hws question

General Discussion
1

heres a question I’'ve been playing around with:

In regard with the typical Paul Jensen styled hws … Now my current plan is to space the the ribs more frequently. say 5 inches apart… now then since the board manily gets its rigidity from the outer-rail (perimeter) plus abit of that from coumpunded structure of the deck and bottom skins… so what if I loose the central stinger alltogether ? that is after I have the ribs glued onto the inner rails and possible deck/-or/-bottom skin?

My thinking is, that if the ribs are close enough to perform the function to keep the deck and bootm apart, then the need for the central stringer is -nada !?? Basicly the only real reason for the central stringer then would be only in the setup stage of assembling ribs to inner rails, after that: redundant

Come on, someone! shoot my theory down before I start trying it out! give it your best shots…

2

Ahm, while I wouldn’t presume to speak for Paul… structurally, I think you’re right, further if you were to increase the deck camber/curve side-side a bit then in combination with the closer frame spacing you might wind up with a somewhat stiffer board than the original. The only question might be what’s called ‘oil canning’ in the metal boat biz- much less likely with the wood/composite skins than with metal plate, but it’s a consideration.

Having said that, I think that if you are going to vaccum bag the skins and such, you may have to make what could be a somewhat complex set of jigs and supports to keep the frames aligned while you’re sticking 'em on ( or at least while attaching the cambered deck, after that it’s not a worry) and putting pressure to 'em.

But this could be something as simple as several pieces of wood ( maybe 3, set at CL and at half the maximum width from CL) kerfed or notched to the rocker line at the various frame locations, frames set in 'em and secured to hold the frames in their proper places, similar to some of the arrangements used in production boat work. The nice thing about this is it’d be reusable and adaptable to later boards.

So, hey, go for it - be interested to see how it turns out.

hope that’s of use

doc…

3

“oil canning” whats it mean?

yeah gluing the skins is a consideration. Fortunately (or unfortunately) vacuum bagging is out of the question for now till I or the school gets $$… so till then its up to “cunning plans” and a sick imagination and lots clamps, weights, bricks, blocks etc.

The bottom skin is easier as I glue it on the form with the proper rocker and then slap it on to the frame. Deckside (with camber) is a mite harder… and am trying to figure that one out still…

If I had the time, inclination, knowldge, skill etc (and vacuum bag )I could try to make a molds for both the top, bottom AND rails… cold lam it with wood strips and voila ! but for now till I get these existing boards done, it’ll be a fantasy I’ll save for later. this all done with wood /veneer strips or thin aircraft ply

4

I made the frame including the stringer for my latest HWS from 1/8th ply and it came out very light. The weight starts to really add up with the deck and rails. Then you have the weight of the glass and resin. You could save a little weight by loosing the stringer but I think you should look for ways to save weight elswhere.

5

yeah, the weight save/loss from the lack of the stringer alone doesnt amount to much, but in the end every little bit counts and adds up. On board number one, I fell into the old trap: I tended to add material strength, alittle bit here, a little bit there and in the end : “whoops, its heavy !??”

So it requires quite abit of discipline to “stick to the original intended plan”

But I’m still trying to work out the kinks… for now I will build up with a central stringer… but a radically lightenend one. After all I need some surface to glue the skins on. The outer rails are the biggest problem for the mooment. Easiest solution is to make em from balsa…But…

6
Keep the stringer...
Layer the rails as you build them out...
Glass the exterior with 4.oz or lighter...
Have fun...!!!...
7

Wouldn’t the weight of double the amount of ribs be more than that of a central stringer? I know the central stringer in my board did not weight much. If I was to loose some weight on it I’d go with thinner glass inside and out on the wood. Vaccuum bagging is really in the wind now, a vacuum bagged lightweight panel of wood with glass on both sides would probably save you much more weight than loosing the stringer. Also, the stringer stiffens the board up lengthwise, something the ribs don’t which might result in the board breaking. You still have the perimeter stringer though, but it’s still something to consider.

