2-3m Sit in Performance orientated yacht

Hi everyone, this is my first post here with an idea thats a bit weird but I keep thinking it would be awsome if you could get it to work. I posted about it a while back over on Sailing Anarchy but the thread went pretty crazy and it was the only way to get real world "performance" e.g. over 6 knots, was to use a multihull or foils or moving ballast, or lean over the side.

My normal boat can probably do 20 knots, so I'm not to concerned about going that fast. The idea behind this is more to learn how to build a boat, then use it to burn around the local lake after work or when I don't feel like putting a trap harness on etc.

The basic concept is a small sit in boat roughly 3m long, not sure about that yet. Bigger then this http://www.youtube.com/watch?v=tWRFd9kQmqY but smaller then a 2.4m. Shaped for maximum performance for its length, carrying a powerful sailplan with a masthead kite, probably assymetric.

I'm no boat designer, however I do have the 3rd edition of "How to design your own boat" coming in the mail, which I have heard is pretty good. I'd like to go a monohull just so I only have to build one hull, and I already sail multis.

My options as far as I see are:
Narrow beam, deep lifting keel. Don't have to slide around to stay on the windward side, could use a sea kayak spray skirt to keep the water out. Less room, could be warmer, but also more cramped. Never going to plane. Will point higher.

Wide, open 60 shape hull, deep lifting keel. Will dig a hole in the water until heeled or moving fastish or both, gotta move around more. Might plane in 30
knots, maybe...

As far as construction goes I've gotta learn how to design the thing first. If other forum users think it could be doable, however it could change depending on money, which is tight as it is for most people that just left school, and where I'm at with my real sailing etc, so theres a good chance it might never happen. The rig would come off something else, I have a decent older cherub mast in my shed. At this stage the easiest way I could see would be too make a strongback out of 2x4's, make ply frames then plank it out of cedar or balsa, or foam if thats possible? What does that much material cost roughly? (like I said, I'm more of a racer, not a builder). However the final boat would be wet, and feel fast due to the closeness of the water, much like if you watch a hobie adventure island video.

Thoughts would be much appreciated for my probably hypothetical idea.

Thanks, Sailingkid

 

Just out of curiosity are you opposed to movable ballast? I don't see how you're going to get "maximum performance for its length" without it-certainly not with a keel! Welcome to the forum!


Minuet + thread here: http://www.boatdesign.net/forums/sailboats/minuet-yachts-2m-fun-boat-can-design-improved-40935.html

Picture: Neil's classic example of movable ballast in a small boat: a bucket of water suspended from a "trapeze" wire/line-

 

genius Youtube! love to follow this Thread

 

Having read the thread Doug posted, it seems this whole ideas been pretty well disscussed. I'd probably bring it down to 30cm bigger then a minuet to accomadate a taller person like myself, then use a simple form of hull construction to make it. For at least the first model I'd rule out moveable ballast, just to keep it within my super tight budget and to adhere to the kiss principle. The minuet style of ballasting which I kind of understood in having the person mostly below the waterline seems like the way to go. The never-going-to-plane theory in the other thread made me think maybe a big non-assy kite would be the way to go so at least your going with the waves and might be able to surf them, as maybe you wouldn't have the righting moment in an unballasted boat to heat up enough to really get an assymetric drawing. Does anyone know approximate costs of cedar or good foam/glass these days? Can you plank foam using the bead and cove method over such tight little curves like you can with wood?

 

Sailingkid, if your current boat is doing 20 knots, you already know what it takes to get real preformance. The term performance is a user defined attribute, so if you state what you want, a target speed, stability range, etc., then you'll get some replies. Frankly staying in the boat means, you'll likely remain at displacement speeds. This is a function of the size of the boat. Even a 9' cat will not carry enough initial stability to remain upright, while offer some level of scoot. In other words, you have to get wet and get out to hold down a small boat, if you expect anything more then displacement speeds. You could develop a unique, limited operation type of craft, such as a proa or canting contraption, but contrivance is usually costly, convoluted and likely to break in trials, as you explore the possibilities of it's preformance potential.

