Why so heavily chamfered HB bows?
1) Saves weight and makes for a stronger structure.
2) Less wind and wave resistance/drag.
3) Less shadowing and air turbulence to the lower sails , especially jib when heeled.
4) Deflects low level air up and to leeward towards the sails. compressed airflow = smoother faster airflow.
5) Delfects spray to leeward as opposed to windward (and eventually in your face at the back of the boat).
6) Spills deck greenwater faster.
Leading edge multihulls have been doing it for quite a while now.....it makes even more sense for a well heeled boat
It's mostly 1).
The number one goal of the designers is to reduce the weight of the boat (foils notwithstanding), but one has to take a key element into account here : The stability requirements of the IMOCA rules.
Basically, on one hand you want to reduce the weight and lower the CG (center of gravity) of the boat minus keel as much as you can, so removing any part of the deck is a good thing. Plus, having one more chine in the topsides reduces structural panels dimensions. This means you can have less carbon and/or core thickness, and therefore less weight and better CG (*). Since the overall CG of an IMOCA is largely under its hull bottom (actually meaning that the weight reduction of ANY part of the boat except its keel is good for the CG height !!!), all this means you can lower the weight of the keel bulb.
On the other hand, the large angle stability rules means you need to have volume in the sheerline at that angle (110°). So the cutout is more or less limited to the part of the sheerline that stays outside of the water at 110°.
PS : The IMOCA and the mini rules work approx. the same : the way their stability rules are written promote high freeboard (at least for the part that's in the water at the measuring angle), because there is no minimal weight. The Class 40 rule, with its minimal weight that is not very difficult to obtain and its prescribed range of stability at 90°, promotes to lower the freeboard and reduce sheerline volume, hence the full-length sheerline cutouts that you see on all Class 40 modern designs.