How to calculate a sail catamaran hull pressure?

How the design pressure for a sail catamaran bottom is calculated? Is it done same way as for monohull? but the weight is divided for two hulls, is it taken into account? Does lighter weight, high slenderness ratio and length reduce the design pressure?

 

Pressure is a function of water height (depth), but there are other pressure variations ie speed, swells and various factors. The same pressure in a 3m filled water pipe is in a dam of water at 3m deep (if at the same height above sea level and atmospheric pressure).

A water tank's pressure increase when you put it up higher, a thicker pipe will not increase pressure, but will flow faster.
See, boats and tanks are exactly the same

Read up on displacement.

 

I'm trying to understand what kind of pressures a boat hull should withstand. The static pressure from just floating in the water is quite easy to calculate, and it seem not to be so high compared to the pressure waves and falling from a crest can cause. I have read that a 40ft monohull could see pressures of 100kpa (10 000kg/sqm)in heavy weather. I'm surprised that the pressure can be so high. Can it be as high for a sail catamaran with the same length and weight?

 

It all depends on what the hull is doing. 100 kPa sounds high, but it's roughly atmospheric pressure at sea level.

If you take a speed boat and start bouncing off waves, you're going to generate some high pressure transients on the bottom. If you have a long, slender hull or hulls which slice thru the wave crests, the pressure transients won't be quite as high.

 

Alik, a frequent poster on this forum, also known as Albert Nazarov, has a good article in the current issue of Professional Boatbuilder about hull pressures used for structural analysis on catamarans and the results of tests he has run.

 

You can approach this is two ways:

1) Try and calculate everything from first principals. - Long, very time consuming, tedious and how will you validate the solution?

or

2) Most Classification societies have good rules with regards to sailing vessels these days. All the work that you would have to do in #1 has already been done for you, and all validated too.

The pressure is also only half the story. Since what is the pressure going to be used for? If it is for structural analysis, then the draft plays a major contributing factor in the final result. As one would expect.

For example. Take a catamaran that is say 25m long and weighs, say 50 tonne with a draft of say 0.50m. With a frame spacing of say 1.0m and stiffener spacing of say 300mm. What is the bottom slamming pressure?

Using any one of the Class societies, in this case DNV (with one or two other assumptions), the pressure is 32kN/m^2. But if i changed the stiffener spacing to 200mm, the pressure is then 33kN/m^2. But if i changed the frame spacing to 1.25m the pressure is then about the same. So not much sensitivity. But if i changed the draft to 0.75m the pressure becomes 50kN/m^2.

The falling off a wave scenario, is taken into account with the Class societies rules, by using a set of prescriptive conditions of the operation of the vessel. Which result in different vertical accelerations to be input to calculate the final pressure.

Thus, a lot of leg work if you try it yourself, which I did decades ago, but as noted, what is there to validate it? Thus Class society rules are your best bet, regardless whether you elect to formally use the rule in the design or not, it is a good indication and importantly a trend, which, as a naval architect, is all one requires for design, trends, not absolutes.

 

Ad Hoc, have you read Alik's article in Professional Boatbuilder, and if so any comments?

 

I have read Alik's article. It is an excellent body of work.

The main thrust of the article is however, at the smaller boat market and from a statistical point of view, is a small sample base to draw concrete conclusions. The cross over between commercial and pleasure in the 15m-ish bracket or so, which it appears this is ostensibly aimed at, is using ISO which is noted a lot in the article, is more prevalent for many designers than Class rules, in the absence of any other guidance.

With regards to Classification societies etc working with designers. They already do this and extensively. We assisted LR with their, at the time, all new SSC rules and provided a lot of design/data for validation and review with LR. That was back in the early 90s. Not just us, either, I know several other companies in the UK (and a few more overseas) at that time, as we all sat on the same various committees. In addition, I am also on LR's technical committee. We constantly review and assist new rules development for LR. I am one of 80 other industrial members, all for this very reason to ensure validity. Thus speaking for LR, their rules are a constant WIP and using their extensive database of hundreds if not thousands of in-service history of vessels for their own internal validation in reviewing new/old rules. If rules are deficient or 'over-engineered', as such, then it comes up for review and must past peer scrutiny of the tech.comm members, as well as LRs internal R&D staff, just like any technical paper. Thus it does occur and a lot. Though many are not aware of it.

 

The sheer strength of the material used as well as the structure that makes the hull rigid determine the pressure per surface area the hull will be able to withstand without bending. Keep in mind that the unsupported area in between is subject to flex under pressure, which may lead to fatigue and failure.

Thickness of a hull contributes to this strength, hence sandwiching to prevent flex, and minimize bulkheads etc. to save weight - as to a thick solid weighty hull. It is impossible to remove all flex, there is always some. A 5mm sandwich wil flex more than a 20mm sandwich, all depends on the size, weight and practicality of the application.

Sure, a flat wide surface will hit a lot harder when it falls off a swell than a narrower penetrating hull, but how long is a string ? In both cases the weight dropped will also play a role, a light vessel will plonk down with less force than a heavy loaded hull. If the force on an unsupported area is more than the sheer strength of the material it will perish, but this is usually a lot, even on a thin skin.

