best stern type

Urisvan, my reply to your email was returned as not deliverable. Possibly you've mis-typed the address, can you resubmit?

This is and has been a debate that has gone on for many generations.

Tests have been done and no conclusive proof has been found to warrant this stern type over others (I'm well aware of the pot I just stirred up).

Double enders have the same design criteria compromises made as all yachts. There are many different types of double enders, as many as there are, keels, rudders, bow profiles, etc. Each of these will have good and bad points about them. Some are clearly more efficient, others less so, but more comfortable to live with. An example of this could be the choice of rudder, an efficient spade or a less efficient, but better protected skeg hung rudder.

In the design process a preliminary set of guide lines is developed, a wish list if you will. The stern type is often covered by other items on the list, like the ability to carry an aft cabin or surf down wind or carry a dinghy on the aft deck.

The double ender is a difficult design feature to get correct. If the deck line is kept full (to provide cockpit space) and the overhangs short (to maximize LWL or limit slip fees), the buttocks will need to "tuck" up, usually rather sharply at the most important point along the run, where it emerges from the water. This has the undesirable affect of making the boat difficult to steer and limits it's windward ability considerably.

In fact, drawing up a double ender, that can perform well requires no small amount of understanding and experience. I have a reasonable amount of this and do love the look and ride of a pointy on both ends kind of boat. I've been on bad and good pointy butted boats, a few in very rough going and would prefer the well thought out double ender, but it's purely subjective on my part. I can offer several reasons, in an attempt to justify this rational, but this would only serve to qualify speculation.

In the end, seaworthiness is much more then stern type and I'd look at many other things before taking the magic pill called a double ender. Stability curves, D/L ratios, capsize screen, point of vanishing stability, (Ted Brewer's) motion comfort ratio, etc. Double enders are wonderful craft and can be designed to serve your needs well, but the boat butt will be a small portion of the equations necessary, while developing up a set of lines you can live with.

Pitching can be caused by one or more of many things. Speak what you know folks . . . No insult intended, just shooting from the hip.

 

These are what i know about pitching,
Violent pitching accours when natural period(Tn) of the boat about the tranverse axis coincides with the wave encountering(this is called resonance). If Tn is low, the boat will meet the resonance situation at smaller but steeper waves, which is undesireable. So a high mast, deep keel and heavy ends are against pitching. The shape of the bow and stern are important too. Less reserve buoyancy means high Tn so it is desirearable, right? But a stern with overhangs will damp the motion so it is good although it adds some reserve buoyancy. It is a property that double-enders don’t have. So they can’t damp the pitching motion effecetively when the motion starts.
For me the seawortiness means, first, to survive in the worst conditions. Regarding this, do you suggest double-enders? Or, aren’t they very different than the others? Or which one do you suggest?

 

Thank you Urisvan,
As I said, "Pitching is a problem only for boats with very high masts and very deep keels." By this I was not referring to what causes the pitching motion ( very high masts and very deep keels are against pitching, of course ), but what suffers from the pitching motion the most, ( except sea-sickened sailors, obviously ) It is the hydrodynamic and aerodynamic performance of the modern light, fin keel sailboats that is suffering, due to the excessive erratic motion of their wing tips.
For me, too, "the seaworthiness means, first, to survive in the worst conditions." Speaking from experience, (as I happened to sail in traditional, heavy displacement, wooden double-enders for about half a century in a sea with very steep waves), I can say that double enders with long keel and low center of effort rigs are perfect, provided one accepts, as a compromise, their limited deck area and reduced pointing ability.

 

I've sailed a few

A double ender as Par says needs more careful design consideration. The transom allows a better run of lines aft, gives a greater reserve buoyancy and best interior volume to deck length is decreased. For trad timber construction the double ender is a much stronger option over a transomed hull. A gentler (long)canoe stern is usually a better option and can provide a strong attachment point for the backstay without resorting to a Boomkin should your rig desire it, however this is dependant on the overall design brief.

Colin Archers are prone to pitching in certain wave trains due to the similar fore-n-aft sections which with a heavy boat can have a fair amount of energy associated with the motion, They definitely operate at slower speeds since the quarter wave they produce is detrimental. Slow and steady unless you get into the pitching oscillation frequencies.

Double enders are renown for being pooped more easily in a large following sea and their aft cockpits can be wet and their drains can be a problem too often pumping as much water in as drains out.....all design considerations.

IMHO

 

"They definitely operate at slower speeds since the quarter wave they produce is detrimental." Mike Johns.
As I understand it, it is the midship section of the hull that determines the quarter wave at most, not the form of the stern. Double enders are usually long keel, heavy displacement boats, so the position, area and form of the midship section of the hull tend to produce the above mentioned detrimental quarter wave. The quarter wave of a light, long and narrow boat is always about the same, be it a double ender or not, isn t it ?

