Replacing a dinghy centreboard with steel

[Greenheart]

I seem to remember asking for advice on this, recently. Sorry to bore with repetition.

I'm thinking of taming a fairly wild racing dinghy (class undecided yet), without slashing the sailplan and making it dull. Mainly I want to be very sure of being able to right the boat after inevitable capsize; so I'm thinking, replace the probably rather tired old ply centreboard, with something like 20mm steel plate, and after some long winter days in the garage with the angle-grinder, cutting a foil-curve into the plate, I should have a 40 or 50kg drop-keel, still in the shape of the original board. That ought to make righting even a Flying Dutchman, into child's play.

I'm not an old-hand at angle grinding, though I've had enough success in the past to feel confident at having a try. I'm just wondering...

- is a particular metal most suitable?

- is a particular paint/finish vital?

- is the weighted plate likely to damage the boat's original centreboard housing?

- is there an easier method of cutting a curve into plate metal?

It's only an idle day-dream, but informed thoughts will be as welcome as ever.

 

[sarabande]

depleted uranium is probably the most affordable really dense metal, with a high density of 19.1 g/cmcubed (68% denser than lead). When I say 'affordable'....

It is used for balance weights in aircraft, so an airfield scrappy is the place to start.

With that density, you would be able to encase it in a wooden sheath of the original shape, with wood being much easier to work.



Just a passing thought.

 

[blackbeard]

Not so much need for DIY ...

Several dinghies had metal centreplates, notably the Bosun (a 14' dinghy much used by the Forces and of notably sturdy construction). In fact a s/h Bosun sounds a bit like what you might need. Not particularly wild, but sails quite nicely (but will not plane (often)).

Note that a metal (or weighted) centreplate will need an effective means of keeping it down even during a capsize.

Also note that adding weight to a fast boat will slow it down substantially and rob it of much of the excitement.

You can also reduce the risk of inversion by a masthead float, these are widely used by sailing schools but will have little effect on performance. A wood mast will have the same effect (but there can't be many of those still around). Or a foam float in the top panel of the sail (unfortunately, less effective with a reef). (Incidentally, you will need an effective reefing system - makes a huge difference when it's blowing a bit.)

Comet Trio had both a reefing system as standard, and a foam filled mast (I think - check) and we have found it to have good manners in this respect and sails quite well.

There is a variant of the Laser Stratos with a 70 kg bulb at the bottom of its plate, I'm told it has the same handicap as a normal Stratos, capsize will be much less likely than without the bulb (though I suspect not impossible), and recovery easier. Also has reefing system. It's a fairly big dinghy and can carry the weight more easily than a smaller boat. Sounds like just what you want if you can afford one.

Order of the Stale Cabbage goes to dreadful modern designs with too much under-floor buoyance so that, unless crew is expert, inversion is inevitable.

Inversion also inevitable if crew tries to remain dry and clings to inside of boat.

And congratulations for considering at this stage the possibility of capsize and inversion; you will be much safer and enjoy sailing more, and bother RNLI less, and are less likely to break a mast.

Anyway, some ideas for you. Enjoy your sailing ..

 

[monkfish24]

Why not look at ti the other way round?

Why don't you try a sedate dinghy like a wayfarer into a more fun boat with a bigger rig?

Wayfarer with a 505 rig could be good fun. Big solid boats for cruising with the stick to give it a bit of go?

 

[monkfish24]

Oh and Lark's also have/had aluminium centreboards, I put a 420 rig on one for the trapeze and that was good fun, it was all I had lying around.

 

[electrosys]

- is a particular metal most suitable?

Ordinary mild steel plate is the cheapest solution - but stainless (from the scrap-yard) would be my favourite. You could always apply a thin (c. 1mm) skin of s/s over mild steel, of course ...

- is a particular paint/finish vital?

If mild steel, then galvanising is usually chosen - but Zinga is good.

- is the weighted plate likely to damage the boat's original centreboard housing?

Possibly - if in doubt, best to strengthen.

- is there an easier method of cutting a curve into plate metal?

Plasma cutter, or oxy-acetylene torch.

 

[Greenheart]

Interesting thoughts, thank you. I used to stuff my Topper's lightweight aluminium mast sections, with polystyrene. I can't recall if it prevented inversion - I didn't flip, often.

Part of my loony plan, is not to need to depend on a crew. I'm not kidding myself that a 50 kilo centreboard will take the place of 85 kilos out on the trapeze, but I've experienced 'easy-to-right' dinghies which could actually be very obstinate whilst on their sides, and having in effect the equivalent of an eight-stone crewman already on the centreboard, will surely hurry the boat upright again, even preventing the masthead going in at all, at times.

