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

[sandy daugherty]

Congratulations on finishing your trip ssullivan! Now you have no cause to doubt your new choice of vessel. To answer your last question, designers want the catamaran to pitch up sooner because they want them them on top of the water more often than not. They don't intentionally put weight aft, owners do. Ask any designer about targa arches and dinghy davits, and you'll see their jaw muscles start to twitch. a boat wallowing around on its heels is just sad.

Our biggest reason to lose the ballasted keel was to go faster.

Big splashes, giant wakes, and green water over the bow are all lost energy. Weigh all that water in your mind and understand that is exactly how much force has to be used to gain back the speed that is lost. Where ever possible, keep that energy directed toward miles made good to weather, rather than splurged on uselessly rearranging the ocean. That's why we want to sail OVER the water, not THROUGH it!

[ssullivan]

Thank you for the thoughtful replies.

I do understand... having bows ride up and over (because we are not constrained by the keel's angular momentum - or inertia), is actually a good thing 90% of the time.

I enjoyed this buoyancy yesterday in some strong following seas under sail. Rick - I understand. While under sail, it's a whole different behavior (especially downwind) since the sails are pressing the bow down as they push the boat along due to their location - above the boat.

I'm glad this difference between catamarans and monos has been discussed and a lot of light has been shed on it. It was quite startling to me at first, coming from 20+ years of mono sailing. Now, I'm used to it and frankly, what Sandy said really makes sense. No need to play "submarine" and soak up your energy when you can keep it for making progress forward.

Thanks again for the replies. Great reading.

[Alan Wheeler]

Sean, you were motor sailing a Cat?? I don't understand. The boat should way out perform what little the motors will ever give you. Your hull was out of the water why? due to the sea state or the fact you were lifting the hull clear with wind strength?

[ssullivan]

Originally Posted by Alan Wheeler

Sean, you were motor sailing a Cat?? I don't understand. The boat should way out perform what little the motors will ever give you. Your hull was out of the water why? due to the sea state or the fact you were lifting the hull clear with wind strength?

The little motors (30 hp Yanmars) were on at a couple thousand RPMs to get that extra half a knot out of the boat that day, as we were trying to make a deteriorating weather window on a leg that would put us into difficult harbor to enter at night. We had a small craft advisory up all day and were in the open North Atlantic. We had little in the way of swell, but had those kind of waves that look *great* for using a surfboard. You know... very steep, curled up nicely and breaking over a little at the top?

The hull was out of the water due to the sea state. The waves were quite sharp and steep, created by the winds that were up for the small craft advisory. Waves were from the stern quarter, so as they passed by, and before the bows went down, the stern was out of the water enough that the through-hulls were sucking air. Apparently, the port thru hull sucked quite a bit of it - enough to overheat.

I got into the harbor assuming an impellor went, but when I took the pump apart in the morning, it was fine. It also pumped water properly, so that was good news... it had just been that we were out of the water enough to suck the air in and overheat the engine.

Lesson?

Don't motorsail in this stuff at all. Sail only.

[Lodesman]

Originally Posted by sandy daugherty

That's why we want to sail OVER the water, not THROUGH it!

You may be right Sandy, but energy is also expended climbing over waves - that's why fastcat ferries are designed to pierce the waves, rather than ride over them.

[BigCat]

Boats don't go through vs. over waves- boats float, and part of the boat is under the water and part of it is over the water. In a wave piercing design, there is a protrusion under water that sticks forward of the above water portion of the boat.

Boats have two motions that are being discussed-the pendulum effect of the weights, which some have called hobby-horsing, and rising to waves. If the two happen to coincide, as they will on some wave lengths and courses, the motion is very rough and uncomfortable.

If a boat is to carry a lot of weight in the ends, it should have higher volume in the hulls above the waterline at the ends, to dampen the pendulum effect.

Boats rise to waves around a center called the LCF, the center of the area of waterline. They pitch pendulum fashion around the LCG, the center of the weights and buoyancy of the boat hull. Simonis comments that the two centers should be separated somewhat in order to reduce pitching.

It is the portion of the hull above the static waterline that has the most effect when it comes to the speed of rising to the waves. A boat with an overhanging bow and a wide beam at the deck will rise quicker than a narrow bow with no overhang, because it has more volume where the wave first meets it.

Boats are flatter aft to make them faster, as straighter buttock lines aft are faster than curved ones. Also, beam at the stern prevents squatting under power, so the bigger the engines, the wider the stern should be. To a certain extent, the width of the stern and the straightness / shallowness of the buttocks at the stern is dictated by the displacement and hull beam. It is easier to draw a fair hull with a wide stern if the hull is wider / lighter, as the buttocks don't have so far to rise from their deepest point.

