How important is relative boat weight?

Actually we do this on landyachts and several larger racing Cats and Tris have waterballast in this way.

It is being done; you were just never around to see it. <img src=

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Wouter

the only place where weight improves performance is in a steam roller

... Uffa Fox <img src=

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water ballast is very different from static weight.

5. get the sail and rig trimmed
6. having a positive attitude
and I would split 2) in
2a. faired rudders, boards
2b. faired hulls *)

My ranking is:
6
4
5
nothing
1
nothing
nothing
2a
nothing
2b

*) long bodies are less sensitive to roughness than short bodies (i.e. rudders and boards) due to Reynolds number effects.

Cheers,

Klaus

4
1
3
2

Water ballast in the transom.... Or what the big Multi sailors like to call them, the 'Oh [censored] Tanks'.

If they had a choice, they would prefer to run without them but are necessary for saftey

Apparently Uffa Fox (designer of the laser dinghy) didn't look around too much.

Many racing mono's have waterballast (Volvo ocean, Classe Mini (the open fleet), large cats and tri like Orange). Many competition landyachters put (bags of) sand in their **** during days with strong winds and competition glider planes use a significant amount of waterballast to improve both airspeed and distance covered before having to find another thermal to regain altitude for the next leg.

Of course keel boats need weight on the end of their

fin

to even be able to make forward progress and sailing ships or a past era (galjons and clippers) used massive rock or brick ballast placed inside the hull on the keelline to stablize the boat, they would capize otherwise. Actually these bricks would be transported in the otherwise empty ship to the colonies where part of it was unloaded and used to build the settler towns. This was possible as the return cargo of species and other goods would make up for the reduction in this ballast.

I guess the designers of all these championship craft are just misguided.

But they don't have that choice as the alternative is to cut down on the saildrive and that loses more performance. Therefor it is not only a matter of safety, it is a matter of performance where they are unwilling to solve the issue by powering down the rig (loose performance)

Wouter

Well put Jimmi.
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C'mon Wout, that's not fair! The man was brilliant.

http://www.uffafox.com/uffabiog.htm

btw- I can't find any mention of his having designed the Laser. I'm not saying he didn't! Just can't find a citation.

There are alot of misconceptions associated with boat weight, some falsehoods even.

First of all, the same water that is pushed out of the water pushes back against the hull on the rear part of the hull and thus negates a very large portion of the

push-away

drag on the front part. The form drag is actually ONLY the net difference between the

push-away

and

push-back

phenomenon and in a frictionless fluid this net difference would be zero, resulting in zero drag. In real life fluids like water are not frictionless and the amount of internal friction forces for a given fluid determines the amount of form drag. That is why the same hull in water has less form drag then say in oil.

Therefor the popular representation ;

each extra lb. of weight in the boat/crew is another lb. of water that water that has to be moved out of the way for each boat length travelled

is misleading as it omits the part where the same additional lbs pushed back later and gives back some of the energy lost on the front part of the hull.

Secondly for some factor to be very important for the overall performance of a sailcraft it needs to constitude a significant part over the overall drag. If a given component only makes up 10% of the total drag then a 25% reduction of that factor only amounts to 2.5% drag reduction of the total with an even less performance gain (= for floating objects typically (1-1.5%)

The team for the C-class catamara Miss Nylex publized some of their research data and I give some of their conclusions regarding drag factors.

Hull drag (subtotal = 35%)

-1- wave/form drag = 15%
-2- skin friction drag = 20%

Sail drag (subtotal = 25%)

-1- form drag = 9%
-2- induced drag= 16 %

Centreboards = 20%

parasitic drag (group of 7 subcomponents like rigging and crew each less the 4%) = 20%

Note that even form and skin drag combined only make up for 35% of the total drag on a C-class catamaran with a highly efficient wingsail. On our catamarans the rig is less efficient and the sail associated drags will increase reducing the relative magnitudes of the hull form and skin drag factors.

