folding multihull hull design

[grob]

I am building a folding multihull (see attachment or www.fourhulls.com), to go on the roof of my car, not having much experience at hull design I have chosen to use a NACA 65012 profile with semi circular tips (bottom and top).

What do people think of my choice, I am hoping that being an aerofoil section it will give low drag and some element of lift so I will not need too much of a keel.

Is this sensible?

[tspeer]

I see several problems. First, the folding mechanism. The compression loads on the mast will make your folding mechanism collapse as soon as the wind hits the sail. You've put the greatest loads on the weakest part of the mechanism - the latches or whatever you plan to use to keep it unfolded - while not making good use of the strongest part. It would make a lot more sense to have the hulls fold bow down with the mast step coming up. But even then, I have my doubts as to the wisdom of putting the highest loads on the folding joints. Even with a solid beam, it's not easy to support the downward force of the mast.

Second is the wave drag from the short hulls and the added wetted area from using several deeply submerged hulls. These hulls will have more drag at both high and low speed than a conventional catamaran. See http://www.maths.adelaide.edu.au/Applied/llazausk/software/michlet/michlet.htm for a program you can use to calculate the drag of the hulls, including their mutual interference.

Third, lift from the hulls is not a good idea. The total lift on the hulls and foils will be the same regardless of the source, because the lift has to match the loads applied by the rig. But there's "expensive" lift and "cheap" lift. The longer and deeper the lift generator, the less drag it incurs due to the lift. Because the hull is shallow, lift from the hull is accompanied by lots of drag - it's expensive in drag terms. So all you're doing with the lifting hulls is trading low-drag lift for high-drag lift. That's not something that's going to contribute to performance.

Cheers,

[MDV]

I also see a problem with the steering due to the weight of the "rudders" and the location of the Centre of Lift with respect to the tiller connection and the rest of the boat, in particular the force that you will have to apply for steering.

Also imagine that your are flying one hull, and you start to plane or surfing down waves, you will have basically only one hull or just the aft hulls in the water which you will reply on for steerage.

The small costs of fitting efficient rudders is insignificant compared to the benefits, and will give you the freedom to design the hulls for minimum resistance.

For the mast, a stayless rig may solve the problems highlighted in the previous post, with the added benefit of simpler rigging. You can put struts from the mast (above the mast step) to the cross beams to provide the locking mechanism against folding.

Regards,

Michael

[grob]

Thanks for your comments,

I have to agree with everything said, In some defense the aim of this boat is to make a folding sailing platfom that will go on the roof of a car, in doing so I have to sacrifice a lot of good practice.

In order for the folding mechanism to work I have to have four hulls and an X shaped cross bar. The rest of the design is a compromise around these two requirements.

As I have to have the cross beams I decided to make use of them by placing the mast load on them, to answer Toms first point I have included a plot of my hinge design, which shows that when unfolded a sleeve carries all the load not the joint.

Can the michelet program calculate wave cancellation effects? and does it use a 3D model of the hull?

As for using the hulls for lift I agree its not the most efficient method, my overriding requirement here is for sailing off of my local beach which has a big shorebreak and lots of stones. I therefore don't like dagger boards. Using hull lift works well on my Hobie 16.

I have my concerns about the stering too, in particuar with regard to the position of the centre of lateral resistance and I plan to compare to try the boat with conventional rudders too.

I am interested in the stayless rig, However I initially dismissed this becuase of the twisting loads it puts into the frame, I am much happier dealing with the compression loads, being a mechanical/structural engineer by proffesion, torsion scares me.

My original qustion was on my use of an aerofoil section for the hull shape, other people have voiced concerns that the fairly blunt end and relatively thick thickness ratio is not a good way to go. Although it seems to be the best shape in air or water I have heard it is not good for surface effects. Is this true or could the Michlet program tell me this.

All the best

Gareth

[grob]

Here is the attachment I forgot

[BrettM]

Have you tried to match the hull "pods" to form a hull shape similar to that of a conventional cat eg nacra etc.

I think that the blunt leading edge of the suggested hull form may cause the boat to be very wet at speed and may also give rise to some directional instability.

Looking at the centre connection shown it seems to rely on "sleaves" to lock each hinged joint. This looks weak to me. I would consider the use of a removable locking device to span across the entire joint and then some.

