Page 1 of 2 12 LastLast
Results 1 to 50 of 79

Thread: Designing a small lapstrake catboat

  1. #1
    Join Date
    Feb 2012
    Location
    England, curently landlocked in Shropshire :(
    Posts
    191

    Default Designing a small lapstrake catboat

    Following on from this thread:

    http://forum.woodenboat.com/showthre...catboat-design

    She is based on the Cape Cod type catboat, but built in glued lapstrake to save weight in the hull structure. She will have a displacement of 1300kg (2860lb), LOD 5m (16'8"), max beam 2.62m (8'9"). She will have removable inside trim ballast, either sandbags or scrap iron cast in concrete, depending how much space there is in the bilges.

    I started by drawing some lines on Hullform. I then started to refine the shape of the hull taking inspiration from the lines of Fenwick Williams's 18' catboat. Hullform is a decent program, easy to use for the most part, and can do basic hydrostatic analysis, which is what allows someone like me to have a stab at doing something like this, rather than having to learn and manually work out all the esoteric maths involved. The joy of computers - when they actually work that is!

    This is what I have so far:




    I then imported the DFX into Sketchup to rough out the backbone, deck and superstructure. This is what she looks like so far:




    As you can see, I haven't got round to modelling the cockpit combing yet. It be a continuation cabin sides as per the classic catboat look. Here is a shark's eye view of the hull and keel:



    That is about it for now...

    Pete

  2. #2
    Join Date
    Oct 2008
    Location
    Walney, near Cumbria UK
    Posts
    17,990

    Default Re: Designing a small lapstrake catboat

    This is Peerie Maa, modelled in Freeship. At a water line of 16' 3" Freeship calculates that she displaces 1090lbs. At 2860 lbs, with all of that wetted surface, I think that your catboat is going to be a slug. Brutal I know, but better make changes now in the virtual world before you get too far down the road.




    and in reality
    It really is quite difficult to build an ugly wooden boat.

    The power of the web: Anyone can post anything on the web
    The weakness of the web: Anyone can post anything on the web.

  3. #3
    Join Date
    Nov 2001
    Location
    Seattle
    Posts
    16,459

    Default Re: Designing a small lapstrake catboat

    It might help to give the boat a longer run. A typical catboat in this length would have about half the displacement you propose, although Fenwick Williams did manage to carry off more weight. Part of the question is how you intend to use the boat.

    Keep in mind, traditional catboats range from this:



    to this:

    The New York ones were much lighter than the Cape Cod ones. By the late 19th century, many had been given a small jib to balance the long boom, as sail area grew.

  4. #4
    Join Date
    Feb 2002
    Location
    Duncan, Vancouver Island
    Posts
    25,013

    Default Re: Designing a small lapstrake catboat

    Don't know much about catboat design but have heard many times that there's a real art to getting it right, given the inherent quirkiness of catboats under sail. But check out this old thread for some details on a much admired lapstrake catboat from a few years ago http://forum.woodenboat.com/showthre...strake-Catboat . A pic from the thread:


  5. #5
    Join Date
    Aug 2010
    Location
    BC Coast
    Posts
    1,812

    Default Re: Designing a small lapstrake catboat

    The main reason I am commenting is to be able to more easily follow the thread. Ian and others have more experience with the type. I have sailed some on catboats, and more extensively on moderate to heavy displacement small boats.
    We sailed Windrush with about 3500 lbs on a 16.5 WL. with a working sail area of 212SFT. She was surprisingly good in light airs. She was deeper and less beamy of course than a CC Style Cat. I like Ians comment about it "I like to joke that Marmalade never met a head sea she didn't want to stop and talk to."
    You will want to sail her to windward a bit free in a breeze, and have enough power to handle the waves. Stiffness for carrying sail, and hopefully no big issues on the helm.

  6. #6
    Join Date
    Feb 2012
    Location
    England, curently landlocked in Shropshire :(
    Posts
    191

    Default Re: Designing a small lapstrake catboat

    The boat is intended for extended coastal cruising rather than daysailing. The fat, heavy displacement Cape Cod Catboat type seems ideal for this purpose. It will also allow for very comfortable accomodation for such a small boat. She needs to be fairly heavy for cruising. She needs to be able to carry a decent amount of gear, food, water, etc, with one or two people aboard. Her beam will allow her to carry more sail without needing a deep ballast keel that would otherwise be needed to make her stand up to her canvas.

    Lets say she carries 240 sq ft of sail. This gives a SA/D ratio of 19.06

    Length is a limiting factor. I decided on 5 meters LOA because mooring fees and harbour dues are charged by the meter here. Also if she is to have the Cape Cod Cat hullform, if she is much over that length her beam would make her untrailerable on UK roads. She has a LWL of only 15'10", giving a hullspeed of 5.33 knots and disp/length ratio of 320.65

    Here is some stability curves I ploted using data from Hullform:

    I plotted a series for the vertical center of gravity being at different heights relative to the waterline. The increments are 0.125 meters or roughy 5". I would aim to get the CG within the lower two, ie 0-5" below the waterline. Assuming I can do that, this would give a righting moment of between 400-442 kg/m (2893-3197 lb/ft) at 15 degrees heel. AFAIK the corresponding heeling moment will be the force acting on the sail's centre of effort multiplied by the distance between the CE and the centre of lateral plane. I'm going to have to research into that a bit more.

