Stretched Flying Saucer Build

Outboard designs up to 14'

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Andy Garrett
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Re: Stretched Flying Saucer Build

Postby Andy Garrett » Thu Oct 31, 2013 6:18 am

I used the Rabl method on mine and had some issues. On paper, it looks very straight forward and intuitive, but in reality, it will create a hull shape which differs from the plans.

Despite what the book says about plywood boats being built without compound curves--only being sections of a cylinder or a cone, that's not exactly true. I built the Zip which has the same hull as the FS. If you look at the two forwardmost frame sections, they are (or should be) rounded from the point at which they connect to the keel and the point they connect to the chine. This would be fine except that at the same time, that very plane (where plywood will eventually be affixed) is also curving from the flat section of the hull-bottom to the bow. It's not much, but it deffinately exists.

As I started to set up for the Rabl method, I spaced the notches at roughly equal distances along the keel and the chine. I just eyeballed it because the keel and chine are different lengths back to the first frame and even back to the transom. I found that it made more sense to equally section those differing lengths an equal number of times so each notch had a counterpart to join with. I found that a yardstick was excellent for this notch setting. It was when I encountered the frames that it became clear that I had gone wrong. I laid the yardstick flat across from the keel to chine right next to the frame(s) and saw instantly that significant material would have to be removed from the frame to get it 'flat' between those to points of fairing. I understood that this would alter the shape of the hull a bit, but I was bound by what I thought was logic--that it must be flat there because the plywood would be bending in a direction 90* to that. I was convinced I was right because plywood will not bend in a compound curve, right?

Later, after applying the plywood, and flipping the hull, I found gaps between the plywood and the frames where I had flattened them. I was shocked. The plywood actually wanted to bend into a compound curve to the degree that it stood out from the frame unsupported after being glued and screwed all around.

I filled the gaps with fairing putty and walked away a bit wiser. I wish now that the Rabl method had never been presented in the book. I wish I had used a sheet of ply to guide my fairing efforts rather than a yardstick which just has too little surface area to offer adequate feedback.

In the end, it all worked out, but I would spare you that lesson. Avoid Rabl on this particular hull.
Andy Garrett

Perhaps the slowest Zip build in Glen-L history...

polarisman14
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Re: Stretched Flying Saucer Build

Postby polarisman14 » Thu Oct 31, 2013 11:06 am

Andy,

Thank you for your informative response. I think I'll probably use the Rabl method at large intervals to get a rough shape and use the plywood per your suggestion to get the final shape of the bevels correctly. To answer my question though, would your spacing of the Rabl notches on the chine and keel be equal from the intersection to "A"? Like from the chine-to-stem intersection to point "A" on the chine, and chine to stem intersection to point "A" on the keel be (arbitrary number) 20" or would they differ but be the same on their respective A to B and B to C, and so fourth?

Thanks,

Matt

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Andy Garrett
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Re: Stretched Flying Saucer Build

Postby Andy Garrett » Sat Nov 02, 2013 7:23 am

The answer to your question is in the diagram you posted, though not very clear.

Measure the absolute distance from the bow to the transom along the stem/keel and divide that number by the number of notches you wish to make. Example: if the distance is 16 feet and you wish to cut 8 notches, you will space them 24 inches apart.

Now, do the same thing along the chine, and whatever the distance, you must also divide by eight. Let's say for example: The chine is 15 feet, and you again divide by 8 because each notch along the keel needs a corresponding notch along the chine. These chine notches will only be 22.5 inches apart.

The key is equal spacing which is uniform between keel and chine even if the distances between notches are different along each path.

Does this help?
Andy Garrett

Perhaps the slowest Zip build in Glen-L history...

polarisman14
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Re: Stretched Flying Saucer Build

Postby polarisman14 » Sat Nov 02, 2013 5:22 pm

Absolutely. That is just what I was looking for--so the key is equal number of notches on both members, and equal spacing between each notch on each member.

I got my planer in the mail yesterday and it works slick. Just did some experimenting on the chines and the results are excellent. Can't wait to go further with this once I go Rabl on this boat's ass.

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Andy Garrett
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Re: Stretched Flying Saucer Build

Postby Andy Garrett » Sun Nov 03, 2013 12:41 pm

Good luck!

I hope you meet with more success than I did.
Andy Garrett

Perhaps the slowest Zip build in Glen-L history...

polarisman14
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Re: Stretched Flying Saucer Build

Postby polarisman14 » Tue Nov 05, 2013 6:57 am

I did some fairing on both chines to get used to the planer since I knew there was plenty of material that had to be removed from them. I need to do the Rabl style measurements and make some notches before I go any further so I don't create more work for myself.

