Sailing Dinghy Design

[SailDesign]

Well, I can't say as I have fully thought out just why my hull is the shap it is yet ;-)
The model started off as a 50' Open Class boat, and was tweaked to 14-ft, widened and had a chine added. That was the starting point.
I know there is little deadrise forward, but the beast can always be sailed heeld in light stuff to reduce wetted surface, so I'm not worried. As far as the bow is concerned, I have always found boats with squirrel cheeks to plane faster and sooner (Johnson 18, "Coyote", and that green Unlimited 30 that did so well for the 4 races it ran )
As the boat progresses (hopefully), I will try to comment on why the changes are made.
In the meantime, I sjhall enjoy watching others' boats appear. This is what I love about design - there is no "right" answer, and progress is made when you can see all the variables at work.

Steve

[SailDesign]

I shall also try to spell-check stuff before I send it......

[Tim B]

I hadn't considered heeling the boat for extra speed, but you're right, I remember reading that Laser sailors heel to windward downwind to help balance and reduce WSA. I can see we won't resolve the question without a lot of references or even tank-testing. The lark has a similar, though narrower hull. Perhaps the answer is higher L/B and outriggers after all. Anyway, I'll be interested to see what we end up with.

Cheers,

Tim B.

[Uli]

Hey,
I'm not sure if you have seen this may it give's you same idears. The boat has been queit successful.


http://www.schiffbau.fh-kiel.de/IfS_...YR_osp_en.html

take care
Uli

I've just seen Polarity's I14 at:
http://www.boatdesign.net/gallery/sh....php?photo=897
Good work Paul!

In regard to Uli's picture, that hull looks a lot like Steve's doesn't it? (see previous page). Notably the bow, and what we presume is a fairly flat area aft. Which brings me back to Polarity's post, which has slight deadrise aft, and an inclined planing surface. This is similar to what I have done on TBI14v1 and TBI14v2 - less so on the latter. It is obvious that two of us are coming at this from similar angles! As Steve says, though, there are no 'right' answers.

I would like to know, just out of interest whether we should go for a full-beam moulding (as in TBI14v1) or a tube-pontoon system (as in TBI14v2). Has anyone sailed both systems?
Do tube-pontoons dig in and capsize boats?
Will a full-beam moulding be too heavy?

Obviously, I am, at this point, right on the edge of my experience, and thus pushing the boundaries. This, of course is what cutting edge design is about. I'd be very appreciative if anyone could enlighten us.

Oh, see my post of 6th June 2003 for definitions of the I14 indexing system.

Cheers,

Tim B.

[Polarity]

Hey Thanks Tim! Glad you liked it

I posted the image in the thread as well - dont know if you could not see it for some reason?

It was only after I posted that I realised I had duplicated your colour scheme though

As I said I have a reason for all the strange stuff (negative sheer and only half a bow for example.) Most of the work went into the bow section, it all being designed to get the boat planing and to keep it upright both before and after.

Regarding the stand offs, I'm going for oval section tubing for less wind resistance (the difference is huge as you probably know) I also have another idea for that section as well...

Cheers!

PAul

[Stephen Ditmore]

Sorry I haven't had much time to check in, but here are two recommended links:
http://www.ovingtonboats.com/content/i14/boatinfo.htm
http://www.mothboat.com/USMMCA/mothhint.htm

I'll try to get & post photos from the U.S. Moth class nationals this Saturday.

[Tim B]

That would be great Stephen, nice to have you back on the thread.

Paul, as you may have noticed from a few other threads, I have a bit of Aerodynamic experience, so I do know the advantage of going to ellipses (possibly even an aerofoil with integral spar), but I wasn't going to worry about that just yet, the most important question is the hull. Drag reduction on the tubing is only a few percent of the entire drag of the craft (a quick sum will/should prove that). We'll come back to the question when we do the centerboard and rudder.

Don't worry about duplicating my colour scheme, if I ever get a built hull (cash permitting) I'll probably use my LARK's colour scheme. White with blue trim.

Cheers,

Tim B.

