My F16-class pontoon design (comments/suggestions?)

[kerinin]

I've been working on the design for a F16-class catamaran, and I was hoping to get some feedback from the (incredibly helpful) community here. I started out using the typical Blade-style wave piercing hull, but the more I thought about it the further I got from those designs.

Last week I decided to start from 'first principles' and see where I ended up. I started by doing a series of airfoil drag simulations using Xfoil to get a rough idea how different mass distributions would affect the drag performance. From those simulations, it started to look like a profile with a 'fat' trailing edge (which is required to mount the rudder) needs to be roughly elliptical in shape, so I did a quick hull design which was essentially an elongated sphere. I'm fortunate enough to have access to Solidworks, so I ran comparison drag simulations on the elliptical design and the 'typical' blade-piercing hull I had been working on to start with. Fully submersed (Solidworks doesn't deal with free-surface wave effects) the ellliptical hull had around 20% less drag, so I decided that was a good starting point.

From there I narrowed the bow to reduce leading edge drag and to reduce wave resistance. This turned out to have about 5% less drag than the original elliptical shape. About this time, I ran across Retired Geek's LR2 design (http://www.boatdesign.net/gallery/showphoto.php/photo/8835/ppuser/13088) which has a very similar design; roughly elliptical section, symmetric about the waterline, rounded gunwales, etc. The LR2 is in its second incarnation, and they recently discovered that the large rocker was creating suction under the bow when sailing downwind. They recently finished some hull modifications and should be testing it out this weekend. I haven't seen their hull modifications yet, so it will be interesting to compare their response to mine. In any case, what I did was to drop the bow 12cm and use asymmetrical ellipse sections for the top and bottom line of the bow. I've tried to maintain a circular hull section below the waterline to reduce the wetted surface.

The final design move was to take shift the max beam aft 50cm. This moves the LCB aft (it's currently at 4%), effectively decreases the hulls curvature and makes the whole thing look much cooler (very important ;) ).

I've attached a couple images. The image that's cut in half is showing the DWL fully loaded and flying one pontoon. The pretty colors are a curvature plot.

I'm curious what people think about the design's performance (I'm trying to to design for aesthetics). One thing I'm very curious about is the pronounced reverse bow. My hope is that this will reduce wave drag, but I'm not sure what the impact of water washing up the bow will have on drag. Would it be better to use a straight bow up to the DWL?

Thanks for any input!

PS - I'm a big believer in the opensource philosophy; my opinion is that more information sharing helps everyone including the person doing the sharing. If anyone is interested I'd be happy to send the digital file - Solidworks, STL, whatever.

[TTS]

You should talk to either Retired Geek or John Lindahl, designer and builder respectively. The have done some pretty serious tests with both the LR2 and LR3 learning quite a bit along the way. I know that the LR3 is currently being modified due to too much drag going downwind. You can email me at tsiders@hopkintoncc.com if you would like John's email.

[kerinin]

Retired Geek was actually kind enough to give me a few pointers over email already (although that was on an earlier iteration of the design). I thought I'd let him enjoy his weekend - I hear they're sailing the revised LR3 today.

I tried to contact John Lindahl a few weeks ago but haven't heard back - maybe he's busy. Probably for the best really - I originally was planning to just pick some plans and build from them and I've had a lot of fun (and learned an enormous amount) designing the hulls, even if the end product ends up being bad.

[bad dog]

I have a few thoughts, although I cannot model or drag simulate them, so they tend to be theoretical and anecdotal. Your model looks like a bloody ripper, so the aesthetics are spot on in my book!

Your use of semi-circular sections below the DWL should give optimal performance in light air and smooth water. I'm not convinced that it is the best shape for all conditions - a flattened vee-biased shape should give better strong wind performance.... it's that question of balance, eh! Then there is the old chestnut of angle of heel affecting the shape as well - are you planning on canting the hulls?

Other thought has to do with the raked bow. Wave drag and hull speed at displacement speeds would indicate a straight stem to DWL - but then you end up with something like The Tool A Class - fast but not pretty. [http://www.thetool.com.au/Welcome.html] The 70s Volvo of the catamaran world - "They're boxy but they're good!" (Sorry Wayne - eye of the beholder and all that stuff - I'll wave elegantly from my ancient but oh so elegant Mk IV as you rip past me). But The Tool does the ugly bow trick for a good reason. I wonder if curves could be used to the same affect?

