Cat designs in regards to Hobby Horsing

[rick458]

Am I correct in thinking, A high performance, narrow hulled, dagger board cat E.G Gunboat, Outremer, etc... will be much less susceptible to Hobby Horsing, than say a fatter hulled Mini keeled cat?

[Cotemar]

Hobby Horsing has more to do with boat length than hull width.

Most cats under 40 feet will have some Hobby Horsing

[catsketcher]

Gday Rick

All cats will conform to waves. It is a good thing that they do, otherwise they would get very wet. What most designers want to do is to design in pitch damping so that the boat doesn't keep on oscillating after the initial impulse has been felt. It is a real bummer to be on a boat that resonates with the impulse and generates higher and higher amplitude pitches.

Like with kids on swings every boat has a frequency that it will rock back and forth at easily. This is probably easier to see sideways. When a swell hits my 38ft cat abeam it hardly makes an impact - the period of the swell and the period of my cat are very different. However when a big stink boat goes by my hulls fall into the trough of the waves and the period (about 1 second) seems to match my boat's sideways resonant frequency and things get very rocky. Thankfully, when the waves pass the motion stops quickly. It is well damped.

The resonant frequency of my cat is a bit longer fore and aft. But if we get into some seas it can pitch up and down a fair bit. The good thing about a daggerboard cat is not the daggerboards themselves but rather the other design features they usually have that help reduce pitching.
- lower CG - this requires less energy to rotate about the centre of buoyancy and so there is less bow and stern immersion needed to oscillate at any frequency
- weight out of ends - a performance cat will usually have voids fore and aft which again reduce rotational inertia like above. Also lighter dinghy and solar arch etc
- Lighter rig but then again it may be higher so this can cancel out
- better sails - often sail drive can stabilise a boat.

On a 2 tonne 31ft trimaran I could feel the dramatic effect of moving a small inflatable dinghy from the front of the main hull to about 40% aft. It never went forward again. Put one person forward and one aft and go out in waves. The effect of increasing the rotational inertia is almost always very detrimental to performance and comfort. Our 38ft cat has the batteries, water tanks and as much heavy stuff as possible between 45-60% of the LWL aft. She sails pretty sweetly to windward in waves.

Both performance and cruiser cats will have pitch resistant and full sterns. Some of the worst pitching you will see is in Wharram like cats with their fine sterns. Saw a 50ft Hitchiker once that was hobbyhorsing like crazy as it had very little damping and the short waves hit it at exactly the right time to act like we do when we swing kids on the swing set - with every wave it pitched even more until the mast looked like a blur. Going a bit sideways would have got it out of resonance with the waves and maybe helped - so would have some wider sterns for better damping.

cheers

Phil

[Seaslug Caravan]

As Mr Catsketcher alluded too, sail drive , or "drive" in general affects a cats ride considerably , "if not the most ( my opinion)".

Light sloppy stuff is a cat killer. Boat speed is your friend. Starting a motor or two, in these conditions if you have a modest sail plan, will control the pitching. These days very few owners tolerate sailing to windward, and boats are designed with big volume hulls to carry all the necessary ?? stuff so pitching characteristics of the hulls are really just a moot point.

Historically some of the worst pitchers were long skinny performance boats and double enders with like wharams.
The distribution of buoyancy fore and aft in a hull is now well understood by designers.

[UNCIVILIZED]

Less rocker in the hull's shape generally helps to cut down on hobby horsing. And a huge factor is the hull's buoyancy distribution, including where the designer locates various amounts of reserve buoyance through the use of hull flares. They play a Big factor in controlling a boat's motion. And of course bow shape, & volume is another one.

[David Hughes]

Excellent contributions from every post in this thread! Thanks!

[barnakiel]

I did not see cats that hobby horse too much. Some smaller older designs do a bit, but not above what you see in a same size mono.

b.

[Cadence]

Hobby Horsing probably is more connected to wave period and length. Longer waterlines are less likely to hobby horse in shorter period waves.

