FPB 64 Series – The Motion Equation – Being Comfortable and Secure

We’ve been watching video brought to us by the Owners of Avatar, the first FPB 64. It has long and steady shots of Avatar heading upwind with waves on the bow quarter. In discussing what the video shows with current Owners, it was suggested that a recap of the design of the FPB Series and how this impacts comfort, and safety might be in order. Now that we have the ability to embed video within these blogs it will be a lot easier to illustrate the design concepts.

This is a long article, with lots of video, so make yourself comfortable and prepare to do some passaging with us aboard Wind Horse and Avatar.

Going Places

From many hundreds of thousands of offshore miles on our own and the experience of our Owners over the years, we know that the key to successful cruising is physical comfort and emotional security. With both of these bases covered, making the passages required to get to new cruising grounds becomes a pleasure (most of the time) rather than something to be endured (or feared). Result: you go more places. We have described the structural approach used with the FPB Series and its factors of safety elsewhere. What we want to concentrate on now is the motion equation.


There are no simple rules or design parameters which dictate how to make a safe, comfortable, ocean crossing yacht. With a multitude of wave shapes and sizes, not to mention crossing wave patterns, the overall situation from a design perspective is chaotic. What we want as owners is a vessel which is an all around design that can cope with a variety of situations.

Designs need to start somewhere and for us this is ultimate stability in extreme weather. In particular we are talking about the ability to handle a breaking sea and with the inherent capacity to recover a full knockdown or capsize. The FPB’s ability to return to upright from a capsize forms the baseline from which all other design aspects flow.

The second design criteria is the ability to maintain cruising speed throughout a wide range of adverse conditions. This affects both comfort and safety. Boat speed reduces weather risks and fast passages are almost always more comfortable than going slowly.

Reduced passage time goes hand in hand with weather tactics. The characteristics which allow you to proceed quickly, even in adverse sea states, also make available a wide variety of weather tactics. All of this, comfort, boat speed, and heavy weather tactics come down to a single factor – steering control. This is the most difficult of all design parameters to get right. You need good control downwind, with seas on the quarter or dead behind you, at the same time you want a fine bow for slicing upwind. The hull shape that does well downwind is not going to be optimized for going uphill, and visa versa. We’ll cover this in more depth a little later on.

Comfort is a complex question, and comfort at sea and in port are not always arrived at in the same manner. If there has to be a tradeoff between these two scenarios and the goal is long distance cruising, the design should be optimized for the ocean rather than the marina.

When you mix these baseline issues with the thousands of decisions that are made in a yacht design there are going to be tradeoffs. The key, for us as designers, and you as owners, is to be clear about the goals for the yacht, and maintain a consistent approach towards this end.

In the text which follows you will find embedded sections of video. These run a minute or two in length. Clicking on the box in the lower right hand corner will play them full screen. Keep in mind when viewing these that the camera shrinks your perception of wave size, typically by a third or more. If you want to see the full length videos from which these are extracted there is a link at the bottom of this article.

There are a couple of things to watch for in these videos:

  • How well do the boats hold their course? Are they veering back and forth or is the course consistent?
  • What does the transverse motion (rolling) look like in response to the waves impacting and/or rolling under the hull?
  • Change in pitch angle and how fast the hull stops pitching once the wave(s) have passed is yet another factor.
  • Does the boat maintain speed, or is it stopped or slowed substantially by the sea state?
  • What sort of fuss (spray) does the hull make as it deals with the waves?

Beam Seas

Lets take the easy (for us) part first, dealing with beam seas. A combination of factors combine for comfort in beam seas. Briefly stated these are:

  • Initial form stability of the hull – stiff (beamy) shapes have a harsh motion – which is one of the reasons for our narrow waterline beam.
  • Stability curve – safest and most comfortable is a soft initial curve which hardens up rapidly past modest stages of heel.
  • Your position relative to roll center – lower is always better.
  • Effectiveness of stabilizing system and the size of the stabilizers relative to the initial stability of the hull.

Now for some video. Lets move half way around the world to the Tasman Sea, between New Zealand and Fiji. In this video segment we are dealing with five to eight meter/16 to 26 foot swells and wind waves generated by a strong gail. Seas are predominantly from aft of abeam, but also occasionally from forward as well, so there is a degree of confusion to the wave pattern. Watch the motion of the boat and the course she steers in these big seas.

This was our first real test after sea trials with Wind Horse and we were frankly amazed at how well she dealt with the conditions and how comfortable she kept us. In particular the lack of roll, even when big seas would sweep under us, was wonderful.

Head Seas

Comfort going into the waves is a very complex equation. It depends on: hull angle to the waves, boat length, natural pitch period, boat speed, and how all of this reacts with wave size and period. What you want to happen is for the bow to slice into the wave cleanly, without a lot of fuss or spray, and then for the wave to pass smoothly down the hull. If the stern is too broad, the wave pushes the stern up and the bow down into the oncoming wave crest. On the other hand, if the stern is too narrow the hull will not have sufficient dampening force and will tend to hobby-horse in place.

We’re going to look at a series of videos demonstrating how these factors work. The first is of the FPB 64 Avatar, cruising at nine knots into 20 knots of breeze. Waves are on the bow quarter. Seas are 80 to 150cm or three to five feet, with roughly a two second period. Avatar’s natural pitch period is right around one second , so this is a worst case scenario (these upwind videos come to us from the very steady hands of Carol Parker). Larger open ocean waves with a longer period, say 12 to 14 seconds, would induce less hull motion.

As you can see from the video, motion in this short chop is moderate. Of course there is a response to the seas, but considering the nine knot speed boat speed and wave shape and period, the pitching is not significant.

You have probably read some of the yachting press articles commenting on how easily the FPB 83 makes its way through the waves, so the above video has side by side comparison between the two designs. The FPB 64 is in the lower left window of the next video while the FPB 83 is in the upper right. The FPB 83 video was shot between the Bahamas and Nova Scotia. The seas with which it is dealing are a little larger, and a lot more stretched out than what the FPB 64 is experiencing. Still, there is a distinct similarity in how these designs react.

Considering that the FPB 64 is 22% shorter than the FPB 83 and carries the same displacement, we think the upwind motion is remarkably close between the two hulls.

There’s an even more interesting video taken during sea trials in the spring of 2010  here. This is in very steep seas, with reflected waves nicely confusing things (for a good test)

Broad Reaching and Running

Where you want a knifelike bow shape upwind the opposite is preferable when the waves are behind you. A shallower, wider bow surfs more easily and brings with it better steering control. If the bow is too deeply immersed the hull will tend to “bow steer”, in other words, the bow locks the boat on course and the rudder has little chance of exerting directional control. This is uncomfortable, forces you to slow down, and as the waves increase in size brings with it the risk of broaching.

The video clip which follows was taken during a North Atlantic gale on a passage between Greenland and Ireland. Note the motion and how little work the autopilot is doing to keep the boat on course.

Although the waves in the preceding video look moderate because of the camera effect we can assure you they were significant. Not only had it been blowing hard for the better part of a day, but there was a 1/2 to one knot counter current stacking up the seas. How the FPB dealt with this North Atlantic gale is self-evident.

And finally, video taken between New Zealand and Puget Sound in the US Pacific Northwest aboard FPB 64-3.

Once again there is a familial resemblance to how the two designs handle running before the seas.

Additional Data

This subject is covered in much greater detail on the Dashew Offshore website, both under the Design Objectives section and under the FPB 64 details starting with the section on Hull Shape.

The videos you have been watching are extracted from longer versions, a complete list of which you can see here.

Posted by Steve Dashew  (May 7, 2010)

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