Reefing gear has always been high on my list of seamanlike essentials. Great to see a small boat which can still set a really well-shaped reefed main and gen, 60% of their full size. So sad to see dismal roller-furling sails that lose their form and upwind efficiency.

The Wayfarer with a 505 rig! That's exactly the kind of idea I've been entertaining. Given what a very reassuring hull form the Wayfarer enjoys, I'm surprised the class's many experts haven't shown an interest in giving the sail area a jolt of nitrous-oxide, for the fun of peak-performance. Imagine a mast 4' taller, perhaps a slab of 35sq ft more mainsail, set up to reef away in seconds when the wind gusts up...and a longer forestay...why should the best cruising dinghy be such a slug?

That said, there are plenty of true performance designs that won't need vast modification to suit my ideal. I think the Osprey and Javelin are both hefty old-school racers, long and broad enough to be comfortable and carry dry-bags of cruising kit. I'm still left with the idea that a weighted centreplate would give the boat an element of stability that the singlehander's luck and skill alone won't always allow for.

Re. choice of material...just a thought...how about buying ten sheets of 2mm (or twenty of 1mm) stainless steel plate, and cutting them to a nice gradual smooth contour, then epoxying them together? I'm not keen to start welding - it looks so damned expensive!

 

[ianat182]

I haven't seen or sailed one for some years now, but the Wildfire Dinghy was a terrific sail and had 250lbs of lead shot in bags that were inserted, before sailing, into the hollow grp keel of about 4' depth. I sailed her in weather that I would not normally go out in F4-5 and she took it in her stride; she was also a very good light airs boat. Ian Proctor designed her with I believe George O'Day's collaboration ,in or about the 1980's. She was about 16-17 ft long. the keel was lifted out for trailing home.
I guess a retractable c/b with a bulb would be another idea for you to think about !

ianat182

 

[Lakesailor]

Wildfire for sale

 

[William_H]

Dinghies

As you have mentioned reefing of the mainsail is the first thing for safety especially single handed. This can be as simple as eyelets in the mainsail then use cord to lash down and out. Set up the reefing before you leave the beech. We had a fleet of 10ft dinghies at our club which could be reefed. it made all the difference for the kids when the wind gets up. Yet it is the only dinghy type I have seen with reefing.

The next line of defence is side decks which will allow the boat to heel right over before water comes in. Dinghies lose their stability rapidly with water sloshing around. Side tanks are the best bet. I am not sure of the arrangement of your dinghy.

Regarding strength of the CB case. This should be strong enough top take the weight of a crew man on the CB with boat laid down. The situation when boat is being righted. I suspect you worry too much about capsize and righting the dinghy. Lazers are of course single handed and are righted quite easily. My son (quite fit) used to say when racing his Lazer that 12 capsizes ina race was the limit. Any more and he would pull out.

I would suggest that you fix the original ply CB then if you want try clamping a slug of lead on the bottom to give it a try. However the weighted keel will help in righting but will not help so much in improving stiffness. It only has righting power at large heel angles. But do get practice att righting the boat.
A steel CB will be heavy to lift up on return to the beach.

I used to teach sailing to kids and first lesson was always righting the boat. I would tether it to the end of the jetty and get them to capsize then right it. One crew would float in the hull so was in the hull when it righted. however these dinghies held a lot of water so had to be bailed before the other crew could crawl over the side. We also taught righting from fully inverted. Two people toes on the under side of the gunwhale pulling on CB. Of course CB must be locked in place. I used to love to get the kids to duck under the hull into the air space. There is a lovely blue light comes from the water.
Of course if you are in UK you will not be so keen to get into the water. good luck olewill

 

[oneday]

I should have a 40 or 50kg drop-keel, still in the shape of the original board. That ought to make righting even a Flying Dutchman, into child's play.

If you are going down this route make sure your board is securely held, I've often seen people stick their fingers in the c/b case when swimming round the boat after a capsize - a 50kg guiloteen coming down on your fingers doesn't bear thinkng about

Also don't forget you'll need clearance for your kicker, unless you are going for a gnav conversion as well..

 

[Touchwood]

A slight alternative plan - a friend increased his dinghy's stability by inserting a stainless steel disc (approx 350 mm diameter from memory) into the wooden centreboard. Somewhat less weight gain than a full replacement, but far easier to fabricate, and retained the aerofoil characteristics of the centreboard. It works well. If you need more details PM me and I'll enquire.

 

[maxi77]

As suggested earlier Bosuns have a metal centeboard. One key feature you need is a means of ensuring the board stays in the slot during a capsise, the original bosun did not have this feature and the original ones were withdrawn from use in the RN until modifications were made. The other important thing is to ensure the board can be fully extended when the boat is either capsised or inverted.

The suggestion of inserting a metal plate in a wooden board sounds quite good

 

[ianat182]

Lakey. That Wildfire dinghy at that price is a steal - C/w outboard and trailer!!!..............!

