Converting to Open Transom

[Alixander Beck]

Are there any specifics to consider when converting to an Open Transom design?

I am restoring this wooden dinghy with a new deck and bouancy. I will add bouancy in the Bulkheads and raise the floor 2 1/2" to add bouancy beneath it.

I have spoken with some dinghy sailers and they have suggested 2 4" "tubes" in the rear bulkhead to help in sailing the boat dry after a capsize.

As far as the construction of the "tubes" and installation I have that under control but was wondering if there are any specific design considerations I should be aware of.

Some pictures...

[Penfold]

If that's the rig you intend to use, I'm not sure I'd be that concerned about a capsize - but if you built the floor aboout 3 or so inches above the waterline and boxed in those side decks to form side tanks you could chop the trans off for sure - what you will of course need is a substantial transom cross beam of some sort to hang the rudder off - but I'm sure you've thought of that. This is my boat - it don't have a transom either!!

[Alixander Beck]

Yes thank you. I did not say that this was a Racer it is a restoration project.

My concern is not whether or not this boat will capsize - becuase it will - My concern is how to empty the cockpit quickly when full of water.

Chopping of the transom is not an option as I mentioned before this is a restoration to original form/design with slight modification not a frankenstein-boat

Your boat looks fast - faster than mine - Congratulations, but I was looking for helpful feedback not braggery.

[Raggi_Thor]

If you want an open transom, can you make the sole sloop backwards to make sure the water exits there?
What about selfbailers like they use in the Yngling? One on each side where the sole meets the hull, at the widest part?

[Tim B]

There is an old trick (assuming that the floor is above the waterline at the transom) of drilling two 1" diameter (25mm) holes, one either side to let the water drain out. Laser 13s and 16s did this with great success as, (i think) does the Laser Stratos. It's a good system and requires a minimal change.

Tim B.

[Raggi_Thor]

I was thinking of the type that "suck" the water out if the boat is sailing.

I think the big drains and raised cockpit sole are a good idea. You might want to consider some sort of rubber(maybe epdm) flaps on the outside so if you had to move aft to work on the rudder it would not necessarily flood-at least not quickly.

[astevo]

best form of flaps are mylar or xray film stuck on with double side tape at the top, bit of vaseline round the edges will help the leaks, but every form of the stern flaps ive ever seen leaks a bit, especially if the stern drags a bit.

other options are a bit of neoprene with a bit of plastic on the back of it to hold it flat, these can handle a bit of elastic to hold them into place, which you can then release if you swim.

[Penfold]

I am very sorry. I thought your post meant converting to an open transom - ie where there was a transom, the intention is to have an open space.

Using tubes to drain thro the aft tank is quite common in racing boats, and like a lot of racing features, it would work very well in a cruising boat. Please do not take my post as braggery. I was meant to be read as 'anything is possible'. Look at the back of an International Contender for great transom tubes, or a Lark or Scorpion for great flaps - one of the best mylar type films I have found to do it is easily available from stationary stores (acetate sheet for making overhead projector slides), secure it with simple gaffer tape is ok, but vacuum bagging tape (good chandleries will be able to get some for you) is better. I think however from experience of rebuilding at least half a dozen Firefly dinghies in the last year - including my own 1951 built one, that doing a proper job on the increase in bouyancy is the key to the challenge. If you design and build it properly it will massively reduce the 'scoop' area as the boat is righted after a swim and you may well find it possible to 'dry' capsize like a laser or a modern dinghy. Watch out you don't overdo the buoyancy however, or build it too low down else you may find the boat floats too high on it's side, in which case you may find you struggle to get back aboard, or even that the boat turtles straight away - which will also tend to happen if the sail area is small. A better idea used by heavier displacement boats is the self bailer - they work well at even low speeds now and if it is windy enough to capsize, you will have the speed to make them drain the boat in no time. An old boat builder once reminded me when I was in two minds about fitting one, that a self bailer weighs less than a plastic hand bailer full of water... and the transom can remain intact...

Draining fast in open boats that 'scoop and keep it in' is always done best with a 2 gallon bucket and a digging the garden action!

I was also not suggesting a frankenstien approach. This sort of job requires skill and consideration of the original - obviously. In our yard we do these sort of conversions all the time and they take days and we never make any money on them - but we do them anyway!

