Sailboat design: loading conditions and factors of saftey

[seanjournot]

What specific loading conditions do multi hull yacht designers use as a standard and what factors of safety do they normally shoot for in a blue water cruiser?
Although finding a waterline, stresses in rigging, and static loading conditions from equipment and buoyancy is fairly straight forward, from the perspective of a mechanical engineer, determining the forces applied to a craft dynamically while at sea is not. How do yacht designers go about determining these dynamic loads? Do most just go with what has worked in the past and use rules of thumb instead? Is there a static analogy that approximates this problem? For example, finding the resulting stresses when supporting the hull with small movable patch areas with some number of G's and a free moment applied to the craft.

[BigBeakie]

Sean, take a bow for having asked an excellent question👍

I have investigated this with a number of Australian multihull designers, and it comes down to how much formal training they've had in Naval Architecture, how much knowledge and experience they've had in material science & engineering, how much they know about composite engineering, and so forth. There is also a body of knowledge on scantlings for multihulls that is largely derived from "racing and breaking".

Needless to say, the designs are done for the expected worse conditions for the use case of the boat. If it is a Southern Ocean racing multi, it is built far stronger & with design and material specifications that have higher safety margins for more demanding conditions than a charter design that is expected to spend its time in more protected waters, and probably motored when the wind pipes up.

For just one example, the mast compression forces can be 1.8 times the total mass of a boat (including full payload) in demanding windward conditions, so a good design uses excellent materials (perhaps epoxy resin with s-glass stitched not woven fabrics and VIP techniques), properly designed and built structural bulkheads that are fully coved and glassed in ( instead of "room dividers" that are glued in for fast production manufacturing), good torsional resistance and build strenth with light weight to ensure a stiff boat, etc, etc, etc.

It is an excellent question that is completely ignored by the vast majority of buyers who have emphasized least cost and perceived "comfort" above other values. But, so be it. There still are really good designs that are properly built, but they are not the most popular boats....they cost more!


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[GordMay]

Greetings and welcome aboard the CF, Sean.

[arsenelupiga]

i believe designers mostly rely on finite element analysis.

[roland stockham]

Not a multi-hull designer or even sailor but from what I have read there is another factor at play. I believe that multies should resist capsize and wave stresses by sliding sideways across the surface. This is the multies version of heeling and spilling wind that monos use to limit stress. Overloading buries the hulls more deeply creating more resistance and therefore higher max stress. Not sure how, or even if this is included in the stress modeling.

[seanjournot]

Originally Posted by BigBeakie

Sean, take a bow for having asked an excellent question👍

I have investigated this with a number of Australian multihull designers, and it comes down to how much formal training they've had in Naval Architecture, how much knowledge and experience they've had in material science & engineering, how much they know about composite engineering, and so forth. There is also a body of knowledge on scantlings for multihulls that is largely derived from "racing and breaking".

Needless to say, the designs are done for the expected worse conditions for the use case of the boat. If it is a Southern Ocean racing multi, it is built far stronger & with design and material specifications that have higher safety margins for more demanding conditions than a charter design that is expected to spend its time in more protected waters, and probably motored when the wind pipes up.

For just one example, the mast compression forces can be 1.8 times the total mass of a boat (including full payload) in demanding windward conditions, so a good design uses excellent materials (perhaps epoxy resin with s-glass stitched not woven fabrics and VIP techniques), properly designed and built structural bulkheads that are fully coved and glassed in ( instead of "room dividers" that are glued in for fast production manufacturing), good torsional resistance and build strenth with light weight to ensure a stiff boat, etc, etc, etc.

It is an excellent question that is completely ignored by the vast majority of buyers who have emphasized least cost and perceived "comfort" above other values. But, so be it. There still are really good designs that are properly built, but they are not the most popular boats....they cost more!


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Thanks; I figured that a "racing and breaking" method was used; I would not be surprised if some designers fit out their boats with strain gauges on hulls/ bulkheads and load cells in rigging also. I don't see how anyone buying a boat would not look into their boats design standards; if you don't know how much punishment your boat can take you can't really work out or manage the risks involved with sailing.
A boat could also be overly built. using extremely strong, and heavy, components where they are not needed only increases the stresses in critical areas since the stress on a boats structure is a function of its mass. Minimizing mass results in a higher factor of safety.

[NevisDog]

Originally Posted by BigBeakie

... If it is a Southern Ocean racing multi, it is built far stronger & with design and material specifications that have higher safety margins for more demanding conditions than a charter design that is expected to spend its time in more protected waters, and probably motored when the wind pipes up.

For just one example, the mast compression forces can be 1.8 times the total mass of a boat ...

Great summary.

Finite element analysis is only as good as the parameters that are input and wind/wave actions on small craft are almost impossible to quantify in any meaningful way (except perhaps for specific conditions - say, Americas Cup cats racing in sheltered waters in winds up to max 20kts). I doubt any designers run around placing strain gauges - much more 'rule-of-thumb' based on experience, or specific rules such as Lloyds, DNV, ABS (though I don't know how these apply to multis).

With all those keels falling off monos recently, you can guess that multi designers have even greater challenges determining safety factors on their designs.

[catsketcher]

This question was posed and delved into over many years on boatdesign.net. Search through the threads to get all of them but here is one.

https://www.boatdesign.net/threads/c...ntlings.42431/

cheers

Phil

[zstine]

Originally Posted by arsenelupiga

i believe designers mostly rely on finite element analysis.

FEA does not define the input loads the OP is questioning. FEA takes user defined loads and determines the resulting stress/strain propagation through your matrix.

