SailNet Community - Reply to Topic

   Search Sailnet:

 forums  store  


Quick Menu
Forums           
Articles          
Galleries        
Boat Reviews  
Classifieds     
Search SailNet 
Boat Search (new)

Shop the
SailNet Store
Anchor Locker
Boatbuilding & Repair
Charts
Clothing
Electrical
Electronics
Engine
Hatches and Portlights
Interior And Galley
Maintenance
Marine Electronics
Navigation
Other Items
Plumbing and Pumps
Rigging
Safety
Sailing Hardware
Trailer & Watersports
Clearance Items

Advertise Here






Go Back   SailNet Community > On Board > Boat Review and Purchase Forum > Sailboat Design and Construction > Hull Characteristics that affect Seakindliness/Seaworthiness
 Not a Member? 


Thread: Hull Characteristics that affect Seakindliness/Seaworthiness Reply to Thread
Title:
  

By choosing to post the reply below you agree to the rules you agreed to when joining Sailnet.
Click Here to view those rules.

Message:
Trackback:
Send Trackbacks to (Separate multiple URLs with spaces) :
Post Icons
You may choose an icon for your message from the following list:
 

Register Now

In order to be able to post messages on the SailNet Community forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.
Please note: After entering 3 characters a list of Usernames already in use will appear and the list will disappear once a valid Username is entered.
User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.
Password:
Confirm Password:
Email Address
Please enter a valid email address for yourself.
Email Address:

Log-in

Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.



Click here to view the posting rules you are bound to when clicking the
'Submit Reply' button below


Additional Options
Miscellaneous Options

Click here to view the posting rules you are bound to when clicking the
'Submit Reply' button below


Topic Review (Newest First)
07-24-2009 11:43 PM
Sailormann
Quote:
So, my question again is what characteristics do matter the most in their effect on seakindliness/seaworthiness of an offshore boat?
That's a question that has been subjected to much discussion, consideration postulation, debate and hypothesis. The end result of this was a weighted table of measurements that attempted to answer the question you posed. You used that table yourself and...

Based on that consensus, the Wauqiez is indeed a slightly less seaworthy seaboat than some others.

However, if you love the Wauqiez and if you believe it is the right boat for you to sail, then buy it.

As you read this there are thousands and thousands of people cruising the oceans. A tiny number of them are sailing Valiants, Cape Dories or Pacific Seacraft. Very few boats/crews are going to come to bad ends and I don't think that they are enjoying their cruising any less because they don't have the "right" boat.

Good Luck !
07-23-2009 03:38 PM
snider
Thanks

[QUOTE=Jeff_H;507900

By the way, I really like the Beneteau 235 which is one of the nicest boats of that size and general type that I can think of.

Jeff[/QUOTE]

Thanks, when I was researching the boat I came accross a thread where you suggested the boat and took notice.
07-23-2009 03:22 PM
Jeff_H
Quote:
Originally Posted by snider View Post
A generous beam would give you more form stability, which would equate to less angle of roll right? (Or did I miss the point?)
You have it right except that you left out the two flip sides, generous beam means more form stability, but at the price of a smaller angle of vanishing stability, and gneropus beam means a smaller roll angle but at a faster roll rate.

By the way, I really like the Beneteau 235 which is one of the nicest boats of that size and general type that I can think of.

Jeff
07-23-2009 02:25 PM
Valiente Even though I largely agree with what has been said, I also own an IOR-influenced old racer and a steel full keeler that looks like a hard-chined Land Rover.

So, as Joni Mitchell noted, "I've seen the world from both sides now".

Perhaps counter-intuitively, I would choose either the Wauquiez 33 (I prefer, with the late Hal Roth, the 35 a little more for cruising) or the PSC 34 (I prefer the much more expensive 37, largely for space reasons). Even though they have the largest "numbers" spread vis-a-vis each other, both models have solid cruiser reputations. I would rather sail in the Wauquiez for the 98% of the time it isn't heavy weather, but I would prefer to heave to in the PSC, because it's probably easier on the nerves. I also think that the PSC would track and self-tend to a windvane and a proper sail set better and you could carry more cruiser crap in it.

The Wauquiez, on the other hand, is of the last generation of primarily tiller-steered boats (which I prefer up to 35 feet) and has a really nice cockpit. So considerations of crew size and functionality come into play.

