A dinghy for my daughter

[Gavin Atkin]

I thought some of you might be interested in a little dinghy I'm been drawing up and am about to start building for my daughter. Key boat dimensions are 129by36in, 325lbs displacement, 56sqft sail. I'm not going to admit to the prismatic coefficient, but it must be better than an Optimist's...

I'm very impressed with the way the Bafter rig reefs and expands without messing up the balance of the boat, btw - one can add or remove a panel without moving the CoE very much at all.

Now, does anyone have any idea how to cut a Bafter sail please?

Gavin

http://home.clara.net/gmatkin/eeksketch.gif

[JimD]

Thanks for posting the boat full size, Gavin :D

[Gavin Atkin]

I should explain that I linked to an image that was 1600pixels across. Sadly, I haven't got much about my person that will resize gifs - something to do with patents, my memory suggests - and having installed the GIMP for windows I can't find the exe file that goes with it. These techies, why doesn't their installer leave a shortcut on my desktop???

I could fire up the Linux but I'm tired, as it's 1.30 or so and I've got to be up to go sailing in the morning.

Still I have managed to put up an image that should give some idea. I'm sorry it isn't better.

Gav

[Meerkat]

Isn't this redundant? After all, isn't her father a little dinghy? ;)

[Gavin Atkin]

I resemble that remark Meer :)

But I am nevertheless rather less than sure how to draw up a Bafter sail with the right curves in it. Maybe there'll be someone sailing a Solo today, which might give me an idea - they've got battened sails.

http://www.solosailing.org.uk/boat/overview.asp

And is it Bafter or bafter please? Is this thing named after somebody or not?

Gavin

[Todd Bradshaw]

...actually I do. Got a magnifying glass?

http://www.imagestation.com/picture/sraid167/p657d792bbf68789e6f349c230640e550/f432fbde.jpg

Dimensions should be pretty close, though I had to measure your drawing of the screen for a base, which has obvious accuracy problems. Square footage seems about right though. If you need a bigger copy I can e-mail you a PDF file.

Here's the deal: You could probably cut it flat, like a batwing, but I think a little draft will perform better, especially if the mast is bending when sailing upwind from mainsheet tension. I added 2% draft, which is moderate to average and which is achieved by adding luff curve. If the mast is really light and bendy the curve should be deepened as noted, because mast bend effectively reduces luff curve and the draft it creates.

Fabric weave is oriented to the leech (sort of a vertical, semi-mitre-cut) to prevent the leech from stretching and flapping. The two independently-paneled sections meet under the top batten pocket. With that batten angle, it's not quite a true mitre, but it's pretty close. Panel widths on the top section are narrower than the vertical bottom panels so that the seams line up better at the mitre, which looks nicer. That long section of upper leech should be hollowed a bit as noted to prevent fluttering. The bottom sections of the leech (between the battens) are most likely too short to need it.

Shape in the foot is created by adding a little foot round and by adding a couple of simple broadseams (seams where the panel-to-panel overlap is gradually increased as you approach the edge). This gives the foot a little bit of a cup shape, rather than being just a rounded edge on a flat span of cloth. Simple, tapered wooden battens would be tied into their pockets. Reef point grommets are placed through the upper edges of the batten pockets so that pulling down on the nettles (ties) pulls down on the battens, bringing the cloth down.

For a luff-to-upper-mast attachment method, I would use a plain, tapered, round mast and weave some rope grommets to put through the head ring and the top three luff grommets like little mast hoops. I'd fit a halyard through a dumb-sheave on the masthead.

The only problem that the design creates is caused by having a tack angle which is substantially less than 90 degrees and which is being combined with a conventional boom. This affects sail raising, lowering and putting in a reef. It's tricky to explain and get a feel for, but here's the nutshell version: The distance between the clew grommet and the tack grommet (along the angled foot) is longer than the distance from the clew grommet to most of the lower luff (from just above the tack corner up to about 6" above the upper batten) and the distance to any grommets that might be placed along that edge for hoops, reef tacks, lacing, etc. Though this looks harmless enough, it often causes the sail to bind at a certain point while going up or down (sometimes big-time and it's even worse on luffs fitted with slugs slides or a boltrope in a groove). Rope hoops are fairly squishy, but even so, if fitted in the rings next to the forward ends of the batten pockets (reef tack rings), they well may bind on the mast when trying to raise, lower or reef the sail.

