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View Full Version : Design this...Build that. - Argh!



Saint
01-04-2001, 12:09 AM
I have designed a 16'kayak with Chine Hull designer. (Hard Chine, 16'x22", 280lb dismplacement.) After committing to a version that I felt was as good as it was going to get, I started on the hull. I am ending up with a different boat than I drew. My bottom panel is tenting higher (Highly technichal boating term <grin> )(assume the hull is upside down) than I have drawn. As a result the hull has a draft at the center that is .8" deeper than the plans indicate.

1. Should I have taken the panel thickness off the depth of the side height (reduce the side panel height by 3/16")? I realize that this is very much an R&D effort and that I am gambling with the results, especially since this the first hull that I am building this way. I am just curious how acurate one can expect to be in replicating a S&G design from plans. To my untrained eye, I actually like the lines that I am ending up with in the wood, The problem is that I end up with a descrepancy between the panel shapes and the resulting hull. Does it really come down to a choice of either redrawing the hull that results from the panels or torturing/modifying the panes to come up with the hull that I have drawn?

2. The real question - when do you stop building to the plans and when do you start building to the wood/boat? OR should the plans really describe to within 1/16" what the hull should be? (This post is not a critisizm against the program.) I am more curious when one lays the plans aside and continues with the boat as per say, rather than try to torture the boat to be identical what someone cooked up with paper and pen? I will end up with some numbers on the plans that are hard and fast (distance of bulhead from bow (my zero point)) and others that I have to take off the hull now (the actual station dimentions). I don't see how I could expect anyone else, let alone me in a few months or years, to build another hull from these plans if they don't know which numbers to believe.



[This message has been edited by Saint (edited 01-04-2001).]

G. Schollmeier
01-04-2001, 01:36 AM
Saint, Just stiching panels together will not give you the hull you drew. No matter how accurate your panels are. A small change in your beam will make a big change in rocker. Even with S&G you need spreaders to maintain your beam. Good Luck Gary

Ian McColgin
01-04-2001, 09:18 AM
While I have some NA friends who use some pretty slick programs & had a little fun some years back manipulating one, I don't know your brand out there. But a likely guess is that the program draws the same way old time lines were drawn - to either the inside or the outside of the planking - and it sounds as if you might have mixed them up. At a guess, perhaps the program went to outside dimension and as you built you measured from inside. This would explaine how you got a bit more deadrise and hull depth, since the error is absolute (panal thickness) and thus porportionally far more serious to the girth than the length. Perhaps you got the beam fixed, so the slightly oversized panels pushed things down a bit.

And you're right - it probably does not matter. Gives you bouyancy to add maybe a nip to your maximum load.

Even when you follow plans stuff goes outof whack. In the early '60's my home yacht club said I could be instructor for the nacent kiddy program next summer and we'd need some boats. A committee settled on the (now ubiquitous) Optimist pram as the perfect group project. True to stereotyping of the times, the fathers would build the hulls and the mothers would make sails.

I coopted the HS shop to make all the wooden parts - panals, chine logs, gunnels, knees, everything - for a dozen boats. All as identicle as my eye (and the eye of a good shop teacher) could get them.

Came the building party. Perhaps fluids a bit less viscous than the glue had something to do with it, but no two boats were quite the same.

Paddle on.

Ian McColgin
01-04-2001, 10:08 AM
I cut it. And cut it again. It's still too short.

Ross Faneuf
01-04-2001, 11:04 AM
After I had 'Ceol Mor' planked and upright (she is cold molded) Dave Swift asked me if I had enough guts to measure each side from a centerline. I did so, and told him the measurements were within about 1/2" of each other (11' beam). He then let me in on the secret that he rarely did better than 1", and that most other builders would tell me the same story. As long as the boat pleases your eye and appears to have the characteristics you want, I wouldn't agonize over it a whole lot.

This doesn't just apply to smaller wooden boats. I dimly recall a story about the old 'Queen Mary' that there was some issue about whether she was the longest ship in world or not (some other rival measured fractionally longer) so the remeasured her and found an extra couple of feet! Now anyone who knows the real story can jump in and get it right...

Art Read
01-04-2001, 02:55 PM
My "Dark Harbor" may belong to a One Design class, but no damn "rules" measurer is EVER getting near my boat! (Be kinda fun to measure all those Havens out there and see how much they "differ". They're all still lovely and are a joy to sail, I'll bet!)

[This message has been edited by Art Read (edited 01-04-2001).]

Saint
01-04-2001, 04:10 PM
I had figured that I had something wrong with my thinking re the inside of the plank/outside of the plank, but my beem is on the mark within an 1/8th. I think I just forgot to take that inside/outside into consideration when allowing for the plank overlap at the chine.

I guess custom built boats are more 'custom' than we think.

Jim Hillman
01-04-2001, 04:11 PM
I found that, while putting together a SNG pulling boat with my father, the ply panels will not behave the way you want them to without temporary molds. She came out O.K. but a little to flat amidships and a little too "V'd" on the ends. Still it rows pretty well. If you're still at the wiring together point, you could loft a couple of temp. molds and screw them in place until you've finished the bottom.

Jim

Andrew
01-05-2001, 11:57 AM
I miss read your post as 8". At point 8, I'd forget about it.

fair&fair
01-12-2001, 06:52 PM
I have just one question: how could your kayak displace 280 lbs?

Tim S
01-12-2001, 07:57 PM
Fair & Fair'
I,m hopefully thinking the 250 lbs displacement is when there is a person aboard..otherwise maybe he's building with iron bark!

Don Maurer
01-13-2001, 11:27 AM
It's quite easy for a kayak to displace 280 lbs. Saint is talking about displacement at the design water line, not the weight of the boat. These two specs are often confused in published data, especially for small craft.

My kayaks are 18' 6" LOA x 22" beam. They weigh 45 - 50 lbs, but the displacement is about 275 lbs dwl, which means they need to carry about 225 lbs of crew and gear. I weigh about 195 lbs and typically paddle with 20 lbs of lead ballast. Without the ballast, the boat feels tippy and doesn't turn as tight.