Multihull Structure Thoughts

We looked at Zingaro in 2014. What was interesting about her was the bridgedeck was glassed on the inside but not on the outside, just paint. Not sure if anyone took the time to glass the underside plywood on the bridgedeck but it would have been important to me. After many years of sailing would it have been possible for the underside of the bridgedeck to become waterlogged then have the bridgedeck to hull tabbing break loose?

 

Smj1. If the plywood was not glassed outside it would very much depend on the quality and finish of the plywood as to its moisture resistance. If it was cheap plywood with no "West type" epoxy on it and just paint then moisture could effect the plywood weakening the joint. If it was good quality plywood with "West" epoxy and or good quality paints well applied then it likely to come down to a structural failure due to wave impact or a weak glass joint.

I have sailed on several cats and tris in rough conditions whilst racing. In fine reaching upwind, the impacts on the lee hull about 25% aft of the bow can be amazing as that hull is depressed lowering the wingdeck closer to the sea and the wave tops often impact the underwing. But when you are travelling at 16 knots plus and slam into the back of the next wave the boat can be slowed to 2 knots in a second. This is when people and boat structures really can be damaged. In all the above cases the actual build of the boat and its materials need to be understood to see if it was a design weakness or a build failure.

No designer I know can design a multi for every condition and sea state out there. They all walk a fine line between weight (read performance) and strength. Some boats are designed for bays others to cross oceans. But to quote Kurt Hughes when an owner of a large charter cat gunned his powerful engines while the rudders were hard over and bent the rudder shafts "I didn't see that one coming". I do not think any designer would have expected that to occur.

I have added a study which included an analysis of a Crowther racing design cat and wave impacts on the forward cross beam etc. Its 198 pages long but you can scan through it to get the general conclusions. Basically the faster you go in rougher seas the harder the slamming effects, the stronger the beam and forward wing has to be.

 

Gday Old Multi

Broken Arrow was abandoned and the crew taken off by the NZ navy. I don't think she was recovered.

One aspect of this style of beam connective is the narrow beam of the hulls and the tight radius of the underwing to hull joint. Also Lock did not continue the bottom flange of the forward bulkhead right through to the outer hull side. So there are three pretty big issues with the design there. That is before you get to construction.

A tight radius on a joint increases loading. Chamfer panels and trenches all increase the effective radius of the beam as it enters the hull, greatly reducing load in the area. Think of rounding corners to avoid stress concentrations.

Spindrifts are very narrow and tall hulls. This give a torque that rotates the hull less "staying base" at the beam connective with which to resist the torque. Less length means increased load for same torque. Modern hulls are wider at the gunwale and reduce load here.

Not carrying the bottom flange of the bulkheads across the hull is asking for the load to go somewhere but not giving it a pth to do so. So the load somehow makes its way into the hull skin. A better way is to carry the whole beam onto the outer gunwale or to locate a bulkhead here with a cutout and reinforce it.

Then there is the huge advance in materials. A modern cat uses unidirectional or muli axial fabrics to cater incredibly well with loads around corners. If I went back in time, I could do a good job, but it would be heavier than today and people didn't know about using glass tow and unis they way we do today.

I am sure Lock knew of these problems because he knew of them when Pennant had issues. He then drew boats that were very different, although none had chamfer panels, he used trenches at the side of the bridgedeck to deepen the beams at the connections on many cats. No Crowther cats I know of had similar beam issues and I think if I had a Spindrift I would do some work on it at the beam ends.

These boats were designed 50 years ago (1970ish) and I am sure Lock would say that he could have done a lot better just 10-12 years later, with the first Windspeed and Catana cats being drawn.

 

I might suggest anyone interested in this subject make a visit to this discussion as well......

Parachute Anchors, Para-Anchor, Sea Anchor
Parachute Anchors, Para-Anchor, Sea Anchor https://www.boatdesign.net/threads/parachute-anchors-para-anchor-sea-anchor.10448/

 

This vessel design peaked my interest a little bit. The aft placement of the mast was the initial draw, and the use of a 'cutter forerig',...all similar to my aftmast rig..



Of course I chose to make my aft sail a roller furling one, rather than a mast attached one. I could have chosen to erected my mast in a vertical manner and attached a aft mainsail as he did, but I was seeking to eliminate the traditional mainsail and substitute a roller furling sail.