I’d rather try two stringers set about six or seven inches apart and loose the ribs altogether… or maybe have only two or three ribs. But 90% of the weight is in the perimeter rails, outer rails, skins and glass anyway. Why try to remove weight from the part that contributes the least weight and certainly the most strength by weight?

regards,

Håvard

8

Hi johou,

I had exactly the same thoughts after reading previous posts by Daniel Hess. I felt that leaving the rocker out might reduce rigidity along the length. So I glued up my rails first on a planshape jig, routed the rocker into them and joined them at nose and tail. I only glued in 3 ribs (made from 2" thick eps sandwiched between veneer) before I glued on the skins this morning, and everything seems just so this evening with a bit of flex from tip to tail.

My only comment is that the rear two ribs (under backfoot and front foot) had some roll in them and the front rib was flatter. The deck skin around the flatter rib has a little give in it, while the convex shape around the tail is much more rigid. A heavily convex deck seems to be the way to go if you’re not using a stringer.

Besides this, although the centre of the board weighs next to nothing, the board is extremely heavy. ALL the weight is in the 1 1/2" wide rails. I haven’t turned them yet (tomorrow morning, and I’m scared! ), but I still don’t imagine this is going to be a feather weight board. Paul’s post about reducing rail weight, I am sure, is the key to a lighter board not what’s inside (which probably affects flex more.) Next time I’ll try and keep the rails under an inch wide, use a lighter wood than meranti ply (lauan??), and roll the ribs more on the deck side to reduce rail thickness and increase deck strength.

Now, I have a question too. I’m sure people will shout at me but I’ve used teak faced ply for the skins. I know teak isn’t good with resins, but it’s the ONLY reasonable quality ply I can get around here. The teak veneer is very thin and the other 2 plys looks like they are kampur (camphorwood?). Is there anything I can do to improve adhesion when I glass it? Wipe it down with alcohol perhaps? I’m using epoxy.

Good luck johou, hope the project stokes you as much as it has me! Thanks to all hws builders on swaylocks!

9

first off… to clarify some points I forgot to mention… oops

These first three boards I’m building are completely un-glassed. reasons being:

A. never worked with glass/carbon/kevlar et al.

B. currently no place to work with glass and epoxies in the amounts required, as the “nifty” workshop space I have availble quite clearly says “no-no” to epoxies (excepting small batches I mix of 4ml-30ml)

C. I am lazy to learn to glass…

D. I like the challenge :slight_smile:

yes I’m aware that making a wooden composite board without any glass may be skirting with disaster and tears…but like I said, I like the challenge, I’m competing (again) for the “village idiot of the year” title and I like doing things contrary to common beliefs :wink: . Yes I suppose I should get my meds checked up on…

anyways… back to my original train of thought . My first board is overbuilt, which is ok for now, it should be pretty much indestructible until the swelling of the wood rips it apart or it gets bashed in the rocks… or…or

The two biggest reasons in my thinking, for the excess weight is /are (in my current boars numero uno “fastfood”)

A: central stringer overbuilt two laminates 12 x 12 pine strips

B: deck and bottom skins 8mm birch ply and 5mm birch plies , respectively

C: solid rails

So now for boards 2 and 3 I’m slowly skimming stuff in the above mentioned areas.

basicly for board three I’m thinking along the following lines:

deck and bottom have the biggest ares and thus biggest proportion of weight. So I skim the deck to 5mm but that means I need to set the ribs (or cross pieces) more frequently spaced… which adds weight …BUT I’m goinna make the ribs even lighter than before ( 5 x 6 mm pine twigs re-inforced with 0.8mm birch ply) the ribes in board # 1 were 6mm birch ply with biig holes in them.

