Part 1 - RNLI Thurrock

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Part 1

TEAM > Tender Safety


KNOWING YOUR DINGHY

FLOTATION:

A dinghy when empty floats high upon the water at the  designed water line for no load. When a dinghy is loaded up with too many crew and too much gear above the maximum load it is designed to carry, it will sink down in the water until it is floating deeper than the designed waterline for the maximum load, with such a load there is a real  danger the dinghy will swamp or become top heavy and capsize.


Freeboard and floating









Figure 1-  Without crew and gear aboard the dinghy will float high upon the water


A dinghy’s hull shape (form) determines its power to float (i.e. the buoyancy of  the immersed part of the the hull) . The hull’s freeboard provides a measure of how well it is floating at a given waterline for the load it has onboard. In general the freeboard  should not be less than 0.2 m (8 inches) at any point with the gunwale sweeping up at the bow and stern (called the sheer line) so as to help encourage the dinghy to lift and ride the waves without taking water on onboard.  

The Design load

The dinghy in figure 1 although designed primarily for use by one person with some gear onboard, has sufficient reserve buoyancy in its rounded  hull form to allow an extra person to be carried. It was not designed as a load-carrying tender to a larger boat; the owner required  a dinghy that could easily be rowed or used with a small outboard motor when answering the call of the running tide as John Masefield’s describes the lure of going afloat in his poem Sea Fever.








Figure 2a – With owner and gear aboard the dinghy floats lower in the water  than when empty but with sufficient freeboard to carry one extra person.

Caution : in the excitement of the maiden voyage the owner has forgotten to wear his life jacket!!


Overloading test

The dinghy in figure 2a was tested for its load carrying capacity by loading it above its design load of two persons with three heavy adults who weighed well over 40 stone with the expected result shown in  Figure 2b.

Safety
:  The test was conducted in sheltered smooth water with the crew all wearing their personal flotation gear.











Figure 2b - With 3 heavy adults onboard the dinghy sank down until it had insufficient freeboard
for safe usage.

Caution:  In an overloaded condition a dinghy must be carefully handled especially if there are waves present which are too high for it to ride over without taking water onboard: in such conditions the dinghy could  ship water onboard when, unless the crew start  bailing, the dinghy could easily become swamped and even roll over


A dinghy for load carrying or rowing?

A dinghy designed as a tender for load carrying to another boat must have a hull form endowed with lots of power to float ( i.e. a buoyant hull). By giving the dinghy’s mid section a flat floor with a hard turn at the bilge , reasonable freeboard and beam , a buoyant but barge like hull form can be created. A draw back with such a hull form is that it is likely to generate significant drag .




Figure 2c  - A barge like hull form (coloured line) contrasted to a more rounded hull form (solid black line).The barge like hull form is more buoyant than the rounded hull form allowing greater loads to be carried.



Contrast the barge like hull form to that of the dinghy shown in figures 2a, and 2b,  with its rounded sections  and not too much beam. The rounded sections , all be it, at the expense of load carrying, can slip more easily through the water. Additionally the low freeboard will not only , make it less prone to being buffeted around in a breeze, it will also help create an efficient rowing position as the oars will not be too angled to the water .Such a dinghy is a delight to row whereas the more barge like hull form will be more difficult.  

Caution:  Dinghies that will not track straight either due to wind effects or otherwise when under oars divert the rower’s attention away from concentrating on the safe use of the craft, because the rower must be continually making course corrections by pulling harder on one oar or the other.

TRIM FORE AND AFT

In a light small dinghy, where on the centreline the crew position themselves  and place gear loads , will affect the fore and aft attitude that the dinghy adopts to her design waterlines: that is fore and aft trim. The behaviour and performance of the dinghy will be affected by the wrong trim.

Good Trim

Good fore and aft trim can be achieved with two persons onboard, by having one crew sit in the middle of the bow seat (bow thwart) and the other in the middle of the stern seat (stern thwart).








Figure 3a  The dinghy is floating level with two people on board and will perform and behave as designed


Not so good trim

With the crew seated on the central and stern thwarts, the dinghy adopts a bow up attitude causing the transom (stern) to become deeply immersed .The raised bow will cause the dinghy to be buffeted around in a wind whilst the immersed transom will generate lots of drag. Rowing the dinghy will be a real chore!








