October 26, 2020

Northern Marine 5706 – Deck Meets Hull


Want to know another reason why Northern Marine is a cut above? See how they marry the hull and deck structures.

By. Bill Parlatore

I have been following the construction of Northern Marine 5706 for some months now. I wrote about some of the systems design and installation to showcase what makes this trawler yacht unique among the yachts coming out of the pleasure boat industry. And I plan to continue this systems discussion as the build progresses. With a heritage of commercial fishing and large working trawlers, every element of the Northern Marine line is a study in big boat construction, adherence to ABS standards, and a rugged, no-nonsense approach to all aspect of the boat.

I asked Stuart Archer, Northern Marine’s general manager, to review with me photographs taken of the critical hull/deck joining procedure, as Covid-19 keeps us at a distance. The process was well documented, however, and I believe speaks for itself that there really is no comparison to other trawler yachts of this size.

Stuart said the hull/deck bonding process involves a couple of dedicated teams, who began work in late July and continue today. It is a very time-consuming effort that can only really be justified by a custom builder. The two-man teams may spend a couple of days on each step along the way, sometimes longer, as they grind, clean, wipe down, fill with structural filler & adhesive, apply mat and several layer of fiberglass material depended on the task.

Once the hull and deck are fully attached, the teams move onto glassing bulkheads to decks, decks to stairways and other structures, and a host of other tie-in jobs besides the crucial hull to deck joint. The laminators continue working around the boat, glassing every joint where components meet, bonding on both sides of all surfaces.

Finished painted caprail and inner hull liner of an earlier build.

Here we see the finished foredeck of another 57-footer, in this case finished with synthetic teak decking and brightly painted caprail.

Stuart said the boats have become less commercial in profile and detail, and the move towards a more yacht treatment and finish is easily accomplished. That is evidenced by not just the synthetic teak deck, but also by the single vertical windlass with single anchor. That is quite a departure from past Northern Marine yachts with big hydraulic cable drums on the foredeck to handle anchoring gear. About 70 percent of today’s yachts have a single large anchor, yet anchoring gear remains hydraulically driven.

Stuart also mentioned the current interest in stern capstans for managing a stern anchor.

The deck prior to decking onto the hull

The complete deck structure, ready to go into the hull. Notice that the crew has started the fairing process to finish the exterior surfaces to show the lovely lines of the profile. This fairing material will be coated with a high build, high density primer. This primer acts as a barrier coat to seal the surface and make it ready for painting.

Note the I-beams through the window frames used to lift the deck structure. I was impressed with the strength of these window frames, capable of handling the weight of the deck structure, which can weigh up to 15,000 lbs. And the total rigidity of the deck, with none of the sagging seen on many other decks once lifted off their frame cradles.

The hull and deck finally together.

The deck structure lifted onto the hull and resting on ledgers on the hull ready for the glassing process to begin. You can see the bow thruster, the location of the port stabilizer, and the solid, full-length keel on the bottom of the hull.

The non-opening portlight, a new addition with 5706, will bring plenty of light into the master stateroom, and is installed as per ABS standards, with a hinged deadlight cover that can be closed for watertight security.

The exterior stairway up to the flybridge and boat deck adds volume to the interior and is one of many ways to customize a yacht to meet the needs of its owners.

The deck after the gap between the hull and deck is filled with structural foam.

The deck sitting on the hull ledgers, ready to be glassed in. The 1.5-inch thick, eight-inch wide structural foam fill the gaps between the hull and deck. It will be permanently glued in with structural adhesive and then glassed over with 2408 biaxial E-glass cloth with ¾-oz mat backing to form a solid structure between the deck, foam, and hull. This 2408 glass cloth is ideal for high strength fiberglass reinforcement. Its dense flat weave provides a superior resin-to-glass ratio and is also the perfect material when the builder wants to eliminate print-thru.

Every joint is glassed over on all sides, top and bottom. As a result, the deck, hull, and other components become essentially one big part.

Another photo of the foam bridging the gap between the hull and deck.

You will also see there is no inside flange for the cap rail at this point, and the hull side is not fair and does not yet have a yacht finish.

The deck structure at transom is shorter to accommodate the tumblehome of the hull so that it will fit into the hull. The structural foam, which might be Core-Cell or Divinycell, will be glassed in from both sides. Note the aft lazarette opening into a space where the hot water system, batteries, and other major pieces of equipment will reside.