I will have a a central stringer but that will be with the same construction as ribs mentined above. so “very light” (lighter than so far)

Inner rail in board number 1 was 2 laminated layers of birch ply :Total thickness 5mm… so for board number three I’m using the same construction as ribs and stringer (ie. 5 x 6 pine “twigs” reinforced with 0.8 OR 1.0mm birch ply) It will look alot like what LP is building (crazy hollow-carbon-kevlar-balsa) minus ofcourse the carbon-kevlar-balsa.

with all this I should be able to drop the weight of the frame to 2/3 of what it is now… I havent weighd board number 1 yet as its still un-finished I’m fervently hoping it will remain under 10 kg (some 20-22 lbs?) I 'm goinna glue the second set of 5mm pine strips onto the frame tomorrow and steam bend the rest of the strips then too.

The outer rails for board number 3 will be as Paul J has sketched above (thank you Paul) .

By no means do I expect my board to be in the same range as LP or possibly even Paul… or then maybe…:wink: I dont know.

The fact that my boards will be unglassed I will have to try to keep in mind the tendencies of wood properties in my construction designs as well… That I think Will be my biggest handicap… And challenge too…

The original idea of this thread was: IS the central stringer truly necessary in HWS boards? well maybe it is… apparently depends on many factors; how much flex ,what kind of rider etc etc…? IF the stringer is not necessary for structural integrity: then loose it, save a little weight in the process.

The idea of loosing all the ribs and using 2-3 stringers instead is a nice one as well… might try that as well.

10

Hi Juho,

'Oil canning ’ happens when you have the frames two far apart or maybe the hull material is too thin, the skin dishes in between the frames, making your board ( or boat) look something like an oil drum. I was going to find a picture, but I got sidetracked by Flower Class corvettes ( if you ever read The Cruel Sea you’ll know what I mean) …alas.

I think that in general, you can’t have too many clamps. Especially for something like this.

Those little ( and inexpensive ) spring clamps like http://www.acehardware.com/product/index.jsp?productId=1290378&cp=1254873.1254946.1304505&parentPage=family&searchId=1304505 are pretty good. I’d wait til they are on sale, or Harbor Freight frequently has them so cheap it’s silly. That and the quick bar clamps like http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=46805 and I think you’ll have no problems.

hope that’s of use

doc…

11

Hi Tomway,

In the boat biz, when we have to glue teak, we usually give it a good wipe with some acetone first to deal with the oils.

Having said that, it’s usually solid teak, so that I’d maybe test it first, to see if the stuff gives the plywood glue any problems.

hope that’s of use

doc…

12

hi tomway,

Actually for board three Im using 1.5mm teak and mahogany veneer for the bottom skin + 1.5mm birchply+1.0mm birch ply for the deck I’m thinking of 2.0mm Teak /Pine/Jatoba+ 2.0mm birch ply… So teak isnt such a bad wood to use. after all boatbuilder prefer it over others for its qualities in resistance to rot. ofcourse the downsides is the weight and oilyness… I dont know about glassing, but generally with glues (epoxies and PVac) we wipe the glue surface to be (on teak) with acetone (cloth/rag: careful though ,its very flammable!) wait a moment for the acetone to evaporate from the surface (which it does very quickly) and then quickly slap on the glue. generally we always try to glue a “fresh” surface ie jsut clean/wiped with acetone or just planed (all woods) as the wood oils and resins seem to react in long term with the air an usually its not good for a good glue seam. Some adhesive brands have their own pre-treatment solutions/mixtures for oily woods as teak. The basic idea is to clean off the woods own natural oils that may act as an barrier against the glue penetrating on the surtace of the wood itself. For this reason we have NO aerosol products anywhere near the gluing areas (silicon, teflon etcs).

with my “non-glassed” boards I only use woods that have been traditionally ok in water, teak, jatoba, nordic pine etc. some woods like oak and ebony are just plain too heavy, though as veneers thay might be ok…

As far as I know… (not much mind youse) if the plywood is ‘marine’ grade, the plywood glue shouldnt be affected by acetone as long as you dont sink it in a vatful of it.

side note. today while by board was on the stand supports (deck and bottom on, innerrails and only 1 mm pine strip as outer railing) I carefully put weight on the board (dont worry, the epoxy has cured for over 1.5 weeks now) and the board seemed stiff as a solid wood plank! the supports were about 30 cm (1 foot) from the tips… pretty stiff Id say, absoluety no flex! though I didnt put all my 65 kgs on it yet.