[rick458]

Great replies. So let me get this straight, Long hulls, pointy bows, flat sterns, and balance favoring the rear help reduce it.
Heavy noses, pointy sterns, and funky flotation locations enhance it.
But confused Sea states, esp swells matching your boats (Period for lack of a better word) you are stuck with it regardless.
Unless you are travelling at good power Via sail or Motor or both and slicing the waves.
Am I closer now?, I had heard, watched or read that the High flotation mini keels being in the center of the hulls would act as a fulcrum.

[catsketcher]

Let's see - a high volume minikeel would probably be about 3m long, 20cm wide and 40cm deep. I just made a quick model on Rhino and I get a volume of 200 litres. On many boats of about 40ft this will be a small part of the underwater volume. On my 4000kg cat it would be about 10%. On a typical heavy condo cat it would be less than 5%. You can get it to be some advantage to by putting water tanks in here to lower CG so tanks in keels could even reduce pitching.

The designer would be able to thin down the mid sections to deal with the increased mid section volume.

As a note - I spent a lot of time watching boats pitch on anchor. One interesting time was at Airlie Beach on a northerly. As the wind picked up and made all of the anchored boats lie to the long fetch the wave length increased. As the length of the wave got to just more than half of every boat's waterline length they started pitching heavily. As the day wore on the wind increased, the wavelength increased and we went from a minor joggle to full on pitching when the wavelength got to about 6 metres (half our LWL). The boat's with pinched in bows and sterns did start a bit earlier but in the end everyone was pitching hard when the wavelength got longer than half the LWL. So size helps a lot too. Monos, cats, tris, all were affected the same.

cheers

Phil

[PaulinOz]

Quote "I spent a lot of time watching boats pitch on anchor."

Did you notice any difference between boats on anchor and those on moorings out front of Airlie Beach ?

Last year we had the use of a charter company mooring ball out font of the marina. These have been gradually converted over the years from a rope to all chain mooring lines. I found it quiet uncomfortable towards high tide with certain wave conditions. Only used it once and went back to the anchor and was a lot happier.

I suspect the shorter scope of the mooring, with zero give in the chain rode was adding to hobby horsing under certain conditions, in he same conditions it did not seem to occur as much on the anchor with a 5 to 1 scope. Come to think of it on the mooring ball we where not using the nylon bridle either (no simple way to attach to chain), so this would not have helped with jerking motion we where getting either.

[zboss]

Originally Posted by Cotemar

Hobby Horsing has more to do with boat length than hull width.

Most cats under 40 feet will have some Hobby Horsing

40 feet LOA or 40 LWL? Or am I splitting hairs?

[downunder]

Originally Posted by UNCIVILIZED

Less rocker in the hull's shape generally helps to cut down on hobby horsing. And a huge factor is the hull's buoyancy distribution, including where the designer locates various amounts of reserve buoyance through the use of hull flares. They play a Big factor in controlling a boat's motion. And of course bow shape, & volume is another one.

Rocker certainly contributes.

[KJThomas]

Originally Posted by zboss

40 feet LOA or 40 LWL? Or am I splitting hairs?

In the end length doesn't make a difference in how much you hobbyhorse. it just changes the resonant wave size in which you do it. A 140 m container ship hobbyhorses in a long swell where an 8m yacht would just go up and down.
Weight and buoyancy distribution makes the difference.

Fine double ends are the worst. Fine bows and fat sterns go along way towards damping pitch. Flared bows accentuate pitch.

Keeping weight out of the ends, especially forward, and low helps. Less momentum to damp.

Moving weights, changing course and or changing speed are effective ways of reducing resonant pitch, and roll.

[Barra]

I agree with the posts on wave frequency and resonance. My boat at 42 feet doesn't hobby horse at all in much at all normally and has all the "right" features described to do so but one incident really sticks in my mind from 2 years back.

We had wind against tide going up the gulf of patras and a sharp fetch was set up of only about 3 feet which would normally not be an issue. This particular day though we hobby horsed like a bronco as each peak and trough was in just the right length to create a fulcrum effect.

Only thing to do was bear off and affect the wave period which when done almost completely removed the action.