Seeking feedback on ski design

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13 years 3 months ago #3134 by Boof Head
Hi All,
I am seeking some feedback on the 2 designs in the attached file.
When I started playing around with KayakFoundary to design my own ski, I was after the following;
* similar stability to my Red7Surf70 (I'm 6'3"&100kgs)
* As fast if not faster than my Red7
* Ability to turn, which my Red7 can't do very well.
* Aiming for an overall weight of 17kgs. Will be strip built from paulownia or western red cedar.

MY PROFILE
I'm a recreational paddler who mainly paddles on flatwater but am spending much more time in the open ocean & riding waves. I participate in a race every now and then with an exit and entry through surf and will generally be in the mid-field. I plan on doing some longer downwind paddles eventually.

Appreciate all comments and if you want the actual design files, go to the KF forum at www.blueheronkayaks.com and look through the ocean racer thread.

cheers, Boof

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13 years 3 months ago #3135 by Boof Head
file was too big to attach. will join a file sharing thingo tomorrow and upload.

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13 years 3 months ago #3141 by Boof Head
www.4shared.com/file/98599147/a64551e1/MOJO.html

Also, if anybody has any drag statistics for a typical surfski over a range of speeds/knots, that would be helpful.

Cheers

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  • Brad
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13 years 3 months ago #3142 by Brad
Replied by Brad on topic Re:Seeking feedback on ski design
Hi
Looking good so far. Some food for thought below:
It looks to be a good deal more tippy than your Surf 70 - may need a flatter area under and behind the seat? Also, you could try a finer entry, reducing both the waterline and deck width at the footrest? I think current designs are well under 400mm wide at the deck and say 330mm on the waterline ...
Does Kayak Foundary give you a curve of areas? The LCoB seems to be about in the middle of the waterline. You could move it aft a bit, again to get a finer entry.
How high will the rails be with a surf ski deck? Check this to make sure it will stay dry, and also the height around the entry point so it isn't too hard to re-mount.
Cheers
Brad

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  • Brad
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13 years 3 months ago #3143 by Brad
Replied by Brad on topic Re:Seeking feedback on ski design
PS
Previous comments refer to MOJO.
I have some issues with MOJO1B - It is a fairly big departure from typical surf ski designs - it may not respond very well to the rudder ...

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13 years 3 months ago #3144 by Boof Head
Brad, I flattened out the bum section and raised the sheer line. Reentry height is same as RED7. I changed the plan view to better resemble a current ski shape. Regarding the LCB, I will probably place on a temporary deck and play around with seat positions untiL I get it right

What do you think?? I don't mind if it is tipier than a Surf70. I know they are fairly stable compared to V10, etc. I've paddled a surf70 pro and I reckon I could get used to something like a pro, v10, mako6 etc..

Here 'tis
www.4shared.com/file/98631615/39e390ed/Brads_MOJO.html

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  • Brad
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13 years 3 months ago #3145 by Brad
Replied by Brad on topic Re:Seeking feedback on ski design
Plan view looks good. Perhaps you could try a little more rocker in the stern, with slightly fuller sections to maintain the same bouyancy back there. Most skis appear to have more rocker in the stern than in the bow. I see the prismatic coefficient has gone down quite a lot since the last version - may need to have fuller sections in the bow and stern - could aim for something around 0.55? If the program allows different Cp for bow and stern, could go for aroun 0.5 in the bow and 0.6 in the stern. Regarding getting the trim right, I have found it possible with three boats out of three to get it right first time by placing a seat (or as a bare minimum a vertical board at your back) on a large board and ballancing that on a broom stick to find the distance between your centre of gravity and the back of the seat (or vertical board). For me it was exactly 300mm. Placing the seat so that my centre of gravity is right on the CoB of the boat seems to work for me.
Cheers
Brad

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13 years 3 months ago - 13 years 3 months ago #3152 by Boof Head
Cp is now closer to .55, less rocker in front. Can't adjust bow/stern Cp seperately. Screen capture doesn't show lengths which are 6.5M OA and 6.43M at the waterline
Almost there, what do you reckon??
Cheers
Last edit: 13 years 3 months ago by Boof Head.

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  • Brad
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13 years 3 months ago #3153 by Brad
Replied by Brad on topic Re:Seeking feedback on ski design
It's a hard call to know when it's done, and a big step to start committing it to wood. Wait for some more feedback - I'm no expert. Some further thoughts though: the waterline level at the tail on most ski's tends to be just touching the underneath of the stern. The V12 photo above for instance shows the stern rising a little higher (ie more rocker) than Mojo, I think. Also, I'm not sure if your design accounts for the thickness of the wood. If the wetted area is 2.17 square metres and the strips are 6mm thick then this adds 12mm to the beam and 13 kg to the design displacement, which could be significant.
It is often difficult to get exactly the design you want (whether your own or trying to replicate another) using free software programmes. You could have a play with some of the others (eg Bear Boat Pro, Delft Ship) before you finalise your design.
Cheers
Brad

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13 years 3 months ago #3160 by Boof Head
G'Day Brad, I've made some adjustments and have a design with stats that lie somewhere between a mako6 & V10. If Length/Beam ratio is anything to go by, then the Mojo design is more stable than both of them (unless i got that back to front??)

