To CANT or not to CANT, that is the question...

Anyone:
Which popular production cats have canted hulls?

Cat designers:
Theory: Why cant the hulls?
How much? Benefits? Drawbacks?
Conditions where canted hulls will help you? Hurt you?
Effect on upwind vs downwind performance?
Effect on tacking/gybing?
Impact of differnt hull shapes on canting - V vs rounded vs flat?
If the hulls and CB's are canted, are the rudders canted as well? What a drag?
Bottom line: is it advantageous and does it "work"?

Cat sailors/racers:
When you sail/race cats with canted hulls vs non-canted do you actually notice a difference? Which do you prefer and why?

I wish I could answer all these questions, but I CANT!!!

Jerry

I think hulls are canted so that when the boat has a slight heel, the leeward hull will be more perpendicular to the water. This allows the underbody (hull for asymmetric hulls or boards and rudders for symm) to generate more "bite" in the water and more lateral resistance.

I don't know if there are drag penalties or bonuses here.

D. Ho
TheMightyHobie18 1067

Wasn't it the Capricorn F18 that had the reverse canted daggerboards (they pointed toward each other at their tips). F18 quickly made a rule change to disallow such designs. The theory that I recall was that they help produce a vertical lift component and also helped lift the boat to windward more...come to think of it the ORMA 60 tris all have these complicated curved daggerboards in the amas that achieve the same inward tilt - I understand that those are necessary because the AMAs do not have enough volume to support the boat alone and the foil is required to keep the ama above the water.

I think all this means that you have to tack your boards now.

Most new A-cats have canted hulls. That angle is calculated so the hull, rudder and centerbord is perpendicular to the water when sailed with the weather hull just above water.

The hull should be used to keep the boat afloat and not act as centerboard. If you use asymetric centerboards or canting centerboards that "tacks in the cb trunk" you can get the hull to travel straight through the water (lower drag).

Using the centerboard to lift the boat out of the water might work in some high speed conditions. But if the drag increase from the centerboard that lifts the hull is higher then the drag reduction from the hull, you loose. So far no A-cats have angled the centerboard in any extreme angle to achive this effect.

Sailing with both hulls in the water is a bad thing for boats with canted hulls.

/hakan

Centerboards that change their angle with respect to the hull centerline are called gybing centerboards. I think 505s have them and some older I-14s had them (do they still?). The action is automatic - the back edge of the board is fixed while the front is allowed to move slightly(clunk!).

The important question is: does using a gybing centerboard improve the lift to drag ratio of the hull and appendages as a system? (not always)


Inward canted boards:

Inward canted boards can begin to work at speeds where the lift to drag ratio of displacement hulls starts to worsen. They work better when the side load on the board is a high fraction of the weight of the boat (hence their use on ORMA tris). They work by reducing the wetted surface of the leeward hull through changes in trim and draft, which reduces viscous resistance.

Boats with a lower side load for their weight may see no advantage from canted boards. A-cats are narrow for their hull length and carry a rig with a high center of effort - so might not be good candidates for canted boards. A wide cat with sail area not limited by a rule is more likely to benefit.

Canted Hulls:

Canted hulls allow the hull to be more upright in the water when the boat is heeled. This results in a slight reduction in wetted surface - which helps reduce viscous drag.

If the hull has slight bow tumblehome or vertical sides the sheet of water on the hull (feather) can remain supercritical (WRT shallow water waves in the spray sheet) for a longer distance from the bow when the hull is not heeled. This reduces the spray drag since the hydraulic jump occurs on a portion of the hull skin that is inclined less forward. Spray drag is not so important for ships, but it matters for small high speed craft.

If the centerboard is kept vertical it's lift to drag ratio will be highest. At lower speed, and for boats that do not have a very high side load for their weight vertical boards are more efficient.

-colin pitts

Thanks, Colin and other posters for your great replies.

Someone wise once said: "As in many things in life, the questions remain the same, only the answers change"... just as another wise(alec)once said "If only it were that simple"...

What's interesting to me is that so often, there is a "theory" behind introducting a new technology (ie, in the case of canted hulls the hydrodynamics should become more efficient when heeled) but in practice this might not play out for many reasons (ie, the effect of canting on performance might be influenced by so many other factors including different hull shapes, the point of sail, different wind, wave, and speed conditions, the presence of gusts/lulls making optimum heeling angle impossible to maintain, or need to sail with both hulls in the water due to insufficient wind negating the benefits on average).

So it's interesting to consider many of these issues (and more) from the "theory says" perspective, but also step back and ask those who've tried to actually compare whether in fact there are real gains made on average. I'd also be interested in knowing if these changes in hull geometry only impact performance or if they also introduce unintended side effects on boat handling (ie, helm, difficulty tacking, etc).

Also, I didn't see anyone respond to the question of what you do with the rudders when you cant the hulls? Do you cant them also? If you do, don't they act inefficiently non-heeled? If you don't cant them, don't they act ineffiently when heeled? And of course, when you don't cant the hulls at all, does this also mean that the (uncanted) rudders have some inefficiency when heeled?

If you CANT stand all these questions, you CANT possibly be interested in learning to sail better or intelligently pick your next boat...

Jerry

Hello Jerry,

Some boats designed to sail at a constant heel angle do have their rudders inclined to be perpendicular to the water surface. A-Scows and E-Scows have their bilge boards and rudders inclined this way.

