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Go Back   SailNet Community > General Interest > General Discussion (sailing related) > Modified Centerboard
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Topic Review (Newest First)
07-01-2005 07:00 AM
Vastbinder
Modified Centerboard

Well, if the Thunderbird can get by with 1/2 inch ply single hull... I could retain the double hull but downgrade to 1/2 inch outer hull and 1/4 inch inner. That would be a 40% reduction in hull weight.
06-30-2005 07:15 PM
Jeff_H
Modified Centerboard

Without a specific design it is hard to be terribly precise about the weight of your hypothetical boat. If I had to roughly estimate the weight of your boat, I would start by comparing it to a known design. For example, I would suggest that we use the 26 foot Thunderbird for a staring point. The Thunderbid is a moderately robust and comparatively heavy plywood boat, especially if compared to more modern designs, but pretty light compared to the more traditional designs of its day.

The Thunderbird has a 1/2" plywood hull and deck. The design weight of a Thunderbird weighs roughly 3,700lbs with 1,550 lbs of ballast. In other words the hull and interior weighs roughly 2200lbs. Based on past design calculations that I have done on similar boats, the hull and deck structure would be roughly 50% of that weight, in other words roughly 1100lbs.

If we compare the Thunderbird to the boat that you are proposing, if your boat had a 3/4" outer hull, plus a 1/2" inner hull that is roughly 2.5 times thickness of the planking on the thunderbird and so your planking and support structure would weigh roughly 2.5 times the weight of the hull of the Thunderbird. If you add the extra beam, the glass sheathing, and the foam core, then the hull/deck structure of your hypothetical boat is probably closer to 3 times the weight of the Thunderbird.

So as a rough guess the you are looking at something on the order of 3,300 lbs for the hull and something like 4,400 lbs for the boat without the keel. The Thunderbird has a 41% ballast ratio. Carried over to your boat that would mean 3,100 lbs of ballast and an overall weight of roughly 7,600 lbs. Even with a less conservative ratio of 30%, you are looking at 1,900 lbs of ballast an overall displacement of roughly 6,300 lbs. Either displacement is at the way overweight end of the D/L scale somewhere in the range of 260 to 330.

You can see that this is in the wildly heavy range of things, if, for example, you compare it to the moderately heavy Dudley Dix designed Cape Henry 21 with a D/L of around 180. I think that your hull scantlings and the resultant of weight of your double skin concept is way beyond excessive. It may be time to rethink your concept for yoru intended purposes.

Sorry Dude!

Jeff
06-30-2005 02:15 PM
Vastbinder
Modified Centerboard

OK, here''s where a bit of my utter ignorance shows through.

I honestly have no idea how much weight I''ll end up with. I''ve no desire to make is unnecessarily heavy. I''m thinking the outer hull to be made of 3 thicknesses of 1/4 inch ply laminated together at cross angles and then sealed in a couple, few (don''t know how many layers is appropriate) layers of dynel and resin. The space between hulls, likely about 4 inches and then a 2 layer (1/2 inch total thickness) inner hull made in the same manner. Add on whatever decking is appropriate, a mast, rudder, centerboard assembly and other than a few minor interior amenities, that''s about it.

I''m sure you''d have a much better idea of how much weight I''d have wrapped up in that than I would. I really have no notion.
06-30-2005 02:00 PM
Jeff_H
Modified Centerboard

The answer depends on the specifics of the design in question. Up until now you have been talking about a heavy 25 footer. By that I assume that you are considering a boat in the 4,000- to 5,500 lb range. Normally, you would want the keel and bulb of a moderate draft fin to weigh somewhere in the neighborhood of 30% to 40% of the boat, or in other words, 1,200 lbs to 2,200 lbs.

I would think that you would want something in the neigborhood of 4 to 4 1/2 feet of draft in the fully lowered position. The canoe body of the boat will probably extend roughly 16" to 18" below the waterline with a vee hull form. You will end up with something in the range of 2 ft. to 2 1/2 ft. or so of draft in the fully raised position (you won''t be able to sail with the board raised that far) and I would think that you are probably looking at a dagger board that would be roughly 2 1/2 to 4 feet long (horizontally) depending on whether you want to be able to sail with the keel in the partially raised position (perhaps 3 feet of depth would be a practical minimum). The shorter (in the horizontal dimension), the less likely it will be that the board will jamb or damage the trunk when you run aground or raise it in the trunk.

I should note that you are now in a territory (heavy displacement, cruising catboat, with a daggerboard with a bulb) where no designer has ventured before but that is not to say that it is not an idea without some merit. (Again you would be way ahead of the game if you scale back the weight and the weight up high a bit.)

Jeff

06-30-2005 01:43 PM
Vastbinder
Modified Centerboard

That''s what I was attempting to explain. Not a hinged centerboard, but a largish daggerboard with a ballast bullet attached.

I take it, that this method is already in use? Aren''t I just the clever little monkey ;-) I just invented something that''s already out there! Oh well, at least I don''t seem to be totally off base.

Say, on a 25 foot wooden boat, likely a rounded hull with some lift at the fore and aft... any notion how long such a daggerboard should be and how much weight in the ballast bullet?
06-30-2005 01:20 PM
Jeff_H
Modified Centerboard

Traditionally, catboats were heavily ballasted with either internal ballast or else a cast lead keel through which the centerboard was lowered. This gave them some degree of stability at low to moderate heel angles. While some Catboats used steel plate centerboards in most cases the centerboards themselves were just ballasted enough to sink when lowered and added nothing to the stability of the boat.

You can''t add a bulb to a hinged center board because you would be dragging the bulb sidewards through the water when the centerboard was partially raised.

Which now brings us gotten back to what I had originally been suggesting, in the "Best Hull Type" discussion, which is essentially a daggerboard with a bulb which would give you both the stability and sailing ability of a fin keel, with the shoal draft of a centerboarder. Daggerboards lift vertically rather than hinge so that the bulb always stays level. It is not all that difficult to design a daggerboard with a bulb that would work well. While a Tripp 26 or a Meleges 24r in no other way resembles what you are proposing to build, and they are far more sophisticated than you are proposing or even need, you might look at some pictures of the Tripp 26

http://www.tripp26.org/stories/2003/06/22/keelRefurb.html

or Melges 24

http://www.yachtworld.com/core/listing/pl_display_photo.jsp?slim=quick&boat_id=1314463&bo atname=24%27+Melges+Melges+24&photo_name=Picture+% 231&photo=1&url=

to see what this arrangement looks like.

The only problem with either a centerboard or a dagger board with a bulb is the space that they occupy within the cabin of the boat.

Gotta Go,
Jeff
06-30-2005 11:18 AM
Vastbinder
Modified Centerboard

OK, right this minute I''m having favorable throughts about the cape code catboat design. It has been noted that the centerboard does not necessarily provide enough stability (at least not as much as would be desirable).

This makes me wonder if one could construct a large centerboard (long from front to back) with a ballast bullet attached to the tip of the centerboard and some kind of simple ratchetting winch mechanism to raise and lower it.

If such a thing could work, could you not gain the added stability similar to that of a ballasted fin but still be able to retract it for shallower waters?

Just think''n is all...

 
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