[bad dog]

ps - I just had another look at the Tool website. I still think the angularity of the bows jars with the lovely subtlety of the curves in the rest of the hulls, but the sheer functionality is obvious. They really are purposeful. The photos don't really show them at work properly - in fact, in smooth water, they bury the straight stem only to about 2/3 of the way up the straight bit - thus maximising DWL.

Question for the gurus on this thread: assuming perfect control of reserve bouyancy, would it be better to have all of the hull below water - torpedo style? Is wetted surface more of an issue here than wave resistance?

[kerinin]

Thanks for the comments bad dog. What causes the better strong wind performance of flattened-v profiles? I've seen some discussion of prismatic coefficient and how deeper hulls have reduced wave drag, but from what I can tell this is due to reduced hull curvature. I should mention that thee will be daggerboards in the final design if that makes any difference.

Thanks for that link too - you're right that the hull is a little ungainly. So I'm guessing that the benefit to the vertical bow is that the bow wave is pushed directly to the side, rather than to the side and over the bow?

[TTS]

From the LR2/LR3 site:
After sailing the LR3 at Lake Hartwell we found there were issues with the boat going downwind. It just wouldn't go wild as quickly as Flyers and A3's. It also had a tendency to pitch more than the other boats. Since the LR3 and the LR2 have a lot of rocker we suspected that to be the problem. And we got that verification when the hulls got a design evaluation at Crain Technologies. There was actually a suction being created that added a lot of drag when going downwind. So we got out a long batten and drew a new rocker line on Ian's LR2. The alterations were evident to those of you who saw the boat at Ft. Walton. Even with all we have done to his boat by adding bow, stern and hiking rails the boat is still 5-6 pounds under weight.
The LR3 is in process now. We are taking an inch of rocker out at the transom and 3-1/2" at the bow. Should make the boat much more stable on those downwind runs and keep it's excellent upwind ability. We did fit our boards and rudders on Matt's Flyer for the NA's and he reports they work real well.

[Rick Willoughby]

..........

I'm curious what people think about the design's performance (I'm trying to to design for aesthetics). One thing I'm very curious about is the pronounced reverse bow. My hope is that this will reduce wave drag, but I'm not sure what the impact of water washing up the bow will have on drag. Would it be better to use a straight bow up to the DWL?

Thanks for any input!

PS - I'm a big believer in the opensource philosophy; my opinion is that more information sharing helps everyone including the person doing the sharing. If anyone is interested I'd be happy to send the digital file - Solidworks, STL, whatever.

Reverse bows are effective at picking up any flotsam that happens to be in your waters. It is annoying to have a plastic bag or weed lodge on the bow and stay there. It does not look pretty either. Easier to remove when caught on the rudder and cannot be seen.

The reverse bow has been discussed before and others have commented on the hazard it poses to other boats because a heavy collision will likely result in a hole below the waterline.

I would be interested in an .igs file of the hull if it is a simple file export function.

Rick W

[CTMD]

Kerinin,

before I make too many comments I'll ask a couple of questions....

1. Is the boat being designed for one crew, two or both?
2. What is the displacement of one hull on your designed water line.
3. Measured from the transom where are the LCB and LCF

[kerinin]

Rick:
Flotsam - there's something I hadn't considered. I'm running some simulations on the model now but I'll upload and IGS when it's done.

CTMD:
1) The boat is being designed for 1 or 2 person crew, that's one of the reasons I was attracted to the F16 class in the first place.
2) The displacement of a single hull is .248m3 (.254 tonnes) give or take - I'm basically designing for a 104kg boat weight and assuming 80kg for two people).
3) The LCB is 2.77m from the bow (so 2.22m from the transom - it's a 5m hull) and the LCF is at 2.83m from the bow (2.17m from transom)

[CTMD]

2) The displacement of a single hull is .248m3 (.254 tonnes) give or take - I'm basically designing for a 104kg boat weight and assuming 80kg for two people).


I'm assuming thats on the dwl. How much reserve bouyancy do you have?


