Transom draining flaps

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Michael4
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Transom draining flaps

Post by Michael4 » Sun Aug 30, 2015 9:32 am

I've done a search of the forum but haven't come to a conclusion

There seems to be a widely held opinion that transom flaps don't really work unless the boat was designed with them in mind and has a flat run aft.

Does the shape of an old Merlin with plenty of rocker and an aft deck make transom flaps pretty useless? Our elderly 'Gentleman's Merlin' has perspex hinged flaps held closed by elastic. The flaps don't leak and normally are well out of the water. All buoyancy is bags.

In extremis is the elastic supposed to be sufficiently weak to allow the weight of water in a swamped boat to push the flaps open or is one supposed to release it by hand?

When we tried to move weight to the back of the boat to shift said water the gunwhales went under rather negating the effect. If I install a central stern buoyancy bag under the deck the boat will float higher when upright (hopefully not too high when on its side) but the flaps will be inaccessible from within and I presume water flow to said flaps will be impaired. If I install longer tube shaped buoyancy bags to run under the deck on either side I suspect the boat will float too high when on its side.

Clearly the answer is not to lose concentration when planing on a dead run...but I don't need to be told that now...

In summary, do I prioritise buoyancy over efficient flaps? or what?

Thanks in advance,

Michael

Two pics from a while ago help to illustrate:

ImageDSC00906 by dralowid, on Flickr

ImageDSC00936 by dralowid, on Flickr
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JimC
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Re: Transom draining flaps

Post by JimC » Sun Aug 30, 2015 11:06 am

Release by hand is conventional.

Never heard the design thing, and can see no reason why it should be true. They work when there's enough speed to generate a bit of suction at the stern. Obviously an old Merlin with buoyancy bags and a thousand gallons of water on board is going to have a lot more trouble getting to an appropriate speed than a boat with tanks all round and only a dozen gallons or so.

Frankly I can't imagine a boat with bags ever floating too high.
I doubt flow rate to the flaps is very important but can easily imagine a bag being sacked onto the ports and sealing them.
Last edited by JimC on Sun Aug 30, 2015 11:09 am, edited 1 time in total.

Michael Brigg
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Re: Transom draining flaps

Post by Michael Brigg » Sun Aug 30, 2015 11:08 am

My own feeling is that transom flaps are only ever going to work if the boat is sufficiently powered up to "sail away" from the water inside the hull, in which the boat can gain speed more quickly than the water it contains. If this fails of course when you inevitably slow down again the water you took with you catches you up and then worse still runs forward and takes you "down the mine!"

A "flat run aft" does help to let the water run aft, but again this will only work if the water can't run forward again. So Bow tanks are usually needed.

The best design for flaps are really confined to the "unsinkable" boats, with a double bottom. The buoyancy in these is kept where it is needed. Above the water when you are on your side, and below the waterline, ie displacing it, when you come upright. This way the hull has sufficient buoyancy to carry the hull weight and crew weight AND keep the water level in the boat above the waterline.

The gradient will that way continue to empty the boat at any speed. Such boats often have no transom at all.

This is said to be a safety issue as there is no air pocket under the hull if it turtles with crew trapped underneath. Deaths (rarely it is true to say) have been recorded.

I think buoyancy bagged boats will always struggle to work with flaps as the bags float up above the water once you are upright unless you strap them down very firmly. Even then they shrink as soon as they cool down in the water, reducing their effectiveness , making them loose again and facilitating their escape like Houdini. (who it was said was able to "Shrink his wrists after he had been tied up!)
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JimC
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Re: Transom draining flaps

Post by JimC » Sun Aug 30, 2015 5:07 pm

Michael Brigg wrote:The best design for flaps are really confined to the "unsinkable" boats, with a double bottom.
But they don't need them at all Michael! I think that's over cooking it. Its all about getting enough speed so that the water gurgling past the transom creates a bit of a hole in the water so gravity can assist the water in the boat to fill the hole.