    Here are the drag curves plotted in Hullform:

    These values seem low to me, but I don't know enough yet to really understand them. It seems that in order to move the boat through the water at a steady speed of 4 knots would require about 12 kg force to overcome friction and wave resistance. Of course the real drag values will be higher, especially a low speeds because this analysis does not include the drag created by the centerboard and rudder.

  7. #7
    Join Date
    Nov 2006
    Posts
    6,177

    Default Re: Designing a small lapstrake catboat

    You really need to get hold of a copy of Fenwick Williams "Design Elements of the Catboat". He walks through designing a twenty footer, lines, displacement, stability, layout, rig, all in separate chapters. A lot of the problems you are trying to deal with have been solved to near perfection decades ago and it's difficult to stray too far from the form without adversely affecting the overall performance or morphing into another type altogether.

    I'll post some of his rules of thumb later today. Things like the length/beam ratio, where the maximum beam occurs, least sheer height, transom height, stem height, deadrise, are almost plug-in numbers, around which the individual boat is designed.

    Your efforts are commendable and worthwhile and a better preparation for reading the book could not be found.

  8. #8
    Join Date
    Feb 2012
    Location
    England, curently landlocked in Shropshire :(
    Posts
    191

    Default Re: Designing a small lapstrake catboat

    Peerie Maa - That's a pretty boat you have there. It looks like she has a ballast keel?

    John W - The Fenwick Williams 18' catboat is very heavy at 3763 lbs. She is carvel built though. Correspondingly she carries alot of sail for such a small boat. Another boat I have been taking insiration from are Ted Brewer's Cape Cod Bay. She is ply-on-frame and displaces 2980 lbs with a LWL of 17'4". The heavier Cape Cod type catboat seems better suited for extended coastal crusing, which is what I want this boat to do.

    Jim D - That lapstrake catboat is beautiful. I certainly think lapstrake offers many advantages over carvel, but it is usually used for relatively narrow beam boats, at least in Europe. The width of planking stock in tradition clinker dictates what sort of hullforms are easily possible, although glued ply overcomes these limitations. I have my reservations about the glued method though, mainly having to work with epoxy, and the fact that all my planking stock will be coming from tropical hardwoods rather than the more ethical use of a local timber such as larch. Wales has a very damp climate well suited for storing traditional lapstrake boats out the water, the issue is more ease of build, strength for weight and the astronomical cost of copper fixings, which sway me more towards the glued method.

    gilberj - It seems that pounding in a head sea can be a problem with catboats. It will take getting used to alright. Unfortunately in my part of the world these boats are practically non existant, so I am unlikely to get to sail in one unless I build my own.

    Jim - I need to order that book though the Catboat Assoc. It might take a while to arrive being an overseas buyer. I admit I have only been going by what looks right to me so far, which is prehaps unlikely to be optimal considering I haven't been on a Cape Cod catboat before.

  9. #9
    Join Date
    Nov 2001
    Location
    Seattle
    Posts
    16,459

    Default Re: Designing a small lapstrake catboat

    Now that I understand your proposed use, I can see that you need to be at the heavier end of the scale. Study the lines of the CC Hanley catboat above closely, he managed to give catboats good performance with heavy displacement. I direct your attention in particular to the raking midsection, with the maximum beam of the lower waterlines well forward of the maximum beam of the higher waterlines. It's part of what enabled him to give the boat a good straight run. I really think you'll have a better boat with a shorter entry and a longer run. Something like this:



    but of course, with a skeg.

  10. #10
    Join Date
    Nov 2001
    Location
    Seattle
    Posts
    16,459

    Default Re: Designing a small lapstrake catboat

    Might be easier to see without the colour. As youu can see, the raking midsection gives you flatter, more stable sections aft.

    Last edited by johnw; 03-16-2012 at 03:15 PM.

  11. #11
    Join Date
    Oct 2008
    Location
    Walney, near Cumbria UK
    Posts
    17,990

    Default Re: Designing a small lapstrake catboat

    Quote Originally Posted by quercus robur View Post
    Peerie Maa - That's a pretty boat you have there. It looks like she has a ballast keel?
    Nope, that is the wood structural keel. She is unballasted, although larger 20' and above Shetland boats did have some ballast.
    It really is quite difficult to build an ugly wooden boat.

    The power of the web: Anyone can post anything on the web
    The weakness of the web: Anyone can post anything on the web.

