View Full Version : 25' trailer sailer
Hello fellow boat builders.
I just thought I'd throw out some thoughts and ideas to everyone and see what bounces back.
I'm in the process of designing a 25' trailer sailer. I'm using a fairly traditional hull shape combined with modern (epoxy, glass and wood) techniques. The hull is a blend types taken from Chapelle's book "American Small Sailing Craft." The lines are primarily the Casco Bay Hampton boat with an extended forefoot to add lateral area and to lengthen the lwl. The sheer beauty of these traditional types and their inherent seaworthiness make them a great starting point (for me anyways) for a new design.
Criteria for the design are minimal weight in transit, shallowdraft, moderate to light displacement, siplicity in operation and maybe even a good turn of speed.
This design would be used for coastal cruising and, hopefully, jumps across the gulfstream for some island cruising.
Minimal weight in transit equates to a water ballast hull with additional lead as required to bring it to it's lines. I realize I just opened a can of worms, but this is not a TP so I think I'll be alright. ;)
Shallow draft is self evident, but ramp lauching is the object.
As far a displacement goes, let's go for "Too big on the trailer and too small in the water."
Simplicity and speed. Hmmm. How about a cat-ketch with eliptical, fully battened sails. Let's throw in some deck stepped, hinged masts for trailering and ease of rigging and I think we are ready to go.
I looking for ideas on the mechanics of the water ballast system as it would apply to an epoxy encapsulated strip built hull. Essentially, a large volume of water must enter or leave an enclosed area readily, efficiently and dependably. My feable mind envisions hinged doors that are always below waterline and that hinge inward so that when the craft is heeled, internal pressure seals the door(s). How about an air fitting and pump to pump air into the ballast tank to lighten the craft for trailering. What if's: we forget to open the door(s) for trailering and the ballast becomes trapped, we pressurize the ballast tank and, again, don't release the ballast doors. Maybe I'm going down the wrong road, but I'm interested if anybody has other ideas.
As far as the rig goes, I like the efficiency of an elliptical sail. The cat-ketch rig allows for two hinged masts the carry the ellipticals. No jib means no effort tacks. Standing rigging would be shrouds on the mizzen, connected to the main by a spring stay and then to the stem by a forstay (I might actually want to fly something up there after all). No shrouds on the main. Should the after mast stand taller than the forward in this case and does it then become the main and do we now have a cat-schooner?
I'd like to hear ideas and also hear what people think. Maybe even have a little fun with it.
Looking forward to responses.
Land locked in Kansas,
learpilot
One difficulty in using these "traditional" designs and shapes are the displacement for the specific construction method will be considerably heavier then the one you intend to employ. This will require additional ballast, which will require a new rig design. The once comfortable motion of the original yacht may be lost with the higher ballast/disp. ratio, leaving you with a snap rolling beast of other unknown qualities. The other way around is a new design in the "spirit of" the type you'd like to emulate, but with an eye on the requirements of your construction techniques and desires in the craft. It would be a rare design indeed to take a 70 year old carvel carrying a 25% ballast ratio (3/4 inside and 1/4 out) and make her into a well handling speedy craft by today's standards, that doesn't need a reef by Beaufort 4. I can think of many other issues that will need substantial engineering to over come, most ruining the qualities of the original design.
Back to the point at hand. I don't mean to disregard your statements. I will give them some thought. I just what to keep the tread on track.
In a water ballast system, obviously you need a hole in your hull. The questions that arise are, how large of a hole, how much reinforcement is needed around the hull hole, if a hinged portal is used on the hull hole, what are the mechanics of such a mechanism.
With a wooden hull, would a doubler the same thickness as the hull plank extending half the hull hole (ballast port) width around the port be suitable? Kind of a cold-molded doubler laid on a 45 and 45 orientation. Or maybe, to negate such a local increase in hull thickness, glass cloth in the appropriate dimension and thickness to carry the loads around the ballast port. Then the hull becomes the core material in a FRP system at the port.
I'm looking at maybe 1000 lbs of water ballast, realistically I think I'm limited to about 850 lbs, due to the hull volume below the cabin sole as my design stands right now. That equates to about 16 cubic feet for the former and 13+ cubic feet for the later. The point being, how big of a port is required for expedient tranfer of ballast. I would lean towards one square foot of opening. Granted, a hinged port would have to be nearly full open to fully utilize the area.
As far as the port door is concerned, I think either a stainless steel or bronze, 1/8" would be adequate. The hull plank that was removed to make the port could then be mounted to the port plate to fair the hull in the vicinity of the port. Strength requirements for the port plate is the question of how much head pressure must it withstand. Greatest head pressure would occur at, worst case, 90 degrees of heel. Vertical accelerations due wave action should be self canceling since the wave itself is supplying the force of acceleration.(?) With an 8' beam and a port no more that 1 foot off centerline, maximum head would be 5 feet. So, for design purposes, 10 foot of head pressure should be suitable margin. Anybody current on their basic physics? I suspect it's not an issue.
More later.
LP
SeaDrive
07-27-2005, 01:46 PM
I think you are tying too hard with your hinged door on the ballast tank(s). A feed tube from a thru hull (with seacock) will do. You will also need an air escape tube (also with seacock/valve). A standard water pump can be fitted to empty most of the water (huff, puff), and the rest drains out.
Places to find ideas: Nimble Yachts Sea Pearl, water ballasted designs by Jim Yound and Phil Bolger. For that matter, check out the water ballasted boats sold by Catalina and Hunter.
Thanks for the input. I was thinking that Hunter or Catalina used some sort of plate system. I was initially was looking at a non-powered system for ballast tranfer. A powered system certainly has advantages. I'll have to explore it. Thanks for the insight.
LP
gonzo
07-28-2005, 08:03 AM
Water bellow the waterline is not ballast. It just lets the boat sink lower in the water. To use it as ballast, you need to install tanks under the deck. Lowering the boat in the water will change the stability characteristics. It may be better or worse depending on the design. Ballast needs to be denser than water. If it is not, you have a neutral or balanced design. A longer forefoot may be a problem. Those boats have a pronounced drag aft. This makes them balanced and well behaved. The centerboard is adequate for lateral resistance.
Gonz,
I have to disagree with your statement about water below the waterline not being ballast. Perhaps it's a point of technicallity, but any mass added to any area below the waterline and maybe above it that is denser than the substance that it replaces is ballast. While water is not as dense as other substances, it does have mass and it will lower the CG of the craft if it is introduced in the right place. I would be hesitant to add weight under the decks as it would actually raise the CG of the craft. It wasn't stated, but I assume you are talking of moveable ballast that you would shift on each tack. Thats fine as long as nothing fails. In that type of system, mounting the tanks in the turn of the bilge would be more effective since their effect on the vertical CG would be reduce and there would be less foreshortening of the moment arm between the ballast tank and the lateral center of buoyency as the craft heeled. (if you're talking shifting ballast) I know L. F. Herreshoff used planking as ballast in his Meadowlark design. He used exceptionally heavy bottom plank in a shoal draft, modified sharpie design. He used a substance that was lighter than water to lower the CG in that design.