My feeling is that for most traditional boats they're going to be ineffective until you've bucketed a lot of the water out of the boat, which in practice usually means when fatigue is overcoming adrenalin in the crew's arms. At that point the bucket can be put down and the transom flaps used to get the remaining water out until you are down to self bailer level.

Never mix transom flaps and bucket though, best one then the other. There's also a definite art to transom flap use, optimising the trim so the bow is as high as possible to keep the water flowing back without stopping the boat.

chris
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Re: Transom draining flaps

Post by chris » Sun Aug 30, 2015 6:38 pm

Well I can't decide either! With MR 507 ( fairly similar to yours). I put a new transom in some 20 years ago when I first restored the boat. Out of ignorance I copied the transom flaps from what remained on the old transom. Originally the transom would NOT have had flaps but someone had added some later which I copied. They are too small, too high and I have always regretted copying them for the new transom. They are held shut with elastic but since they are well above the water level it has not worried me that the elastic perished some 8 years ago. All they do is let water in when you launch.
However....
I do believe that any boat must be retrievable without outside assistance and this has been tested several times over the years most notably during the Birkett Trophy on Ullswater a few years ago. We capsized and righted it without too much problem - even saved our mars bars and apples from floating away, but yes, merlins will hold a lot of water. Forget following the race course but sail in the fastest direction you can, bailing like mad ( throwing the water over the stern rather than the side helps ) Tiller between the knees etc to steer. Only release the transom flap's elastic once you are moving otherwise you will let more water in and be prepared to close them if you slow down. We have one decent bailer in the hull and one old piston tube bailer which is hammered shut. With a good dose of adrenaline we were dry in about 5-8 mins - sorry the boat was dry, not the helm and crew. If, when you capsize, the water is over the centre board case then the first job is to stuff something in the gap. A piece of cloth, sponge etc you keep for that purpose or items of the crew's clothing or body parts if all else fails. Personally I have arranged the bouyancy so that the boat does not take in enough water to cover the case. So transom flaps can be usefull on these boats.

The other advantage of transparent flaps is that in the case of the crew being trapped under the hull in an inversion the crew has some day-light and does not panic. There is a good air space and they can compose themselves before diving out.... Yes, we have done that two or three times ( just to check of course).

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trebor
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Re: Transom draining flaps

Post by trebor » Sun Aug 30, 2015 9:44 pm

I have transom flaps on my Enterprise, I thought they only opened when their was water behind them, their is a self bailer fitted, I always assumed this would drain boat once underway.
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JimC
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Re: Transom draining flaps

Post by JimC » Mon Aug 31, 2015 2:30 am

trebor wrote:I have transom flaps on my Enterprise, I thought they only opened when their was water behind them, their is a self bailer fitted, I always assumed this would drain boat once underway.
Basically (and over simplified):

A bucket works in any wind conditions and should be used when the boat is so full of water that you can hardly move. Its unlikely to be usable when sailing normally, and won't shift the last few gallons. All boats with buoyancy bags should carry a bucket. Most/many boats with bow tanks and generous sized full length side tanks will get away without a bucket.

A hand bailer works in any wind conditions, shifts less water than a bucket but can be used while sailing along and to get the last of the water out. Every non-self draining boat should carry a hand bailer.

Transom flaps work at moderate speed when the boat still has a lot of water in provided that you trim the boat and set the course to suit the working of the transom flaps, not where you want to go, and provided there's enough wind to get the boat up to a moderate speed. The elastic is to pull the flaps firmly shut: it won't act as an effective valve, so the string needs to be released and recleated as necessary.

Self bailers work when the majority of the water is removed, and the boat will sail at reasonable speed. They will work on any point of sailing provided the boat is making a reasonable speed through the water, and do not require special trim, but below a given boat speed/wind strength they won't work. They will also deal with spray coming on board in almost all conditions.
Last edited by JimC on Mon Aug 31, 2015 3:56 pm, edited 1 time in total.