  12. #12
    Join Date
    Aug 2010
    Location
    BC Coast
    Posts
    1,812

    Default Re: Designing a small lapstrake catboat

    We had a rather burdensom 14 ft lapstrake gaff cat dinghy for many years. It was not as wide as a CCCat in proportion, probably about 5 ft. She was not ballasted, but probably could have handled perhaps 200-400 lbs either inside or as a shoe outside. In the dinghty racing on the lake she did not loose by much to all the others, mainly Albacore's though we seldom finished in the top 1/3 of the fleet. I did some sailing with a friend on a Bill Garden catboat and with another friend on a Whitholz 17 ftr. My Father built a LFH Cat Yawl (keelboat with 3 ft draft).
    Boats with a lot of beam or belly tend to have more trouble punching into waves, particularly short steep near coastal waves thean a longer finer hull. Starting the sheets and sailing a little fuller is the main answer. Reefing at the right time is another part of the answer. I have more than once seen boats reduce sail and speed up in a blow. 6 points or so off the wind is still getting to windward in recreational sailing weather. I do not think you will find it punchy with the sails a little fuller.

  13. #13
    Join Date
    Feb 2012
    Location
    England, curently landlocked in Shropshire :(
    Posts
    191

    Default Re: Designing a small lapstrake catboat

    After a bit of tinkering trying to get longer run aft:


    The problem here is that my design has a pretty much a straight rabbet line (probably the wrong term on a glued clinker boat as she will not have a rabbet but you know what I mean) from stations 1 to 6 where she reaches her max draught:


    The original reason for this is to ease construction. The keel would be a straight run from the start of the curve of the outer stem to about halfway along the deadwood where it terminates and the bottom edge of the deadwood angles up slightly to reduce draught (this is from before the above revisions, but you get the idea):

    I'll have a crack at moving the deepest point of the rabbet line further forward. It'll require a bit of thinking about as to how the backbone will fit around it.

  14. #14
    Join Date
    Jul 2009
    Location
    Sunshine Coast, Queensland, Australia
    Posts
    940

    Default Re: Designing a small lapstrake catboat

    From Chapelle

    The shape of the run is important where speed is concerned; a full run will destroy the good qualities of a model otherwise excellent. It was often a fault in boats having their breadth of beam well aft that the run was short and full. The Hampdens obtained a long run in spite of the position of their greatest beam by use of the "raking midship section," indicated by the change in position of the greatest beam on each water line shown in the half breadth plan. By this means, the run could be carried well forward and a clean wake assured.



    ---------------------------------------------------------


    You might find this interesting in general as well

    http://www.anaburen.nl/catboat/index.html



    Last edited by Sayla; 03-16-2012 at 05:29 PM.

  15. #15
    Join Date
    Nov 2006
    Posts
    6,177

    Default Re: Designing a small lapstrake catboat

    As promised, here are some of Fenwick Williams rules of thumb for designing a Cape Cod catboat.

    Lowest freeboard should fall about two thirds to three quarters of the waterline length from the forward end.

    Freeboard at the transom about one and a third times the lowest freeboard...maybe less, depending.

    Bow freeboard can be the sum of the lowest and the stern freeboard.

    A little rake aft on the transom.

    A little tumblehome to the stem, which results in a little tumblehome in the hull sides forward.

    The foremost point of the stem about a third of the way up from the waterline.

    Lowest point of the rabbet and maximum section between station 5 and 6. (assume a ten space division)

    Rabbet meets the waterline at the bottom of the transom.

    A nice sweep to the rabbet.

    Beam a little less than half the length.

    There's more. Keep in mind that this is for a Cape cat of about 1930, not a Gil Smith, or a Hanley or many of the other variations.

  16. #16
    Join Date
    Feb 2007
    Location
    Halifax Harbour, Nova Scotia, Canada
    Posts
    1,881

    Default Re: Designing a small lapstrake catboat

    Quote Originally Posted by Jim Ledger View Post
    As promised, here are some of Fenwick Williams rules of thumb for designing a Cape Cod catboat...

    Beam a little less than half the length.

    There's more. Keep in mind that this is for a Cape cat of about 1930, not a Gil Smith, or a Hanley or many of the other variations.
    This came up in a recent thread (which I can't find now, of course!) and sparked some heated discussion. IIRC, it was pointed out that catboat length/beam ratio changed over time, as the type was refined, and as they developed as racing craft vice working craft.