 

To remain:
* simple to build
* inside the boat
* max performance for length,

the minimum weight bulb keel (just sufficient to right flattened boat + margin) and DSS for "sailing stability" looks like the way to go.

The lifting keel is already stated as part of design brief.
The DSS foil physically is similar to one more daggerboard to build in to boat -not too expensive or too complicated that is. The scientific part of it could prove more difficult -foil shape, angle of attack, span, area. Contact with author of DSS could prove helpful.
As to the rest -symmetrical spinnaker look a little overcomplicated for fun sailing on so small a boat.

 

Hull speed for a 12' boat is approx 4.64 knots in displacement sailing. Higher speeds may be achieved surfing down a large wave (probably too big for micro-displacement sit-in boat).

I doubt DSS would significantly affect performance in a displacement boat this small, other than make tacking a logistical nightmare in a sit-in boat. Even if it showed a 10% improvement in speed, it would then reach a not-really-staggering 5 knots. Shuttling a more or less horizontal board back and forth on every tack (2 inches or so under your behind) seems like a lot of work for less than a knot improvement. You would have to create some type of Rube Goldberg cranking mechanism to move the board, dramatically complicating things in a tiny boat with little to no room for complicated mechanics.

As the hull speed of a boat gets higher, the real benefit from DSS increases. 10% of 18 knots is almost 2 knots - which is significant and perhaps worth the complication and cost. In a 12' boat I doubt the benefit is worth it, because it may not significant enough to "feel". Taking a simple, unballasted displacement hull like the Minuet, adding a lifting bulb keel and then adding DSS just seems like a lot of work, cost and complication for a trivial speed increase. Unnecessary weight is the enemy, so why add a keel bulb if it is not needed?

Contacting the owner of DSS would certainly result in increased cost as the designer is certainly due their technology licensing fees and consultation costs.

I agree with your comments about an asymmetric spinnaker, prod, launching/retrieval and sheeting in this small a boat. If SailingKid wants speed, he can opt for the 29er-I14-49er he sails for the day. Making an overcomplicated tiny displacement hull seems out of sync with his original post looking for a simple homebuild sit-in ride when he doesn't want to trap.

You can make a pig fly with sufficient thrust, but that doesn't mean you should. Putting DSS on a small displacement micro design seems like grafting a Formula One rear downforce wing on a Yugo. Might look cool, but probably won't make enough of a difference to be worthwhile.

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CutOnce

 

Dynamic Stability System

Actually, if somebody was willing to try DSS experimentally on a small boat they would get the help they need at no cost. When a foil is moving it generates lift-even at 4 knots. A twelve footer, though, is capable of extremely high speed off the wind-see the 12' video on "Great monohull video's" on this page. DSS could replace the second crew on a twelve footer at planing speeds. Upwind at hull speed DSS won't generate a whole lot of RM-say around 158ft.lb.-but there is a lot more to DSS than just RM-it positively affects the handling of the boat upwind and downwind. Downwind at 8.6 knots it will generate 194lb of lift or 521ft.lb. of RM at a RA(righting arm) of 2.7'. And much more as speed increases. And I know-I just got thru doing a lot of research winding up with a design for a 15 footer with DSS + movable ballast for a single(heavy) person who sits in the boat w/o having to move-a modern replacement for a 2.4 meter. I got the help I needed from Hugh Welbourn but it helps if you have a pretty thorough understanding of how DSS works-Hugh doesn't provide classroom instruction.
Relying soley on DSS for upwind RM is not a very productive idea. Every DSS system so far has used a fixed keel + movable ballast(crew). A dinghy would need some form of movable ballast.
A twelve to fourteen + footer is an excellent range where DSS + movable ballast would work very well together.
Smaller than 12 not so much.....

 

Apples and oranges, Doug. A 12' / R Class type skiff with two traps, featherweight planing hull, racks and T-foils is NOTHING like a comparatively stable micro displacement hull where the crew sits-in and basically forms the bulk of the ballast below the water line. Just because the length may be similar is no basis for extrapolating performance projections.

