Shape or Profile of Timber Coaming

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Shape or Profile of Timber Coaming

Postby DSGregLS » Fri Dec 26, 2014 10:01 pm

I am about to start the shaping of the port and stbd timber cockpit coamings. Although my Australian DS1 did not have coamings as standard, I like the look of the DS1's in the gallery which have plenty of timber details. My question relates to the longitudinal shape and how that impacts on their purpose, which was, I believe to increase the freeboard, particularly towards the aft end of the cockpit as this is where most writers here report water coming on board.
If this was their purpose, why is it that most examples I have seen on this website rise towards the front of the cockpit, where freeboard is greatest, and appear lower, relative to the deck, towards the aft end where due to lower freeboard, the risk of taken on water when healed is greatest??
Is it just that ascetically this appears better or is there a reason? My boat has the standard swivelling jib sheet eye and cam cleat on the side deck which will limit the height of the coaming forward and therefore reduce the height even further aft.
Any advice re this profile before I start shaping the coamings would be appreciated.
BTW I am not concerned that the coamings will make "sitting out" uncomfortable as it is not my intention to race. That I have done for over 50yrs. Now I intend to take it easy.
Cheers, Greg
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Re: Shape or Profile of Timber Coaming

Postby Baysailer » Fri Dec 26, 2014 11:29 pm

Greg,

The coamings do add some structural reinforcement for the side decks but in my opinion the shape and how much they stand proud of the deck or their shape is more aesthetic. So, I think you could shape them to whatever is pleasing to your eye. I left mine original but did lower them some for comfort, more toward the rear since my jib lead is routed through the coaming.

Where I see water coming from is more toward the bow and it runs along the deck until it bails out further back. The sprayed water makes a stream which will get your butt wet if you're sitting on the deck or hiked out. To cure that you could add some spray diverter splash boards like KC did, I think there's a picture of it in his gallery.
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Re: Shape or Profile of Timber Coaming

Postby GreenLake » Sat Dec 27, 2014 3:36 am

I agree, it's style rather than substance.

I have the unmodified version of the coamings and mange to sit out despite the full height. Sometimes I simply use a block of foam (on a lanyard) to elevate the sitting position, at other times, I simply manage somehow.

From around eight knots of wind or perhaps a bit more, I find it takes sitting out on the side deck to maintain balance without spilling wind. Especially if I am by myself. In stronger gusts, I would certainly lean out a bit, racing or not. Still a far cry from hanging from hiking straps with your but over the side...

On the other hand, with sufficient crew weight I've sailed up to around 20kn winds (in gusts) with everybody inboard (but then spilling wind in every gust). Even with crew like that I might choose to sit up on the side deck for better view. When in full cruising mode with both stores and crew together adding to more than the empty weight of the boat, the DS begins to feel and handle a bit like a keelboat (an internally ballasted one) and instead of sailing her upright, I would sail her at a constant angle of heel. (Still with mainsheet in hand - even "ballasted" the DS remains a dinghy after all).

Any, my point is that he full coamings don't necessarily force you to only sit on the bench and further, that there are situations where you might want to sit there, even if you are not racing. (I remember one moonlight cruise - solo - where I rigged the tiller with a bungee to keep her on course, and just sat up on the side deck all the way forward holding on to a shroud and enjoying the night sky - can't get further than that from racing, really, but the wind was just right to benefit from a bit of extra leverage for the live ballast).