    Tom

  17. #17
    Join Date
    Nov 2001
    Location
    Seattle
    Posts
    16,459

    Default Re: Designing a small lapstrake catboat

    Quote Originally Posted by quercus robur View Post
    After a bit of tinkering trying to get longer run aft:


    The problem here is that my design has a pretty much a straight rabbet line (probably the wrong term on a glued clinker boat as she will not have a rabbet but you know what I mean) from stations 1 to 6 where she reaches her max draught:


    The original reason for this is to ease construction. The keel would be a straight run from the start of the curve of the outer stem to about halfway along the deadwood where it terminates and the bottom edge of the deadwood angles up slightly to reduce draught (this is from before the above revisions, but you get the idea):

    I'll have a crack at moving the deepest point of the rabbet line further forward. It'll require a bit of thinking about as to how the backbone will fit around it.
    If you look at the designs in post#3, you'll see that C.C. Hanley, one of the greatest of the Cape Cod catboat builders, abandoned that system, while Gilbert Smith, one of the greatest of the New York catboat builders, retained this feature, and made it work by giving the boat a very hollow bilge where the boat was planked down to the straight keel aft. This was a feature of the early New York catboats and jib and mainsail boats (which sailed as a sloop in summer and as a catboat, with just the main from the sloop rig and the mast moved to a forward step) and of the sandbaggers, which like the Cape Cod catboat were derived from the New York type. Here's an 1868 jib and mainsail boat:


    The Cape Cod boats did not retain this feature, perhaps because their greater displacement made it more limiting in terms of giving the boat a decent run.

    I like your new set of lines better, but I still think you're making the entry too sharp and the run too round. Having the beam farther forward, the way Fenwick Williams did, will probably make the boat slower to develop weather helm as she heels, and flatter sections aft will give her better initial stability and less tendency to hobby horse in a seaway, a problem many short, heavy boats have, especially with the weight of the mast forward. It's part of why the straight run is needed even when the boat isn't close to hull speed.

    Anyway, that's my theory.

  18. #18
    Join Date
    Feb 2012
    Location
    England, curently landlocked in Shropshire :(
    Posts
    191

    Default Re: Designing a small lapstrake catboat

    This is after tweeking the rabbet line to get a raking midsection like in the CC Hanley catboat. I have got rid of much of the hollow in the bow. This should also help keep her from pitching excessively:


    She has harder bilges and is much stiffer: righting moment is 521 kg/m at 15 degrees heel compared to 442 kg/m with same CG previously. I have added 50kg to displacement to keep her sat on the DWL after these changes. Seems like she is going in the right direction thanks to the valuble advise from the good folks on this forum.

  19. #19
    Join Date
    Nov 2001
    Location
    Seattle
    Posts
    16,459

    Default Re: Designing a small lapstrake catboat

    Yes, I think that might pitch less.

    Fenwick Williams did it with a shorter run, but he kept the beam surprisingly far forward. By the way, have you seen this blog?

    http://buildingthecat.blogspot.com/2...oductions.html


    or this thread?
    http://forum.woodenboat.com/showthre...Brewer-catboat

  20. #20
    Join Date
    Nov 2006
    Posts
    6,177

    Default Re: Designing a small lapstrake catboat

    Here is a photo of the lines plan for the twenty-five foot Brewer catboat. Although a bigger boat than you're designing it might be of some use for comparison and study.


  21. #21
    Join Date
    Feb 2002
    Location
    Duncan, Vancouver Island
    Posts
    25,013

    Default Re: Designing a small lapstrake catboat

    Doesn't look like anyone has yet mentioned Charles Wittholz, another recognized name in catboat design from an 11 foot cat dinghy to much larger cruisers

  22. #22
    Join Date
    Nov 2001
    Location
    Seattle
    Posts
    16,459

    Default Re: Designing a small lapstrake catboat

    Quote Originally Posted by JimD View Post
    Doesn't look like anyone has yet mentioned Charles Wittholz, another recognized name in catboat design from an 11 foot cat dinghy to much larger cruisers
    I'm partial to his rigs.


  23. #23
    Join Date
    Feb 2012
    Location
    England, curently landlocked in Shropshire :(
    Posts
    191

    Default Re: Designing a small lapstrake catboat

    I have the study plans for the Wittholz 17' catboat. I really don't like the look of hard chined boats though. The Wittholz 17' is very similar in profile to the larger Brewer catboat, and seems to conform the the classic proportions that Jim posted earlier:

    She has that classic low freeboard and sweeping sheer, the problem of which is that on a boat of this size the cockpit sole ends up below the waterline. Maybe it is not imortant to have a self-draining cockpit on a boat that is not intended for the offshore sailing, but I would feel better having one. If main reason for the sweeping sheer of the Cape Cod catboat is a holdover from it's workboat origins, then retaining it in a pleasure craft has more to do with aestetics and tradition than outright practicality.

    Here is the Lyle Hess catboat built in Holland that Sayla linked to:


    She has a much flatter sheer than the Brewer and Wittholz catboats, in fact I was stuck by how much she looks like my initial rough design in that respect. I would like to find a happy medium, enough freeboard for a dry, self-draining cockpit, but keeping as close to the classic sheerline as possible. It will involve working out the dimensions the cockpit in Sketchup and juggling back and forth between that and hullform to get the sheer right.