Like all things in boats it is a compromise between weight and strength, you have to give some to get some. You cannot grow up and be a boat builder, you have to choose one

For my own reference, the boat must be able to be supported on land under it's own weight by a reasonable mechanical support without flexing or bending. Maybe there are other ways to improve, but if the hull can withstand that it should make it in most conditions on water.

Next time, ask easy questions.

 

What I mean by this is the supports must not press dents in the hull,
If one of ie four supports are removed the boat must not sag or twist.

 

Yes the weight is divided between the two hulls thus the pressure is distributed along the two. Lighter weight means also that the hull will not be as deep so the pressure is less. The deeper the hull is immersed, the greater the pressure. The keel, the deepest portion of the hull has the greatest pressure.

With a slender boat, pressure is distributed along the length BUT if it is going to go fast, the front part has greater pressure due to slamming of the waves. If the boat heaves because of the adverse sea condition, a vertical acceleration force will be generated contributing to slamming.

I don't have a copy yet as it is still in the review stage but for sailing cats design pressure, I believe it is discussed in ISO 12215-7 (multihulls).

If you want to understand more about design pressures including slamming, download a pdf copy of Marine Composites by Eric Greene and Associates pages 83 to 93 http://www.ericgreeneassociates.com/images/MARINE_COMPOSITES.pdf . I have tried some of the formula presented in the book and it comes close to Lloyd's method.

You can also download a copy of LR regulations for Special Service Craft but I must warn you, the calculations are tedious as it covers the type of boats, range of operations, and the dynamic loadings all spread out in several chapters.

Pressures varies greatly depending on the location and scantlings as AH has pointed out. Great if you have the software as it replaces the finger walking.

 

Hello RX,

I have gone through the ericgreen composites pdf, some nice work and I always wonder where someone get the time to put something like that together. I thought boat builders spend their time counting their money

I still do not see how one can calculate the pressures optimally ? given the huge variation of conditions that may or may not be experienced. Or is this just a reference ?

Perhaps it depends what the application is for - ?

If you want as light a boat as possible then calculate the material strength for a specific environment ie for racing in limited weather/water conditions ?

If it is a work boat then add a load limit and again works only in favorable weather/water conditions ?

If it is a work boat with varying loads and must be able to withstand unforeseen weather/water conditions ?

I saw that video where they attempted a new record hence sailing was done in a storm, the one where the nav looked like he was crying while the nut cases driving this thing through those waves were riding it like an explosion - surely a vessel like that has to be designed with a few additional features to keep the parts together ?

Or can they all do that ?

 

Hello Fanie,

Marine Composites is like a bible to me. What is great about it is they include reference to test results and studies.

Pressures on the panels cannot be optimal as approach varies and the complexities of application varies. Even strength calculations for the supporting members that carry the panel varies with each class rules. E Greene's coverage on the pressure is only a basic coverage of the principles, not intended for design purposes.

I find the LR SSC rules a complete guide to design as it covers all aspects of how the boat is going to be used such as inland waters, near shore, and offshore to name just a few. The formula used is also unique as it covers the operating envelope, the stress distributions, etc. For example, using the LR formula, you can calculate the pressure at any point of the boat, below waterline or above, referencing from the lowest part of the boat, the keel. To do that, you have to take seriously the design guidelines.

I trust Class Societies rule as they constantly upgrade their studies and if something breaks according to their design, they immidiately conduct study why.

So how do they predict unforseen weather conditions? In a way, they don't because the design is for a specific design limit. Exceed that and the boat breaks. You can only hope that the safety factor included in the formula is not exceeded. There is what we called, an operating envelope also and is included in the design. If you design the boat to speed of 20 knots in 4 foot waves, you don't drive it at 30 knots in that condition because the stresses will be much higher.

Racing boats are quite different. The materials are pre-tested to assure the claim stress are on par with the manufacturers claim. Calculations are also more tedious as almost every critical part meets the strength criteria. Some employ statistician, to predict the probability of an adverse condition that would most likely happen and design the strength around it. If it breaks, they can always say, "the conditions are far worse than we have anticipated", otherwise, they have a bad engineer.

 

Thanks RX, so the idea to stand back and "feel" the concoction with female instinct weather it will live up to expectation is not completely right, after calculations, but it's not that wrong either. I was worried I may be missing something

 

2AdHoc: I agree that one boat might not be representative. But we actually did such measurements on 3 catamarans... And some interesting findings:

1. Data from Allen-Jones indicate similar load distribution, refer to attached graph (see Allen R.G., Jones R.R., “Considerations on the Structural Design of High Performance Marine Vehicles”, SNAME New York Metropolitan Section, 1977.)

2. LR SSC Rules - design pressure distribution changes gradually from 'not high speed boat' to high speed boat, especially at stern. It is not likely to happen in reality.

3. In ISO12215-5 pressure distribution does change with increase of speed.

4. RS Rules for high speed craft - catamaran section shows different pressure distribution (similar to enclosed A-J graph), it was obtained for cats with FnV starting from 0.2 and up.

So, I believe for craft in semi-planing mode the loads from classification societies are exaggerated. One might agree or not, this is opinion based on our measurements and other known facts.