In an ultra-lightweight planing skiff like an R-Class or 12' skiff if you think the crew can be replaced by DSS you might want to reconsider. You are moving constantly on the wire - in and out, forward and aft. You have to anticipate gusts and lulls, lifts and headers just to keep the boat upright. There is little possibility that a static and significant lift component on the leeward side of the boat can do the same things.

DSS may be worthwhile on larger keelboats where things happen a lot slower and the boat isn't actively trying to assume it's only stable position - upside down. I'm perfectly willing to concede that DSS may be a worthwhile technology in the right circumstances, but a micro-displacement hull may not be one of them.

The point of these threads is to provide sage advice and opinions to help the original posters with their questions. For a first time homebuilder looking for a sit-in ride like the Minuet (but a bit bigger), adding DSS and bulb keels to the mix seems excessive.

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CutOnce

 

Actually, under "max performance for the length" in original post could be interpreted as requirement to sail well above hull speed. In this context, DSS foil (together with other design characteristics) could possibly provide just enough power/weigh ratio to reliably plane quite often.
The key word is "possibly". I do not claim it will be everything.

If we put more weight on "diplacement speed only", than simplicity without any more extra appendages sound more sensible.

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The statement of requirements has some internal contradictions here.

 

The point is that DSS foil lift is not static, because it operates close to surface.
Same principle works for foil-born motor boats on surface-piercing foils: the deeper the foil is submerged, the greater the lift.
Here we will have the situation, when increase in heel=>increased submersion of DSS foil=> increased lift=> increased righting moment.
I do not say it all will happen automatically or without some cut-and-try.
I say it should be possible, because basic effects work in the right direction.

 

I disagree.

As a DSS foil, it becomes less effective as it is further immersed. The boat is rotating on the "roll" axis, and the angle of the foil to the surface increases. As the foil to surface angle increases, the lift vectors change and part of the lift that was vertical is now increasingly horizontal - increasing leeway. VMG will suffer, and the vertical lift component degrades as more and more of the foil is angled downward - and when at 90 degrees to the surface, the lift is 100% horizontal and leeway is maximized. At 45 degree angle to the surface, the vertical lift component approaches HALF it would at 0 degrees - and the horizontal induced leeway would be HALF the original vertical lift. It's just basic vector math.

DSS works effectively only when it is close to the surface generating maximum vertical lift. Like all lifting foils, smooth un-detached flow is most effective - therefore the turbulence ensuing from dramatic lateral pressure changes from gusts and lulls will affect function.

I'm not arguing boundary layer versus deep immersion fluid dynamics - I'm just considering lift vectors. Think about it.

All this hypothetical armchair discussion would be best served by tank testing with load cells measuring things.

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CutOnce

 

DSS is a thoughly tested and proven technology. Most DSS boats are designed to operate at around a 10 degree angle of heel upwind. Significantly more than the design angle of heel creates a lift component to leeward-not good. Downwind it doesn't matter. On a small boat it is difficult to get the design right with the chine low enough so that the foil is one
chord below the surface @ 10 degrees. This requires a curved foil OR two fixed foils( fixed removably in side trunks).
SK said nothing about a "displacement" hull though he did mention "never going to plane" that he said he gleaned from the thread I referenced. He must have missed the part about the little (8') New Zealand boat that planes off the wind. Which means a boat as small as 8' could be designed with
DSS primarily for off the wind and the thing could carry lots of offwind sail(big asy spin).
He also mentioned that he has a 12' Cherub(DA thread) sitting in his garage. Cherubs are sailed two up like the 12' skiffs. So if you put one person in a keel or 50% of one person in a sliding ballast system + DSS you have a boat that performs very well-about as close to "max performance for its length" as you can get
in a "sit-in" 12 footer. Saying "max performance for its length" for a "sit-in" boat means there will have to be some development effort,no ifs ,ands or buts.
And none of this is worth a damn w/o carefull, thoughtful design. Just slapping something together is not going to do too well.
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Frankly, I think DSS is beyond the scope of someone who has never designed a boat before and who is not interested in development so the options are:
1) a Minuet type sheltered water displacement design where the crew is the only ballast designed from scratch as a first effort,
2) converting an existing dinghy to a sit-in type with the addition of appropriate ballast. So the dinghy has to have the capacity for the crews weight plus ballast. May require reduction in the dinghies rig to reflect available righting moment.
Ballast probably in the keel since a sliding ballast system would require some development(not much-see the "bucket" dinghy above).
3) the simplest for SK seems to me to be converting his Cherub? Sliding ballast or a keel with very reduced rig commensurate with available righting moment.
4) for a high performance "sit-in" monohull the combination of sliding on-deck ballast and DSS would produce a new type of sit-in boat with superlative performance way surpassing any other current sit-in monohull. And it would be exciting to sail as well. It would require some development time but anything really worthwhile does-especially IF you're trying to achieve something that is not currently available. See Trapwing picture below as just one example of sliding ballast in a sealed wing:
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rough sketch of "Little 60" sail plan for a 12 footer: Note- 80lb sliding ballast wing just ahead of mid-boom mainsheet. DSS foil 1.5 sq.ft,6" chord sticks out to leeward 3'. For this size boat two DSS foils fixed(removably) in side trunks will work as opposed to a sliding DSS foil. The windward foil is always clear of the water.
Simplifies a DSS installation. Still requires that chine is low enough for foil to be one chord below the surface at 10 degrees angle of heel.
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click on image-

