Open boat buoyancy

T

Trevor57

New Member
Joined
1 Oct 2009
Messages
10
Visit site
Does any one have advice on built in buoyancy for an open boat?
Looking at my latest accusition, it seems a little lacking in this department.
http://www.ybw.com/forums/showthread.php?t=215389
All new boats built to the European Recreational Craft Directive seem to have much more. Is this euro over-kill, or should I be building some extra watertight compartments? If so how big and where.

Trevor.
 
Last edited:
What sort of sailing are you going to be doing and where?

I have one boat that would sink if swamped, and one that would float.
 
Once she's fixed up I'll probably keep her on Bala and maybe trail up to the Lake District or the West of Scotland for holidays. Nothing too adventurous planned, yet...
I'll swamp her in shallow water and see what happens when I get chance.
I'd hate to spend all winter repairing her to have her sink under me.
T.

Swimming is the recreation of last resort.
 
Last edited:
I f you can check her wt. (weighbridge less trailer, use bath scales for trailer) Then you need at least the equivalent volume in the boyancy tanks in litres to the wt. in kilos, pref quite a bit more. Wooden boats are a bit easier, as they are positive. GRP is not.
A
Technically you could work out the density of GRP and the big steel plate and adjust for that. But a just floating boat is no use in a swamping.
 
Yes, I'd like to have enough so when swamped the top of the centreboard case is above water to give me a reasonable chance of bailing her dry.
Question is whats the best way of achiveing this?
T.
 
Inflatable buoyancy bags would be one solution, a triangular one under the foredeck and a couple of long sausage types. They would need to b well secured of course, either under a shelf structure or strapped to the bottom with webbing or similar.
 
I've been having a think on what needs to be done on Cygnet and while I'm doing the other jobs I may as well make a proper job of the buoyancy.
She's got a couple of small tanks under the side benches but the total is probably less than in Josie my 9'6" sailing dinghy / tender, maybe enough to keep her just afloat but thats all.
My thoughts are to add more watertight compartments in sheathed marine ply up to the level of the centreboard case top. I can't see any point in going higher, but correct me if I'm wrong.
I need to build roughly one litre per kg but how do I work out the volume of such an odd shape? Can I do this without ending up with more watertight compartments than boat? Also how strong do I need to make them bearing in mind that strength = extra weight = need for even more buoyancy? Sounds like a vicious circle.
Any comments?
T.
 
Last edited:
Up here in Lewis we sail some traditional dipping lug open boats and have fitted flotation to these. We used foam boxed in below the thwarts, and then courtesy of the local fire brigade conducted a full swamping test:

http://www.flickr.com/photos/ansulairetrust/534758305/in/pool-sgoth

http://www.flickr.com/photos/ansulairetrust/534758299/in/pool-sgoth

http://www.flickr.com/photos/ansulairetrust/534758309/in/pool-sgoth

Following these tests we found that the swamped boats had very little lateral stability, so we tied in some large fenders along the sides of the boats.
 
Nice boats..
I took a look at all your photo's on flika.
I sailed out to Staffa myself a couple of years ago, magical.

Back to the point. I stripped the floor out this afternoon, and there are two sizable ingots glassed in either side of the foward part of the center board case!
I think I better get her weighed before I make any decision on buoyancy.
This is a serious little boat not a fall over at the first puff dinghy.
T.
 
please bear in mind when you build your airtanks that they should be placed where they can help with two, sometime conflicting, tasks:

1 to prevent water building up inside the hull and thus prevent the boat overturning (i.e. to displace water to the outside of the hull by e.g. having the cockpit floor self-draining).

2 to assist the return of a partially capsized hull the right way up.

In the first case, you need tanks low down in the hull, so that if suddenly swamped the tanks "lift" the boat upwards out of the water.

Secondly, you need air tanks which exert a "turn-over" effect on a hull which is capsized. For this you need the air near the gunwhale so that there is a strong righting moment.

Do a websearch on "Roamer dinghy" for a detailed discussion on bouyancy placement.


Tyhere is also some useful discussion on a SeaRaider - which is quite simialr to your boat - here
http://www.swallowboats.co.uk/content/view/68/
 
Thanks for that. I knew the Roamer dinghy but had forgotten the details.
I like the concept of the traditional self righting lifeboat but I don't really want to sail one. I hope the ends of Cygnet are high enough to do the job without the necessity to build upwards. A scandinavian floating banana shape may have it's advantages!
I'm going to investigate the concept of adding a sealed (self draining?) false floor and building a watertight locker in the bow and making the current lazerette locker watertight. This is in addition to the two small tanks currently fitted below each side bench.
I'd like to add some buoyancy at full beam gunwhale level to give her some roll stability when swamped, but the ballast (and small tanks) should have much the same effect without intruding into working space.
The idea of putting a removable hatch between the cockpit and outboard well to get rid of water quickly - Sea Raider design - is really neat as long as the levels can be got right.
Much thought is still needed, but a design concept is gradualy comming together.
Watch this space....
T.
 
If using bags pay particular attention to lashings. In a sinking boat, a bag makes strenuous efforts to escape. A few years ago I capsized on Loch Ness during the Great Glen Raid in a 3/4-decked boat whose large stern bag I had not inspected properly since buying the boat. The bag escaped through a surprisingly small gap and bobbed off down the Loch in a scene highly reminiscent of the 60s TV show 'The Prisoner'. Seeing just the forward 2ft of your boat out of the water on a loch 750ft deep is a sobering experience.
 
I have a clinker dinghy and avoided fitting saftey-bags as I didn't want spoil the period feel of her. I have often wondered about her buoyancy. I fondly imagine that wood will be just about bouyant and the aluminium centre board and rudder blade would just about drag her under. I often put a canoe bow bag under the forward thwart "just in case".
On the lake as long as a boat doesn't actually sink there is a fair chance of an easy recovery.
 
You've got more faith than me!
I've been sat on an upturned hull in a gale, grateful that it's still floating, more than once.
On one occasion I got asked if I needed help from a rib safety boat, and replied that the boat was more stable this way up and I was happy to hang on until conditions got better. The conditions did and I was the right way up and sailing again five minutes later.
You can't beat a nice buoyant hull when the weather starts being silly.
T.
 
You must log in or register to reply here.

Share this page

Top