Thanks doc,

on the oil canning term. I suspected as much but wasnt sure.

One never can have too many clamps. here at school I counted 64 normal sized clamps (not the over 1 meter models and the mini-mini ones) when I was gluing the wing spar for the aircraft a coupl years back, I pretty much used up all the clamps, the spar was 5.5m long. had to borrow an sturdy I-beam to use as a “table” to support it all… A lot of weight in all those clamps!An air-hose wouldve been lighter but would’ve been difficult to maintain an even pressure for 24 hours. plus alot more work building the jig! actually I was bordering on the same problem when gluing the deck and bottom skins, fortunatley epoxy doesnt require all that much clamping pressure (at least the epoxy Im using)

I’ll be needing alot of those small spring clamps with the frame for board number 3… thats still down the road some…3-4 weeks so theres time to scrounge the local “el cheapo markets”

will try to get new pix online during the weekend.

thanks for all the input thus far!

13

Juho and Doc: Thanks for the acetone advice. Gave it a brief test on some scrap bits of wood just now and didn’t seem to be a problem with the adhesives.

Not going to be turning the rails today. Last night I thought I’d crack open a beer and read through the hws threads again. 4 hours (and quite a few bottles later) I noticed the screen was getting a bit blurry. This morning my hands are shaking a bit too much to hold the planer with confidence! Didn’t realise there was so much to read!

Juho are you going to post some pics? Sounds like you are going to have some pretty colours with those wood combinations.

All the best,

Tom

14
Quote:

Last night I thought I’d crack open a beer and read through the hws threads again. 4 hours (and quite a few bottles later) I noticed the screen was getting a bit blurry.

(chuckling - been there, got the t-shirts, plural, and will probably be adding to my wardrobe regularly) you know, that sounds like a plan - snowing here, a fine powdery snow such that it’s a great night to kick back and relax. Have to say, I want to see how your boards go as well. Especially ‘work in progress’ stuff. and on that note… time for a beer doc…

15

Very interesting concept, however I’d like to point out one potential problem.

The 2nd moment of area refered to as the variable I has a very low value.

Second moment of area

From Wikipedia, the free encyclopedia

The second moment of area, also known as the second moment of inertia and the area moment of inertia, is a property of a shape that is used to predict its resistance to bending and deflection.

It is derived with use of the parallel axes rule. The second moment of area is not the same thing as the moment of inertia, which is used to calculate angular acceleration.

one of the most common shapes used in engineering is the I-beam. This shape has a very high I value (second moment of area) for its size.

The way an I beam works: bending stresses are greatest at the furthest edeges from the center of the beam. Thats where the flange at the top and bottom reside. There is lots of material there to carry bending stresses. Shear stress is vertical stress which is concentrated in the center of the beam, but is far less than the bending stress.

There is a vertical shear web in an I beam to handle the shear stress.

In a HWS, Bending stresses are handled by the skins, but by eliminating the spar, you have no structure to handle the shear stress in the center of the board.

Imagine if you take two parallel sticks and hold them an inch apart from each other and bend them. Imagine each stick as a representation of the skin of a HWS. The orientation of one stick compared to the other will change as they are bent. The shear web is the structure that will keep each part of each stick in the correct orienation to carry bending stresses. Try it out, attach a vertical shear web beteween the 2 sticks. It doesnt have to be strong, however the strength of the 2 sticks will be dramaticly greater because now they can work together.

In a stringerless HWS, I would fear that the rails will hold together fine, but the middle of the board could buckle in between the ribs because there is no shear web. Perhaps you could design a very simple light stringer which is designed to carry ONLY the shear stresses between the top and bottom skin, while letting the skin carry the bending stresses. Now imagine the profile of your board if you ONLY look at the shear web and the skins. You have a large flat I-beam. A great engineering shape!!

