Preface: This discussion should not degrade into a Krogen versus Nordhavn argument, but rather an examination of the hull efficiency and it's impact on fuel consumption. Listmembers, There has been some discussion on fuel burn on Milt's boat and the feeling the numbers were higher than they should be. Recently I was discussing the topic of full displacement efficiency with Lary Polster of Krogen. Here is part of our discussion: Fuel Burn To help explain the difference in fuel economy it is important to note that there are differences amongst the displacement types. Jimmy Krogen, N.A. offers further insight beyond the obvious factors of horse power and displacement weight in his “Categorization of Full Displacement Hull Forms”. We hope this helps trawler enthusiasts better understand what James S. Krogen understood 30 years ago, end to end symmetry and light displacement to length ratios provide for better sea keeping and fuel economy in the full displacement realm. This concept is backed by a recent survey of Krogen 48’ owners that reported 3-3.4 gallons per hour at an average speed of 7.75 knots. Larry Polster Vice-President Kadey-Krogen Yachts The Categorization of Full Displacement Hull Forms (for long-range mono-hull motor yachts of near optimum Pc) Motor yachts are commonly categorized as either full displacement or semi-displacement, each having pros and cons. Full displacement forms are not commonly categorized but important distinctions do exist. Useful categorization can be made based on two form fundamentals: longitudinal symmetry and displacement to length ratio (D/L.) A form will tend towards having either high or low longitudinal symmetry and, tend towards having either high or low D/L. Prismatic coefficient (Pc) is also a very important form fundamental. The optimum Pc for displacement-hull operating speeds ranges from about 0.50 to about 0.70 with the ideal typically around 0.60 to 0.65. Since the majority of designs fall within this range, Pc will not serve much use in making distinctions. It should however be the filter through which less suitable craft are sifted out prior to selecting a full displacement hull form based on the categorization presented here. Longitudinal Symmetry- This is the degree to which the stern shape matches the bow shape. A barge is highly symmetrical but it doesn’t have a very good seagoing bow to begin with. A double ended motor life boat is a better example of the kind of symmetry desired, a nice pointy bow followed by a nice pointy stern. The canoe stern shape is another good example. By the way, some yachts exhibit a “canoe stern” look-a-like shape above the water but have relatively flat sections and broad waterlines below the water. A hull with good symmetry will have v shaped sections and sharp waterlines at both ends. Pros for Symmetry: An asymmetrical hull with an immersed transom or relatively flat sections aft will tend to roll more in a following sea by virtue of the leverage wave action has on the hull buoyancy present at the extremes of the vessel’s under water beam. For typical hull forms this roll will cause the vessel to veer off course (yaw) in a direction opposite the roll. This roll induced yawing can tend to degrade course keeping. Therefore, symmetrical forms track better in waves. V shaped sections aft slice following seas rather than surf them resulting in lower pitch accelerations and less slamming. Sharp waterlines aft offer less wave making resistance and eddy current drag than the broad waterlines of asymmetrical hulls resulting in better fuel economy at full displacement speeds. Pros for Asymmetry: An asymmetrical form by virtue of its broader waterline and flat sections aft will resist the squatting tendency of the symmetrical form at higher displacement speeds (i.e. above S/L of 1.34.) Therefore this form can be driven faster than a symmetrical form of equal weight and horsepower. This form is taking a step closer to the semi-displacement type but is still considered full displacement by virtue of its full length ballasted keel and its concentration of underwater volume near amidships in keeping with the optimum prismatic coefficient desired for displacement speeds of operation (typically less than 0.65.) Broader waterlines aft increase waterplane inertia producing a vessel that is less prone to list over to one side when imbalances arise such as unequal fuel distributions or sideways wind pressure against a deckhouse. The broader waterlines and flatter sections of the asymmetrical form will typically allow the cabin sole aft to be extended further out toward full beam width thereby creating a significant gain in useful interior accommodation space. Displacement Length Ratio (D/L) - This measurement indicates whether a given displacement (under water volume) is carried over a long waterline length (LWL) or a short. If carried over a long waterline, D/L is low. If over a short, D/L is high. Low D/Ls tend towards lean and slippery forms with fine ends. High D/Ls tend towards full bodied and less efficient forms with blunter ends. The higher the D/L, the more the hull will approach a barge shape (prismatic coefficient approaching 1.00.) So remember, high D/Ls are to be kept in balance with optimum Pc. Note that D/L reflects two important hull efficiency variables: waterline length and underwater hull mass. Since under water mass (volume) can be equated to a vessel’s weight, inaccurate conclusions are sometimes made using D/L: 1) Low D/L means light weight- Sometimes but not necessarily. Consider two yachts having the same weight but differing LWLs. The one with the longer LWL will have the lower D/L and be more efficient without any loss in displacement (weight.) 