What causes a yacht to pitch?

[AndrewB]

Saw a video clip of my yacht a couple of days ago, and have come to the conclusion that it pitches more than most, throwing the sea up and probably losing speed as a consequence.

What causes this? Can anything be done to reduce pitching?

 

[Salty John]

You need to take weight out of the ends to reduce pitching.

 

[bobgoode]

Reduce the mass at the bow and stern as much as possible. This will reduce the inertia of the bow and stern resulting in less burying when the bow pitches down hence less speed will be lost.

 

[richardabeattie]

It's not just the weight distribution but the hull form. A fine entry and narrow stern will pitch more.

 

[landaftaf]

mmm - interesting concept.

I was wondering if this remedy has any theoretical or practical proof - as in my experience and schooling this would exacerbate the problem not reduce it

 

[Talbot]

whilst the concept of weight removal from the ends will normally help to reduce pitching, It is not always correct. You can actually have too little weight in the bow thus the wave just throws the bows up, rather than allowing it to pierce through.

 

[BlueSkyNick]

Aren't you agreeing with richardbeattie ie "...will pitch more." and "..... would exacerbate the problem...." are the same thing, to me. /forums/images/graemlins/confused.gif

 

[landaftaf]

no - having wieght distributed at each end increases the longitudinal moment of inertia. this helps dampen the pitching moment in theory.
in fact a fine profile frd eases the entry of the bow into a seaway as the waterplane area is reduced, so reduces buoyancy and thus the pitching forces - but I wasnt going to get into this heavy stuff
if pitching is excessive then alter the period of encounter so that you dont get composite pitching - the natural pitching period compounded by the bow digging in as it naturally comes down into the water.

 

[hylass]

[ QUOTE ]
Can anything be done to reduce pitching?

[/ QUOTE ]
1°) Change your old steel anchor for a "new design" in Aluminum /forums/images/graemlins/smile.gif

2°) Reduce your heavy mooring chain to max 40' and add a Nylon Rope.. /forums/images/graemlins/grin.gif /forums/images/graemlins/grin.gif /forums/images/graemlins/grin.gif

 

[Ships_Cat]

There seems to be alot of confusion about pitching. As two others have said, pitching is generally reduced by distributing the weight longitudinally in the boat ie out to the ends as that increases the moment of inertia about the boat's CG. This is contrary to the common view but is easily imagined if one compared the resistance to rotational accelerations of a car wheel, for example, over a bicycle wheel which has a much lower MOI.

What I think confuses this is that distributing the weight out, slightly reduces the boat's speed, but while important in a race boat it is of little consequence in a cruising boat. However, there was an interesting discussion document on the MacNaughton Design Groups site some time ago (now disappeared) which made the observation that sailing on the wind in pitching type seas a race boat might be better off with weight distributed more in order to reduce pitching and the energy loss from that (if I recall correctly it was Olin Stephen's observations they quoted).

Traditional boats normally have their weight widely distributed towards their ends purely as a result of their construction and lines. They are generally regarded as not being inclined to pitch and with low accelerations.

Comments from designers are also confusing because some say that weight should be concentrated towards the CG of the boat "to reduce pitching" but in the next breadth say that a boat has good seakeeping abilites because its longitudinal and transverse MOI's are high thus reducing rolling and pitching accelerations. Clearly one cannot have a high MOI against pitching by concentrating weight at the CG unless one puts alot of weight (or draft) into the keel which may be fine for a race boat carrying alot of sail area and not oriented towards comfort, but not neccesarily so for a cruising boat.

The tendency to pitch is magnified if the frequency of encounter with the waves is close to the natural pitching frequency of the boat. To increase the natural frequency one decreases the MOI and the opposite to reduce the natural frequency. Rythmic pitching is normally only an issue in coastal waters and some say that one should aim for a higher natural frequency for the boat than the frequency of encounter of these coastal waves which means concentrating the weight to the CG. But again, I think that is probably driven by the desire to avoid the small speed loss incurred by a high MOI but does result in high accelerations (especially for the foredeck hands and is perhaps why one reason why they occasionally go over the side).

The cost of greater accelerations I do not think is the applicable solution for a cruising boat. It was something I thought about alot before selecting a design for our current (40 ft) boat as our home waters (Cook Strait) are noted for their short wavelength steep seas (so high frequency of encounter and pitching moments when into them). We carry alot of weight in the bow of the boat (apart from miscellaneous items such as fenders, includes 2x 60 lb anchors, 1x 40lb anchor, 100m or so of 10mm chain, 110m 25mm warp) even though the boats lines are quite fine forward.

It works for us and we have never experienced rythmic pitching or any other pitching problem of concern even though we get conditions with very strong pitching moments.

Obviously hull shape is a major factor too. I have never sailed a Roberts 38 but am familiar with them. I suspect one of the hull lines issues with them with respect to pitching is comparatively low buoyancy in the stern sections and longitudinally a distinct "rocking horse" shape. That is pure speculation by me and may be wrong and also does not distract from the other strengths and popularity of the design - no boat is perfect.

John

 

[oldharry]

[ QUOTE ]
Saw a video clip of my yacht a couple of days ago, and have come to the conclusion that it pitches more than most, throwing the sea up and probably losing speed as a consequence.


[/ QUOTE ]

Taking a rather different angle to the question: '...more than most'. Do you mean she pitches more than other boats of the same class, and is she actually slower than other identical boats?

Or is it just that boats of other classes were doing better that day?

Every boat I have ever owned has had particular combinations of wind and sea that she just does not like, and which will cause her to jump around and slow down, and just a small change in conditions has allowed them to settle down again and get on with it.

I am no expert, but it seems to me to be a quite specific combination of wave frequency and height which seems to accentuate the normal pitching motions of a given hull design - almost as though it hits a sort of 'hull harmonic'. Different hulls respond in different ways, and the more 'sea kindly' ones seem to be able to manage it better. I have had tubby boats that could hardly make headway in certain conditions, and at the other end of the scale a boat with long fine entries that would seem to just stop and bounce in certain types of sea, and needed a lot of wind to get going again, although the rest of the time wickedly fast! /forums/images/graemlins/smirk.gif

Could it be that, on the day the video was taken, your boat had just hit that particular combination?

Also were you actually aware that you boat was not performing so well before you saw the video? If not, then I wonder if the clip you have seen was just taken at a wrong moment when your boat had hit a funny series of waves?

I think it a little rash to assume there is something wrong on the basis of the boats performance over the few moments it was being filmed! Unless of course the video was simply confirming your opinions based on x hours seatime.

One curious feature of small boat handling I have often noticed is the entirely subjective and false impression that other boats are not having it so rough, and are always sailing closer winded than me.

Sometimes they are.... /forums/images/graemlins/mad.gif

 

[Mirelle]

It may be illusory...

Seeing one's own boat on video is a bit disconcerting as she always seems to be pitching far more than you would think on board her.

My last boat would pitch into the same hole and grind to a halt every few minutes when working to windward; this one very seldom does so - maybe once or twice a season. It may be related to wave size.

 

[AndrewB]

Thanks for the very helpful replies guys. Perhaps it's most convenient to assume this is just an illusion, or just to do with the sea that day. But as several suggested, I might try moving the anchor chain from the forward anchor locker to the central bilge to see if it makes a difference.