Keep thinking.
Brett

[tspeer]

Originally posted by grob
...
As I have to have the cross beams I decided to make use of them by placing the mast load on them, to answer Toms first point I have included a plot of my hinge design, which shows that when unfolded a sleeve carries all the load not the joint.


Have you calculated the bending loads those sleeves will have to take? Look at the size of the beam under the mast of a comparably sized beach cat, and consider that many of them are arched in addition to taking the loads in bending. It really does need all of that strength. And you are going to have to react that bending moment across just the arm from the center of the hinge to the edge of the sleeve. Finally, don't forget the dynamic loads - they can be signficantly higher than the static loads.


Can the michelet program calculate wave cancellation effects? and does it use a 3D model of the hull?


Yes. Yes. But it doesn't handle hydrodynamic lift or induced drag.

... the fairly blunt end and relatively thick thickness ratio is not a good way to go. Although it seems to be the best shape in air or water I have heard it is not good for surface effects. Is this true or could the Michlet program tell me this.

Yes, this is well within Michlet's capabilities.

Cheers,

[Peter_T]

It is a fastastic idea.!!! Certainly can be explored.

I worked on tug/barge connections and have also deal with articulated connection of barge to barge. The forces acting at the connections are very large. We even started with a rigid tug to barge connections which costed the most but have no relative movements. The articulated connection later took over the market, ideal for less severe sea conditions. At that time people may think that was not real. But it was possible to engineer to work.

The joints in your folding rig for catamaran pleasure boat for family car transport, will have to spring or shock mounted. Means to disengage the joint restraint have to be provided. Most likely hand hydraulic system can be used. Make it to hold just two people may be a good start.

On the other hand to simply the issue, why not take the pontoons separate and have the frames foldable for mounting. There is a plastic pontoon in the market by "Wilson" http://www.plasticpontoon.com/

[Tim B]

I like the general idea, though like most people who have replied, I would be very wary of having two hulls on either side. Could you set the hull up, so that you could bolt it together at say, the centerboard case (assuming a centerboard is included), with a bulkhead on either side of the joint, and some (carbon?) joiners across the joint. That would allow you to 'fold' the hulls. As for the central joiners, they needn't be that large if they were one-piece, but if they were bolted on to the hull (to a bulkhead extension) then you could quite effectively 'fold' (well, dismantle) the entire yacht. I may point out though, that while any of the ideas posted would fit on a car roof, remember that the mast is invariably the biggest problem. A two-piece mast may well be an answer to that. Other-wise, there's no point in folding the hulls because you don't gain anything. It just gets heavier, more complex, and there's more chance of water ingress. If I were you, I'd KEEP IT SIMPLE!!!

Good Luck and Cheers,

Tim B.

[grob]

Tim,

You and others have raised some concerns about the four hull setup of my design. Is it better to have two hulls or four hulls, actually to quote blackadder “opinion is divided on the subject, All the other captains say it is, I say it isn’t”

I will try and explain my thinking. The four hull design was originally developed for one good reason:-

a lack of cash – The cost of a mould for a 5m boat is expensive so I designed a 2.2m symmetrical mold.

As many people have pointed out having extra hulls creates additional bow and stern waves which, it is believed, will slow the boat down.

However a monohull is not generally slower than a trimaran, it is the hull shape that is more important than the number of hulls. Cats and Tri’s are faster as they have an efficient hull shape compared to a monohull because they do not have to worry about lateral stability. I have taken the concept one stage further. By spacing out my hulls laterally and longitudinally I don’t have to worry about longitudinal stability either.

I am not an aerodynamicist I am an engineer but one thing I remember from my lectures is that for low drag you want a round front and a sharp back and that is not what I see on multihulls, they have sharp fronts and blunt backs. That is why my hulls are aerofoil shaped.

Others have also said that four hulls will have a larger wetted surface area (WSA) . In fact the opposite is true, four short fat hulls have less WSA than two long thin ones. In the case of a 5m boat like mine its about 15% less. In fact no matter how long the boat the WSA remains the same with a four hulled design and so the advantage gets bigger as the boat gets longer.

Of course this could all be bunkam, but I have put my money where my mouth is (as you can see from the build pictures on my website) and I will have some answers by the end of the summer.

