Preliminary Boat Design Geekery
Nov. 16th, 2010 05:08 pm![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
catsittingstillWhen last we left our intrepid boatbuilder, I was seriously considering two designs--the Vayu, a Rushton model, and the Bear Mountain Rob Roy.
I was kindly made free of the good drafting table at Carson-Newman (I think the art instructors are hoping some sort of "monkey see; monkey do" process will take place with some of the art students when they see it in use) and have played "set down the dots" and "connect the dots" with the offsets for the Vayu as taken by Orvo E. Markkula in 1967.
The Vayu plainly has a shallow Vee hull, which I like (at least I liked how it worked out in the Wee Lassie II.) A shallow Vee hull tracks well, and tends to be fast (though a shallow arch hull tends to have less drag than a shallow vee.) The Vee may be more shallow than Constance's --if not this model would be a bear to turn, except that I'm going to shrink it anyway. The thing about the Vee is that unlike the Wee Lassie II, where the Vee melts into a gentle curve that continues, getting gradually looser, all the way to the gunwale, in the Vayu the Vee goes up for a while, uses a modest curve to turn a corner, and then basically goes straight up to the gunwale. I don't know how that will behave when it's tilted so the gunwale is near the water.
The Vayu has minimal rocker (about 5/8 of an inch if I'm remembering right--the drawings are still at CN.) It has a lot of sheer, though, which a boaty way of saying that the bow and the stern sweep up a lot higher than the center of the boat, so the gunwale will have a (very shallow) U shape. On the one hand, sheer should mean that waves from the end are less likely to come in the boat and cozy up to you, so the boat handles waves better. On the other hand, sheer gives a lot of impromptu sail area for the damn wind to play with, making the boat harder to keep on course in a wind. (It will probably slow down in a headwind, too.) I'm really not sure how these characteristics will interact, but the stems of the Vayu are like 20 inches tall, when the middle is about 11. Which made it a pain to draw, too--my husband's 18 x 24 drawing paper wasn't big enough and I had to cut an unused corner off the sheet for the stations and transplant it onto the sheet for the stem to make it big enough.
If I go with a stem that high, it will be a pain in building too--either adding extra strips above the first strip to build up those extra 9 inches and then trying to cut them freehand to a smooth curve, or bending the hell out of the ends of the first strip and then using lots of cheater strips--and every single cheeter strip is four long skinny triangles to saw and, more to the point, fit without gaps, which isn't easy. Hmm. Or I could just lower those stems. That would work too. If I decide to do the Vayu I'll probably go in and multiply all the figures for height and width by 0.9 or so, and multiply the inter-station distance by 0.9 as well. I understand that you can run into issues doing this--a 1/10th scale model doesn't sail the same as the full sized boat, for example. But I suspect that cutting everything by 10% will probably work--and the Vayu is too much boat for me at its current size.
Also, one of the measurements for at least one of the stations can't possibly be right. I've had dots I had to leave out of the line, but the line usually passes within a sixteenth of an inch or so of the dot (though you'd be surprised how much that can change a profile.) But this one is off like half an inch. If it's real, then the stations nearest the bow and stern have half an inch of tumblehome in a really tight curl. The thing is for that to happen, the gunwales would have to twist 45 degrees in 22 inches, and then the same amount back in the next 22 inches. I just can't see that as being either possible to construct, or a reasonable nice looking design. It would just be weird. So at least one of those points is wrong and I will have to decide which to leave out.
The other design I'm checking out is the Bear Mountain Rob Roy. I've only drawn the first three stations of this so far, and I'm going to need to do 11, because unlike the Vayu, where 1 profile makes 2 stations except for the middle station, the Rob Roy is asymmetric and I don't get any 2-for-1 stations. Also it looks from the book like there should be 2 more stations right near the ends of the boat, but they don't give any numbers for them, so it's hard to be sure.
The way Bear Mountain is doing their offsets is damned confusing, but I think I have it figured out. They give their numbers as feet-inches-eighths. So 13 5/8 inches is written 1-01-5. And I have figured out that when they have a + by their number? They mean add a sixteenth. Ooookay.
At least one more day, maybe two, of lofting. It's not hard, but it takes time, and the darn cement floors in the Art building are making my knees ache.
Note that there will be more boat design geekery when I have finished lofting the Rob Roy.
I was kindly made free of the good drafting table at Carson-Newman (I think the art instructors are hoping some sort of "monkey see; monkey do" process will take place with some of the art students when they see it in use) and have played "set down the dots" and "connect the dots" with the offsets for the Vayu as taken by Orvo E. Markkula in 1967.
The Vayu plainly has a shallow Vee hull, which I like (at least I liked how it worked out in the Wee Lassie II.) A shallow Vee hull tracks well, and tends to be fast (though a shallow arch hull tends to have less drag than a shallow vee.) The Vee may be more shallow than Constance's --if not this model would be a bear to turn, except that I'm going to shrink it anyway. The thing about the Vee is that unlike the Wee Lassie II, where the Vee melts into a gentle curve that continues, getting gradually looser, all the way to the gunwale, in the Vayu the Vee goes up for a while, uses a modest curve to turn a corner, and then basically goes straight up to the gunwale. I don't know how that will behave when it's tilted so the gunwale is near the water.