  24. #24
    Join Date
    Feb 2012
    Location
    England, curently landlocked in Shropshire :(
    Posts
    191

    Default Re: Designing a small lapstrake catboat

    I've been cleaning up the lines a bit this morning. I think I am happy with the sheer I already have. The lowest freeboard is between stations 7 and 8, roughly two thirds aft of the stem. Freeboard at station 7 is 0.621 meters (just over 2'). If the cockit sole is .15 meters (~6") above the waterline then I have .471 meters (just over 18") depth in the cockpit:


    With the height of the combing (say 6-8") this should allow for reasonably comfortable seating arangements I hope.

  25. #25
    Join Date
    Nov 2006
    Posts
    6,177

    Default Re: Designing a small lapstrake catboat

    That Hess boat certainly is practical, there's no getting around that.

    Cockpit drains in catboats are mainly for rainwater and spray. If you ever get a serious amount of water in the cockpit, you're foo...well, you're in a lot of trouble.

    You mustn't neglect the windage factor. A high-sided hull catches a lot of wind while providing no lift when you're trying to go to windward.

  26. #26
    Join Date
    Oct 2008
    Location
    Walney, near Cumbria UK
    Posts
    17,990

    Default Re: Designing a small lapstrake catboat

    Quote Originally Posted by quercus robur View Post
    Here is the Lyle Hess catboat built in Holland that Sayla linked to:


    She has a much flatter sheer than the Brewer and Wittholz catboats, in fact I was stuck by how much she looks like my initial rough design in that respect. I would like to find a happy medium, enough freeboard for a dry, self-draining cockpit, but keeping as close to the classic sheerline as possible. It will involve working out the dimensions the cockpit in Sketchup and juggling back and forth between that and hullform to get the sheer right.
    That is one ugly boat. She needs about a foot sawn off of the freeboard. The cabin head room could be retained with tricks like high camber in the coach-roof beams, and possibly a full width trunk. All in all that is a boat that needs to be longer for its intended use.
    It really is quite difficult to build an ugly wooden boat.

    The power of the web: Anyone can post anything on the web
    The weakness of the web: Anyone can post anything on the web.

  27. #27
    Join Date
    Nov 2006
    Posts
    6,177

    Default Re: Designing a small lapstrake catboat

    You might want to rethink your tag line, Nick.

  28. #28
    Join Date
    Oct 2008
    Location
    Walney, near Cumbria UK
    Posts
    17,990

    Default Re: Designing a small lapstrake catboat

    Quote Originally Posted by Jim Ledger View Post
    You might want to rethink your tag line, Nick.
    I'm not saying that its impossible. But you have to be devoid of any aesthetic taste and have to be willing to work at it.
    It really is quite difficult to build an ugly wooden boat.

    The power of the web: Anyone can post anything on the web
    The weakness of the web: Anyone can post anything on the web.

  29. #29
    Join Date
    Oct 2008
    Location
    Walney, near Cumbria UK
    Posts
    17,990

    Default Re: Designing a small lapstrake catboat

    This is a really short version of a real boat:

    That looks right and works.

    That other thing doesn't.
    It really is quite difficult to build an ugly wooden boat.

    The power of the web: Anyone can post anything on the web
    The weakness of the web: Anyone can post anything on the web.

  30. #30
    Join Date
    Nov 2001
    Location
    Seattle
    Posts
    16,459

    Default Re: Designing a small lapstrake catboat

    Quote Originally Posted by quercus robur View Post
    I have the study plans for the Wittholz 17' catboat. I really don't like the look of hard chined boats though. The Wittholz 17' is very similar in profile to the larger Brewer catboat, and seems to conform the the classic proportions that Jim posted earlier:

    She has that classic low freeboard and sweeping sheer, the problem of which is that on a boat of this size the cockpit sole ends up below the waterline. Maybe it is not imortant to have a self-draining cockpit on a boat that is not intended for the offshore sailing, but I would feel better having one. If main reason for the sweeping sheer of the Cape Cod catboat is a holdover from it's workboat origins, then retaining it in a pleasure craft has more to do with aestetics and tradition than outright practicality.

    Here is the Lyle Hess catboat built in Holland that Sayla linked to:


    She has a much flatter sheer than the Brewer and Wittholz catboats, in fact I was stuck by how much she looks like my initial rough design in that respect. I would like to find a happy medium, enough freeboard for a dry, self-draining cockpit, but keeping as close to the classic sheerline as possible. It will involve working out the dimensions the cockpit in Sketchup and juggling back and forth between that and hullform to get the sheer right.
    I think the world of Lyle Hess's cutters, but Nick is right about that boat. Looks like the client insisted on a liter in a pint pot. Make very sure your boat doesn't end up like that. After all, it's a 16 footer, you only need sitting headroom. And it will be wide enough to sprawl back and use less headroom sitting than you'd need in a narrower boat.

    And by the way, not all the Wittholz catboats are plywood.


    http://www.ashmarboatworks.com/projects.html

  31. #31
    Join Date
    Feb 2012
    Location
    England, curently landlocked in Shropshire :(
    Posts
    191

    Default Re: Designing a small lapstrake catboat

    OK, so I have not being spending much time on the design recently. The biggest change I have worked on recently is the sheer, and I have come up with an initial sailplan, backbone, rudder and centerboard design, and from that worked out lead with the centerboard in different positions.