3) The LCB is 2.77m from the bow (so 2.22m from the transom - it's a 5m hull) and the LCF is at 2.83m from the bow (2.17m from transom)

Quick Weight estimate.
Item. Weight LCG LMMT
Hulls 40kg 2.2 88
Mast 16kg 2.5 40
front beam 5kg 2.5 12.5
back beam 5kg 0.5 2.5
steering assembly 10kg 0.2 2
prod, kite, etc 5kg 4 20
sails 10kg 2 20
boards 4kg 1.8 7.2
misc (tramp etc) 9 1.8 16.2
Total 104 2.0m 208

your LCG 2.77

required crew location 3.27m fwd of transom.

The above numbers are very rough. but if you hash them out in more detail you'll be able to calculate the required LCG. For your initial calcs set your crew's front foot on the front beam (so crew's LCG is say 300mm behind beam and skippers is a further 600 behind that)

[kerinin]

Here's that IGS file. The shape has changed a tad since those images, but nothing major.

[kerinin]

I'm assuming thats on the dwl. How much reserve bouyancy do you have?

Yes, that's on the DWL. I have roughly 100% reserve buoyancy - there's .243m3 above the DWL.

Quick Weight estimate.
Item.LCG

? I'm not quite sure about the LCG - I don't have great info on the mass distribution of the mast, sail, rigging, etc. I could figure out the rough LCG for the pontoon, but I suspect mast placement will make that number irrelevant.

[kerinin]

Disregard that last part - the post was clipping some of your content.

Those numbers help a ton, and I don't particularly like the idea of being that far forward, so thanks for pointing that out! Looks like I need to shift some buoyancy around.

[CTMD]

Kerinin,

Now that you've got a preliminary hull you need to start a weight estimate. Set up a spreadsheet and list every item on the boat. Using the net you should be able to calculate or find the approx. weight of each item. For example the F16 rule has a minimum "mast tip" weight, double that and you have an approximate mast weight. The sails will be something like Apen 09. so if you sketch up a sail plan and then find what that weighs per meter you'll have a sail weight. The F18 (yes i know you're doing an F16) rule has max weights for the foils this weight would do until you've actually designed them.

The idea is to work on what we called a design spiral. You've done a hull so now incorporate that into a general arrangement/sail plan. Using that info do a preliminary weight estimate and determine where your LCB need to be. Then go back and modify the hull shape. Then design the structure. Then update the weight estimate then modify the hull shape etc. etc. etc.

[Rick Willoughby]

I compared your hull against the lowest drag hull for 10kts displacing 254kg. I am assuming this would be the case of reaching with one hull just lifting with 2 on board. The speed selection is not based on any experience with the F16. If the selected speed was lower the block coefficient would be smaller.

You would need to consider a few points of sailing and the expected speed to do a proper analysis but I find it useful to know what the lowest drag displacement hull can achieve as a starting point.

Rick W

[Ad Hoc]

"...What causes the better strong wind performance of flattened-v profiles.."

The hull, when heeled, acts like a lifting surface, owing to the 'V' at the keel. Obviously not a pure foil, but a "flat" lifting surface that generates, in some instances, lift, over that of a smooth 'U' shape which does not.

[CTMD]

Rick, any chance of showing us what you consider the optimised hull shape for 10 knots?

[Rick Willoughby]

Rick, any chance of showing us what you consider the optimised hull shape for 10 knots?

I attached the .igs file of the hull. This is the lowest drag hull. Only the underwater portion is relevant. The draft is 230mm. I have not considered any aspect above the waterline. The hull is straight out of Godzilla. I would normally do some fairing and then recheck the result because I do not like the bumps.

I have avoided the word optimum as there are a number of factors to consider in addition to drag. Turning ability being one that is obvious for something that races around buoys.

I have had great success with Godzilla so I take a lot of notice of what it produces. Getting the design speed right is an important factor of course.

Rick W

[CTMD]

sorry didn't notice the second attachment.

[Ad Hoc]

drag values are meaningless without:
LWL, Beam, Draft, Speed, as a minimum.
And what drag, total, Rf, Rw ??
A graph showing lines is just that a graph showing lines...without any real world reference it has no meaning

[Rick Willoughby]

I compared your hull against the lowest drag hull for 10kts displacing 254kg. I am assuming this would be the case of reaching with one hull just lifting with 2 on board. The speed selection is not based on any experience with the F16. If the selected speed was lower the block coefficient would be smaller.

You would need to consider a few points of sailing and the expected speed to do a proper analysis but I find it useful to know what the lowest drag displacement hull can achieve as a starting point.

Rick W

Note that I have edited this post.

In loading the .igs file I did not turn off symmetry so I had a double hull. Made the Michlet file export nonsense.