I also noticed he chose a single central centerboard mounted in a central nacelle. I did so as well, but I also chose to make that central nacelle a 'backbone' strength member to assist in making the vessel stiffer fore-to-aft,...a weakness in catamarans.
Two different CB arrangements





MISC ITEMS of note:
There is a nacelle structure down the centerline of the vessel that acts as a bottom truss member, acts as a wave splitter, and provides a mounting for two asymmetric centerboards, thus eliminating any daggerboard or centerboard penetrations into the main hulls. And everything,…cables, bearings, boards are all above the load waterline…serviceable in remote areas.

Bridgedeck centreboard why don't they work?
https://www.boatdesign.net/threads/bridgedeck-centreboard-why-dont-they-work.57051/page-4#post-795051


I do NOT think his strengthened gunnels are going to do that good of a job,...you need an I-beam effect, not just a single beam member.

 

This tri is designed by Paolo Lodigiani and Matteo Costa in 2005. Th tri called Trillo and is 24.6 x 19.5 foot weighing 1680 lbs. The mast is 32 foot high with an approximate sail of 225 square foot main and 125 square foot fore triangle. The length to beam of the main hull is 8:1. According to the designers “Compared to a catamaran a trimaran is more agile, enjoyable to sail, evolutionary and fast with light winds.” The numbers indicate the tri will have reasonable performance.

TRILLO is a fast and reasonably comfortable day cruiser. The main objective in designing the boat was economy, ease of construction and simplicity of handling. Special attention was focussed on the tri’s retractable beams, with mounting, dismounting and transporting.

The designers proposed two versions, one with a round bottom and one with a chine hull. The difference in construction time and required skill by the builder is not very much, but according to the designers, there is not even much difference between the two versions as far as performance is concerned. The round bilge should be just a little better. The flat bottom hull version will offer a number of advantages to internal space.

The chine hull is flat plywood construction with stitch and glue taped chines and stringers on plywood bulkheads. The round bilge main hull uses strip plank construction with epoxy glass on both faces. Again, plywood bulkheads, deck etc. Crossbeams are aluminium and are telescopic sliding in tubes. It is according to the designers, the simplest and most functional approach for an amateur. The same logic applies for the centreboard, rudder and all other details. I do not know if any have been built but it looks interesting. Sorry about the lack of jpegs but little further information is available.

 

I don't mean to be disrespectful but I don't get the engineering of this mast set up. The rear longitudinal staying base is so small that the load on the backstay and therefore the compression loads on the mast would seem to be huge with just normal forestay loads. Normal multi stays are swept aft by 30 degrees which multiplies the vertical vector by 2 to hold the mast up. So on a square run, to stop the mast being pushed forward by 1000 N you need 2000 N of stay tension. (But there are two "backstays" so each only has to take half the load. For the mast to stay in position the components of the mast load (X and Y) must be equal and opposite. The forestays are much better aligned at pulling forwards (X component) but the backstay is much less effective in acting in the X component so it has to increase load to get the same torque or load. By going to a thinner staying base (as seen from the side ) the compression and staying loads must increase greatly. The longitudinal staying base looks like about 10 or 12 degrees. This means that to stop a load of 1000N pushing the mast forward on a square, the stays have to be tensioned to 5000N (2500N each) (12 degrees) with a corresponding increase in mast compression. This is a lot more load than the normal type rig. Have there been any rigs like the one drawn that have been successfully sailed?

 

@ catsketcher
perhaps look thru this rigging force analysis....
Aftmast rigs??? https://www.boatdesign.net/threads/aftmast-rigs.623/page-23#post-414479



I have yet had anyone find fault with this vector analysis.

 

Gday Brian

I read through some of the thread and saw that there was a tri that used such a rig. Did it go well? I don't follow your rig analysis. It doesn't look the same as what I am used to in books like Skene's. It doesn't seem to deal with torque.

My worry about the force on the rig comes from the substantially differing angles of the forestay and backstay. Both stays have to exert the same force fore and aft, or the masthead would move. The forestay has an angle of 33 degrees to the masthead. So the forestay exerts 53% of its force into a horizontal vector or X component. The backstays have an angle of (if I get the angles right ) about 8 degrees to the top of the mast (The torque has to be exerted at the top of the mast otherwise it will move so even thought the stays attach to the mast lower down the effective staying base is about 8 degrees). At 8 degrees the backstays have an effective X component of 0.14. So you need to increase the load by 7 times to get a set X vector. To get 1000N pulling forward you need just under 2000N in the forestay. To cater for this load in the backstay you need 7000N in the backstay and a sizable increase in mast compression. As the forestays are long and have two jibs on them there will be substantial torque on the rig pulling the masthead forward. This has to be opposed by the backstay or the masthead moves. There is no mainsheet tension to assist with the forestay tension. Jumpers and extra stays do not help with resisting the torque from the forestays, dropping the backstays down the mast increases staying angle, which lowers load, but reduces effective torque arm, which increases load, so there is no gain there.

