The last thread on the "ultimate beach cat design" talked a lot about sail area versus righting moment and why a 10' Tornado has such a huge advantage over the 8' wide boats of the same length (some even carrying more sail area). The generalization that I interpreted out of it was: "if you want more sail area, make the boat wider". O.K., this makes a lot of sense but at what point does that additional sail area begin to make your hull length susceptible to pitch polling. Certainly, I couldn't make my 6.0NA 12 feet wide, add another 100sq ft of sail area, and expect to keep from hearing "eeerrrrr-EEEEEEE...DIVE! DIVE! DIVE!" everytime I turned the thing downwind. What is a reasonable ratio of sail area to boat length? Do the new taller higher aspect ratio sail plans lend themselves to more submarining problems off the wind?

I don't know about sail area to beam issues, but in a recent Multihulls Magazine article, a designer (and I forget off the top of my head who this was) related that for catamarans, there is an ideal length to width ratio of 2:1. The claim being that more than 2:1 (like a 20' x 8.5' boat) doesn't have the ideal righting moment, while less than that(like a 20 x 12+ boat) leads to a potential for pitch-poling. The designer was talking about big boats, not beach-cats. But at 20' x 10' a Tornado would seem just right. That all doesn't take trapezing into account, of course.

The last 20 foot design was Marstrom's M20. He made the width 9 and 1/2 feet. My understanding is that with 340 lbs on the wire, you could burden the leeward hull and increase your drag... so he made the boat a bit narrower to optimize the design.

I am not sure how choices in hull shape influences this calculation for beam width given a fixed crew weight.

Hi Jake,
First let's review a little bit.
Using the Tornado Sport as a chinning bar, to have equal speed in light winds a boat,X, must have equal sail area to weight ratio than the TS. That means if boat X is heavier than the TS by 10%, for example, then boat X must have 10% more sail area than a TS just to be equal in sail area to weight ratio or power to weight ratio. The highest performance way of adding sail area for sailing to windward is to add mast height and increase sail aspect ratio at the same time as the sail area increase. Let's say this results in boat X having a mast 10% taller mast than the TS. The centerboard area on boat X needs to go up by 10% also to keep the sail area to centerboard area in balance. Now we have boat X powered up to the same sail area to weight ratio as the TS and it took a 10% increase in sail area, mast height and centerboard area to get there with no further increase in weight.
Now let's look at the righting moment situation. The TS with a 10 ft beam has 6100 ftlbs of righting moment and 251 sqft of sail area for a righting moment to sail area ratio of 24.3. Now we want boat X to have at least an equal righting moment to sail area ratio as the TS so it will have equal speed when the wind blows over 12-15 knots. Boat X has 10% more sail area than the TS, therefore its max righting moment must be 1.1x6100 = 6710 ft lbs. If boat X is 8.5ft wide and the compliment in righting moment is gained with wings, then the wings must be an additional 3.6 ft outboard of the hulls and not weigh anything. So at this point boat X, which is 10% heavier than a TS, must have 10% more sail area, 10% taller mast and 10% more centerboard area and also have 3.6 ft wide wings on each side to be competitive with the TS in 'all' wind conditions. Now we have a boat X that might stand a chance of being competitve with the TS.
So how about the pitchpole tendency of boat X??? Boat X has 10% more sail area than the TS and that sail area is 10% higher above the center of bouyancy than the TS. Is it more prone to pitchpole than the TS???
First let's understand pitchpole. Two criteria must be met before pitchpole can occurr. 1) The pitching moment from the forward component of the sail force must be equal to than the restoring moment from the hulls. This is called 'verge of pitchpole'. 2) A very sudden and abrupt increase in hull drag occurrs and trips the boat and makes the boat rotate in a violent forward cartwheeling motion, sterns over bows. When this happens, we sailors call this pitchpole. This very sudden and abrupt increase in hull drag occurrs when the fore deck goes underwater and the sleek bow that was splitting the water becomes very blunt as the foredeck becomes the bow and parts the water. The cartwheeling motion of pitchpole is a result of the the conservation of momentum, Physics again. The boat system mass times velocity of translation is converted into rotational momentum, mass times rotational velocity. This can only occurr when criteria #1 and #2 are met. Now let's go back and look at criteria #1 again.
Boat X has 10% more sail area and that sail area is 10% higher above the center of bouyancy in the hulls. This tells us that boat X has 21% more max pitching moment than the TS. This is pro pitchpole for boat X. Now let's look at the max restoring moment of boat X relative to the TS. The restoring moment is the sum of the platform restoring moment plus the people restoring moment. The restoring moment, in general, is equal the the weight times the distance from that weight to the center of bouyancy in the hull. Now the boat naturally tries to resist pitchpole. As the forward component of sail thrust increases, the resulting moment forces the bows down and this makes the center of bouyancy migrate forward in the hull and increase the distance between the center of bouyancy and the weight, platform and people. This is an automatic increase in restoring moment. If boats were long enough and bows were tall enough, catamarans would never pitchpole. (Look what they did to Playstation's bows.) Back to boat X and the TS: Let's assume that boat X and the TS have similiar bow heights, therefore both boats, hull shapes, can make the center of bouyancy migrate the same distance forward toward the bows. If the rigs, main beams, on both boats are at the same fore and aft location along the hull, 10 ft for example, then it is likely that the center of gravity of the two platforms are in the same place. Now remember that boat X is 10% heavier. This means that boat X has 10% more platform restoring moment, platform weight times distance from the center of bouyancy to the platform center of gravity, than the TS. This is more pitchpole resistance for boat X. Now let's consider the people restoring moment. At the verge of pitchpole the sailors are usually as far aft as they can possibly get on the boat. Let's assume that the sailors are at the same aft position. Since the centers of bouyance of these two boats are equally forward at verge of pitchpole, the restoring moments, people weight times distance from center of bouyancy to their center of gravity, are equal.
So in summary on the pitchpole question we can say that boat X has 21% more max pitching moment due to the larger sailplan and 10% more restoring moment due to its heavier platform which is only 3% in total restoring moment. Therefore Boat X has an 18% greater tendency to pitchpole than the TS. Another way to put it is to say that boat X will pitchpole with 18% less forward component sail thrust than the TS.
Bill
PS Jake, there is alot more to this which I will get into another time.

