sail area vs ballast

yes, it is true that "as the boat gets smaller, the crew weight accounts for a much larger proportion of the total displacement," gonzo.

i think stephen is referring to an idea that tom speer mentioned early on in this discussion: "Sail area scales as size^2, displacement scales as size^3. So your 1/3 scale model would have 1/9 the sail area (203 ft^2) and the weight would be 1/27 (3.7%) full size to get her to float on her lines."

by these numbers, the boat should weigh 3.7% of 56,000 lbs. actually, a little less since this isn't quite 1/3 size. it's all greek to me; stephen's idea to stick her in the water and ballast her to her lines will give the definite amount of ballast needed, but we need to come up with an approximation in order to determine what to do about the keel.

 

I'll have to correct my displacement number, then. If she's a 1/3 scale model of a 56,000 pound boat then the displacement should be 56,000 lbs*(1/3)^3 = 56,000/27 = 2074 pounds. If you'll be using it mostly in fresh water multiply by 0.975, giving you 2022 lbs. If you want her to float on her lines with her crew on board you should deduct the weight of the crew, say 372 lbs, leaving 1650 lbs. Then you deduct the weight of the reinforced hull, deck, interior, engine, batteries, rig, all hardware and equipment. Do you know anyone with a construction company? Do they have a crane with a scale? Do you live near a truck weighing station?

If you scale down the 1800 sq feet of scale you get 1800/9 = 200 sq. ft. It may be that if you hike out you will be able to carry this, as Gonzo suggests. The thing to be aware of, however, is that stability scales with (1/3)^4, so if you don't hike you would only have 1/81 the stability of the full sized boat, even if you did have the same ballast to displacement ratio (which you won't). Meanwile the heeling moment will be 1/27 that of the full sized boat. That assumes the same apparent wind velocity, however, and you wouldn't see the same apparent wind velocity even if sailing in the same true wind because you wouldn't be going as fast and because of the wind gradient. And if, in addition, you simply sail in lighter winds than the big boats, then you might be O.K with 200 sq. ft.

The thing about hiking is: it works best when the boat is relatively flat. Fixed ballast, on the other hand, is most effective when the boat is heeled, so the question becomes: how do you plan to sail this boat? I think the mini 12s are fun for people (esp. those with physical limitations) specifically because they DON'T have to hike. Instead, they keep their weight low, contributing to a low overall center of gravity.

Now let's recalculate your boat's metacentric height. It should be about 0.038*182*40^3 / (2074*27) = 7.9 inches. This is lower than I said previously because I used 48" as your beam when I should have used your waterline beam, which you said was 40". This means your boat's metacenter is only about 8" above your center of bouyancy, putting it near the waterline. This means that if the center of gravity were to be at or above the waterline on your boat it would simply flop over on its side. With your hullform, a low center of gravity is critical!

 

won't be hiking. there will be two steering stations: one amidships with a wheel-line-drum setup and the other a tiller off the rudder post near the stern. mostly singlehanding and a poor swimmer, hiking probably isn't a good idea, anyway.

i had read that scaling down means a greater proportion of ballast is necessary but 1/81 stability seems scary even with the 170 square ft of sail i had guesstimated.

if by mini-12s you mean the 2.4 class, they are tres cool and look like a whole lotta fun. i'm surprised they aren't more popular. i'm trying to set this boat up so most everything can be done from midships.

 

After going through the numbers I'm returning to the thought that the 150 sq ft Yngling rig is probably going to end up being right for you if you add a second set of spreaders and diagonal lowers to compensate for your smaller shroud base.

Be sure to note the last paragraph of my previous post, which I was adding using "edit" while you were replying. It's interesting that the form factor commonly used to approximate waterplane moment of inertia is close to 1/27, and 27 cu in of sea water weighs 1 pound. If you use inches and pounds as your units you can therefore drop the factors, leaving:

BM = Lwl * Bwl^3 / Displ

where:
BM = metacentric height in inches above the center of bouyancy
Lwl = waterline length in inches
Bwl = waterline beam in inches
Displ = salt water displacement in pounds
Note: this value is approximate. The exact value will also depend on the distribution of waterline beam into the ends of the boat at the waterplane.

 

thanks stephen. i don't know how to crunch the numbers as you're doing. in fact, i'm not sure what some of the symbols mean but i'm finding your conclusions very interesting and helpful.

if the metacenter is near the waterline, it would seem to my amateur mind that a light rig is important. i know carbon fiber is light but not affordable. i was hoping to simplify by having a mast strong enough to obviate spreaders but it seems weight aloft will be a disqualifying consideration for a big mast.

and these numbers would indicate, to me at least, that those who have advised leaving the keel as it is are correct; the lower the ballast the better, especially, in this design. there doesn't seem to be an option for raising the ballast [?].

 

I think you're right on all counts, but I also think Andy (was it Andy?) was right when he pointed out that your narrow shroud base will work against lightening the rig too much. Given a choice between two sets of spreaders and a heavier mast section I think the two sets of spreaders will be the lighter answer, and multiple spreaders are in keeping with meter-boat practice.

