BUOYANCY by Frank Chesworth
BOUYANCY by Frank Chesworth
Ever since the start of production of the Devon Yawl there have been, from time to time, questions concerning the buoyancy characteristics of the boat – to quote Michael Quick “Yes, they can be a little nose heavy….”. Incidents in the more intensive racing scene coupled with the Editor’s boat depicted swamped in the last Newsletter, and the impending EC Recreational Craft Directive (94125/EC)) have motivated us to activate our long planned buoyancy trials.
In nearly every case of a Yawl becoming swamped this has occurred during racing when the boat is carrying full sail and is being pressed to the limits, the boat is hit by an unexpected strong squall and the crew have failed to release the sheets in time. To be swamped when cruising or pottering would suggest that a greater amount of sail is being carried than is prudent in the prevailing or forecast conditions. Please note that one of the Association’s recommended safety measures is to fit ball bearing cam cleats, such as Harken 150, to the genoa sheeting arrangements because of the ease with which sheets can be released from these cleats.
The areas of the above mentioned EC Directive that we are addressing in our trials are set out in:
Annex I.G Boat Design Categories. (i.e. The Yawl is likely to be in this category) “INSHORE: Designed for voyages in coastal waters, large bays, estuaries, lake and rivers where conditions up to, and including, wind Force 6 and significant wave heights up to, and including, 2 metres may be expected.”
Annex 3.2 Stability and freeboard. “The craft shall have sufficient stability and freeboard considering its design category according to section 1 and the manufacturer’s maximum recommended load according to section 3.6.”
Annex 3.3 Buoyancy and flotation. “Boats of less than 6 metres in length that are susceptible to swamping when used in their design category shall be provided with appropriate means of flotation in the swamped condition. “
To generate some basic controlled data and to investigate possible ideas on improvements Richard Box, Ian Canadine, Frank Chesworth, Derek Gouldsworthy and Garth Webster met on the 19 August at Rutland Water with the intent of swamping various boats with different buoyancy arrangements and applying simulated design changes. The Yawl’s used for the trial were:
DY38 a standard boat with no bulkhead and the only buoyancy provided by the standard side tanks. DY188 a deluxe boat with sealed bulkhead with the hatch clamped closed and standard side tanks.
DY329 a kit boat with bow buoyancy bags to half height of under deck area, some under floor foam, and standard side tanks with additional buoyancy bags in ends of aft side lockers.
The trials went well, helped by the bosun at Rutland whom we thank for his assistance and the loan of a large pump for initial flooding (A) and more importantly for pumping the boat dry as there is not the adrenaline to move buckets when the boat is alongside a pontoon! – Note Association rule 9.0.viii to always have on board a 9 litre /2 gallon bucket for bailing. Thanks also to Richard Box’s boys who did valiant work and plunged into the water at the slightest request.
Two sets of conditions were simulated. Static or neutral conditions which is where water has been introduced and a stable state achieved with water neither draining in or out via the centreplate slot. Swamped when the boat is filled up to the deck level. In the swamped condition the static or neutral state is restored either through the slow process of the water draining out through the centreplate slot, or accelerated by use of the bucket. Obviously in the swamped state a stable boat is essential to prevent the side decks dipping below the water surface and thus take in more water. The problem to address is buoyancy distribution to reduce excessive boat movement as the crew is moving and bailing.
The worse case scenario is best illustrated by DY38, the standard or basic Yawl. In neutral conditions DY38 floats with the average water level only just below the top of the side tank, but slightly bow down (B). With a crew member aboard towards the stern she floats more or less level, but if the crew moves forward (C) the foredeck goes right under and the side tanks aft are half out of the water. If the crew weight is on the aft deck a similar effect is achieved but less dramatic as the centre of gravity is around the centreplate. When a swamp was simulated (D) the boat filled up with the deck level with the water. The problem with the standard boat is the small quantity and distribution of the installed buoyancy.
All this does not appear too good but it must be remembered that all boats will fill up to deck level if water comes in faster than it can drain out. The drain in the Yawl is the centreplate slot.
Some solution to the basic problem was demonstrated in the trial of DY188. In neutral conditions (E) the sealed bulkhead keeps the boat well up in the water with the side tanks well out of the water despite the crew weight. The water level is around 2 to 3 inches over the winch. If weight was added to the aft deck as with DY38 it sank below the water level. Swamped the extra buoyancy from the sealed bulkhead supported two crew (F). we then simulated adding further buoyancy by placing a 50 litre container under the stern and xx litres adjacent to the centreplate to simulate extra “under floor” buoyancy. (G) shows the neutral position with the boat well up in the water.
Finally DY329 was flooded and (H) shows the water near the top of the side tanks with crew standing well forward on the foredeck. With crew weight on the aft deck this went under water as with DY38 and DY188.
This report has not detailed what improvements can or should be made. We would add that the boats currently built by Devon Yawl Ltd are fitted with a sealed bulkhead and side tanks so have already overcome many of the problems illustrated here.