Mark,
When any catamaran sails on one hull, the total boat plus people weight is carried by the leeward hull. It doesn't matter if the boat is 5ft or 10ft or 50 ft wide. The total weight carried by the leeward hull is the same.
Bill

*I am not a boat designer or a physicist, the following are merely my thoughts on the matter*

What some people are maybe forgetting in this wide vs narrow debate, is that with a wider boat the weight of the hulls is also futher out, helping increase righting moment. That's why it's not the same as putting 2' wings on each side of the narrow boat so the crews can trapeze at the same distance from center.

Also, when the comment was made that the easiest way to make a boat faster was to make it wider, they weren't talking about increasing sail area or anything else, because the point of making the boat wider was to help it say upright in heavy winds in the first place.

Width shouldn't effect a boats ability to pitchpole all that much, since it doesn't help you move further aft to keep the bows up. However a boat that is so wide that capsizing is unlikely can be pushed hard enough to get the bows to dig in instead.

One designer's comment that I remember well: "When you're not sure wether the craft will first capsize or pitchpole, you've got it right." Granted, he was talking about maxi cats.

Another great post by Bill. Keep this up and you will have material for a book. I would appreciate your comments on the new generation of wave piercing designs. It would seen these boats seek to avoid that sudden increase in drag you spoke of rather than providing more boyancy in the bows. The most extreme examples I have seen is Morelli and Melvin's new A cat and perhaps "Team Phillips"...Thanks Dan

at some point the boat becomes more stable side to side and less stable fore and aft. my boat is 13' 6" and she will drop the
bows before she will fly a hull. she has enough reserve buoyancy to resist a pitch pole at this point, but it feels weird to have
the transom flying by 3' or so and the bow just inches off the water.

Hi Keith,
I don't know who wrote the article you read but there is no 'ideal' or perfect length to width ratio for catamarans. Catamarans of all length to width ratios can be pitchpoled and turned over sideways. The responsibility to avoid these undesireable situations belongs to the sailors on the boat. For turnover the sailors on the boat can see and feel the windward hull lifting out of the water and the boat tilting prior to turnover. They are being given notice. The wider the boat the more slowly this action occurrs and the more time is available to take corrective action and avoid the turnover.
Pitchpole is more subtle but there are signs that the boat is getting close to pitchpole such as the bow being driven lower and lower until it goes under solid water, not foam or spray. At the moment the bow and foredeck go under solid water there is an abrupt increase in hull drag and the boat stops quickly and does a forward cartwheel maneuver. So to avoid pitchpole, watch the bow!
Hull design has much to do with pitchpole. The taller the bows, the more wind it takes to make a boat pitchpole. The more streamlined the foredeck is, high arch side to side, the smaller the increase in hull drag when the foredeck is pushed underwater. Hulls designed with a flat foredecks pitchpole with a snap roll, very quickly. Hulls designed with a foredeck that is shaped with a high peak in the center will pitchpole much more slowly, sort of a mushey pitchpole. Sometimes this hull shape will recover from a near pitchpole situation even when the transoms are 4 or 5ft up in the air if the sailors can hold their positions on the boat and not fall forward.
Bill
PS. I put a SC20 together once that was 20ft wide and it sailed fine. To windward it was a rocket. Reaching don't trapeze. Downwind same as a normal SC20, watch the bows.

