Drawing a lapstrake hull

I plan on trying something new, at least to me. I am drawing a boat that will be 18 1/2' LOA, 6'2" beam and probably lined for 6 strakes. Instead of drawing the hull sections as normal, I am interested in drawing the midship section to determine volume of dislacement, etc. and then draw 6 strakes on this section which would be drawn on top of the profile view. I would then draw all the plank lines on the profile filled in and faired to the midship section. At that point the look of the boat would be largely determined and the heights close to locked in. At that point I could draw the rest of the sections and then transfer those heights and widths to the half-breadth view and use those lines to fair up the sections. Then I could put a few diagonals through the sections (a bit of a pain with the sections split up and put on their individual station lines rather than stacked as usual) and check lines for a final fairing. I have just completed a midship section so would love any feedback from folks. Thanks.

Cheers,
Clint Chase

 

Clint, from distant memory of the lines plans I've seen of clinker(naval cutters, whalers, Blacksland-Chapman putters) the lines where kinda like a bearding line drawing- without the strakes like the mold stations/ body plan, the designers most likely made allowance for the extra volume of the strakes in the displacement calcs & omitted them for clarity of fairing the lines & lofting. Six strakes a side dont sound like enough for the beam you've mentioned & remember to make your sheer strake a little wider so it dosn't look sqibby when a sponson/rubrail is fastened on. sounds like your on the right track but & all the best from Jeff.

 

TX Jeff, yes six strakes is few but will do it OK, I may try 7 strakes for kicks. I want as few as possible w/o making the hull look chunky. I'm going to go for it and start drawing in the lap lines on the profile. First I have to decide on # of strakes and line off the miship section. Here goes nothing.

Cheers,
Clint

 

Are you clear on the method for making flat templates from your drawing for the planks?

 

Clint,

I'm a former owner of an 18' Cruisers lapstrake runabout. Dimensions were close to what you mentioned. I recall more than 6 strakes showing, but then I remembered that that design had a clipper bow with lots of flare.

One question: Are you planning full lapstrake planking to the keel, or, for example, a hard chine hull with plywood bottom and lapstrake only above the waterline? I've seen the second type, although most are full lapstrake. The number of strakes seems to vary, also. I just found this thread http://www.boatdesign.net/forums/showthread.php?t=20975&highlight=this boat showing photos of an old 19' lapstrake hull, including a shot near the transom. 4 strakes above the waterline show at the transom, so there are more than 6 in total there, and even more as you move forward. Note the photo illustrates what Jeff mentioned about making the sheer strake wider because part of it will be covered by the rubrail. The photo below is of a Lyman. Blow it up enough and you can count 12 strakes at the bow.

Opinions on the number of strakes will vary. I guess the best thing to do is draw out the whole thing in profile and see how it looks. Your eye will tell you a lot.

 

Clint, like Charlie says some more info on the hull style/ purpose would help & also what kinda planking you gunna use- if its ply I suppose you can have wide planks but if its solid planks their width & run in the hull style will dictate the No. of planks, maybe as your design evolve a 1/5 scale half model will help you work it out & get a practice go on land bevels & geralds & really help with planking widths & the run of it,but maybee you got a "fancy program" for the computa to do it? Regards from Jeff.

 

Glued-Lapstrake design process

TX for comments. My main question was not reagrding plank width and # although that is part of the answer. This will be a glued-lapstrake plywood sail & oar boat. I could go as few as 4 if I wanted to...no problem just makes a wide hul with a lot of freeboard look "chunky". My main question regards how well it will work to divert from the usual method of drawing and fairing a hull, i.e., draw the profile, draw a half-breadth with sheer, make a midship section, get that right and then fill and fair lines to this predetermined midship section. That is what I am used to. But I am trying to do away with waterlines and buttocks and only draw the lap lines on the profile and in half-breadth and use those real lines to fair the hull. Maybe throw a couple diagonals through the sections to check for fairness at the end. So no waterlines and buttocks. The procedure, I think will be to draw the midship section and get that right (check), draw my transom (check), decide on # of planks (5-6...TBD) and tick those off on the stem profile and transom and MS section drawn right on profile of course. I now would have three points to fair to and can fill in all my lap lines. This is without having determined any of the sections except at midships. At that point I may then do the same filling in of lines on the half breadth and draw all the lines fair with the ends and middle of the boat as control points. I'd then make my sections from these two drawings and tweak to get the shape I want and use a few diagonals to make sure it is all fair.

I am making this up as I go along so any help is great. Anyone else used this method to draw a hull and can give some direction?

 

Sounds good to me Clint cos it sounds kinda like a "multi" chine boat where the ply planking happens to have a square lower edge & happens to lap the previous plank (& stringer ?) & WL's & Buttocks look kinda kinky & useless for fairing on a multichine where they cross the "corners". All the best fom Jeff.

 

Strongly recommend you purchase Ian Oughtred's book "Clinker Plywood Boatbuilding Manual". It has an entire chapter on locating planks to create a good structural as well as an aesthetically pleasing job - and even I can understand his "how to".

 

Yes, thank you for the reminder...I have my Oughtred manual at work where some colleagues are learning from it...I have begun lining off on the midship mold and transom and will soon line off the profile view. I have settled on five strakes, I may add another depending on the profile view.

 

Clint,

In my opinion you will have much grief trying to fair each plank to it's neighbor with no guide. The usual practice with a lapstrake hull is to fair (loft) a round hull form and then line out planks (per Oughtred) on that surface.

There is no "Fancy program" or otherwise software to loft lapstrake hulls. In Rhino I would (I have) create a fair single hull surface and then lay curves on this surface (with planking reduction). Wiggle your curves around till they suit, then build your developable plank surfaces between the curves. Painful.... quicker and easier on the loft board.

Tad

 

Clinton,

You can do lapstrake lofting of the strakes on software. The boat I'm building now was done that way, but not by me. Ithen did my own lining off because I wanted to learn how. I did it on the middle mold first and then by moving the ribbands (stringers) around until they looked right over the whole boat from different angles. I used Oughtreds book for a start and then to suit my own eye.

I am using 8 strakes and think that 5 would probably look cranky, especially if you try to get some power into the boat by having a hard turn of the bilge. Fewer strakes make the outside angle between adjacent strakes greater and make the overlap thicker and more pronounced. More strakes allow a better looking boat.

 

I haven't found any software that lines off a hull to reasonable satisfaction. The screen is too small and the sweeps too easy distorted, because your eye is too close to the work to see them properly. I line off hulls by hand then scan this, which I've found the only way to really get fair and more importantly sweet sweeps in the planks.

 

Here is a little power boat I designed in Rhino and the builder lined off the ply planking in Pro E. I think it turned out quite well.

 

Well, I may take Tad's advise, fair up the body plan with round bilges, then line off the hull in profile and make sections multi chined. We'll see...in th meantime I have a question. I am having a hard time really settling on the midship section, about how hard to make the bilges, in other words what to use for a waterline beam. My overall beam is 6'2". I want the boat to be rowable for aux. use, getting back to the ramp, etc., but I may have to row for hours on end in our local RAID like event (WB's Small Reach Regatta) or for when my little electric outboard I intend to use in a well loses battery power. At the same time I want some initial stability so that when my kids are in the boat she won't be too tender. I don't feel I have enough experience yet in these small sail and oar craft to make a call on the waterline beam...right now I have it drawn with about 58" waterline beam and 6'2" overall beam which provides nice flare to the sides, which I like for secondary stability. Any thoughts?

Cheers,
Clint