Tuesday, April 15, 2014

Progress, life gets in the way a bit.

First, there have been several requests for the little double enders vital statistics.

Sail area is 7.1 sq m.  Thats  76.5 sq ft.

Length is 4.45 m   - 14ft 7in
Beam is 1.55m  - 5ft 1in
Weight should be around 75 kg  165 lbs
Draft 175mm board up  7in
1.1m board down.  3ft 7in

I dont expect to have to build a proper boat trailer for this, I figure that I will be able to just lift the bow up onto the end board of a handyman trailer, pick the stern up and push the boat forward. 
I'll put plastic imitation grass on the end board to make it slide, and will slot the board to take the skeg which will make it stay centered as I push it.
Managing it on the beach will be with a couple of inflatable sausage shaped fenders used as rollers.

I had an “Off day” yesterday. Not every day goes as planned, sometimes  ones frailalties get in the way.
So there was not much done, and I’ve decided to take my own advice and just build when I feel like it.  No pressure, and it makes it much more fun than trying to meet deadlines.

I did though go to a Folk music club meeting last night, its only the second meeting of a brand new club and it being a new group means that the social structure is not so entrench and I’ll fit in more easily.  I used to play several instruments, ( badly but enjoyed them) and should really get one or more out and practice. This will be an incentive.

I’ve also joined a Writers Club, while I have written a great deal of technical and non fiction, writing fiction is very different and I’m doing a little of that for fun and to make friends here in my new home area as well. There is no need to be lonely.

Back on boat things, I’ve roughed out the foil shape of the daggerboard.  Its three laminations of 9mm plywood which is easier for most people than making it up from strips of solid wood.
 I use an angle grinder with a 40 grit sanding disc to form the shape up where it transitions from the rectangular shape that fits the ‘case, to the foil shape and rough from there down to the tip with a power plane.  Next is the hand plane, the dark glue lines of the plywood make it easy to get the section constant all the way down, and its at this stage that I go from “eyeball” to fitting the template that I’ve made from the foil section I drew on the plans.
That section by the way is not an NACA section, its one that I got from a hydrodynamics software suite that I have, its designed for high lift at moderate speeds and to have a very high stall angle, the aim being that it wont stall when tacking at slow speeds.
While SEI should be moderately fast for a somewhat trad styled boat the fact that its double ended means that it will be slower than boats with near parallel sections from the widest beam aft, that plus even a really fast small boat is not fast in terms of true speed through the water (compared with say a 70 ft ocean racer) so the medium speed section is more appropriate to the use.

 Laminating the three layers together, use whatever happens to be handy to put pressure on the glue lines.

Fitting the template to the foil.
From here, I’ll knock the bumps off with the Festool Rotex sander then coat the ‘board with System 3 fairing compound and carefully fit it to the template.
I wont glass it, but have put a strip of hardwood down the leading edge by cutting the middle layer of plywood back 20mm and laminating a piece of mahogany in there. That will reduce impact damage quite a lot.

Up to the stage where you see it, that shaping of the foil took about an hour, the rudder blade is next…



  1. John, could you comment on this from your post above:
    "...double ended means that it will be slower than boats with near parallel sections from the widest beam aft..."
    Is this also true for a pure rowing boat? Or is it true only for sail boats?
    I'd also like to understand the underlying rationale for why double enders are slower vs parallel sections running aft. I would think they would be faster because of the reduced surface friction.


  2. There is a point where wave making becomes more a restriction than surface area friction, and the square stern boats will make their stern wave further from the bow wave, the distance between the two being what sets "hull speed".
    Double ends work ok on heavier boats, or on rowing boats, and to some extent surfboats, but if you want a really fast sailing boat check out the Sydney Harbour 18 ft skiffs shape.
    In general the wider stern contributes a lot of stability which enables the boat to carry more sail, plus manages the stern wave form better.
    Its a long and complex subject though, and if you want to pursue it there is a book called "High Speed Sailing" buy Frank Bethwaite that would help explain a lot of it.


  3. Thanks, John...I'm thinking of a pure rowing boat of 12 feet... either double ended (DE) or square stern (SS)... The DE would have constant deadrise stem to stern of let's say 20 degrees... The SS would have 20 degree deadrise forward trailing out to let's say 5 degrees at the transom.
    Given the above, the SS would have more surface area friction (all other factors being equal).
    If I understand you correctly, the stern wave generated by the SS will be further aft than that generated by the DE... this will result in a higher "hull speed".
    But the difference can't be very much... Let's assume the distance between the bow wave and stern wave on the SS is 14 feet (Hull speed = 14^-2 (3.74) X 1.3 = 4.86) and for the DE 12 feet (Hull speed = 12^-2 (3.46) X 1.3 = 4.5). So the difference in "hull speed" is .36 knots based on my asumptions above.
    And, more importantly, I'm not going to be rowing at "hull speed" for very long. So at cruising speed, let's say 3 knots, the "hull speed" is not a factor in this scenario. But the skin friction IS a factor.
    Do I have this right?

  4. Tom, this is a long and involved subject, and gets really involved when the more esoteric elements of hull design are considered.
    But do remember that SEI is a sail and oar boat, so there are a whole set of conflicting requirements to be worked out in the shape, the result will inevitably be quite different to a pure rowing boat.
    I'd suggest for your rowing boat, that you draw your shape and do the numbers, being only 12 ft long you'll row her close to hull speed so her prismatic c/f will ideally be higher than say a 16 ft waterline rowing boat.
    Try for 0.54 for starters and see what that looks like.
    Keep the shape fairly rounded which will minimise wetted area.
    I cant really think of one book that will give you all of the information that you'll need, I have an extensive library including access to several professional societies literature and each book or publication seems to have one or two interesting points to make.
    But do read everything that you can, the more of those "interesting points" that you can gather the better.


    1. That's interesting that you say use a 'rounded' hull to minimize wetted area. I was curious about displacement vs. wetted area as a function of deadrise... created an Excel spreadsheet and found, for a constant displacement, the least wetted area was a 45 degree deadrise for a "V" bottom hull... Here are a couple of examples... all with a constant displacement...
      85 58%
      75 29%
      65 12%
      55 3%
      45 0%
      35 3%
      25 13%
      15 29%
      5 58%
      John, if you want the spreadsheet, send me your email address to
      tlclarke at optonline dot net

    2. After thinking abut this, I remembered that the maximum volume and minimum surface area is a sphere... So, of course, rounded hulls will always produce less surface area for a given displacemnt.
      I think I'll extend this spreadsheet to included 'round bottom' hulls, adding it to the "V" bottom hulls currently in the spreadsheet.