Sorry for thread drift,but that would be better than a Wayfarer for the OP and no mods to do.

ianat182

 

[reeac]

You'd need tackle to raise the centreboard. My 1894-design dayboat has a 150lb. steel centre plate and a system of blocks with the line being cleated on the aft. end of the casing within reach of the helmsman. The original system featured a bronze winch onto which a steel cable was wound but a previous owner thought that that was too hazardous for her childrens' fingers.

 

[Greenheart]

I'm still thinking of an entirely newly-constructed plate, probably using ten or twelve sheets of 2mm thick stainless steel, stuck or welded together, edges ground to aerodynamic perfection. Possibly a little too flash, if it gleams and flashes and mirrors like Sir Lancelot's armour? Ah, well, it shouldn't be seen very often.

I'm alive to the necessity of keeping that 40 kilo guillotine safely in the 'lowered' position, especially when the boat's inverted. I'm thinking there'll need to be a 'fixed' lowered position, which I can 'bungee' firmly, up or down, so that while there's so danger of the plate scything back into the slot under gravity, there'd still be some 'give' in its position, in case of going hard aground.

Maybe I could put on a cleatable loop of 14mm Marlowplait, that always stays hooked securely, locked, either in the up or down position. No danger of any dismembering descent by it accidentally dropping this way or that.

 

[Slowtack]

Everything is conected..........The much heavier centreboard will stiffen the boat and also incease pressure on the rig as it will heel less in a breeze. If the boat does capsize it will be more stable in the inverted position than was previously the case, and more difficult to right. I would look at a similar purpose built hull length ballasted centreboard boat and assess the other differences in design and construction eg mast, rig, chainplates, beam, overall weight etc. MIght be cheaper and better overall to buy than retrofit.

 

[Lakesailor]

My "new" boat has an alloy centreplate. I wouldn't get too hung up on the aerofoil section. It's going to be doing 5 knots. Not a great deal of "aero" effect at that pace.

I doubt the weight will be much advantage either. Get a fatter crew. Or lean further out.

The plate is to prevent leeway. Ballast, and hull form, will make you more stable. But a heavy plate will be a pain in the arse and not make you that much more stable. Unless you can get your ballast ratio up to more then 30% or so.

 

[Greenheart]

Thanks, Slowtack...I remember that I heard that having a hefty leaden foil sticking up out of the water actually increases the vessel's inclination to stay upside-down...but I'm not sure I paid attention long enough to believe it. I can see the sense in the thought though.

But surely, given that we're talking about a boat no bigger than a large dinghy, the crew's ability to induce initial heel, as crewmen everywhere who recover dinghies from inversion, always must, their approach to the task will be made rapidly more effective, once the keel's additional eight-plus-stone mass is thrown more than about 15 degrees out of vertical? Suddenly it won't be one shivering twelve-stoner on the centreboard, it'll be more like someone of Peter Griffin's build.

Lakesailor, you've a humourous streak. And I approve of your philosophy. But, my boat's gonna go a lot faster than 5 knots. And, I'm not content with my righting-effect, even when I'm leaning out, standing on the gunwales. And I singlehand a lot. Interestingly, since you said a 30% ballast ratio is basic, I see my 40kg keel plan is co-incidentally about 30% of the total...

 

[trapezeartist]

Some designer has carefully worked out all the parameters, and then you want to hang 40 or 50 kilos underneath it! I really don't think that's a good idea.

The last time I looked, the Y&Y Classes Guide listed something like 130 different classes of dinghy. There must be something in there that suits your requirements, without going to all the trouble that you are proposing.

You want something with a good performance that is easy to right. What about a 470? It has a PY of 970ish and it's completely unnecessary to stand on the board to right it. Just lean on your elbows and up she pops. You'll need a mast float to stop it turtling, or just accept that you have to pull it up from fully inverted occasionally. It's not fast, but neither is capsizing. It's not a big deal either.

The 470 is a double-hander, of course, and you're talking about doing some single-handed sailing. If you're a decent weight you could consider a 420 and put on a long tiller extension so you can helm from the trapeze. The optimum total crew weight is only about 110 kg, so one reasonably large person on the trapeze isn't going to fall too short.

The best way of going single-handed though, is to buy a single-hander. If you want to trapeze, there's the Contender, though the boom is quite low for anyone of unathletic proportions. You could recut the sail to get the boom higher. Then there's the RS600, which has a reefing arrangement as standard. If you're prepared to hike, what about a Solo? Not as fast as the others but no doubt as fast as a fast boat carrying 40kg od extra weight. Or you could go down the Laser route. Personally I hate them nearly as much as Flying Fifteens, but 200,000 Laser sailors can't be wrong (so it must be me).