2) Low D/L means a less substantial structure- Not necessarily. This assumes an approach whereby weight is reduced to gain a lower D/L. As seen in the above example, a lower D/L can be achieved without reducing weight- including structural weight. In other words, structural material does not have to be removed to gain an efficient D/L. 3) Low D/L means less displacement and therefore less living and provisioning space- Similar to example 1, two yachts with the same displacement will have equal weight carrying capacity and therefore can have equivalent living and provisioning space. Determine your required living and provisioning displacement and then distribute it over a long waterline rather than a short for an efficient D/L. 4) Low D/L means long and narrow - Often but not necessarily. Now consider two yachts with the same LWL and beam water line. One with rounded sections, wide keel and full ends. The other with leaner sections, narrow keel and fine ends. Obviously the latter will be of lesser displacement than the former and have a lower more efficient D/L- without having resorted to a long narrow form. Pros for Low D/L: Low D/Ls result from either long LWLs or streamlined underbodies, or a mix of each. Both form characteristics serve to improve hull efficiency and therefore fuel economy. Longer LWLs permit higher displacement speeds and streamlined sections result in a more easily driven hull through the speed range. For example, if a given displacement is stretched over a longer waterline two thing happen: The LWL increases (higher hull speed) and the ends get finer and sectional areas less full, i.e. streamlining (less hull drag.) Of course, reducing a vessel’s fully loaded weight (displacement) will lower D/L and improve economy but the fully loaded displacement requirement is usually already more or less locked in by the voyaging requirements (living quarters, fuel supply, provisions etc.) Pros for High D/L: High D/L permits the required displacement to be carried in a shorter length. This can be driven by practical concerns such as dockage space requirements or perhaps because a smaller length vessel will typically be easier to maneuver and maintain. High D/L usually results in a waterplane area that increases in fullness at the ends or sometimes along its entire length. Since initial stability (not range of stability) increases exponentially for such fattening of the waterplane area, high D/L yachts have the capability to carry more weight aloft. This brings about the potential for more living space by virtue of the permitted larger deck houses and the ability to carry heavier gear up high such as a yacht’s tender up upon a cabin top. Written by: Jimmy Krogen Naval architect for Kadey Krogen Yachts, Inc.

This type of discussion really highlights the difficulty of getting actual data. Lately, there have been a few people who candidly shared their experiences with the list with some second-guessing. Why? Because the expectation was very optimistic. Fact: the first national magazine write-up of the N47 by Power & Motoryacht estimated something like 9 kts @ 3gph. Fact: Krogen aggressively claims their designs are "form stabilized" and stabilization is good, but almost optional (DeFever has made similar claims). On the NAR when the hydraulic stabilizers on the Krogen 58 failed, I wonder if the crew would agree, or if they were too busy hanging over the leeward rail. There is no free lunch. The key point is that boats in the open ocean behave differently than boats on day-trips in protected or inland waters. Scott is quite correct - it's not a "Brand X vs Brand Y" discussion. It's a "real world" versus "hype and expectation" discussion. Peter www.SeaSkills.com --- scottebulger@comcast.net wrote: > Preface: This discussion should not degrade into a > Krogen versus Nordhavn argument, but rather an > examination of the hull efficiency and it's impact > on fuel consumption. > Listmembers, > There has been some discussion on fuel burn on > Milt's boat and the feeling the numbers were higher > than they should be. Recently I was discussing the > topic of full displacement efficiency with Lary > Polster of Krogen. Here is part of our discussion: > > Fuel Burn > > To help explain the difference in fuel economy it is > important to note that there are differences amongst > the displacement types. Jimmy Krogen, N.A. offers > further insight beyond the obvious factors of horse > power and displacement weight in his “Categorization > of Full Displacement Hull Forms”. We hope this helps > trawler enthusiasts better understand what James S. > Krogen understood 30 years ago, end to end symmetry > and light displacement to length ratios provide for > better sea keeping and fuel economy in the full > displacement realm. This concept is backed by a > recent survey of Krogen 48’ owners that reported > 3-3.4 gallons per hour at an average speed of 7.75 > knots. > > Larry Polster > Vice-President > Kadey-Krogen Yachts > > > The Categorization of Full Displacement Hull Forms > (for long-range mono-hull motor yachts of near > optimum Pc) > > Motor yachts are commonly categorized as either full > displacement or semi-displacement, each having pros > and cons. Full displacement forms are not commonly > categorized but important distinctions do exist. > Useful categorization can be made based on two form > fundamentals: longitudinal symmetry and displacement > to length ratio (D/L.) A form will tend towards > having either high or low longitudinal symmetry and, > tend towards having either high or low D/L. > Prismatic coefficient (Pc) is also a very important > form fundamental. The optimum Pc for > displacement-hull operating speeds ranges from about > 0.50 to about 0.70 with the ideal typically around > 0.60 to 0.65. Since the majority of designs fall > within this range, Pc will not serve much use in > making distinctions. It should however be the filter > through which less suitable craft are sifted out > prior to selecting a full displacement hull form > based on the categorization presented here. > > Longitudinal Symmetry- This is the degree to which > the stern shape matches the bow shape. A barge is > highly symmetrical but it doesn’t have a very good > seagoing bow to begin with. A double ended motor > life boat is a better example of the kind of > symmetry desired, a nice pointy bow followed by a > nice pointy stern. The canoe stern shape is another > good example. By the way, some yachts exhibit a > “canoe stern” look-a-like shape above the water but > have relatively flat sections