Definition
Deadrise is the angle, measured in degrees, between the hull bottom and the horizontal plane at the transom. It is the single number that most strongly predicts how a planing hull behaves in chop; higher deadrise means a softer ride, lower deadrise means a flatter, more efficient hull.
Background and use
Three deadrise bands cover most superyacht tenders and chase boats. Shallow-V hulls (10-16 degrees) plane early, run efficiently at moderate speeds, and turn into beachlanders, calm-water day boats, and certain catamaran cross-deck designs. Moderate-V (17-20 degrees) is the workhorse range for limousine and open tenders, balancing efficiency against ride quality. Deep-V (21-26 degrees) is the chase-boat and offshore performance range; the steeper deadrise slices through chop with less slamming but pays a fuel and stability penalty at low speed.
The number is misleading on its own. A boat with 24 degrees of transom deadrise but a flatter forefoot will pound differently from one with the same transom angle but consistent deadrise running forward (a "constant-deadrise" hull, the design language pioneered by Ray Hunt). The full picture comes from the deadrise distribution along the keel, which is why specification sheets quote both transom deadrise and forward (often at amidships) values for serious offshore boats.
Strakes and chines modify the picture again. Lift strakes running along the bottom add planing efficiency and dampen roll without changing the headline deadrise number, which is why two boats with the same transom deadrise can ride and behave very differently. The right deadrise depends on the boat's mission. A Mediterranean limousine spends most of its life at displacement speeds in flat water; high deadrise is wasted. A Bahamas chase boat covers fifty miles in two-metre chop daily; high deadrise is essential.
Related considerations
- Always pair the deadrise figure with intended cruising speed; deadrise that pays at 35 knots punishes you at 12.
- Higher deadrise hulls roll more at rest; specify trim tabs or interceptors to manage.
- Catamarans do not have deadrise in the conventional sense; their ride characteristics come from hull shape and bridge-deck clearance.
- Confirm constant-deadrise versus variable-deadrise design when comparing chase boats; the implications for offshore comfort are real.
- Deadrise affects beaching capability negatively; deep-V hulls cannot nose up to a beach the way a shallow-V can.