Figure 3b  The dinghy is floating bow up affecting her behaviour and performance.


Bad trim

A trim problem can arise when attending to the outboard (see figure 3c) and also may continue once underway. (see figure 3d)

When attending to the outboard motor

Should the crew stand when attending  the outboard motor ? Not really, because if he should loose his balance he could fall overboard or cause the dinghy to swamp or even turn over – and if the engine had started, the revolving propeller could cause serious injuries to the crew.








Figure 3c  Standing up to attend to the outboard is not recommended : the crew has got away with it because of the dead smooth water


Kneeling on the dinghies bottom boards is a safer position as it will lower the top weight of the body (see swamped load testing) and even then the body weight should be kept within the boat.

Dangling over the stern, to turn the fuel on or off, is also very undesirable, as besides reducing the dinghy’s ability to resist heeling , it reduces the dinghy’s freeboard at the transom: if this becomes too low it exposes the dinghy to the possibility of a wave coming on board over the transom (called pooping) and eventual swamping

Avoiding bad trim when underway

Under way with an outboard the dinghy should be trimmed level. For single handed out boarding, a tiller extension is required, preferably connected to a twist throttle, so the crew may operate the dinghy  from approximately amidships (in the centre of the dinghy).





Figure 3d   The crew in picture 3d is trimming the dinghy level by operating the engine from nearly amidships with out stretched arm, but from this position  the engine cannot be controlled or stopped.

Caution: Outboard engines should be fitted with a kill chord for stopping  the engine in an emergency–this will minimise the risk of the sharp high revering propeller from cutting into a crew who has fallen overboard!!!!  Note: The excellent 25-year-old engine shown in picture 3d was built before these desirable safety items were introduced. More modern ones come fitted with twist grip throttles, kill chords, while  commercially made extensions are available and well worth the investment.



HEELING

Knowing how a dinghy resists and recovers (i.e. transverse stability) from an  athwartship heeling force such as a crew or gear load offset from the centre line is crucial to the safe operation of the dinghy. It will affect the way a crew boards the dinghy.


Informal heeling test

When afloat in a strange dinghy for the first time it is wise to test its resistance to heeling. An offset  heeling force may be applied  by pushing down on the gunwale whilst also carefully moving  off centreline.








Figure 4a  A two man crew afloat for the first time in a new dinghy. They are investigating the heeling characteristics by performing  a simple informal heeling test.



More formal tests

A more formal test can be conducted , by sliding off the centreline across the middle thwart to the gunwale or using  weights to replace the crew:  the heeled dinghy’s gunwale should remain  above the water by at least 10 mm ( 0.4 inches).









Figure 4b  The dinghy under test is the sailing version of that being tested in picture 4a. but with    50 mm more freeboard, which allows for an increased angle of heel before water will come over  the gunwale.










Figure 4c  Conducting a similar  heeling test to figure 4b but with lead and concrete weights  replacing the crew. A mock  outboard motor is used to prevent oil from  seeping into the fresh water lake.


CAUTION: A dinghy tested for heeling with  lead or concrete weights instead of people(see picture 4c)  is likely to resist the heeling effect of a larger  load. This is because the weights are lower down than when a person provides the heeling force and so will  create a smaller heeling moment than a person of equivalent weight.

Tips for boarding a dinghy

Make sure the dinghy is well secured to the jetty or the vessel before boarding.

Any crew members already onboard should help those boarding one at a time .If possible they should also hold onto the other vessel or jetty so they can steady the dinghy if unexpected heeling forces occur.


To prevent heeling the crew should :-
a. step into the middle of the boat  avoiding if possible, steeping  onto the centre thwart
b. aim to position the feet so they end up straddling the centreline.
c. think about which way he wants to end up facing and step of on the appropriate foot to achieve this.
d. sit down as quickly as possible using a hand on the outboard gunwale to steady themselves with – not the inboard gunwale or the hand and fingers risk being squashed between the dinghy and the vessel or jetty.



