Pitching Peformance of Cruising Yachts

[Ships_Cat]

Because of the heavy conditions and steep, short wavelength seas we often encounter in our home waters I take an interest in information I find on the pitching performance of yachts. I recently came across this forum post http://www.macnaughtongroup.com/_disc5/0000025b.htm from MacNaughton Yacht Designs which I found very interesting (others may find the same).

Our own cruising boat is designed along the lines he outlines with a low ballast ratio (less than 30%) and we carry alot of weight in the bow for the reasons outlined in the post (normally 2x 60lb anchors, 1x 40 lb anchor, approx 105 m 10mm chain, 110m 20mm warp, assorted mooring lines in an approx 10 tonne, when empty, fin keeled, semi-balanced rudder boat).

As he says the results can vary depending on the boat. We find it works well in our own boat which is quite fine forward with no flare and alot of beam aft to get displacement and form stability, and can sail closehauled at hull speed into steep 30-40 knot rip type seas (and motor with 50 hp at 5-6 knots into similar, vs 7 knot cruise in smooth water). We have never experienced rhythmic pitching although we have non rhythmically pitched to a stop motoring into very steep standing waves.

I would be interested in any real life experiences along the lines I have set out with various hull shapes and weight distributions in yachts that others may like to pass on.

Thanks in advance.

John

 

[Evadne]

I wonder if you'd agree with the summary is that changing the weight distribution changes the resonant frequency, and if this is slower than the wave frequency then the boat will sail faster. It is less obvious from this argument why a boat with less distribution would be faster, in different sea conditions, except for the case when the slower pitching boat's frequency matches that of the waves.

Our boat has a high ballast ratio (around 40%), narrow beam, long overhangs and cut away forefoot. We don't particularly try and concentrate the weight and I've never noticed pitching to be a problem, indeed she usually cuts through the waves, which always sounds good, but gives quite a wet ride in heavier weather. I think it makes her faster than a more modern design of the same LWL in stronger winds but I think that the ballast ratio and hull form are far more influential than the weight distribution.

 

[boatmike]

A very interesting and often misunderstood subject John I agree. Having been "exposed" as you have I know, to the opinions of some very well respected Naval Architects it's surprising how attitudes vary. Probably another illustration of why there are so many varied designs out there and no such thing as a perfect boat! Pitch and roll frequency will be affected by hull shape, mast and keel mass and the distribution of internal stores and fitments. Obviously if a yacht has lots of reserve bouyancy in the bows and stern this will resist pitching and a beamy soft bilge hull form will resist rolling but the problem with two much reserve bouyancy as any multihull freak will tell you is that too much reserve bouyancy especially in the stern will cause severe lift and in extreme cases, when already hard pressed downwind, pitchpoling can result. I think the easiest way to look at this is to think of the boat as a pendulum. My own cat has long low aspect ratio keels which add nothing to the resistance of momentum which is why it hobbyhorses like most cats do. Basically, the original Polynesian cats and prous which enabled them to cross all the way from Asia to your back yard had very little reserve bouyancy in the ends which meant that a wave would actually break around and over the bow or stern rather than lift it. The philosophy of the Prout brothers and others was to emulate this and the symmetrical hull form on my Snowgoose (pointy both ends) is an attempt to resist the effect of a following wave to lift the stern. I believe actually that this is why there are a lot of them about and they have a good reputation for easy motion at sea even if they are not the fastest. Yes they hobbyhorse, but not like a lot of modern wide sterned cats do..... To get to your point, the pitching or hobbyhorsing effect is at it's worst in a cat because it does not have a heavy keel hanging down giving a pendulum mass to resist it. So what happens with a cat? Well sometimes the opposite to that which you expect. Traditional monohull teaching is to keep the weight of stores out of the ends of the boat. With PEREGRINE I have found that keeping weight out of the ends actually increases the tendancy to pitch and slows her down. At one time, like most cat owners in the persuit of speed I carried the lightest anchor I could get away with on rope cable. I put very little weight in the forward lockers and always carried dinghys etc on the top of the coachroof. Now I carry two large anchors with chain all the way. Always carry my dinghy and outboard on davits, and have spare fuel etc stowed in cans right up forward. Yes I know it sounds wrong but I have damped the hobbyhorsing tendancy. When I hit a large wave I tend to lift more slowly not get chucked up in the air, and don't come crashing down in the next trough. Because the motion is more kindly I can in fact carry more sail without worry. In fact the mass in the bow and stern actually stops the bows burying on the rebound as it were and keeps the boat on an even keel.....
I would not advocate overdoing it however and think that the load taken in bow and stern should be carried in proportion to the reserve bouyancy of the design. Many modern monohulls have enormous reserve bouyancy in the stern driven by the desire for accommodation. These boats definitely trim better with weight carried aft to counter the effect of waves lifting the stern. The traditional long slim deep keeled sloop of yesteryear with lots of bow and stern overhang detests weight in the ends however as there is no reserve bouyancy to counteract it's effect.
At least that's my take on it John. Perhaps because it is more of a problem for a cat I have a different viewpoint but I see no reason why it isn't true for "half a boat" too /forums/images/graemlins/smile.gif

