Thread: Change Centre Plate & Centre Case Design - Possible??

Originally Posted by HappyPappy

My question is will a 34mm centre case gap under the boat cause turbulence with water rushing up and could it be enough to effect the performance of the boat to any appreciable degree?

The mechanism for raising it and lowering it is something I haven't got to yet.

Hi HP,
My TS had a wooden centreboard ballasted with lead just enough to give it neutral buoyancy protruding through half a ton of lead keel bolted to the outside of the hull. From memory, the C/B slot was 45mm or thereabouts. No problems with drag. Your biggest problem will be raising and lowering 100kg without busting your muscles (up) or splitting the wooden keel (down). A pivoted 100kg will go down with a fair amount of kinetic energy which will have to be braked or it could end up under the foredeck! Having the ballast inside the hull will reduce the strain on the centrecase and make your life easier.
Cheers
Graeme

I'd think using lead would be a better choice for adding weight to the board. If you cut out a substantial hole, in the lower portion of the board, which was flush filled with lead, you'd be able to dramatically increase the weight of the board without any width change.

My natural tendency is to recommend people not do this. The obvious reasons of centerboard case reinforcement have been noted. but other, less obvious and more dangerous issues crop up. With the lowered CG and ballast/displacement ratio, your rigging fittings, wire sizes and hardware selections need to be checked. This is because you've increased the ballast on the board by 60% and the total ballast by 15%. This provides a much greater hunk of leverage, that the rig and all it's little bits must resist, when you're heeled way over in a sudden gust of wind.

Speaking from experience, you'll likely be fine, though technically your rig will break in lower wind strengths, then was envisioned by the "old man" when he penned up the rigging scantlings. How much lower of wind strength is the new rig breaking point? Who knows, without some calculations, but the rig as designed, is a bit of over kill, which is the general approach we take towards home builts (it's not that we don't trust you to make odd deviations from the plans or anything).

Your understanding of "stability" is typically miss-placed. I'm not trying to insult you, but it's common for people to think that the ballast provides the bulk of the stability in a ballasted boat. In some boats it can, but not this one. The vast majority of the stability in this boat comes from her shape. The ballast helps extend her range of "positive righting arm" so she's less likely to capsize.

Don't get me wrong, I have designs that without the ballast in the boat, would flop over on it's side and sink. On the other hand, I have many designs that don't use ballast at all. The use of "internal" ballast, in this particular boat, is an old school method (and one I still employ when necessary) used to help "trim her up" on launch day. By shifting around the internal ballast, you can get her to sit dead bang on her lines, as pretty as can be. If the ballast is fixed (bolted on) you better have built the boat precisely as designed or it's not going to float where intended and the waterline paint, will look "off" with the trim differences. Movable, internal ballast permits you to adjust the fore and aft trim a wee bit and "fix" her on launch day or after sea trials.

A popular design of mine uses 1,000 (454.5 kilo) of ballast in a fixed keel casting. 200 pounds (91 kilo) is used internally, for trim (20%). I've attached the "bricks" that are also made to use as the internal ballast. They're shaped to "nest" and interlock to a degree, helping hold them in a neat pile in the bilge. Of course they're lead and weight around 6 kilo each. These can be easily moved around until you get the boat to trim up properly. I mention this particular design, because I just preformed a metric conversion on her and this was one of the "e" files I had to create and send out to builders in metric dominated areas. I still have great difficulty with metric, but do enjoy the ease of a 10 base system.

As an interesting side note, the way I got this shape for a trimming ballast brick was to blatantly steal it. I've been using this shape for many years. I mentioned that I had stolen it from a Billy Atkins design while talking to noted designer Jay Benford some years ago (okay a few decades now). He laughed hard, as he'd actually sat a desk, behind Billy at the Atkins design studio, in the early 60's as an apprentice. He told me that Billy had stolen the shape from the "old man" with I was to assume was L. Francis Herreshoff, who had hijacked it from his father Capt. Nat. I now own a 1960 John Atkins design (Billy's son). A 40' box keel, power yacht that has about 2,000 pounds (about 150 bricks) of these exactly shaped bricks in it's bilge. Funny how things seem to "come around".

Thank you graemet and PAR

That's a real interesting story PAR and thank you for your detailed answer, its much apprecaited.

I hear what you say about misunderstandings about stability, and I appreciate your comments. I am learning all the time.

With this particular one (Hartley TS21), here is part of a letter written by Richard Hartley himself to a home builder back in the 80's who asked him about doubling the width of the centreplate, i.e., from 3/8" to 3/4" ...

Richard Hartley wrote ...
"If you have a 3/4" thick centre plate it will add to the stability of the boat, but you would need to make the slot in the keel and the space in the centre case 1.1/8" wide not 13/16" as is shown on the plan.
Do not hesitate to write for further information.
Yours sincerely,
Richard Hartley"

This is what got me thinking about not just simply increasing the width of the centreplate but instead selectively choosing the extra width to be towards the bottom.