The designs that I'm using as a starting point for this design have no centerboard. Since I'm going to be building this design, no centerboard means I don't have to build it and ultimately, it's means not having to use a centerboard. I am incorporating drag into the keel. This is a cruising design, so the pronounced forefoot should add to directional stability (and lwl) leaving maneuvering to be done while under power in congested areas.
As always, I appreciate input and look forward to other points of view and technical expertise.
LP
(may the bottle never explode)
Jocko
07-28-2005, 12:41 PM
wrt your sailplan, it sounds very much like one used and written about by Chris White - but as a multihuller. His book explains the details, I believe he built the rig himself, if I'm not mistaken. I found a link for the book "THE CRUISING MULTIHULL By Chris White at:
http://boatbooks.co.nz/multihulls.html
if it does any good. Good luck. :)
That is very close to what I'm looking at. I'm targeting around 300-350 sq. ft. of sail divided between two masts. I haven't run the numbers yet, but I'm looking to maintain a decent aspect ratio and keep the CE as low as possible.
LP
gonzo
07-28-2005, 04:40 PM
Water will lower the CG. However, a design that needs ballast for stabilty won't handle well without it. Hampton boats carried a considerable amount of rock ballast. As for the forefoot giving directional stability, that may not be so. A sharp forefoot makes a boat tend to broach, the opposite of directional stability.
Water ballast isn't a good idea for a cruising boat, especially as you envision it. Water makes a very poor form of ballast (it takes up way too much room and moves around) It only is useful if used to lighten trailering loads or to increase the righting arm like that used in racers (though this fad is fading) Some production craft do use it to lighten the trailering load, but frankly the amount of water taken on by these boats can easily be handled by a small pickup, SUV or mid sized car if it was replaced with a solid material. In these boats, I believe it was more a marketing ploy to emulate the latest racing trend, more so then to decrease trailering duties.
I think what Gonzo was referring to is the free surface of liquids effect. The shift in the center of gravity will always be unfavorable in the system as you have suggested, because the water will run to the side that is heeling. This is why bilge tanks and pressurized systems are used in racers, to take advantage of the movable weight, placing in on the windward bilge. Even baffled tanks that are full will have a negative effect on the CG, not as much as a bilge half full of water, but a measurable effect in any case. If your keel was hollow and the water contained within it, you could use it to reasonable effect, but if athwartship tanked in the bilge, not so much.
LFH's Meadowlark design was an uncommonly light design of very shallow draft, limiting the ballast options available to him, so typical of the era, he used progressively lighter dimensioned planking stock, moving from the garboard to the sheer strake. This was a common practice and still enjoys favor in several construction methods.
There's been considerable historical development of the Casco Bay Hampton boat and it became quite extreme eventually, ruining the type. The forefoot was deep. I don't understand the logic of increasing it further, unless you want a Dhow, which is an interesting craft, but tests and development have proven the concepts have less advantage then other, more widely accepted techniques to gain maneuverability and good tracking in shoal craft. These craft were also quite burdened by there trades and their shapes. The concept of pivoting on the bow and raised quarters has been proven inferior by many designs over the last couple hundred years.
Have you any sketches of your proposed modifications to the lines of these vessels?
PAR,
Here is a screen shot of the lines as they stand. The sail plan is tentative. I reviewed my resources and found I was making reference to a different craft. I liked the comments that Chapelle had about the sailing qualities of the Toulinguet boat. It has the raking stem. My lines follow the Casco Bay craft fairly closely. Sorry for the confusion.
I'm still not quite ready to give up on the water ballast idea. The issues you present are volume and sloshing. My intention was to create a ballast tank under the cabin sole. Historically, a place that collects water anyways.
:D So, why not make the cabin sole the pressure bulkhead and have every cubic inch of volume below the cabin sole used to capture ballast. I see some secondary advantages to such a system. First, the cabin sole then reinforces the hull at approximately the waterline. Slightly below it in fact. Should a person encounter a floating or submerged object, where else would you want reinforcement. Should your encounter breach the hull, it would be of no consequence. You're already full of water below the waterline. The pressure vessel is still intact. Second, my intent is to always sail with a full ballast tank. I only mentioned bilge tanks earlier to expand on a point. Granted there may be a small volume of air that does not evacuate when filling the ballast. I see that as a design issue. Something as simple as a cabin sole that inclines to the bow with an air vent in the area of the stem. A third point would be that since water is neutrally buoyant in water, if a person wanted to make the vessel positively bouyant when swamped, there would be less of a volume required for floation material.
I'll willing to admit that water in not the best material for ballast. In fact, it's a compromise at best. But that is what boat design is all about. In this day of $2+ a gallon gasoline, that 800-1000 pounds of water ballast looks pretty good. Thats a 25-30% reduction in trailer weight.
Actually, all off this discussion has led me to a very simple system for ballasting, that should the idea of water ballast fail, there will not be a large investment in time of materials. Invision this. Ports not in the hull, but in the keel throught the deadwood. The deadwood would never leave the water unless on the trailer so by essentially stuffing a "cork" in the air vent located at the bow, the ballast is trapped by one simple devise. Attach an air pump to the vent and you can evacuate the ballast in preparation for trailering. Should the whole thing fail, plug all of the vents and ports, bolt on the additional lead and be on your merry way.
As always, I'm open to input. Keep sending me posts. I'm obviously inexperienced at boat design and probably am viewing it though rose colored glasses.
LP
gonzo
08-02-2005, 02:37 PM
It seems, at first sight, that the center of floatation is well aft. Without a great deal of ballast aft it will float down by the bow. Water won't do it. This design was supposed to have a bunch of rocks in the deep bilges aft. You flattened the run so there is no place to put ballast unless it is exterior. Boats that use flooding water tanks are rather flat in the bottom and have balanced displacement fore and aft.
Gonzo,
If you look at the lines, yes, the entry is very fine. I like it that way. If you were to rake the stem and shorten the waterline by a foot or more and redraw the waterline to that point, I think you'll find that the center of buoyancy is not so far aft, compared with other models. There is also a fair amount of flam in the forward sections. So, as the bow pitches down, reserve buoyancy comes into play. The interesting thing about filling the bilge with water ballast, is that the ballast distribution matches the displacement/buoyancy distribution. The water ballast in this design is part of the ballast equation. Additional conventional ballast will be added to bring the vessel to its lines.
