are monohulls just overbuilt?

[peterchech]

I've been looking at alot of multihull designs, and comparing them to monohulls. Though i now own a multi, i have sailed on monos and realize they have their advantages as well. I dont want to get into the multi vs monohull debate, that's been done to death. But i have noticed some interesting things comparing designs.

Generally, multihulls need to be built as light as possible to realize their potential. Therefore, despite hitting high speeds and stresses, and despite having long narrow hull forms that dont lend to strength and stiffness, they are built extremely light. A thirty foot trimaran can be built of 1/4 inch ply in some instances. Even putting aside the extremes, a similar displacement monohull nearly always has much thicker scantlings than a multi. This is true even in the case of unballasted, or lightly ballasted, monohulls.

Is this necessary? Isn't a wide hull form theoretically stronger than a narrow one? Shouldn't it require less, not heavier, scantlings? Are monohulls just built heavier than is necessary, because they suffer less from being heavy?

[Ian McColgin]

Nope.

[Daniel Noyes]

I dont think performance mono hulls are built any heavier than performance multi hulls... it's just that nearly every multi hull is a performance boat and lots of mono's are party barges/ floating houses/ work horses, rather than go fast machines.

also a multi hull for a given size is a much smaller boat than some mono hulls, take a 18' catamaran and compare it with a 18' Cat boat... two similar lenght boats but the Cat boat is twice as "large" (or more) a boat as the catamaran...

also going fast actually reduces stress on the hull and rig to some degree, most ice boats suffer break downs on days with bad ice, because they are sailing slower there is lots more pressure on their hull and rig.

[Woxbox]

I've got to agree that the scantlings on light go-fast monos do push the limit. Take out the ballast (if there is any), and the performance monos don't weigh that much. You see this in dinghies back to the original Thistles, for example. The Thunderbird was ply over stringers way back. The modern open ocean types are just big eggshells. The cruising boats are another matter.

Of course it's not common, but both monos and multis have broken up when the construction proved too light.

[Roger Cumming]

I think the comparison between multi and mono hull must be made on the basis of displacement, not length, just as when two monohulls are compared. For equal displacement and type of service (e.g.,racing boat, cruiser), scantlings should be comparable.

[peterchech]

That's why i am comparing displacement not loa. Gotta disagree that speed reduces pressure on hulls. My boat never gave me problems till i hit 7 knots, now i had to spend the weekend adding a bulkhead and other reinforcements. The whole hull started to flex and various components began to bend/stretch/slip/crack...

[gilberj]

When the object of the game is high performance, mono or multi..... light, engineered, structure is important. there are costs involved..You may have a strong stiff structure with low puncture resistance, or highly vulnerable if grounded . These structures are more expensive to build and to maintain. Look at the inspections and maintenance on aircraft....I used to race in the Great Lakes....Another boat of the same class and rating in the area had high performance kevlar racing sails. We with the exact same hull and sail plan could not compete at all......If racing is important you want the best you can afford. If you are a cruiser the difference can be as low as a few percent in expected boat speed, or considerably higher. Traditionally, increasing the scantlings was more or less synonymous with strength and longevity.

[BarnacleGrim]

In an ocean racing yacht the possibility of catastrophic failure is acceptable, if it increases the chances crossing the finish line first. This is not the case for cruising boats.

What kind of difference is there in scantlings in cruising and racing multihulls?

[skaraborgcraft]

any boat carrying a ballast keel,ie most mono hulls will have to have bigger scantlings to carry this load,especially if built in wood. Modern exotics not included. Hull skin of a mono or multi of the same displacement should be pretty equal to gain the correct punture resistance. I know a 45ft cat built of 6mm ply and sheathed with nylon (cascover),it was far too light, after one particulary heavy passage it was found that most of the ply on the seaward side of one hull had fractured,and it was only the very light sheathing that kept the water out. It would have been better to have built the hull with right materials to start with. As said,racing is one thing,cruising is another,as are monos-multis. Cheers

[Bill Huson]

Safety factor? If a multi-hull breaks it will float. If a monohull breaks *glub -glub*.

