Hobby horsing vs slam

[johnalison]

I'd have thought narrow fore and aft sections would increase hobby-horsing as they offer little buoyancy. A fuller midships would then act as a pivot.

I think that is true, and they also corkscrew in a quartering sea. My HR is sort of in between compared to modern designs and is generally well-mannered but there is always going to be the odd sea that catches a boat out and a hole can appear for the bow to drop into. Looking at some new boats without a forefoot I get the impression that the crew may have to put up with some discomfort going to windward.

 

[jimbaerselman]

Each boat, if disturbed in pitch in flat water, returns to its steady state through a diminishing series of wobbles in pitch. A sort of mini hobby-horse. The frequency of those wobbles is the boat's natural frequency in pitch. The frequency depends on the boat's pitch inertia (the distribution of fore and aft weight, mainly) and the buoyancy at each end of the boat which restores it to its natural level.

Light ends make for a higher frequency, heavy ends makes for a lower frequency . Flat ends make make for a higher frequency (and also induce slamming) while wedge ends (catamarans) make for a lower frequency.

When the timing of waves arriving from the forward sector is the same as the boat's natural frequency, instead of dying away, hobby-horsing will build up to high levels, and the boat will slow markedly. Cure is to change course a little so that waves arrive at a different frequency from your natural frequency. That usually implies paying off if you're close hauled. The gain in speed makes up for the fact you're not pointing so much to windward.

Waves arriving close to your boat's natural frequency make a complete mess of your windward polar diagrams.