New Project - Analyzing Design Pressures

Long time reader, first time poster. I am a hobby builder currently building a 37.5' offshore power catamaran. I have the laminate schedule, core specifications, framing spacing etc from the original designer. I am currently trying to understand more on design pressure and unsupported panel size. I have been playing with vectorlam for some more insight into design pressures for different locations on the hull bottom and tunnel. The beam on the boat is 11.5' and designed heavily loaded displacement is 15600 lbs with a max speed of 55 knots. The boat is anticipated to draft 1'-9". The boat is being built in foam core and epoxy. My focus right now is just confirming that the design pressures I am seeing in vectorlam are close to what I can expect. I want to see what slam pressures on the tunnel are along with the bottom. When I run these numbers in vectorlam I have used DNV-GL and ISO 12215-5 (more applicable to what I am doing).

I am consistently getting design pressures on the bottom running surface around 8 psi and around 9 psi on the front of the tunnel. My question initially do these seem to be in the "ballpark" for a boat of this size and speed?

 

Welcome to the forum.
A pressure of 8 or 9 psi may be a bit low for the inside bottom-forward part of a 55 knot boat. But nothing can be said without having more information about the boat.
Working with composite materials, the thickness of the laminate by itself does not say anything conclusive. It is necessary to look for admissible values of the thickness/weight ratio. But, above all, a layer-by-layer analysis must be analyzed to verify that none of them exceeds their design pressure.

 

Thank you. I realize there is a lot more analysis to do with my panels and laminates. I have ran some numbers with the Vectorlam panel module with the laminate schedule specified to get a basic understanding of my unsupported panels. I’m just trying to get a good starting point to confirm I’m not way off base. I’ve spent more time looking at the bottom and have not spent a lot of time looking at the tunnel yet. If you had to make a conservative guess on pressures on a tunnel like that would expect. I plan on analyzing the tunnel in more detail. Thanks!

 

Here we discussed it:
Professional BoatBuilder Issue No. 157 - Professional BoatBuilder Magazine https://www.proboat.com/professional-boatbuilder-issue-no-157/
See 'Impacts on cats'

 

thanks I’ll check it out.

 

Unfortunately, no structural data on speed boats have been put into the statistical regression of ISO pressures. I wouldn't apply the design pressures required by the ISO standard for boat whose speed @MLDC is greater than 45kts, in the scope of boat building.

Nevertheless, I've double- checked the ISO response to the numbers of your boat, and the pressure that I get are far greater than the 8 to 9 PSI you are talking about. In fact, I got values more than twice as high. ISO pressure values are calculated from a base pressure by applying coefficients :
- representing the panel location and longitudinal angle
- representing the panel aspect-ratio
- representing the characteristics of the boat

You've already provided characteristic numbers for the boat. What is the size of your unsupported panel ? What is its location from AP ? What is its height above WL ? What is its longitudinal angle ? What is the height of the deck above WL at your panel location ? To give you an idea of the ISO response, for a panel 15"x60" at 50" above WL, 32' from AP, I got 22 PSI of ISO design pressure. Could be too much conservative, I agree. But anyway, this is also very different from your results. Basically, the number I got is equal to the total weight of the boat, applied on this single panel.

 

Who said that?

 

> Officially ...

ISO12215-5:2019 8.3 Table 7. The quote following the definition of the Kdyn does restrict the use of the standard for recreationnal crafts and charter crafts with a maximum speed of 50kts. Working boats with an higher maximum speed should be analysed using the annex J, that increase the Safety Factor.

> From my experience into the GT18 working group for ISO12215...

a communication in 2006 (Source : "Error Log-FDIS Part 5)" shows the boats that have been evaluated for the determination of the expression of the Kdyn, in comparison with the Ncg of the Bureau Veritas. Not plenty as you can see... To take the difference between BV and ISO into account, Kdyn formulas have been "tweaked", since no test campaign have been realized to establish the 2019 standard.



Speed values higher than 50kts can be found when applying the ISO12215:2019 Annex J. table J.4, but as far as I know, these calculations have only been made to introduce the Annex J. into the standard, allowing small rescue/patrol boats to be assessed. As far as I know, no scantling calculations have been checked. Again, We only compared the reduction speed table of ISO with the BV one (source : 2021-12-06 WG 18 Errata -improvements ISO 12215 ), checking if the speed reduction requirements are the same. Regarding the scantling, and the real usage of these boats, the idea was to let these formulas be applied by users, and to get feedback from them, for future adjustements.

 

Alan, from the tables you posted aGC matters, not the absolute speed. I hope we all understand that.
Most of CSs do limit the aCG, as it is physically hardly possible to drive the boat at 8g. Given this, maximum aCG is the function of sea state and speed, not the speed only.
ISO should have limited aCG, not the speed.

 

yes Alik, you're right. I hope that any confusion that could be made from my post could be avoided following your remark. I would like to summarize it :


With the following remarks :
- Scantlings of boat whose maximum declared speed is above 50kts or any scantlings of working boats whose speed, at any sea state, is higher than 48kts, have not been fully checked during the process of revision of the ISO12215:2008 to ISO12215:2019
- I fully agree that it would have made more sense to limit the aCG for every boat, instead of having limited the speed for recreationnal & charter boats.

Since the ISO is now entering a new revision phase, and with your permission, Alik, I would like to submit officially your proposition to the working group.

 

Oh no no we spent so much time to program it Kidding.

Yes sure can submit. If you need any formal paper from us, pls let me know.

 

Thanks Alik. I will indeed bother you for a formal paper. Cheers,

 

Thanks Alan,

So this is what I have been looking at. I wanted to analyze three spots in the tunnel. Furthest aft, midship and a couple feet off the tip of the bow where the the panel size is the largest.

Aft location. Panel is 12" above the waterline, 3' from AP, sitting almost perfectly horizontal at 2 degrees, deck above wl is 17"
Midship. Panel is 15" above waterline, 16' from AP, 5 degrees, deck above wl 19"
Bow. Panel is 45" above waterline, 34' from AP and 14 degrees, deck above wl 60"

I was working conservatively on the speed. If the boat hits 60mph I'll be very lucky. Realistically it's a under 50 knots, especially with a 15600 load. Currently all three panels I'm looking at on the tunnel are approximately 24" by 24" panels.

I input the numbers you used above in vectorlam's IS 12215-5 panel tool and got these results. What am I doing wrong?

 

Yes design loads can be subjective, we did comparision for few rules recently, for some projects. Especially on catamaran wet deck structure.
Interesting to note, that ISO12215-7 prescribes 'displacement' design loads on catamaran wet deck, even for planing cats. I dont think this is correct approach.

This is the reason we keep design the loads outside of our SigmaLAM software. The loads are defined separately, using Excel files made for different class rules or standards.

 

Indeed, most humans will complain at anything above 1g, and going to 2 or even 3g, it would be impossible to 'drive' the boat.
But most CS tend to use this Acg, as instantiations peak slam pressure, which do occur. Since CS are more concerned about the safety of the boat, per se..
For safety of those onboard...that's the realm of statutory rules