Underwater lines for a performance cruiser

[cat man do]

Didnt need to get to the galley for a feed, whoever was sleeping in the back cabin would have been fine.

Dave

[Nordic cat]

Originally Posted by cat man do

Instead of adding more rocker, more draft, more dragm why wouldnt you just go same accom. on longer hulls ?

More seakindly, better ride and faster.

Dave

That's exactly what I am doing, started off with a 43 footer that is now 49, but added a for cockpit....

Alan

[catty]

Now these were interesting sterns. From memory all the range irrespective of length came out of the same hull mould with very little aft rocker. Basically if you ordered the 11m version the builder pulled out the tape measure and chainsaw and lopped the hulls of at a bit less than 11m and then added a severe bustle like section to bring the sterns clear or the water. Owners certainly didn't need to deploy a drogue to slow them down.

[cat man do]

Originally Posted by Nordic cat

That's exactly what I am doing, started off with a 43 footer that is now 49, but added a for cockpit....

Alan

I went the other way, 50 foot performance with 40 foot accom.

8500kg to dwl but should be about 4700 - 5000kg dry

Dave

[BigCat]

"Instead of adding more rocker, more draft, more dragm (sic) why wouldnt you just go same accom. on longer hulls ?" I am a big advocate of that, as you can see from my website, but this was generated as a response from someone who wants an Oram 44. I pointed out that the ones pictured on Oram's site had all been overloaded, and commented that slightly increasing the rocker was an easy fix to keep a boat on its lines. As a designer, I can assure you that lengthening the transom is not such an easy fix, and it plays hell with a lot of numbers designers try to control. Lengthening the whole boat is not as easy, either.

[cat man do]

Originally Posted by BigCat

"Instead of adding more rocker, more draft, more dragm (sic) why wouldnt you just go same accom. on longer hulls ?" I am a big advocate of that, as you can see from my website, but this was generated as a response from someone who wants an Oram 44. I pointed out that the ones pictured on Oram's site had all been overloaded, and commented that slightly increasing the rocker was an easy fix to keep a boat on its lines.

And yet the rocker in your drawings is quite flat in comparison to other designs

As a designer, I can assure you that lengthening the transom is not such an easy fix, and it plays hell with a lot of numbers designers try to control. Lengthening the whole boat is not as easy, either.

As a designer ?????

Hold on, are'nt you a real estate agent ?

Dunn and Dunn, Realtors



As a builder (of boats) I can assure you that lengthening the transom IS a relatively easy fix and the ones I have done and seen done have inproved performance and helped prevent squatting.

Dave

[BigCat]

Originally Posted by cat man do

And yet the rocker in your drawings is quite flat in comparison to other designs As a designer ????? Hold on, are'nt you a real estate agent ? Dunn and Dunn, Realtors As a builder (of boats) I can assure you that lengthening the transom IS a relatively easy fix and the ones I have done and seen done have inproved performance and helped prevent squatting. Dave

Like Joshua Slocum, I do whatever it takes to keep body and soul together. I also have a bachelor's and master's degree in psychology, and worked for many years as a counselor and psychotherapist. ***** ***** ***** ***** ***** ***** ***** ***** ***** ***** I spent years sailing Batwing across the Pacific, etc. etc. I started studying yacht design in 1971, after I had great difficulty controlling my Ericson 35 fin keel sloop (monohull) in north Pacific storms while surfing down 25' waves sailing from Hawaii to Seattle in September and October. I decided that my next boat was going to be easily handled and sea worthy--and it was. I "finished out" Batwing, buying a hull, main bulkheads, and deck, and sailed her from Seattle to L.A., to the Marquesas, the Society Islands, both Samoas, Tonga, New Zealand, Fiji, Tuvalu, Kiribati, the Carolines, the Marianas, New Guinea, Palau, the Philippines, and Hong Kong. A subsequent owner sailed her around the world, and back to the Seattle area. The third owners are now cruising Central America in her. You can see a drawing of Batwing on my website. She has a junk rig designed by Hasler, unstayed masts, a pilot house, and a traditional profile. ***** ***** ***** ***** ***** ***** ***** ***** ***** ***** Rocker is a function of displacement, all else being equal. The catamaran design I posted on my site is quite light for its length, and so it doesn't have much rocker. ***** ***** ***** ***** ***** ***** ***** ***** ***** *****If I were confronted with a finished boat that needed improvement, I'd lengthen the stern, too. The subject, however, was designing boats, and modifying the design before building. I keep getting misguided comments from people who haven't read the thread. Folks, if you want to comment on my (or for that matter, anyone's) posts, please read the thread first. Context is the key to comprehension.