Boat X center of drive from the sailplan, with 10% taller mast, is only 5% higher, unless I'm badly mistaken?

Hi Andrew,
I'm trying to keep the math and logic easy to understand. Let's say that our base boat has a 30ft mast and that the center of effort,CE, in the sailplan is at 40% of mast height plus 3ft above the hull center of bouyancy,CB, to the bottom of the sail. The CE of this sailplan is located at 40%x30ft + 3ft = 15ft above the CB of the hulls. Now our 10% taller mast on boat X will be 33ft tall with a CE at 40% of mast height plus 3ft from the hull CB to the foot of the sails. This measuerment is at 40%x33ft + 3ft = 16.3ft.
The ratio of 16.3/15 = 1.087 or about 9% higher center of effort in the sailplan of boat X.
If I ignore the height from the CB to the bottom of the sailplan and simply take the ratio of the heights to the CE in the sailplans based on mast length alone, I would take the ratio of 40%x33/40%x30 = 1.1 or a 10% higher CE in the sailplan with the 10% taller mast.
You are right in that the CE only moves upward half as much as the top of the mast did but since we are taking the ratio of the two CEs, the 40% factor cancels out. So the CE moves up the same percentage as the mast top moved up.
Good Sailing,
Bill

Hi Dan,
Thanks for the compliment. First I want to say that in my experience 'all beach cats are wave piercers'. I sail in the ocean so that is what I see. The foredeck of a boat can go underwater for two reasons. 1) A wave taller than the bows, two times as tall or more for example, can pass down the hull, bow to stern, and put the foredeck underwater for a short period of time. If there is enough bouyancy in the forward sections of the hull, the bow will lift and the wave action will actually push aft on the hull for a moment. When the bow is being lifted higher than the transom as the wave passes, the transient bouyant force actually has an aft component to it relative to the forward velocity of the CG of the boat. There is some recovery of lost boat speed as the transom goes higher than the bow as the wave passes but the loss, bow up, is greater than the recovery, stern up. Also the sail/rig is made to pitch, changes relative velocity rapidly, as the boat goes over and through the waves and this reduces sail thrust.
We can reduce the hull pitching up action due to waves by reducing the volume in the front end of the hull. If we reduce the width of the hull, we upset the displacement distribution of the hull. So how else can we reduce the hull front end volume? Reduce the height of the hull at the bow! Doing this will reduce the response of the hull to wave action; it will reduce hull windage while sailing to windward; it will reduce hull surface area which will reduce hull weight. These are the plusses, +++. There is one big minus, ---.
2)There is one very important situation which occurrs while sailing downwind where hull volume in the front end is very valuable. This situation is "verge of pitchpole". The max forward sail force, maximum pitching moment, that a hull can support occurrs when the waterline is at the top of the bow. As the forward sail force increases, the bow is pushed downward which causes the transient center of bouyancy to migrate forward in the hull. This increases the distance between the hull instantenous center of bouyancy and the boat plus sailor center of gravity. This is the "restoring moment" which opposes the "pitching moment". The taller the bows are the deeper they can be driven into the water and the further forward the transient center of bouyancy can be made to migrate so the larger the restoring moment and the larger the sail force can be and the faster the boat can be driven downwind in windy conditions. So there's the trade off, reduced response to wave action vs reduced restoring moment to prevent pitchpole. A noteworthy point relative to A cats is that 18ft is a 'long hull' for a 'one man boat'. With that in mind it may be possible to reduce bow height some without too much loss in restoring moment. Note that Marstrom has not lowered the bow on his A cats and his are amoung the best and fastest.
One other comment: If we take an 18ft hull and put two people on it and double the sail thrust which doubles the pitchpole tendency, we probably should not consider the downward sloping foredeck and reduced bow height trade off on this size boat. A downward sloping foredeck is still draggy when forced underwater if it is flat on top. A high crown foredeck is much less draggy when forced underwater and it is inherently stronger and stiffer.
Good Sailing,
Bill

>>But at 20' x 10' a Tornado would seem just right. ratio 2:1

How about a a 16,4 ft X 8,2 ft ? Yes we have this area covered.

