‘Building’ the perfect powerboat
Posted on 24 February 2009
Written by Eric Sorensen
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Seaworthiness, safety, engine efficiency and unsinkability are at the top of this writer’s design brief
When Soundings editor Bill Sisson asked me to devote a column to musings about my ideal boat, I thought this could be fun — and I was right. Though it’s hard to say it all in a column-length article, here’s a basic outline of what I would look for in my next boat.
First, what’s important in my ideal boat? Seaworthiness, seakindliness, passenger safety, propulsion efficiency, family-friendly habitability, multifuntionality and aesthetics — pretty much in that order. To provide plenty of comfort and range, my boat will be 46 by 12 feet overall (not counting the integral swim platform), which is essentially the same size as a more typically proportioned boat measuring 40 by 13 feet, 8 inches, only much more comfortable at sea and easier to propel than its short, fat cousin. It will be unsinkable, built of resin-infused and cored fiberglass, and have a semiplaning hull form that will cruise easily on plane at 11 to 16 knots (16.5 knots at a speed-to-length of 2.5).
I would use a hull form that has a clean, fairly flat run aft to allow the boat to develop the dynamic lift needed to easily get up on plane. Fine sections forward — with a sharp half-angle of entry — deliver a pillow-soft ride in a chop. Light displacement helps keep the boat dry, in part because bow immersion when pitching will be reduced.
The hull would be easily driven and have a useful range of operation from steerageway to full speed — in other words, there essentially is no hump speed at which the boat is laboring to get on plane. Like a trihull pontoon boat, it should just speed up sequentially as power is applied, with little bow rise — or wake — to show for its efforts. In fact, a small wake is indicative of an efficient hull, as wake is what’s created by wave-making drag.
The hull will have a keel to provide grounding and impact protection for the running gear. A swim platform will make boarding from a floating dock easy and safe. The hull shape will also prevent snap roll, just a gentle rocking sensation and no possibility whatsoever of pounding up-sea.
Construction methods and materials are selected to minimize weight while creating a stiff, strong and durable structure at reasonable cost (no exotic materials needed). If this were to be a one-off boat, I would have it built using a male mold with Airex or Core-Cell foam coring laid over the mold and then fiberglassed on both sides. I’d specify vinylester resin throughout the laminate; vinylester has excellent physical properties, including toughness, elongation, adhesion and resistance to osmotic blistering.
If my ideal boat entered series production, laid up in a female mold, the hull would be vacuum infused in one shot after the gelcoat is applied and the skin coat sprayed up over the mold. This means the outer layers of fiberglass laminate, the core, the inner layers of fiberglass laminate, and the stringers and bulkhead landings would all be infused at once, creating a true one-piece, primary-bonded hull. By using infusion to reduce and precisely control the amount of resin in the laminate, weight can be reduced and the structure’s strength and integrity, especially the bond between the core and fiberglass skins, improved over an open-molded laminate.
With the whole hull cored, and no worries about skin-to-core bonds, the result is a quieter, stronger, stiffer and condensation-free, thermally insulated boat.
Superstructure and topside
The one-piece deck superstructure would be built the same way as the hull — resin infused — since minimizing weight topside is going to be especially important to reduce rolling. Keeping topside weight down also reduces pitching in a head sea and bow steering in a quartering or following sea, since the inertia of the deckhouse will be less with the boat’s gyradius reduced.
The center of gravity is important to control, but so is the distribution of weight. If you have, say, an anchor locker with lots of chain in the bow, along with a generator and battery bank back aft in the lazarette, the CG may be the same as the next boat. However, with this distribution of mass in the ends of the hull (or added weight up high in the superstructure) it will pitch more than the boat that has its heavy weights concentrated amidships. Many, perhaps most, production boats built today would benefit from having another 10 or 15 percent of hull (but not superstructure) length at the waterline just to make the entry finer and improve the ride.
My boat would have 1.25-inch-diameter, 32-inch-high stainless steel rails forward and 30-inch cockpit coamings with deep toekicks aft. The transom door would be built of structural fiberglass (not plastic) and open outboard (to resist pressure from boarding seas), mounted on heavy hinges with a positive, heavy-duty latch. Eleven 12-inch cleats (three forward, two pairs of spring cleats per side, two aft cockpit stern cleats, and two pop-ups on the swim platform) provide plenty of mooring options. Also, the swim platform would have plug-in staple rails aft so it can double as a safe-for-the-kids fishing or water sports staging area, while being removable so you can use it to stow the tender.
An Ed Dettling-inspired (www.dettlingyachts.com) natural ventilation plenum would be designed in below the windshield at the helm to provide a steady supply of dry sea air to the pilothouse. The boat also would have a pair of dorades to provide plenty of dry air below, an anchor/chain washdown system, a single anchor pulpit holding two anchors, an emergency battery up high at the helm to power radios and GPS, lips around the integral fuel tanks to contain small fuel spills, and a chill-plate freezer-refrigerator that only requires daily recharging to stay cold.