Let's talk about the mechanics of water ballasting. In a moment of clear thinking (or clouded depending on you point of view), I saw, say, a half dozen 1" holes penetrating the center of the keel and/or deadwood. Just for grins, let's line them with copper to kept the critters at bay. Then at the bow, we'll have an air vent leading to topsides or perhaps to inside the chain locker. This vent will have a valve for control of entry and exit of ballast (or air, again depending on your point of view.). Maybe, there will be a second vent aft, just in case the bow sinks. (or something) All other thoughts aside what would be the inequities in such a system. An addition of an air pump connected to a vent pipe would aid in evacuating the bilge in preparation for trailering.
LP
capt'n ron
08-02-2005, 07:22 PM
if a rookie might add a thought?... every thing i've read so far makes me agree with PAR. alrhough water is ballast, as any amount of wieght below the c/g will enhance the pendulum. the lower the better. water, i will agree is a "not so good" a ballast in the case of a boat of this size, it just isn't heavy enough and takes up too much room to be effective.i think the real evaluation here is the height of ballast volume. rocks, lead, even concrete is many times more dense than water so it will take up space that is lower in the boat in volumetric comparison to water, a major consideration in the effectiveness of the ballast.
gonzo
08-02-2005, 09:51 PM
A common mistake is to expect the boat to float as drawn. You need to do the displacement/weight distribution calculation and it will show how the boat will float. If it doesn't float on her lines the ballast needs to be shifted. With water, in that design, it won't be easy. You may put bulkheads to compartmentilize the water, but it is the opposite of what you are trying to do. For a 'water ballasted boat" you need a design that is appropiate. The one you are showing would only work with a denser ballast that can be localized.
Your idea of water, free flowing into the bilge will require it be sectioned into rather small compartments, each with it's own vent and valve. You'll still be subject to the effect of free surface liquids and have a poor density ballast material to boot.
After looking at your lines, I too do not think your boat will float as you've got it drawn. You need to perform a time honored routines center of masses and displaced areas before you can have any idea of a target to shoot at. I'm not sure of the software you're using, but the centers and area calculations should be available. If your "winging it" using a drawing program, then you'll have to resort to the old fashion way with a calculator, pen and pad (you'll love the center of masses by hand)
For what it's worth, that waterline shape will not help you with speed, nor those buttocks.
Actually, the center of buoyancy in this design is quite tolerably in place at 52% aft along the lwl. My current boat is very bluff in the bow and it drags its transom. Problems that are remedied in this design. Actually, I don't see a problem with the buttocks. It's a smooth transition to a gradual slope rising to the waterline. Waterline shape has less of an effect on speed that do the diagonals, which would reveal any humps or hard spots for the water to flow around. I'm not looking to build a minitransat. I am looking to built a traditional "in spirit" design that incorporates many of the style/design aspects of an earlier era (raking mid-section) while applying modern materials and construction techniques.
You keep talking about "free surface liquids." What free surface liquids? All of the water is fully contained. The best example I can give you would be to take a Coke bottle and immerse it in water, top up, cap off, until it is full. Invert it and slosh it around to your hearts content, but keep the open submerged. Slosh is obviously a bad word since the is no sloshing taking place at all. The liquid is fully contained and the center of mass is constant. Having free openings in this configuation is the same as having no openings at all. If the openings were exposed to free air, then there would be a problem. Thus the reason for having the ports at the lowest part of the keel. There will be no free air in the ballast tank, no free surface liquid, no shifting ballast and no change in the center of mass of the ballast. I agree that water is not the best material for ballast, but I have a very specific design criteria I'd like to meet. 1000 lbs. times 1000 miles equals a lot of work expended. If I am completely missing the mark, in regard to why my ballast is going to be unstable, then let's digress to single syllable words that will help me understand the situation. Straight and simple, I see one shortcoming to water ballast. Density. The center of gravity issue just has to be dealt with, but it's a known. The volume has been dealt with. The distribution is dealt with through additional conventioal ballast as mentioned earlier. There is one issue that I'm surprised that no one has brought up. The moment of inertial of such a large distribution of mass.
Let's go ahead and digress right now. I guess I need to know exactly what a free surface liquid is. Perhaps the name is misleading. I just had a little brain-storm that left some free surface liquid on my head. Actually, I started thinking about eggs and electrons. :idea: How do you tell if an egg is hard-boiled? Spin it. If it spins, it's hard-boiled. If it doesn't, it's not. Would that be your free surface liquid? Electrons->eddy currents?
Sorry if I'm hard-boiled --- I mean hard headed. Some design elements are going to stay and others may go. Either way, the scrutiny is welcome, even though it may hurt. :(
LP
P.S. Here's my current boat. Eve-N-Temper
gonzo
08-03-2005, 07:02 PM
The original design has hollow bilges to allow placing the ballast low. You took that away and replace it with wood- a bouyant material. Then replace a dense ballast with water, again less density. If you want traditional, why don't you stay with the original design. 1000lbs are not a great difference if they will make the boat perform properly. On the other hand, design a hull that has inherent form stability and doesn't need a dense ballast.
The few manufactured trailer craft that employ water ballast use a hollow keel, to contain the additional ballast in a narrow compartment, below the CG. As I previously pointed out, is an acceptable use, because the free surface (of the liquid) is small and narrow (relative to beam) and helpful (weight below the CG) developing additional righting leverage because of it's location in the keel. Another method employed is to pump to a windward tank, again as an assist to existing ballast. The successes of canting appendages will quickly displace the water pumpers in the next several years and fad following trailer sailors will sport canting foils shortly there after.
Free surface of liquids has a bad effect on dynamic stability because the volume and the free surfaces are separate from the hull. Only in compartments of relatively small area, can these effects be managed and not done so at the expense of the main portion of ballast. Because the volume and free surfaces are separate from the hull, their roll period differs from that of the boat. When the roll periods are out of phase with the general motion of the boat, a beneficial effect is gained. When they're in phase with the craft's motion, their heeling moment is added to the work effected by the general heeling moment and the critical dynamic stability angle is reduced.
In theory, your idea sounds good, fill the bilge, cork it (and hope it doesn't pop the cork or leak) and bingo, instant ballast. In use, you'll never get the bilge completely full (very necessary for safety reasons) without pumps and valves. The act of walking to the foredeck or sitting over, where ever you decide on placing this cork (likely a valve) will shift the center of buoyancy just enough to insure a reasonable air bubble remains in the bilge. Even with baffles, which do help control sudden center shifts as the boat pitches about, you'll find yourself in a world of "frequency phase". Of course this will not occur on a nice calm Sunday morn's sail, but just when you really don't need to broach, which it typical of most things I figure. The thought of a completely filled vessel of water not having air is a false one. Water has a lot of air (oxygen) in it, but it's "captured" not free like the stuff we breath. The oxygen can be 'freed" with agitation, which will occur with the normal motions of the craft, as the water is compressed all over the internal structure of the yacht.
