sail area vs ballast

thanks andy. you make some good points. i hadn't considered the mast section. i just thought i would pick up a used one somewhere.

the daggerboard idea is a good one, but if shortening the keel by 12" or so compromises the stability too much on paper, we'll probably leave it as configured. i notice the 2.4m boats have a little skeg in front of the rudder. not exactly a fin but easier to fabricate.

i like your cabin idea. yes, it must be the proper dimensions to give the correct profile. if the cabin is too shallow to allow a steering station inside, one can rig a seat in the companionway with the helmsman's head and shoulders protruding. it wouldn't be too difficult to rig a dodger for wet days. with a hinged cabin and side curtains, this could give both cabin space at anchor and aesthetics under sail.

the longitudinal stiffness question is one i had intended to solve mostly with copious amounts of epoxy cloth and ring frames in lieu of other ideas. thanks for your thoughts.

 

Regarding Andy's post: many raceboats with overlapping headsails have chainplates well inboard of the sheer line. I'd still like to have the actual numbers on how Tohbi's shroud base compares to that of a Yngling, the most comparable one-design I'm aware of. If a standard Yngling mast does not have enough transverse moment of inertia one option would be to add another set of spreaders.

 

a mast's "transverse moment of inertia?"

well, i've just purchased "principles of yacht design" as recommened by the list members. now, as soon as i find an interpreter i'll be able to discourse with those who know what these terms mean.

all i know, stephen, is that the beam is 48" so the chainplates will be right around two feet from the mast. if the boat weighs 600 lbs and we allow 300 lbs for one crewman and gear, the ballast will probably be close to 1100 lbs and will be centered about 14" below the lwl. that's my guesstimate with this keel.

if the keel draft is reduced, the cg will, of course, rise but by a lesser distance. so, it gets real confusing to me. i remember a post offering the opinion that it will take a proportionately greater ballast than the full-sized boat to make this model stand up to a proportional sailplan.

then, there are those who think the project is inadvisable, that it won't perform similarly to the bigger boat, no matter what we do. this discussion is terrific because we're kicking around so many ideas. when we get some numbers back from the fellow who has agreed to help, it will probably be more definitive. until then i'm going to bury my nose in this book and see if i can figure out what you guys are talkin' about. whatever we do, i'm getting a heck of an education.

 

The width of the mast is the main component of its transverse moment of inertia. I think the Yngling's beam is about 6', so Andy may be right that your shroud base will be narrower. You may need a slightly beefier mast than the Yngling (or perhaps you could use a similar section but add a 2nd set of preaders and another diagonal shroud). My guess is your mast section won't end up being far from the one on your Venture - nothing to get alarmed about.

Where the boat as a whole is concerned I think you'll come out O.K. if you retain the existing draft and get the ballast as low as possible in the keel. I think you risk ruining a potentially good boat if you try to reduce the draft much, though. Windward sailing is what these heavy displacement racers are good at - but you need both keel span and a low center of gravity. My advice is to use your Venture when you must have shoal draft, or trade it in for something like this:
www.by-the-sea.com/edey&duff/edsakon.html
if you yern for a traditional aesthetic.

 

oh, so high transverse moment of inertia means "fat mast." that, i can understand.

i'm tending to agree with you about retaining the original configuration of the hull, but i want to exhaust potential alternatives, as well. a small compromise to achieve less draft is acceptable; the problem is determining, incrementally, what reduction in draft/rise in cg will do to handling and stabiliity so we know how far to go.

there has been some question about longitudinal support. i think most of the stringers can be substituted by epoxy. i am a bit concerned about the lack of carlings, however. i'm going to have to crawl inside to take another look; i don't remember any type of stringer in that area.

that is a beautiful design. do you think edy and duff will trade straight across for the venture? .....sakonnet is advertised as having a bulb keel but the drawing showed a shoal keel with a centerboard. wonder what that means?

 

Yngling deck photo

Here's a photo showing the Yngling's shroud base. If you're in a light wind area and you retain the existing draft you might be able to go to a slightly larger sail area than a Yngling, but this would put more pressure still on your mast section unless you go to multiple spreaders and diagonals.

