‘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.

Criteria

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).


Hull design

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

This long, narrow, light-displacement Nigel Irens Rangeboat is easy to push, requiring just 80 hp to make its design speed of 15 knots. If you want a 20-knot-cruise planing boat, find one with hard chines; the water flow separation at the chine corners will make it more efficient at higher speeds. 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

This 25-foot Shearwater has a beam of just 7 feet, 3 inches, which along with its intelligent, lightweight construction allows it to cruise at up to 16 knots and hit 22 knots wide open with a fuel sipping 60-hp Yamaha.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.

The Rangerboat's keel is a big help when the bottom of the harbor gets too close to the boot top. The cutaway reduces drag and protects the running gear.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.



Accommodations

Just as many boats today have too much deckhouse for the hull, they also try to cram too much into a 5-pound sack. My philosophy on accommodations is to avoid the temptation to cram too much into a limited space. Many boats have too many berths for their size, with the result that none of them are big enough to actually comfortably sleep an adult. With this in mind, my 46-footer would have a single stateroom forward with a 78-inch-long berth that’s 60 inches wide for the majority of its length. Overhead would be a big hatch to serve as an escape route in the event of fire or flooding aft. The stateroom has private access to a split head — the head and sink to one side and the shower on the other — and there would be access to the head from the main cabin as well.

Many builders try to fit too much sleeping capacity in too small a boat. The Rangeboat's versatile pilothouse berth converts from a bench seat by folding the seat back down.The galley would be a few steps up at the companionway, which brings it closer to the pilothouse without intruding. It takes full advantage of the interior volume below the windshield, and its raised position creates extra storage space below. Also at the galley would be large windows to provide a good view from the sink. I would avoid some of today’s trends like the plague, such as those cute bowl-shaped sinks that toss their contents at you the first time you hit a wake.

Hand-holds would be liberally provided throughout the interior, and there would be no hard corners to get hurt on. Headroom in the cabin at the companionway would be 6 feet, 8 inches, same in the pilothouse. The companionway opening would be 24 inches wide and the stairs evenly spaced, with a gentle rise and run for safe and comfortable passage.

If you think of a sedan-style boat as being divided between forward, lower cabin, pilothouse and cockpit, the majority of my boat’s length would be given over to the pilothouse. That’s because I like sunlight and the cheery view through large windows. This would be balanced by a clearly conflicting goal to situate the helm station farther aft in the hull, where the ride is better. The pilothouse — or saloon, if you insist — would contain a pull-out bed for the occasional guest, along with a couple of inflatable mattresses stowed away for the grandchildren. A companion seat opposite the helm would be up nice and high so my co-pilot has just as good a view ahead as I do at the wheel. Besides being a thoughtful touch, this comes in especially handy when coming into an unfamiliar harbor and trying to pick out buoys or other aids to navigation.

Helm station

The boat would have its primary helm station in the pilothouse and a second on the flybridge. The windows would have narrow mullions to minimize obstructions to horizon visibility, and the height of the deck at the wheel (along with the boat’s running trim and bow height) would be carefully engineered to provide an unbroken view of the horizon ahead, including when the boat is coming up on plane. Steering will be an effortless three turns from lock to lock for excellent responsiveness. Running lights would be up high where they can be spotted from vessels farther away.

Electronics flats, angled so they face toward the operator, would be close enough to reach without the contortions and stretching needed on most boats. The design would comfortably accommodate a seated or standing operator. The dash area under the windshield would be a dark, flat color to eliminate windshield glare, and the weather decks would be light gray or tan to minimize glare in sunlight. Depending on how the initial sketches worked out, I’d also consider having a Western-style, forward-sloping windshield, which give a big advantage in visibility ahead, especially when running at night. Red lighting would be provided to maintain night vision in the dark. An adjacent watertight door would provide quick access forward.

Access to the bridge is via open stairs from the cockpit. The bridge will be minimalist in size, in keeping with the boat’s beam, with seating for four. It would be situated well aft atop the pilothouse, since farther aft is where pitching motions are less pronounced. There’s no substitute for driving the boat from the bridge on a beautiful day or when you just want the added height of eye for improved visibility.

Seaworthiness and safety

The boat would be hard to sink, with compartmentation and use of foam creating positive buoyancy if the hull is breached in one area. The cockpit would be well above the waterline at full load, providing plenty of height for the diesel, along with a good measure of reserve buoyancy in the event of a boarding wave. Very large transom scuppers would be sized for — and the bottom of the transom door designed to supplement — rapid deck drainage.

Touching here on horsepower (underscoring how propulsion impacts seaworthiness), selecting a drivetrain according to the desired hull speed is an excellent starting point. But anyone who has spent much time in heavy weather (loosely defined here as when the seas are noticeably higher than the boat) knows that brute power can mean the difference between returning home unscathed and not.

If my boat needs 260 hp to achieve its optimum cruise and top speed, I might opt for an extra 25 percent margin, say 325 hp, so I have a better chance of being able to accelerate out of harm’s way or stay comfortably on the back of a wave running an inlet. As we’ve discussed elsewhere, speed definitely affects seaworthiness; it’s tough to dodge a breaking wave at 9 knots.

Propulsion

My boat would have a single common rail diesel, such as a 330-hp Volvo Penta D6 (think low noise, smoke and vibration), with a joystick controlling the engine, steering and thrusters for precise docking. A large, efficient, slow-turning propeller in a shallow pocket will be fitted to reduce shaft angle and overall draft and allow a larger prop.

If you’re wondering about pod power, the hydrodynamic drag of pod and sterndrive propulsion are not significantly lower than a well designed inboard at this boat’s cruise speed, so a well-designed inboard can be just as efficient. At 16 or 18 knots I would much prefer the traction of a large, slow-turning inboard prop; I’d just make sure the grounding skeg, shaft struts and rudder are carefully faired to minimize drag and propeller water flow disturbance. For a 25- or 30-knot cruise boat, pod power becomes a strong candidate for me.

A noisy boat is unpleasant to be on board, so acoustic insulation would be designed to keep noise levels to 74 dBA or less in the pilothouse when running at full power. This job is made easier because the common rail diesel is quieter and smoother-running than older technologies. With wave noise against the hull minimized by the cored construction, this will be one quiet boat at cruise — think sailboat quiet. A fixed fire extinguishing system would be installed, along with bilge alarms and an engine-driven emergency pump.

So there you have it — a quick sketch of my ideal boat. I might even become pleasantly irrational momentarily and have a varnished teak transom or natural teak washboards, since a little wood adds a lot of soul to a fiberglass boat. This boat will be easy to own, environmentally friendly and economical to operate, and safe and extraordinarily comfortable to be aboard, with a versatile layout and timeless beauty. See you offshore!


Eric Sorensen was founding director of the J.D. Power and Associates marine practice and is the author of “Sorensen’s Guide to Powerboats: How to Evaluate Design, Construction and Performance.” A longtime licensed captain, he can be reached at This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

 

This article originally appeared in the March 2009 issue.