Water as part of a trailer born vessel's ballast is a good idea and can be well executed, with simple, off the shelf pieces. I figure your vessel is in the area of a ton and a half displacement. This will require a tandem axle trailer and reasonable sized car or truck to pull it. This is within the realm of a mid size, V6 powered SUV, big six powered mid size car or light duty truck. Towing with something smaller then this isn't fun, because you'll be towing something heavier then you. Towing a boat that's 800 pounds lees, still means you'll be towing very near your cars weight, so you'll want a reasonable tow car/truck. I know gas isn't cheap, but towing a 2,200 pound boat on a 800 pound trailer with a Celica is going to chew up more gas then towing a 3,000 pound boat on the same trailer with a mid size pickup or SUV, plus kill the brakes and the engine pretty quickly on the Celica. Think about trying to live with half of your ballast requirements in water (a third would be better) you save weight and maybe a smaller car. A general rule about towing is to pull with more then you're dragging if you want it to live for a while.
PAR,
Care to talk about trim change with heel. I'm curious what is acceptable.
To regress though, the center of buoyancy is purposefully aft. I may not be up on all of the reasons, but since the center of gravity is aft (due to crew, engines ,etc..), the center of buoyancy needs to coincide with the C.G. The waterline is also taken from theories for speed at the turn of the century and are thus in corporated here. It's the "spirit of" yacht thing.
So, to continue, I've got the displacement of my design at 4100 lbs. I'd like to keep the hull weight under 2000 lbs. (sans ballast) so that I have about 1000 lbs. of useful load. Enough for two people to cruise. (?)
Earlier, you were talking about trim changes with a Center of Buoyancy so far aft. As the design stands right now, the CB is 16.5" aft of midships. At 15 deg. of heel, it shifts 4" aft. That equates to the bow sinking 1.25" and stern raising by the same amount. Unless my trig is failing me, that is a 0.6 deg. trim change, bow down. That doesn't seem like a lot, but then maybe I'm not being critical enough. I'm aware of the increase in weather helm in this situation. If this trim angle is unacceptable, what kind of trim changes are acceptable?
Thanks for the input.
gonzo
08-14-2005, 03:36 PM
Is that the CB with or without water?
The CB doesn't change based on the type of ballast I use. The CB is based on the distribution of displacement when the vessel is brought down to it's lines in water. The water ballast tank will always be full, except for trailering.
I guess the answer is with water. However, if I dump the waterballast idea and go with lead ballast mounted to the keel, the CB will still be in the same location.
gonzo
08-15-2005, 12:18 AM
Yes it changes. Observe the ballast on older boats with long keels. It is usually lead or iron in the forward part of the keel. That is to change the CB and make the boat float on its lines. Water ballast won't do that unless you have narrow very high tanks with atwarthship dividers.
Gonzo,
You're confusing a couple of concepts. The Center of Gravity (CG) is what changes with the movement of ballast. The Center of Buoyancy (CB) or Center of Flotation (Xf) is fixed with the design of the lines. Granted, if a ship is not floating on her lines, then it's possible for the CB to be in a different location and yes the center of buoyancy shifts as heel angle changes, but the trim changes until the CB is aligned with CG. Obviously, that is the trick. Too align the CG with the CB, or vise-versa, and that's where additional trim ballast comes in to play.
gonzo
08-15-2005, 08:55 AM
CB and CG have to be aligned vertically for the boat to float on its designed lines. With water ballast evenly spread, it won't. This means the CB will shift. CB and CG will align themselves naturally. The trick is to make them do it where you want them to.
Gonzo,
Sounds like we are argueing the same point.
My statement: "but the trim changes until the CB is aligned with CG."
Your statement: "CB and CG will align themselves naturally."
My statement: "Obviously, that is the trick."
Your statement: "The trick is to make them do it where you want them to."
My Statement: "To align the CG with the CB, or vise-versa, and that's where additional trim ballast comes in to play."
Your statement: "CB and CG have to be aligned vertically for the boat to float on its designed lines. With water ballast evenly spread, it won't."
My statement: "Granted, if a ship is not floating on her lines, then it's possible for the CB to be in a different location"
Your statement: "This means the CB will shift."
I certainly appreciate the input. My statement though is that, if a vessel is floating on her lines, then the CB is in a fixed location and the goal is to put the CG at that location. It doesn't matter how spread out the ballast is. That ballast will have it's own CG, and all computations can be made as if it were a point mass at that location. The complexities arise when you start introducing accelerations to the mass(ballast). A long, spread out mass will have a different moment of inertia than a small concentrated one.
As far as shifting CB goes, every time the vessel pitches or rolls, the CB changes. That why a properly designed boat floats upright and returns to that position when it is moved from that position. To reiterate, If the CG is in the wrong location, the vessel will trim until the CB is aligned with the CG. In which case, the CB does shift, but in designing a watercraft, the CB becomes a fixed location, A target or "bullseye" so to speak.
Sorry to go "basic" on you, but I feel that my understanding of CB vs. CG are adequate. What I am looking for though, is an understanding of an acceptable pitch change with regard to heel. The CB in my design is 16.5" aft of midship at no heel. At 15 deg. of heel, it shifts aft by 4". This cause a pitch change of 1.25" down by the bow and up by the stern by the same amount. This equates less than a degree of pitch change.
You (LP) and Gonzo seem to be having difficulty over each others syntax, basically saying the same thing. Any object that floats will adopt a natural position that will place the CB in alignment with the CG. This may not be where you'd like it to be, but it will occur, like it or not. Neither are fixed (though we'd like to think so) but move around continuously. The true trick, is to design your immersed areas to permit the craft to float on her intended lines. Any mass changes, for any reason, will force the two centers to find a new common axis to live on. An old Greek worked this out some time back and it's well understood.
As far as pitch, I'm not sure what your referring to. Your suggesting the CB slides aft 4" at 15 degrees, which effects the trim, but it will also move to leeward. I'm not sure what you're asking frankly. Are you talking about trim changes, because of the distortion of the hull's waterlines when heeling?
My view of your hull shape suggests you've got the waterlines down reasonably close to the model you're after, but you've removed considerable area in the bilge, where the original form was "built down". I can understand the need for the volume reduction, given modern construction methods and materials, but that area of the hull probably provided most of the sailing qualities of the hull. Bill Atkins was rather famous for his shapes in this area and it attributed to the fine sailing qualities of his yachts as a result.
PAR,
Thanks foe the reply. I was looking forward to your input.