Your voyage, your call. I think if they pass survey, both are viable small liveaboards...but they are, by modern standards, pretty compact boats.
07-23-2009 10:57 AM
snider
Slowly coming around

Thanks Jeff, I've always enjoyed reading your posts. Until recently though I've also always had the exact opposite opinion. Lately though as I'm getting more experience I think your posts are whittling me away and I find myself agreeing with you (wow did I really say that?) I pride myself on always challenging my thoughts and beliefs and I think you've converted me.

You sound like you're describing the mini 6.5 until you mentioned narrow beam, A generous beam would give you more form stability, which would equate to less angle of roll right? (Or did I miss the point?)
07-23-2009 10:29 AM
Jeff_H This is a topic that keeps coming back like a bad penny. It is not that difficult if you take your time with it. On the other hand this has been a topic that was ignored for by designers until fairly recently. It has come in to the forefront in the 25 years since the Fastnet disaster. It has also become an important frontier in racing yacht design (which will ultimately provide a lot of useful information for the design or cruising yachts) because of the realization that large angles of rotation and quick motions are slow. With the advent of smaller more powerful computers and miniaturized accelerometers full sized boats have been instrumented and motions measured. As a result theories on the factors involved in the dynamics of motion are being more clearly understood.

At the same time there has been a clearer understanding of what makes for seakindliness. If you look at motion that wear people down and cause seasickness it is both the speed and the angle of rotation. Marchaj’s slightly dated tome ‘Seakindliness the Forgotten Factor’ includes a chart that shows a navy study on the causes of seasickness and both rate and angle are equal factors. A more recent study indicated that individuals have different thresholds for discomfort and that the triggers of discomfort are purely individual as well. In other words, some people are bothered by the speed of the motion more than amount of the motion and other people are affected more by the amount of motion than speed of motion. And still others are equally affected by both. On that basis a boat that rolls through wide angles slowly is no more universally seakindly than a boat that snap rolls through small angles. This means that an ideal boat from a motion comfort standpoint would tend to roll, yaw, or pitch slowly and through a small angle.

To agree on terms, I will take a moment to define the basic terminology (at least as I will be using these terms). There are six types of motion:
-Three directional: surge (for and aft), leeway (sidewards), and heave (vertically), and
-Three rotational: Pitch (fore and aft), yaw (rotation as seen in plan view) and roll (which of course is side to side)

The amount and speed of motion a boat experiences is related to the amount of energy that is imparted into the boat, the ability of the boat to store that energy, and the ability to dampen that stored energy.

If we look at the issue of absorbing energy, most of the energy imparted into a boat is through simple physics, mostly due to inclined planes and impact. While there is comparatively little to do to minimize the absorption of energy due to inclined planes, much can be done to reduce the imparted energy due to impact forces through hull and foil shaping. Vee’d bow sections and finer bow sections reduce the impact loads of frontal seas for example. Another example that comes out of tank and instrumentation of actual boats which shows that a heavier boat passes through the waves while a lighter boat tends to move over waves. This suggests that heavier boats therefore experience greater impacts. This is somewhat mitigated by the greater momentum of the boat but depending on hull shape those impart greater energy into the boat. On the other hand going over waves a lighter boat is more likely to absorb more energy due to heave (vertical motion).

There are three ways that boats develop stability; form (also called initial stability), ballast, and dynamic. Of the three, dynamic has the least relevance to displacement boats as they rarely have enough speed to create a useful amount of dynamic stability.

As the name implies form stability derives from the shape of the hull of the boat. A shallower and wider hull generates more stability than a deeper narrower hull. Visualize a piece of wood lying in the water. On the flat it has a lot of form stability. On edge it has next to none. This is because stability in any boat comes from the distance between the center of gravity (the balance point for all of the weight in the boat including those things that are part of the boat and those things that can be moved around) and the center of bouyancy (the single point that is the center of all of the volumes under water). In a wider, shallower boat the center of bouyancy moves more quickly toward the low side for small increases in heel angle. A boat with a low vertical center of gravity tends to positively increase the lever arm between the center of gravity and the center of buoyancy at smaller angles of heel and therefore develop stability more quickly.

