Ballast, Planing and Hull Shapes

[Big Red]

Following up on my earlier thread ("My Next Fantasy" which was gettin' too big for my modem), I have another couple of questions? Now lets see...

Firstly, someone mentioned that the Haven 12.5 is hard to build. Why is that? Could a reasonably skilled beginner build one?

Secondly, if you have a sailboat, is it feasible to adjust ballast if you think the boat needs it? Say if it has a hard time in a chop, could you add more ballast within reason? I mean, it will float lower and possibly go slower. It might ride better though, right?

See I was looking at the Glen L 17 and it seems
unballasted, could I add ballast to it? By the case perhaps?? ;)

Which makes me wonder about planing hulls in sailing boats. What kind of boats can plane? I guess they would have a similarly designed hull to a planing powerboat right? With a big flat bit somewhere?

But a powerboat has its planing surface near its source of thrust. Do sailboats have the planing surface near the mast/masts? Hah, I know they don't, so why?

And finally, are catamaran hulls displacement hulls, cause thats what they look like? How come the go so quick? Or are they planing hulls?

I know thats a lot of questions, I just can't help myself. I gotta know ;) Please feel free to add in your .02

Red.

[Ian McColgin]

Red, you might try beginning in some of the basic texts such as Brewer's to get an idea why certain compromises are made.

Some thoughts:

A well developed and shapely boat can be a bit harder to build in carvel or clinker than a boat designed for large plywood panels. Also, some boats have a bit of twist in the planking, especially if she's a sharp narrow bow at the waterline and below, maybe a bit of hollow, and is full above. This can call for steaming the planks.

In general, a boat is easier to build to the extent that the planking can be wide and can lie down on the frames with only a bend, no twist. Sometimes the method can make what was hard fairly easy, if a bit tedious. Cold-molded building calls for less woodworking skill than carvel. It can be more expensive in material given the use (and wastage) of epoxy. Some of that expense can be made up for in that the hull wood may be far less select. In one experiment of Monomoy surf boats made by a school or museum or something out in California the results were surprisingly close.

Ballast is great fun. Not only can you always play with its location, but in smaller boats you will perforce because most ballast is 'live' - your crew. But a design that does not call for ballast is rarely improved by adding any.

Some catamarans actually do manage to plane despite the very narrow hulls. If she's flying the weather hull and the water is breaking somewhere abaft where the mast is, she's planing.

Most cats don't actually plane. In displacement mode, the boat's 'hull speed' in knots is about 1.3 times the square rood of the waterline length in feet. At this speed the hull's wave formation makes only two waves - bow and stern (or first wave in the wake) - and the stern wave is a bit behind the transom. Thus, the boat is essentially sailing up hill to get over her bow wave. This required ever increasing power to accomplish as going just a bit faster makes bigger waves - a bigger hole to climb out of. Planing is a little like the soundbarrior - you get out and over the problem. Without planing, it's a law of diminishing returns and you get to the point where the hull is just not big enough to hold more sail area or more engine. That's "hull speed." Not like the soundbarrior in that it's not a 'hard' number based on the medium, but a soft number based on both the medium and the hull's wave formation charactoristics. The sound barrior varies a bit with pressure and temperature, of course, but at those speeds, who's counting. A very narrow hull makes very small waves. It does not take disproportionatly more power to go faster. A cat's hulls are exceptionally narrow.

G'luck

[Keith Wilson]

Someone mentioned that the Haven 12.5 is hard to build. Why is that? The Haven 12-1/2 is Joel White's modification of the Herreshoff 12-1/2, AKA the "Buzzard's Bay Boy's Boat". It follows the construction of the original as closely as possible, with shallower draft, a little more beam, and a centerboard. The original 12-1/2 was designed by N.G. Herreshoff for fill-in work between big boats at the time when the Herreshoff yard was the premier yacht building facility in the US, and the time when "if you have to ask, you can't afford it" was true. It cost at the time at least twice as much as comparably-sized boats from other builders.

The construction of the 12-1/2 incorporates a lot of fussy details that were the trademark of the Herreshoff boats, and absolutely NO compromises for easier building by amateurs. Remember, these bats were sold to people who would brag about how much they paid for it, and there had to be something to point to to justify the price. These details were standard procedure for the Herreshoff yard; they had the tooling, jigs, and machinery, so it was really no more costly to build - custom-cast bronze hardware, a building jig with a mold for every frame, a lovely molded sheerstrake instead of a normal rubrail - Everywhere you look on the boat you see this sort of thing. They were no problem when the boat was built in series production by Herreshoff, but they sure do add dollars and hours for the amateur builder.

OTOH, it sure is a nice boat. I've often thought that someone could take the same hull shape and radically simplify the construction - glued-plywood lapstrake perhaps? - and end up with a boat that was 90% as nice for half the work.