The CB change I was mentioning is when the vessel is heeled to 15 deg. while maintaining pitch and displacement. The fuller after body creates more buoyancy there. By my calculations, the bow will sink by 1.25 inches and stern raises by the same amount. This puts the CB back at it's original location. So in this case, the bow trims down by the bow when heeled. My understanding is that this is common and is one of those many compromises that are part of the design process. My question is, "How much trim with heel is acceptable?" This will cause a forward shift in CLR and cause a certain amount of weather helm because the bow is more deeply immersed and the stern is less immersed. Guess I'll have to run numbers from some other designs
I'm curious what you mean by "built down." Are you talking about at the turn of the bilge or down at the keel. I was looking to reduce displacement a bit and have read that a deep V with little keel does not have the best sailing qualities. With a draft restricted hull, I felt that having keel exposure was important and so have compromised beween displacement and keel area.
I think I've the Bill Atkins name before, but have studied any of his works. I'll have to track down some of his work. Do you have any leads.
Thanks again.
asathor
08-16-2005, 11:28 AM
I am not an expert on anything including traditional boats but I just sailed a brand new "old" workboat of the size you want (it sailed very well considering the antiquated rigging) so I have a few ideas to add. I have also been trailering a shoal draft Freedom 21 for 18 years and that I do know something about.
Bear with me as I state some design considerations and assumptions:
Since you want to use fiber glass you really don't need your keel to be one continous "log" for strengt (like the old boats).
You are expecting to give up hull dept to accomodate voluminous water ballast - so it doesn't really matter to you how that space is used, right?
when you trailer you need low draft in the front of the boat to get on the trailer - the back of the boat floats a lot higher.
If the "keel" does not keep the boat from getting off the trailer or back on the trailer it is NOT in the way.
You are willing to build you boat strong enough to be on a trailer with rollers.
You are satisfied with the way small full kealed boat sail and are only hoping for incremental improvements
And where does it take us?
A "Full Swing Keel" or should we say "Jack Knife Keel"
Let me explain. At some point near the front of the boat you put a giant (strong) hinge in yor keel so that it can drop down further like the blade on a jackknife.
When at rest on the trailer the keel fits nicely in its trunk in the bottom of your boat.
When you launch the keel it is latched in the up position - after you float/roll off the trailer you release it and it drops down to make a lower keel.
When you go on the trailer you simply winch the boat up - the keel will fold up and out of the way - off you go.
If you want to beach it you can add a mechanism for raising it if you feel you need to.
You can add as much lead as you can stomack in your keel - in the botton where it actually helps.
You still have most of the storage under the floor for growing mildew on old life jackets or cooling beer.
Build properly you can race across reefs instead of riding the Bronco at the local Cowboy Bar without fear of fallling off (it might spill your beer), because it will autoretract when pushed up.
Your boat will sail exactly like the old deep full keel type it looks like - just better.
The "new" boat types will have to trim their sails to keep up.
There aren't many hard and fast rules that apply to your question about an optimum trim angle at 15 degrees on this hull form.
You have a bit of a bastard there and direct comparisons are difficult, so the form needs dynamic study (shove it through some software) to determine what, where and when things will occur. Deciding if these qualities are desirable is pure speculation, though experience with similar modifications or types can increase your chances of success and it's this, that is the hallmark of good yacht design.
I like the tucked in or wine glassish stern sections and deadrise there, but don't like the quick rise in the run. I think you should flatten out the midship section a bit more and I can't see enough of the forward sections to comment, but would recommend they be "U" shaped, rather then "V", you can always toss in some flair at the top of the forward frames to provide the "look". The tumblehome aft is a nice styling touch, helping keep the "in the theme of" thing going. A boat with these recommend revisions will produce a craft with the motion you could expect form your base model, though it will be a livelier boat due to the D/L reduction, not one that is quirky.
Built down sections refer to the reverse curve found in many older designs as the planking approaches the garboard and keel. This reverse provides displacement down low, that is foil shaped, deepens the bilge for stowage, lowers freeboard and cabin top height needs plus can dramatically soften up the ride qualities of a yacht. On the other hand, it adds wetted surface, increases planking difficulty, increases drag and a few other less desirable traits if weight and speed are more important in the design compromises made to complete the brief. Since you've drastically reduced the immersed volume, you'll be better of with a "U" forward section, firm bilged with flattish midship section and "V" shaped stern sections. This will give you some of the qualities found in your base model. The high D/L in the original hull form contributed to it's sailing attributes a great deal, making a soft riding, solid feeling craft that great security could be had in. Your version will be quicker in most every regard, but should retain a lot of the sailing qualities, except in heavy air, where her lower D/L will slam you around a bit more.
People that make generalizations like ""V" sections not being fast, or not sailing well, frankly haven't been on many boats of different type or hull forms. I was on a L. F. Herreshoff - Bounty copy a few years back, spent the whole afternoon battling a fleet of plastic cruiser/racers. The winds got up into the 20's and they were reefing down, changing headsails and tying things down, we were laughing at not spilling our beers and debating, rather calmly, when we should begin to think about a reef in the main. At the end of the day we all got together at the dock and they were all very wet, we were quite dry. Their sloop and cutter rigs got us to windward, but we kicked they butts once freed up a little. We had mahogany, raised panel seating and dining areas to enjoy, they sat on sail bags, full of wet rags and plastic settees with sopping wet cushions. We had a very easy slosh of it, they battled hard for the few gains they got. Which yacht would you like to cross the puddle in?
The point is, a lot of the design decisions made are reflected in the type of sailing you want to do. You have a cruiser there and will win few matches against modern production craft, but you'll be closer to what you want, then they likely will be. At this point you need a full hydrodynamic work up and as much comparison as you can with similar types (good luck) to see if you're in the ball park for your desires in this little yacht. Stop worrying so much about the latest arcane developments in theory, engineering, structure and whatever else. You focus should be calculating reliable, practical, solid scantlings for this vessel, after you've developed it's shape for it's intended service, which should match you needs as closely as the necessary compromises will permit.
Mark 42
08-18-2005, 03:00 PM
...And where does it take us?
A "Full Swing Keel" or should we say "Jack Knife Keel"
Personally, I like the retractable keels on the modern boats (e.g. Melges 24),
but they don't lend themselves well to traditional shape hulls.
I would angle the keel so seaweed doesn't always stay wrapped around it.
...And where does it take us?
A "Full Swing Keel" or should we say "Jack Knife Keel"
PAR,
I am greatful for all of your input towards my project.
I love the wineglass stern of a shapely yacht. I can't say that my design is shapely, but maybe I can pass along a little of the elegance and beauty of a bygone era. Some of the defining elements of this design are to correct the shortcomings of my current boat. First, in pounds to a stop in a chop (powerboat wakes). Secondly, it drags it's stern through the water. Makes a heck of a racket back there.