A good place to look for confirmation of the performance of ideas like canted hulls, daggerboards and rudders in practice are the least restricted development classes.

Talk to you later
-colin

Jerry-
Jim Boyer, co-designer or the Taipans and Auscats, told me he felt the biggest advantage of the canted hulls was actually the canted rudders, which offered more control when "wildthinging" an A cat downwind.

Kirt

Cant (kant). Deffinitions please.

Tip, Lean, Slant, Tilt? Toward which direction? (Lee, Weather, Forward, Backward, Fore, Aft, Lee, Weather, Side, Up, Down, etc. ???)

Colin and Kirt,

Thanks guys, interesting. Part of the reason for this post is that canted hulls is often cited as a selling point for cats that have them and cited as a "limitation" for the competition. Sounds good in principle. But in reality who knows? I can't imaging that if you optimize things for the ideal heeled state that "everything else" doesn't suffer as a result. Slower tacking and two canted rudders in the water when not flying a hull don't necessarily sound so hot. I'm looking for someone to report "it really makes a difference and the downside is minimal/nonexistant" or "it helps a little but there are drawbacks".

Is this another marketing ploy like wave piercing hulls?

You CANT ignore this issue much longer...

Jerry

Thanks, Hakan. Interesting.
By the way, what are assymetric centerboards and why do they get the hull to track straight?

Jerry

Jerry

We cant the hulls on our products, we originally didn't but found that in choppy conditions, water would climb up the outside of the lee hull when flying a hull quite high, this would catch on the outside of the rear beam and you could feel the boat judderring over the waves. We canted the hulls and it stopped, we could tell no difference in the behaviour of the boat in any other way, so we ran with it for this gain only.

I will try to answer your points

How much cant: 4 degrees did it for us

Benefits see above, the gain in choppy conditions is certainly no more than 1 %

Drawbacks no measureable difference in tacking, boat feel, two hull in the water sailing, only drawback is it costs more to build.

conditions for benifit, upwind choppy conditions only

no drawbacks

effect on tacking/gybing nil

Impact of different hull shapes: I can only comment on ours,

Cant rudders as well? we fried it both ways and could not tell the difference.

Is it advantageous? well we found that in one type of condition it is for us, and with no downside it is worth it.

Boyer produced gave a statement about canted boards a few years ago along the following lines :

Mk 3 A-cat and Mk 4 a-cat are identical in all but canted hulls and canted boards. The speed of both is identical but in the midrange of windspeeds the mk4 has a slightly better steering characteristic.

In other ranges the mk 4 is fully identical to the mk3 and in some cases the steering and handling is slightly inferieur.

Wouter

john, how or why is it more expensive to build canted hulls?

John,

Thanks for the great reply. It's so helpful to get that kind of insight into the design process, so clearly stated, particularly from the designer himself.

From your experience it sounds like there could be no advantage to canting on hulls with more freeboard since the rear beams may clear just as easily without it?

Interestingly, I could see how your experience may NOT apply to other hull shapes than yours (which is why I originally included hull shape in the question). I notice (if I am seeing straight!) that your hulls have more roundness to them, particularly on the sides, than many other designs. Since the transition between the bottom and sides is so gradual I would think your hull's characteristics (when both hulls are in the water) would be simialr whether canted or not, hence no downsides regarding drag, tacking, etc. One could imagine if a hull had a sharper transition between bottom and sides that there potentiall might be some different behavior or even downside to canting in some conditions (a more suble example perhaps of deeper v hulls such as H14/16).

On a related note, even without canting I would think that a sharper trasition between bottom and sides could potentially introduce some subtle unpredicatble behavior when flying a hull as the heel angle varied. Anyone actually observe this?

Jerry

Thanks, Wouter

Jerry

Jerry

This isn't a free board issue with our boats, although I grant you it may be down to the shape of our hulls, we found that the water was riding up the outside of the hulls, we have pronounced shoulders in the beam tray area, it was these shoulders that the water was interfering with, canting the hulls didnt raise the free board on this side of the hull, it just stopped the water flowing up the hull.

As to other effects, I think you need to bear in mind that a catamaran hull spends hardly any time with less than 4 degrees of lean in any conditions, so they are very rarely travelling on a level keel through the water.

I think this is one of those areas where there are so many variables that it is very dificult to theorise what is best, the only way to tell is to try it, then try some other angles and see what works best in a variety of conditions for the hulls in question.

As to why it costs more, you either need 2 different moulds to produce the hulls or you need an extra step when fitting out the boat.

You guys have missed the important part of the hydrodynamic equasion.

It looks cool and gives us something to bs about while drinking a cold one after a race.

Thanks, John.

>> I think this is one of those areas where there are so many variables that it is very dificult to theorise what is best, the only way to tell is to try it, then try some other angles and see what works best in a variety of conditions for the hulls in question.

And that's why you CANT blame me for asking you designers these questions Thanks!


Jerry

One comment I would make on the canting plate issue is there can be a small disadvantage when close to marks of the course.

I sail a Euro Inter 17 in the UK and when sailing with these funny boats with only one hull they need to give you a few more inches room at the mark as when the boat is flat, the bottom of the plate is actually outside the foorprint of the Hull......Can be messy if you catch a plat oin a mooring line at speed......