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Old 04-01-2009, 10:49   #1
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Transom !

hi,im new here! can i ask how does a transom ,whether it's flat or curve, affect a boat or cat in any way tt is possible...

thank you for your comment and replies if any
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Old 04-01-2009, 11:15   #2
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Originally Posted by zang3lz_2 View Post
hi,im new here! can i ask how does a transom ,whether it's flat or curve, affect a boat or cat in any way tt is possible.
That’s a really good question. A major part of the transom is how the designer wants the boat to look and what sort of effect he’s shooting for. There is a really good book, Yacht Style, you might want to check out. There is a hydrodynamic aspect too, but I think you'll find it defies a simple answer because you start having to address things like wetted surface, canoe body and a host of coefficients.

The popular trend now in costal cruiser is a walk off transom or transom with a swim platform incorporated. Hunter has some good examples of this. I think this is an extension of what monohulls started with swim platforms but catamarans took a step further with the sugar scoop transoms.

In the racing circuit, as in Open 50s and Open 60s, the design is more of a very broad end which is really flat. In part this is so it can drain fast but also because there are often two rudders in parallel. Actually, the two rudders are function of the width, not the other way around.

Anyway, good question and welcome.
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Old 04-01-2009, 14:36   #3
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Aloha Zang,

www.cruisersforum.com/forums/f47/hey-folks-could-i-get-an-opinion-21831-2.html

You might try the site above for more discussion on what you've asked. One thing I know for certain is that everyone has a preference and sometimes it just doesn't make a lot of objective sense. Sometimes just for looks. I prefer a flat narrow stern because you can mount a windvane to it easier.

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Old 05-01-2009, 07:45   #4
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thanks maren n skiprjohn for ur comment and replies, greatly appreaciate e immediate response from u people!!!

hmm, i search through e net, n links ur gave bt couldn't find a ans tt is relevant to me.. mayb ill try harder. yeap i do know part of it is down to personal preference..but i do wonder does the shape of the transom affect the boat's stern waves it creates, and will it cost any drop in efficiency...any drag etc..

hmph, i reli do not know what will different part of transom do apart frm one's preference..
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Old 05-01-2009, 08:56   #5
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Two cents worth:

I am under the impression that the shape of the transom above the waterline has no effect at all, other than aesthetic.

On the other hand, changing the angle of the transom can alter the waterline length, which has an effect on hull speed.

For instance, the Bob Perry-designed Mirage 26 has a transom that is longer at the coamings than at the water line, giving the boat a waterline length of 21 feet eight inches.

The Mirage 27 (also by Mr. Perry) has essentially the same hull, but with the transom sloping the other way, giving a waterline length of 23 feet, five inches.

Sailing USA's hull speed calculator (Hull Speed) shows that the 26 has a theoretical hull speed of 6.25 knots, while the 27 can reach 6.5.

Connemara
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Old 05-01-2009, 09:06   #6
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thank u connemara i understood all tt u have said, as i read it before..hmm so in ur impression, the transom do have little effect besides longer waterline length...hw about curve vs flat transom in water...any effects.. thanks,,hope ya reply!!
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Old 05-01-2009, 10:36   #7
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Well, I think it is not right to look at the transom shape as independant of the hull shape. Most designers will design a hull to a point out beyond their projected LOA. This is the same way you do it when you loft a set of lines by hand. It insures that your hull is fair. You extend the the hull shape as far aft as practical. Then, when you have the hull shape you want, you chop it off to conform with your projected LOA. So the shape you get at the transom will reflect the shape you have developed in the aft sections. The traosnom shape is a pure function of ther aft sectional/hull shape.

You can chop the stern off in a variety of ways. You can just take a flat plane thru the sterns sections. Or, you can take a radial cut, pure radius or elipse, thru the stern sections. That will give you a prettier cambered transom than a plain flat cut.
Typicaly for a 40' boat the radius at the transom will be between 15' and 20' radius as determined by the designer's eye.

However, and here's where it get tricky, if I am trying to design a very modern hull form but I want the boat to have a traditional aesthetic I might want the transom to have the traditional, i.e. conventional rake as opposed to the modern "reveresed" transom. In that case I might end up with a very ugly transom. So I will back track a bit and begin to sculpt the stern sections, compromising performance, so that when I cut that transom in I get a pleasing shape. Nice, fast, broad and flattish stern sections combined with conventional transom rake will usually result in a very ugly transom. I like to introduce a hint of hollow in the sectional shape aft, extending to the transom, just to make the tansom more shapely. The hollow ( think wineglass tansom) does not do the performance any good but it sure makes for an eye sweet transom shape, more complex and more delicate.

We could write a chapter on this.
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Old 05-01-2009, 12:49   #8
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Aloha Zang,
Maybe you could ask precisely what it is that you are wanting to know? It does effect the boat, storage, appearance, but not so much performance unless it is underwater at hull speed. If it is truly underwater at hull speed then a flat transom is way less efficient than a tapered transom. The counter under the transom if flat horizontally will slap at anchor and will be quiet if it is rounded and tapered up from the rudder.
Any of this helping yet?
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Old 05-01-2009, 19:19   #9
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(Grumbles about the shape of ducks underwater)

Transom shape is also, to a small degree, affected by the material of which the hull is constructed. Fibreglass is somewhat floppy, and various techniques in construction are used to improve its stiffness including shapes - a gently curved shape is stiffer in construction than a flat plane. But lots of little jogs like clinker-style fibreglass is even stiffer yet, although it adds plenty of surface area and tends to break up laminar flow of water by the hull.