So anyway, I've put in a very fine entry with a fair amount of flam to start the bow back up in a controlled manner. My concern is that it might be too fine and the bow would have a tendency to get buried. I'd like to take your suggestion on a "U" shaped section forward. I've tried to in corporate that idea, but one of the ways to increase lateral area is with the prominent forefoot of the Casco design. Blending the forefoot into the forward sections produces a "V" section by nature. It's kind of a convoluted section. Lower portion flaring from the keel to the waterline, then flam to the sheer.
I've put some powerboat theory into the mid and aftersections of the craft. From midships aft there is a constant deadrise of 18 deg. I don't expect to plane in this craft. The constant deadrise section of a planing craft produces less drag than the changing deadrise sections of planing craft from the turn of the century (and possibly through the mid part of this century). I'm probably applying some theory erroneosly, but by maintaining a constant deadrise, the accelerations on the water surrounding the hull are minimized by such a section. BUT, does that really matter in a displacement hulled craft? Actually, I had forgotten that I had put that much deadrise into the hull. The sections in the afterbody of the Caasco are closer to 14-15 deg. Maybe I should target that. I like a craft that "talks" to me. So, having a a craft that sails on heel gives me security. Gut feeling tells me that a "deadrise" hull will heel sooner, but will shift it's lateral CB earlier, as the weather bilge will rise above the surface sooner, and will begin to stiften earlier than a hull with flatter sections. Sounds like a contradiction to me. :) I guess that's where a numerical analysis comes into play. If you were to flatten the midsection, would you strive to maintain the current displacement or would you accept a greater displacement and the possible requirement for more ballast. If this weren't a trailer sailer, I'd go with the added displacement and ballast.
I left the built-down sections out for two reasons. 1) Ease of construction, 2) Reduction in displacment. I think it would be great to have the sections of the Casco type. Actually, I think the wetted surface would be reduced. I'm looking to strip-build this craft. I just couldn't visualize the transition at the garboard. I could start the built-down sections and finish them with an epoxy fillet at the keel. Just a thought.
I'm open the the hydro-dynamic work up you've mentioned. Any suggestions? My location is land-locked in Kansas. Not too many NA's around here with the right kind of software.
In the meantime, I'll try to fatten...I mean flatten the midsection, reduce the deadrise a few degrees and shift the heeled CB forward a bit (fatter foreward sections, Argh).
Thanks again for all of you insight.
P.S. The water ballst might go by the wayside. Maybe there is still hope for me. :p
Right now, I have no intention of having a swing-keel. I feel like I have sufficient lateral plane area, but I'm not ignoring the idea. I'm looking for simplicity in the build and in sailing. Thanks for the input. All things are always subject to change.
asathor
08-18-2005, 11:58 PM
I would agree that simplicity is great but looking at the map of Kansas I see a lot of interesting reservoir lakes most most of which follow river valleys. To sail them you want a boat that goes high on the wind comfortably otherwise you will be tacking until after the cows come home. To me that means a reasonably sophisticated rigging and sails and some kind of lee board, blade, keel - whatever it takes for your hull. My little Freedom (shoal) does not get up to full speed until I tack at 120 degrees (60 each way on the true wind) going closer only yields a better VMG when the water is flat (with my old sails - new would be a little better). Also shallow hull/keels tend to stall and generate far too much leeway without speed.
If Kansas is anything like the Dakotas you will have plenty of wind so trailering is your primary weight/displacement limitation and you do not have to have the tallest rig in town. A cutter rig with a full roach main with battens may be an idea that could give you a lot of power with a low sail plan. You can size the main to match some of the catamerans and buy cheap used sails.
Asathor brings up a good point, in that lake and river piloting requires a fair amount of close quarters maneuvering. That long, shallow, draggy keel isn't going to help you much in this regard.
I'm not sure if the deeper forefoot is going to help with your concerns over getting slapped around by chop, as you current boat does. Small craft get beat about, mostly because they lack the mass necessary to carry through. Shallow forward sections can make matters worse, but that isn't typically a problem with the older shapes you seem to have an interest in.
You sketch looks to carry a reasonable amount of draft to provide sufficient lateral plane without trying to add more in the forefoot. A more centralized appendage, still with the moderate draft and well shaped forward sections can be had with a reduction or increase in the displacement. I'd be inclined to try to maintain the qualities of the boat (built down sections and all) by moving the waterlines around to get the desired displacement.
Log on to http://www.atkinboatplans.com/ and take a look at some older hull shapes that you may enjoy. The displacements will be higher then the standards currently employed, but this can be changed to suit your needs and you'll retain the sailing qualities of the craft.
As currently designed your boat will not have the qualities of the model you're working from. Altering these older designs and retaining the abilities of the vessel can be quite difficult, but not imposable.
sharpii2
08-24-2005, 05:57 AM
PAR,
I am greatful for all of your input towards my project.
First, in pounds to a stop in a chop (powerboat wakes). Secondly, it drags it's stern through the water. Makes a heck of a racket back there.
Actually, I had forgotten that I had put that much deadrise into the hull. The sections in the afterbody of the Caasco are closer to 14-15 deg. Maybe I should target that.
I like a craft that "talks" to me. So, having a a craft that sails on heel gives me security. Gut feeling tells me that a "deadrise" hull will heel sooner, but will shift it's lateral CB earlier, as the weather bilge will rise above the surface sooner, and will begin to stiften earlier than a hull with flatter sections.
Sounds like a contradiction to me. :)
In the meantime, I'll try to fatten...I mean flatten the midsection, reduce the deadrise a few degrees and shift the heeled CB forward a bit (fatter foreward sections, Argh).
Thanks again for all of you insight.
P.S. The water ballst might go by the wayside. Maybe there is still hope for me. :p
Dear LP:
A couple of things.
A 'V' section is only more stable than a flat one because it is possible to get the ballast lower without also much lowering the sectional center of bouyancy. Since ballast is usually more dense than water, the needed amount of it usually fits nicely into the lower part of the 'V', where as the CB stays in the upper part of the 'V'. There is no contradiction. The 'V' hull will heel sooner and to a greater degree than a flat section. But. But, if there is enough ballast, and the 'V' section is deep enough, The 'V' section will obstinately refuse to capsize. It will let the wind knock it all the way over on its side, then pull itself upright at the first let up. (A deep, ballasted, fin keel on a shallow flat bottomed section, by the way, would do the same thing)
The flat, straight sided section, on the other hand, will be initially stiff, especially in your normal sailing range of heel, but will quickly lose stability as the windward chine rises. There are two reasons for this. One, that the flat section to float rightside up at all without an enormous amount of ballast, needs to be relatively shallow. And two, when it is this shallow, it can stand a very high center of gravity and still be stiff enough to carry sail. This can make it very dangerous. It will feel perfectly secure until it's too late. It's interesting to note, however, that the same section, but ballasted and much deeper, is even more tender and even more resolute about not flipping than the comparable 'V' but is so much heavier that it requires much more sail for its given beam and sails at a much sharper heel. When it does this, The leeward chine presents a sharp 'V' to the sea so it will, paradoxily, pound less. Phil Bolger's earlier sharpie designs had this characteristic.
