Small Day Boat Refurb Discussion Topic - Buoyancy

[ningcompoop]

From a quick (3am!) calculation, 500kg of GRP would need roughly 150-200l air* attached to give it positive buoyancy (dependant on the amount of glass in the lay-up, more glass=more dense). If you have reasonably accurate figures for the weight of the hull, and weight and type of ballast, you can get a more accurate picture of how much buoyancy you need.

(*I've just worked out that's roughly equivalent to 2500 waterbuoy inflatable keyring thingies - there, that's your buoyancy sorted!)

John

(Edit - just seen you already have figures for ballast, etc! Time for me to get to bed!)

 

[fisherman]

I think fore and aft tanks with access plastic hatches in the top, for a bit of storage. Under deck if you can genuinely keep the water out, but it always leaks you can have drains into the pump well. Grinding old GRP off is painful, I find if it wasn't very well done you can often split it off with a chisel, no dust and quicker.

 

[TSB240]

I used to sail and my Brother still has a 20 foot open day sailing boat. It was originally sold without any additional bouyancy.

In the aftermath of one being driven under whilst racing in a wind against tide situation in which it sank in less than a minute some felt the need to instal fore and aft bulkheads
.
These had minimal effect and in subsequent submarine escapades it was found to delay the inevitable sinking by a few more seconds.

Providing enough bouyancy for half a ton of lead, a heavily laid up hull and mast head rig proved completely impractical.

The class decided that all boats should have an effective bilge pump and a back up of a scared crew and bucket!

None were ever lost for more than a couple of days.

 

[testmonkey]

how about retaining original self bailing design, re-fit the floorboards and fill the voids with closed cell pour foam like here http://forums.iboats.com/forum/boat...r/290828-pouring-foam-in-s-and-out-s?t=283667

 

[CreakyDecks]

How about lining the bottom of the hull with polythene, bolting the floor down and then pouring in foam, with the objective of the foam bonding to the underside of the floor but not to the hull, allowing the floor and foam to be removed as one unit? It would obviously have to be bolted down VERY securely to be of any use!

 

[Lakesailor]

I think fore and aft tanks with access plastic hatches in the top, for a bit of storage. Under deck if you can genuinely keep the water out, but it always leaks you can have drains into the pump well. Grinding old GRP off is painful, I find if it wasn't very well done you can often split it off with a chisel, no dust and quicker.

I can attest to that I did 1 1/2 hours yesterday and just done 2 1/2 hrs today. Anyway it's done now.

I used to sail and my Brother still has a 20 foot open day sailing boat. It was originally sold without any additional bouyancy.

In the aftermath of one being driven under whilst racing in a wind against tide situation in which it sank in less than a minute some felt the need to instal fore and aft bulkheads
.
These had minimal effect and in subsequent submarine escapades it was found to delay the inevitable sinking by a few more seconds.

Providing enough bouyancy for half a ton of lead, a heavily laid up hull and mast head rig proved completely impractical.

The class decided that all boats should have an effective bilge pump and a back up of a scared crew and bucket!

None were ever lost for more than a couple of days.

That is my thinking. Nothing on the boat has any neutral buoyancy. Even the centreboard and rudder blade are solid grp. The ballast is a 100 kg block of resin with scrap metal embedded in it. So out of the 400+ kgs displacement I reckon at least 350 kgs needs compensating for. That's 350 or so litres, or a lot of milk bottles. These figures are bit of a guestimate as different resources quite varying figures, but I am going with the BMIF figures.

how about retaining original self bailing design, re-fit the floorboards and fill the voids with closed cell pour foam like here http://forums.iboats.com/forum/boat...r/290828-pouring-foam-in-s-and-out-s?t=283667

The boat had already had foam pumped into the cavity beneath the floor. It may not have been closed cell and had become sodden. The whole interior had been cut out before I got the boat.The floor was ply with a grp liner bonded on top. It's all been cut away and not easy to replicate. Besides which I think the floor level (above water line) was too high. I like to be IN a boat not ON it.
I am putting bearers in to support the floor, probably about 4" lower than previously, with drains between sections. There is already a Whale Gusher fitted. The floor boards will be removable. I am glassing over the 38mm hole the cockpit drain was installed through. I did think of a brass drain bung, but can't really see the point as I have a bilge pump and will have removable floors.

