LOA). It is simply multiplying the last result by .618 again, as many values as you need, never reaching zero. The second set is simply the numbers calculated backwards or measuring from 300 down in other words.

Length All 300
185.400
114.577
70.809
43.760
27.044
16.713
10.329
6.383
3.945
2.438
1.507

229.191
256.240
272.956
283.287
289.671
293.617
296.055
297.562
298.493


Your bulkhead will harmonically balance well with a combo of the fist phi number and one of the last. Wider equals more dampening force. Example.....

at 300 it would be

Center at 185.4
forward wall at 189.34 (185.4 + 3.94= 189.34)
back wall at 181.46 (185.4 - 3.94= 181.46)

So the bulkhead here is 7.88 thick centered at 185.4

 

Hang on i will get the numbers you can play with from my spreadsheet

 

20
//////////////////////////////
12.360
7.638
4.721
2.917
1.803
1.114
0.689
0.426
0.263
0.163
0.100

15.279 (reversed)
17.083
18.197
18.886
19.311
19.574
19.737
19.837
19.900
//////////////////////////

4.25
//////////////////////////
2.627
1.623
1.003
0.620
0.383
0.237
0.146
0.090
0.056
0.035
0.021

3.247 (reversed)
3.630
3.867
4.013
4.104
4.160
4.194
4.215
4.229

Play around with these number and you will find that they just SEEM right. Naturally pleasing to the eye even. The same balance your eye knows instinctively.

 

CaptBill,

I hate to admit it, but I am not sure what to do with the numbers you just posted.

 

You just build a grid over a top view of your hull.
The 20 meter group you align vertically , the 4.25 go horizontally..
Each value has a line on the grid. It is only the first 2 or three values that you will probably need. The rest, or the smaller values, will be used like we did calculating the 300 earlier (finding good widths/stance of stringers, set upon one of the prime or larger values) generally.

So draw lines from top of page to bottom at ...

12.360
7.638
4.721

and side to side at....

2.627
1.623
1.003


Actually, you want to halve the 4.5's to account for the Y symmetry of your hull and measure from/to the centerline.

 

I beg your pardon, I've lost the thread of this... What dampening force?

 

Me too...

 

Ok, when you find the harmonic center (at exactly .618%) you just HALVED the vibrational force of the energy wave, instead of elswhere (thud)... so the energy wave is now 1/4 strength fore and aft (1/4+1/4=1/2= octave jump). So, essentially, with a very thin bulkhead you are HALVING the energy wave. THEREFORE the width or the spread /thickness of the bulkhead will proportionally absorb more vibration the wider you make it. So DAMPENING is technically what is happening, when set right on the harmonic point. This is what one should aim for and hopefully have in a boat....it should be carefully considered just like Mr. Stradivarias wisely considered in his violins. Same exact dynamics apply to both boats and stringed instruments.

 

Looks like you're considering only the vibrational aspect. How about static or quasi-static case, when hull is not vibrating but has to resist external static loads (like, well... hydrostatics, wave action etc.) ?