Form stability has several problems. First of all at large angles of heel, usually approaching 90 degrees form stability drops off dramatically. This is the point where it can be needed most. Second form stability tends to give the boat a quicker motion because more energy can be imparted into the hull either from impact or inclined planes. Lastly, in the extreme conditions of a blue water passage, a boat that depends on large amounts of form stability also tends to be more stable in an inverted position.

Ballast stability has mostly positives associated with it. All other things being equal, the deeper and heavier the ballast the more stability a boat will have.

Ballast that is heavier and deeper also gives the boat a slower, more comfortable motion. Of course like most things in yacht design, there are some tradeoffs in this area as well. If the ballast occurs at the end of a deep keel, the boat cannot get into as shallow water. As you start to shorten the keel length you substantially give up performance for the same stability because you either end up with a low aspect ratio foil or you end up with a big bulb or wings also increasing drag. No mater what the ads say, nothing performs like a deep fin keel. Of course deep fins have their own compromises but that is not the point of this.

Another factor that comes into play is the issue of the loads imparted into the boat. The larger the loads relative the more motion there will be. But because the loads involved can be so large the amount of load felt by the boat tends to be proportionate to its weight and exposure. In other words all other things being equal a heavier boat will have higher impact loads, but also proportionate resistance to those loads.


One thing that has been determined that the single most important factor controlling comfort in seakindliness is length. All other things being equal a longer boat will have a more comfortable motion. While sources do not universally agree whether it is waterline length or overall length that is critical, the general agreement seems to be the longer the boat the more comfortable the boat will be. That said a short waterline boat compared to overall length boat will have a greater range of pitching motion and may have a quicker motion as well, because the reserve buoyancy of the ends can suddenly come into play jerking the pitch to a stop and back the other way.


Next comes form stability as a culprit. Form stability allows a light boat to have a lot of initial stability but at the cost of poor ultimate stability. Form Stability’s has two affects on seakindliness, it allows more energy to be imparted into the boat and form stability results in a lot quicker motion as the boat quickly builds stability to resist the rolling motion. Form stability comes from wide shallow hull forms be they light like the IOR boats of the Fastnet era or moderate like the Moorings 38 mentioned earlier or heavy like the Island Packets. Of course without some form stability the boat would sail at large angles of heel (look at the English lead mine cutters for example) and that is not very seakindly either.

When we look at the relationship of a boat’s weight and seakindliness we are really talking about the effects of moment of inertia. The inertia of a boat has several affects. A boat with a large moment of inertia takes more energy to get the boat in motion. Once in motion, it is harder to stop from moving so this stored energy of a boat with a lot of inertia causes a boat to roll further than a boat that has less inertia. Of course a low inertia boat will accelerate and de-accelerate quicker.

The amount of inertia that a boat has is a result of the weight of the boat and the distribution of the weights. A small weight located a long distance from the axis about which the weight is rotating can have an equal inertia to a much heavier object closer to the axis of rotation. Since weight is linear but distance from the roll axis is to third (or fourth power), distance from the roll center is far more critical to the overall moment of inertia. If we consider a heavy boat, that has a short heavy rig, a lot of weight in the hull and a lot of low density ballast in a long shallow keel, its roll moment of inertia may actually be same or less than a much lighter weight boat with a deep bulb keel and a tall light rig.

Another factor that had little study in any detail is dampening. Dampening is the ability of a boat to absorb the energy once it has been imparted to it. The two best examples are the affects of a deep keel and a tall rig. If you visualize these rotating in a circular motion the air pushing against the sails and keel far from the hull creates a resistance to rotation and supresses the effects of inertia. In other words, dampening results in less motion and slower motion; a definite win/win situation.

So where does this leave us, in terms of seakindliness, a light weight boat, with a fine bow, and a narrow beam, Vee shaped hull sections forward merging into elliptical hull sections aft, with a deep bulb keel and a tall light rig can have very admirable seakindliness characteristics and would be a much faster boat in all conditions than a heavier displacement full keel boat.

Obviously there are trade-offs being made here but seakindliness or weatherliness isn’t one of them. Properly designed, the ability to carry the weight of cruising gear is not lost either. The price comes with deeper draft and these boats do require greater care in their engineering and greater skill to build.

