Comparing Trawler Displacement Hulls
Shopping for any large ticket item can be confusing. Fans of Selene, Nordhavn, Kady Krogen, and other brands will always be able to explain, usually convincingly, why their particular favorite is “better than” other available choices. To some degree, preferences are subjective. Does exterior styling outweigh interior layout? Is an extra knot or two of cruising speed more important than maximum fuel economy? Given a finite amount of space with which to work, should the engine room be a greater priority than the master stateroom? There is no universal and always correct answer to these or many other factors considered when choosing among displacement hulls.
New displacement hull trawler prices begin at several hundred thousand dollars, and often run up into seven figures. With that kind of money at stake, most buyers will hope to make the best possible and most fully informed choice. Competing marketing claims, testimony from equally loyal current owners, and inconsistent expert reviews are confusing. With only a short sea trial prior to typical purchase, how can a shopper realistically and objectively compare competing hull designs and anticipate performance?
A series of mathematical ratios is useful when considering design differences between displacement hulls. Some of the major manufacturers freely publish these ratios, but some of them can be easily calculated from commonly available information in sales brochures or on-line listings.
The D/L ratio
Displacement, divided by waterline length, equals the D/L ratio. Essentially, the ratio defines how large a load is carried by each foot of waterline. The same amount of weight, distributed over a longer waterline, creates a low D/L ratio. With the same amount of horsepower applied, hulls with a lower D/L ratio will cruise at a faster speed than hulls with a higher D/L ratio. With a displacement hull, there is no significant advantage in using an engine that will drive the vessel faster than nominal hull speed at 70 or 80 percent of rated RPM. Boats with a lower D/L ratio can be equipped with a smaller, lighter, more economical engine.
Boats with a higher D/L ratio will require a larger HP engine to reach hull speed. The engine itself will weigh substantially more, as will the fuel needed to provide adequate range.
To calculate the D/L rating for any vessel, divide the displacement expressed in long tons by the cube of 1% of the waterline expressed in feet. That sounds more complicated than it is. Let’s work through a couple of examples.
A 66-foot Selene displaces 151,200 pounds. The water line length is 61-feet, 6-inches.
1. Divide 151,200 by 2,240 to determine the number of long tons. (Result: 67.5)
2. Determine 1% of the LWL, expressed in feet. 61.5 X .01 = 0.615
3. Cube the result of step 2. (0.615 X 0.615 X 0.615 )= .2326
4. Divide the number of long tons (step 1) by the cube of 1% of the waterline (step 3).
5. The D/L rating for a vessel with a 61-foot, 6-inch waterline displacing 151,200 pounds is 290
Let’s compare that 66-foot Selene to a Nordhavn of similar size.
A 64-foot Nordhavn displaces 185,000 pounds. The waterline length is 59-feet, 2-1inches.
1. Divide 185,000 by 2,240 to determine the number of long tons. (Result 82.5)
2. Determine 1% of the LWL, expressed in feet. 59.16 X .01 = 0.5961
3. Cube the result of step 2. (0.5961 X 0.5961 X 0.5961) = .2118
4. Divide the number of long tones (step 1) by the cube of 1% of the waterline (step 3)
5. The D/L rating for a vessel with a 59-foot, 2-inch waterline displacing 185,000 pounds is 389
What conclusions can be drawn by comparing the D/L ratio for the Nordhavn and the Selene? The Selene carries less weight per foot of waterline length, and if cruising at the same speed as the Nordhavn should be slightly more fuel efficient. The Nordhavn, on the other hand, will appeal to people who associate maximum weight per cubic foot of volume with a more sturdily built boat. In an era where modern materials and layup techniques are rapidly updating traditional fiberglass technology with equally strong but lighter weight composites, that may not be as true today as in years past.
The S/L ratio
The S/L ratio compares speed to length, and in many respects is consistent throughout all available displacement hulls.
Displacement hull speeds are determined by two factors. The most significant consideration is the length of the waterline. When underway, a displacement hull operates in a well-defined area between the wake created by the bow and that generated at the stern. Naval architects have long appreciated that the distance between the bow and stern waves changes with the speed of the vessel. A mathematical formula for “hull speed” establishes that the square root of the waterline (expressed in feet), multiplied by 1.34, equals a vessel’s theoretical hull speed.