    Following on from what people here have advised and not wanting a, er... barrel shaped boat, I decided to forget having a self-bailing cockpit, she is not intended for the open seas after all. This allowed me to tweek the sheer to one more in keeping with the classic catboat form:

    This will give a min freeboard of .56 meters (~22"). As you can see I have modeled a mast as far as the hounds. I'm working on an initial overall diameter of 190mm for a hollow Doug Fir mast. The dotted line running from the hounds is 10 degrees off the line of the mast, and would be the ideal minimum for the forestay. Unfortunately the angle will have to be more acute unless I am willing to have a bowsprit. I like the simple idea of using a chainplate on the stemhead for attaching the forestay, and I really don't want a non-reefing bowsprit and all the extra standing rigging that would require.

    The sheer is now much in keeping with the proportions that Jim posted earlier. This is what the hull looks like in the water:



    With the sailplan from the first picture, I took the underwater profile and picked a fore and aft position to mount the the centerboard based on the designs I have been looking at. The centerboard is 4' long by just over 2' wide.

    I calculated the CLP with 100% of the rudder area included. The rudder design I have added here may be too small for this boat I am not sure yet. Some NAs do not count the rudder at all in calculating the CLP, I am not sure why. IIRC Ted Brewer includes 1/3 of the rudder area and he recommends fitting a bigger rudder to balance a boat with too much weather helm, presumably in order to move the CLP aft rather than just for the sake of getting more purchase on the water when she is trying to round up. It is odd then why the rudder area is not considered by many to be important enough to be included in these calculations.

    The most lead I have is 7.56% with the board half down to move the CLP as far aft as possible. To get more lead I would have to move the centreboard pivot further aft or shorten the boom - which is not really possible without making the rig either too tall or too undercanvased. According to Ted Brewer I should be aiming for about 13% lead with a design like this. All the catboat designs I have seen so far obviously have much less than that, though. The problem is that Brewer is making general rules of thumb for sailing yachts, and the CC catboat is not typical of sailing yachts. Ian (I think) metioned that catboats don't tack so well. My logic says having the centerboard where is is would help because it can be in the half-down position when on a close reach to minimise weather helm and lowered all the way when tacking and brought back half way again as her bow comes though the wind onto the new tack.

  32. #32
    Join Date
    Nov 2001
    Location
    Seattle
    Posts
    16,459

    Default Re: Designing a small lapstrake catboat

    Now, that's a handsome boat. One thing to look out for is to make sure there's enough centerboard in the case when the board is down that it doesn't crack the bottom of the centerboard case. Don't ask me how I know this.

    The way it's drawn at the moment, which I realize is probably not how you intend to build it, the water has a lot of leverage on a very small area at the bottom of the case. If you have some forward rake on the front of the board when it's up (top of the board when it's down), that will help.

  33. #33
    Join Date
    Feb 2012
    Location
    England, curently landlocked in Shropshire :(
    Posts
    191

    Default Re: Designing a small lapstrake catboat

    John - The tips from you and Jim on catboat design have been really useful in getting this design going right. I can see what happened with the Lyle Hess catboat, it was clearly designed from the inside out. The quarter berths are the culprit there, they cannot really work on a boat so small. The cockpit is tiny as well, a problem I also see but to a lesser extent with the Wittholz catboat. Everything on a boat is a compromise, and on a boat in this size range there is little point trying to make it comfortable or roomy below decks. I am currently doing a bit of rough work on the layout. I should end up with a cockpit of around ~6'x7' with the main bulkhead on station 6. Once all pretentions to bluewater cruising or cramming in as many berths as possible are thrown out the window a world of possibilities opens up.

    I am probably just re-inventing the wheel with this project but I'm enjoying it

    As far as the centreboard goes, I am not really sure how it will work out yet. The design as shown means that the lateral and torsion forces will mostly be absorbed by the keelson on each side of the slot. I haven't given the design of the centreboard case much thought yet. I choped a bit off the top of the board so that when the board is up it takes less space in the cabin. My initial thoughts are that if the after third of the board is tapered then that top corner will not ever be in contact with the CB case sides anyway. I guess if the lateral forces are enough the garboard seam on a trad carvel or lapstrake hull could open up, and the movement could also cause the CB case to crack or leak. The CB case sides are usually a weak point, but if the forces are mainly taken by the backbone, and if the backbone structure is up to the job, then the CB case itself is really just there to stop the water coming in rather than to hold the CB true to the centreline.
    Last edited by Peter Vella; 03-29-2012 at 04:11 PM.

  34. #34
    Join Date
    Nov 2001
    Location
    Seattle
    Posts
    16,459

    Default Re: Designing a small lapstrake catboat

    Thanks, I'm glad my input was helpful. One of the great things about catboats is the huge cockpit, so you shouldn't miss out on that.