 

the cross section of that mast doesn't appear to be very large. what are the design parameters for mast strength?

 

so, i'm reading into stephen's yingling comment a bit, is it advisable to look for a complete rig from another boat [mast, sails, etc] that approximate the dimensions desired, rather than build one from components?

 

If you find a comparable boat that is succesful-didn't demast in normal use- it is a quick and easy way to do it.

 

concrete keel?

i had intended to add lead to the inside of a reinforced fiberglass keel but, on reflection, that seems a waste of hydrodynamics. better to build a keel in which the ballast forms the greatest percentage of the keel's size.

if one were to build a form and add lead scrap [wheelweights, cutup pieces of lead, etc], could concrete be poured into the mixture to act as the adhesive? what additives to the concrete are recommended?

i don't have the equipment to forge lead and hiring a company to build a keel is too expensive. all suggestions are appreciated.

 

There are different kinds of concrete with various densities, but it's not as dense as you might think. I'm recollecting the number 144 lbs/cu ft for a common type, while lead is about 810 lbs/cu ft (or is it 710? It's been a while).

The first thing you need to know to determine your proper mast section is your boat's maximum righting moment (a measure of its stability). If you know the initial righting moment per degree heel you can multiply this by about 40 for this type of boat. The best way to determine your initial righting moment is with an inclining experiment after the boat is ballasted and most of your other work is complete.

Your shroud base, whether your mast is deck or keel stepped, and the mast height, specifically the distance between support points, matters. Each point where shrouds attach to the mast counts as a transverse support point. The angle the shrouds make with the mast also matters, which is where the spreaders come in.

That's all very complicated, though. If you can find a boat that has similar stability and a similar shroud base, and a rig you'd be comfortable with, you're pretty much there. Andy was right to point out that you will have a narrow shroud base. My point was simply that a multi-spreader rig is sometimes an alternative to a beefier mast section.

FYI the Yngling has a main + foretriangle area of 150 sq ft. If you scale up a Norlin Mk 3 mini 12 to the size of your boat you get 200 sq ft by my calculation. I think you're going to want to be between those two numbers. A Rhodes 19 has a sail area of 175 sq ft, but I think has a lower aspect ratio main than a 12 meter, and has a wider shroud base than your boat. The mast section might work, though, with dual spreaders. That's very seat-of-the-pants; it might do for a weight estimate, but I'd be better to do the math right before going out and buying it. The largest section I can imagine you having to use would be similar to that of a J/24.

 

At 200 sq.ft. crew weight is more than enough to keep the boat upright. The ballast for that sail area shouldn't be more than 300 lbs or so. The boat weight you quoted of 2500 lbs is excessive for that sail area. Are you sure that is the right weight? Sailing dinghies with that amount of sail usually weigh 150 to 250 lbs.

 

The boats are extremely different in their stability characteristics. Let's say two sailors hiking out sail a 505 with a 2500 ft-lb righting moment at a 5 deg heel angle. A narrow, heavy displacement keelboat is going to operate most efficiently to windward at a heel angle of nearly 30 deg. Even if the crew is on the windward rail they may not be to windward of the boat's center of bouyancy by much. By my calculation the metacenter will be about 1 foot above the CB. Let's say the center of gravity is 1 foot below the CB. The righting arm will be 2sin30 at 30 degrees heel, which is 1 foot. Multiply that by the displacement and you get.... well wha-da-ya-know.... 2500 ft-lbs. Maybe the person who suggested a 505 rig wasn't entirely off base (though I still prefer keelboat-to-keelboat comparisons).

 

the full-sized boat built from this model [almost 1/3 size] had a displacement of approx 56,000 lbs. approx 70% of that was ballast.
sail area was approx 1800 sq ft.

i know these numbers don't correspond proportionately with 2/3 reduction in size but, for those who can crunch the numbers, they may help.

 

My point is that as the boat gets smaller, the crew weight accounts for a much larger proportion of the total displacement. This means the original design is a keel boat but yours has movable ballast.