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PostPosted: August 3rd, 2007, 1:51 pm 
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Location: Belleville, ON
CG should be slightly at or slightly aft of the centre of buoyancy. Its easier to paddle and steer a boat that's trimmed slightly low in the back than one that's low in the front.

Your dotted line position looks better to me (missed it in first look). Like the load "pad" idea, although it can probably be quite thin/light and still be very effective. Even a couple extra layers of glass would be a big difference.

Personally I'd place it permanently for the no load position and distribute the load as necessary when paddling with a load.

As for carbon in the lathe blank I'd have it just in the centre with a good 3/8"-1/2" material around it on all sides so that it would never make contact with the chisel on a lathe. It'd take the fine edge off the chisel even if it didn't grab or misbehave otherwise.

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PostPosted: August 3rd, 2007, 3:00 pm 
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Rapt wrote:
CG should be slightly at or slightly aft of the centre of buoyancy. Its easier to paddle and steer a boat that's trimmed slightly low in the back than one that's low in the front.


You read my mind!!!

Also worth mentioning, trying out the Wee Lassie I was struck by how sensitive such a small boat can be to load distribution. But when out alone, with no wall-size mirrors to paddle past, it ain't easy to see just how the boat is floating in the water. My hunch is that it was slightly stern heavy when I liked it best but still don't know for sure. Similarly, when canoeing with my wife in the tandem we put all the gear pretty much in the center, then my 140 lbs in the stern seat and her 110 in the bow seat, which is further abaft the fore stem than the stern seat is forward of the aft stem. We always felt good about the trim.

Re. the load pads, maybe the best course is to plan on a season of trying out. I could stick a foot or so of industrial velcro directly to the hull, both sides, and carve a single "socket" directly into both pads. Changing the position would not be the pinnacle of convenience, but that would encourage finding a single position that worked well for both paddling and portaging. If such a position could be found and proved satisfactory over a season of use then I could remove the velcro and secure the pads in a more permanent fashion, either double-sided, thin, closed-cell foam tape or even epoxy if my confidence level warranted. Even epoxied it wouldn't be too big a job to shave them off a slice at a time. The single-socket pad would have the added advantage of being sized solely for load distribution--no need to add length and weight for alternative positions. Methinks perhaps 2" wide by 4" long, thickness of zero along the outboard edge and as necessary for a level surface on the inboard edge. This is comin' together.

bob

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PostPosted: August 22nd, 2007, 10:47 am 
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Finally got the data on layup strengths and weights as published by Gougeon Bros.

Posted in its own thread.

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 Post subject: Negative Progress
PostPosted: September 20th, 2007, 1:09 pm 
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The pursuit of my dream solo tripper has suffered a serious setback. I discovered a substantial blunder in calculating displacement. The error is so great that tweaking the design-in-progress is simply not practical to meet my goals. I'm left with abandoning Rushton's Wee Lassie as a design departure point and coming up with a hull shape that displaces more water below the load waterline without significantly increasing total hull area, and weight, or producing an unsafe or hopelessly sluggish boat.

The solution that I'm looking at increases Wee Lassie's beam at the lwl and is much fuller below water--more "arch" and less "vee."

Lines and specs are at the following addresses. *.bmp, *.dxf, and *.vlm are produced by Ashlar DrawingBoard ver. 1.00 and show lines of the new design and comparisons with Wee Lassie and my initial adaptation. *.xls and *.ods are produced by OpenOffice.org Calc and give offsets and a few calculations based on them.

Addresses:

http://www.vermontel.net/~bobmoran/HUI-7.BMP
http://www.vermontel.net/~bobmoran/HUI-7.DXF
http://www.vermontel.net/~bobmoran/HUI-7.VLM
http://www.vermontel.net/~bobmoran/Hui-7.ods
http://www.vermontel.net/~bobmoran/Hui-7.xls


Comments and suggestions will be greatly appreciated. I really don't want to build a sea anchor.

Bob

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Last edited by Graybeard on September 20th, 2007, 1:16 pm, edited 1 time in total.

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PostPosted: September 20th, 2007, 1:16 pm 
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I really don't want to build a sea anchor.


Or a log to roll I expect.


