The Bouncing Bows - Why Didn't You All Tell Me?? :)

[Edward Teach]

All POWERboats will behave thus. The driving force is coming from the prop all the way aft. If you added a huge, heavy, tall mast to a powerboat, especially one with the added weight of a rollup in the mast maiin and a roller furling jib (thousands of pounds on a boat this size) it would of course pitch uncontrollably when beating into the waves. Everyone knows in any powerboat the stern is the only comfortable spot in any kind of rough stuff.

Pitching is reduced by getting as much weight as possible out of BOTH ends, NOT by adding weight in the bow. These condomarans that pass for sailboats these days have all the weight aft, such as v drive engines, big hardbottom dinghies hanging on davits, for that matter giant davit/solarpanel/swimplatform/windgen contraptions mounted at the stern. Then you get yo yos that will add gensets, watermakers etc in the bow and there you go. This abortion will now pitch like mad in pretty much ANY waves. But it will have a very comfortable interior for sitting at the dock. It's a matter of priorities.

A SAILboat, however, has the driving force up high (C/E of the sailplan) thus tending to DEPRESS the bows when driving hard. One doesn't SAIL straight into the wind, right? Hoist some sail and crack off kids, the boat will stop pitching. Move all that crap out of the bow and stern. Throw most of it overboard, in fact, you don't really need it. It's a SAILboat, try SAILING once in a while. Why is anyone trying to motor dead upwind in steep chop?

Also, the Chesapeake is notorious for steep, nasty chop especially when the wind is against the tide. (just like every shallow bay)

My boat has fairly small engines mounted relatively far forward. This detracts from the interior layout in the hulls. I have hardly any interior wood trim, no A/C, no genset, small engine driven watermaker and small centrally located water and fuel tanks. You probably would laugh at the interior layout compared to some french condomaran while you are sitting at the dock. When I pass you doing ten knots upwind you may not laugh so hard. I can blast through 8 foot waves under sail without taking on green water, or even slapping up under the bridgedeck, and not pitching much at all. The other day when it was blowing 20-25 my friend on a jetski could not keep up with me under sail upwind, as he was being completely launched by the waves while I was slicing through them like butter (4-6 footers) My anchors are carried just forward of the mast under the main beam, NOT at the bows. My bows are completely empty for ten feet aft to the first crash bulkhead, and there is nothing but a levac head in the next ten feet.

[Edward Teach]

Originally Posted by Lodesman

With respect Gord, I disagree. I know it's the common mantra of the multihuller that "all weight must be kept central", but I submit that has nothing to do with reducing pitching motion. With all the weight concentrated around the longitudinal centre, the bows (and by extension, the sterns) will be easily moved by any waves/swells. Weight in the ends would give them greater inertia, and they would be less apt to pitch. Of course, once the pitching started it would oscillate more and take longer to stop, thus necessitating fuller bow sections - more buoyancy results in a quicker return to equilibrium. Just my hypothesis.

Kevin

Nope, you have it exactly backwards.

[btrayfors]

Sean,

Couple of things.

1. When your pic was taken, it appears that the wind was no more than about 13-15 knots. At 15 kts, one would expect to see quite a few whitecaps; there are almost none in the pic.

2. The Chesapeake Bay can be one of the most treacherous bodies of water one would want to sail, especially if you are caught off Pt. Lookout where the Potomac River enters the bay. The river adds its current to the confusion, and if you have, e.g., a north wind and an incoming tide the waves can build very fast.

3. The Chesapeake is very shallow. This adds to the problem: wave periods are very short, and the waves are very steep. I've been off Pt. Lookout in such conditions with my boat....a 28K lb seagoing monohull...and it can be very, very uncomfortable going against the waves. Motion is erratic and violent, with bows alternately heaving out of the water as the stern settles into the narrow trough, then plunging into the next wave. Changing headsails in these conditions guarantees you'll get a dunking with solid water across the bow, even when the wind doesn't seem so strong.

4. That said, cats are notoriously bad performers to windward. I used to routinely sail past them in the BVI, while they were plugging to windward with both engines running. Great off-the-wind, not fast downwind, and just plain sluggards going to windward. These were cruising cats, of course, of the charter kind. Racing cats are a different ballgame altogether.

5. Given the poor windward capability of many cats -- and even well-found monohulls in the conditions sometimes found on the Chesapeake -- it often makes sense either to fall off the wind a ways or, better, choose another destination downwind. Many experienced Chesapeake sailors I know will choose the latter, unless it's absolutely necessary to keep slogging to windward.

