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Dear all

Could I ask for your help in understanding how the deck is drawn in small sailboats. I am not exactly sure what or how to ask as my knowledge and understanding of this is very shallow but I ll try.

I think I have understood that the camber of the beams is a parabola and that there are many ways that this can be drawn. My main problem though is the deck centreline.

The way I understand it, it is essentially a straight line form the stem to the transom, having chosen a crown height at max beam. Due to the deck furniture and the very visible sheer, this straight deck centreline is hidden from view. However, it seems that most decks are truly curved.

 

1. Is there a rule of how curved the deck centreline can be?

2. If the centreline is curved, what happens to the king plank that needs to be straight?

3. Suppose I draw a slightly curved centreline that pleases the eye and then arrange the beams using the same way to figure out the camber. Is it likely that the deck planks will sit nicely or will a lot of fairing be needed?

4. Is the crown height measured perpendicular to the horizontal level or to the sheer? In the first case the beams will meet the deck planks at an angle and will need fairing but in the second will sit flat. Or is this not an issue to the relatively small rise of the sheer?

5. Is it better to go for a straight centreline or is it better to have a bit of curve?

 

Apologies if I don't make much sense!

 

Vaddoc

donrobinson likes this

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Posted (edited)

Vaddoc -

 

I confess I'm having a bit of trouble understanding your question. It sounds as if you are attempting to design your own boat rather than working from a set of existing plans. If that's the case, I am not qualified to advise you. However, if you're working from plans, then the information you need should be in those plans. I am guessing that by a curved centerline, you mean that it follows the line of the sheer as opposed to curving from side to side. Of course, the centerline will be straight from bow to stern when looked at from above (in the half-breadth plan), but may be curved up and down (often higher at bow and stern than at midships) when viewed from the side (sheer plan).

 

In real construction, deck beams are typically notched to fit over the sheer clamp or deck clamp (depending on which the boat has). Since the clamp follows the line of the sheer, that means the tops of the deck beams will be parallel to the line of the sheer. Therefore, fairing of the deck beams would not be necessary. Depending on the level of details in the plan, the amount of crown may be specified, such as 6 inches in 10 feet. That has not been the case for the models of small boats I've built, but there is often a cross-sectional view provided showing a deck beam so you can see and measure what the crown is.

 

There is an excellent book on small boat building by Howard I. Chapelle called Boatbuilding. I believe it is out of print, but it's usually easy to obtain a used copy in good condition. It is specifically geared to small sailboats although, of course, it's meant for full-size boats, not models. But it's a very good reference. That book will likely provide you with more and better answers than I can, but I hope this helps to some degree. Note too, that everything I've said above are broad generalizations, not hard and fast rules. Any given designer or boat builder may follow his own set of practices.

 

Cheers -

John

Edited by jhearl
mtaylor likes this

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Thanks John

I have a full set of plans which were (I think) a bit inaccurate so I had to do quite a lot of lofting. However, in the plans there is very little info regarding the deck. It seems that all naval architects assume that this is a very easy part and do not elaborate!

 

I have tried to find some info on the subject but there is very little on the net. I think I ll need to get Chapelle's book. Still, The deck centreline does not seem to always follow the sheer, some times it can be actually almost a horizontal line. Others go for a variable crown towards one or both ends. But even if we suppose it follows the sheer, the crown will need to get more shallow as it reaches the bow and stern otherwise the deck will rise unnatural high. In my boat, the crown should be around 19 mm but the top of the beam selves is 3 mm down from the sheer

 

Then there is the king plan that is pretty long and needs to be straight, but that section of the deck curves upwards.

 

I think I need to study...

mtaylor likes this

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Like a highway, the crown is specified for the nominal width. Narrow the road and not adjust the crown will result in a very high crown. You wish to move water from the deck, pick the widest part and determine how much crown you need. With the chord distance from outboard to outboard and the perpendicular distance up at C/L to the crown, you can compute the radius of the circular curve needed, now use that radius for all your deck beams, just use shorter segments of that curve as you move forward and aft from the widest point in the deck. The rate of change will remain constant the length of the hull and there will be no twisting of the deck boards as you plank that deck. That is how I would do it, but keep in mind, I have never built a ship, but I have designed and staked for construction lots of roads during my time as a Land Surveyor.

 

jud   :pirate41:5a513c5babf3d_DIRECTFROMCEARCLICK075.thumb.jpg.33d69821d48d57e0d97fae58d7ad924c.jpg

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Jud

 

This makes a lot of sense, I understand what you say. This will work brilliantly if the deck centreline is a straight line. Will it be the same if the centreline has a slight curve to follow the sheer (not fully, to a degree)?

 

Vaddoc

 

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Yes, unless you use a step to break the C/L, then use the same radius on both sides as usual.

jud

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I think I get it know

The way to do it is to decide on the crown height at max beam and decide on a method to draw the camber. Then you apply the same camber along the boat and different cron heights are produced. As long as your seer is fair, the deck centreline that will be produced will be also a fair curve and the deck planks should sit nicely.

mtaylor and jud like this

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I am posting the conclusions I ve reached, as others may find it helpfull

 

The first thing to decide when arranging the deck is the crown height or camber. It is usually between 1/2 and 1 inch per foot of beam. Then, find the maximum beam and draw an arc or parabola. There are a few ways of doing this but I used a 3D program.

Following this, use the same arc for all points (beams) across the length of the hull. Keep the arc vertical to the waterline and not the sheer (bevelling/fairing will be needed for the beams to sit on the beam selves and the planks to sit on the beams.)

Provided the sheer is fair, the deck  centreline that will be produced will be fair as well.

Nothing is easy in boatbuilding!

 

Vaddoc

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