review of available options

7/27/2019 Review of Available Options

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Few would argue with the premise that a vessel operator should have as much control over an

underway vessel as possible. As designers, we provide control over boat speed by way of throttles.

Likewise, we provide control over the direction of the vessel by a combination of steering and, at

lower speeds, side thrusters and gear selection, Yet some would say that active trim-control devices

are not necessary on properly designed vessels. They maintain that such accessories as trim tabs

(flat planes at the transomsee Figure 1), Interceptors (vertical planes at the transomsee Figure

2) and liquid ballast tanks, or power trim on outboards and I/Os, would not be required if the boat

were properly designed in the first place. While that may be true in an ideal world, no designer or

vessel operates in such a fantasy realm; good design is full of compromises between conflicting

real-world demands. In the case of dynamic trim (sometimes called running angle), the designrequirements are to: minimize resistance and power, or maximize -speed; optimize ride quality at

sea, whether for passenger comfort, payload protection, or weapons control; maintain adequate

visibility, particularly over the bow at intermediate speeds; and avoid dynamic instability. [For

more on this topic, see "Planing -Hull Stability," PBB No. 31, page 20Ed.]


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Figure 1Trim tabs are nominally flat plane surfaces in which the immersed area stays

constant and the angle varies.

There are also operational factors to contend with. Vessels carry an assortment of payloadsbe it

people, cargo, fuel, water, or suppliesthat vary in weight and location, not only from trip to trip,but from time to time during any one trip. Vessels are also subject to a wide range of wind and

wave conditions, from dead calm to .survival condition, often coming from the wrong direction.

Finally. there's the amount of surrounding water to consider, as water depth and channel width are

significant factors in both resistance and dynamic trim.

In the absence of any control over all these external factors, there needs to be some way beyond hull

form to control trim. This is not to say that the designer and builder are relieved of responsibility for

developing a hull-form that trims naturally within predetermined bounds. To achieve the best in all-

around performance and to minimize sensitivity to off-design conditionswhether created before

or after delivery from the builderwe recommend that once careful weight calculations are

completed, the hull be designed to operate with a bit more positive, or bow-up, trim than is ideal for

resistance. This allows the bow to ride high in severe following-sea conditions, helping to keep the

weather deck dry. It also allows some margin for error in the designer's longitudinal center ofgravity (LCG) calculations, or for unforeseen changes during construction or operation. Trim tabs

and interceptors induce a positive lifting force at the Stern, creating a bow-down or negative

trimming moment, and reducing the dynamic trim angle,

Figure 2Interceptors are nominally vertical planes that are lowered into the flow stream at

the transom to increase immersed area. They generate a lift force at the stern of a boat to

create a bow-down trimming moment.


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The designer has the greatest opportunity to develop a balanced boat design when given unrestricted

freedom by the owner or builder. Unrestricted freedom, however, often conflicts with reality. At the

beginning of a project, the designer has a variety of trim-control techniques at his or her disposal,

hut the number of design tools diminishes as construction proceeds. Selecting the appropriate

longitudinal distribution of chine beam and deadrise angle of the hull with regard to the LCG is

very effective when developing hull lines. Careful placement of fuel, water, and other tankage

relative to the LCG provides an opportunity to manage trim as loads change. Depending on thedesign speed of the boat, hook or rocker can be designed into the after body buttocks 10 influence

running trim. From the outside of the hull, hook is concave curvature (downward at the transom)

and creates a bow-down trimming moment when underway, reducing running trim. Rocker is

convex curvature (upward at the transom) and creates a bow-up moment, increasing running trim.

Good design entails making informed choices to meet all operational requirements. When a planing

craft's design has progressed sufficiently, light and full-load displacements and LCGs can be

established with some certainty. The designer then has the option to vary overall dimensions

slightly. Such subtle geometric refinements can make significant contributions to dynamic trim and

thus to resistance, visibility, dynamic stability, and ride quality in waves.

The simple hull diagrams in Figure 3 show bottom-geometry variations, which can result in

dynamic bow-up or bow-down trim moments, Hull geometric features that develop bow-up

trimming moments are used for high speed craft, features to develop bow-down trimming momentsare most frequently used for craft that operate at semi-planing speed or slower. These suggestions

are approximate, because hull-loading and LCG contributions also need to be considered.

It's clear that running trim can be influenced both by designed-in fixed features and operator-

controlled systems. Our intent is to offer guidance as to which design conditions call for rocker or

hook, as well as suggestions for sizing trim tabs and interceptors.

Measuring running trim for a full range of boat speeds during sea trials provides a valuable database

lor evaluating and/or diagnosing operating characteristics of existing craft. Trim can be easily

measured with electronic devices. Figure 4 shows a speed-trim graph obtained from model tests and

scaled to a 14,000-lb, 31' boat (6,350 kg. 9.5m).