For example, let’s examine the hull speed of a long range cruising trawler with a 49-foot waterline.
1. Determine the square root of the waterline. (7 x 7 = 49, so the square root is 7)
2. Multiply the square root of the waterline by 1.34
3. The result is the “hull speed” of a displacement hulled vessel with a 49-foot waterline. 7 x 1.34 = 9.38 knots
It’s worth noting that most boats achieve optimal fuel efficiency at factors closer to 1.0 to 1.2 times the square root of waterline length. Hull speed should not be considered the most efficient speed, but rather the maximum speed at which one should attempt to drive a displacement hull.
The second consideration in the S/L ratio is available horsepower. A displacement hull will not achieve hull speed if it is underpowered. With the physics of momentum considered, it requires surprisingly little horsepower to drive a displacement vessel to hull speed. The Caterpillar Engine Marine Analyst Manual recommends a power to weight ratio for displacement vessels at 509 pounds vessel displacement per net horsepower delivered to the propeller.
For example: A Kadey-Krogen 48’ AE displacement trawler displaces 56,450 pounds.
Dividing 56,450 by 509 reveals that the Kadey-Krogen 48 can be properly powered by 110-HP, net, at the propeller. Kadey Krogen equips the 48 with a 200-HP John Deere engine, ensuring that the engine will not need to run at full capacity to supply 110-HP. No power train is 100% efficient. A 200-HP rating leaves some power available, in reserve, for situations when weather conditions or unusual loads (towing another vessel, etc.) require additional power to maintain hull speed.
By way of example, if the same 56,450 pound Kadey-Krogen were equipped with only a 95-HP engine, that engine would be required to run at full capacity when cruising and still might never achieve hull speed. Running a smaller engine at wide open throttle seldom yields better fuel efficiency than a larger engine throttled back to less than maximum power rating- but operation of the smaller engine is likely to prove more costly by virtue of a shorter lifespan.
On the other hand, there would be no benefit to powering the Kadey-Krogen used in our example with a 400 or 500-HP engine. Unlike semi-displacement hulls, long range trawlers with true displacement design cannot, in any practical sense, be pushed over the bow wave to get “on plane.” An over-sized engine would merely add to the weight and purchase cost of the vessel and occupy more space in the engine room than required.
The A/B ratio:
Trawlers are evaluated by the A/B ratio. “A” is the amount of mass above the waterline, and “B” is the amount of mass below the waterline when the boat is running under normal load. Robert Beebe evaluated the question carefully in “Voyaging Under Power”, the 1975 book still considered the ultimate authority on long distance cruising. Beebe’s conclusion, confirmed when his book was updated and revised by James Leishman, was that the mass above the waterline of a seagoing vessel should not exceed 2.6 times the mass below the waterline.
A practical approach for estimating the A/B ratio involves viewing an accurately scaled profile drawing of the vessel in question. Comparing careful estimates of the profile above the waterline with the profile below the waterline will create a usable A/R ratio.
A/B ratios are controversial. Some naval architects take issue with the entire concept. Critics correctly observe that the factors considered in stability studies don’t include an A/B ratio. The factors do, however, consider a vessel’s center of gravity. The lower the center of gravity in the hull, the less likely a boat will rock excessively in a beam sea or capsize. Voluminous superstructures perched on a shoal draft hull are more likely to have a higher center of gravity.
A high A/B ratio will have some ramifications that are completely separate from stability. The greater the amount of surface above the waterline, the more “sail area” is subjected to wind to confound close quarter maneuvering. On the positive side, more mass above the waterline permits cabins with more headroom and larger windows, and may even raise the main deck enough to increase the vertical clearance in the engine room.
The sea is no respecter of brand names. Storms do not defer to ad campaigns or marketing hype. Informed and intelligent selection of an ocean going trawler intended for long range cruising can be somewhat simplified by comparing and contrasting the afore-mentioned ratios and understanding their effects on predictive performance of a hull.