[RodSBT]

Safety factor? If a multi-hull breaks it will float. If a monohull breaks *glub -glub*.

...Assuming the mono has no built in floatation like the multi. ...on the other hand...

[peterchech]

Again, point is not to get into the mono vs multi debate, just to clarify whether im crazy in thinking many monohulls are overbuilt by multihull standard, even ones without any or with little ballast. Here's an example.

Reuel parkers ply adaptation of a 16' stickup sharpie. I dont know the displacement, but it can't be more than 500 or so pounds unloaded, probably less. Scantlings call for 1/2 inch hull bottom and 3/8 inch sides. Compare that to my 24' outrigger canoe, about 450 lbs displacement unloaded. It can be built with 1/4 in hull bottom and 3/16 inch sides. It is clearly a much" larger" boat, with more forces acting on it due to a long but slender hull and relatively high speeds. Yet it is built much lighter. Why can't the sharpie be built in 1/4 inch?

[Paul Pless]

just to clarify whether im crazy in thinking many monohulls are overbuilt

yes - you're crazy

[Bill Huson]

Again, point is not to get into the mono vs multi debate, just to clarify whether im crazy in thinking many monohulls are overbuilt by multihull standard, even ones without any or with little ballast. Here's an example.

Reuel parkers ply adaptation of a 16' stickup sharpie. I dont know the displacement, but it can't be more than 500 or so pounds unloaded, probably less. Scantlings call for 1/2 inch hull bottom and 3/8 inch sides. Compare that to my 24' outrigger canoe, about 450 lbs displacement unloaded. It can be built with 1/4 in hull bottom and 3/16 inch sides. It is clearly a much" larger" boat, with more forces acting on it due to a long but slender hull and relatively high speeds. Yet it is built much lighter. Why can't the sharpie be built in 1/4 inch?

Wow. I see your point. A 16' hull with 1/2" (12mm) bottom and 3/8" (9mm) sides? Put an iron plowhead on the bow and call for ramming speed! That thing is a tank.

[Paul Pless]

overbuilt?

[callsign222]

The sharpie can be built in 1/4". The 15'6" mono-hull Goat is entirely built of 1/4" ply and calls for no fiberglass except to tape the chines, and 4 internal frames and a chinelog. I built the Goat knowing almost nothing about other homebuilt boats, and the more I compared my GIS to other boats of comparable size, I was struck with how heavy everything else is or how they are being built.

[Eddiebou]

Wow, what is that thing?

[Woxbox]

A Moth
Great stuff on the U-tube, too.

[Breakaway]

The crab skiff is a workboat, or at least a derivation of one. So thicker bottom equals ability to take more abuse. I'd bet the extar weight of the 1/2 and 3/8 combo increases its stability as well. She'd be fine, but more "active" boat built in 1/4/"

Kevin

[peterchech]

Another example, chebacco. Thick scantlings, little or no ballast. Compare it to a scarab 22 trimaran in ply. Similar displacement/loading/size however u wanna look at it, scarqb is built in 1/4 inch ply, chebacco in 1/2 inch.

[CapnJ2ds]

Nope.

Succinct - and correct.

[brucehallman]

IMO. The weak link in performance boats, (typically) is the tension connections of the wire stays to the hull. Or, the compression of the mast. The scantlings of the hull skin isn't really an issue. Structurally, this is a truss system. Multi hulls, being wider, can avail some benefit from the wider truss angles possible than with narrower monhulls, and the tension connections (chainplates) can see less stress. Ditto for the compression in the mast and at the mast step.

[Paul Pless]

IMO. The weak link in performance boats, (typically) is the tension connections of the wire stays to the hull. Or, the compression of the mast. The scantlings of the hull skin isn't really an issue. Structurally, this is a truss system. Multi hulls, being wider, can avail some benefit from the wider truss angles possible than with narrower monhulls, and the tension connections (chainplates) can see less stress. Ditto for the compression in the mast and at the mast step.

don't forget keels and rudder appendages, both seem to 'fall of' at alarming rates in performance oriented boats

[johnw]

IMO. The weak link in performance boats, (typically) is the tension connections of the wire stays to the hull. Or, the compression of the mast. The scantlings of the hull skin isn't really an issue. Structurally, this is a truss system. Multi hulls, being wider, can avail some benefit from the wider truss angles possible than with narrower monhulls, and the tension connections (chainplates) can see less stress. Ditto for the compression in the mast and at the mast step.

