Hull Form

Trawler yachts designed for ocean cruising should have hull forms optimized for economy of travel, as fuel consumption is by far the major running cost. In many ways other considerations should be subordinated. Unfortunately in many vessels excessive demands for spacious accommodations and restricted draft have severely compromised this aim and have resulted in craft with poor propulsive characteristics and sea keeping ability. It is worth noting that hull forms optimized for least resistance also have the best characteristics for sea-kindliness.

Sailing vessels dominate the cruising market. By their very nature, in order to make reasonable passage times under sail, their hull designs have had to be optimized for least resistance. This has resulted in, what we would consider from a power vessel standpoint, poor accommodation arrangements, even in the largest yachts. 

For this reason a trawler yacht hull with the proportions of a sailboat would have little to offer.
An example is shown in fig. 1; a vessel from around 1912 with the proportions of a sail boat.

Vessels of this general type proliferated in the early part of last century. At the time it was thought that this hull form offered the best solution to the limited horse power rating of engines then available. In the case of this particular vessel: 8 bhp. 

While such a hull form offers very little resistance, if sized to offer the same volume as our Watson 48 model it would need to be over 70ft in length. The resulting vessel would provide limited accommodation options and very poor deck space arrangements. Also, and more importantly, it would suffer from exaggerated pitching and be exceptionally “wet” in any kind of sea. Mooring fees, which are generally charged by length, would also be proportionally higher. Plainly because of the conflicting demands of spaciousness and desire for high propulsive economy the trawler yacht hull has to be a compromise. 

There are principle factors affecting a power vessel hull form and resistance and very severe penalties suffered by compromising some of these factors. 

The resistance of a hull can be divided into two main components: 

(1) Form Resistance – commonly called wave-making resistance. This is the resistance caused by the shape of the hull moving through the water. At the steaming speeds of trawlers this resistance amounts to approx. 75% of the total. 

(2) Frictional resistance – This is the resistance caused by the friction on the wetted surface of the hull. Typically this is around 25% of the total. 

Of the two components we shall consider the form resistance because it is influenced strongly by the shape of the hull. The frictional resistance is a function of wetted surface area only and is not influenced by shape. 

In order to compare form resistance’s of vessels of differing lengths it should be noted that the specific resistance of two hulls is not proportional to their speeds, but to the square root of their waterline lengths (?L). It is therefore convenient to express speeds non-dimensionally as V/?L. In other words at a speed of V/?L = 1, a 50ft WL hull would have a speed of 7.07kn and a 100ft WL hull would have a speed of 10kn, yet all other things being equal, both these hulls would have the same specific resistance. 

It is convenient to describe resistance in terms of specific resistance, which is the ratio of the resistance in lbs. divided by the vessel displacement, i.e. Lbs. per tonne of displacement. The reason for this is that vessels of any length, all other things being equal, have the same specific resistance at equal speed length ratios (V/?L). 

These curious relationships stem from the fact that form resistance depends on the length of the wave system generated by the hull. At a speed length ratio of V/?L = 1.34 the wave generated by the hull has a length crest to crest the same length as the hull waterline length. 

Hull shape can be defined mathematically by a series of coefficients of form. The principle coefficients are:

L/D Length to draft ratio. 
B/L Beam to length ratio. 
B/D Beam to draft ratio. 
? (L/100)3 Displacement length ratio. 
Cp Prismatic coefficient. 
Cb Block coefficient. 
Cm Midship section coefficient.

All these coefficients have a powerful effect on the resistance of a hull but in a particular design many are fixed by overriding considerations. 

L/D This ratio is often fixed by the demand for shallow draft. Restricting the L/D has a profound effect on the propulsive efficiency. 

B/L This ratio is generally fixed by the demands of accommodation and stability. 

B/D High B/D ratios result in higher specific resistance than low B/D ratio. Once again high B/D ratios are caused by restricted draft. 