Phil Bolger designed a taped-seam plywood boat along these lines, much as his Bobcat is similar to a Beetle Cat, It looks a lot better than you would imagine, although of course it's not at all the same shape.

[Tonyr]

You don't need to add ballast to the Glen-L 17. I built one some 15 years ago, and recommend it as a good starter family boat (our kids were between 6 and 10 when we had it).

It is a nice, simple, traditionally designed hard chine plywood boat with good manners. I did not find it to pound in any weather I should have been out in.

It will sort of surf off the wind, but it behaves best sailed within its limits as a displacement hull.

Tony.

[Venchka]

Further discussion of the Haven 12 1/2:

Then there is the matter of the keel. Accumulating lead. Building a mold. Melting lead. Pouring lead. You get the picture.

And then there is the matter of performance. At the risk of being branded a heretic, the Haven 12 1/2 seems a bit under canvased to me. A few numbers:

LOA: 16'0", LWL: 12'6", Beam: 6'0", Draft: 18" (board up),

Sail Area: 133 sq.ft., Displacement: 1409 Ibs.

If you sail where you always have a nice breeze, no worries. If you live somewhere with light air much of the time, you might have a problem.

And there is always the matter of the weight of the boat out of the water if you can't leave it in the water during the sailing season.

The boat is quite popular and many folks don't seem to mind these problems.

[ 04-13-2004, 04:17 PM: Message edited by: Venchka ]

[Big Red]

About the Haven first... 1400 pounds doesn't seem too heavy. I mean as a towing weight. It seems a bit heavy for a 16 foot boat though.

Unfortunately Venchka, I don't know enough about
boats for the figures to mean a lot just yet. I just know how big 16 foot is cause my Dads boat was that big. Pretty big as a kid let me tell ya smile.gif

What I am more concerned with is draught, as in getting it off the trailer. Not all ramps are created equal. And I have seen the odd yachtie struggle before to know I don't wanna be there :(

Without inviting scorn for my heretical beliefs either ;) couldn't it just be built simpler. I mean, forgo the specially cast components
(not the keel obviously) and just use generic products from a chandlers. And modify some of the fancier bits of timberwork to something simpler?

I thought it was available as a clinker hull. I will have another look later.

So you've built the Glen-L 17 Tony. Well, without being too forward, how about some pics!! :D How did it go, easy to build, were the plans good?Anything you want to tell me about it, I would be glad to know?

Red.

[bainbridgeisland]

Planing sailboats:

They don't usually look much like powerboats because they are extremely low powered by comparison. They cannot afford to throw away drag in exchange for keeping the deck dry for example. Also, all sailboats generate power in proportion to stability (righting moment). Thus, the hull of a planing sailboat often reflects the need for stability.

Planing sailboats need to be lightweight. The upper limit is around a displacement/length ratio of 150. At this burden, a sailboat with the right shape and in the right condition can plane on a reach. Typical planing sailboats have displacement/length ratios of 80 to 100. Extreme planing sailboats have displacement/length ratios around 40. The extreme boats can make top speed of about 4.0 times the square root of the waterline length. They can easily plane upwind too.

The shape of a planing boat depends mostly on the displacement/length ratio, the wind strength they are optimized for and the method of obtaining stability. They generally have relatively flat buttock and diagonal lines. The higher speed boats often have chines, especially aft. Boats with stability aids like racks, sliding seats and trapezes can be narrower than average, thus producing less drag and therefore higher speed.

Planing sailboats usually have the planing area from about 1/3 back from the bow to the transom. This is to keep the bow fine for less resistance in waves and when going upwind.

[ 04-14-2004, 03:30 PM: Message edited by: bainbridgeisland ]

[Tonyr]

Red, I mistyped 15 years - it was more like 30 plus years. Time flies when you are having fun.

Pictures of our Glen-L 17? My computer is so old that it can not even receive high speed internet, let alone handle things like photos and graphics. However, we do hope to upgrade to a much better machine/scanner/digital camera package later this year when our friendly local telephone company promises (again?!) to improve line speed.

The Glen-L 17 saga was interesting, in that it is designed conventionally plywood on frame, but I built it using a sort of bastard self-invented stitch and glue method, using polyester and lots of fibreglass, not epoxy (which was not then known to me or available locally). I rigged it like a Drascombe Lugger yawl, since I wanted a safe, simple, divided rig I could handle solo with the kids too small to be able to help. We tend to get some sudden and violent summer thunder squalls, so have to be ready for them at any time. I liked being able to drop the main quickly, and ride it out on jib and mizzen.

We really liked the boat, sailed it for several years, and then found that as a family we grew out of it and went F****s and bigger. Now the two of us are back to power (a 24' is a-building) and a 17' row/sail Whitehall to keep me honest (and fit).

Tony.

[ 04-14-2004, 03:31 PM: Message edited by: Tonyr ]