New Technology:Quant 30-Twin DSS Foil Equipped Keelboat

Quant 30

Here is a great picture of the 30, in build, showing the twin curved DSS foils nesting inboard:

 

Smart

 

Only one thing - is it really work?
As I calculated, the 2m L and 1 sq.m. DSS foil of 6.5 mini produce some significant lift only after 5 - 6 m/s boat speed (appr. 500 kgm moment). But the canting keel work more effectively; is it justified to get more resistance of the foil (appr. 20 kg on 6 kts, up to 60 on 10 kts) ?
Where did I go wrong, please...

 

Dss

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Yes, it has been proven many times over to work exceptionally well. You can see the performance results(including video) of the Quant 28 DSS that has a racing record second to none here: http://www.boatdesign.net/forums/sailboats/quant-28-foil-assist-keelboat-dss-38421-8.html Start at the end of the thread......

 

Advantages of a DSS foil over a canting keel:

The foil produces a mix of lift and righting moment (RM)

As the wind increases, the increased boat speed gives you an increase in both lift and RM.

Extending and retracting a foil can be done with a lot less effort.

Strangely enough, I see one of the biggest potential advantages for a racing DSS boat is when they are not using the foil at all. In light air, the design above is fully retractable to eliminate any extra wetted drag. The weight of the foil system adds a very small amount to the overall boat weight. Because the foils add RM and stability to allow a taller rig, this becomes a big advantage when the entire fleet is in under-powered conditions.

There are of course downsides. Foils can pick up seaweed or debris. Foils can stall, ventilate and cavitate. Foil design needs to be very strong, high lift and low drag. This is not easy to get right. Fancy foils shapes add even more challenge. Foil systems can be complicated.

DSS works. DSS probably has the potential for significant improvement in performance with continued development. For now, DSS is probably only worth the effort in a very limited range of applications.

 

Dss

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Limited applications if you consider all boats designed from scratch for it between 20' and above 100'. Boats under 20' may benefit(such as my 14' Crossbow) but to date the system is used on larger boats, including a 25,28,30, 50, and soon a 100 footer. DSS is still legal in the Open 60 class but rule changes will have to be carefully analyzed by Hugh.
Applications as an add-on to existing designs are not a good idea but I imagine Hugh looks at each case separately.

 

"Worth the effort" is the important qualifier.

If you just want to go faster, a multihull is probably a better choice.

If you want simple & easy, DSS is not a good choice.

DSS is only worth the effort if you need or want what it gives. However, you have to be willing to deal with the extra complexity and the limitations.

DSS is just one of many choices that can be made when trying to design a boat for a specific goal.

 

There are a lot of complicated trade offs with this bit of kit. I was wondering if anyone has developed any rules of thumb or nondimensional quantities that can be employed. The system of hull and DSS seems to resist nondimensional reduction.

1. Is there a nondimensional speed which the boat needs to reach before deploying the DSS? Can it be a function of DLR only?

2. Practically speaking, to what extent can the DSS be set up so that the increase in sail power does NOT require the usual increase in reserve buoyancy at the bow?

3. For a fixed cross section, ie. easier to build and easier to run in and out, what sort of L/D ratio and aspect ratio can be achieved using a glass/epoxy over foam, and how rigid should the structure be? (rigidity, aspect ratio, and absolute size are related)

4. For a hydrofoil to earn it's keep, a sanity check is that the reduction of hull surface area should be at least 4 times the plan area of the foil (I think this is because an optimal foil will have about equal induced drag and friction drag.) In DSS, this is complicated by several factors when trying to relate DSS to the hull. Perhaps it would work better if DSS could be related to the change in rig area that is required to justify its use. Not exactly an easy thing to do, but better than trying to compare DSS hulls with nonDSS hulls, I suspect.

5. Does water ballast make any sense in DSS boats? perhaps simplified water ballast systems, maybe just two small tanks in the stern, will go a long way to help ameliorate the systems costs of DSS. Comparing DSS to water ballast systems is one area where I think some straight forward computations are possible. Any analyses out there?

 

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You need Hugh to answer some of these questions-he's the only one with the design and testing experience to answer accurately. Unfortunately, he's tied up now.
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--What do you mean by the "usual increase in reserve buoyancy at the bow?
--Based on what you understand, why do you believe water ballast "will go a long way to ameliorate the systems cost of DSS"?
--Tom Speer published the 4 times planform rule of thumb for the viability of "foil assist" a few years ago here. When you have a foil designed to increase RM(with the secondary effect of reducing wetted surface), I'm not sure the rule of thumb is accurate-it's complicated ,as you say. With DSS the foil will sometimes plane on the bottom side of the foil and that has been found to be effective-which seems somewhat counterintuitive to me.

 

Quant 30 DSS

She's sailing! Pictures from Michi today. This is the first DSS boat to have twin independent, retracting DSS foils. Note the folding racks.. She's probably going to clean up on the lakes like her little sister did:

 

Quant 30

Here is an article from the Daily Sail on the Quant 30: http://www.thedailysail.com/inshore/13/65155/0/first-impressions-of-the-quant-30

 

Quant 30

Latest pictures and race report from Michi:

 

Quant 30

Pictures from Michi of three Quant 30's racing together, illustrating one of the strongest advantages of DSS: light air! In heavier air when the foil or foils are used the boat has an obvious advantage-extra righting moment due to the foil. Less obvious, for a boat designed from scratch for DSS, is the larger sail area available in light air-hence their remarkable light air speed:

 

Quant 30

Heres a video of the boat:
http://www.youtube.com/watch?v=CZS_sOa-L1o&feature=youtu.be

 

That is extremely impressive. Especially the footage of the boat two sail reaching.
Hugh's idea could be fantastic on an IMOCA 60. Does anyone know if there are any rules that get in the way of that?