 

[sarabande]

Lakey

what issues, if any, are there with leaving yer yacht on a mooring in heavy rain for a few days ? Any self-draining capacity, or are you going for a little automatic pump ? I looked at the diagram and noted SD, but can't see it on the pics.

 

[ningcompoop]

Using your boat specs figures (900lb weight), allowing 20kg for the mast and rigging and substituting the density for cast iron for the resin/iron ballast, I work out you need a minimum of about 190l of (air) buoyancy to keep the hull afloat- this should be quite 'do-able' - for example, if you could fit a block of foam (suitably protected), 200cm long, 50cm wide and 10cm thick under the floor, this would give you about 90l buoyancy (making allowances for the weight of the foam).
Can you estimate much space you've got in the fore and aft tanks?

 

[onesea]

Lakey,

Agree I am with you with no buoyancy the route.

The thought I was trying to make about buoyancy is to make it effective so you can recover from a filling.

Having ditched assorted dinghies in my time, I always hated the ones where they had no stability when full of water... IE the buoyancy low down (under floor boards etc) so whilst supporting your weight and the boats weight and how ever much water she could lift she has a big free surface and no stability. She would probably be more stable upside down.

Hence my thought for buoyancy under side decks to supply stability, in the event the boat is swamped.

 

[Lakesailor]

Using your boat specs figures (900lb weight), allowing 20kg for the mast and rigging and substituting the density for cast iron for the resin/iron ballast, I work out you need a minimum of about 190l of (air) buoyancy to keep the hull afloat- this should be quite 'do-able' - for example, if you could fit a block of foam (suitably protected), 200cm long, 50cm wide and 10cm thick under the floor, this would give you about 90l buoyancy (making allowances for the weight of the foam).
Can you estimate much space you've got in the fore and aft tanks?

I'm not sure how you work out190 litres of air. That would equate to 190 kgs of water wouldn't it? So you may float the mast, boom and furler foil plus the ballast, but surely the grp has negative bouyancy. Even the rudders that fell off the Toppers at the Activity Centre sank faster than you could reach for them.

I'm going to go down the route of the fore and aft chambers. I will take some rough measurements to assess their capacity.

 

[Lakesailor]

Lakey

what issues, if any, are there with leaving yer yacht on a mooring in heavy rain for a few days ? Any self-draining capacity, or are you going for a little automatic pump ? I looked at the diagram and noted SD, but can't see it on the pics.

No self-drain. It was all removed along with all the floor and bearers by the previous owner. I have an empty hull, but ready to sail as it has everything else in place.
there is a Whale Gusher manual pump (you can see it on the starboard gunwhale in the pic in the OP), so a few days on a mooring would be no problem. I had thought of a little float-switch pump and a motorbike battery with maybe a small solar panel to top up with. There is already a second skin fitting outlet on the port gunwhale.

 

[Seajet]

I'd definitely fit the auto pump, something may happen preventing you getting out to her for longer than expected...

If finances allow the solid state pumps with a built in sensor like the whale supersub are far superior, float valves are the invention of the devil and never really trustworthy in my experience.

 

[ningcompoop]

Showing my working out!
Total boat weight = 900lb = 410kg.
Say rigging is about 20kg, ballast 200lb, 91kg, this leaves about 300kg of GRP.
Density of GRP* 1540kg per m3; therefore 300kg has a volume of about 0.195 m3, or 195 litres.
Density of Aluminium* 2660 kgm3, 20kg of rigging has a volume of.... 0.008 m3, or 8 litres
Density of cast iron* 7210 kgm3, 91kg of ballast has a volume of... 0.013 m3, or 13 litres **I've used the figure for cast iron as I don't have one for the ballast mix, if anything, this should give you a bigger safety margin, unless your ballast is uranium.
Total volume displacement of hull is... 0.221 m3, or 221 litres.
The hull, submerged, will displace a mass of 221kg water, this gives it a negative buoyancy of about 190kg; 190 litres of contained air would give a (roughly) neutral buoyancy. This will also depend on the accuracy of the original figures, as how much a designer or manufacturer thinks a boat weighs, and how much it really weighs may not be the same thing.
You'd want to build some safety margin in, especially if you're going to add a chunky outboard or other fittings; obviously the larger the positive buoyancy, the higher the hull would float if awash.

Can I also point out, I've not studied design or engineering since school, and if my calculations are completely wrong and/or I'm completely deluding myself, feel free to point this out!

*Figures taken from Nicholson's Boat Data Book.