[BigCat]

I forgot to mention another little project of mine, which can be seen at Metal Art Sculpture, Steel Sculpture by Tim Dunn . I started that company, building it up from scratch, and designing all of the artwork and products, as well as doing all of the business management. After a couple of years, I started selling artwork to Costco, a 60 Billion dollar a year company. I like to create things and do new things, which probably shows, if you follow my wake.

[terhohalme]

Could we define "keel rocker"? Havn't seen any unambiguous one.

Rocker should measure in relation of draught. Just like other parameters of hull are proportional.

Here is my proposition:

[Nordic cat]

Originally Posted by terhohalme

Could we define "keel rocker"? Havn't seen any unambiguous one.

Rocker should measure in relation of draught. Just like other parameters of hull are proportional.

Here is my proposition:

Good idea terhohalme!

Then my design has about 90% forward and 140% aft. See below

Regards

Alan

[BigCat]

Hi, Terhohalme-So Catharsis would be 10% fwd and 50% aft? Sure, why not? -I'm steelsil2 when on junkrig : A forum to discuss the design, building

[Lodesman]

Hello BigCat,

I didn't forget our discussion - it took some time to finish the reading assignment And of course everytime I came back there were more posts with more links - very interesting reading (thanks to all). Thanks for pointing me to the other thread, but I was already following that discussion. I don't think it answered many questions, but it did help me decipher what you meant when you said:

Once you have chosen your waterline length, beam, and distribution forward and aft (the PC,) where are you going to put these cubic feet if not into rocker?

If you want the keel profile to be a simple curve, and want to keep the forefoot and transom above the WL, then I agree the only way to increase displacement, without changing LWL or BWL, is to increase the rocker.

I don't claim to be a designer, but it seems to me there are several options to increase displacement - increase length, beam and/or draught. Increasing rocker is really a variation on draught increase, but with a change in underwater profile. I would think that just adding rocker could change the handling characteristics - at least increasing the tendency to pitch.

When you look at the profile of a sailing ship hull, you are seeing the keel, not just the hull. I think you will find that sailing ships have rocker, which is a description of the hull shape, and that the part of the profile that makes them look as though they don't are actually mostly keel.

In some cases you are right, but not all. Certainly longer ships would have no rocker, as the imbalance in buoyancy would tend to 'hog' the ship and there would be no benefit to having it; a rounded forefoot and cut-away stern should not be confused as 'rocker'. If I find any pictures I'll post them.

For a double ender to not have rocker, you would have to have the sterns plunge into the water down to the bottom of the hull, down to what used to be called the garboard strakes in the days of wooden ships. I have never seen a catamaran (which is what we are discussing, btw, this is the "multihull" section of the board,) with such a hull form.

I think Rob (harryproa) Denney addressed the first point -obviously a double-ender can be designed without rocker. As to the second point, I am well aware that this is the "multihull" section; I don't think that should preclude generic discussion about boat design or using monohull examples to illustrate a point. I have the impression that a lot of designers are fixed to rules and formulae that while valid for monohulls, may not be true for multihulls. My readings tend to support my gut instinct that rocker is not as important in multihull design.

I don't actually know how valid concerns about bow steering might be, but I assure you that many have expressed such concerns. The effect the sails' center location has is on vessel trim. When running, it lifts the stern(s), depresses the bow(s), and on monohulls, swings the CE outboard when heeled, causing weather helm. This effect would cause deep bows to become deeper, exactly when you are most concerned about pitchpoling. This is why sailboats are much more likely to pitchpole than powerboats. If you want to read about bow steering, google 'bow steering forefoot' and you will find lots of references...

I did the google search, but didn't find anything definitive. I believe it was Robert Cateran who replied in this thread about his experience with bow steering - thanks. If I read correctly, it didn't seem too onerous. I wonder if running in a steep sea where the rudders might come out of the water, if having a deep forefoot would maintain directional stability and reduce the tendency to broach? Running will depress any bow, not just the full ones - would think the solutions are a higher Cp, fuller bow (dare I say maybe a bow-bulb), flare (or flam, as you call it - can't find this def'n anywhere btw), larger (deeper) rudder or rudder placement.

Anyway, all the reading was very informative and this last point makes a good segue (if you can excuse what is becoming a ridiculously long post) - I checked out your catamaran design. It's an interesting design, but I wonder why you chose to place the rudders so far aft? I like the idea of the skegs, but placing them that far back would surely expose them to damage, especially if you have to med-moor. The other question I had related to your numbers. Now I don't have a fancy computer program to use, just my calculator so hopefully the math is right; I wonder if I'm missing something in my calculations? You claim a hull draught of 14" @ 45000 lbs displacement. So each hull has to support 22500 lbs. Divide by 64 lbs (/cuft of seawater) = 351.5 cuft. Divide by DWL 62'9" = 5.6sqft. Divide by BWL 5'2" = 1.08' or 13". That's 13" of draught for a rectangular block - I haven't yet applied a hull form or prismatic coefficient. Assuming a Cp of .65, the draught would need to be 20" (13/.65). I don't know what your deadrise is, but from the diagrams it looks to be about a 30 degree angle, which would add almost another 9 inches to the draught. Did I screw up the calc's somewhere?