The biggest fault I can find with your proposed design is the extreme double wedge shape. When this shape heels, The bow tends to bury because the heeled center of bouyancy tends to be much further aft than the level center of bouyancy. This has a very bad habit of making the boat round up into the wind. My boat had this fault and I once cracked a rudder stock trying to keep it on its course during a strong gust. For this reason, you may find yourself reefing due to controllability reasons long before you do so for stability. This can be lived with. Just know what you're getting into. The long, sharp bow may cleave the sea nicely. But that may be all it wants to do. Just because a little more of something is good, doesn't neccessarily mean a lot more is better. I would move the maximum beam forward by about half a station (5% of the boat's length).
Other than that, I would make no changes.
Of course, a full preliminary wieght study needs to be made to determine where the center of ballast weight needs to be. Since this boat is to be strip planked, the the cg of the hull itself is a major concern. Also, whether or not you are going to use an inboard or strap an out board to the transom needs to be taken into account as well as how many people you will usually have in the cockpit. My guess is that the center of ballast may have to be forward somewhat of the max Beam.
As for water ballast.
It's interesting to note that the original boats used stones for ballast. And the stones were probably granit. I read somewhere that granit weighs approx. 175lbs/cft. Now water weighs 62lbs/cft which is, of course, much less. However, that is not the whole story. The original boats had only internal ballast. Other than plain soakage, there was no weight on the keel structure at all, making it neutrally bouyant at best. If you took a 3x3in square steel bar of about ten ft in length (300lbs) and made it the very bottom of your keel structure, you could, perhaps, use water for the other 700lbs of 1000lb estimated ballast.
The hope is that the vertical distance of the cg of the steel would at least somewhat make up for its lack of weight. It would provide the lion's share of the righting moment where as the water would provide the wave splitting heft. Maybe with this combination you can get away with fact that it would have a combined density of slightly less than half that of the granit stones. Making the keel hollow by making it out of two 1/2in plywood sides would further help the cause by eliminating approx. 100lbs of bouyancy down where it would do the most harm.
This would reduce your water ballast tank to approx. 10cft capacity while trimming your launch ramp weight by approx. 23%.
When I started my own water ballasted project, I decided that I didn't want to deal with valves of any kind beneath the wl. In exchange for that, I was willing to put up with long filling/draining times of at least 1/2 hour each. This way, I needed just little holes in the bottom as well as a single bleed tube on the top of each tank. My boat was to have four tanks. By capping the air bleed tubes, I hoped to cut the drainage rate by at least 1/2. Now the tanks would take at least an hour to drain out. That and the narrow, high sided proportions of the boat were supposed to insure she self righted long before she lost much of her 1400lbs of ballast.
I am sure that for your project, this would be unacceptable. My boat was to be trailered very infrequently (which is why I abandoned water ballast to begin with). But, by the looks of it, yours would be trailered all the time. I would suggest for you a single one way valve which would have a pull string that would end at the top of a much larger bleed tube that would have cap on it. The draining system would work much the way an average toilet works ;). You would first get the boat on the trailer, pull the string, then slowly pull the trailer up the ramp. A fill/drain rate of about 40gal/min should be quite sufficient. Within 3 minutes it would be drained dry.
Well. That's my $0.02 worth.
Best of luck on your project.
Bob
P.S.- Did you build that gourgeous looking boat, you now own, yourself?
Bob,
Thanks for the input. I did mean hydrodynamics. I can work my way through the hydrostatics with out too much trouble. I'm re-evaluating the whole water ballast idea. If I can gain even 3-4" of head room in the cabin, I think it will be worth it. I'm also looking at building the garboards down as PAR suggests. I'm hoping to still keep the displacement under control. I initially had trouble visualizing the construction, but have designed the solution. I'm also going to move the CB forward slightly and move the lines around until the heeled CB is better behaved. The Casco type also had a bit of a foil shape to the keel. I might be able to hide a swing-keel/centerboard in there. Maybe some rocks instead. :) Anyway, I've got some drawing and calculating to do.
Oh, and "yes." I built my current boat. It's a Karl Stambaugh design. A Meadowbird. Originally created by S. S. Rabl as a "Titmouse."
sharpii2
08-28-2005, 06:14 PM
Bob,
Thanks for the input. I did mean hydrodynamics. I can work my way through the hydrostatics with out too much trouble. I'm re-evaluating the whole water ballast idea. If I can gain even 3-4" of head room in the cabin, I think it will be worth it. I'm also looking at building the garboards down as PAR suggests. I'm hoping to still keep the displacement under control. I initially had trouble visualizing the construction, but have designed the solution. I'm also going to move the CB forward slightly and move the lines around until the heeled CB is better behaved. The Casco type also had a bit of a foil shape to the keel. I might be able to hide a swing-keel/centerboard in there. Maybe some rocks instead. :) Anyway, I've got some drawing and calculating to do.
Oh, and "yes." I built my current boat. It's a Karl Stambaugh design. A Meadowbird. Originally created by S. S. Rabl as a "Titmouse."
Dear LP:
Have you done the displacement calculations yet? I could not read the text on your drawing. It appears that your design is approx.
8ft wide.
By my calculations it should displace at least 4500lbs, maybe as much as 5000lbs. You might reduce the Beam a bit and keep the deadrise proportionate. If you did that, your displacement would go down considerably.
By my estimation, reducing the Beam by 1ft would reduce your deisplacement by at least 1000lbs.
This way you would also be able to move the Max Beam forward some and still keep the same finess in the bow. Wave penitrating power comes from either sail carrying power, or heft or a combination of the two. Bow finess helps too.
What I mean by heft is wieght in relationship to Beam not length. By making a narrower boat, it is possible to have plenty of heft while, at the same time, having a relatively low D/L ratio. Multis, with their slim hulls, use this trick all the time.
Monos with good heft (with proper CG location) are not only easier to self right but also harder to capsize in the first place. Joshua Slocum's spray had quite a bit of heft dispite its enormous Beam. I doubt, however, that she had a CG low enough to recover from a capsize (a knock down of more than 90deg.) But because of her heft, as well as her long keel, and prudent handling by her skipper she ended up being so succesful that, even today (with some modern improvements such as external ballst) she is much admired and copied.