This figure points out the significance of having operator trim control throughout a range of speeds.

At or below hump speed, a boat's ability to accelerate to planing speed is improved by a bow-down

trimming moment. Once on plane, resistance can be reduced and speed increased by applying astern- down moment.


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Figure 3Simple hull diagrams showing bottom-geometry variations, which can result in

dynamic bow-up or bow-down trim moments.

Figure 4This speed-trim graph. obtained from model tests and scaled to a 14.000-lb. 31'

boat (6,350 kg, 9.5m), points out the significance of having operator trim control throughout a

range of speeds.

There are, of course, safety limits involved with applying trim control. Too high a running-trim

angle may cause- porpoising or visibility problems, Too low an angle may result in loss of dynamic

stability, leading to yaw and roll problems.

Hook and Rocker


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Hook and rocker buttock curvature are defined by radii with their axes below or above the hull

bottom, as shown in Figure 5. The theories of Bernoulli are available to those interested in a

detailed hydrodynamic explanation. Simply put, hook increases dynamic pressure, which lifts the

stern. Rocker reduces dynamic pressure, which sucks the stern down.

Figure 5Hook and rocker buttock curvature are defined by radii with their axes below or

above the hull bottom.

Design displacement and maximum speed can be used as criteria for determining when hook or

rocker should be incorporated into the hull lines. Figure 6 provides guidance in this area. When

design conditions fall between the regions for hook and rocker, it's best to use straight after body

buttock lines.

Figure 6Design displacement and maximum speed can be used as criteria for determining

when hook or rocker should be incorporated into the hull lines. The figure above provides

guidance in this area.

If the designer decides that hook or rocker in the hull lines could be advantageous, the next question

is, how much? Model-test data offer some direction. Results of one comprehensive series of tests

are given in Figure 7- The base of the graph is a ratio of projected chine length to buttock radius,


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where zero represents straight buttock lines. The vertical scale indicates the hull-resistance-to-

weight ratio- The curves represent conditions of constant dimensionless speed (volume Froude

number, Fv) and constant trim.

Figure 7Derived from a comprehensive series of model tests, the results shown here can

help a designer determine how much hook or rocker to add to the hull lines.

The speed contours were selected for their significance;

Fv = 1.8 approximate hump speed Fv = 2.8 low planing speed Fv = 4.5 high speed Fv = 6.0 very high speedFrom this figure, you can see that hook and rocker have only a small influence on hull resistance for

speeds below Fv = 2.8, Thus, they can be employed to control trim to improve visibility, enhance

ride quality, and reduce wetness with little or no impact on boat speed. At higher speeds, however,

dramatic improvements in boat performance are possible by avoiding hook in favour of straight or

rockered buttocks.

In plan view, hook or rocker should be greatest on centerline. It should extend about 15% of chine

length forward of the transom and about 80% of the chine beam outward from centerline, We

recommend that bow-up trim changes induced by rocker should not exceed 1 at maximum speed,

in order to avoid porpoising, If the maximum boat speed greatly exceeds- Fv = 2,8 and some bow-

down moment is desired, we recommend avoiding hook in favor of operator-controlled devices

such as trim tabs or interceptors.

Consider the following as one example of a design approach. Assume the boat dimensions below

for evaluating hook or rocker:

LOA -31'(9.5m) overall length LP - 29.5'(9m) chine length BOA - 10.0'(3.05m) overall beam BPX=8.5'(2.6m) maximum chine beam =12,5 midship deadrise W = 14,000 lbs (6,350 kg) half-load displacementIf the boat has installed power for low planing speed (Fv = 2.8, or 23.1 knots), the hull with straight

aft buttocks might trim at 4.2. To reduce the running trim to 3 with hook, then LP/R =-0.27 at

centerline would be required, as per Figure 7, Thus, the hook radius at centerline, R = -109' (33.2m)

[29.5/-0.27], should he faired transversely into the unaltered hull lines covering a bottom width of


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413

3

2

490W

M= * * 150010000

P

P

PX P

PX

L

L

B L

B

+

M = moment per degree, in foot * pounds

W = displacement, in pounds

LP = projected chine- length, in feet

BPX = maximum projected chine beam, in feet

V = volume of displacement, in cubic feet (ft3)

To calculate the corresponding lift force required:

MFr

X

=

Fr = lift force required, in pounds

M = moment per degree, in foot-pounds

x = horizontal distance between longitudinal center of gravity (LCG) and transom of boat, in feet

To calculate attained lift for trim tab(s):

20.125* * *t

Fa A V = Fa = lift force attained, in pounds

t - trim tab angle, in degreesA = total projected area in plan view of trim tab(s), in square feet (ft

2)

V = velocity of water over tab(s.), in knots

Figure 8 A stern configuration, showing tab, interceptor, and angles.