35% may sound like a large portion but we still haven't completed the analysis yet. 1% additional weight doesn't increase the hull related drag by 1% of 35% = 0.35%, it is less. This is because of the non-linear dependence of volume (weight) and surface area upon size. We all know this to be true as a bottle that is twice as large will have 4 times the surface area and 8 time the enclosed volume (weight).

Assuming the hull can be optimized for a given boatweight and crewweight then for each 1% of additional weight you only have to entlarge the hull by 0.33% resulting in similar fractions (0.66%) of increased hull area and crossectional area. Also the fractions became smaller within increased additional weights, again due to the non-linear behaviour.

Basically (a simple example) an additional kg on a 100 kg boat + 75 kg crew (0.57% weight increase) results in a hull drag reduction from 35.00% to 34.86%. Which on the total drag of a sailcraft like Miss Nylex results in a less then 0.140% reduction => ... => on average 0.1% performance increase for a waterborne craft (= about 3 second per hour racing). Of course this is for lightweight boat with a singlehanding crew with everything else like sailcut and windconditions being perfectly equal. When looking at doublehanded crews on heavy boats the difference becomes MUCH smaller as 1 kg difference here will be a much smaller fraction of the total. In effect the solo sailor on a lightweight singlehander is the worst case scenario. However, allowing different sailcuts (formula classes instead of One-design) is so potent that it is able to complete ofset this weight dependancy, as proven over and over again in the A-cat class where 85kg skippers are competitive with 65 kg skippers on a world championship level. That is how small the dependence of weight in the larger picture can be and why OD garanteed equality in performance is such a myth. In several cases the OD character of a class makes the differences in performance worse !

We should now do the analyis on 1% difference in sailarea and see the difference between that and the dependency on weight. That tells an interesting story as well and why water or sand ballast can be an advantage on sailboats and landyachts.

Wouter

Do you want to just check up on Uffa Fox and correct comments?
Uffa Fox did wonders for the development of lightweight planing dinghies. I dont think Bruce Kirby will be too pleased about about the laser comment either.

I haven't looked it up but I may well be wrong about Uffa Fox having designed the laser.

The rest of my posts still stands however.

Wouter

Bruce Kirby designed the Laser.

Wouter,

Its a lot simpler than that buddy. You can have as little or as much friction as you want. It doesn't really matter. Is all about the sum of the forces (F) which includes your very impressive friction component.

So i F equals 1 newton just for aguments sake. Then that will acclerate 1 kg at 1 m/ssq. The more mass an object has the slower it accelerates. Simple physics. Forget your crazy friction analysis it doesn't mean [censored] in the end.

So if your boat is heavier. Your going to have to come up with that extra power to keep up with the lighter boat coming off the start line and in general manouvers around the race course. Thats how real life works Wouter. We dont race in straight lines in some dutch test tank.

Sue

I was sailing on a really narly day in Newport, it was the day that one of the 40ft racing tri's went head over.

We stuffed it going up wind and my crew stuck the trap ring thru the hull getting the boat back up.

Upwind on the tack with the hull with the hull full of water was unbelieveable. The boat was fast and flat in 30knots, with bigger puffs, Luckily we sailed back on a tack with the holed hull in the air, but the hole was forward, and we kept the bow up, and the hole out of the water, so the water that was in the hull stayed in the hull. Back at the ramp we discovered that the hull was almost FULL of water. Unbelieveable, I cant believe we didnt lose the shrouds with that much weight.

I've always thought we could/should have a way to suck up water on the tack and use that as ballast in the windward hull.

Bill

Sue,

Are you explaining junior grade secondairy education physics to me and then misapplying it ?

You really have no idea of what I do in my professional life do you ?

of which the drag forces are an very important part as the accelleration is only the result of the net difference between drag forces and drive forces. When both are equal and opposite to eachother the boat stops accellerating and had achieves max sustained speed (=topspeed).

Newtons second law of motion is therefor not a factor with respect to sustained straight line speed (=topspeed) which is what racing catamarans spend by far most of their time doing.

Of course, in reverse, a heavier boat decellerates (slows down) less then a lighter boat in the lulls and when encountering waves. A light boat may win at the start-line but will loose in the lulls by the same token.