And the leverage of the ballast.

The 70' America's Cup Class hulls, minus ballast, weighed in at about a ton or less. How much would a 70' performance cat weigh?

[peterchech]

Again again again im not comparing racers to racers, im comparing regular old daysailor/cruiser multihull vs monohull. Most multihull cruisers are no more built to race than any monohull cruiser, yet they are so much lighter not counting ballast. My point is that given the inherent strength of monohull forms, they should be able to be built lighter than multis if anything, yet they are consistently double the scantlings of a comparable (and usually longer loa) multihull. A compression mast only supports my point, since it is easier to support a mast that goes into a hull than one that is supported by crossbeams like a catamaran.

I thing the goat island skiff is an excellent example of a monohull that is built light and has excellent performance partly because of this. My point is, why dont more monohull designers clue into this?

[brucehallman]

My point is, why dont more monohull designers clue into this?

Tradition? Monohulls have a longer tradition, tracing far back to workboats. And, in workboats, thick scantlings make sense for the durability. Plenty of things we see in modern boats defy logic, and are the way they are simply because of tradition.

[JoshuaIII]

I think the torsion that happen on a boat is harder to deal with on a monohull. The 2 big beam on a multihull that keep the hull together help a lot to resist it which would be why the hull and scantling need to be bigger on monohull.

[Paul Pless]

okay, what are the typical scantlings on a wharram?

[Daniel Noyes]

Again again again im not comparing racers to racers, im comparing regular old daysailor/cruiser multihull vs monohull. Most multihull cruisers are no more built to race than any monohull cruiser, yet they are so much lighter not counting ballast. My point is that given the inherent strength of monohull forms, they should be able to be built lighter than multis if anything, yet they are consistently double the scantlings of a comparable (and usually longer loa) multihull. A compression mast only supports my point, since it is easier to support a mast that goes into a hull than one that is supported by crossbeams like a catamaran.

I thing the goat island skiff is an excellent example of a monohull that is built light and has excellent performance partly because of this. My point is, why dont more monohull designers clue into this?

A multi hull is nothing with out speed, even a cruiser/daysailer multi hull (I dont know of a whole lot of designs in this genre) is built with speed as a primary concern, if a multi hull doesnt have speed what does it have?...not much.

for a mono hull daysailer/cruiser durability and the ability to cary a load becomes a major part of the design, compare a multi hull to a mono the mono has much larger and flatter hull pannels that can be damaged by landing on a rock or hitting an obstruction, also these large pannels need thickness to feel stiff under foot.

you mention the GIS scantlings, I have sailed a GIS and the planking is very thin, it flexes considerably under foot and moves perceptably as you walk around and it flexes with the waves, it may be strong enough but it does not have a solid feel, I think while this lightness is appreciated by performance/racing sailors it would make a daysailer customer wary about how the boat would handle hard useage by a group of enthuiastic but inexperienced day sailing guests.

by contrast on your out rigger the largest unsupported span of a hull pannel is likely only a foot or two Vs. close to 3/4 ft for the GIS so you can get away with a thinner pannel...

[Paul Pless]

A multi hull is nothing with out speed, even a cruiser/daysailer multi hull (I dont know of a whole lot of designs in this genre) is built with speed as a primary concern, if a multi hull doesnt have speed what does it have?...not much.

stability
shallow draft
efficient hull form
positive buoyancy
sea kindliness
deck space

[johnw]

Again again again im not comparing racers to racers, im comparing regular old daysailor/cruiser multihull vs monohull. Most multihull cruisers are no more built to race than any monohull cruiser, yet they are so much lighter not counting ballast. My point is that given the inherent strength of monohull forms, they should be able to be built lighter than multis if anything, yet they are consistently double the scantlings of a comparable (and usually longer loa) multihull. A compression mast only supports my point, since it is easier to support a mast that goes into a hull than one that is supported by crossbeams like a catamaran.