It is a bit like a monohull owner trying to reduce sidestay loading by adding large spreaders to the rig. It won't work because the sideways (heeling) torque is resisted by an equal torque at the deck. You can increase the spreader length if you want but as the only torque that resists the rig torque is produced at the deck, it is the distance between the mast and chainplate that matters. The same occurs fore and aft as well - increase the lever arm to get less tension. Torque = Force x distance. If you reduce distance then the force goes up, you can't get around it and that is why multis have such wide staying bases - to reduce load in the rig wires. Reducing staying base in any orientation increases load.

A typical rig has extra sweep back on the backstays than this drawing and a more symmetrical arrangement of forces from forestays and backstays. Mainsheet tension helps deliver a tight forestay and a lower forestay attachment, which you get with a fractional rig, reduces the torque from the forestay.

It would be pretty easy to make a simple mockup of the rig with spring balances and check out the fore and aft loads. Then we would not be debating, just observing data. Then again, this is probably not the thread for a detailed consideration of this idea but I would be interested in the mockup.

cheers

Phil

 

Lets talk about rigs for multihulls. They can vary from Asian based free standing mast junk rigs to full rigid wing masts. Each can function on a multihull with varying degrees of efficiency, ease of handling and cost. Now we will focus on the 3 components as separate items in a “generic” way. I will focus on each specific rig type in later posts.

Component 1. Efficiency. Tall and thin generally is more efficient for any rig. A rigid multi element wing sail generally has the highest lift and the lowest drag. The down side of this model is the need to constantly trim the sail and the difficulty of reducing (reef) the sail area. There are reefable soft sail wings available (one example has been developed by VLPL

Next on the list, is a solid wing mast ahead of a fully battened fat headed mainsail. This is used mainly on EG day cats. They need constant attention to get the highest efficiency but you can reef them if required.

Next comes the soft wing mast. Either a rigid frame shaped leading edge with soft sail around a rigid often free standing mast. The aft end of the sail is a single thickness. The entire rig can achieve high efficiency if the sail can have a lot of halyard tension. A variation of this model is the Wharram wing sail rig. Again in all cases they are sensitive to trim to get the best efficiency.

In each of the above cases the shape of the leading edge of the wing is very important. A sharp leading edge means constantly adjusting the wing angle to get the optimum angle of attack for peak efficiency. This really means it needs to be adjusted for each speed and shift variation of wind speed. A more rounded leading edge means the wing requires less adjustment but is theoretically slightly less efficient. In reality a more rounded leading edge ends up being more efficient overall as humans cannot keep up with the minor wind variations.

The next problem is soft or reefable wing sails fabric can be “round” in light to moderate airs but often deform in heavier airs unless halyards are pulled really tight.

In all cases a small jib in front of a wing of any variety can help smooth the airflow onto the leading edge of the wing. This allows the wing to have a steadier airflow onto the wing which means it needs less adjustment to keep at full power.

What does this all mean. A rigid wing can produce twice the power of a fixed mast fractional rig BUT it must be adjusted continuously to gain that advantage. It is futile to have a wing mast and have it at an inappropriate angle to the wind. The drag increase negates any advantage of the wing mast.

This is the reason that fractional fixed mast rigs can perform as well as wing mast rigs in offshore sailing in eg cruising mode.

The real need for any rig is adjustability. Are you capable of adjusting its draft of the sail, reducing or adding sail area as required? Can you eg move the tack of your foresail, move a boom out to 90 degrees. Do you have the right fabric weight for the wind conditions etc.

This is getting to long and we are only half way through efficiency. More tomorrow.

 

that Trimaran experiment.

I'm just going to address in this posting that trimaran you mentioned.

I will make a copy of your additional comments so I can study them in more detail, and give you a reply.


Trimaran situation doucumented back on page 3 of that Aftmast subject thread...
Aftmast rigs??? https://www.boatdesign.net/threads/aftmast-rigs.623/page-3
Mast Aft Trimaran Failure?

Brian said:

Did you find that in his logs? I've not read them as there appears to be almost no mention of the sailing capabilities of the vessel.


Guillermo said:
Yes, he says that somewhere, but it's difficult to find among the very long logs. The general feeling I got is that the boat as a whole was a complete disaster (And I have my doubts about the mental sanity of the man....)
As I wrote you in a private email, I also questioned the sanity of this gentleman, but that’s all another subject.