The same transom area has now been swept with a structural adhesive and then glassed over with mat and 2 layers of 2408.

The aft cockpit is now structurally one rigid piece. It is ready for installation of cockpit cabinets and other finishing treatments, which will include a form of non-skid or synthetic teak.

The side deck being glassed.

Notice there is now an inside caprail wood frame which they will use as a temporary mold to lay up the caprail flange where the laminator’s hand is. It will later be recreated in foam and fiberglass for a permanent caprail. Two layers of 2408 biaxial E-glass go over the side deck, structural filler, and hull side to complete the topside bond of the deck and hull.

The wood frame of the caprail is removed showing the newly formed inner caprail flange.

This new flange serves as the landing place for the new fiberglass caprail. The area will be thoroughly glassed in with a ratio of 30% resin to 70% laminate for ultimate strength.

While this image may be out of sequence, it shows the underside of the deck.

You can see the fiberglass work done to tie the deck and hull together from underneath. A flange off the hull extending out about a foot to ensure the deck resting on it will have maximum overlap. Structural adhesive on the flange bonds the deck and hull pieces together, then the team puts a 45-degree angle on the inside edges of the flange and deck which is then glassed over to permanently secure the hull flange and deck together, as well as bulkheads and other tabbed structures. This results in the absolute best in bonding hull and deck together from both sides.

It is interesting to compare this approach to traditional pleasure boat building. While this is time-consuming, it provides the most rigid structural bond possible, essentially making the hull and deck one structure.

Such a labor-intensive approach is simply not practical for production boat builders, who typically use a combined adhesive/mechanical methodology.

There may be a couple of inches of overlap between the hull and overlapping deck, which provides a narrow surface for applying a bedding compound (either a flexible product, such as Sikaflex, or permanent adhesive like 3M 5200), and the hull and deck are then mechanically through-bolted together every six to eight inches. This has been successfully used in pleasure boat building for decades and is standard practice for many popular trawler yachts.

There are numerous YouTube videos that show this adhesive and mechanical process. Even though this may be standard practice, it does not offer the same ultimate rigidity and structural bond as glassing over a foot of overlap of deck and hull. And when the bolts and nuts are tightened during construction, this introduces the possibility of squeezing out much of the bedding compound from the mating surfaces, creating a dry spot.

Additionally, after years of use, and perhaps thousands of miles in different climates, many sealants and adhesives lose their elasticity and adhesive qualities, or the chemical bond weakens.

Ask any owner of a top-quality yacht who has noticed, after some number of years, the saloon portlights, for example, begin to leak as the bedding compound fails. This is the nature of boats, and the same can happen on the hull to deck joint, allowing water to find its way into the boat. While 3M 5200 is the industry standard for permanent adhesive, can it be expected to uniformly hold up after decades of structural movement?

Glassing both sides of all joints and mating surfaces ensures the ultimate in structural integrity and water tightness. There won’t be any twisting of the hull and deck in heavy weather, possibly stressing bulkheads and other tabbed attachment points, and potentially creating leaks and joint failures over time.

If you have sailed offshore in heavy weather in a production boat, you no doubt experienced the hull and deck working in a seaway, often making it difficult to close or open interior doors as they flex at sea.

A hull cutout for the thruster.

Note the thickness and consistency of each layer that comes from the resin infusion process. The laminate schedule is a combination of fiberglass mat, Kevlar, and multiple layers of fiberglass cloth. The hull is solid fiberglass up to one inch above the waterline, near the keel will be double this thickness.

The caprails are made to match the wood patterns.

The caprails are built in sections from high-density structural foam, shaped to the required pattern. Then they are overlaid top and bottom with two layers of 2408 glass. These sections will be glassed together as one piece, sanded smooth, and painted to the owners’ desired color.

End view of the cap rails prior to installation. Note the wood pattern they follow below.

As finishing touches such as the caprail go on, the laminators move into the boat and begin on overhead liners, building the boat’s hard top, mast, and the fiberglass cabinetry that goes onto the flybridge and in the aft cockpit.

All the fiberglass components will be glassed together top and bottom to make a single, supremely strong structure that will wear the Northern Marine nameplate. It does not get any better than this when it comes to securing the deck and hull structures together.

As Stuart Archer pointed out, only a custom builder like Northern Marine can justify such time-intensive attention to detail.

Which is why, in the world of bluewater passagemakers and expedition trawler yachts, Northern Marine is second to none.