 

[Peppermint]

Re: Pitching and the wet ride

One of the major sources of pitching is a buoyant bow section.

I spent some time on the harbour launches in Southampton and gained a bit of first hand experience with the crews. The SP boat, that we see leading shipping past the yotties, is a modern purpose built design. It gives a remarkably dry ride but, from the crews point of view, the ride is so harsh as to be life threatening at any real speed in a swell. They've added extra weight but it didn't help much. They sing the praises of the old Nelsons which were as wet as submarines but gave a soft ride in any conditions. Of course these guys are sat indoors, so a wet ride doesn't bother them much.

I'm told that some of the Laurent Giles designs got as near to balancing a dry ride with a soft ride.

 

[Ships_Cat]

Dave & Mike (together)

Yes the conventional wisdom is that one must keep the weight out of the ends but I think that originates because it imposes a speed penalty - best I can determine is about 2% when pitching isn't an issue, and so that penalty is unimportant in cruising boats. Cruising boats which sail beyond smooth waters obviously have to cope with pitching seas.

So the thing that particularly caught my eye in the linked post were the comments regarding 2 S&S boats and Olin Stephen's comment, the faster one being the one with the heavier mast (has same effect on moment of inertia as putting weight in the ends of the boat to reduce pitching).

I think, but don't know, that Dave's boat is one where from its fine lines it is not inclined to pitch so weight distribution is not so important {Edit: I think Tom's comment which I now see makes the same observation}. The linked post alludes to hull form affecting the usefulness of distributing the weight out to the ends but does not make any predictions. Perhaps someone with a similar type will add their experience.

On the cats Mike you are aware of my fondness of cats (especially power ones as they help me earn some sailing money /forums/images/graemlins/smile.gif) but sailing ones are not very common around here. There are just 2 in our marina one of which sailed originally from Oz about 2 years ago and the other just an around 50 foot recent build, just been rigged and not gone anywhere yet. The ex Oz one, around 40 foot, is one of those that I would not fancy going far in - some production boat which is all superstructure.

Thanks for the feedback.

John

 

[boatmike]

Just to elaborate John, I think that in relation to longitudinal weight distribution what is true of the cat is true to a lesser extent for any boat. It's just more so in a cat so it illustrates what is going on more easily. Also the speed issue is interesting. Granted that carrying too much sail can slow you down in a monohull, often, especially downwind, it's the sea keeping ability and motion of the boat in a seaway that causes people to reef early. Anything that reduces pitching will enable you to make headway and ease the motion enabling more sail to be carried or if under power more throttle to be maintained. It is therefore (especially on a cat) important that one balance the DYNAMIC trim (which is what this is all about) to get the easiest motion through the water, without which one will be shortening sail to take the way off a pitching boat before it is really necessary. Peppermint is right that the old Nelsons (built by VT by the way) were an easy ride although wet and the argument is the same for a power launch as it is for a sailboat in this respect. They would plough through without reducing power rather than ride up over. The reason though was not that they had less bouyancy forward but that they carried more weight forward to counteract it. It's a matter of periodicity. The bows WILL lift on a Nelson in a big wave to avoid swamping but little
short bastards will simply break over the bow because they are too quick to lift the bow and by the time the old girl has started to react to the wave she is in the next trough.......
By the way I have not always sailed catamarans. I once had a 23 ton gaff rigged oak framed cutter built in 1922. I could carry full main and jib on her in anything up to a F7 on a broad reach and still be doing 9Knots..... Stopping now.... that was often a problem!!!