[Stephen Ditmore]

I don't have a picture yet, but I'm going to take Paul up on his challenge to talk through my hull shape. I see my Moth design as a chance to try a hull shape that can one day be scaled up to become anything from a Mini Transat to an Open 60. I also want it to be 80% developable so that it can be built largely from prefabricated flat panels. I plan to use a constant chine radius, a fine bow and broad stern yielding a not quite wedge shaped planform with all curvature in plan being between stations 2 and 9 (mostly between 3 and 8). There will not be a hard knuckle at the intersection of the bottom and the stem. Instead, the radiused chine will intersect the CL plane to form a quarter ellipse. From there to station 2 the bottom will be dead flat, and will continue to be flat along the centerline back to about station 5.5. Beginning at station 5.5 the bottom will develop into a shallow V. This shape is exactly opposite of the usual V sections forward and flat stern, but it should alleviate the tendancy of a fine bowed boat to trim down by the bow as it heels, and in my experience V shaped bow sections sometimes send a gush of spray to leeward and practically stop the boat going heeled into waves unless the helmsman steers into the wave (sending a sheet of water into his own face).

The Laser eventually proved itself a better boat than the Force 5, and it didn't need hard chines to do it. The top Open 50s and 60s also do fine without them.

I plan to give my Moth tube racks and enough freeboard to keep them out of the water. I'm not sure what Tim's been referring to in the I-14 rule concerning a measurement section, but I don't think it applies to Moths.

Paul Bieker's success depressing the stern wave with a rudder mounted hydrofoil is interesting, yes?

[Tim B]

You'll find that I14 Rule 2 defines section data for the hull. Available from the I14 site, mentioned earlier in this thread. You'd have to reference the Moth class rules, but I doubt section data is defined. The depression of the stern-wave with a rudder mounted hydrofoil is interesting, but I suspect we could end up creating more drag than we save. If anyone would like to research it and tell us the answers, please, be my guest... good little PHD project actually.

Cheers,

Tim B.

[Stephen Ditmore]

I think I'm correct in saying that Bieker designs equipped with the foils have dominated recent I-14 competition.

Attached are the International Moth rules. It's interesting that U.S. Moths seem to be narrower than the I-14s or the Australian Moths.

[Tim B]

Moth Rule 3. iv is the closest I can see to any section data. This rule dissalows hollows in the hull. There are no specific points that must be satisfied, unlike the I14.

Sorry I haven't had any time to draw anything new yet, but I've had exams and I'm waiting for my new computer (I've finally given up on the 486!!!). I might sketch something out by hand at some point.

Oh, and you're not meant to sail a LARK single handed in a force 3! so my guess is that whoever's going to sail this I14 (though not the moth - see class rules) is going to need a crew!!

Cheers,

Tim B.

[Stephen Ditmore]

That's the first difference between a Moth and an I-14, of course. The I-14 is a two person boat, while the Moth is a singlehander. I had a great time Saturday. In the U.S. the Moths race in three classes: Modern, Classic, and Vintage (the Vintage boats have to have been built before a partricular year. Otherwise they're the same as the Classics). The Modern Moths are allowed to have racks, large, full battened sails, and asymetric chutes. Trapezes are prohibited on all Moths. The Classic Moths may not have wings or racks, and no hollow in section more than 1" is permitted. The dominant hull is a fully developable design called the Mistral, which has a V shaped midsection.

The Classic Moth rules can be found at:
http://www.mothboat.com/CMBA/About/m...trev101802.htm

Photos of a new Moth design under development can be seen at:
http://gregunit.tripod.com/cg/cg.htm
http://www.mothboat.com/CMBA/Building/cgraves01.htm

[Jim Walsh]

You may be interested in the 12ft skiff Woof shape, no restrictions on hul shapes except length 12ft, beam 6ft and weight 90 lbs, Rigs have no restrictions.

[Tim B]

Thanks Jim, the sections you've posted look almost like an average between the two hulls I suggestted. The major problem is this though:
Rule 3 Shape and Depth of section 2134mm aft of bow

At this section:

(a) The outside of the skin shall not be higher than 200mm above the outside of the keel at a beam of 1100mm.

(b) The top edge of the hull or gunwale assembly shall not be less that 508mm above the lowest part of the hull.

For the purposes of this Rule, any hollow in the keel or keel band shall be bridged by a straight line from which the measurements shall be taken.

At these measurement points the hull shall pass the following test:

A pan-shaped instrument consisting of a flat disc of 305mm diameter with a vertical lip of 13mm depth internally shall, when placed on the hull, touch at two or more points on the lip and nowhere on the disc. This restriction shall not apply to boats first registered before 1st January 1991.

This rule is what poses a considerable problem. Simplistically, the hull must take account of that shape. In complex terms, the nice smooth spline must fit three points, and, if not to have excessive curvature, the aftmost point must be well out from the center-line. Anyway, I'll have some time come the start of July, so I'll have another bash at the hull then. I think a round-bilge hull with an almost straight extension to the shear might work. Anyway, I'll see when I have some time. Thanks for the sections.

Cheers,

Tim B.