'Sugar scoop'-style transoms can allow easier boarding opportunities, swimming access, a place for an on-deck shower, even better cockpit drainage. They can be ugly as heck, utterly unusable in design or execution, always slice a chunk out of the hull volume that could otherwise be used for carting around junk you don't know how to get to (or not), and might invite waves and people you don't know aboard as well.

So... A transom is a compromise in boat design, giving and/or costing performance, strutural integrity, and aesthetic value.
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Old 06-01-2009, 04:02   #10
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Quote:
Originally Posted by bob perry View Post

Nice, fast, broad and flattish stern sections combined with conventional transom rake will usually result in a very ugly transom. I like to introduce a hint of hollow in the sectional shape aft, extending to the transom, just to make the tansom more shapely. The hollow ( think wineglass tansom) does not do the performance any good but it sure makes for an eye sweet transom shape, more complex and more delicate.
I am definitely not a boat designer but have a background in aeronautics. Hydro efficiencies and aero efficiencies are similar because the medium are similar.

"Nice, fast, broad and flattish stern sections..."

Zang - There are two aspects. What is in the water and what is out of the water. Bob is describing both.

Look at some of the most modern race boats at speed from the stern. The water behind the boat is flat and disturbed as little as possible. This is a sign of great efficiency. In fact these boats often to not have a transom, as noted, to allow water to drain immediately out of the cockpit.

In cruising boats we care about efficiency to some degree. We also like broader aft sections for internal space considerations.

My boat has a very flat transom in the water. At speed there is great turbulence behind the boat. However she does have a fairly flat bottom.

There are shapes that pinch the aft end of the boat in order to reduce turbulence. It is interesting to look at many boats in overhead plan and see how the shape is not very symetrical at all on most boats.

Bob describes "cutting" the boat at the design LOA. That is extremely informative.

Finally the shape above the water is purely aesthetic but vastly important. As an aero guy I could comment about above the water line aero but as boats don't close in on mach 1, I won't - LOL...
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Old 06-01-2009, 09:05   #11
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thanks to all above hu commented

well right now,at least im a bit more enlightened.

btw im researcing info on my already built boat,a cat,it is hard chine, flat bottom and rocker slightly after bow to slightly before the aft and it has a flat transom..
before we build it we had very little, yes and i mean very very little info on naval and boat related stuffs.however, before we present our proj on this fri, we would need to discuss how we build and design the hull form, but our transom is already build and we woulld like to know what effects it has on the shapes of it.. we are just merely copying the typical flat bottom boat's transom without understanding it..what i want to know more is towards the performance side...not the storage nor appearance..

hi ex calif,i see ur a singporean, im working on my Sing poly final year proj btw

amgine, well im a fren of depthofit, we're working on the same project, jus different area. thanks for ur help all this wile dou


regards
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Old 06-01-2009, 09:27   #12
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Actually I am Ex-Californian but living in Singapore. Haven't taken the plunge for citizenship quite yet.

Well I certainly hope you are getting plenty of sailing in. This Saturday the Vovlo boats are having their in-shore prep races. I understand it will be along East Coast. We are contempleting floating out and seeing what we can see.

Good luck on your project.
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Old 06-01-2009, 09:55   #13
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Quote:
Originally Posted by bob perry View Post

We could write a chapter on this.
Thanks Bob for that informative primer.

Just curious…..for an offshore cruising boat…. How much influence does “reserve buoyancy” play into you final transom design?

Do you use a simulation program of dynamic stability with following wave periods to fine tune the eventual transom shape?
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Old 08-01-2009, 09:04   #14
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I don't worry about "reserve bouyancy". In fact, that term has always bothered me. I feel strongly that what you do in the last 10% of the boat's LOA will not make or break the boat's seakeeping abilities. Stern shapes to me are all about extending sailing length and in doing so I get to move volume aft so you can call that "reserve bouyancy" if you like. I don't. On the practical side, sterns can be also be about what you are trying to accomplish with the cockpit or perhaps and aft cabin in a center cockpit boat. I know you often hear even some well known designers justifying canoe sterns because they add reserve bouyancy but I find just the opposite, Making the fanny pointy removes volume. That's one reason the Valiant stern works. It pulls as much volume aft as possible while still ending in a point. In smaller boats stern shapes may also be trying to add stability. Look at modern sport boats with chines aft.

If there is any part of the stern design where I will compromise the shape I need for performance it is the sectional shape aft. I will generally avoid a flat section at the stern to prevent "marina slap". I will ad some deadrise to the stern shape so that I get about 10 degrees of deadrise at the transom.

For your second question the answer is no. I do no calculations and I know of no calculations that would be of any use in determining the seakeeping qualities of yachts. I have however, tested performance in a seaway when doing tank testing.

This is a fun topic.
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Old 16-01-2009, 16:42   #15
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Does transom shape have any effect in a following sea? Does one shape over another make it easier to be pooped?
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