Easy Pick-up Buoy - Buoyancy Calculation.

[Vegable]

So that my wife can steer the boat onto the swinging mooring with ease I've made up a pick-up buoy using a Polyform CC3 buoy. (The blow up ones with a 40mm hole right through the centre.) I've then stuck in an 8' length of 35mm white plastic domestic waste pipe through the centre of the buoy high enough to be seen taping against the bow of the boat from the steering position.
Apart from trial and error, is there a simple formula to calculate how much ballast to hang below the buoy to keep the whole thing upright?
Mike

 

[Kukri]

Wind will be more important than gravity so I’d suggest overdoing the weight.

 

[rob2]

I'm sure it should be possible to calculate, although it would involve calculating the effects of windage on the top hamper as well - so probably still end up doing a bit of trial and error!

Do you intend to use the buoy purely as a pick up? The waste pipe could well become embrittled after a season or two so you might benefit from feeding a rope up through it with a handle on top so as to take most of the load off it in use. Of course, this will also slightly increase the instability and require a little more weight at the bottom...

Rob.

 

[ShinyShoe]

I tried a calculation. I dont think the answer was right. It needs a decent weight. And ideally well below the buoy (like the keel of a boat). All sorts of conflicted forces at play - tide, wind, ball shape, 8 foot of tube - what force is needed to balance just that?

We use similar for sailing club start line race mark. Ours are aluminium 40mm tube. I'd say 6ft above the buoy, 2 ft below. Good dod of chain hanging off that - doesn't keep it upright when the breeze gets up and the water starts flowing. Best is a piece of 110mm 45cm drainage pipe filled with concrete. Even it isn't perfect in blow!

 

[sarabande]

The total mass of the pickup buoy needs to be kept as low as poss to make lifting on board easy. So the counterweight should be well below the waterline to reduce the need for a heavy weight. perhaps an 8 or 9 ft pole top to bottom ?

An old carbon fibre fishing rod or piece of carbon tubing would be strong enough, yet light. 35mm waste pipe seems a bit big.

 

[[70521]]

When I had a swing mooring I got one of these, picked up from the cockpit. Other colours are avaiable.

I'd also recommend Duncan Wells's book, "Stress Free Sailing" for some fantastic tips on short handed sailing.

 

[Vegable]

Many thanks for all your thoughts. And for others who might be thinking of doing similar I'll explain my reasoning in full.
I've had the buoy for a while as a panic fender when berthing goes a bit wayward. The 35mm pipe only cost £9 for a 3 or 4 metre length and fits snuggly in the 40mm slot in the buoy once it's blown up. Also with a bit of Dayglo on the end it's very easy to see on approach....especially as no one else has anything similar. The pipe is easy to handle with cold hands which is another reason for choosing it and makes it easier to pull onboard. I agree that it will probably get brittle but the boat is on the mooring for a much greater time than the buoy is in the water and the buoy is kept down below when not in use, and it only cost £9 someting in the first place.
At the moment I've got about 2 metres of 8mm chain folded in half dangling from it and thats too heavy at the moment. So it'll just have to be trial and error with various odd bits of weights then weigh that and cut the chain to match the weight. Having teak covered ply decks (Westerly) having chain dangling below the buoy means the counter weight is flexible and isn't going to gouge chunks out of the deck. Having a length of rope coming out of the top sounds a good idea and will experiment and see how it works, the pipe might get stuck on the rubbing strake though, got to get the whole lot through the pulpit.
But thanks once again
Mike