Hulls like Chris White's with comparatively narrow and deep hulls will inevitably have narrower sterns with steeper buttocks than the more common catamaran hull shape with a wider and shallower hull.

If a boat's lines are to be fair, then the hull beam and displacement will have a big effect on the bow and stern shapes- a wide, heavy hull will have fuller bows and sterns.

Designers will strive to put the center of weights and buoyancy at about 10% aft of the center of the waterline, because that makes for a faster boat. Long waterlines make for faster boats, and so catamarans often have no overhang forward. Derek Kelsall thinks forward overhangs look better, and ride up docks better if there is a docking mishap, and so he gives his boats overhangs forward.

[beiland]

Originally Posted by Hud3

The absolutely WORST, MOST UNCOMFORTABLE sailing we had on the entire voyage was in the Chesapeake on our final leg to home.

The breeze picked up to about 20 from the NE, and kicked up some short chop, only 3-4 feet, but the waves were such that our bow was going 15' in the air and plunging down to scoop up green water.

I ended up having to tack "up wave" instead of "up wind", finding the right angle on the waves to minimize the insane pitching. BTW, I have 9,000 lbs of lead in the keel.

Many people don't realize how shallow most of the Chesapeake Bay is, and that produces that short steep chop. The Delaware Bay is the same...nasty going up in a northernly breeze.

I think the posting at #20 got it right as to handling the situation.

[beiland]

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!

CORRECT !!...good basic explaination

[beiland]

Originally Posted by BigCat

Boats don't go through vs. over waves- boats float, and part of the boat is under the water and part of it is over the water. In a wave piercing design, there is a protrusion under water that sticks forward of the above water portion of the boat.

Obvious you are not familiar with these technologies:
VSV (Very Slim Vessels)
Very Slender Vessel (VSV)
Long - Skinny Power Boats - Boat Design Forums

...there are more sites...just don't have time to reference them

Boats have two motions that are being discussed-the pendulum effect of the weights, which some have called hobby-horsing, and rising to waves. If the two happen to coincide, as they will on some wave lengths and courses, the motion is very rough and uncomfortable.

If a boat is to carry a lot of weight in the ends, it should have higher volume in the hulls above the waterline at the ends, to dampen the pendulum effect.

Boats rise to waves around a center called the LCF, the center of the area of waterline. They pitch pendulum fashion around the LCG, the center of the weights and buoyancy of the boat hull. Simonis comments that the two centers should be separated somewhat in order to reduce pitching.

You need to separate the concepts of pitch and hobby horsing as several on this thread have tried to point out.

What induces a pitch motion does not necessary correct a hobby horse situation. Double-ended monos are a prime example...like the old Colin Archer types. Pitch will be enhanced with full bows, and surprisely, no mast. The real trick is to get this pitch motion dampened out as quick as possible...unbalanced ends help quite a lot. Long slim hulls are more of a problem in pitch and hobby horsing. As you point out separating the two centers is also very effective

Designers will strive to put the center of weights and buoyancy at about 10% aft of the center of the waterline, because that makes for a faster boat. Long waterlines make for faster boats, and so catamarans often have no overhang forward. Derek Kelsall thinks forward overhangs look better, and ride up docks better if there is a docking mishap, and so he gives his boats overhangs forward.

I agree with Derek...forward overhangs and 'graceful' bows look better

[44'cruisingcat]

Originally Posted by ssullivan

Ok,

I saw a very large, new 47-55ft cat today pounding up the Chesapeake with me. Its bows were *leaving* the water completely on every other wave, while mine were flailing skyward.

I've been asking about this for a long time, but got very few responses. Is this a weakness of cats that nobody wants to admit or talk about??

Actually I did tell you. Ages ago, I said there will be some conditions where your old boat would be more comfortable. Bear in mind though, you're in a boat that is 11 feet shorter now. Pitching is certainly one area where length is going to make a difference.

As other have said, You would have been better off taking the waves at an angle. Even better with some sail up.

I've motored straight into 30 knots in a 40 foot heavy displacement steel mono, and sailed into 30 knots in a 38 foot lightweight cat. Both in very messy, shallow water. The cat was more comfortable - because were were sailing. We were hitting the waves at 30 - 45 degrees instead of head on.

With some sail up in the same conditions, the mono was improved out of sight too.

It's really just a matter of learning your new boat.

[ssullivan]

Sorry... may have forgot that you mentioned bouncing bows in those initial discussions on the comfort of a catamaran vs a mono.

Also, the title of the post is meant to be taken with a smile... slightly in jest...

Thank you for all the helpful information.