I thing the goat island skiff is an excellent example of a monohull that is built light and has excellent performance partly because of this. My point is, why dont more monohull designers clue into this?

Actually, the compression forces on a monohull mast should be much greater than on a multihull, because the base for the stays is narrower.

There are monohull designers who have built lightly, either for speed or for less expense. But the leverage of the ballast keel and the narrow base of the rig work against it. Remember how some of the AC class boats folded up in the middle? That's what happens when you build certain kinds of monohulls too light. I've heard of a Fisher Island 31, built to Herreshoff's Rules for Wooden Boats, being lost when it fell off a wave and stove in its side on water. That's what happens when you take a boat built lightly for inshore racing and take it transatlantic. It's not that monohull designers are idiots, it's a matter of not wanting to kill their customers.

Monohulls are designed to carry weight, but because they carry weight, they have to be built strong enough to carry the weight. There, that's simple enough, isn't it?

[Woxbox]

if a multi hull doesnt have speed what does it have?...not much.

You beat me to it, Paul. My response was: comfort.

Speaking as someone on his third multihull, in my mind, multis are about comfort first and speed second.

[Paul Pless]

You beat me to it, Paul. My response was: comfort.

Speaking as someone on his third multihull, in my mind, multis are about comfort first and speed second.

It feels funny doing that as I generally prefer monohulls to multis.

Generally. . .

[Paul Pless]

Oh. . . regarding skin thicknesses on monocoque structures, I believe very generally speaking that the larger the interior volume (especially as measured by sectional area) of the structure the stronger the skin has to be. Comparing comparable displacement monohulls versus multihulls will usually show that monohulls have very much larger sectional areas than do multis, thus one more reason that monohulls tend toward heavier scantlings.* Does that make sense?



*I'm not an engineer, but I play one on the woodenboat forum.

[johnw]

Again, point is not to get into the mono vs multi debate, just to clarify whether im crazy in thinking many monohulls are overbuilt by multihull standard, even ones without any or with little ballast. Here's an example.

Reuel parkers ply adaptation of a 16' stickup sharpie. I dont know the displacement, but it can't be more than 500 or so pounds unloaded, probably less. Scantlings call for 1/2 inch hull bottom and 3/8 inch sides. Compare that to my 24' outrigger canoe, about 450 lbs displacement unloaded. It can be built with 1/4 in hull bottom and 3/16 inch sides. It is clearly a much" larger" boat, with more forces acting on it due to a long but slender hull and relatively high speeds. Yet it is built much lighter. Why can't the sharpie be built in 1/4 inch?

I designed and built a monohull for myself, a 17' sharpie built in 1/4 inch plywood. If I had it to do again, I'd make the bottom out of 3/8 inch ply, because a wide, flat surface like that needs more strength than the bottom of a catamaran hull. I can't imagine building a 16' sharpie with 1/2 inch ply, unless I really did plan to go scraping over the oyster beds for a few years.

The Snipe class (15' 6") called for 3/4 inch cedar or 3/8 inch ply for construction, and they seem to last forever, unless they rot. I don't see the point of using anything stronger than 3/8 ply for the stickup skiff. On the other hand, durability is more important to some people than speed or light weight, and if you want a boat to carry through a tack, weight is a plus.

[Daniel Noyes]

stability
shallow draft
efficient hull form
positive buoyancy
sea kindliness
deck space

All of the above are just as available in a Cat boat as a Catamaran... with the possible exception of "efficient hull form"... wich really just means fast

[Daniel Noyes]

You beat me to it, Paul. My response was: comfort.
.

oh, that kind of "multi hull"...

[Paul Pless]

All of the above are just as available in a Cat boat as a Catamaran... with the possible exception of "efficient hull form"... wich really just means fast

Yes but, catboats, with few exceptions, make poor (unsafe even) bluewater sailboats, many multihulls are as at home offshore as they are inshore. . .

Also in certain conditions a catboat's form stability would probably be at odds with comfort, whereas a catamaran or tri would take those conditions in stride.