You cherry pick concepts and then simply ignore the reverse effects to argue your case. A failing grade skill level even for a junior secondairy level pupil/student.

Wouter

I would like to know what Sue is doing in her professional life though.
It is well known to the regulars here what Wouter do <img src=

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Basically the loss of performance due to increased weight was more then offset by the increase in performance due to ability to hold down a more powered up rig. According to Uffa this is comparable to sailing a steamroller.

I rest my case.

Wouter

Sue,

Life is not as simple as you say. But okay, I will use your simple formula f=m*a.
If your boat accelerates slower because it is neavier, then it deaccelerates also slower.
Let us assume, both boats (one is 10% heavier) sail with 17kts. Both boats are hit by a wave (or a lull or make a manoeuvre or whatever) at the same time.
The speed of the normal boat slows down to 15kts. (17kts - 2kts/s * 1s)
The heavier boat slows only down to 15.2kts, right? (17kts-0.9 * 2kts/s * 1s)
Now the boats accelerate again.
After one second the normal boat reaches again 17 kts. (15kts + 2kts/s * 1s)
The heavier boat accelerates slower, but because it has only lost 10% less speed, both boats reaches at the same time the speed of 17kts. (15.2kts + 0.9 * 2kts/s * 1s)
During all this time, the heavier boat had have an equal or a higher speed than the lighter one, and hence traveled a longer distance.
It is logic that a boat has to accelerate after each de-accelertaion. Hence from start to finish we have n de-accelertions and n+1 (the +1 is at the start of the race) accelerations. So the hevaier boat makes n times something good and looses only one time something.

Of course life is not only F=m*a, the hydro- and aerodynamic characteristics cannot be neglected. But even in your simple model, a heavier boat is faster.

Not only to Sue:

So far, it is an interesting discussion with good arguments from all participants. Let us discuss in civilised way, without insulting each other please.

Cheers,

Klaus

I think you will find that Bruce Kirby was the designer of the Laser.... But I might be wrong.... I have been known to spread crazy talk in the past <img src=

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The designers of the above craft were not misguided, they just had an understanding of how sail boats work... unlike some on this forum...

I will attempt to explain but dont freak out if you get lost along the way, its complex and involves thought <img src=

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on a Displacement boat (not a performance catamaran/multihull as the displacement rules dont fully apply to such a craft) once you reach hull speed then weight is good. it helps stabilise the boat and allows you to carry more power. it also helps with keeping speed during direction changes such as tacking. more momentum into a tack equals more speed on the way out.

in the AC boats they have huge water bladders to fill with ater once the wind speed is such that the boat will be travelling at hull speed upwind. On the Farr 40's we put as much water as allowed in the bilge if the wind is strong enoug for us to reach our max VMG upwind.

Here, I have a simple way to look at this and exclude the discussions about physics, fluid dynamics etc.

Look at the US Sailing Portsmouth numbers and compare these three boats; M20, CFR20 and I20. They have different rigs (uni vs. sloop) but carry similar sail areas both upwind and down wind (I know the M20 is 1' wider but it's still a good reference point). I probably should have pulled the Texel numbers and compared the I20 to the Eagle 20 Carbon but.... I did this....

Key
Boat - weight, SA up x SA Down, Portsmouth numbers

M20 - ~255lbs, 24m^2 x 25m^2, 58.0 61.0 59.0 57.5 55.0
CFR - ~265lbs, 24m^2 x 25m^2, 58.0 61.5 58.2 58.1 57.3
I20 - ~390lbs, 24.25m^2 x 24m^2, 59.3 62.0 60.2 58.5 57.5

Notice that in light/med conditions the light boats are rated faster. As wind speed goes up the gap closes for the I20 and the CFR but not the M20 (the beam starts paying off).

I think the conclusion should be obvious. Lighter is faster. (don't rake me over the coals a/b the sloop vs. uni, I know I know....)