 

[Lakesailor]

Your workings out are far superior to anything I would have done.

I have just measured the two chambers. The sealed bow one and the open stern locker. The bow one would work out as 105 litres if it wasn't for the pesky foot of the tem curving under it. So lets be pessimistic and say 50 litres. By extending it upwards to the shelf under the foredeck it would add something along the lines of 150 litres to the 50 litres, so I'd have 200 litres in the bow.
The stern locker is very much an averaging of the dimensions I took as the stern narrows slightly and pinches nearer the waterline. However, being pessimistic again I get it to about 325 litres, say 300 litres to be conservative.

That's a total of 350 litres or 500 litres if I extend the bow locker upwards.

That would seem to be enough to keep the boat awash if it had a catastprophic event. That's all I would be looking for. I really don't think you could hope to right it at the time, but with a bit of help it could be pumped out and got ashore either on a slip or a crane.

 

[Lakesailor]

I'd definitely fit the auto pump, something may happen preventing you getting out to her for longer than expected...

If finances allow the solid state pumps with a built in sensor like the whale supersub are far superior, float valves are the invention of the devil and never really trustworthy in my experience.

Yes, I hadn't investigated auto pumps, but as the intention is really to dry-sail it, that is a thought for the future.

 

[Deleted member 36384]

No buoyancy, just for aesthetic reasons, wear a lifejacket/buoyancy aid. The sealed floor can actually be a negative if you do ship water as it keeps it higher up instead of lower down. My Wayfarer is like this and while I have self bailers, when they are closed it's surprising how much water gathers on the low side, above the sole. On versions where there was no sealed floor most of this water was sloshing about below the sole, having less effect.

 

[Greenheart]

Even the rudders that fell off the Toppers at the Activity Centre sank faster than you could reach for them.

Was that because it was bolted to the metal rudder stock? I particularly remember my Topper's daggerboard floating, possibly because it was polypropylene like the rest of the boat. I thought the rudder was, too.

I'm finally nearing the stage of boxing-in literally hundreds of 2-litre bottles I've accumulated over the year, so that if the Osprey takes a dive, she'll always come back up.

Looking at 3000 Newtons of buoyancy in lemonade-bottle form at home, it was hard to believe there'd be room for them aboard. In fact, I could easily stow dozens more...to the extent that the weight of the empties (about 8kg) might become an issue if I were racing.

But the boat is old, and ashore after brief periods afloat I've watched drips from untraceable leaks in principle buoyancy tanks, so to my mind, with distance-daysailing in mind, it seems smart to fill every accessible void with lightweight, very robust, totally free, nonabsorbent buoyancy. If it took up useful space, I'd object; but for me it's a no-brainer.

I love the new boat Lakey. :encouragement:

 

[William_H]

I think it essential that you have buoyancy but that the buoyancy be near the chines. That is under side seat buoyancy. I think the best way is to seal up the seats. Plastic Inspection ports around 100mm diameter should be fitted which enable you to drain the tank if it leaks or even use it for stowage.
It is vital that the buoyancy be at the sides and not under floor or in the bow. This is because if any water gets into the boat (and it surely will) if it can slosh to the sides where it will reduce the stability of the hull to the point where you won't be able to sail it with a lot of water in it. The side tanks maintain the buoyancy at the outside edges so keeping stability as good as possible. Firther the seats if sealed will tend to stop water from getting into the hull in extremes of heel. Thus the higher the seats the better. good luck olewill

 

[fisherman]

Ideally under the side seats, maintain stability, unlike under floor. Nidaplast is your friend in place of ply. It can be bent easily until the first layer of GRP goes on, could be used to make side seats, and with no through hull fastenings in that area less likely to leak, but I would fit drains anyway.

 

[Lakesailor]

I think we are complicating things a bit. This is a 17 ballasted day-sailer with a centreboard. It's certainly not a dinghy.

Think more of a small cruiser sailer with the coachroof removed.
I sail on a lake.
If it were to be swamped (hardly likely) or capsize I'm not bothered about it floating in the best attitude for righting. I just thought it may be an idea to stop it actually sinking so that it could be recovered.

I am not really wanting to build as I rather prefer the open feel of slatted seats. I think the workings out that ningcompopp did suggest that my two available spaces in the bow and the stern would achieve some positive buoyancy.

I was rather hoping it to be something along these lines, well, a bit anyway.