 

[ShipsWoofy]

Re: Pitching Performance of Cruising Yachts

You have just contradicted everything I have been taught about loading my cat.

This is going to require more reading and playing with my load distribution.

I am currently doing everything I can to remove weight from the stern by loading as close to midships as possible, which is quite difficult as this is where the main accommodation is.

One problem I have, with too much weight aft is that she buries her arse when under engine power, symmetrical hull shape, maybe from what you are saying I should be less worried by this and rethink my storage.

 

[Talbot]

Re: Pitching Performance of Cruising Yachts

I used to minimise the weight forward on my cat and suffered severely from the bounce. I have discovered as boat mike, that additional weight reduces the bounce considerably, but does provide a wetter ride.

 

[Roberto]

fwiw, I was taught that there should be a delicate balance between weight and volume distribution, pitching and speed.

Crafts having full, U shaped fore and aft sections should have comparatively lighter weight at their extremities, in order to allow the boat to lift when approching a wave: a heavy + full bow section would dissipate a lot of energy against the wave and the speed would seriously be affected

On the contrary. weights on crafts having sharp V fore and aft sections should be more longitudinally distributed, to allow the boat to pass more easily through an approaching wave (with lovely submarine effect): a fine, unloaded bow section would have a very limited dampening effect on pitching.


Can t remember the exact references, one paper was from Finot if I recall correctly

 

[boatmike]

Re: Pitching Performance of Cruising Yachts

Hiya Woofy!
I think most cats bury their arse under engine! Mine does too... So what? Would you rather she buried her bows? I dont think so! Take courage from the knowledge that the Heavenly Twinge (sorry!) is a really good little seaboat for its size but does suffer more than most from hobby horsing due to the hull design. It will look after you in crappy seas though. I once read of a lone sailor with very little experience deciding in a HT that his end had come in mid Adlantic in a storm. He pulled all the rags down. went below and consumed a large bottle of Scotch... 3 days later he was woken by a US coastguard cutter boarding him off Cuba. He had apparently survived a hurricane with nothing worse than a bad hangover to show for it!
Dont go overboard moving weight around until you know the effect. Ignore what she does under engine. Is she trimmed down by the stern when on her mooring? If so try moving stores forward to correct the static trim. She should float pretty level on her marks. Then see if it makes any difference. If she has a tendancy to bury her bows to windward get the weight off immediately. All boats are different.
Experiment. There are no hard and fast rules!

 

[boatmike]

Sorry Roberto but I think this is totally opposite to the truth. Boats with greater reserve bouyancy in the fore and aft sections can take more load there not less....

 

[Evadne]

Re: Pitching Performance of Cruising Yachts

I always knew that all boats are different, but reading the conversation on trimming cats just shows how much. An old fashioned monohull like mine just does not respond to trim in the same way, I just hadn't appreciated how different that makes us before now. I guess a cat has less ballast than most monos, which in turn gives you more control over the weight distribution. I doubt that moving things around will do much more than give me a bad back! Now if only I could stop her rolling on a dead run....

 

[Roberto]

yes of course I agree about load carrying,

but consider you are trying to push a big volume through the water (ex when a wave hits a full section bow), if the volume is very light (bow without anchors&chain), the wave force will make it lift easily, and little forward energy will be lost
if the same big volume is heavier (full bow chain locker+2 anchors) then the same force applied by the travelling wave will not be able to lift the bow, which will be forced to pass through the wave, reducing forward energy and speed

if otoh the volume is small, adding weight allows the thinner bow section to pass through the wave (speed is +-maintained ) with little induced pitching; the boat will of course have more inertia, but finer sections will not reduce speed as they would present less resistance through the water.