My own personal project would be 20ft long, 5ft wide, and displace 2800 to 3000lbs. 300 of which would be concrete and 600 of which would be sand bags. The good news is that she should self right from a 140deg capsize. The bad news is that she would have such high sides and deep deadrise that she would only be able to carry around 150sft of sail. This would make her, in performance terms, a dog.
That being said, she would also be convieniet, well mannered, and have good to excellent course keeping characteristics. And she would split channel chop rather than be stopped by it. Everything I would want for a mini voyager.
Her vertues on top of her vices make her a perfect custom boat. Not one many people would want to copy.
So too may be your boat. Just be sure you are aware of your hierarchy of requirements. Boat design, like economics, is a dismal science. For everything you get there is something you must give away.
Good luck on your project.
Bob
P.S- Incase you are curious; here is my 'heft formula'
20*displ. vol./Length*Beam^2
With moderate ballast(33%) and moderate draft(1/2 Beam) My ratios are:
0.75 is low, for good to excellent sail carrying but poor wave penetrating,
1.00 is modrate, for decent sail carying and decent wave penitration, and
1.50 is high, for poor sail carrying and excellent wave penitration.
Hey Bob,
Actually , my displacement is where I want it. Right now it is coming in at 4200 lbs. I also think that I have a relatively fine hull form. It's just over 3 to 1 L/W ratio with a 25' LOA to 8' beam. Waterline beam is at 7' and that drops to 6' at 15 deg. of heel. The prismatic coefficient of the canoe body is 1.13. I'll leave you to play with that one. While a narrower boat would be fine for performance, I feel I'm compromising interior volume already. I just redrew the lines with fuller forward sections, built down garboards, a foil section in the keel and a gentler rise in the aft buttocks. I haven't redrawn the waterlines yet. I'll post it when it's cleaned up.
I've got the LCB forward a bit more. Until I do a definitive balance calculation, I'm not going to dink with the hull shape any more. On a craft this size, crew will have a significant effect on balance, so I see an aft LCB as being desireable. The nature of the aft lines gives increased buoyancy rapidly which will compensate varying crew weight and should I encounter a large following sea, the additional reserve buoyancy should keep me from being pooped. Yes, yes, I've read too many heavy-weather stories. The problem is, I still get the same bow down pitch change with the new set of lines. It may be one of those conpromises I have to accept. Thats also where I'll get a lot of sail carrying power.
I get a 1.14 by your formula. Pretty close to my prismatic coefficient.
You'll have to convince me of your heft theory. I see wave penetration effected primarily by hull form and inertia (weight). My current boat is unballasted and bluff, but not excessively so. It grinds to a halt pretty rapidly in the right type of wave system.
Thanks for the input. More latter. See ya.
sharpii2
08-29-2005, 07:52 AM
Hey Bob,
Actually , my displacement is where I want it. Right now it is coming in at 4200 lbs. I also think that I have a relatively fine hull form. It's just over 3 to 1 L/W ratio with a 25' LOA to 8' beam. Waterline beam is at 7' and that drops to 6' at 15 deg. of heel. The prismatic coefficient of the canoe body is 1.13. I'll leave you to play with that one. While a narrower boat would be fine for performance, I feel I'm compromising interior volume already. I just redrew the lines with fuller forward sections, built down garboards, a foil section in the keel and a gentler rise in the aft buttocks. I haven't redrawn the waterlines yet. I'll post it when it's cleaned up...
I get a 1.14 by your formula. Pretty close to my prismatic coefficient.
You'll have to convince me of your heft theory. I see wave penetration effected primarily by hull form and inertia (weight). My current boat is unballasted and bluff, but not excessively so. It grinds to a halt pretty rapidly in the right type of wave system.
Thanks for the input. More latter. See ya.
Dear LP:
My 'heft ratio' is not at all related to the Prismatic Coeffiecent(CP). The Prismatic Coeficient is based on your boat's displacement volume/the area of your largest underwater hull section*your boat's waterline length. CP's of more than 1.00 are therefor impossible. A typical CP for a sailboat is between 0.50 to around 0.58. Since your boat has a long keel, which may account for a significant portion of your boat's displacement, it is probably advisable to include the keel in that calculation. It would be interesting to see how others feel on this issue.
With my 'heft ratio', the entire 'bouyant body' of the hull is included. This is because often the whole hull has to get through the wave, not just the underwater sections. So. For your boat that would mean: 4200/62=67.5. 67.5*20= 1350. 1350/8*8=21.1. 21.1/25= 0.844.
0.844 is a little less than half way between 0.75, which I consider light, and 1.00, which I consider around average. Not too shabby for a trailer sailer. Compromises. One must also keep in mind that the displacement you are talking about may be the boat sans people and provisions. If so, you can probably add at least 500lbs to your displacement.
I would be interested to know what the Length, Beam, and Displacement of your present boat is. A friend of mine bought a full ended daysailer. It too pitched a lot when confronted with a chop. But sailing it in a strong wind was a revelation. No sore arm trying to keep it on course like my old boat. I read somwhere that a wave train of the right frequency will stop any boat. I think it works on the principle of 'harmonic resonance' or something like that. I think they said the cure is to angle either into or out of the wave to change its relative length.
Thanks for your response.
Bob
Just for grins, I thought I'd upload a couple (or three) of images. The hull form is essentially correct. The drag has not been added to the keel. Nice to see it in 3D though it is still only a work in progress.
sharpii2
09-01-2005, 09:08 PM
Looking pretty cool. It would be interesting to see a traditional boat type brought into the 21st century to meet modern needs. It will be interestng to see how it does.
Jocko
09-02-2005, 12:05 PM
envisioning the holes/vents makes me think that I've seen this system elsewhere - submarines. On the same idea - why just a vent? why not a small compressed air tank?
2 thoughts - a vented ballast tank could/might slosh/fill/empty with the craft's motion/heel
a shallow boat, as depicted, would benefit from lateral displacement of ballast -> moving water to the high side.
The single vent would have a valve. If I were to use this system, I'd launch the boat with the vent closed. Once floating free of the trailer, I'd open the valve and move aft, alowing the bow to rise. Once the tank was filled, I'd go forward to the valve and close it, thus trapping the fluid. Since the top of the ballst tank would be below the waterline, it would fill completely if designed properly. In a previous post I also suggested attaching an air compressor at the valve to empty the tank prior to retrailering.
I'm looking for lo-tech here. One valve and maybe an airpump. Murphey is everywhere, so I try not give him reasons to hang around.
I'm on the fence right now with regard to a water ballast system. I've not had a very warm reception with my concept, possibly for good reason. It's not a racer, so I don't want to deal movable ballast.
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