For the OP, I'm not sure what you want to do... design and build a DIY cat, evaluate what's on the market or just an intellectual exerciser? If I were designing my own boat, I would probably see if I could reverse engineer a similar size boat that has a good reputation. particularly see if I could find the laminate schedule, which some builders may be forthcoming with to a perspective buyer, while others may hold that info in confidence. You could get hull thickness from people who have put in thru-hulls and 'empty' displacements, less blast and equipment, can provide some info about how much glass and resin went into the boat.

You will likely find more help here... https://www.boatdesign.net/

[seanjournot]

Originally Posted by zstine

FEA does not define the input loads the OP is questioning. FEA takes user defined loads and determines the resulting stress/strain propagation through your matrix.

For the OP, I'm not sure what you want to do... design and build a DIY cat, evaluate what's on the market or just an intellectual exerciser? If I were designing my own boat, I would probably see if I could reverse engineer a similar size boat that has a good reputation. particularly see if I could find the laminate schedule, which some builders may be forthcoming with to a perspective buyer, while others may hold that info in confidence. You could get hull thickness from people who have put in thru-hulls and 'empty' displacements, less blast and equipment, can provide some info about how much glass and resin went into the boat.

You will likely find more help here... https://www.boatdesign.net/

I would like to design and build a 40ft + cruiser; I've been dreaming about doing that for the last 15 years. I should be in a position within the next few months where I can start saving one to two thousand dollars a month to make that a reality inside of 8 years. I will of course start with a smaller project; maybe a bare bones 32ft coastal racer that I can hone my sailing, boat design, and fabrication skills on without worrying about sinking everything I own. Right now I guess this is just an intellectual exercise.
I don't believe that you can compare the laminate schedule between two hulls; it's not an apples to apples comparison even between hulls with identical waterline lengths while modifying an existing design, like changing a sail drive boat to a classical inboard or the head/galley/ cabin layout, could invalidate all of the engineering that went into the original design.
I have looked for laminate information on catamaran hulls so that I could roughly estimate total material costs but since this is what most of the engineering goes into figuring out that information is a closely guarded secret. My best estimate so far for just the hull of a 45ft cat comes out to around 80,000$ while fitting it out adds on another 70,000$; I don't think that's very realistic though since a similar yacht sells for 300-500 thousand dollars. Then again 150,000$ doesn't take into account 2000-4000 man hours and a decent profit margin.

[44'cruisingcat]

Unless you build something with an extremely basic finish, 2000 hours won't get you far.

Most kit catamarans will take 4000-6000+ hours, built to a reasonable standard.

A one off will be longer. What method of construction are you planning?

A friend of ours is a quite well known yacht designer, who has been designing and building boats for decades. When he was looking to build a catamaran to circumnavigate, he decided to build a kit catamaran rather than go through the hassle of designing it first. It's really a matter of finding a design that suits your wishes.

[NevisDog]

Originally Posted by seanjournot

I would like to design and build a 40ft + cruiser; I've been dreaming about doing that for the last 15 years. I should be in a position within the next few months where I can start saving one to two thousand dollars a month to make that a reality inside of 8 years. I will of course start with a smaller project; ...

Wow! that sure is ambitious. One-off designs are hardly an economical proposition for most buyers. Mass production spreads the hefty design costs over hundreds of hulls. Mass production provides feedback to the designer on what works and what should be improved - whether weaknesses in the structure, shortcomings in the accomodation layout, problems with the rig, sail balance, steering issues, etc, etc, etc.

When a well-known yacht designer chooses another firm's design of catamaran (see previous post) I think this says a lot about the chances of getting it right first time.

With your knowledge, I'd suggest use it to evaluate existing designs?

[Maniew]

For what it's worth, here is my story:
I went the same route for a number of years until I got too old! (actually I am 75 young at my next birthday) I have a passion for designing boats, designed, built and sailed several boats. All of them below 5 meters LOA. Now I am running out of time to do the big one! The one that will take me across the Blue! So I've decided to use an existing design by a reputable designer. There is a kit available for this specific Cat. I function in Rands (to make matters worse) hence words like "budget cruiser" became very attractive!
The boat I am contemplating to build (actually I will have it built this time!) is the Proteus 106 (LOA 10.6 m) designed by Angelo Lavranos. It has 4 sleeping areas (I wont call it bedrooms) 2 Double bunks and 2 3/4 size. We will be 3 people doing any ocean crossing, but it can easily accommodate 6
What I love about this boat is no 1: It sails well, it is Spartan, no "bells and whistles" yet quite comfortable (human comforts) and it doesn't cost an arm and a leg to build!

[NevisDog]

Originally Posted by Maniew

...The boat I am contemplating to build (actually I will have it built this time!) is the Proteus 106 (LOA 10.6 m) designed by Angelo Lavranos...

That's one affordable ocean-crosser!

[seanjournot]

Originally Posted by 44'cruisingcat

Unless you build something with an extremely basic finish, 2000 hours won't get you far.

Most kit catamarans will take 4000-6000+ hours, built to a reasonable standard.

A one off will be longer. What method of construction are you planning?

A friend of ours is a quite well known yacht designer, who has been designing and building boats for decades. When he was looking to build a catamaran to circumnavigate, he decided to build a kit catamaran rather than go through the hassle of designing it first. It's really a matter of finding a design that suits your wishes.

Resin infused foam core construction using female molds; this method should allow for a rapid build and fast fairing of parts before layup while using a cheap and fast to build one off mold. It looks as though most of the man hours for any boat project are used during fairing, painting/ sanding/ finishing. In the end I may end up buying a license from a professional yacht designer; it will still be at least two years until I can start construction.