Thats enough for me to start building unless I've toatally missed something and somebody sets me straight.
KayakFoundary makes adjustments for strip thickness. I may even put a plywood deck on if that saves some weight. Did you use 3mm for your deck? and if so is it strong enough without glass?

thanks for he help,
Craig

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13 years 3 months ago - 13 years 3 months ago #3161 by Boof Head
I think I'm in the ball park?
Attachments:
Last edit: 13 years 3 months ago by Boof Head.

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  • Brad
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13 years 3 months ago #3165 by Brad
Replied by Brad on topic Re:Seeking feedback on ski design
I used 4mm, with glass on outside and carbon on inside, but probably overdid it. I am sure 3mm ply would be fine. I prefer to glass everything, not only for strength/stiffness but easier to get a good finish too.
Cheers

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  • Brad
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13 years 3 months ago #3166 by Brad
Replied by Brad on topic Re:Seeking feedback on ski design
PS have you considered using aircraft birch ply? It's expensive but very light and available in very thin sheets. Might use it one day to make my ultimate ski : )

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13 years 3 months ago #3171 by Hiro
Why not use two layers of balsa, cross-grained for more strenght.

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13 years 3 months ago #3172 by Boof Head
Havn't looked into the ply option too much but normal marine ply wouldn't conform to the current design of deck. was thinking that some "bendy" furniture ply with 4oz glass on top. might work. However its hard to beat strips for tricky curves.
Cheers

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13 years 3 months ago #3173 by YBA/Jim Murray
Final strength is not from the wood form, but from the sandwich construction of glass/wood/glass that gives you a sort of torsion box.
I would think you can have your 17Kg surfski but the seat and trim will have to be carefully looked at. Maybe the seat should be fiberglass or composite?
Two cloth layers should be put on the outside-I always used 6oz on canoes. Only one layer is needed on the inside, and that can be lighter cloth- maybe even 2oz.
I almost always built with western red cedar unless I could find eastern white-it is a bit lighter. You can use most anything, but as light as you can get while being easy to work.
Most of the amateur class marathon canoes (17') I made came out about 19Kg or so. That included all the trim, and that is where weight adds up.
It sounds like you are on a good track.
Finally, western red cedar dust is carcinogenic and can cause allergic reactions to some people. You need ventilation plus a mask.

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  • StuartXpat
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13 years 3 months ago #3178 by StuartXpat
Replied by StuartXpat on topic Re:Seeking feedback on ski design
All this talk of laminating with wood sounds great and I'm sure the end result is awesome. But what about the traditional carved foam blank method. People have built skis like this for years and I think its a lot quicker and cheaper than frame construction. You could still use a program like Kayak Foundry, just cut reverse forms to help with the shaping.

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13 years 3 months ago #3179 by YBA/Jim Murray
You can paddle the strip built boat and use it as a plug for a mold, if it works well.
I don't know how much time it takes to make a foam plug or if it can be paddled. A person can get pretty quick at strip building though. The design time for either would be the same.
Building can be an interesting challenge, but one can't paddle much while he is building- my attitude now
A lot of really good paddling innovations have been developed by guys like Boof Head and Brad fooling around in the home shop. Keep it up guys!

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13 years 3 months ago #3182 by Shaun
I used foam block to design a whitewater kayak and it was very quick and easy. I used 2" insulation foam and used it to cut the forms. I cut the first form the length of the desired boat, and then I cut forms for every foot or so (cut 2" out of the middle of each form) and glued those to the lengthwise form with oversized foam between them to make a complete block. Then I just sanded everything down to the size of the form blocks and I had my plug. I painted and waxed the plug, then layed up fiberglass over the entire plug, let it cure, then cut it at the seam to take it off the plug. Put the two halves together, and I had my first prototype. Any desired changes can be made to the plug, and another prototype made. Once you have your desired design, the prototype becomes the mold to make your boat. It worked very well and gave me prototypes that were simmilar weight to a finished boat.

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13 years 3 months ago #3183 by Boof Head
will be using Paulownia rather than WRC. A bit cheaper and a bit lighter although doesn't look as nice IMO. Have just come in from laying some strips down on a sea kayak that i have to finish before i can start ski ands I'm using WRC for that

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