As to whether my boat is faster than yours or not - I'm sorry but I don't understand the comparison - I was mearly pointing out that open transoms (as your post puts it) normally mean NO transom and that it is a very ordinary thing, done on race and cruising boats the world over - and the photo was intended to show that very wet boats benefit from the design very well indeed.
No offence or braggery was intended in any way. Please accept all due apologies if that was how you read it - and I hope these comments from someone who's been 15 years a boat builder are of better use.

[Tim B]

In my experience, LARK flaps can be absolutely pants. It's not helped by the rules stipulating a minimum distance from the transom floor (which is not the lowest point). I got so fed up with the flaps on my LARK that I just glued some perspex into the transom and now use a self bailer and 1/2 a milk bottle. It seems to work much better, considering that I don't get a lot of water in the boat anyway.

As long as you have the with the floor and drain principle, it's not going to flood as long as the floor is above the DWL. The only way it COULD flood is if you were sailing backwards (another reason for me not having flaps on the LARK). Even if it does flood a little, it's going to drain out pretty quick as long as the floor slopes down aft.

Tim B.

[Penfold]

You know Tim, you're right. Come to think of it, I sealed my Lark transom flaps up and didn't use them because when you're sitting in the right place up wind (ie as far forward as you can get just about), the bottom of the flap opening would have been about 75mm above the water, and if you filled up after a capsize, they let more water in than out unless you were planing, and as anyone knows if you've got a boat full of tide you can't sheet in too hard for fear of the rig getting bent (a common problem with Lark rigs too) I did also have the problem of them nearly filling the boat up when I was reversing in traffic - and when sitting around between starts hove-to with my feet up eating lunch! good to see how a flap works tho- even better as an example of how class rules mess things up sometimes!

[Alixander Beck]

Thanks for all the suggestions.

Tim B. - the boat does already have the 1" drains which work well for emptying the boat after sailing, but they take forever if the boat is full in the chop, not to mention they sit at the bottom of the transom.

Penfold, Appologies for my hasty descisions, your comments are useful and appreciated. Thanks.

I think I may just increase the bouancy and leave the open transom on the back burner for now. Maybe take another look at it after evaluating how she sails with the added bouancy.

Penfold, Could you please elaborate on the increase of bouancy possibly causing turtling? It's something I'd like to avoid at all costs if possible.

[Penfold]

Sure... Consider the boat lying on its side, capsized. In an ideal world you want the sails on the water so it stays stable with the centre line of the boat about 6in above the water so you can reach the centreboard and get her up. If the centre of bouyancy (c of B) is in the sole of the floor, there can be a tendancy for this to rotate the hull around the centre of gravity - which in most dinghy hulls is around the top of the centre board case sort of area. The hull will always try and rotate until the c of g is directly under the c of b... Sounds like a difficult thing to do right? Actually it's quite easy - you just have to work out the centres of volumes of the buoyancy compartments correctly to ensure that the c of g of the hull is lower in the boat than the c of b when the boat is upright, then when on it's side the hull will try to right itself (won't be able to because of the rig being full of water of course) so that in a capsize the boat will stay on it's side. Working it out is easy in metric because of course 1 litre of air will support 1kg of mass etc...

[Alixander Beck]

Ah I see, So by that explanation there should be more bouancy in the side decks that on the Sole, to keep the bouancy higher than the CoG, which would be the steel CB in this case?

In this picture are the ducts those rectangular sections at the bottom of the transom? and is that a cap over the tubes that this man is holding?

Thanks Penfold!

[Penfold]

It looks like a waterproof inspection hatch cover he's putting on to the back tank so not a tube really... But yeah, the triangular ducts are drain tubes down from the cockpit - which has a false floor in this class so the boat drains easily.

Aim to spread the buoyancy around... a false floor will keep the boat dry but very much reduces head room when crossing the boat, and of course you have the problem of how to repair hull damage... Making side deck 'tanks' is a good way to stiffen the boat, give fixed buoyancy and will probably help reduce inversion in a capsize... gotta go now, finished work early for a change and a weekend of sailing ahead of me! - good luck, Penfold