[peterchech]

Oh. . . regarding skin thicknesses on monocoque structures, I believe very generally speaking that the larger the interior volume (especially as measured by sectional area) of the structure the stronger the skin has to be. Comparing comparable displacement monohulls versus multihulls will usually show that monohulls have very much larger sectional areas than do multis, thus one more reason that monohulls tend toward heavier scantlings.* Does that make sense?



*I'm not an engineer, but I play one on the woodenboat forum.

That makes alot of sense. But it's nothing that can't be solved with a bulkhead supported floor no? And wouldn't a wider hull be naturally stronger than a long narrow one? Maybe not on the unsupported skin, but certainly on a floor?

And as far as carrying weight, I don't think most monohulls are designed to carry more weight generally, especially if you're comparing similar unloaded displacement daysailors/cruisers. They just suffer the consequences of being overburdened better than multis. A boat floating well below its waterline is not a good performer, and is possibly unsafe, no matter the type, one could argue...

Good point about the compression mast johnw. But again, just seems like something that can be designed around with proper bulkhead placement.

[johnw]

And wouldn't a wider hull be naturally stronger than a long narrow one?

I would have thought the opposite. The bottom of my sharpie would be fine at 1/4 inch if it were half the width.

[peterchech]

I would have thought the opposite. The bottom of my sharpie would be fine at 1/4 inch if it were half the width.

True that stiffness suffers over a wider beam if there is just an unsupported floor. And I could totally be wrong here I dropped out of engineering school to pursue law, so I'm much better at arguing a point than at scientifically finding the truth ... so perhaps the wider hull form is inherently weaker than the long narrow one, I am truly curious.

But to speculate, I do know that a major problem multis have is twist in those long narrow hulls. Larger catamarans may support the hulls 3/4 of the length or more with a bridgedeck, but that is a rather unseaworthy and expensive way of adding stiffness if taken too far, and few trimarans use this solution.

The crossbeams are a major stress point as well, since all that twist that may hit a monohull is multiplied by several times (due to the beam of a multi) and then concentrated on the crossbeam connections. So again, the forces acting on a multihull just seem far more extreme than on a mono.

A few low bulkheads may help stop the flex in the GIS floor, or in the floor of any small mono. I think round bottomed hull forms would largely eliminate this problem as well, so GIS or a sharpie may be a bad example...

[peterchech]

PS johnw any pics of ur self-designed sharpie?

[callsign222]

If the GIS is considered to have a flexi floor, then 420's, FJ's, and other comparable fiberglass boats of the same size have the same flexi-floor characteristics... just for comparison's sake.

[slidercat]

And as far as carrying weight, I don't think most monohulls are designed to carry more weight generally, especially if you're comparing similar unloaded displacement daysailors/cruisers.

Peter, I don't think this is actually true, though I wish it were. Whenever you compare two boats of approximately the same displacement, the multihull will be significantly larger. Or to put it another way, two boats about the same size? The mono will have more displacement.

There are probably several reasons for this, but you can demonstrate the first one easily yourself. If you draw out the waterplanes of a cat, say, and a monohull: two cat hulls of say 20 feet long with a maximum beam of two feet, and a monohull 20 feet long and four feet of beam, then lay the cat hulls over the monohull waterplane, you'll see there's a lot of waterplane missing from the cat hulls as they converge toward bow and stern. Most 20 foot monohulls will have more beam than that, too, but most fast cat hulls will not.

Then look at cross sections. If you lay your cat hull cross sections over the mono cross section, you see a lot of extra section on the mono, particularly if both have a little flare.

Add to that the fact that most monohulls will have more rocker than the cat hulls, if the cat hulls are at all fast.

Stripped of much of the whargarbl, the reason a multi is faster than a mono in most cases, for similar sized boats, is that the multi has less boat in the water, so less drag. Less boat is another way of saying less displacement.

Of course, I could be completely wrong, and I frequently am.