and broad waterlines > below the water. A hull with good symmetry will have > v shaped sections and sharp waterlines at both ends. > Pros for Symmetry: An asymmetrical hull with an > immersed transom or relatively flat sections aft > will tend to roll more in a following sea by virtue > of the leverage wave action has on the hull buoyancy > present at the extremes of the vessel’s under water > beam. For typical hull forms this roll will cause > the vessel to veer off course (yaw) in a direction > opposite the roll. This roll induced yawing can tend > to degrade course keeping. Therefore, symmetrical > forms track better in waves. V shaped sections aft > slice following seas rather than surf them resulting > in lower pitch accelerations and less slamming. > Sharp waterlines aft offer less wave making > resistance and eddy current drag than the broad > waterlines of asymmetrical hulls resulting in better > fuel economy at full displacement speeds. > Pros for Asymmetry: An asymmetrical form by virtue > of its broader waterline and flat sections aft will > resist the squatting tendency of the symmetrical > form at higher displacement speeds (i.e. above S/L > of 1.34.) Therefore this form can be driven faster > than a symmetrical form of equal weight and > horsepower. This form is taking a step closer to the > semi-displacement type but is still considered full > displacement by virtue of its full length ballasted > keel and its concentration of underwater volume near > amidships in keeping with the optimum prismatic > coefficient desired for displacement speeds of > operation (typically less than 0.65.) Broader > waterlines aft increase waterplane inertia producing > a vessel that is less prone to list over to one side > when imbalances arise such as unequal fuel > distributions or sideways wind pressure against a > deckhouse. The broader waterlines and flatter > sections of the asymmetrical form will typically > allow the cabin sole aft to be extended further > out toward full beam width thereby creating a > significant gain in useful interior accommodation > space. > > Displacement Length Ratio (D/L) - This measurement > indicates whether a given displacement (under water > volume) is carried over a long waterline length > (LWL) or a short. If carried over a long waterline, > D/L is low. If over a short, D/L is high. Low D/Ls > tend towards lean and slippery forms with fine ends. > High D/Ls tend towards full bodied and less > efficient forms with blunter ends. The higher the > D/L, the more the hull will approach a barge shape > (prismatic coefficient approaching 1.00.) So > remember, high D/Ls are to be kept in balance with > optimum Pc. Note that D/L reflects two important > hull efficiency variables: waterline length and > underwater hull mass. Since under water mass > (volume) can be equated to a vessel’s weight, > inaccurate conclusions are sometimes made using D/L: > 1) Low D/L means light weight- Sometimes but > not necessarily. Consider two yachts having the same > weight but differing LWLs. The one with the longer > LWL will have the lower D/L and be more efficient > without any loss in displacement (weight.) > 2) Low D/L means a less substantial structure- > Not necessarily. This assumes an approach whereby > weight is reduced to gain a lower D/L. As seen in > the above example, a lower D/L can be achieved > without reducing weight- including structural > weight. In other words, structural material does not > have to be removed to gain an efficient D/L. > 3) Low D/L means less displacement and > therefore less living and provisioning space- > Similar to example 1, two yachts with the same > displacement will have equal weight carrying > capacity and therefore can have equivalent living > and provisioning space. Determine your required > living and provisioning displacement and then > distribute it over a long waterline rather than a > short for an efficient D/L. > 4) Low D/L means long and narrow - Often but > not necessarily. Now consider two yachts with the > same LWL and beam water line. One with rounded > sections, wide keel and full ends. The other with > leaner sections, narrow keel and fine ends. > Obviously the latter will be of lesser displacement > than the former and have a lower more efficient D/L- > without having resorted to a long narrow form. > Pros for Low D/L: Low D/Ls result from either long > LWLs or streamlined underbodies, or a mix of each. > Both form characteristics serve to improve hull > efficiency and therefore fuel economy. Longer LWLs > permit higher displacement speeds and streamlined > sections result in a more easily driven hull through > the speed range. For example, if a given > displacement is stretched over a longer waterline > two thing happen: The LWL increases (higher hull > speed) and the ends get finer and sectional areas > less full, i.e. streamlining (less hull drag.) Of > course, reducing a vessel’s fully loaded weight > (displacement) will lower D/L and improve economy > but the fully loaded displacement requirement is > usually already more or less locked in by the > voyaging requirements (living quarters, fuel supply, > provisions etc.) > > Pros for High D/L: High D/L permits the required > displacement to be carried in a shorter length. This > can be driven by practical concerns such as dockage > space requirements or perhaps because a smaller > length vessel will typically be easier to maneuver > and maintain. High D/L usually results in a > waterplane area that increases in fullness at the > ends or sometimes along its entire length. Since > initial stability (not range of stability) increases > exponentially for such fattening of the waterplane > area, high D/L yachts have the capability to carry > more weight aloft. This brings about the potential > for more living space by virtue of the permitted > larger deck houses and the ability to carry heavier > gear up high such as a yacht’s tender up upon a > cabin top. > > > > > Written by: > Jimmy Krogen > Naval architect for Kadey Krogen Yachts, Inc. > > > _______________________________________________ > Passagemaking-Under-Power Mailing List >