Accumulation of marine growth on ship’s plating in warm tropical seas is often a cause of significant reduction in ship’s speed and impairment of vessel’s performance. Such accumulation produces particularly negative effect on speed characteristic when vessel remains waiting at anchor in tropical water for a period of more than 3-4 weeks. Obviously, this natural factor also affects positions of the shipowner and the charterer under time charter contract in a way that it can lead to loss of time resulted from underperformance and expenses plus time losses associated with hull cleaning.
Per Mocatta J in Cosmos Bulk Transport Inc v China National Foreign Trade Transportation Corporation -  1 All ER 322 at 334:
… the whole purpose of the description of the vessel containing a speed warranty is that when the vessel enters on her service, she will be capable of the speed in question, subject of course to any protection which her owners may obtain if there has been some casualty between the date of the charter and the date of delivery affecting her speed which, under an exceptions clause, protects them from liability in relation to a failure to comply with the warranty. From the charterer’s point of view the speed warranty is clearly of very great importance in relation to his calculations as to the rate of hire it will be possible for him to pay, a matter on which his decision would be affected by his calculations as to the time which the ship would take to complete the one trip voyage for which this time charter engaged the [vessel]. From the business point of view, I think it is clear that commercial considerations require this description as to the vessel’s speed to be applicable as at the date of her delivery whether or not it is applicable at the date of the charter.
In The Didymi  2 Lloyd’s Rep. 108 per Bingham LJ at p.117:
If the vessel had under-performed, the basis for which the charterers contend would lead to the charterers being very inadequately compensated. The purpose of the stipulated conditions was to provide a yardstick which was then to be used to assess the loss or gain in other conditions more usually encountered. So in my judgment the procedure envisaged was clear and involved these stages: first, assess the vessel’s performance in good conditions as defined on all sea passages from seabuoy to seabuoy, excluding altogether any period of slow steaming at the charterers’ request; secondly, if a variation of speed from the stipulated norm is shown, that variation should be applied with the necessary adjustments and extrapolations to all sea passages from seabuoy to seabuoy in all weather conditions, but excluding periods of slow steaming at the charterers’ request; thirdly, if there is a variation of consumption from the stipulated norm, that variation should be applied, with the necessary adjustments and extrapolations, to all sea passages from seabuoy to seabuoy in all weather conditions, including periods of slow steaming at the charterers’ request.
In The Gas Enterprise  2 Lloyd's Rep. 352 per Lloyd LJ at p. 366:
Once the average speed and the average bunker consumption have been calculated, the figures are to be used for the purpose of seeing whether and to what extent there has been a resulting loss in time or saving in time and a resulting excess bunker consumption or saving in bunker consumption.
Paragraph 4 of the clause [BEEPEETIME 2, Clause 5] provides a yardstick which is to be used to measure the extent of the vessel's under-performance or over-performance throughout the course of the time charter in respect of both speed and bunker consumption.
Once the vessel's under-or over-performance has been measured using the contractual yardstick, it is then necessary to calculate, in the case of speed, the resulting loss in time or saving in time and, in the case of consumption, the resulting excess bunker consumption or the resulting saving in bunker consumption. Once it is accepted that in all cases involving a claim by charterers for loss of time or for a loss resulting from excess bunker consumption, … , it makes no sense to construe that paragraph restrictively so as only to confer a remedy on either party for those parts of the chartered service where the weather conditions are of force 4 or less. The obligation imposed on the owners by pars 1 and 2 is one which relates to "the period of service under this charter", and is not limited to fine weather periods only.