Caution:  Boarding a dinghy requires a sense of balance, concentration on the job in hand, and lots of common sense – it is a serious operation and not a time for relaxing and  playing about!!!  

FLOTATION, HEELING AND TRIM WHEN SWAMPED

When recovering from unwanted events such as a swamping or capsize it is useful to know how many crew members the swamped dinghy may support, and how stable it is. Armed with this information the crew can decide whether  to climb back onboard either over the gunwale or the stern in the event of a capsize.

Simple practical tests can be used to gain a feel for how the swamped dinghy will float, trim and heel.


FLOTATION WHEN SWAMPED:

One crew aboard

In the test being  conducted in figure 5a  the dinghy was swamped by pouring in buckets of water.







Figure 5a The swamped dinghy safely carrying a load equal to a crew of one  represented   by a dustbin filled with water weighing in excess of 75 kg  (the weight of a standard man). The gunwale is well clear of the water.


In this test the dinghy floated with the swamping water level to the top of the dagger plate case, because the  buoyancy of the hull ( its power to float) had been supplemented with air bags strapped firmly and  low down within the hull. These gave the hull sufficient power to float so any  excess swamping water above the dagger plate case could drain out through it!

Minus the air bags the natural buoyancy of the hull would not alone have had  sufficient power to support the crew load and water already on board; water would have started flooding  in through the dagger plate case  until the dinghy was wallowing  with its gunwale awash.

For a dinghy to float with one crew onboard when swamped it must have sufficient flotation gear fitted which may be in the form of air bags, solid closed cell foam, or built in tanks.

CAUTION: if the dinghy in figure 5a had been built in GRP instead of wood extra flotation gear would have been required. This is because GRP is less buoyant (i.e. power to float) than wood.
The same applies to plywood . It is less buoyant than an equivalent solid piece of wood because of the weight of the glue used to bond the veneers together  making up the plywood.


Could more crew come onboard?

The swamped dinghy had insufficient flotation gear to safely carry a crew of 3 men represented  by  dustbins filled with water weighing in excess of 75 kg  (the weight of a standard man) allowing  water to flow  up through the dagger plate case into the dinghy causing the dinghy to float lower in the water.









Figure 5b The dinghy was well and truly swamped by pouring in buckets of water  and with a crew load of 3  on board   it started to sink with water flowing in through the dagger plate case. As it became low in the water the tester in the fore ground had to stop  the dinghy from taking a graceful roll to port---the instability was caused by offset loading and the top heaviness of the dinghy..

HEELING AND TRIM WHEN SWAMPED:








Figure 5c The swamped dinghy with the equivalent dry weight of 25 kg  placed aft trimming the dinghy down by the stern.








Figure 5d The swamped dinghy with the equivalent  dry weight of 25 kg  placed forward trimming the dinghy down by the bow. The stern is not so low in the water due to the longer waterline making the hull more buoyant.




To test the dinghy’s athwart ship resistance to heeling (i.e. transverse stability) and fore and aft trim in a swamped condition a heeling load equivalent to a dry weight of 25 kg.,  was suspended over the side by rope attached to the thwarts. [dry weight is a weight of over 25 kg so as to compensate for the buoyancy (i.e. upwards lift ) associated with its immersed volume ].

The testers at first had doubts that such a large weight could be supported. The swamped dinghy supported the weight because the righting moment of the dinghy ( i.e. its ability to resist the heeling force of the weight suspended  over the side) was enhanced by the weight of the large volume of swamping water contained within the dinghy. This should be born in mind when trying to climb back into a dinghy, which is well swamped, as it may be possible to clamber over the gunwale rather than the stern in such cases. With less water onboard the dinghy would have adopted a larger angle of heel and trim and  probably would  not have supported such a large load.

Safety note:  These tests were all carefully planned with all known risks carefully evaluated before hand  because they were carried out in deep water.

Caution: For small amounts of water in the bottom of the dinghy the above result will not hold. The bilge water would run down into the lower bilge as the dinghy heeled which in most circumstances will increase the heeling moment (Called free surface effect) and may cause a swamping or capsize.

Tip: keep the dinghy dry by bailing out any water in the bilges.

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