Bill

[Therapy]

Originally Posted by sandy daugherty

Wave period is a function of water depth, becoming shorter and sharper in shallower water. Non ballasted boats have sharper motions because they weigh less, and have less inertia to overcome. Most objects tend to move around their center of gravity when their equilibrium is disturbed, unless there are other limiting factors*. Objects with the weight maximized in the center and kept light at the ends are more easily moved out of equilibrium, but restabilize quicker.

If you had an analytical bent as a child you might recall that it was easier to ride a teeter-totter with just two kids, and it was a chore pushing back up when ther was a lot more weight on it.

Designers strike a balance between pitching motion and recovery from deep bow penetration by changing the fineness of entry and motion damping aft bouyancy, and current thinking says that a flat section aft is faster because of some wave generating dynamics. These are things Naval Architects argue about.

That all means that us catamaran sailors need to get used to the fact that our boats sit on top of the water, and have sharper motions becasue we more closely follow the top surface of the water than boats with ballast because they get a smoother ride from the pendulum effect of the keel (even in pitch) and the inertia of greater mass. Chris White likes a fine entry and long bows for less disruption in waves, while other designers like fat forward sections to prevent submarining. Your catalac was designed more for interior volume at a certain length than for rocket ship performance, which would not have sold well in that era. You have a lot of boat for a small sacrifice in top speed.

The worst place for a 34' Cat is in 16 to 30' of water depth in wind driven waves, because the wave period almost matches the boat's waterline. A much longer cat bridges these waves, but at 45 to 65' finds itself suspended amidships in the same water. If it's heavy in the ends, it will start hobby horsing too, and can have a pretty wild ride.

The solution for the Chesapeake Bay (where more than half of it is between 16 and 30', with confused waves) is 1. a longer boat, but not too much longer, 2. turn 30 degrees to the principal wave direction, and 3. slow down. If you had really been in a hurry you would have taken highway I-95!

Someone else is sure to point out that this delimma applies (to a lesser degree) to shoal draft monohulls in the same hull length.

*Designers can move the dynamic axis of pitch forward or aft by changing the finess of entry, and the the sharpness of reaction can be changed by the relative positions of the dynamic pitch axis to the center of gravity. Nobody wants you to put a big weight in the bows!

Wonderful explanation and very succinct.
Thank you.

Now, has anyone taken a mono and a multi out on the same day and actually measured how much they differ in how high they move up and down at the bow and stern? No one I know of. I have my suspicions it is not that much different. I have no science for it, just my observations.

If a wave is 10 feet tall and goes by both boats will rise by 10 feet at their center line. There can be no more than a couple of inches difference.

What about angles off of level as looked at from the side? Any one measured these. I don't see much difference do you?

The only difference I see is in the speed of the reaction to a wave force except that some of the energy dissipated in the mono is shown as a roll from side to side.

[Lodesman]

Originally Posted by Edward Teach

Nope, you have it exactly backwards.

OK Blackbeard, you have my attention - now convince me. You yourself contradict this in the first paragraph of your previous post. The aft end of a powerboat is the most comfortable spot, is what you said. Maybe that's because there's a lot of weight back there with engines, fuel, etc. It takes more energy to lift a heavier object - basic physics. If a wave has a defined amount of energy it will lift a lighter object higher/faster than a heavier object. You say it's backwards, then prove it.

[Joli]

Originally Posted by Therapy

Wonderful explanation and very succinct.
Thank you.

Now, has anyone taken a mono and a multi out on the same day and actually measured how much they differ in how high they move up and down at the bow and stern? No one I know of. I have my suspicions it is not that much different. I have no science for it, just my observations.

If a wave is 10 feet tall and goes by both boats will rise by 10 feet at their center line. There can be no more than a couple of inches difference.

What about angles off of level as looked at from the side? Any one measured these. I don't see much difference do you?

The only difference I see is in the speed of the reaction to a wave force except that some of the energy dissipated in the mono is shown as a roll from side to side.

Chances are that for equal length boats the weight and surface areas are different enough that the ride will be different mono versus multi in the same conditions.

One trick in IMS is/was to add additional lead in the very shallow bilge centralized to add straight line speed and "punch" to force the bow into and through the waves. I was recently on a 60 carbon IMS boat that had pallets of lead ready to be loaded into the bilge, not ballast per say since the bilge is so shallow but weight for momentum.

I gotta shake my head, build a $3 million dollar shell out of carbon and load it with lead because you need the weight to get through waves. Pretty extreme just to lighten the ends, no bouncing bows though.

[ssullivan]

Originally Posted by GordMay

Two observations (Catalac 10M & Chris White 55) hardly constitutes proof positive that all catamarans do it (pitch, hobbyhorse, bounce).

