search.noResults

search.searching

dataCollection.invalidEmail
note.createNoteMessage

search.noResults

search.searching

orderForm.title

orderForm.productCode
orderForm.description
orderForm.quantity
orderForm.itemPrice
orderForm.price
orderForm.totalPrice
orderForm.deliveryDetails.billingAddress
orderForm.deliveryDetails.deliveryAddress
orderForm.noItems
The marine surveyor should note that the midship section bottom right in Figure 1 is said to be slab or wall sided NOT because the sides are called walls (they are, in fact, called sides) but because, like the walls in a house, they are flat and upright. The curved section forward of the parallel mid body is called the fore ship or entry and that abaft the after end of the parallel mid body is called the after ship or run.


When surveying such a boat, it is very important that the main and secondary dimensions of the hull be measured. The job takes about 5 minutes. The marine surveyor should remember that he is paid to record in his report items that he himself has measured, weighed, tested, whatever, NOT what somebody else has told him. So- called reported dimensions are not evidence, they are legally hearsay and usually undefined.


In the


event of the surveyor being sued, recording reported dimensions in his report would be a gift to the prosecuting counsel.


To illustrate the transverse metacentric stability of a narrowboat, the following dimensions for a typical modern vessel were chosen:


Item


Length Overall Length Hull


Length Waterline Breadth Overall Breadth Waterline Depth Overall


Draught Forward Draught Aft Freeboard


Displacement Weight Displacement Volume Block Coefficient


Symbol


LOA LH


LWL BOA BWL DOA TF TA f


Δ CB Table 1 Typical Narrowboat Dimensions 36 | The Report • March 2019 • Issue 87


The boat’s controlling dimension is the Breadth Overall which, in order to use the locks on much of the canal system, must not exceed six feet ten and a half inches or 2095 mm. The breadth that controls the stability of the boat, however, is that on the waterline which, after subtracting the thickness of any rubbing bars each side is usually about six feet eight and a half inches or 2040 mm but may be several mm less.


To understand why a ship or boat stays upright when afloat, it is necessary, first of all, to understand some naval architecture terms. Figure 2 shows the midship section of a narrowboat with wall sides. She


is floating upright in still water. G is the centre of gravity and B the centre of buoyancy which is the geometric centre of the underwater form. G is vertically above B which, in turn, for most narrowboats is about 0.75 times the depth of the hull. G, for an average narrowboat is usually at or very close to the waterline and both B and G are on the boat’s centreline. The weight of the boat and everything on her – the displacement and acting downward through the point G – is equal and opposite to the force of buoyancy acting upwards through the point B. Both are given the symbol Δ which is the Greek letter delta and corresponds to the English letter D.


Δ W Δ Figure 2 Initial Positions of Centres of Buoyancy and Gravity in Still Water


Dimensions Imperial


52’ 0” 50’ 0” 48’ 9”


6’ 10½” 6’ 8½” 3’ 9” 1’ 4” 2’ 2” 2’ 0”


12.00 t


432.00 ft3 0.7922


Dimensions Metric


15.85 m 15.24 m 14.86 m


2095 mm 2045 mm 1.14 m 0.41 m 0.66 m 0.61 m


12.88 te 12.88 m3 0.7922


If now a small weight w kg is moved through a distance of d metres across the deck from port to starboard, the boat will heel to an angle of θ degrees. θ is the Greek letter theta and is pronounced as a soft th as in thin. See Figure 3 above. The underwater volume will remain the same, but its shape will alter. That will cause the centre of buoyancy to move to position B1. The weight of the boat will remain the same and will continues to act downward through the point G, but the buoyancy force, although it will remain the same in magnitude will now act upward through the point B1. The vertical line though the new centre of buoyancy will cross the centreline of the boat at the point M which is called the metacentre. The distance, GM is called the metacentric height and


G B


L


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70  |  Page 71  |  Page 72  |  Page 73  |  Page 74  |  Page 75  |  Page 76  |  Page 77  |  Page 78  |  Page 79  |  Page 80  |  Page 81  |  Page 82  |  Page 83  |  Page 84  |  Page 85  |  Page 86  |  Page 87  |  Page 88