I appreciate all the feedback I have been getting and thanks in part to comments on this and other sites I have ditched the rear hull steering and the idea of putting the mast on the folding mechanism and also have a number of ideas to try to stop the boat folding up while at sea!!


All the best

Gareth

[Tim B]

Might I just voice one concern...

I have not heard of much work done on multihulls with one behind the other. Your comment on drag reduction is valid for an aerofoil flying through a fluid, the surface creates bow/stern waves which should be minimised (now going outside my experience), so generally it is a reduction in wave drag that gives a large increase in speed.

As far as the moulding goes, I'd have expected two 5m mouldings to be of a similar price to four 2.2m mouldings. I may be wrong though. I'll be very interested to see what happens later this summer.

Cheers,

Tim B.

[grob]

Sorry Tim I wasn't clear enough, it was not the cost of the hulls that I was reducing but rather the cost of the mould or more accurately the plug that is used to make the mould.


Making a plug and mould is a costly and highly skilled process you only need to make one per boat and if that mould is small and symmetrical then it can be used to create both sides of the hull (left and right or top and bottom).

[Tim B]

I see. Sorry. It is obviously a much more cost efficient method, but it will limit you to something that is symetrical in several axes, thus not really giving as much control over hull shape as you would like. Give it a try anyway, and do keep us updated.

Cheers,

Tim B.

[terhohalme]

What if you modify the shape of the hulls so, that the same side hulls meet each other with the streight edge. Then the two hulls act like one hull (hydrodynamically more sensible) and you still have the advantage of cheap small mould.

[grob]

What if you modify the shape of the hulls so, that the same side hulls meet each other with the streight edge

Thanks, that is allready covered in my patent, but you would need the hulls to be symmetrical front and rear as well as top and bottom. Probably not a big problem, especially for a proa.

[gonzo]

Are the foil section symetrical? If they produce lift in opposite directions the resulting forces cancel out. However, they produce a lot of drag. Another thing to consider, is that wave resistance is not addessed by NACA foils. A hull is in two mediums, which creates a completely different environment. NACA foils, at anything but the slowest speeds create a huge wave resistance. Also, they handle poorly. As you correctly pointed they are aerofoils, not hull forms. A solution to the folding system is to have forward and rear sections that end in a flat. They can be joined to create a longer waterline. This gives a higher hull speed potential. The four sections can be identical. Another advantage is that there can be locks between the forward and aft sections so it takes a lot of the stresses from the pipe frame.

[grob]

NACA foils, at anything but the slowest speeds create a huge wave resistance. Also, they handle poorly

Is this opinion or do you have some evidence for this statement, most surface piercing rudders are NACA profiles at the waterline, if they are slow, create large wave drag and handle poorly why are they used?

I have been looking for any research on wave drag and hull (or rudder/foil) shapes, and have found nothing other than those that use simple ratios, that seem to tell us that hulls go just as quick going backwards or forwards, which to me is absurd.

[gonzo]

A rudder has a small area in the water/air boundary compared with a hull. Therefore the wave resistance is negligible. A hull is coming in and out of the water continuosly so it's submerged section changes. Also, the boundary area of the rudders is in the water exit from the hull. It is mainly eddies, which changes the conditions. Another problem that NACA foils don't adress is cavitation. They are designed as airfoils. At high speeds water behaves almost as a solid forward and a gas aft.

[grob]

Sorry to press you Gonzo, but I am looking for some information on what creates wave drag, How do you know about wave drag on NACA foils?

[gonzo]

Any object moving through the water surface creates waves. The length of these waves is proportional to the speed. The other aspect of wave forming to consider is wave shape. A blunt form will create a huge bow wave. At very low speeds it is efficient. Think of a supertanker doing fifteen knots, one third of it hull speed. Compare it to a thirty foot sailboat at seven knots. The first has water flowing along the sides and the stern with several small waves forming. The second has only one wave between the bow and the stern with a big through in the middle. NACA foils were not designed for this surface turbulence. Also this description is only for flat water. Enter the sea waves: the vessel stars pitching, rolling and yawing. The hull shape is continuosly changing. The NACA foils only work well if the water flow starts at the blunt edge and leaves cleanly at the sharp edge. Proper hull designs account for the changing conditions. In other words, they are a compromise that works well all around.