Imho, considering the two main factors: weight inertia, and pitching forces (mainly determined by hull volumes), one can have:
1.a high inertia with high pitching forces (ex longitudinally distributed load + full fore and aft sections)--> boat can easily be stopped by waves, speed is reduced
2.low inertia with low pitching forces (ex weight amidship in a thin fore and aft section boat)--> as a wave approaches, the reduced weight inertia will make the boat pitch rapidly, speed may not be reduced too much but comfort is
3.high inertia with low pitching forces (ex. thin section hull with weight at the extremities)--> the boat goes through the waves, speed is maintained because sections are thin, and pitching is limited
4.low inertia with high pitching forces (ex. weight conc. amidship in a hull with full extremities)--> good compromise for speed and reduced pitching

Comparing weight distribution in a given hull, point 4. is better than 1. (a full section boat is better with weight concentrated amidship); and point 3. is better than 2., a thinner boat is better given more inertia (load at the extremities)

Of course wave period, relative weight amounts, etc etc there are one million other factors, but I don t know, this looks sensible to me and more or less agrees with the behaviour of a few different monohulls I have seen

cheers rob


[edit]

I just found this Finot site link , sorry it is in French it gives a brief summary of their idea about weight distribution and hull volumes

 

[boatmike]

Re: Pitching Performance of Cruising Yachts

Ah now! Thats one of the reasons we sail cats!

 

[boatmike]

Not looking to argue Rob but I think you are blinding yourself with science and missing the obvious. Only ever move weight to the extreme ends of a boat if there is good reserve bouyancy there to counteract it. If you move weight to the bow of a vessel with little reserve bouyancy there you will overcome that bouyancy and could bury the bow inducing pitchpoling in extreme cases. I am talking about increasing the dynamic resistance or "damping" the tendancy of some designs to high frequency rotation around the centre of bouyancy WITHIN the capacity of the reserve bouyancy. This reduces the boats tendancy to rise in a short stopping sea but still allows the bows to lift in a big sea as they must. You seem to be ignoring that the basic objective of displacement designs is to remain afloat. Most of us don't drive wave piercing fast cats or submarines.........

 

[Ships_Cat]

Yes I agree with the reserve buoyancy bit too Mike. In our own case we get that from the bow overhang plus a high freeboard (1.4 m at the stem) - obviously it can be had from flare as well.

Again, in our own case, because of the fine forward sections the bow trims down very readily with weight forward and sailing on the wind but getting to ShipsWoofy's trim under power I don't think that weight forward has any real effect on the stern trimming down in our case when under power.

I have never measured how much it does so but under full power the transom probably goes down about 200mm and about 150mm at cruise (to the extent that the transom drags). I think in most cases the trim under power is mostly affected by the hull form than by weight distribution, but that may be different for very light vessels such as sailing cats with small water planes (I would not know, however real Ships Cat tells me his weight distribution is just fine /forums/images/graemlins/smile.gif).

John

 

[boatmike]

Re: Pitching Performance of Cruising Yachts

Correct Dave.
Actually a cat has no ballast at all (or shouldn't have) so moving stores around makes a difference that you would not notice in a ballasted keelboat.

 

[boatmike]

Quite right John. Most sailboats squat their tails down under power anyway. The hull shape is designed to resist a force that comes from the centroid of the sail area some way ahead of the mast and above it (dependant on the sail plan) The relationship between this and the hydrodynamic centre of resistance if the hull is what is important under sail and a balance between keel and sail is what the hull is designed for not engine power which is applied from a point a lot farther aft and lower down. This results in an inevitable rise in the bow and squatting of the stern. With a powerboat (like those we have both been involved in building of NZ design) the hull is designed to resist this moment, but the very things that make a good powerboat hull make it a crappy sailboat and vice versa. Thats why I told Woofy to ignore the performance under power and concentrate on performance under sail. First step get the static trim right when floating free and stationary. The squatting of the stern is exacerbated in cats like Woofy's and mine that have a symmetrical hull form. What makes them good under sail makes them poor under engine. Even here though moving some weight to the bow will help but only if the static trim is not upset and there is plenty of reserve bouyancy remaining in the overhang and flare of the bows as you suggest.

 

[tonyran]

Phew!