 

Please note, that when foil is nearing the surface, lift created is getting smaller and smaller. So all else being equal, foil deployed to leeward will increase its lift and righting moment, if heel, and hence immersion of foil is increased, up to a point.

Here we obviously talk about different ranges of heel angles.

If we assume heel angle range from 10 to 45 degrees, you are absolutely right. At 45 degrees of heel, lifting and leeward components will be equal.

If we assume heel angle range from 10 to 15 ( maybe 20 degrees on rare occasions) degrees , vector direction change is not so large to be catastrophic. Say, normal sailing heel is 10 degrees with foil horizontal, ~1m span, ~200mm chord, immersed for ~200mm (lets assume the same 2-3m boat here). If heeled 5 degrees more, downwind component of lift will became ~9% instead of 0. Not too good, but not catastrophic either. Immersion at the tip will increase for at least 90mm, making it from 1 chord to ~1.5chord. Lifting force, especially closer to tip (where it is most effective for righting the boat) will increase and increase the righting moment, restoring equilibrium, or at least giving the crew some time to react. But that is only start of story -as gust hits, not only heeling force is increased. Driving force, and hence the speed is increased as well. As lifting foil will move trough the water at greater speed (force =C*(speed^2)* ... ) the lifting force and righting moment will increase again.
From DSS boat tests in action, reported here and there, I could conclude, that just described sequence of events do actually take place in reality.

.... All this hypothetical armchair discussion ....

As to this, let me inform you I am considered qualified enough to be hired as a skipper for yacht races, including some international ones, and get some sailing yacht/ship design works, besides of my daily job at NA office.

 

Agreed.

There was no personal component to my "hypothetical armchair" objection. It was an indictment to the general oversimplification of technologies and the habit of issuing proclamations of "thoroughly tested and proven" with the only citations being "I read it on the Internet" without public access to research data and peer review. Again, please note this is not an indictment of DSS or it's owner's claims, just how that preliminary promotional/technical presentation somehow becomes gospel.

Every technology and idea may have specific cases in which it is beneficial - and a lot of cases in which "not so much" may apply. I've been consistently attacked for trying to look at all sides (including liabilities) of new technologies (even this balanced and fair response will probably generate a frothing retort from Florida).

In this specific case of a somewhat scaled up "Minuet" style design, I'm not certain DSS is worth exploring. I have reservations about DSS as a crew replacement for high performance dinghy/skiff sailing as it is a reactive not anticipative technology. If skiff crew on a trapeze respond a couple seconds AFTER a gust hits, the only performance increase will be in their swimming abilities.

I would be glad to be wrong about this, but as I stated clearly all along, these are my opinions.

Do I think DSS may fit well with "sportboats" and larger? Absolutely. Things move slow enough here that my issues with dinghy/skiff sailing are far less relevant.

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