Pitching is generally caused by too much weight in the ends of the boat. Accommodations and storage areas that extend much forward of the mast, solid fiberglass decking forward, engines and storage too far aft, not enough “empty” stern or bow hull area extending aft and forward accommodations & storage all contribute to increased pitching.

Ahhh.... my antagonist posts his negative posts again.

Gord, please...

I have a photo of it on a FP in this thread if you scroll back.

I'd say watching a FP, a Catalac, a Chris White Atlantic 55(?), a Prout and a few other does indeed constitute proof. Lay off me, alright?

[ssullivan]

I also wanted to say that this is otherwise a very productive and interesting conversation. Resting now... long day. Made Massachusetts today!

[ssullivan]

Originally Posted by Lodesman

OK Blackbeard, you have my attention - now convince me. You yourself contradict this in the first paragraph of your previous post. The aft end of a powerboat is the most comfortable spot, is what you said. Maybe that's because there's a lot of weight back there with engines, fuel, etc. It takes more energy to lift a heavier object - basic physics. If a wave has a defined amount of energy it will lift a lighter object higher/faster than a heavier object. You say it's backwards, then prove it.

Yeah, I I have trouble with this one too.

If I can recall my days of freshman physics, there is a little concept called "rotational inertia."

The heavier the bows and sterns are, the less the boat will pitch in response to a wave and the more it will punch through. The lighter they are (for the same overall weight boat), the more they will fly up and over the oncoming wave.

I have trouble seeing how this concept is different in boats than it is in standard physics.

[Walter1]

Greetings,

We have a '75 Pearson 28-1, hull #4. Launched today and filled the water tank only to find a leak under the tank. Tank will be easy to remove. My question is can this plastic be repaired or do I order a new tank.

If I was going to cruise extensively I would want the reliability of a new tank but we mostly daysail and the beer usually lasts.

Fair winds and thanks,

Walter

[ssullivan]

Well, I woke up this morning understanding exactly why the bows do their bouncing thing on cats.

Sneuman, colemj and others are speaking out of experience. They know the bows ride up and over, leaving the water, just like in the photo of the Fountaine Pajot. They own cats or have been on them. Those that don't own cats and really haven't been on them in these conditions can theorize, but please don't dispute that cats do this when motoring upwind. It's not a question of if (Gord), it's a question of why.

Well, I woke up understanding exactly why.

Half of it is already up here in this post. As the poster said, they share a lot of features with power boats when fore/aft symmetry is examined.

So, here's the answer:

A keel boat has the keel - an absolutely enormous amount of mass located a fair vertical distance from the fore and aft center of rotation, but *exactly* on the boat's center of gravity and center of rotation horizontally speaking. The keel's mass keeps the boat sitting flat and level as a wave starts to push the bow up. Only when the wave has imparted enough energy (through the buoyancy of the bow) to the angular momentum of the boat, does it begin to pitch upward, like a cat. This pitch motion (on a keel boat) takes a lot more force (and has a lower frequency of oscillation) due to the enormous weight of the keel acting as a damper.

One poster mentioned all the weight aft, and they were also right on target. Cats have engines, engine rooms (with all that heavy stuff), more hull weight, people, etc...etc... all aft of the companionway door. This significant weight, unlike the keel of a keel boat, is *not* located at the center of gravity, nor is it located at the center of rotation fore and aft, as the weight of a keel is. They have little to no weight, relatively speaking, in the bows. This makes for a fore/aft rotational axis of rotation approximately *at* the companionway door on most cats. In fact you can clearly see the weight distribution (fore and aft) on a cat by looking at its fore/aft rotational axis while it motors to windward in even a light rolling 1-2ft sea. The rotational axis is *defined* by the weight distribution of the boat.

Just as with a power boat, all the weight is significantly aft of the center (fore and aft) and this causes the rotational axis to be aft of center.

So, the cause of this motion is that there is no keel, or at least that the weight is not centered, nor located any distance from the center of rotation, as it is on a keel boat. The position of the axis of rotation is defined by the mass distribution, so the axis of rotation is further aft.

Visualize this:

Cats are fatter aft and thin and skinny forward. Why? Because they need greater buoyancy aft to keep all that mass floating level. If the hulls were symmetrical fore/aft and we had the same mass distribution as they do now, they would float with their sterns down.

If you put a bunch of weight in either end, you will have a boat that behaves *exactly* like a keel boat, in that its angular momentum is greatly increased. This may not be desirable, because every cat designer out there has designed their boats to have very light bows that fly up and over waves. Not sure why.... but they have. Any guesses as to why flying bows are needed on a cat when they are not on a keel boat?