One solution is to make the sail loose-footed as shown, so that the outhaul can be backed way off or even detached while raising, lowering or reefing the sail. No outhaul tension means no binding (this is why iceboaters with their highly raked masts and sub-90 tack angles always raise the sail and then install the boom). Another option is a lacing or jackline system on those lower luff grommets that can be uncleated and eased, so that the luff can temporarily move back away from the mast, easing the binding force without having to mess with the outhaul and then re-tension it, once the sail is in position.

I would probably deaden a chunk of lace line to the luff grommet next to the upper batten, spiral-lace downward through the grommet by the lower batten and cleat it or tie it off down below on the mast. Then you simply give it some slack when you want to move the sail up or down and re-cleat it. The tack ring on the bottom corner isn't affected by all this and could have a rope hoop, be connected to boom jaws, a gooseneck or whatever. Some small batwing and gunter rigs with sub-90 tack angles get around this potential problem by simply not having any attachment points along the lower luff. In this case though, you're looking a span of about 55" with no mast hoops, which is a bit much.

So, that's the plan Stan. It's not a hard sail to build or prototype for a gifted amateur as long as it doesn't have to be really fancy and I see no reason that it won't work as long as the details are planned for and taken care of.

[Gavin Atkin]

Todd -

That's completely wonderful. Thank you sooo much.

I hope others also find a use for this info...

Gavin

[rbgarr]

Yes... very interesting!

[Gavin Atkin]

And Todd: do you mean there's no need for a boom? That's how I read it, I can see why it's true and for the purposes I think it's just perfect - it could NOT be better.

I think this could turn out to be a seriously lucky project...

Thanks again. Why aren't all kids' boats like this?

Gavin

[Todd Bradshaw]

It should be noted that sailplans similar to this one are also often built as gunter rigs, though other than easy mast stowage for multi-purpose and tripping boats, I don't believe there are any advantages to the gunter. The one-piece mast is likely to give somewhat better sailshape and more consistent performance as conditions vary.

The boom/no-boom question certainly came to mind while doodling on the plan. If you could get the right sheeting system figured out, I'll bet this sail would work reasonably well on most points of sail with no boom at all. However, since the sail's foot on the dinghy plan extends back to a point where there just isn't much to anchor the mainsheet to, it's probably not practical as the sheet will need to be angled aft quite a bit in order to work well. It might be quite workable though on boats with more hull extending aft of the sailplan. The clew corner is going to curl in a bit on a deep reach or run (like on a spritsail or boomless standing lug), but runs aren't generally all that exciting anyway.

Boomless Bafter-ized canoe, for example:
http://www.imagestation.com/picture/sraid167/p136fdd36c45ed48542257870998a2058/f43174ba.jpg

Of course, if tou tell people you "bafter-ized" your boat they're going to think you have a speech impediment.

You might be able to replace the dinghy's boom with a hefty batten (more like a small spar that lives inside a sleeve sewn to the sail's bottom, with a mid-length cut-out where the sheet ties on). Essentially, you would be using the batten and the tack ring's mast hoop to take over the jobs of the boom and the gooseneck. How much performance you would lose, compared to having the adjustability of a boomed, loose-footed sail is hard to say, but there is a decent chance that the system would have enough play in it to get around the binding problem when raising or lowering sail. I think I'd prototype it with Tyvek or polytarp first, before chopping into a roll of Dacron, as there might be something I haven't thought of which could cause problems.

[Gavin Atkin]

...Hadn't yet thought of the sheeting issue. But in my conception of this thing there will be a spar at the foot of the triangular area of the sail, and that's instinctively the right answer to the kid head meets boom too often problem.

I think it's worth a try.

Huge thanks again.

G