    Most people who get blue-water capable boats end up daysailing and coastal cruising, I've notices. Few of them ever spend a night at sea.

    As for designing and building your own, I'm all for it. I did it with my sharpie, and is was a great experience which I hope to replicate at some point. I'm spending a lot of time dreaming about my next boat.

  35. #35
    Join Date
    Feb 2012
    Location
    England, curently landlocked in Shropshire :(
    Posts
    191

    Default Re: Designing a small lapstrake catboat

    This weekend I have modeled the deck, cabin trunk and combing. I am quite certain that I have the final hull shape and sheer, so this will be the last time I do this. It is quite time consuming modeling the camber of the deck and the lines where the combing meets the deck and cabin roof. There is 1.28 meters of headroom where the main hatch is going to be, so with the hatch in place there should be a little more.


    I have been thinking more about the centreboard. I found this online NACA profile generator:


    http://www.ppart.de/aerodynamics/profiles/NACA4.html

    NACA0004 gives me a max thickness of 1". The final foil shape would have to have less taper aft to avoid a feather edge, so it would not be a true NACA profile but based off it. As you can see from the marks I added only the forward half of the board's profile will be in contact with the CB slot with the board all the way down. To increase this contact area I would have to extend the CB area above the pivot point. There would be no point adding more area on upper corner of the trailing edge unless I used a straight board rather than a profiled one. Also the CB case will be unsupported by floor timbers at it's forward end above the cabin sole, so it doesn't seem to me that adding more board and case in that direction will add much to it's ability to resist the forces acting on the CB, but it will make the case much more intrusive in the cabin.

  36. #36
    Join Date
    Nov 2006
    Posts
    6,177

    Default Re: Designing a small lapstrake catboat

    Here are couple of observations about the centerboard you might want to consider. First, catboat boards are never let down to the extent you are showing in your drawing. Second, if you have only one inch of board thickness to play with, you should be thinking about a non-traditional construction, as edge-joined wooden boards impose severe limitations on the amount of shaping possible. Most cats use a square edged board anyway, and opt for robust construction over delicately shaped foils and suffer little loss of performance.

    Any chance of tweaking the low point of the sheer down a little? If you look at all the examples, the sheer shows a considerable rise as it approaches the transom. Gives it that "springy" look.

  37. #37
    Join Date
    Jan 2008
    Location
    Anacortes, Wa
    Posts
    317

    Default Re: Designing a small lapstrake catboat

    Ditto what Jim says regarding the sheer. I think that low point should be a touch more forward and maybe an inch or so lower. I love catboats, had a Herreshoff America. You're getting pretty close and the improvements have really helped.

  38. #38
    Join Date
    Feb 2012
    Location
    England, curently landlocked in Shropshire :(
    Posts
    191

    Default Re: Designing a small lapstrake catboat

    The previous drawing shows the board all the way down. On a close reach the board will usually be in a half-way position like this:

    I was thinking to cut the CB from the same 1" okume ply that the transom will be made from, and sheath it in fiberglass. Getting the profile faired on ply will be easier than with solid timber and it will be dimensionally stable so the gap between the board and the sides of the slot in the keel can be smaller. I admit it may be too delicate even when glassed and from doing further research it seems the thickness should probably be closer to 8% of chord than 4% considering that this is not some high-tech kevlar racing boat. I would however like the centreboard to break before the case or the keelson moves and spring a potentially catastrophic leak, so I don't want the CB too overbuilt. I have started a rough schedule of materials already, but I should probably not let thrift dictate the design too much, and I should get whatever material is necessary for the centreboard rather than trying to make it from leftover materials from other parts of the boat.

    As for the sheer I see what you guys mean, but I'll have to do a bit of chin stroking and pacing about before I make further changes to that.
    Last edited by Peter Vella; 03-31-2012 at 12:54 PM.

  39. #39
    Join Date
    Nov 2006
    Posts
    6,177

    Default Re: Designing a small lapstrake catboat

    A word about the centerboard pin. It's tempting to place the pin low to get the board further forward, but you have a few practical considerations to contend with. I've seen catboats with the pin outside the rabbet, but this strikes me as a weak construction. Putting the pin high enough that it's inside the boat, and able to be withdrawn without hitting the planking, seems to be a better idea. The keel in this area is supported by the floor timbers against possible strain.

    Don't be afraid to redraw things. If you think it's a lot of work now, wait till you built half the boat and decide to change something.

  40. #40
    Join Date
    Nov 2001
    Location
    Seattle
    Posts
    16,459

    Default Re: Designing a small lapstrake catboat

    A big, square centerboard let down to a leading edge angle of close to 45 degrees seems to work pretty well for catboats. The less the leading edge angle, the less efficient the board, the steeper the angle, the less likely to kick up instead of breaking on a reef or sandbar. The low aspect ratio seems to work well with the rig, and with a boat that doesn't have a jib to back to complete a tack -- it's good to have a board that doesn't stall easily, like a high-aspect board does. My theory is that the board should stall just a little easier than the skeg, so that the boat will bear away after a tack, but Jim and Ian can tell you more about that. A lot of my catboat sailing has been in Beetle Cats, which seldom get caught in irons unless you sit too far aft and get the transom dug in and towing half the ocean behind you.

