“The hull is much closer to the canoe shape of a yacht than the usual displacement powerboat. Long and narrow, just like their sailboats, the vessel is eye-catching for its purposeful lines…”
The hardest part of a hull design to get right is the bow. The tradeoffs that need to be made between upwind comfort and performance and downwind steering control are the same for power and sail. This is an area where computer programs and tank testing are of little value. What you need to do is call on experience and then visualize how the design in question will work in the waves.
To do this you need to understand the design objective, which is what we want to talk about in this blog. We’ll use some photos from the third (above) and fourth (following) FPB 64s under way in New Zealand to illustrate.
If you are designing for open ocean work priority has to be given to maneuverability in big waves, particularly those from behind. This is a key ingredient in making the boat safer in heavy weather, and more comfortable in every day passage making.
Rounded shallow bows work best in this regard.
At the other end of the spectrum to have maximum comfort heading into waves you want a more V shaped hull, with more immersion. The problem is that too much V and/or immersion locks in the bow, making it difficult to steer. This becomes a major problem when headed off the wind, with seas on the stern quarter or directly aft.
Too much bow in the water and the rudder cannot correct for the waves. This uses more rudder (drag and rolling result) and makes the ride uncomfortable as the boat wanders back and forth. As seas build the risk of broaching comes into play.
The FPB 64 has a moderate amount of V shape, and more immersion than Wind Horse, the FPB 83 prototype. This is good in terms of upwind comfort. But compensation has to be made to maintain steering control. This comes in two forms. The first you can see above. From the side, what we call in profile, the forefoot is cut away reducing the “bite” of the bow. The second item is the rudder, it is larger in scale than the FPB 83.
We do pay a wetted surface drag penalty for the big rudder, but in moderate and larger waves this is offset by the rudder working at lower angles of attack to keep the boat under control. And when maneuvering in tight quarters the big rudder pays major control dividends.
There is a final benefit to this cutaway forefoot. Debris – think logs, ice, containers – will slip more easily under the bow than would be the case with a deeper, more abrupt forefoot.
If you have not visited our streaming videos recently you will find lots of real world examples of up and downwind steering in big waves – click here for these videos.