If you’re good with math, you can do a few calculations to find out exactly how strong your board will be.

http://en.wikipedia.org/wiki/Second_moment_of_area

16

thanks LP,

…So after the long explanation it comes down to “buckling” and then onto the term “oil canning” which is what you’re pointing out, right? I’m familiar with the basic design concept´but have never dwelled with the numbers, as I’m not very adept in complex math. The way I normally plan things is picture the design in my mind, slap a 2 x 4 on the side and if it works, fine! If its not strong enough , slap two 2x 4s on the side on the second time around. :wink: …

Anyhow, I wont be able to try the stringerless design for another year. After the trials of board number 3 (only 126 mornings left!) which will be the lightest in design which I’m going to build for this summer, I will know IF that design is strong enough… If so, then I will skim more weight off for board number 4… (maybe)

but thats all down the road…

I was going to steam bend some aspen strips for the perimeter today, but noticed that the strips were too short, so I need to make scarf joints instead.

17

If I may -

Rather than thinking of this as an I-beam, consider it as an internally stiffened box girder or as a monocoque structure with internal formers, I think that’s closer to the way it works in practice, one with a D-like cross-section.

Now, with the compressive and tensile actions of a wood/fiber/resin skin, consider how it’ll act. Let me run a little gedanken-experiment past you … assumptions inlude the structure being built as light as possible, structural members as thin as possible - little or no stretch in the skins under tension, some buckling and deformation likely in compression.

If the skins are bonded, at center, to a structural member, the initial stiffness is gonna be higher, no doubt about it. But when you get to where the shear loads on that glued joint to the center member are kinda high, the glue joint ( or the wood immediately adjacent to it ) is gonna fail, maybe crack along a fair length of the board.

Depending on the wood, you’ll then get buckling in that center member, a thin ply would probably be least prone to it. This will act on the other bits and pieces with unpredictable results. If the deck is built rugged enough so that it will accept ‘stomp loads’ then it’s not gonna buckle a lot.

On the other hand - if you go without the center member and subject it to the same stresses, what’s perhaps going to happen? The skins buckle a bit on the compressed side, between frames, maximum buckling on the deck, but again that was reinforced to accept the stomp load. The rails are in effect solid built up wood, they won’t move and if there’s sufficient bond between the skins and the rail structure that’ll act to keep it together. You might get a concave bottom for a bit as the center of the bottom flexed up under pressure/tension- it does, after all, have the longest perimeter/length at center so that’s the way it’s likely to go.

But note that there’s really low shear on all the wood-skin joints, possibly excepting the rails/skin joint. The glass layer over that substructure is in tension so that’d help too, tension is what fiberglass does best.

Dunno, will be interested to see how this goes…

doc…

18
Quote:
(chuckling - been there, got the t-shirts, plural, and will probably be adding to my wardrobe regularly) you know, that sounds like a plan - snowing here, a fine powdery snow such that it's a great night to kick back and relax.
Ahhh... sounds ideal - wood fire? Well, I'm definitely not going to have any beer tonight as I've got to get those rails done tomorrow. And the cold is no excuse for me. It's 25c here tonight!!
Quote:
The 2nd moment of area refered to as the variable I has a very low value.
On second thoughts, maybe there IS one in the fridge [unsure] ... Took a few photo's of my project this morning. Still at blank stage, immediately post gluing skins on. This one is of the deck side. ... and this one of the bottom (teak ply) and a bit of rail. You can see the uneven density of the meranti ply used in the rails. This all from a single sheet. There's alot of good things about living in Indo, but quality control isn't one of them! Selemat! Tom


19

Forgot to say. She’s a 6’7" x 19 1/2" x 2 1/4" single fin.

What are the dimensions of yours juho?

20

the intended dimensions are 6’4" x 22 x 2½"… I’m probably going to make boards 1-3 with the same external dimensions and rocker and shape… except for board 3 which will have a curved deck (camber) and some V on the bottom. This way I can hopefully ‘try’ to figure out aspects of board building as well as try out some different internal constructions.

oh yeah… single fin…no-glass.

link to early photo archive: http://www.dlc.fi/…/6_4_retro/index.htm