Respectfully
Jeff
07-23-2009 07:06 AM
gog Great posts, thanks! We are new to the game and are in the process of sorting out what we will buy. The advice here is really good. The pacific seacraft has been a fantasy of mine for over twenty years, but I am really pretty ignorant when it comes to sailing and sail boats, so getting out there and checking them out in person makes a lot of sense.
07-15-2009 05:06 PM
SecondWindNC Good post, OsmondL.

To put it simply, you just have to get on the boats yourself and see what you like and what you don't. A boat that's set up the way you want it is going to have just as much impact on comfort as small differences in hull characteristics.
07-15-2009 04:42 PM
Ericb
An answer of sorts

Very well said OsmundL - I wish more people would realize that a boat, a design, a specific rig will never be perfect - just do the best you can and actually do it!

"So, my question again is what characteristics do matter the most in their effect on seakindliness/seaworthiness of an offshore boat?"

stipakb I will try and answer your specific question repeated above....

The best description I ever heard that I would apply to a sailboat I was going to buy is: "She was moderate in every aspect." If you keep that in mind along with what OsmundL said you should be fine.

Good Luck!
05-12-2009 10:16 PM
OsmundL
Quote:
Originally Posted by stipakb View Post
Thanks for any comments!
Well, maybe not
Brian, I waited a long time to see one of the usual culprits give you the low-down here. I am, frankly, tired of hearing the same pros and cons repeated over and over again, so I stayed out.

But nobody has replied yet. Did the Swine Flu floored everybody?

So, I feel like answering in a way you least of all wanted, and to hell with it: If you are a designer, the numbers and profiles would interest you in minute detail, and I apologize if that is your interest. But if, as you say, you want a boat to sail to Mexico, how about going with the boat that appeals most, that appears comfortable within, that is in a reasonable condition, so forth?

I say this because I’ve been through the same mill: a few years ago, I made up huge spreadsheets with vital stats of probably 40 boats, calculated my own ratios and read every footnote on the infamous “capsize-ratio” and its likes. It was an obsession – my boat should have the closest thing to a perfect hull. Except, it had to be cheap also. I was looking in the Albin Ballad range (a couple of foot less than your choices, give or take), and in the end I came up with an odd choice: the BostrÝm 31 from 1970s, a good hull for rough seas, of unmistakable IOR heritage. It was by all accounts underrated and good value. I scoured B31 clubs in many countries, I knew it all.

This was before I got closer to a few in real life. That cockpit was much more cramped than I envisaged; it had wheel steering but the mechanisms weren’t convincing; most B31s were owner-built and had varying standards; the winches looked just dinky; all had the old boat smells.

I still like the B31, but the fact is all the number crunching had in the end only created a monster, a larger-than-life B31 that met or almost met all criteria – especially when I conveniently overlooked the less favorable data.

I am fairly sure a few in these forums can tell you the Pacific Seacraft 34 is a lovely boat. The others will also have someone rooting for them. Sure as the sun sets, someone will diss the Wauquiez 33 and call it a dog. Listen to them all, but in the end this is fact as far as I am concerned:
Any of these boats would take you to Mexico, and you will like one more than the rest regardless of advice and numbers. “Motion-comfort” rating my ass! There is so much more to comfort – and some will be up to you, for instance how well you stow gear onboard and how much weight you add. If you really like what you see and enjoy being there, it compensates for a lot. And to be brutally honest: in really bad weather, I defy any manufacturer to call his boat “comfortable.”

You have already done a great job in studying the hull profiles and numbers, you already have some feel for what you want – and perhaps that was the important part of the exercise? It means you can approach your choice with more confidence, it means you have perspectives on how numbers fall and what matters; it is all part of the judgment.

I didn’t answer your question, sorry
This thread has more than 10 replies. Click here to review the whole thread.

 
Posting Rules
You may post new threads
You may post replies
You may post attachments
You may edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On


All times are GMT -4. The time now is 08:42 PM.

Add to My Yahoo!         
Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.
SEO by vBSEO 3.6.1
(c) Marine.com LLC 2000-2012

The SailNet.com store is owned and operated by a company independent of the SailNet.com forum. You are now leaving the SailNet forum. Click OK to continue or Cancel to return to the SailNet forum.