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PostPosted: September 20th, 2007, 1:21 pm 
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Right!!! ....or a log to roll!!!

BTW, if you tried to open the files I may have been fixing the URLs. I think they work now.

bob

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PostPosted: September 20th, 2007, 1:30 pm 
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As a general note, increasing beam increases wetted area faster than increasing length as a way to increase displacement. (Assuming similar section and prismatic co-efficients.)

But your lines look pretty good to me. Although I'd be tempted to have a smidge more rocker than you have shown (I see none in your drawings and I'd opt for something like 0.5"-0.75") Also I'd go for a somewhat fuller prismatic co-efficient that is somewhat fuller curve in the plan view fore and aft and and slightly less of a "kink" at the midships point and with slightly less max beam... Its hard to say because everyone's tolerance for stability and tracking are different. :)

Regardless I don't think your plans show a sea anchor, but it wouldn't be as fast as something a little longer and leaner...

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PostPosted: September 20th, 2007, 2:15 pm 
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Rapt,

Agreed, increasing beam increases wetted area, and displacement, faster than increasing length. It was the increase in displacement that I was after. I was also keeping in mind the ratio of total hull area, and therefore weight, to displacement.

The filling out of the ends of the below-water waterlines is a good suggestion. Doing so will increase displacement which could be compensated by a slight slimming of those same lines amidship.

I hesitate about introducing rocker. (You're right; there's none.) Keep in mind that this boat will be 12' or less in length and therefore more maneuverable than a boat three or four feet longer--and it will be powered with a double-blade so the advantages of rocker in turning heeled over won't be readily available. What would you expect rocker to contribute to this boat?

I agree that longer and leaner would help it win races on the water. On the other hand, short and fat will be lighter for a given displacement and therefore faster on the portage (where few races are held.) :D

bob

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PostPosted: September 20th, 2007, 2:43 pm 
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Graybeard wrote:

The Nymph does look interesting; much like a cross between Rushton's Sairy Gamp and his Wee Lassie. I also find it interesting that he has gone the further step of reducing strip thickness to 1/8".

In a boat this size, total weight of wood in strips 3/16" thick would range from 8.4# with White Cedar to 11.04# with Redwood. Lopping off a third of that by using 1/8" strips would be significant and worth considering if the end user can be trusted to treat the boat with adequate care and caution.

Bob


I saw Nick's prototype for the Nymph at the Wooden Boat Show in July. It is really sweet looking and feels like a feather when you pick it up. Plus, the Kevlar/graphite cloth inside looks so cool. I'm going to build one for my wife when he starts selling the plans next month.

Word of caution, however. He places his station molds at 10" rather than 12" with the 1/8" planking. He says it wouldn't lie fair otherwise. That means you will have to re-draw the body plan for 10" spacing and build a couple extra stations molds. Also, the bead and cove edges won't be a possibility, so you will have to bevel the strip edges by hand with a plane. Not that hard, but it will need to be done.

I'd go with the white cedar. You should be able to get it easily out your way. It is significantly less dense that WRC and is less prone to splitting. You'll encounter lots of knots, but the stuff can be had so cheap that you can just cut around a lot of them, or scarf them instead..

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PostPosted: September 20th, 2007, 3:22 pm 
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Well...

Even at 12' it'll be a bear to make turn without some rocker especially if you aren't going to heel it like a kayak... Rocker makes it easier to turn without heel, not the other way around. If you have no rocker then you heel the boat to get the side curve to simulate rocker while you're turning.

If you look at this link http://www.placidboatworks.com/SpitFIRE.html you'll see this 12' boat has 1.5" forward and 1" rocker aft, and using a double blade paddle novices have no trouble with tracking yet it turns fairly nicely when desired all while keeping it perfectly level.

As for the rest, longer and leaner is also about efficiency while paddling. You wanted to avoid a sea anchor, so I suggested you want a longer boat for ease of paddling, unless you intend to paddle very slowly...

Even for novice/slow/weak/relaxed paddlers 12' LWL is really shorter than optimum. For more experienced paddlers its easily pushed into a much less efficient realm. 14' is a lot closer to optimum if you can tolerate the trade in stability that the narrower beam gives. And even at 14' its not super narrow.