The proposed hull is almost identical to what Godzilla can produce so it is very good under this condition. Even slightly better than Godzilla under 5kts.

Now I am wondering how you achieved it? I have never been able to do better than Godzilla.

It would be interesting to compare in condition of equally loaded hulls but that will be another day.

Rick W

[kerinin]

The two hulls seem to have virtually identical performance; what difference there is I would say falls within the margin of error for the simulation algorithm.

As I said, the whole design is essentially some nudges and pulls on a spherical shape; it would appear that elliptical curvature does a good job in reducing pressure gradients on a narrow hull. The horizontal bow profile is the result of trial and error; it's essentially a degree-3 spline that started as an ellipse.

Another thing I just remembered reading in the Michlet documentation as I was thinking about how I might have accidentally outperformed the algorithm is that wave drag decreases with the square of depth. So just based on that, it would seem that burying the fore mass of the hull and using a 'raked' bow might serve to reduce wave drag? Does Godzilla produce any raked bow shapes? (I haven't opened the IGS file yet).

I'm sure someone's mentioned this, but I've been taking the Michlet results with a grain of salt; the drag numbers Michlet produces are fairly different that what Solidworks is producing in similar flow situations, so I've been concentrating mostly on the differences between hulls.

[kerinin]

The hull, when heeled, acts like a lifting surface, owing to the 'V' at the keel. Obviously not a pure foil, but a "flat" lifting surface that generates, in some instances, lift, over that of a smooth 'U' shape which does not.
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I might start a thread (or look for one) about this, because I keep hearing about dynamic lift and it doesn't make much sense to me. I'm mostly confused by the role the water surface plays in generating this lift.

[Ad Hoc]

kerinin

It is somewhat analogous to that of a deep V planing hull when banked, see attached. Does this help?

[Rick Willoughby]

.......
I'm sure someone's mentioned this, but I've been taking the Michlet results with a grain of salt; the drag numbers Michlet produces are fairly different that what Solidworks is producing in similar flow situations, so I've been concentrating mostly on the differences between hulls.

I have validated the Michlet result on a variety of slender hulls for froude number up to 0.7.

With the most accurate power measurement we have used I was getting within 2% of design estimate against actual from pedals to boat speed. Leo even adjusted the viscosity limits so we could get values to suit the cold lake water testing:
http://www.youtube.com/watch?v=0yk7tkX9mOY

I am impressed with what Godzilla produces and you have done well to get virtually identical results in a first attempt.

Rick W

[kerinin]

Rick:
I just took a look at your IGS file; that's very interesting, I wonder if that design is optimal due to some quirks of the algorithm. I'm a brick-and-mortar architect in my day job, and one of the things we've been discussion around the office is the possibility of using genetic algorithms as a design tool for buildings; giving the computer a set of spaces and adjacencies and allowing the computer to produce a set of spatial arrangements that optimize whatever constraints we set - it's called 'generative design'.

Looking at your hull shape and mine, it reminds me of something that often comes up when dealing with algorithms like GODZILLA; local minima (http://en.wikipedia.org/wiki/Maxima_and_minima). Gradient descent algorithms such as what's probably used for GODZILLA are very susceptible to 'valleys' in the optimization curve that are surrounded by less optimal values but which aren't the 'true' minima for the curve as a whole.

I would be curious to see what GODZILLA produces if you use my hull as a starting point; would it converge on your optimal design (in which case your hull probably isn't a local minima) or would it converge on something different?

This is getting interesting... :)

[Ad Hoc]

"..Leo even adjusted the viscosity limits.."

I don't understand this comment, care to explain?

When performing resistance calculations one cannot calculate without knowing the Reynolds number, this requires the value of the viscosity of the fluid which is also related to the temperature of the fluid. It is the first thing one checks when doing tank testing....so i don't understand what is being suddenly adjusted?

[bad dog]

Gee you blokes have been busy! One day at work and look at all the feedback! Some of these posts have some really good info and are beyond what I can usefully remember from my days enrolled in Naval Architecture at UNSW back in 1974, which was tug boats and trawlers - so I'll contain my comments to 'user experience' rather than 'theoretician' (for fear of making a goose of myself).

Re raked bows: strongly raked bows seem to shed all but plastic bags quite readily (why are bags still legal?). Plumb bows do not: seagrass builds up on mine all the time. I reckon the steeply reverse raked bow that drives hard with the bow 100mm below waterline have a terrific wave drag advantage - once again this just seems intuitive. Hull length and Froude numbers go into the mix somewhere, but I still wonder if there is a theoretical explanation?

Re vee in hulls: as I understand it, and corroborated at least in part by Ad Hoc above, is about planing area. Yes I know cats like F16s and As don't truly plane, but flat sections generate more lift than curved sections. Ron Given's designs have relied on this for years (Paper Tiger et al), and most current As have a definite shallow vee flattening of the aft sections. The benefit of the boats that have this (compared to those that don't) is evident especially off the wind.

Re rocker: interesting comments on the LR2 and LR3, which are longer hulls with less crew weight. F16 two-up must do a bit more work than an A with 75kg crew! Light air upwind in my humble experience calls for more rocker, crew weight way forward. This is where the non-vee (and perhaps heavier) hulls can match it with any others. Turn the corner at the top mark and everything changes - the flatter vee hulls just streak away, which the LR2 experience backs up. So maybe this decision is informed by the local conditions you are expecting to sail in most: heavy/light air, choppy/smooth water, (and maybe plastic bag strewn/pristine! ;-)

[Rick Willoughby]

........
Looking at your hull shape and mine, it reminds me of something that often comes up when dealing with algorithms like GODZILLA; local minima (http://en.wikipedia.org/wiki/Maxima_and_minima). Gradient descent algorithms such as what's probably used for GODZILLA are very susceptible to 'valleys' in the optimization curve that are surrounded by less optimal values but which aren't the 'true' minima for the curve as a whole.

I would be curious to see what GODZILLA produces if you use my hull as a starting point; would it converge on your optimal design (in which case your hull probably isn't a local minima) or would it converge on something different?

This is getting interesting... :)

Leo advises that there are two lowest drag hulls but I have never seen this. It does not matter what hull you start with it will arrive at the same form each time. Sometimes it can take many attempts to get going so I usually start with a hull that I know will be nothing like the optimum so it kicks off in a few seconds.

I normally use the 7 function hull form but I have the 18 function series as well and this will produce quite convoluted hulls similar to things like the Ward Optimum Symmetrical Ship form depending on what objective function is set.

The bumps on the hull give the lowest drag but fairing them out does not add much drag.

Rick W

[Rick Willoughby]

Attached picture is my ultimate reverse bow. The boat is doing around 6kts. It is pre-GODZILLA days when I was trying all sorts of hull forms.

The nose is 100mm below the surface and there is no hint of a bow wave. Taught me that it pays to look aft when you are considering wave energy.

Michlet supported the fact that it was a dog after the event despite no bow wave.

Rick W

[kerinin]

Rick:
I had been ignoring the GODZILLA portion of the docs - I guess I'll have to take a look so I can understand the process.

A quick googling didn't turn up anything for 'Ward Optimum Symmetrical Ship' - do you have a link? That sounds interesting.


Bad Dog:
Thanks for the pointers on V-hulls, I think it makes a little more sense now. I'll have to chew on that for a while - it explains some questions I had watching some A-cats in action which seemed to be practically flying out of the water.

[kerinin]

Michlet supported the fact that it was a dog after the event despite no bow wave.

Was that due to the reduced aspect ratio of the hull?

[Rick Willoughby]

.......
A quick googling didn't turn up anything for 'Ward Optimum Symmetrical Ship' - do you have a link? That sounds interesting.

....

Here is a reference to the original paper:
http://www.dtic.mil/srch/doc?collection=t3&id=AD0623551


This is a reference to earlier work of Ward and others:
http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA094129&Location=U2&doc=GetTRDoc.pdf

Leo posted a .mlt file of the Ward OSS on the thread I previously referenced. The hull has virtually no wave drag at the design speed but it is not the lowest drag hull. The main interest is how a couple of bumps can eliminate waves.


On the topic of the orange boat - it was too short for the target speed. Things I found out once I started using GODZILLA.

Rick W

[Ad Hoc]

"...Leo advises that there are two lowest drag hulls but I have never seen this. .."

This doesn't sound like a very quantitative approach or explanation, yet you claim the program is perfect and better than tank testing...!

A "technical" statement like that requires validation.

[Ad Hoc]

"...Here is a reference to the original paper:
http://www.dtic.mil/srch/doc?collection=t3&id=AD0623551.."
This is dated 1965.
So when the authors make a statement thus "..demonstrate a decided advantage of such direct wave survey methods over the classical Froude method of determining the wave resistance.."..if it is so much better and more accurate, why has this "new" method not been adopted world wide by test tanks and taught at Uni's?..been over 40 years! Doesn't sound like validation to me....unless you can show otherwise with other references of this "new" method that is so much better?

[Rick Willoughby]

....
A quick googling didn't turn up anything for 'Ward Optimum Symmetrical Ship' - do you have a link? That sounds interesting.


...

I dug out the Ward OSS file from Leo's original post to save you hunting around for it. See the attached.

Rick W

[Ad Hoc]

"...Michlet supported the fact that it was a dog after the event despite no bow wave..."

Kerinin

There is always a "bow wave" from a body moving through a fluid, and wash. Just because one cannot see it, does not mean it is not there. Any simple study of Lord Kelvin about a single pressure point moving in a fluid in a straight line will tell you that.
What has emerged in recent years with regards to wash is the qualitative documentation of solitary waves first discovered by Russel in 1838! As the name implies, consists of a single elevation of the water level above the original undisturbed water level.

[waynemarlow]

I'm assuming thats on the dwl. How much reserve bouyancy do you have?



Quick Weight estimate.
Item. Weight LCG LMMT
Hulls 40kg 2.2 88
Mast 16kg 2.5 40
front beam 5kg 2.5 12.5
back beam 5kg 0.5 2.5
steering assembly 10kg 0.2 2
prod, kite, etc 5kg 4 20
sails 10kg 2 20
boards 4kg 1.8 7.2
misc (tramp etc) 9 1.8 16.2
Total 104 2.0m 208

your LCG 2.77

required crew location 3.27m fwd of transom.

The above numbers are very rough. but if you hash them out in more detail you'll be able to calculate the required LCG. For your initial calcs set your crew's front foot on the front beam (so crew's LCG is say 300mm behind beam and skippers is a further 600 behind that)

The above weighs are quite a bit out individually but never the less the class weight is 104 kgs. Hulls are now less than 26 kgs, masts about 12, beams about 7 front and 5 rear. Its surprising how all the other little bits add up. Check on the Catsailor F16 forum where this has been discussed regularly.

First thing to consider when designing a F16 is which is it going to be, a single hander or a dual hander. We have worked out that a boat can't be both. My Stealth is a great single hander but a much slower dual, the Viper is over weight by about 30kgs but is volumous to say the least and is doing well as a dual hander. The loss of one sail is enough to accentuate the higher volume and hence extra drag to such an extent that in my belief the boats have to be designed primarily for one purpose or the other.

I love the LR2 design but sadly I think it won't succeed in the F16 world and its not the eliptical design, its the bow design. We have a 3.5m prodder holding down a 17.5msq very very flat spinnaker ( almost a screacher ), the upward forced are enormous. Those designs which have diagonal wire braces mounted from the front stays are now breaking 4mm D12 ( which is stronger than 4mm wire ) due to poor triangulation angles ( we are limited to max luff length and height ), the only way to get better angulation is to move the bottom of the stay foward to the bow area, now if that stay is now in the water it will have a lot of drag. Perhaps a modified nose section would be enough to get these angles better. This upward pull is great until it starts to go wrong. If the bow does go down the speeds are enough to trip the boat really quickly, those without T foils seem to be planning on the hull one minute and the next the bow just dissappears and over you go. All recent design such as the Falcon and Viper have large volume well ahead of the beam and with each new generation of boat that volume is getting further foward.

As to the veed hull laying on its side to get a better planning aspect, yes the stronger the wind and faster we go ( we are regularly now up around 20knots with the spinnaker ) the recommendation is to fly the hull higher to get a more flat section in the water, last years WC winner was the best at this and left everyone in his wake, it takes balls to do it but it does work in rougher water.

Crew weight is more than what you think, very few crew with sailing gear and harnesses etc are going to be less than 8okgs per person, more like 90 - 100, the buyers of these boats are typically not in the flush of youth and tend to have a little added girth.

There is mixed feelings about the worth of T foils on the rear, they do work extremely well and should be considered in any design.

The F16's are little hotrods, they are not subtle like the A's but boy are they fun and you can wheel them up and down the beach on your own. Any more questions fire away.;)