WHITE PAPER: A Seaworthy Flat Bottom Boat? Impossible!

by Mal Low, Naval Architect, SmallTugs LLC, Gloucester, MA USA
www.smalltugs.com

Many comments about flat bottom boats can be heard on the internet Forums or at the harbor-front pubs, especially by those myriad "experts" whose contact with water is, and has been, limited to the pitcher by the barmaid's hand. These same "experts" will freely offer "expert" opinions on everything from space travel to the horticulture of night bloomers; let them loose on the anonymous internet and they go crazy. Just because they have 100,000 Posts and comment on every Topic doesn't make them "experts".

Pelikan, Pouch, Paunch, Powder Puff’r, Porker and the other single chine SmallTug™ hulls have a rocker "flat" bottom of unique form accompanied by a generous beam and a separate keel. (The latter is sized for the drivetrain installed ie: prop diameter and shaft angle, and is determined by the builder, so the draft will vary accordingly.) The SmallTug hulls are all quite seaworthy and capable in any sea into which a vessel of their approximate LOA might wisely venture.

To be clear: SmallTug hulls are full displacement hulls; if you want higher speeds than the time-honored displacement hull speed formula gives then you don't want a tug hull and you don't want a full displacement hull. Tug hulls are beamy for their length, blunt bowed (I don't ever want to hear the word "prow" unless you are talking about a Moorish galley), with the maximum beam carried far forward and aft, minimal freeboard especially at the stern, with oversized rudders and high gear ratios driving large slow propellors. Those are not your typical small craft design points. Let's face reality: folks like the appearance of tugs and the "romance" of their work ethic but when it comes time to build one many really want more speed than a true tug hull can give. So what they end up with is a compromise that is neither fish nor fowl. It sort of looks like a tug above water but the below water portion is totally unsuited to work duties. That's fine, maybe, for those that just want a cute pleasure craft.

Please inspect the attached images of an extremely capable flat bottomed tug (one on which I served) that regularly crosses the Atlantic and Pacific, towing oil rigs and other structures as well as performing ocean salvage operations. She has even rounded the Horn with a tow under the capable hands of Capt. Latham Smith. Note the "flat" bottom. This particular tug, Elsbeth II, is one of 4 sister tugs of similar design used extensively for these purposes. She is designed to take advantage of her draft in accessing inland waters, rivers, and other shallow waters as part of deep ocean tows and salvage operations in any depth. My SmallTug designs are of a similar hullform theory although designed prior to the existence of the Elsbeth series.

Latham Smith's "Elsbeth" ocean tugs are almost single chine "shallow draft" triple screw monsters with wonderful Wartsila diesels (Elsbeth II) and engine monitoring alarms made from Radio Shack burglar alarms. A grand piano in the saloon and a caged parrot in the galley. Home cooking to die for. I miss those days. The stability and sea-keeping abilities of those tugs were really the sparks that encouraged the creation of my continued series of single chine tug hulls. You will be pleasantly surprised at what your single chine hull is capable of.

The reason some older tugs are round or deep-vee bottom is to make space for the large and heavy engines and their required bunkerage. But, as you see in the Elsbeth II pics, even 6000 Hp (3 x 2000 Hp) can be accommodated in a proper flat bottom design. Key word: "proper".

The single chine SmallTug™ hulls can be altered in LOA by careful factoring of the offsets by about 10-15% to obtain varied sizes. This factoring must be done with SmallTugs assistance as one common factor for the X, Y, and Z dimensions is not possible. You can start with the 21 ft. LOA hull or the 28 ft. LOA hull and go up or down from there. Although your budget may, you will never regret the largest hull that makes sense for your use. The saving grace of going large is that the single chine SmallTug hulls are quite easily driven with very small engines and therefore are exceptionally economical in all ways.

BTW: My double chine hulls are not suitable for simple enlargement. Some hull forms can be "stretched" easily some can't.

BTW2: Sailing vessels rely on the sails to provide stability in windy conditions, as well as a deep ballasted keel to prevent being pushed off down-wind, and a ballasted hull.

BTW3: All hulls will potentially require some ballast for trim purposes and to get her down to her design lines depending on the weights of the accommodations and equipment you install.

SmallTugs hulls have outstanding stability in all seas and therefore are perfect working platforms for any use as well as normal tug duties. The limiting factor will be the stomachs of the crew; the motion is more abrupt in these hulls in a heavy sea as they tend to follow surface conditions rather than drive through them as a vee hull would. Nevertheless they ride like a duck and are quite safe in any sea suitable for safe navigation by comparable sized vessels. You must always think safety in weather conditions regardless of hullform and practice prudent seamanship. Always, no matter how big or what style your vessel.

The single chine SmallTugs hulls are extremely stable in any seaway, what is called “stiff”. But note that there are two positions of high stability: rightside up and upside down. Lacking a deep ballasted keel, like a sailing vessel, the righting moment will change abruptly from positive to negative in rolls much beyond 90 degrees. Broaching is to be avoided. This is not usually a problem unless you habitually steam broadside to the wind along the crests of breaking seas in a half-gale. In weather like that you should be safe in port like every other prudent small craft sailor.

Remember these are tug hulls; flat and beamy. They are not, nor are they meant to be, passagemakers.

See T. Smith's words here: http://www.smalltugs.com/ownerspage.htm , about his SmallTug in heavy seas at the very bottom of the page under "Comments".

The biggest negative you will find with one of these single chine hulls is putting up with the comments by the local dock-walkers and the "well-wishers" who will watch your building process: they will all have an opinion about how much more seaworthy a vee or double chine hull will be. Invite them to go with you for a sail later (and dump them overboard with or without a flotation device).

The SmallTug™ single chine vessels were specifically designed for ease of construction by a backyard builder and have enough curves when finished and floating to look just like any other well-found boat. Do not draw any conclusions from the underbody shape.

The more traditional SmallTugs double chine hulls are designed for primarily commercial operation and are quite heavy duty and therefore much more expensive to build and operate than the single chines. They do provide more interior volume for high capacity bunkerage, larger twin engine installations and full sized enginerooms for commercial operation equipment like multiple generators, transfer pumps, and redundant systems.

Another example from Ocean Tug and Barge Engineering Corp., Milford, MA:

“2008- The RUTH M. REINAUER represents a new concept in AT/B design. Equipped with double skin fuel tanks, the first on a boat her size, the RUTH features our new "FacetTug" hull shape with is designed to make double skin fuel tanks in small vessels easier to install and also allows the tug to be built in panel lines, or in less sophisticated yards in developing countries, that do not have the extensive plate forming and bending capabilities.

This tug is highly maneuverable, being fitted with twin NAUTICAN nozzles and triple shutter rudders. The 4000 HP tug, the first of four, was delivered in 2008 and constructed by Senesco Marine in North Kingstown, Rhode Island.”

There are many, many examples of seaworthy single chine “flat bottom” commercial vessels in use today and history is full of examples of successful voyaging in all kinds of seas with these hardy types. They are especially prevalent in the commercial market where vessels must be used daily in all seasons and all weather and yet must be economical to build, economical to operate, efficient in terms of crews and cargo, able to operate in “thin water”, and long lived with varied maintenance. A SmallTug™, while smaller than most, is nevertheless a bona fide member of this service.

Mal Low, Naval Architect -- January 2008, Gloucester, MA USA

SmallTugs LLC

PO Box 7147 Gloucester, MA 01930-5847 USA

www.smalltugs.com

No duplication or reproduction by any means is allowed without prior written permission.