One more anecdotal account. I have sailed with some upper upper level sailors. If weight didn't matter, I don't think they would be so anal about using small light lines and keeping the garbage off the boat.

well... yanno..
Uffa Fox was as instrumental as Bethwaite Snr, Charles Sparrow, John Spenser, Ben Lexcen and Richard Court in the development of high speed sailing. Debate all you want but I am confident the marine architects will back my statement.

Uffa if I recall basically made the International Canoe class the speed machine it is.. He was the father of the modern planing dinghy with his International 14

Avenger

. Her record stands today winning almost all the races in the first year she entered. Only Darkie is in her class as a 14teen legend. He was involved in developing the trapeze..

Maybe if land yachts used a trapeze they wouldn't need the sand ballast.

Im not an engineer.. I only vaguely recall my sckoolin but isnt there a formula that equates force is equal to mass times acceleration? So for a given force the greater the mass the less the acceleration..
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From http://stanschreyer.com/?cat=7

Of course, the C-Class is now most widely known because of the wing sails which all boats use. The first wing sail appeared at a C-Class championship in 1974 on Miss Nylex, and it was made of spruce and balsa. Miss Nylex won the title that year, but was defeated by a soft sail boat (Aquarius V) the following year, mostly because Aquarius V was lighter.

So the less efficient rig with greater drag from the rig beats the more efficient boat with no hull drag penalty and weight has nothing to do with it! Or maybe they weren't sailing in a tank and had a non- uniform wave pattern. Maybe like me when I go sailing I cannot optimise my hull shape for different crew weights. I have to cope with the non-optimised increase in drag. At a 10 knot average your 1kg example still gives a significant boatlength difference. let alone the opportunity for position on the water. I am sure that most sailors would not quibble about your 1kg difference but 10, 15 or even 20kg! I do however find that when I have a lighter crew/boat combination I power up earlier and once overpowered I can use rig control to reduce power and drag from the rig.

Your posts are very interesting, Wouter. Could you send me a pm where you do a complete analysis of the drag forces and the driving forces acting on a cat moving through the water? thanks!

One important point to keep in mind is that weight can not be considered by itself. You have to consider the power to weight ratio. Look at two of the fastest sailing craft in the world- Yellow Pages and the windsurfers. One weighs about 350lb and the other weights about 30. Both have achieved virtually the same speed while the first being about 10 times heavier than second. It's all about power-to-weight.

This is why having a heavy boat is generally not beneficial, because the added weight contributes mostly to drag and little to righting moment (which equals power). However having a heavier crew (movable weight) contributes to more power generated from the rig (as long as the wind is strong enough to support the fully hiked crew).

sm

No doubt that the power to weight ratio is important, but comparing beach cats to a windsurfer or yellow pages to me seems wrong. Yellowpages travels at nearly 50 knots, and as the wind-drag goes by the velocity squared, the wind-drag is by far the biggest drag component in those speeds. At 15-20 knots however, I think the picture is rather different.

Air and water are both fluids, so the hull drag goes up with respect to velocity squared as well. That's why planing and hydrofoiling craft are able to get up to these high speeds- they reduce surface area as their speed increases. So in this case, a catamaran does not act the same way as a planing craft because it's surface area does not decrease with increased speed.

My point however was that you can't look at weight alone because here we have two craft (YPE and windsurfer) that are able to travel at roughly the same speed yet one weighs about 10 times as much as the other. We could throw Hydroptere into the equasion and probably have another boat that weighs about 10 times what YPE weighs.

The critical issue (and one that effects catamarans as well as any other sailboat) is that the total weight essentially contributes to both drag and power. The trick is to maximise the power-to-weight(drag) ratio something that YPE and windsurfers have done quite well. This means having minimum boat weight, but not necessarily minimum overall weight.

sm

Sorry, insufficient spare time.

Besides I've typed up large sections of it already on this forum in the past; maybe do a search ?

Also writing out a COMPLETE analysis of drag and drive forces is a major project in itself.

Wouter

Finally some persons who are using their brains !

You guys are spot on, Smiths_cat and SRM !

And both of you give perfect example of where an overly simplified rule of thumb like

less weight is always better

falls flat.

Wouter