Once trim-tab dimensions and angle are developed to achieve the desired lift, interceptor

dimensions for equivalent lift can be calculated. To simplify the calculation, it is assumed that the

trim tab and the interceptor are the same width.

First, calculate an equivalent angle for the interceptor and then derive a depth, using Figure 8 and

the formulae below:

20.175 0.0154i t t

= +


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t= trim tab angle. in degrees, for a defined chord lengthangle not to exceed 15

i = equivalent geometric angle for intercepto

sini

d Lc =

Lc = defined chord length, in ind = interceptor depth, in inches

Other Considerations

Details of planing trim tabs or interceptors should be considered prior to finalizing the design. To

maximize the effectiveness of the system in transverse leveling, or correcting running heel, it's best

to place the tabs or interceptors well outboard, usually just inboard of the chines. In some cases, we

have noted that placing tabs adjacent to the hull centerline can aggravate any tendency toward

dynamic instability.

On most small craft, tabs and interceptors can be attached to the transom, extending beyond the

length of the hull itself. If the transom is curved, it's necessary to create a flat surface for mounting

the unit, either by adding a shim or including a suitable spot in the mold. For sport fishing boats and

boats with integral swim platforms, where snagging fishing lines or legs is a concern, trim tabsshould be recessed under the hull. Cavities must be created, matching the dimensions of the tabs as

closely as possible so that the tabs are flush when retracted.

Whether mounted on the transom or under the hull, trim tabs should generally be in line, at least

initially, with the hull bottom when fully retracted. This can be checked with a straight batten and

adjusted by properly positioning the actuation cylinders or by placing physical stops in way of the

tabs. Any unintended up or down angle remaining in the fully retracted position will create effects

similar to rocker or hook, raising or lowering the running-trim angle.

Figure 9 Pressure drag increases with running trim angle, but viscous drag decreases. So,

for each hull bottom configuration and speed, there is a certain running-trim angle that

minimizes total drag and thus required power. Trim tabs or interceptors can help achieve this

optimum angle. The graph here illustrates drag curves, at a given speed, for prismatic planing

surfaces with 0, 10, and 20 of deadrise.

If the boat is fitted with underwater exhausts, it may influence the placement of trim tabs or

interceptors. The flow path of the exhaust gases should be predicted or observed at the transom, and


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the units placed away from the exhaust flow. Placing them in line with the flow reduces their

effectiveness and in extreme cases, may render them totally useless.

Boats with propeller tunnels require special consideration. If the tunnels intersect the transom, the

available width for trim-tab placement is reduced. If possible, it's best to place the entire tab area

outboard of the tunnel. which sometimes necessitates tab planes that are- longer and narrower than

usual. When area outboard of the tunnels is extremely limited, placing a second set of tabs inboard

of the tunnels may be required.

Figure 10 Running trim and hull deadrise both have a significant effect on wetted surface,

as welt as on the location and direction of spray. This illustration shows deadrise angles of 0,

5 10, and 20. and running trim angles of 2. 4, and 6.

It's also possible for under-hull tabs placed immediately adjacent to tunnels to lead to ventilation of

the tunnels or rudders. drawing air into the tunnel cavity from the surface of the water at the

transom. This can result in "blowing out" the rudders and/or propellers, so it's a good idea to leave

at least 1" (2.54cm) of intact hull bottom between the tunnel and the lab. A downward-turned edge,

or fence, on the lab side next to the tunnel can also help avoid the ventilation problem.

On boats with propeller tunnels, small changes to the aft end of the tunnel can sometimes affect

running trim and speed dramatically. Opening up the tunnel area at the transom will create a bow-up moment, similar to rocker. Wedges at the trailing ends of the tunnels, either designed in initially

or fitted after testing, will create a bow-down moment, similar to hook. [For more on stern wedges,

see "Stern Flaps," PBB No. 70, page 81Ed.]. Because of the accelerated water flow in tunnels,

very little area and angle is needed for these wedges to significantly affect running trim (and

sometimes speed). It's difficult to get accurate results from model tests for such details, so full-scale

testing on a prototype or completed boat is recommended. PBB

About the Authors:

Donald Blount, president of Donald L. Blount and

Associates INC. (Chesapeake, Virginia), is a professional engineer, a Fellow of the Society of

Naval Architects and Marine Engineers, and the author of numerous technical articles and researchpapers.

Dudley Dawson. president of Dawson Marine Group (Greensboro, North Carolina), is a

professional engineer, technical correspondent for Yachting magazine, and a contributing editor of

Professional BoatBuilder magazine.

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