  41. #41
    Join Date
    Nov 2006
    Posts
    6,177

    Default Re: Designing a small lapstrake catboat

    When you tack a catboat, you have to let it fall off a lot to regain speed, then edge it back upwind. Beetle cats probably not so much, but larger cats have to do this. They will never regain speed pointed up too far.

  42. #42
    Join Date
    Feb 2012
    Location
    England, curently landlocked in Shropshire :(
    Posts
    191

    Default Re: Designing a small lapstrake catboat

    Jim - I haven't worked out how the pin will be removed yet. I want to pin to pass through the keelson as that would be strongest, but it limits how high it can be. It could pass through the case logs instead I suppose, but I will only do that if I can't get it to fit in the keelson without fouling the planks. The advantage of having the pivot in the keel below the rabbet would be no chance of leaks, although the keel would have to have a decent moulded dimension to support it. It seems an inherantly weak place to have it. One idea would be to design and cast a sort of removable pivot bracket that slots into a recess in the keel and is held in place with long bolts that are inserted at the top of the case, down the case sides, and screw into the bracket. This idea is beyond my ability to impliment though.

    John - I will probably make the board thicker than in the profile shown above. 6% of chord would be 1 1/2", could prehaps go to 8% at 2". Thicker would mean more drag but less tendency to stall.

  43. #43
    Join Date
    Feb 2012
    Location
    England, curently landlocked in Shropshire :(
    Posts
    191

    Default Re: Designing a small lapstrake catboat

    After tweaking the sheer a bit more:

    The green is the sheer of the old hull. I have added about 2" to the stem, 1" to the transom, lowered the min freeboard and moved it forward. Here is a comparison of the two in perspective:

  44. #44
    Join Date
    Nov 2001
    Location
    Seattle
    Posts
    16,459

    Default Re: Designing a small lapstrake catboat

    Usually, the pin goes through the logs.

  45. #45
    Join Date
    Feb 2012
    Location
    England, curently landlocked in Shropshire :(
    Posts
    191

    Default Re: Designing a small lapstrake catboat

    A quick update - I have rejigged the deck to fit the new sheerline, revised the cabin trunk and fleshed out the rig somewhat:


    The rudder probably needs more cowbell.

    On a sidenote next week I will be sailing on Eve of St Mawes, a pilot cutter designed and built by Luke Powell of Cornwall. If I have time I might see if I can visit his yard and take some pics. They make some beautiful boats there:


    http://www.workingsail.co.uk/

  46. #46
    Join Date
    Nov 2001
    Location
    Seattle
    Posts
    16,459

    Default Re: Designing a small lapstrake catboat

    the boat's looking good, and I like the rig you've drawn for it. Sailing on a Luke Powell pilot cutter sounds fantastic, you must post pix!

  47. #47
    Join Date
    Nov 2006
    Posts
    6,177

    Default Re: Designing a small lapstrake catboat

    If you look at the lines of a few Cape cats, you will notice that the keel curves upwards from the front of the centerboard to the stem. The forefoot area is very shallow. This, I believe, makes the boat nimble and quick to come about. It also probably handle better downwind in a chop. A deep forefoot in a following sea, especially on a short, beamy hull, will dig in while the stern is lifted.

  48. #48
    Join Date
    Apr 2007
    Location
    Anacortes, WA
    Posts
    10,211

    Default Re: Designing a small lapstrake catboat

    Quote Originally Posted by quercus robur View Post
    Here is the Lyle Hess catboat built in Holland that Sayla linked to:

    Wow. Just wow.

    If I owned that poor beast, the very first thing I'd do is trick it out in WWI dazzle camouflage like HMS Argus here. It wouldn't help any, but at least it'd be a distraction.

    If this post did not meet all of your needs, please consult this thread for more options.

  49. #49
    Join Date
    Nov 2001
    Location
    Seattle
    Posts
    16,459

    Default Re: Designing a small lapstrake catboat

    Well, at least it has the shallow forefoot Jim recommends!

  50. #50
    Join Date
    Feb 2012
    Location
    England, curently landlocked in Shropshire :(
    Posts
    191

    Default Re: Designing a small lapstrake catboat

    I had a great time sailing on Eve of St Mawes last week:



    On Sunday we saw Freya out on her sea trials. She is the latest pilot cutter by Luke Powell, launched on Saturday 7th:



    Unsuprisingly I didn't see any catboats in Falmouth, but there are plenty of very pretty wooden boats there generally, especially compared to most other ports which are full of plastic boats and little else. All in all it's a great part of the world, it's got me reconsidering where I want to relocate to.
    Last edited by Peter Vella; 04-20-2012 at 04:40 PM.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •