Small planing craft

An interesting article i came across about the physics of windsurfing vs sailing craft.

average joe - windsurfer blog: Jim Drake's Windsurf Physics http://joewindsurfer.blogspot.com/2008/04/jim-drakes-windsurf-physics.html?m=1

Looks like some definite advantages of windsurfer vs sailboat re planing.
Also interesting how 'much' happens behind the mast.

Has me pondering whether replicating the physics in a small super lightweight single hander dingy might work?

E.g. 10-12' boat using a large windsurf rig and employing a canting mast system/rake (Probably have to utilise outriggers).
Rig way back - below 50% of boat length.
My gut feeling is that it might be a blast on a reach/planing but possibility not so good on other points of sail?

Interested to hear any one else's thoughts

 

Welcome to the forum! I've thought for a long time that the advantages of a planing hull could be put to good use on small trimarans. There are already quite a number of successful planing monohulls-some of which plane upwind- but very few multi's use planing hulls so far:
High Performance MPX Foil/Self-righting Trimaran-The Test Model https://www.boatdesign.net/threads/high-performance-mpx-foil-self-righting-trimaran-the-test-model.36058/page-186#post-839318

 

Hi Doug, that's super impressive!
Importantly, it 'looks' fast - that's always a good thing.
Have subscribed to the thread for updates.
Any plans to go to full size prototype in near future?

Re monohull's, I agree, i guess there are some planing variants already existing - Silly to reinvent a wheel.
Rather - proposing to develop a boat which utilises modern 'off the shelf' windsurfing rigs and being specifically optimised (geometry wise) for this purpose.

 

The size of the advantage of the windsurfer depends very much on how you assess it. The short and wide Formula board that Jim used in that article, for example, is likely to be beaten by an Optimist or Sabot a lot of the time. It's an excellent performer in open water and planing conditions, but terrible when not planing. It's also very difficult to sail in the conditions many windsurfers love, which is reaching in strong winds. The Formula board carries a huge 12m2 and like slalom and other shortboards it often relies on sail pumping to get planing in marginal conditions. So how do you assess the true performance advantage of a type, when that advantage only exists in specific conditions and the type is very slow in other conditions?

So..... if we look at boards that can perform in all conditions like a dinghy (ie all the way from going upwind in a 3 knot lull on a river to sailing downwind on the ocean in 25 knots) and don't rely on pumping, we see that their advantage is much smaller. It's basically impossible to find boards that are closely comparable in dimensions etc to a dinghy. One of the closest comparisons would be the original Windsurfer to a Laser Radial - one is 3.66m, the other 4.2m, but they both have the same sail area and the Windsurfer is about 4% quicker overall if the windsurfer sailor doens't pump. The Kona One Design windsurfer, a slightly shorter longboard with a sail of varying size but averaging around 8m, is similar in overall speed if you don't pump. At a guess, I'd say that these two are the same speed as a Spiral dinghy around the course in light winds which makes sense since they are similar in sail area and dimensions.

Another comparison is the pre-foiler Moth with the former Olympic Div 2 Lechner, a round-bottom board almost 13ft long and about 66cm wide with a 7.3m sail, or a 3.8m Raceboard of the same width. The Lechner is basically identical to the Moth in speed under about 8 knots in my experience and is then significantly faster, making it something like 8% quicker than a Moth. The Lechner is similar in all-round speed to the International Canoe which has 25% more sail and 4' more length. My guess is that you could chop down a Lechner to a Moth's length, add the extra sail and it would still be significantly quicker than a Moth.

The Raceboard types are now carrying 9m sails and are faster than a Div 2 board in planing conditions but slower upwind in light stuff. Since they can pump all the time it's hard to make comparisons. Without pumping and around a typical dinghy course I'd say they are similar in speed to the Lechner. Note that these two boards are long and skinny - the short and wide approach like the Formula board is extremely efficient when planing, but only when planing. The longboards have much more versatile performance although they are much slower than the Formula board most of the time when planing.

There has been at least one attempt to make a Formula Windsurfer-like dinghy, the DDS. In depressing fashion, the acronym was a dig at dinghies, because it meant Dinosaurs Don't Surf. It claimed to be very original although actually in some ways it was like one of the entries in the 1960s singlehanded dinghy trials! I think in both cases they were very slow upwind, especially in light winds. It turned out that dinghies weren't dinosaurs - they were actually better in many ways.

The advantage of raking the rig to windward is actually pretty small. The odd thing is that lots of windsurfing technique relies on reducing and controlling windward rake, yet people who don't windsurf see it as an advantage. The lift isn't huge and it comes with disadvantages. Your projected area is reduced, your apparent aspect ratio is reduced if I recall correctly. You also develop too much lift quite early on in the piece. In longboards the lift tries to capsize the board. In shortboards, the lift can try to lift the board into an uncontrollable jump. So you see people using long harness lines, changing their stance and sometimes even hanging onto the uphaul line in an effort to REDUCE windward rake.

I tend to think the only reason people started thinking so much about windward rake was because the early boards carried so much of it. Ironically, the windward rake on such boards is not really for lift - it's to move the C of E to windward to counteract the weather helm of those boards. Later boards moved the CLE and CLR but then you end up with a board that tacks slowly, which is frustrating in itself. The lesson seems to be that nothing comes for free.

Similarly, the huge planing lift developed by the short wide Formula-style board develops so much lift, and the instability of the slender hulls of the longboards, means that windsurfers can't carry centreboards that are as big as that of a dinghy, which hampers the boards significantly when going upwind in light airs.

Having the rig way back also brings issues. You end up with a nose in the air doing nothing and with less waterline. That's why longboards have mast tracks so the whole rig can slide forward upwind and in light airs.

Ironically, if you take many of the windsurfer "developments" and apply them to a dinghy, you come close to ending up with a scow Moth, which is a great boat but not all that fast by many standards. In the end I tend to think that the only real all-round speed advantage of the windsurfer, pumping apart, is that it's easier to balance on a narrow hull with the windsurfer rig than it is with a normal rig.

From what I read, by the way, the foiling windsurfers can sail amazingly low downwind in medium winds, can use quite small sails, but still lack the top-end speed of conventional performance racing windsurfers. The foiling Moth seems to be faster in overall performance than any racing windsurfer - the Moth would be slower under about 6-8 knots than a Lechner and similar to a Raceboard, probably, but vastly faster thereafter in reasonable winds and if well sailed. The Formula board would be comparable in performance in a breeze but as noted, it's very slow in light stuff.

 

=============================
Thanks-I'm hoping to do a crowd funding project to get WOLF flying. There are a couple of recent windsurfer threads in this forum -one with a little about foiling windsurfer design-you might look into that if you're interested. Planing, whether with a windsurfer or dinghy, is a blast but foiling can be too-there are a lot of choices these days-good luck with whatever direction you decide to go. Don't ignore multihulls either-lots of food for thought!

 

I missed this bit.

Most windsurfing rigs are designed very much around gust response, specifically gust response with a hand-held rig where it's easy to overload the back hand. Because windsurfers often pump and/or are centred around reaching in high winds, the rigs are also generally designed for very low drag at the expense of having low lift and often not pointing well.

If you put these low drag/low lift rigs onto a boat hull they tend to under-perform. Boats need a higher-lift rig and because the drag of the hull is high, the windsurfing rig's advantage of lower drag is lost. As Mark Drela has pointed out, the lift/drag ratio of the rig isn't really relevant - what it relevant is the lift/drag ratio of the entire craft.

You can really notice these issues when you put something like a speed/slalom rig onto a 18kg 12ft 9 in Raceboard. The speed/slalom rig is essentially gutless and low-pointing when it's faced with the task of the higher drag of the bigger board, especially in light winds or going upwind. It would be much, much worse if the rig was trying to drive a boat hull. The big Formula rigs can drive longboards pretty well, but that's because they are enormous.

The same sort of thing happens if you try a longboard rig on a slalom board, in reverse. The longboard rig has lots of lift but lots of drag, so the small low-drag board never really gets a chance to reach its top speed because of the aero drag. What is also fascinating is that the tight leach, deep-draft high-lift sail that drives a longboard to windward so well can be terrible when it comes to getting a shortboard upwind, because the aero drag is too high for the small fin to overcome.

What really underlines these lessons is the fact that you can switch rigs and boards back and forth within seconds, and you really see how the lift/drag characteristics of the rig must match the hull. So there's a pretty direct correlation between how close a windsurfer rig is to a normal dinghy in terms of use, and how close it is to a normal dinghy rig in terms of design. If you design a rig to drive a larger hull upwind and downwind from 3 to 25 knots you make the leach tighter and the sail fuller, taking it further away from the typical modern rig but giving it more lift for the area at the expense of reduced top-end speed.

One final interesting point is that windsurfer sailmakers keep on claiming their sails are light. In fact the full battens make them very heavy. About the lightest windsurfer sail made, for its size, was the original Windsurfer one which is less than 2kg despite being made in dacron. A modern sail of the same size can be well over twice the weight because of the number of battens. The interesting thing is that these numbers are rarely brought out because few people have weighed old sails and new sails. And the typical monofilm windsurfer sail is actually quite fragile in some ways, so it may not last well if used in a boat.

 

Hi CT249,

I appreciate your experience, detailed response and thoughts on the topic.

I can't find any pictures of the DDS boat you mention.

You raise some good points.