Michael4
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Re: Transom draining flaps

Post by Michael4 » Mon Aug 31, 2015 9:42 am

Based on the advice above I conclude that being able to shift enough water to get her moving is priority No:1. Therefore I have invested in two buckets and put an old buoyancy bag under the stern deck to help her float a little higher when upright.

Tesco mop buckets seem appropriate since they are a sort of 'D' shape allowing one different options to relieve the monotony of bailing. The buoyancy bag is quite big (ex N12) and pretty much fills the space.

I have rigged a line over the top of the bag so I can still open the flaps from the inside. The bag may restrict water flow to the stern to some extent but by this stage we should be in recovery mode and making our own way back to the shore.

It is essential that we are able to sort ourselves out without outside assistance, a busy harbour can all of a sudden be a big and lonely place mid-week on a dull, grey day.
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Michael Brigg
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Re: Transom draining flaps

Post by Michael Brigg » Mon Aug 31, 2015 10:11 am

If you are bailing hard for more than minute then you will suffer fatigue, especially with a big bucket. Like the 400m runners, if you bail to hard for more than 45second at an Anaerobic pace, then "Jelly arms" will hit you at almost exactly 40seconds.

I would favour a pair of robust scoop bailers, quite small, one in each hand. You can scoop with alternate strokes and maintain a continual stream of bailing. Also less likely to be obstructed by thwats, paddles, spinacker poles etc.
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davidh
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Re: Transom draining flaps

Post by davidh » Mon Aug 31, 2015 11:24 am

Okay, with particular respect to Merlin Rockets, the story of transom flaps is one that is luckily documented. When I was researching for Real Magic, the book of the Merlin Rocket, I was able to not only find documentary evidence but to talk with some of the participants - so we have 'primary source' material on this.

The first transom flaps were retro-fitted to first one, then a number of existing boats, a development that was an unqualified success. So, I was able to dig down and get to the bottom of the story, and then write it up in full. But the bottom line, it worked across the full range of boats.

But a further thought - having just spent a fortnight working at the Fireball worlds; the fireballs now sport transom drain tubes - again a move that was relatively easily retro-fitted to older boats.

D
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Re: Transom draining flaps

Post by SoggyBadger » Mon Aug 31, 2015 6:26 pm

Some people seem to manage without transom flaps or bailers :D

https://www.youtube.com/watch?v=qA8bmfjGwfI
Best wishes


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chris
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Re: Transom draining flaps

Post by chris » Mon Aug 31, 2015 6:42 pm

Very clever! quess it might work on an open decked 14, but not on a boat with side decks.

My bucket is a square small waste bin as it fits the hull shape well. My other bucket is one of those soft flexible tub-things sold for the garden etc. Works very well indeed.

I can't see two small hand bailers being much use in a situation where the boat is seriously flooded. They are OK for keeping you feet dry.

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Re: Transom draining flaps

Post by Pat » Wed Sep 02, 2015 6:37 pm

We have transom flaps on half cut MR2121 and finally worked out how to get the boat to empty when filled with water.
We sail on a reach by backing the jib to compensate for the weight of water trying to rotate the boat about the mast and also stop it coming up through the centreboard case - stuff the slot with something and bale out furiously until the level is down.
This worked for us at Roadford when the safety boat failed to attend and we even finished the race although when we first filled up we were under water from the shrouds back!
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Graham T
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Re: Transom draining flaps

Post by Graham T » Wed Oct 14, 2015 5:52 pm

I have just retrieved my late aunts sailing diaries from her flat in Broadstairs. She kept a record of every race in and modification to their Merlin Rocket 1088 right from collecting the part completed kit from Chippendale in the winter of 1959. In the first sail they half filled with water and so bought a pump the following week. There are many references to pumping out until 1965 when she writes " self bailers fitted at last!". In the pre season fettling for 1966 they cut drain flaps in the transom "for speedier draining after capsizes".
Graham T
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