Continuous curve vs curve-straight-curve

Discussions of proa and tacking outrigger design & technology not tied to a particular design. Anything from the accomodations to different steering solutions and wingsails.
RiskEverything
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Joined: 30 Nov 2015, 01:57

Continuous curve vs curve-straight-curve

Post by RiskEverything » 06 Jan 2016, 02:11

Is there any reason these boats need to have a continuous curve along the bottom and sides?

For instance, could a 40' proa be 20' of straight section with 10' of curved section on either end? Perhaps some other proportion would be more acceptable?

Could a 40' proa be stretched to 50' with a 10' section added in the middle. As in, if there were a 40' mold that was bolted together in the center, could a 10' straight section be added or removed to make a 50' boat from the same mold?

I figure that once they get tall enough for standing headroom, about the only dimension that really needs to change is length. A 40' boat with a 12:1 L:B would have 3'4" beam at the waterline. A 50' boat with the same waterline beam would have a 15:1 L:B. Both are acceptable.

It seems to me that 40' makes a good cruiser for two, but 50' could cruise four. That extra 10' is essential for fitting two double-bunks in the lee pod.
John
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Joined: 05 Aug 2015, 05:57

Re: Continuous curve vs curve-straight-curve

Post by John » 06 Jan 2016, 04:06

Intuitively I would guess that the higher the aspect ratio of the hull, the less noticeable the straight section would be. The angle of entry at the bow does make a difference for displacement hulls. So you would be giving up a little bit there. Your Cp would be higher, so might gain a little back there.

I suspect that Skip and/or Rob could put some numbers to this.

-John-

[I have been entertaining the exact same thought process with regard to boat length. (Without the straight section though.) Take a boat with the proportions of Jzerro and stretch it another 8 feet, but change little else. Get another two spaces in the lee pod and another 8 feet of cabin space. Don't change the width of the hull or the height of the cabin. I haven't run the numbers yet but I suspect that the displacement gained would outpace the weight added. Unless the extra length meant that the hull thickness needs to be increased...]
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Skip
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Re: Continuous curve vs curve-straight-curve

Post by Skip » 06 Jan 2016, 17:57

It is an apple and orange comparison in some respects but I have done some comparisons in cutting apart and adding a section to a multiman racing canoe versus a new boat to the same length. Theoretical differences in drag were less than 0.5% the new boat being slightly better. In this case it wasn't carried any further, the ruling authority decided that "unlimited" meant no more than six paddlers so the push for 7,8,9 or however many paddlers went away. Interesting enough another approach at the time was to design a boat from scratch with a removable section in the center so as to be able to mold a 5 man or a 6 man from the same basic layout (without a the center section being parallel). I wasn't able to to do the deal, any variation from truly fair even when working at l/b ratios of 24:1 or more were painfully evident.

In our proa case the parallel center section has some merit, little downside in terms of drag and easier implementation of interior, construction and use. Each to us own, I'd take a long hard look at how it would look before committing to a project.
John
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Joined: 05 Aug 2015, 05:57

Re: Continuous curve vs curve-straight-curve

Post by John » 06 Jan 2016, 19:27

Skip wrote:... I wasn't able to to do the deal, any variation from truly fair even when working at l/b ratios of 24:1 or more were painfully evident.
Skip,

Painfully evident aesthetically?

-John-

P.S. Good to see you around. I was beginning to worry that you had floated away...
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Rob Zabukovec
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Re: Continuous curve vs curve-straight-curve

Post by Rob Zabukovec » 06 Jan 2016, 22:38

I have to admit to being a continuous curve fan. Nothing in nature is dead straight, so it seems artificial to me to do it to a yacht. Unless you have a good reason like getting multiple sized boats out of one mould or are designing car carriers.

Sidecar was stretched from 8.5metres to 9.5 metres using the same cross section and profile. The ama stayed the same. Easy to do if you have CAD. And if I were to start Sidecar again, I would probably look at going to 10.5 metres. Length in a Proa is everything.
RobinBennett
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Joined: 08 Feb 2015, 22:36

Re: Continuous curve vs curve-straight-curve

Post by RobinBennett » 06 Jan 2016, 23:31

I'm sure you're aware that this is pretty much how the wa'apa is designed. So far I've only built the two bow sections, and the join is virtually parallel. The middle section is only 2mm wider in its middle than its ends. It might not be mathematically perfect, but it works well enough.
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Skip
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Re: Continuous curve vs curve-straight-curve

Post by Skip » 07 Jan 2016, 00:27

John wrote:
Skip wrote:... I wasn't able to to do the deal, any variation from truly fair even when working at l/b ratios of 24:1 or more were painfully evident.
Skip,

Painfully evident aesthetically?

-John-

P.S. Good to see you around. I was beginning to worry that you had floated away...
Yes, was just ugly, every time I've tried to take a shortcut in a fully faired boat it just as obvious as hell. Bear in mind that a true racer would do ugly three times over if it made his boat quicker. These guys mostly build their own boats and it's a lot of work. All else being equal they like for the boats to look nice (and fast).

Still around, no building or boating during the winter, did get a commitment from my friend John Wright to crew on the 2016 Texas 200 in June and I've signed us up. Probably not much going on til after the Everglades Challenge in March that John has entered again.

Skip
RiskEverything
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Joined: 30 Nov 2015, 01:57

Re: Continuous curve vs curve-straight-curve

Post by RiskEverything » 08 Jan 2016, 03:28

By "straight" I simply mean that the stations won't change from one to the next for those 10 feet. It wouldn't actually be slab-sided...

This *is* strictly to get two boats out of one mold. Actually, out of half a mold. Since proas are symmetrical around the midship I would design the hull to be symmetrical around the centerline as well. The cockpit and the lee-pod would be identical on the bottom. Make two pieces from the mold, spin one around, and 'glass them along the keel and bow. Saves money and space, costs a little more time and effort.

Since the straight section would be where the pod and cockpit are, they should help to "hide" the lack of local curvature in the hull...
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Amati
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Re: Continuous curve vs curve-straight-curve

Post by Amati » 10 Jan 2016, 02:18

The main downside of this shape, AIUI, is that it accentuates the wave resistance hump. Although at higher Bruce numbers the Prismatic might be cool.
old school
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Re: Continuous curve vs curve-straight-curve

Post by old school » 10 Jan 2016, 17:53

If you are aiming for Pc as high as 0.7 it helps to do this.

Aesthetics ? .......whether a boxy slab sided hull looks any better than a curvaceous one with a straight run midships, is merely a matter of opinion.
-Jeremy
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