? This coefficient is fixed for a particular design by the weights of hull, machinery, out fit and fuel load. 

Cp This is the ratio of the volume of displacement to the volume of a prism of the following dimensions: area of midship section x waterline length, and is really a measure of how the displacement is spread over the waterline length. This coefficient has the most profound effect on Form Resistance and must be optimized at all costs. 

Cb This is the ratio of the volume of displacement to the volume of a block of the following dimensions: draft amidships x beam amidships x length waterline. In any design Cb should be as low as possible. A vessel with a low Cb has a vee shaped bottom amidships. A flat-bottomed vessel has a high Cb. Shallow draft vessels generally have a high Cb. This coefficient has an important bearing on the interaction between hull and propeller. 

Cm This is the ratio of the area of the underwater midship section and a rectangle with the following dimensions: draft amidships x beam amidships. This coefficient has a strong relationship with Cb and should be minimized for the same reasons. 

As can be seen many of the hull coefficients are unavoidably fixed in a particular design, but the most important one from the standpoint of Form Resistance, Cp, is variable or should be in any design. 

To demonstrate the Form Resistance of a hull and the importance of Cp, we have drawn up a table for a hypothetical 50ft WL vessel with specific resistance’s at various Cp`s plotted against V/?L (speed length ratios). The optimum Cp for each V/?L is shown in the right hand column.

V/?L

V knots

Specific Resistance lb`s/Tonne

Optimum Cp



Cp = .50

Cp = .60

Cp = .70

 

0.6

4.24

3.41

3.41

4.03

.540

0.7

4.95

4.72

4.77

5.37

.530

0.8

5.66

6.39

6.39

7.84

.530

0.9

6.36

8.52

9.02

12.22

.525

1.0

7.07

11.00

13.92

22.52

.525

1.1

7.78

16.62

19.22

33.32

.530

1.2

8.49

30.52

28.02

42.02

.580

1.3

9.19

56.30

45.19

54.19

.620

1.4

9.90

86.60

69.60

73.60

.640

1.5

10.61

125.30

106.30

103.30

.660

The resistance figures in this table demonstrate, for a displacement hull, three things unmistakably.

  • The very low resistance’s at lower speed length ratios.
  • The sharp rise in the rate of increase in the resistance at about V/?L = 1.25.
  • The penalties to be paid for not having the Cp correct for the speed.

Refering to (2) above this table shows clearly that speeds above V/?L = 1.3 (9.19knots) become increasingly impractical. If this table were extended to higher V/?L`s the resistance figures would rapidly approach infinity. At about V/?L = 1.65 a planning hull form must be chosen, one that can reduce its displacement hydro-dynamically and hence its specific resistance. Unfortunately planning hulls are quite unsuitable for ocean crossing powerboats.

At speed of V/?L = 1.34 (9.47knots in this case) is what is commonly termed “hull speed” and should be considered the maximum design speed of a passage maker. At this speed the hull will have generated a wave system the same length as its waterline.

A cruising speed of V/?L = 1.34 however would be very uneconomic for a Trawler Yacht and a better choice would be a speed of V/?L = 1.2 (8.49 knots) before the resistance upturn begins. At this speed it can be seen from the table that the resistance is almost halved.
The optimum Cp for V/?L = 1.2 is 0.58 and for V/?L = 1.34 it is 0.62. Therefore a Cp of 0.60 is a prudent choice for a Trawler Yacht.

The penalty for choosing the incorrect Cp is plainly obvious from the table. Take our cruising speed of V/?L = 1.2 for instance. If our vessel has a Cp of 0.70 instead of 0.60 this would mean a 50% increase in resistance (i.e. A 28.02 lb./tonne against 42.02 lb./tonne). An increase of 9% would result from choosing a Cp of 0.5 instead of 0.60.

In summary it has been shown that many of the factors affecting resistance are fixed in the design but that the variable factors should always be optimized. There are a couple of other features that should also be considered.

