Small Boat Designs - 10' to 30'
Tad Roberts Yacht Design

Design Comments

With a deadrise angle of 17 degrees at the transom, Sitka Spruce is just barely a "Deep Vee" planing hull. Today designers consider any boat with deadrise over 16 degrees at the transom to be a deep-vee.

  One of the very first successful deep-vee's was Moppie, built in 1960, she had 24 degrees of deadrise, was designed by C. Raymond Hunt, and maintained 23 MPH through 9-10 foot seas and thirty knots of wind to win the Miami-Nassau race in 8 hours. She was a radical boat at the time, other planing boats of the day had very shallow vee bottoms that were almost flat at the stern. Moppie's deadrise was almost constant her whole length, meeting the chine well above the waterline, which made her tender at low speed or at rest. The narrow vee shape also needed more power for a given speed than the flatter bottomed boats. But by wining Miami-Nassau, breaking the old record by 32 minuets, in truly rough conditions, Moppie established a whole new class of boat. Sitka Spruce and all her relatives are direct decedents of the Hunt deep-vee.  

  The planing hull is supported almost totally by hydrodynamic forces (lift) created when the bottom strikes the water at an angle (trim) and a velocity (boat speed). The flatter the bottom, the more efficient it is as a lifting surface, and the faster the boat. But a flat bottomed high speed boat will pound itself to bits in any amount of sea, it's also difficult to control, and the pounding is very hard on crew.  

  The acute deadrise of the deep-vee solved the pounding and handling problems of the shallow vee hulls, but they lacked the lift necessary to really get up and go. So Mr. Hunt also came up with "deadrise angle compensators", the small horizontal strips seen on virtually all vee bottom boats, which he borrowed from seaplanes. These are now known as "lifting strakes" and provide lateral stability and flow separation, reducing frictional drag.  

  There is no published data I know of concerning the most efficient positioning of lifting strakes, designers try whatever they think might help the hull. We can see what the outer one is doing because it's exposed when the hull is on plane, the inboard ones we're less sure about, though it seems a good idea to have them inboard up forward where deadrise increases. So Sitka Spruce has two, one almost full length at 3/4 beam and parallel to the chine, a short one inboard running back to station 7, plus a narrow horizontal chine flat.  

  One more neat thing about vee bottom boats is they can be built of plywood, as Sitka Spruce is meant to be. A developable shape in plywood is not perfect for a small powerboat, but it's pretty good. If the boat has much beam the bow ends up a bit full for my taste, but balanced against cost and ease of construction it's a worthwhile trade. Plans are in aluminum.

   Sitka Spruce is an outboard fisherman. At my weight of 1800 lbs a 300 lb, 90 HP outboard motor will push her at 32 knots (top speed) and she'll cruise at 26 knots using about 56 HP. These large outboards are engineering marvels, but have ceased to be inexpensive. Luckily there's always someone who needs next years model and has to take a loss on last years, and makes Sitka Spruce attainable for those with a winter worth of weekends to hammer her together.