When I was first made aware of his original website and his rig calculator, I was very surprised at the exact wording on his site that copied the wording right out of my website verbatim. Here is a gentleman who never once had contacted me about this subject he had such a great interest in, nor ask any permission of me, nor given any references on his site back to mine. I wrote him a letter about these matters, which he chose to ignore answering, and instead took down some of his web pages from his site.

In that letter I also expressed some concerns about his interpretation of my design. I was particularly concerned about his lack of a resistance element to the forward force that will be generated by the upper aft facing jumper strut over which the masthead backstay passes. In contrast I have provided a triangulated (diamond) jumper strut arrangement to help counter this forward pressing force. I have also provided two additional backstays at this point to counter forestay loads.

…photo of vessel under discussion: http://www.boatdesign.net/gallery/showphoto.php/photo/6863/cat/500/ppuser/399

In other words I was concerned about his variation of my design. But since he had chose not to communicate with me about my concerns, and since he had already built it, there was not much left to do but wish him the best. Maybe his mast section would stand up to the loads, but I seriously doubted it. I was also concerned about his disproportionably long, lower mast panel.

Another of my concerns was his use of three headsails. To my way of thinking this placed the headsails too close together to be practical. We know from past cutter arrangements that should we have the headsail and the staysail too close to one another it became difficult to maintain the trim on both such that they were always fully engaged. Plus, it became more problematic in a tacking situation. And he obviously had not sailed enough to recognize the balance that can be had with a jib-mizzen combination.

Your reference to the rig being replaced with a conventional one was rather brief, leaving the impression that this mast aft experiment was a total failure.
Guillermo said:
quotes: The guy wrecked close to Malaga…..He rerigged later with a conventional bermudan rig……Be careful about the trust ability of that calculator. The guy dismasted….The general feeling I got is that the boat as a whole was a complete disaster
I don’t think we can assume this just yet with the VERY limited information we have. All I see so far is a failure of a piece of ordinary rigging, “a Norseman backstay fitting failed, crystallized in the threads. That could happen with any rig. Certainly it appears to be a backstay component, and if he had only a single backstay with this 3-headsail arrangement that would be a real problem. But lets not condemn the mast aft concept in general based upon this flawed design experiment.

And finally I might even have to give some credit to this fellow. At least he went out there AND DID IT, more than can be said for many dreamers.

regards, Brian

PS:
I don't know exactly where his Norseman fitting was located in his rigging plan. If he had only one single backstay on his rig that was clearly not enough. And I am unsure as to what size rigging wire and fitting were utilized. I have a feeling that his fitting was faulty, and/or his installation of it. The reason I say this is in reading his log I can find nothing really heavy or extreme weather-wise that should have brought the rig down unless it was greatly underdesigned by his own design calculations.

 

I've forgotten exactly how various books presented it, but I went back to good old vector analysis. ie...

Use of Scaled Vector Diagrams to Determine a Resultant
Vector Addition https://www.physicsclassroom.com/class/vectors/u3l1b

Net Force Problems Revisited https://www.physicsclassroom.com/class/vectors/u3l3d

Brian replied: Okay I think we are starting off from the wrong perspective already. You seem to be viewing the image from a different perspective than myself. Who says true vertical can only be viewed with respect to perpendicular to water level? I am choosing to view my 'vertical component' as direct in-line component with mast tube itself,....and then my 'horizontal component' as perpendicular to that vertical. Now look at my masthead a little closer,..




My forestay and my backstay are both very close to one another in their angles of force. They might be made to be 33 degrees each, or as I have chosen here the forestay is 33 degrees and the backstay is 25 degrees.
If I choose an arbitrary force amount for my forestay (length of the vector) I need to match that force equally in the opposite direction,...shown in that diagram. The masthead is in equilibrium so we need directly opposing forces.

Now you see the difference that each of those vectors transmit in the perpendicular direction (the perpendicular direction in this analysis is directly in-line with the mast,...mast compression force). Yes we have added a little extra compression force to the mast tube itself, but not that much extra. Its as though we had a longer boat with a backstay located further aft. And we have added a rearward force to the base of the mast that the boat's structure has to account for.

Can we agree thus far?

 

I believe you are correct in switching the discussion back to the original discussion of Aftmast rig. I copied my latest reply to you over there.