Kevin

[terhohalme]

Well, this wasn't unambiguous either... So, the height from base line (= deepest draft) up to the endpoint of keel line proportional to draft.

I'd like to say that Catharsis 26 has keel rocker 70% fwd and 120% aft.

[BigCat]

Hi, Kevin

Well, lets see-you have covered a lot of ground, and your comments are very perceptive, too. I agree with many of them, though not all.
***** ***** ***** ***** ***** ***** ***** ***** *****
The fact that this is a slightly modified hard chine design, and the fact that it is meant to be a charter boat licensed to carry 149 passengers has a lot of effect on the design. The fact that it may vary in displacement by almost 50% has governed a lot of my choices. I have striven to do something for which there is no guidance in yacht design manuals--design a sail boat which will perform well at wildly varying displacements. This is a major reason for the "v" bottom-it keeps some boat in the water at the ends, as you raise and lower the waterline by as much as one foot. It is also a reason why there is very little bow in the water at 45,000 pounds of displacement-I want to keep the the bow depth to something reasonable when the boat is 10" deeper in the water and weighs up to 69,000 pounds. I am not troubled by this, as I see successful examples of sailing catamarans with no bow underwater to speak of, and those with up to say, 12% of the freeboard forward under water at the bow, while the boat is at rest on its marks.
***** ***** ***** ***** ***** ***** ***** ***** *****
"If you want the keel profile to be a simple curve" - This is a hard chine "v" bottom design, with the chines rounded to a 2' radius, so yes, I want a simple curve because I want a 'developed' hull surface. I am making my own sheet material, and bending it to shape on a cradle. I got the idea from Derek Kelsall. I don't use his patented method of achieving compound curvature, however. I notice that Kurt Hughes is doing this now, as well, except for the underwater part of his hulls, and that he hasn't been gracious enough to credit Derek.
***** ***** ***** ***** ***** ***** ***** ***** *****
Length: I don't want to change the length, because I want to be under 65'--This boat is meant to be commercially licensed, and the laws give you a lot of incentive to keep it under 65'. Even under 65', you have to jump through truly astonishing amounts of hoop and spend vast amounts of time and money complying with the relevant laws, to be licensed to carry 149 passengers.
***** ***** ***** ***** ***** ***** ***** ***** *****
Beam: I don't want to increase the beam because the beam is wide enough to give me the interior I want. Wider = slower, as long as your hull form isn't extremely deep and narrow. You will remember the "K Factor" discussion.
***** ***** ***** ***** ***** ***** ***** ***** *****
Rocker and pitching: I keep hearing claims about rocker causing pitching, but I see no reason to believe it. Why would rocker increase pitching? Pitching is the vertical lifting of the bow about the axis of the CB (or perhaps the CLF) followed by its vertical plunging. Adding or subtracting rocker slightly increases or decreases the vertical location of the CLB, but not by much, especially with a "v" hull form. The bow lifts because it floats, and a wave passes by. It drops for the same reason. So, motion here is going to be governed by the buoyancy underwater at the ends, which is controlled by the P.C., and by the beaminess at the ends, which is not numerically defined. If you are trying to keep the PC at a desirable place (and everybody seems to think mine is more than high enough already,) and widen the bow as much as you can, you aren't going to have much depth of bow. The buoyancy of the ends (freeboard, flare, and overhang,) above the DWL also comes into play, as the passing wave reaches above the DWL forward as it passes. If your encounter with the waves is rhythmic enough, you can set up "pitching" which is essentially a pendulum effect. This pendulum effect requires that the weight distribution in your boat (pendulums have a natural period) coincides with your encounter of the waves. So, this will vary with wave length and your course and speed. Designers center weight in order to decrease the likelihood of these periods coinciding (the pendulum period and wave length.) It is also said that buoyant sterns (the fairly wide shallow sterns in vogue) dampen pitching by causing a sharp increase in buoyancy which counters the slower motion of pendulum pitching. I hope this is clear, because I doubt if I can make it any clearer. I have a stack of design books on my shelf a couple of feet long, and none of them pay any attention at all to rocker. It's all CLB, and CLF, and CLR, and PC, and so on, which is another reason for doubting the importance of "rocker." The main effect I would anticipate from increasing rocker would be that the boat would turn a little easier. Multihulls are bound to have less rocker than ballasted monohulls because they weigh less, have higher P.C.s, and because their displacement is distributed over two hulls.
***** ***** ***** ***** ***** ***** ***** ***** *****
Rocker and Harryproas: Not only are Harryproas very light and double ended, but Rob has used a P.C. of .8, which is pretty close to being a rectangle to begin with, as a P.C. of 1.0 is a rectangle, so yes, they would be pretty rocker free.