I'm not sure it's even debatable that weight in either end will cause the boat to pitch more - it can't. If you put weight in the ends of a boat, it will rotate more slowly about its fore/aft axis of rotation. This means a slower, less quick rise of the bows over waves and more of a "punch through" the waves, and of course more water over the bows. Weight in the center (close to the center of rotation) will cause the bows to rise quickly and sharply ( and the sterns too) when they are acted upon by a force. This happens because centering your moveable weight in the boat will reduce the angular inertia. It's the same principle as a figure skater increasing their angular rotation speed by tucking in their arms to spin very quickly. How do they slow that rotation? By putting their arms out, or distributing their mass to the ends, thereby decreasing angular rotation speed. It's conservation of angular momentum at work.

It's standard Newtonian physics... kid's stuff. Nothing complicated. Here's the link:


Angular momentum - Wikipedia, the free encyclopedia

For those who didn't have a career in math/science, sorry about that math in the link. Intro physics texts have this concept well laid out and explained in simple terms without using calculus. Calculus is not necessary to understand angular momentum. It can be described more simply, but I can't find a link for that.

All that said, you can get used to this motion, and I don't see it as anything that makes a cat less sea worthy than a similar mono. Rode out a pretty gnarly one yesterday evening in the open ocean and the cat performed beautifully. Thankfully, it was all between 90 and 270 degrees apparent. Smooth sailing...

The key difference here between a cat and a mono is that the mono has the keel, which greatly increases angular momentum (and more importantly for this topic, angular inertia). This is why they ride up and over waves differently from each other.

[Joli]

Originally Posted by ssullivan

This makes for a fore/aft rotational axis of rotation approximately *at* the companionway door on most cats. .

So then you beg the question...........why build a cat with a forward cockpit?

Congrats on making Massachusetts.
Ense petit placidam sub libertate quietem

[MidLandOne]

Nice to know that there is someone else here who didn't sleep through Physics 101 .

Originally Posted by ssullivan

Any guesses as to why flying bows are needed on a cat when they are not on a keel boat?

As I put in an earlier post it comes about in power cats because buoyancy is taken well forward in the hulls and at least the same reason will apply to sailing cats. This is a function of the need to transition to the pointed bow on a narrow beamed hull (ie if one transitions to the pointed bow from well back you end up with very pointy hulls indeed) and also to give water plane area forward to stop the bows sinking in seas (including pitching the bows right under which narrow hulled power cats will also do if there isn't sufficient reserve buoyancy from freeboard or flare) or when loaded.

On the load point power cats used as ferries and whose passengers board over the bow generally keep their foredecks small so passengers cannot gather there and sink the bows down when loading because even though buoyancy is carried well forward (and power cats are generally beamier than saily ones) that buoyancy is still low compared to a beamier hull shape such as for a mono.

With respect to the force required to accelerate the bow upwards and that being dependant on the longitudinal moment of inertia, it pays to remember that moment of inertia is proportional to the mass x distance from the point of rotation squared. So, for example, doubling the distance of the mass from the point of rotation increases the longitudinal moment of inertia by four times.

As far as I know pitching behaviour is difficult to model accurately on computer due to the complexity of all that is going on. In manual analysis it is sometimes claimed that the boat rotates about its CofG but I think that will only be correct if the whole boat is out of the water. In my view as the bow rises out of the sea and the buoyancy moves back towards the stern then the centre of rotation moves back too and other attempts at analysis claim that.

[Adaero]

All of the cat designers keep weight out of the hulls due to the danger of pitchpolling.
If you re-look at your calculations of a cat surfing down from the crest of a wave at say an angle of 45 deg with a lot more weight in the bow. At the bottom the bows are going to dig into the face of the next wave. So your new rotational moment is from the point of the bow which for this example is now fixed into the wave face with all your heavy bits in the stern stuck up in the air at 45 deg. Acting on the heavy bits will be the mass of the wave trying to continue the rotation. You will pitch pole quite easily in this scenario.
By keeping the bows light you will have bouyancy forcing the bow up out of the wave face to counteract the rotation of the stern coming over the top.
This won't help with your bouncing bow problem but might go to explain the reasons why.
I did sleep through physics 101 so could be talking out of my backside but as an engineer the above makes sense, at least to me.
Regards
Adaero

[rickm505]

Originally Posted by Adaero

All of the cat designers keep weight out of the hulls due to the danger of pitchpolling.

Great discussion and everyone seems to appreciate the problem. The invisible elephant in the room , so to speak is the propulsion method of the boat. This discussion seems to gravitate more towards a Cats motion under engine power.

Adaero brings us back to the primary design function of a sail boat. Designers keep weight off the ends of a cat to prevent pitch poling under sail. When a Cat has a jib deployed, it stabilizes the bow, and actually keeps the bow in the water, at least on my boat it does.

Pitching bows under power is a consequence of the intended design of the boat.