Cheers

I don't like these "boiler plate" types of board boats, they suffer from a grossly inefficient board, though the weight is welcome.

If I was to do anything with the "additional thickness" I would be to adding some foil shapes to the board. If this included some weight so be it, but shape would be my primary goal. Even a flat sided foil shape, like those used by Mik Storer on some of his designs, is much better then a flat plate.

I'm not sure what your focus or interests are on this additional board weight is, but it's not going to significantly improve the stability of the boat, unless it's a substantial amount. If it's a substantial amount (at least enough to really improve the stability curve) then you'll have to up the rigging scantlings and possibly the spars too. What is the goal of adding weight and mass to the board? As currently designed, the boat is self righting (barely). Is there something further you'd wish from the stability curve? It's easier to hit a target if you know precisely what it is, so you can adjust your aim correctly.

If Hartley gives a tick to the thicker board then it will be OK. Particularly that we can build and attach centreboard cases a lot more efficiently (and leakproofly) than in Hartley's day.

The boat is a nice one too. One of the first dinghy form yachts really.

MIK

Originally Posted by Boatmik

If Hartley gives a tick to the thicker board then it will be OK. Particularly that we can build and attach centreboard cases a lot more efficiently (and leakproofly) than in Hartley's day.

The boat is a nice one too. One of the first dinghy form yachts really.

MIK

Thank you MIK, much appreciate your comments. The 10mm plate as in the plan (actually its 9.75mm) is around 75Kg. If welding extra plates at the bottom is not the right idea then I'll perhaps just make the whole thing 14mm wide. This means it will end up about about 110Kg - not quite double width as Hartley says is OK but a fair bit heavier anyway.

Are there any resource about the newer ways to do the centre case?

Undertaking a build is a daunting task and the centre case/centre plate is one area that I think is so critical its not funny so any advice on this subject would be like gold to me. I am a little worried about whether I can get this right.

Are there any companies that can build the centre case/centre plate complete, for my fitment???

The plan shows a type of 'horn' welded onto the centre plate which is used to leaver the board up and down. But I have seen photos of a number of more recent (from the 80's and 90's) Hartley TS21's with an enclosed case and winch with centre plate having no horn. But how to do this, and how to properly attach a cable to the plate when the centre case gap will only be 2mm or so wider (to minimise side-ways movement) than the steel plate itself? And most important of all how to incorporate a mechanism that 'locks' the plate in position for safety. If a 110Kg steel plate should ever become untethered I hate the think about the damage it could cause.

I have found some excellent build information about using epoxy instead of mastic to fix the centre case to the hull (the obvious) on various websites, like from the Hartley association and user community sites and this one too.

I'm in Cairns and there aren't any TS21's up this way. If I want to see a real one in the "flesh" its going to mean a long plane trip - thus I'm doing all my research on the internet. Pretty risky really.

Everything looks manageable, with the exception of this centre case/centre plate issue. So if I can very politely ask for any tips, or if you can point me in the right direction I would be eternally grateful.

The lot needs to be done right of course, but I do believe the centre case/centre plate in particular deserves a huge amount of extra attention or even be pre-made by a company who knows all about it.

Best regards

Peter

I'd agree it's appears that making a thicker/heavier centerboard is okay, in regard to the structure, but I still question why?

I've got 107.1 degrees for the point of vanishing positive stability, in my data base for this design in "light" trim. This drops quickly if loaded normally with a crew of three and their stores. She carries a capsize screen (SSV) well over 2 (2.4) so capsize is very likely if loaded up for a lengthy cruise, but most will not have the boat in this condition, preferring to sail lightly loaded.

Adding some weight to the centerboard might (don't hold your breath) get the point of vanishing positive stability over 110 degrees, though a measurable amount, it's very unlikely you would notice the difference in your butt, when heeled over in a brisk slosh to windward.

On the other hand, if the centerboard was foil shaped and made twice as thick in the process, then you'd have a very noticeable improvement in performance, affecting improved maneuverability, pointing ability, much decreased centerboard noise and vibration, etc.

Again, the goal of the added weight?

Hello PAR,

Thank you for your detailed comment.

PAR, I am not technically minded nor knowledgeable in these matters (that's why I'm here I guess)

The purpose of me posting the first message is to gain some advice on my thoughts to make the vessel safer, i.e. to give it more ballast down low. To improve the resistance for it to topple over. I know the weight of the steel centre plate is not the only thing that influences this but sheer logic suggests more weight down low under the water will provide more resistant to the very forces wanting to turn it turtle - although that's my logic mind you - I could be very wrong.

Originally Posted by PAR

Again, the goal of the added weight?