Might try a large powerful formula rig as you suggest to help curtail extra weight of the hull.
I'm thinking a final weight estimate of hull being between 20-30kg for 11'
That's if I go with lightweight EPS core and sandwich skin.

Taking on board your comments - Possibly I may end up with a craft which is too specialised/narrow performance range.

If so, that may be acceptable as reaching in planing mode is my main focus.

I'll never forget the first time I did a powered up broad reach on a windsurfer - sensational - and completely different to kite surfing for example.
Would be nice to try replicate similar experience using a sailboat platform, and possibly, make sensation more accessible to someone who hasn't gone through the long learning curve required for windsurfing.

 

Its sometimes thought that Sailboat and Windsurfer designers operate in completely different realms and don't interact, but by and large that's just not the case. Many designers and innovators do both, and anything new and promising in one area is speedily tried in the other. The error is to look at a sailboat rig or hull designed 60 years ago and think that represents the state of the art. It doesn't.

It could be argued that the development of practical hydrofoil solutions has made pure planing sailcraft obsolescent, since if one wants a craft that only operates satisfactorily at high speed then dynamic lift from an immersed hydrofoil is way more efficient than dynamic lift from a planing hull, but on the other hand if one wants a more versatile craft suitable for all conditions then a modern development dinghy style does the job better.

I don't know that I'd describe the learning curve for very high performance sailboats as being any shorter than that for windsurfing. The Australian/UK Cherub class I used to sail, for example, would bounce over the wavetops at 20 knots with the best of them, but the level of skill required not to go somersaulting base over apex as you ploughed into the back of the next wave was as great as any sailboard.

Try stuff by all means. We learn by our successes and failures, and design/build projects are fun and educational. An individual specialised craft can do things within a limited field that no more rounded design can match. I was always very dismissive of inclined rigs, but the Sailrocket team demonstrated that there is something there within a very specialised arena. But you mustn't be disappointed if your reaching demon turns out to be a devil when you want to get up the estuary against the current in order to be home for supper! Read Bethwaite's books - he tried a bunch of weird creations before settling on more conventional craft and there's a lot of good stuff there even if you do need to allow for a certain excess of Sydney-centricity [grin].

 

Sounds like a skinny low–rider Moth with a windsurfer rig. The hulls were down to 1okg or less with a relatively large and highly developed rig (8.25 m2), but I think sailboards were faster. They're sailed flat or healed slightly to windward so I don't think a canting rig would help.

I guess you want outriggers for stability, but less boat in the water is always better where speed is to be optimised. If sailed like a sailboard, the outriggers would be a draggy annoyance.

Moth's banned sailboard type rigs so as to stop the class turning into a sailboard, so never went down that path.

I have a friend who built a carbon scow with canting rig, I don't think he was any faster than the other scows (but spent a lot more money). Foiling Moths have also played with canting rigs (to cant it upright when healed to windward), but again I don't think there was significant benefit for the added complexity.

 

Haha, this though had crossed my mind before you mentioned it
I did some research re Bethwaite - he sure left a legacy of great designs.

Hi OZFred,
Kind of - except thinking of some more volume so I can stuff it into the back of ocean swells without worrying about a nose dive...
see below sketch

Started out by scaling up a new formula windsurfer board approx. x 2
which equal a 3.25m x 1.5m boat hull (may go longer yet?)
A 'planing' flat exists on the rear 2/3 of the hull which has well defined/hard chine for water release.
This planing flat is equal in size to a typical fast large windsurfer.
(I have tried this planing flat concept on a super wide windsurf board before and it works ok for reducing drag)

A deepish concave (with double) under the mast (to promote lift and planing) fading to a flat vee at stern for clean water release
and rounded off hull sides/bottom like a modern racing sailboat.
To take some of the slamming out of the front of the hull there is a rounded off nose section / bottom.

The plan is to sail it as 'flat' as possible for reaching and then put it on it's 'side' for upwind work.
Maybe duel rudders and or centreboards? and small trim tabs if required to try and keep it flat during reaching.

Not sure if anyone else thinks so but is starting to look a bit like mini transit (scow)?
Ironically it equals about 1/2 scale dimension wise

Will it work? who knows?
feel free to offer any comments/thoughts (be brutally honest

 

Have you seen the Bethwaite HSP's? Basically a planing trimaran.... This one used a planing main hull and planing amas that weren't designed to be sailed on-only for intermittent contact. Seems like he had a lot of fun with the singlehanded and doublehanded versions....

 

You may want to study the US inland lake scows, which are a somewhat similar concept.

 

Haven't had a chance to start this project due to a 20 footer I am still madly building - but will happen eventually.
Found this interesting article on archive which designs small sailboat and comparison to windsurfer
Development of a hydrofoil sailing dinghy http://web.archive.org/web/20121010122640/http://www.btinternet.com/~sail/boatbuild02.htm