At this point we'll call it an even 80 hours!

polarisman14
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Re: Stretched Flying Saucer Build

Postby polarisman14 » Mon Nov 18, 2013 10:24 am

No actual updates, but let's call this a "update about an update, soon." Today I will be using a cloth measuring tape and getting the chines, sheer, and keel marked for the notches to be filed into them for the Rabl method. I may even start in on filing those notches if I have time. I am thinking of doing 20 sections per frame just to make it easy for fairing.

A friend of mine is coming over on Wednesday and we are going to put a couple hours into fairing together, me with the electric hand planer and him with the belt sander and some coarse grit on the opposite side. Hopefully by the end of the night Wednesday the boat will be at least ready for side planking if not bottom planking as well. I think I'll get a lot of my motivation back once I am in a position to apply fiberglass and paint and make the flip.

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jprice
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Re: Stretched Flying Saucer Build

Postby jprice » Mon Nov 18, 2013 11:12 am

A little late to this discussion here, but my experience was the same as Andy. Rabl seems straightforward, but in practice doesn't always work out well. I'll be interested to hear how it ends up working out for you.
Thankfully, thickened epoxy in a caulk tube took care of any of the gaps after the flip. :)

polarisman14
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Re: Stretched Flying Saucer Build

Postby polarisman14 » Mon Nov 18, 2013 2:06 pm

That's a great point and I'm sure I will end up doing that. Maybe I'll spring for the 3m 5200 stuff when it comes time as that's a structural adhesive and can also work to fillet areas, along with being sandable and paintable.

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jprice
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Re: Stretched Flying Saucer Build

Postby jprice » Mon Nov 18, 2013 2:20 pm

Yeah, I suppose that could work, but would be a bit pricy. Although I don't think it fully hardens. It stay's rubbery, doesn't it? Not sure I'd want any flexing (or the ability to flex) in those areas.

Anyway, I'm a big fan of the west system empty caulk tubes. $3 for a 2-pack. :D

polarisman14
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Re: Stretched Flying Saucer Build

Postby polarisman14 » Tue Nov 19, 2013 6:57 am

Sweet that works. The place that I bought the wood from also sells West System epoxy products so depending on their pricing and availability I may go with their stuff. Who knows, maybe we'll be able to work out a package deal price or something.

After making the marks for the Rabl technique I can see how you guys say it doesn't work, at least for the back half of the boat with the side planking. There is a constant curve from top to bottom so beveling everything so that the straightedge would lay flat would cause the sides of the boat to be completely flat and therefore not have the tumblehome that the transom has.

So, for the most part I am just making the notches on the frames from frame 0 to frame 4, ahead of that I am using the Rabl technique as the vertical curvature is barely there in the front of the boat but the wood has to curve severely from the side-in to meet with the chine.

Does this make sense?

gdcarpenter
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Re: Stretched Flying Saucer Build

Postby gdcarpenter » Tue Nov 19, 2013 3:27 pm

I don't know rabl from rubble but my ZIP ZIP HOORAY seems to look fine.

Yes, cut all the 'notches' where the sheer and chine meet the frames.

Start at the transom, the easiest place to begin, simple curved tumble home. Always have a 'scrap' piece of hull ply, say 6" or so wide, and long enough to span sheer to chine, and keep bending this ply sheer to chine to once you 'picture' how the ply needs to go on in a smooth curve you can tell how to fair.

Kinda like sculpting'David' out of a chunk of marble, just remove what doesn't belong there.

I used nothing but constant test bending the ply over the framing, sitting and thinking, often beer in hand, then removing some more material. Get you feet wet at the easier back end and by time you get to the front you will likely have a better feel and understanding of what to do. Good luck.
This is my first, last and only boat build.

http://www.gdzipbuild.blogspot.com

polarisman14
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Re: Stretched Flying Saucer Build

Postby polarisman14 » Fri Nov 22, 2013 6:57 am

Yeah I'm about a quarter of the way done and it is coming along well. I am just having problems picturing how the "shoulder" progresses as it moves from frame 4 forward to frame 5 1/2.

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Gayle Brantuk
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Re: Stretched Flying Saucer Build

Postby Gayle Brantuk » Fri Nov 22, 2013 11:48 am

The Zip and Flying Saucer are designed using Conendric development. With the Rabl method of fairing, the spacing of the points on the keel/stem and chine must be equal. The frames have nothing to do with this process. You are using the marks to determine the bevel required in the keel/stem and chine so the plywood will mate to these areas.
http://boatbuilders.glen-l.com/3820/fairing-by-the-rabl-method-2/

polarisman14
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Re: Stretched Flying Saucer Build

Postby polarisman14 » Fri Nov 22, 2013 12:07 pm

^^Gail,

I see what you mean--I was just saying that starting at the transom and moving forward that it's around frame 4 that I am having issues seeing what the fairing shape in the wood is supposed to be doing.

So what you are saying, is that regardless of the difference in length of the keel vs chine or chine vs sheer, the spacing between the rabl markings must be equal (IE you have 10 points marked out on the chine, sheer, and keel and the distance between these points is a foot on each one, etc.?)


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