 

We mainly focused on mainsails yesterday. Today we will discuss the efficiency of headsails. Rule 101 is either have a tight forestay or a very good sailmaker. Why? Multihulls have a problem, they bend in all sorts of ways. When a cat or a tri bends fore and aft the forestays go slack, putting more draft (fullness) into a headsail and altering aerodynamic shape by shifting the point of maximum draft. This means less forward drive, more side (overturning) moment and more drag. In short you go slower!!

In many situations (mainly light airs) fullness in a headsail is ok. But as wind strengths increase, you want your headsails to hold a constant aerofoil shape or flatten slightly to provide more drive, minimize the overturning moment and minimize drag.

Now we enter the world of racing yacht headsails. Either a racing yacht has many headsails for each wind range that suits the characteristics of the yacht. The boat may have an adjustable rig that can alter the tension on the forestay, this allows a reduced number of headsails as you can flatten or increase fullness in a headsail by allowing the forestay to be taut or slack. Rig tension can be altered by several techniques. Hydraulics or manual rig tensioners on the rear stays, runners or as in some racing yachts have a hydraulic ram under the mast base that pushes the mast up. I know in a 28 foot monohull racer, the forestay sag can be controlled from 70 to 400 mm depending on the rig tensioner hydraulic pressure.

Now we come to the problem of bendable multi’s. Multihulls bend in crossarms, hulls fore and aft etc. Even if you put hydraulic rams on backstays to tension forestays you can only go so far before the bending forces take over and things start to break. Monohulls generally have better keel line structures and broader continuous deck structure and can be loaded up by hydraulics more, therefore have more rigid forestays. Multihulls generally have lighter structures and cannot take the same rig/forestay tension loading.

The reason for the very good sailmaker is if a forestay sags (slack) increasing fullness in higher winds, sailmakers can adjust the cut of the sail to compensate. The problem is the sailmaker either needs to know the characteristics of the specific boat well or they have to use their experience of similar craft to adjust the headsail cut to suit the forestay sag (slackness).

Translation of all this, the longer the forestay the more sag (less taut) the forestay will be no matter how much tension the rig has in it. High load rigs with big headsails and small mainsails suffer the most. EG it can be a Prout cruising cat or Crowthers 60 foot Shotover racing cat. They all suffer the same problem in stronger winds especially upwind they do not perform as well as big main smaller headsail rigs. (Yes, Prouts have inefficient keels, high windage hulls and low underwings compared to Shotover daggerboards, but I can assure you Shotover hulls were fast, its rig was its relative weakness). Headstay sag can vary from inches to feet depending on the length of the forestay and the relative rigidity of the multi.

Now we get to roller furling headsails. This requires either multiple roller furlers in a cutter rig or a brilliant sailmaker to get any form of efficiency out of a partially furled headsail. An average furling headsail works well at full size and declines in efficiency the more its reefed. The 2 problems: first problem is the cloth weight to handle higher winds mean a full size headsail has very heavy cloth for light airs and the second problem is when the sail is partially furled its shape is compromised. How badly its compromised depends on the sailmaker and the type of furling equipment used. Irrespective of the furling equipment used you must carry a separate storm sail that can be rigged independently for the problem times.

Finally, the type of sail cloth also effects efficiency. Dacron’s major virtue is its longevity but it stretches and distorts over time. Hybrid cloths hold there shape better but when they fail, they often fail badly in a shorter time. Now you have the dilemma, do you go for high performance foresails made from Kevlar, Carbon etc then use hydraulics to maintain a tight forestay and build a multi that is longitudinally stiff to power the rig up. Or do you do what the majority do and accept lower rig efficiency using Dacron type cloths and a pretty fixed rig depending on good hull shapes and foils to get reasonable performance. More to come.

 

You are using the same old tried arguments for big full battened mainsails with little fractional rig jibs. It this rig was the 'saving grace', why don't we see it more universally utilized on monohulls?

Go out an observe a lot of cruisers out there. Ask them why they are not raising those mainsails so often. If they have even a half way decent headsail, ask them what sails they prefer to use out crusing.

The subject of backstays has come up recently. Have you taken notice that we don't have backstays on our fractional rigged multihulls. Rather we have 2 shrouds that are set partially to rearward beam of the vessel, that are trying to act like shronds and backstays both,...and they are so shallow with respect to the mast that they are likely to have a poor capability to maintain any forestay tension,...and only one of them works at a time.

BTW I was NOT promoting the use of partially furled sails, but rather I sought to break the vessels total sail are into 3 sails, that could be used in a variety combinations like a good ketch can.,....full sail with all 3, split rig under jib-mizzen combination, or just under mainstaysail alone. I refer to my rig as a single-masted-ketch.