Reduced wetted area is reduced hull area and if you're really pushing the weight, total hull area can easily be reduced by lowering the shear a bit in areas where its not necessary for seaworthiness...

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PostPosted: September 20th, 2007, 3:24 pm 
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Hornbeck boats (http://www.hornbeckboats.com/index.htm) also make Rushton-like boats out of modern materials. My wife loves hers because of the light weight and it easily carries her and about 100 lbs. of gear (never tried anything more, but I don't think we've maxed the boat out).


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PostPosted: September 20th, 2007, 7:13 pm 
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Rapt,

OK, I'm convinced. Like my design, I'm not easy to turn but given enough convincing argument I do eventually come around. It'll get some rocker. And thanks for not just throwing up your arms in disgust. BTW, do you think that the introduction of a modest amount of rocker warrants adjustment of all the stations and waterlines or am I safe simply introducing rocker with shims on the strongback as Ted Moores suggests in Canoecraft?

Spitfire is indeed a very appealing boat to me. Many of it's features are in fact very similar to what I've tried to design into my own. The biggest difference is in boat weight, Spitfire coming in at 23 lbs while I'm aiming at only marginally over 15 lbs. To reach my weight target I'm sacrificing strength/durability to an extent that a commercial builder would probably not dare put on the market for fear of getting a bad reputation for poor durability. If I can I'll get up to Lake Placid and take a look.

I agree with you entirely that a fast design is also a design that moves at moderate speeds with less effort, i.e. is more efficient. By the same token, however, as speed decreases the influence of hull shape decreases and the portion of total resistance attributable to friction, a function of wetted area, increases. In the final analysis, hull design has always been as much art as science, as Chapelle concludes at the close of his magnificent book The Search for Speed Under Sail.

On the question of length, I don't disagree with you about the performance of a 14' boat compared to a 12' boat while keeping hull sections identical. But those two extra feet add more than two pounds to boat weight, even with my minimal weight-per-square-foot layup. Taking two pounds from freeboard would put the boat below my comfort level. At 67 slow is not a new experience--I'll take it over discomfort. Anybody wanting to stretch it can simply enter a new value in the "station spacing" cell of the spreadsheet I put up and see the consequences in displacement, hull weights, and wetted surface.

Again, thanks for your input. I do appreciate and respect it even if I don't always incorporate your suggestions.

bob

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PostPosted: September 21st, 2007, 8:01 am 
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No worries... I understand that part of designing your own boat is that you are designing it and no one else. :)

As for adding some rocker I'd probably just tweak the station heights if its less than an inch you're adding to a symmetrical hull. You can do this either by an even curve from one end to the other, or by keeping the centre portion flat and curving the ends. I'd be tempted to do the latter... say divide it into thirds or thereabouts...

The Spitfire's weight is a little higher than your aiming for, for sure, but it is a bomb-proof hull for flatwater in my opinion. I wouldn't do serious white water with it, but lets just say that there's no discernable flex even when I get in and try to push it hard. So its much tougher than absolutely necessary.

Something like Nick Schade's Nymph is more likely to give you what you'd like... Or use foam core construction... It'll give greater stiffness for the same weight as wood or less weight for the same stiffness.

I wasn't suggesting same sections just stretched, but rather the same sectional co-effcients but with the length, beam and draft tweaked to maintain displacement... Easily done in a design program, like Bearboat that is co-efficient driven, but a "bear" to do manually...

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 Post subject:
PostPosted: September 21st, 2007, 1:10 pm 
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Battenkiller wrote

[quote="Battenkiller Also, the bead and cove edges won't be a possibility, so you will have to bevel the strip edges by hand with a plane. ]

Just curious why you would think bead and cove would not be possible?

Sincere query...

Sundown


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PostPosted: September 21st, 2007, 10:25 pm 
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Sundown wrote:
Battenkiller wrote

[quote="Battenkiller] Also, the bead and cove edges won't be a possibility, so you will have to bevel the strip edges by hand with a plane. ]

Just curious why you would think bead and cove would not be possible?

Sincere query...

Sundown


On my Wee Lassie, I used 1/8" strips. I tried to bead and cove, but it didn't take. If I would have really played around with it, I might have been able to get some kind of bead and cove, but it wouldn't have been pretty.


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