 

Xarax
I was referring to the Colin Archer.

But in general as Par explained it depends what those lines do aft of that mid-ships section, you could for example sweep them back into a wide flat transom or even into a bustle, the double ender particularly the Northern type has little option but to tuck those buttocks up and the quarter wave is quite different for each configuration.

cheers

 

thanks Mike, i noticed a lot of things by your experience. first you said that a double-ender could be pooped more easily by the following seas. i guessed it. by also hearing it from you is a good thing. and all other comments about canoe types, northern types are all valueable.
but i still have questions:
"Colin Archers are prone to pitching in certain wave trains due to the similar fore-n-aft sections which with a heavy boat can have a fair amount of energy associated with the motion" Mike Johns.
as i said before, i think a double ender is more prone to pitching, because of lacking damping abilities of their sterns. but i think being heavy is not a reason of pitching. isn't it?
and i heard that, in the big following seas, the wave can hit the transom stern and may cause to lose the control of the boat. is it true?
and double-enders have balanced ends,so when the boat heels its bow doesn' sink, so i think they don't yawl. it is a desireable quality because yawling can exite rolling also. isn't it?
by suming cons and pros, i believed that balanced ends, and a moderate counter stern is the best. what do you think?
cheers

 

"it depends what those lines do aft of that mid-ships section" Mike Johns
Could you please elaborate a little more on this ? I can almost see these lines "pushing" the water flow in a way that produces a deeper hole in the waves, but still I can not understand how this affects the length of these waves. The water flow around the aft sections of a hull is not easy an easy thing to follow, I suppose... I suspect that the flat surfaces of the transom hull help the stern wave to push the boat a little more up and a little more forward, so the stern wave crest stays lower and further from the stern, so the wave is now longer than steeper, so the wave trap leave some room to the hull to move faster...

 

No but the mass-energy in the system makes it a detrimental characteristic which is why we want good damping in a heavier boat. These older designs can be improved upon in both performance and seakeeping characteristics.

No generally a breaking crest on the stern quarter of any vessel has just as much tendency to skew the vessel. The overall response will depend on all hull design factors and seldom on just one characteristic.

It's 'Yawing'
Heeling balance has more to do with beam and the fineness of the bow, it is easy to produce a balanced transomed vessel .

Moderation will usually produce a moderate performer with moderate vices, ie a good crusing boat. Trimming down by the bow is not an issue with most moderate boats regardless of hullform.

All transomed boats with the transom terminated above the waterline are 'double enders" just the deadrise angle aft can be lower with a transom termination producing a fuller waterplane aft.
You must also consider the heeled waterlines. It may be helpful to consider the attached flow and where it is going to go after separation ie after it leaves the hull surface.

I’m sorry I don’t have time for a detailed tutorial but it may pay to get hold of a few books on the subject or start a new thread on general hydrodynamics. I hope this helps anyway.

Cheers

 

Quantifying stern geometry

I know that this thread is a few weeks old, but as I was following it, I got to thinking about the "U-ness" or V-ness" of stern (and bow) geometries. I've always had problems with subjective terms like these, so it prompted me to write an article about the vertical prismatic coefficient (Cvp) in our latest "Marine Performance Technology Exchange" newsletter that is pertinent to the subject.

You can get the newsletter online [PDF, 415 KB]:

www.hydrocompinc.com/knowledge/techexchange/tex0701.pdf

I'll be curious to hear if it proves useful for anyone.

Regards,

Donald MacPherson
HydroComp, Inc.

 

I printed it and still have to read it. Looks to be over my head, but enjoy learning.

 

Thanks for the article. It was very informative.

 

CVP is not the whole story...

Glad you found it informative. As I dig deeper into this, I have concluded that CVP is not the whole story for some things. For example, a high B/T barge has a value of CVP that suggests "U-ness", and this CVP relationship with pitching makes sense as long as the hull remains submerged during motions - effectively ignoring slamming. Also, if you consider the implications for using this as a measure of water flow, a barge's CVP would suggest WL flow rather than the buttock flow that it truly has.

I'm working on (what I believe to be) a new coefficient that takes the average B/T into account. It equals

AWR / ((LR * VOLR)^0.5)

where,

AWR = waterplane area of the run
LR = length of the run
VOLR = volume of the run

and the run is defined as the part of the hull from the aft-most position of maximum section area to the stern. I've only evaluated this for a limited number of hull form geometries, but I'm optimistic that it can be a useful parameter for a variety of hydrodynamic evaluations, including drag and hull-propulsor interaction (e.g., wake fraction, thrust deduction).

My biggest hurdle right now is what to call this coefficient.

Regards,

Don MacPherson
HydroComp, Inc.