On the subject of traditional shoal draft designs check out the Egret at
www.parker-marine.com/28shegretpage.htm
I'm thinking the original Egret was an L. Francis Herreshoff design. Does anyone know more about the origin of the Egret?

 

i don't remember 'egret' being a herreshoff design but i've been interested in sharpies for some time. they are a 4/1 length to beam ratio [our project boat is over 5/1] without a deep, ballasted keel.

one would think a hull that narrow wouldn't be capable of carrying much sail but sharpies are reputed to be fast. is form stability such a deciding factor, even on these narrow hulls?

 

As you'll see looking at the metcentric height formula, displacement is in the denominator - it actually HURTS metacentric height. In the end righting arm is multiplied by displacement to get righting moment, so displacement ends up largely cancelling out of the stability picture except in as much as it effects center of gravity. The two things that influence stability most are center of gravity and waterline beam. A lighter weight boat will be more easily driven (less drag). The waterline beam relative to fairbody draft and (therefore) overall displacement is usually pretty good on sharpies. That they're typically long relative to their midsections causes them to excel at reaching.

Someone asked me a while back if he should pay $2000 for plans for boats resembling the Lewis & Clark "keelboats." I pointed out that plans for Parker's 36' Egret, which is quite similar, could be had for $275. I think Egrets are an excellent choice for sailing lakes & bays.

 

Another advantage of a light displacement boat, is that it needs less sail area for the same speed. This makes them easier to handle.

 

weight & shape

stephen wrote: "A lighter weight boat will be more easily driven (less drag)." on the face of it this is certainly true, but what about shape? i have a venture 21, another 21' boat weighing 1200 lbs with 170' sq of sail. beam is 6'10".

this is a fast boat but will it be faster than our project boat at 2000 lbs, with similar sail area, and 4' beam? certainly, the beamier venture can plane and will be faster on a run, but the heavy displacement mini cup racer should outpoint her, if not outreach her, and will have a very different motion.

an extreme example would be a lighter boat, shaped like a garbage scow, compared to an ac racer. of course, the heavy ac boat is going to be faster, but it isn't lighter; the narrower shape is just more efficient. the trend today is to ultra-light, top-skimming racers, but in times past it was an opposite design mentality and those old boats were reportedly very fast [if you discount planing].

so, i'm only guessing [in my amateur mind], but i don't think speed is going to be our biggest challenge. right now, the problem is all this ballast and finding a rig that will stand up to it. don't forget that this is going to be a trailer boat and spreaders will be a real pita to deal with. is there an alternative?

 

Why will spreaders be a problem?

 

my, albeit, limited experience with spreaders is that they are fine for boats kept on moorings or slips, but can be a problem when the mast is frequently lowered and rised. especially when the mast is down, they tend to hang up on things, bend, and generally just complicate matters.

i would prefer a double set of shrouds if necessary, at masthead and fractional positions but, like the keel question, what i want isn't always the best avenue to pursue. what do you think?

 

I was looking back on the thread, and couldn't find wheter the mast is stepped on the keel or the deck. I it is deck stepped, you can have a diamond rig with only two shrouds.

 

actually, there has never been a mast and, because the interior is so tight already and the beam so narrow, a deck-stepped mast would be most convenient.

a diamond rig is better news, of course, but how can we get away from spreaders altogether? or can we?

what do you think of the rig in the "4m single handed keelboat" thread in the design forum?

 

Re: weight & shape

Actually, physics hasn't changed over the years, but fashions in boat rating systems have. "Fast" is generally considered only in the context of the rating system, and the key to winning handicapped races is not to build a fast boat, but to build a boat that's not quite as slow as her rating says she is. The typeforming influence of rating systems is the biggest factor in determining what sorts of boats are built in a given era. Just look at the rise and fall of the IOR and what's happening in the Med right now with IMS racers.

Even in the past, the fastest monohulls were scows. A-scows (good shot of the hull shape at http://www.melges.com/ ) have been racing for the Felker Cup since 1885 (http://www.ilya.org ), and until the recent advent of multihulls were the fastest boats in the world. Hull shapes haven't changed that much, although modern materials have radically changed their construction and rigs. An old 38-foot wooden A-scow can hit speeds of 15 - 20+ kt that a 78 ft AC boat can't even dream of. Scows are fast even when not planing, because when heeled they present a long, narrow shape with greatly reduced wetted area. Look at how much of the boat is out of the water in the Melges shot.

A 60 ft scow would beat the pants off an AC boat in light winds and heavy, and be far more nimble on the starting line to boot. Just like Dennis Connors' 60 ft catamaran beat Michael Fay's big mono. Even offshore the fastest monohulls, like the Group Finot-style racers, are essentially sea-going scows. Unless you put artificial constraints on the design, you can always design a light boat that will be faster than a heavy boat of comparable size.