[peterchech]

Ray i am only comparing similar unloaded displacement boats. Thus my 24' outrigger canoe gets compared to a 16' sharpie. The reason for higher speeds in multihulls is that they use narrow waterline beam to defeat the limitations of hull speed. Monohulls may do this as well, but generally have to plane to do so (haha moths dont count)... however, my understanding is that multis do indeed have more wetted surface per displacement and as a consequence can actually be slower than monohulls in light airs.

[johnw]

PS johnw any pics of ur self-designed sharpie?

There are several here: http://www.scribd.com/doc/21456843/B...pie-Black-Swan

This will also give you some idea:


Even after adding stringers and a second layer of 1/4 inch ply to the bottom in the most stressed part, it's probably lighter than it would be with a 3/8 inch bottom, but weight is sometimes an asset in a monohull--the heavier bottom would make things happen a little slower when a gust hits, and make the boat more relaxing to sail. I went with lighter construction because I have a bad back, and would be pulling the boat up on a dinghy dock, so I knew about that compromise when I designed the boat. The problem is that I'm not okay with the amount of flex at the pivot point for pulling the weight of the boat onto the bunks on the dock.

[skaraborgcraft]

they may not be wood,but if you look at the displacement figures,and hence the scantling/layup of the old fibreglass catamarans liker the heavenly twins and the old catalacs,i think you would find them comparable to the same displacement monohull. The key point has already been made,monos carry ballast and so structurally need to be heavier built to take the loadings. Or put it another way,you could happily build a large mono,with skin-on-frame,exceedingly light scantlings, but you wouldnt want to hit anything with it,even an egg will survive an ocean storm....until it comes across something harder than its shell......

look at the original Piver trimarans and Wharrams and you will find scantlings used for the same displacement as some monos.

[John E Hardiman]

And I could totally be wrong here I dropped out of engineering school to pursue law, so I'm much better at arguing a point than at scientifically finding the truth ...

Stick to law, but to be honest I first thought this was a troll. The whole multi vs mono argument is never going to be solved, just like the "big end" or "little end" question.

The real reason that cats are built lighter is because they HAVE to be in order to support the non-buoyant crossdeck. And monos are built heavier is because they CAN be to save construction time and cost.

It's all about resource (time and money) management. For the same cost and half the time I can build 3 times the usable displacement in monohulls than in multihulls. True fact. The ability to use stiffer and larger panels is a reasource saver that offsets the heavier weight of the structure. All the other differences between the hull design are trivial and downright disingenuous. Everything that Peter listed:
stability
shallow draft
efficient hull form
positive buoyancy
sea kindliness
deck space
can be found in both types of hulls and every example can be matched by a counter example.

Go read my explinations in the following thread http://www.boatdesign.net/forums/mul...lls-13511.html Especially my post #70. A lot of opinions, a few facts, some engineering, and a couple of cat fights.
In in the end I am neither pro or con with respect to multi or mono; but there is NEVER a single best hull choice, only hulls that better fit the needs and requirements.

[Paul Pless]

at last - a professional responds

[peterchech]

Stick to law, but to be honest I first thought this was a troll. The whole multi vs mono argument is never going to be solved, just like the "big end" or "little end" question.

The real reason that cats are built lighter is because they HAVE to be in order to support the non-buoyant crossdeck. And monos are built heavier is because they CAN be to save construction time and cost.

It's all about resource (time and money) management. For the same cost and half the time I can build 3 times the usable displacement in monohulls than in multihulls. True fact. The ability to use stiffer and larger panels is a reasource saver that offsets the heavier weight of the structure. All the other differences between the hull design are trivial and downright disingenuous. Everything that Peter listed:
stability
shallow draft
efficient hull form
positive buoyancy
sea kindliness
deck space
can be found in both types of hulls and every example can be matched by a counter example.

Go read my explinations in the following thread http://www.boatdesign.net/forums/mul...lls-13511.html A lot of opinions, a few facts, some engineering, and a couple of cat fights.
In in the end I am neither pro or con with respect to multi or mono; but there is NEVER a single best hull choice, only hulls that better fit the needs and requirements.

Wow like the 8th time someone is trying to turn this into mono vs multi...

I didn't want to post this question on boatdesign.net because i was pretty sure the guys over there would turn it into a "catfight"...