For structural strength, I think anything on the order of 3-4", even mounted flush with the side deck, would give the required support. But I read between the lines that you like the original looks, so go for it.
~ green ~ lake ~ ~
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Re: Shape or Profile of Timber Coaming

Postby DSGregLS » Sat Dec 27, 2014 4:47 am

Hi Baysailer and Greenlake, thank you both for the prompt and considered replies. Baysailer I take your point that any coaming higher than the deck will limit run-off from the deck, but I was mainly concerned with raising freeboard at the lowest point which appears to be about 2 ft forward of the rear of the cockpit. For that reason I've decided to shape the coaming to remain about 2 inches above the deck for the full length.
Greenlake you make points about when I might still want to "sit out" however I'm hoping that with the installation of a full width traveller I can reduce the angle of lean without needing to spill too much wind. Remember too I have a BBQ plate for a CB and I imagine my boat a bit stiffer than the US versions. I'm hoping that a coaming that doesn't rise will not be too unpleasing to the eye.
Cheers Greg
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Re: Shape or Profile of Timber Coaming

Postby GreenLake » Sat Dec 27, 2014 2:25 pm

Keeping "run-off" out is definitely one function of the coamings. To that end, they are installed with a foam strip (I used some closed cell foam tape for outdoor or marine use). Otherwise the water will just drip behind them.

However, I've occasionally manged to "bury the rail" briefly, and in that situation, the coamings do extend the angle of heel just a bit. Really can't say whether the water was highest near the front or the rear of the cockpit. In flat water, the height of the water around the boat is determined by the stern wave. At speeds close to hull speed, the first crest of the stern wave is a short distance before the transom, further forward if you are sailing slower. The trough would be around the middle of the boat. (Once you add wind waves, this picture isn't quite as simple any more).

So, not only is the freeboard lowest, the water is the highest at some point at the rear of the boat. Interesting. However, it seems to not present problems in practice. (And whether a straight-line shape will look acceptable that's really up to you :) ).

As for the traveller, the difference is that it allows you to maintain sail shape while spilling wind. (Same effect as you would with a high purchase vang used up-wind). With a mainsheet only, the minute you reduce tension the boom not only goes out, but it does go up a bit, opening the leech. So, yes, that gives you more control.

As for the BBQ plate, how heavy is it? The standard CB is 25 lbs (before subtracting any loss due to the immersion). Any additional weight you add that low will definitely add to the stability - but mainly when the boat is heeled substantially. The weight has to be off the centerline before it can develop a righting moment. Its not unusual for keelboats to be sailed with the rail buried (about 30 degrees) which means the center of gravity moves off-centerline by about half of its depth below the center of buoyancy. And, as you can see from the diagram, the center of buoyancy itself shifts outboard in the other direction because of asymmetric immersion.

772

At smaller angles of heel, shifting the "live ballast" has much greater effect, because the lever arm is longer and you can directly increase the distance from the center of buoyancy and therefore increase the lever arm further.

However, when it comes to recover from a capsize, I could see solid advantages to a bit of extra weight down there. The lever arm would be optimal at those angles and might be enough to prevent, or at least slow down any tendency to turtle.

We are all curious to hear how she sails with that modification.
~ green ~ lake ~ ~
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Re: Shape or Profile of Timber Coaming

Postby DSGregLS » Sat Dec 27, 2014 4:51 pm

Hi Greenlake,

Here is a photo of the steel centreboard and these are the dimensions. I found these in an earlier post from Brad which was posted by Graham, the PO of my boat. BTW I have been able to contact Graham, although as he never got to sail the boat before selling it, he was not able to provide much info.

LENGTH: 47 5/8 INCH 1210MM
BREADTH: 15 INCH 380 MM
THICKNESS 3/8 INCH 10MM
BOSS AT PIVOT : 1 5/8 INCH THICK
WEIGHT: 70 LBS
CASING DEPTH: 16 ¼ INCH

I understand the standard CB is only 24lbs so at 70lbs these Australian boats should be a bit stiffer. Returning to the issue of side deck stiffness, I can only say that the Australian boats must be made of stronger stuff. Without any coamings to stiffen the side deck at present, there is absolutely no flex. I weigh 175lbs and bouncing on the side deck there is no movement. From some recent holes drilled to fix cleats etc, I estimate the deck at over 12mm (1/2inch) thick. The foredeck is 16mm (2/3inch) thick, measured from a piece I removed to fit a hawse pipe. No wonder Brad suggested their weight would make them quite uncompetitive with US boats.
Cheers, Greg
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