Cockpit Bailing

How close to the waterline can a the cockpit floor be on a 8-9 meter pocket trawler, (displacement, 35HP, 4-5 Ton)? Primarily used in sheltered waters but able for rare fair weather island hops. Are there methods to reduce this distance and what are the drawbacks of getting too close to the waterline with the cockpit floor? Are there accepted rules of thumb? Thanks

 

Many production boats have no more than an inch or so. When they are so close, they require some kind of device to prevent backflow. There are several types, for example flaps or floating ball valves.

 

All commercial ships, not pleasure, are required to have a minimum freeboard, which can not be decreased by many "floating ball valves or flaps" to be placed. It is regulated by the International Convention on Load Lines. Some ships, by their small size, they are not covered by this Convention. In this case, the depth is calculated for the most unfavorable condition for which meet all the stability criteria applicable to the ship. That depth is what defines the minimum freeboard of the vessel.
The determination of the conditions of freeboard, or equivalent measure of security, for pleasure boats, is even more laborious. A good guide can be found in ISO 12217.

 

Freeboard

Is freeboard the height of the hull above the water or the height of the deck above the water?

 

Yes freeboard is usually the height of the Gunwhale above the water at its lowest point !!

The height of the deck above the waterlevel outside is the greatest concern as water always finds it way back inside !.

 

ABYC has standards and freeboard limits/formulas to weather deck.

 

ISO does not apply in the USA. Check these links for pertinent regulations:
http://www.uscg.mil/pvs/docs/Stability/Which is your T-Boat.pdf
http://cfr.regstoday.com/46cfr178.aspx
http://www.abycinc.org/standards/interpretation.cfm

 

Install some very efficient Scuppers on your rear deck. On my build the rear deck is 7' x 8' and I have 2 scuppers per side 4.5" x 11" -3/4" above the sole(rear deck)
with 2 quality sump pumps under the deck at the transom.

 

There is a lot of work to be done but I understand that ABYC does have an ISO committee to see if the standards can be harmonized.

 

Yes, but is a voluntary compliance to make it easier to sell boats in the EU mainly. Eventually they are supposed to be equivalent.

 

Welcome to the forum Courterjoinery.

I think the issue is cockpit sole height above the LWL isn't it? I consider 6" to be the minimum for a self bailing/draining cockpit, though I've seen closer. With the 6" sole height, you can install back flow equipped, flap style scuppers as well as floating ball. I'd reserve open scuppers for a higher cockpit sole.

If you don't have or desire a self bailing cockpit sole, then the height above the LWL is immaterial. You'd slope the cockpit sole forward and use pumps to remove boarding water. The water could be contained in the cockpit or permitted to drain to a sump, in either case a bilge pump will handle excess.

Courterjoinery, it might be better if you where a bit more specific about your needs and what you're trying to do. Are you building new, doing a remodel, interested in raising or lowering a cockpit sole, etc. Will the vessel be for "hire"?

 

Boarding Water Pumps

Thanks for the help. 6 inches sound prudent, If pump(s) fail or if power is down in an emergency, the weight of that much water in the cockpit would concern me. Also, how fast could pump(s) clear a cockpit which filled unexpectically? Is filling unexpectically even a normal concern or a possibility?

I am planning to build a plywood/epoxy trawler for personal use.

 

Yes, flooding is a concern in certain sea conditions. It depends on the design, the area of use and conditions you expect to encounter.

 

A moored or berthed boat, with a non-self draining cockpit can have it's pumps overwhelmed in a big thunderstorm, but given sufficient time, they'll catch up and get the cockpit bailed. This is also true of a boarding wave. You can calculate how much time they'll need with the volume of the cockpit footwell. I like to see self draining cockpits with scupper big enough to get a fully filled footwell drained in less then a minute. This is an eternity as you slosh along underway in a storm, so faster is better, but often impractical. Pumping out several hundred gallons in a minute or less can be impractical too, but board waves aren't especially common either.

The best approach is to make some hard decisions about how the boat will be used and the general sea states it will operate in. Set your gear up for this. Setting up for every eventuality, usually isn't practical or necessary, though back ups and some planning, in the event it does occur isn't unwise. So, a fair weather boat in relatively protected waters, doesn't need deep ocean cockpit bailing equipment.

 

If it comes to fishing vessels (trawler), there are regulations that require placing openings, of a certain minimum area, in bulwarks so the cockpit was empty as quickly as possible. In Europe, at least, apply the Torremolinos Convention. Although only requires vessels of more than 24m, for smaller boats is usually applied, also, this regulation.
Although this was not the original question, everything is related, freeboard and quick emptying the cockpit, with the issue of security.