  • Transom immersion – deeply immersed transom sterns can increase resistance markedly.
  • Half angle of entry – This is the waterline half angle of entry at the bow and should always be minimized. This is however very difficult in high B/L ratio craft. Full waterlines cause a premature pressure buildup at the bow. In the interests of forward accommodation spaces many vessels have overly full waterlines forward.

What steps can be taken to improve a design where the aforementioned factors are incorrect?

The short answer is very little. Some designers try to negate some of these poor features by fitting a Bulbous Bow. For high Prismatic and Block coefficient shallow draft hulls they have been proved through tank tests to be of benefit, with decreases in resistance of up to 15%. Simply adding a cylindrical bulbous bow to a hull is however not sufficient, it must be properly faired into the hull lines otherwise the laminar flow at the bow is affected.

Ultimately it is better to concentrate on optimizing the important coefficients we have discussed, as the gains to be made are far higher.

HULL TYPES

Hull shapes tend to be of three types:

  • Round bilge.
  • Double chine.
  • Single chine.

We are not aware of any authoritative tank tests having been made on hulls to compare the specific resistance’s of the three hull types, but from our own observations their resistance’s increase in descending order.

Round Bilge hulls are most readily executed in molded GRP hulls but add to the cost of steel and aluminum hulls.

Double Chine hulls can be designed to very closely resemble the features of a round bilge hull. We believe the resistance of this form is only marginally higher than that of a round bilge hull. Double chine is our preference for steel and aluminum hulls.

Single Chine hulls really only have a useful application in fast planning craft and coastal trawler types. Certainly this form has no place in an ocean capable trawler yacht. There are two main reasons for this:

  • It is very difficult to design a pleasing hull form because the single chine forces a strong linkage between topside shape and bottom shape where this should never be the case. li>
  • By definition the hull ends up with a sharp bilge. Water does not like passing around sharp edges. The only way to avoid this is to attempt to align the bilge with the flow of water along the hull. This is impossible to predict short of undertaking extensive model tests, and in any case this flow is ever changing at sea.

There is argument made that a single chine has a roll dampening effect in a seaway. This is probably true to a certain extent but the same effect would also increase the resistance of the hull in a seaway.

Single chine vessels are perceived by some to be cheap to construct. In fact the cost difference between single and double chine is only marginal and in the main scheme of things the additional cost is infinitesimal. Single chine hulls only look cheap.

Planning and Semi-Displacement Hulls.

Many vessels offered as Trawler Yachts are of the planning or semi-displacement type. Neither is suitable for making ocean passages.

Planning hulls are able to overcome the steep rise in the specific resistance as shown in our resistance table by reducing their displacement hydro-dynamically. The bottom hull form must therefore be designed as a lifting surface which demands a high Cp (0.70-0.80) and Cb and shallow draft. At speeds above V/? = 2.5 (17.68 knots in our 50ft example) planning hulls can have good propulsive efficiencies.

The powering of these hulls is however extremely sensitive to their displacement length ratio ^ which means their hull weights and fuel load must be minimized at all costs. For this reason the vessel is often weakly constructed.

In practice speeds of V/? = 2.5 can not be maintained except in the calmest of conditions which are rarely met at sea. They must therefore operate most of the time at the lower speed length ratios where all the hull form coefficients are incorrect. The effect of this is plain by comparing the specific resistance’s for Cp = 0.6 against Cp = 0.7-0.8 in our table.

Many readers will have noticed the exaggerated wave making effect of a planning hull travelling at lower speeds.

Semi-displacement vessels are presumably designed with form coefficients suitable for speeds between V/?L 1.34 and 1.65. The trouble is these vessels tend to fall between two stools, being neither suitable for planning nor lower cruising speeds. It is best to avoid this type for economical ocean cruising trawler yachts.

Last Updated (Wednesday, 15 September 2010 09:25)