Its not added weight, it is part of the required extra 227 Kilos of ballast Hartley's say I have to install on the floor just in front of the centreboard case. My thought was to put a little of this required extra weight into the steel centre plate directly thus putting it down under the boat (instead of all of it on the floor in the boat) and thus, to my reasoning anyway, making it more efficient, i.e., can contribute better to resisting the forces that want to topple her over.

Richard Hartley has commented that doubling the swinging steel plate's thickness (which would make it double the weight) will add to the vessel's stability.

I don't really want to double the weight of the steel centre plate but I want to head in that same direction, i.e., that will see me "add to the vessel's stability", by increasing the plate's thickness somewhat. I was thinking from 10mm to maybe 14mm or 16mm. Then I thought maybe position this extra weight towards the very bottom of the centre plate itself by welding or riveting extra plates onto it (the subject of my initial post message).

I am finding it hard to follow our comment about a foil. How can you make a 14mm thick piece of sheet steel 55 1/2 " long and 27" wide into a foil shape? The steel plate swings up into the centreboard box (the slot is only a few mm wider than the pate itself) when not in use and swings down when in use.

I can trim all edges so they are sharp as a knife but I'm just not able to follow you about the foil bit. Sorry, I know I'm showing my lack of knowledge here.

My apologies for not following you on this.

Pappy, it not as "intuitive" as it might appear in regard to stability from weight. The extreme majority of the stability of this craft comes from her shape, not the ballast. The ballast adds enough to raise the PVS over 90 degrees, which makes her self righting. Unfortunately, at 107 degree, this isn't a guarantee, particularly if well loaded. Even if you doubled the centerboard weight, you only raise this figure by a degree or two, which doesn't insure anything. For real security and assured self righting ability you need around 120 degrees. Hence the rub.

Richard Hartley's comments on "increasing the stability" with a doubled board are correct, but he's omitted the hair splitting aspect of this decision, which is a couple of additional degrees of self righting ability may not be worth the trouble.

My point is you can make a thicker board that weighs much more then the steel plate board, that also incorporates foil shapes. This of course wouldn't be a steel plate board, but could be made from a number of materials and methods. I'd recommend a strip planked, 'glass sheathed board, with a lead insert. This would offer the weight you desire and the foil shapes that will benefit the sailing abilities of the yacht.

In fact, if I was building this design, I'd convert her to taped seam, save a bunch of weight and building materials, (and building time), toss this savings in ballast (attempting to get well over 30% ballast/displacement ratio, instead of the low 20's as she is currently) and better appendage shapes, to really have some fun. With the considerably higher ballast/displacement ratio (a 50% increase over stock) you would have a "stiffer", seemingly safer (this is more skipper dependant then any design feature), that could also could carry more sail area (faster).

I'd also consider a stub keel to house much, if not all of the centerboard, to prevent or limit it's intrusion into the cabin spaces. Then again, this wouldn't be my first picnic and making this level of modifications wouldn't intimate much.

Yes, we're on the same page, as I understand you're building the centerboard (center plate as he calls it) version. You must remember how old these plans actually are. We've (the design industry) made some advances in building techniques (water tight centerboard cases for example), materials and techniques, plus some of these can offer huge weight and labor savings, compared to the way the plans show things.

In short, I don't think the trouble of making a wider board case and a thicker steel board, is worth the limited benefits it brings to the table. A note for future reference, don't grind the plate to a razor sharp point, you'll actual make it worse, not better.

Thank you to PAR and Boatmik for your kind replies.

And I hear what you say about the age of the design PAR. There are many newer designs out there offering faster performance, better self righting abilities that can be built using very latest techniques and methods.

But on the other side of the same coin the Hartley trailer sailer designs are about as well proven as you can possibly get. They have the most amazing reputation all around the world, even though their designs are a little dated. You don't get into that position without very good reasons. At the end of the day they are relatively easy and straight forward to build for someone like me who is severely lacking in boat building knowledge and experience ... but learning quickly.

I don't know whether this is going to sound strange but the TS21 feels like it cuts through the water a little smoother than some of the newer designs that are as light as a feather. Some of the newer ones you bob up and down like a bloody cork in even the smallest of chop, feeling every single miniture wave - quite uncomfortable and unsettling. But on the TS21's I've been on (years ago) I clearly remember them riding through things much more smoothly with seemingly less bobbing up and down motion (if that makes sense). Maybe because of the weight, I don't know, but its a specific observation I've made myself many times and one of the many reasons why I've chosen to build a Hartley TS21.

Originally Posted by Boatmik

... Particularly that we can build and attach centreboard cases a lot more efficiently (and leakproofly) than in Hartley's day.

... and ...