***** ***** ***** ***** ***** ***** ***** ***** *****
Modern ship design: I have paid little attention to it, and certainly haven't made a study of it, so I won't comment.
***** ***** ***** ***** ***** ***** ***** ***** *****
Flam versus flare: You can't find it? I googled flam + flare and found it on the first entry, at flare: Definition, Synonyms and Much More from Answers.com - complete with drawings illustrating the difference. As the drawing shows, you can find flam, flare, and tumblehome on the same boat. Tons of those long-keeled ketches made in Taiwan in the 70s had all 3, in different parts of the boat. Give a boat all 3, add some belaying pins, lanyards and dead eyes, throw a little teak trim here and there, and you have "character." I am picky about making the distinction because flam and flare have very different effects on, for example (speak of the devil!) pitching.
***** ***** ***** ***** ***** ***** ***** ***** *****
Monohull designers: Yes, that is surely what lead to "Alan" sighing and comparing me to someone who thinks he can do brain surgery without training. That and an unfortunate tendency to be condescending in a public place without first making very sure of his ground. I notice we haven't heard from him since. Actually, I have been studying monohull yacht design for 35 years. My copy of Skene's Elements of Yacht Design is completely worn out, and I used to have (Look it up, you whippersnappers!) a planimeter and "whales." In those days, a computer was something programmed with punch cards, and it was kept in a refrigerated room and attended by men in white lab coats. Really. I swept the floors in a place that had one of these when I was a college sophomore.
***** ***** ***** ***** ***** ***** ***** ***** *****
Bow steering: As I said, I have no personal opinion, but I note that many are concerned about it. I found scores of entries on the subject. As a thought experiment, imagine a bow 3 times deeper than the rest of the boat, rushing down a steep wave. Not a reassuring thought, at least not to me.
***** ***** ***** ***** ***** ***** ***** ***** *****
Rudder location: Yacht design manuals encourage placing rudders as far aft as possible (though under the hull, to prevent ventilation,) essentially due to the leverage of the distance between the rudder center and the CLR. Another thought experiment should make it clear--if you placed your rudder just aft of the middle of your boat, wouldn't you expect less effect from it? Another issue is draft. The farther aft you go, the deeper your rudder can be for a given draft, at least if you have rocker. I started out with rudders aft of the boat that could be flipped up, rather like a beach cat. So, if you think my rudder looks vulnerable now, consider the improvement. As for med mooring, I have done that exactly once, in Papeete, Tahiti. I don't plan on doing it again, but if I did, I'd put the bows towards the quay and use a stern anchor. It's much more private to do that, too. Otherwise, you are cheek and jowl with passers-by in your cockpit. Well, not in this design, as it has a pilothouse and a deck above the aft beam that conceals it, but on most boats.
***** ***** ***** ***** ***** ***** ***** ***** *****
My design's displacement and the required hull volume: Well, the keel and skegs are quite large, and give 2.5 tons worth of volume between them, and you haven't taken this into account--but you're right that I have been sloppy when interpolating from my calculations to lighter displacements. All of my accurate calculations were made for 69,000 pounds of displacement. My main concern for lighter displacements have been to satisfy my eye that the shapes are reasonable and normal-looking. If you took a typical modern catamaran and removed half of its displacement, you would have very little waterline left, and then rocker would indeed lead to behavior like a rocking horse. This is obviously undesirable, so I have focused on making sure that at each load condition, my hulls looked fairly normal. From my point of view, it is actually a good thing that the boat will float deeper in the water at a 45,000 pound displacement, as it means that the lighter waterline and sections will look more like the fully loaded boat. --Well, you've given me a workout, for which I have time because there is lots of 'hurry up and wait,' when getting a boat approved for commercial use by the USCG. Just getting them to answer the phone in a post 911 world is a challenge, as they have been given lots of extra security work and no extra resources.

[44'cruisingcat]

Originally Posted by BigCat

There are photos of other 44s on his site. On none of them can you see any sign of overhang at all, in those photos which were taken while the boats were in the water. I suppose that they all were loaded with a year's supplies?

Selective eyesight? on Bob's site homepage the picture of "Just Add Water" has overhang clearly visible.

Bob Oram Design

As for the other boats, in fact they most probably were indeed heavily loaded - Outahia is a full time liveaboard boat and Xtra Chilli, who'se owners live 5 minutes away from me, cruises for 6 months at a time.