The Vayu has minimal rocker (about 5/8 of an inch if I'm remembering right--the drawings are still at CN.) It has a lot of sheer, though, which a boaty way of saying that the bow and the stern sweep up a lot higher than the center of the boat, so the gunwale will have a (very shallow) U shape. On the one hand, sheer should mean that waves from the end are less likely to come in the boat and cozy up to you, so the boat handles waves better. On the other hand, sheer gives a lot of impromptu sail area for the damn wind to play with, making the boat harder to keep on course in a wind. (It will probably slow down in a headwind, too.) I'm really not sure how these characteristics will interact, but the stems of the Vayu are like 20 inches tall, when the middle is about 11. Which made it a pain to draw, too--my husband's 18 x 24 drawing paper wasn't big enough and I had to cut an unused corner off the sheet for the stations and transplant it onto the sheet for the stem to make it big enough.
If I go with a stem that high, it will be a pain in building too--either adding extra strips above the first strip to build up those extra 9 inches and then trying to cut them freehand to a smooth curve, or bending the hell out of the ends of the first strip and then using lots of cheater strips--and every single cheeter strip is four long skinny triangles to saw and, more to the point, fit without gaps, which isn't easy. Hmm. Or I could just lower those stems. That would work too. If I decide to do the Vayu I'll probably go in and multiply all the figures for height and width by 0.9 or so, and multiply the inter-station distance by 0.9 as well. I understand that you can run into issues doing this--a 1/10th scale model doesn't sail the same as the full sized boat, for example. But I suspect that cutting everything by 10% will probably work--and the Vayu is too much boat for me at its current size.
Also, one of the measurements for at least one of the stations can't possibly be right. I've had dots I had to leave out of the line, but the line usually passes within a sixteenth of an inch or so of the dot (though you'd be surprised how much that can change a profile.) But this one is off like half an inch. If it's real, then the stations nearest the bow and stern have half an inch of tumblehome in a really tight curl. The thing is for that to happen, the gunwales would have to twist 45 degrees in 22 inches, and then the same amount back in the next 22 inches. I just can't see that as being either possible to construct, or a reasonable nice looking design. It would just be weird. So at least one of those points is wrong and I will have to decide which to leave out.
The other design I'm checking out is the Bear Mountain Rob Roy. I've only drawn the first three stations of this so far, and I'm going to need to do 11, because unlike the Vayu, where 1 profile makes 2 stations except for the middle station, the Rob Roy is asymmetric and I don't get any 2-for-1 stations. Also it looks from the book like there should be 2 more stations right near the ends of the boat, but they don't give any numbers for them, so it's hard to be sure.
The way Bear Mountain is doing their offsets is damned confusing, but I think I have it figured out. They give their numbers as feet-inches-eighths. So 13 5/8 inches is written 1-01-5. And I have figured out that when they have a + by their number? They mean add a sixteenth. Ooookay.
At least one more day, maybe two, of lofting. It's not hard, but it takes time, and the darn cement floors in the Art building are making my knees ache.
Note that there will be more boat design geekery when I have finished lofting the Rob Roy.
![[identity profile]](https://www.dreamwidth.org/img/silk/identity/openid.png){kind=small}
no subject
Date: 2010-11-17 09:18 pm (UTC)I find it enjoyable to see/hear/read a geek of a subject I don't know. I can almost but not quite understand what you are doing, it sounds fascinating. Have fun!
no subject
Date: 2010-11-17 10:53 pm (UTC)The part that is weirding me out is that offsets for station 1 and 13 (right at the ends of the boat) aren't given. The stations are there in the illustration, but no figures are given for drawing them.
"If it's true to a plus mark it's true," doesn't have the same ring.
Date: 2010-11-16 11:24 pm (UTC)Re: "If it's true to a plus mark it's true," doesn't have the same ring.
Date: 2010-11-16 11:41 pm (UTC)So if the Bear Mountain plans called for 11 *1/5* inches that would actually be quite a bit harder to deal with, because I don't have any ruler marked in fifths of an inch. But since I need to bend over and peer at the paper with the columns of measurements on it, find the right column and the right row in the column, read the measurement, carry the measurement in my head to step up to the drawing--measure it out on the drawing and mark it... well, it just seems complicated. It did get quicker when I quit trying to read the measurements out of a book, and just photocopied that page and taped it to the drafting table next to the drawing I was working on. Something about not having to move as far between the page and the drawing made it faster and easier.
But oddly enough I remember lofting the Wee Lassie II as being much easier. Instead of columns of numbers, Mac MacCarthy had little pictures of each station--too small to use, but the numbers were marked next to the points drawn out as they would be on the final stations--knowing roughly where your next point was made measuring it out easier, somehow. Maybe it was engaging the "shapes" part of my brain to help the "numbers" part of my brain. I guess the Bear Mountain folks didn't go that route because it takes about twice as much room on the page.
Re: "If it's true to a plus mark it's true," doesn't have the same ring.
Date: 2010-11-16 11:50 pm (UTC)I have a rant in my astronomy class about how the English System constantly shifts, from base ten to base 5280 to base ten to base 12 to base ten to base 2.
Re: "If it's true to a plus mark it's true," doesn't have the same ring.
Date: 2010-11-17 12:12 am (UTC)no subject
Date: 2010-11-16 11:51 pm (UTC)I took a look at the boats on the Bear Mountain site; some very pretty wood there. I doubt you will be disappointed if you build their Rob Roy.
no subject
Date: 2010-11-17 12:12 am (UTC)The Rob Roy is pretty; I'm a bit concerned by the amount of rocker it seems to have, though. If it's too easy to turn, it won't track straight. One of the reasons I'm drawing it out in full is to have a better look at it.