Originally Posted by PAR

... You must remember how old these plans actually are. We've (the design industry) made some advances in building techniques (water tight centerboard cases for example)

I am now super keen to do this. Can either of you kindly point me in the direction of where I can source information on how to incorporate this newer water tight "leakproofy" way to do the centre board case in the Hartley TS21??

I hope I'm not asking for too much, but any advice on how to connect the cable to the centre plate and how to create a brake system that stops the centre plate in either direction for safety would be extremely helpful. The centre case slot will only be a few mm wider than the steel plate itself.

Thank you so much for the way you've both helped me with advice. Can't tell you how much I appreciate it.

The case logs need to be "bedded" (term used improperly) in a thickened pile of epoxy. This will marry the bottom of the case to the keel batten, preventing leaks. Depending on design, I might add some fillets and biax for additional strength.

You don't need a brake for your centerboard lanyard, though you will need tackle or a winch. The lever can slam home on some stops (place a rubber pad on the "land zone") or you can use swaged sleeves on the cable lanyard as stops. Much depends on the engineering in the hoisting system. Since this one has an arm rising through the top of the case, I'll assume it's tackle and fairly unsophisticated, so thus a simple arrangement. Personally, I'd never use this arrangement on a boat of this size. The case is more then large enough to house all of the board and hoisting tackle. I'm just finishing a 15' daysailor, which has a centerboard and all the hoisting gear is inside the case, except for a single line that runs to a cam cleat on the outside of the case. If there's an issue, just remove the case "cap" screws, lift off the cap and have a look see.


Sourcing information about the "leakproofy" case is a mater of good building practices and being aware of your materials abilities. The best thing you could do is read all the Mik Storer's site has on epoxy use. Also download the user's guides from West System and System Three. These will give you a good overview on how to work with epoxy as a coating and adhesive. Glen L. Witt's book "Boatbuilding with Plywood" would be a good start.

Thanks PAR for your advice. Will do.

Originally Posted by PAR

Personally, I'd never use this arrangement on a boat of this size. The case is more then large enough to house all of the board and hoisting tackle.

Totally agree. Mine will have a little winch and cable. This apparently is an approved system from the Hartley 18 and 21 association. The centre plate no longer needs the 'horn' sticking up and I can enclose the case fully, perhaps like the way you have described.

While a better system it does mean the steel centre plate is free to thrust up into the cockpit if we go over. Actually I think yachting regulations say the centre plate can not be free like that so I will need to develop some type of braking or holding system somehow ... which annoyingly brings me back to thinking the stupid 'horn' sitcking up was not such a bad idea in the first place.

I love that term "leakproofy". Every vessel should have a "leakproofy" rating

Cheers

Maybe in your country, but ours doesn't include such a board locking requirement.

The likelihood of capsizing a boat of this size and configuration is fairly remote. You have to screw up pretty bad to get into this kind of trouble. Engineering something who's eventuality may never come, in the life of the boat is over thinking an issue. In other words, lets say this boat lives 40 years and in all the 40 years it may never heel more then 45 degrees, well within it's self righting abilities. In fact double this angle and you're sill going to self right, so, don't get carried away with things that aren't likely to happen.

Lets take a different example, catastrophic rigging failure. This can occur, does occur, is surprisingly easy to make occur and I've personally brought down considerably more then my share of rigs. The mast, spreaders, other spars, sails, wire, lines, etc. all comes crashing down onto the boat. It happens and isn't an uncommon thing. In fact it's much more common than a full up capsize or severe knock down (in a boat this size and configuration, not a daysailor which you can capsize with loud sneeze). In light of this very possible eventuality, do you build a "roll cage" over the cockpit to protect the crew. Well, this seems an over reaction, but I can tell you, people have been hurt and killed in incidents like this. The rig not only jeopardizes the crew, but the boat as well, so maybe a 6 mm plate steel deck and hull side to prevent punctures when the rig does come crashing down?

Do you see the downward spiral of ass covering that happens when you attempt to engineer like this? My point is don't over think the issues. The usually mistakes a novice builder makes is to "over build". The logic they use to justify this, is to make things stronger. In fact, in most cases they actually make things weaker, decrease load capacity and increase things like point loading, stress risers and limit load transmissions. The typical results are the "beefed" up pieces, cause neighboring pieces to break and a cascade of load transmission failures, spreads these load paths to things (other structural elements) never intended to withstand them (read more breakage).

Good building techniques, proven methods and materials will get you a much better boat, then re-engineering and making things "stouter" arbitrarily.

I was told about a Hartley TS21 whose centreplate smashed through the top of the centrecase, then smashed through the cabin top. However, this was when it came off the trailer at highway speed and rolled. The good thing is the boat was repaired.
Locking down the plate has been a contentious issue amongst TS18 & 21 skippers at times. I have no method of locking the plate yet. I've never seen the need, to be honest. We've never capsized but run aground regularly, so I may leave things as is for the time being.