Wouter

Hi Wouter,
I'm not sure what 'just right' is. It is not a nautical/boat design term that I am familiar with. I don't follow what it means unless it means 2:1 boat length to beam ratio. There is no magic ratio or perfect ratio or just right ratio that I know of.
If a Tornado was 12ft wide, it would be the same speed/performance as today's Tornado in light and medium winds. When the winds go higher than 15knots, the wide Tornado would be faster to windward, faster on a close reach, faster on a beam reach and the same speed downwind.
Any time there is enough wind to utilize the additional 20% in righting moment, the 12ft wide Tornado will be faster.
Now, there are some conditions where the over use of righting moment can drive a boat to pitchpole. This is no different whether the boat is 10ft wide or 12ft wide or 20ft wide. You still have to watch the leeward bow and make your decision. The sailors operating the boat have to make the call as to how much righting moment to apply. Righting moment to a sailboat is just like horsepower to a race car. The operator has to make the decision as to how much power to apply. In a highly powered race car sometimes the driver has to back off the throttle to maintain control. So it is with a high powered catamaran sailboat. Sometimes you have to get out of the throttle a little to maintain stability. That's part of the skill and fun. Monohull sailors don't know what we are talking about.
Bill

Bill,
I believe the article was by Malcom Tenant (sp?), but I haven't had a chance to dig in and retrieve it. Again, he was dealing with larger boats where trapezing crew didn't come into the equation. And I think the meaning of 'optimum' was a balance of righting moment and perceived safety for pitchpole, in that there would be other hints to depower before driving a bow in - so this also was related to the level of skill in the crew. Again, this was for larger boats where the consequences of going over are larger. Still, I found it to be an interesting take.

I know you have way more research and design time on the bow/deck shape thing with respect to pitch pole, but I feel (and this is from my very limited pitch-pole experience) that the deck lips of a shoe-box style construction method seem to contribute way more to the pitchpole causing drag than the flatter decks. In the absence of the lips, flatter decks are certainly next in the order, but now having been on boats without the lips I'm of the opinion that they are the bigger evil. The tripping drag seems to initiate far sooner and in situations where the flatter deck itself wouldn't be as much a problem due to the angle of attack in the water.

I've been toying with the idea of widening my NACRA 5.8. It seems that the bow shape is pretty resistant to pitch-pole (lots of volume and narrow on the top), and that with some extra righting moment it could handle a fair bit more horsepower. Any thoughts on the NACRA 5.8 design in this regard?

I noticed that Performance Catamarans sells 11' cross-bars for converting a NACRA 5.5 to an 18sq. It seems like they would go on a 5.8 pretty easily. Has anybody else tried this?

Hi all,
To Bill and all the other posters, I would just like to say Thanks for posing and answering such stimulating questions.
I have one of my own. We have pretty much established the relationship between centres of effort, buoyancy, hull and deck forms etc in their effect on PP resistance, but I was wondering about one aspect of dynamic lift. O.K I have seen threads on foil assistance either in the form of T foils (as seen on the Australian moths, the Stealth F18HT and F16 and on many I14 skiffs) and inclined foils as used on the ORMA 60 tris. There is no doubt both these devices will allow the boats to be pushed harder downwind although in extreme conditions the foils may work against you. But what about concavities in the hull under the waterline forward of the main beam. Sailbooards have being using subtle concavities for some time to promote early planing and recently I tested a seakayak with slightly concave rear sections to help lift the stern when paddled hard. This very fast kayak the result of computer modelling and tank testing. Could the acceleration of water under the hull on a beach cat provide enough useful lift at speed to hold the bows higher and so allow the boat to be driven harder? Bill have you done any work on this?

Cheers

Simon Fisher

great question! Kinda like this?




And this?



This is all from an earlier thread located HERE

>>>If a Tornado was 12ft wide, it would be the same speed/performance as today's Tornado in light and medium winds. When the winds go higher than 15knots, the wide Tornado would be faster to windward, faster on a close reach, faster on a beam reach and the same speed downwind


Every kid knows that there are two limits to how far he can push his parents. -1- His mother -2- his Father. Often his mother is the more strickt but when his father gets angry it is often the most severe. Now every kid now knows that either one blowing up limits his range of pushing the boundery. Which ever he reaches first will stop his gains. I mean he know that his farther still gave him some slack when for example his mother reaches her liit of patience sooner. So some potential has been lost. Now what is he can shift some burden towards his father in this situation and move the apparent limit up while keeping the limit of his father the same. Now teh kid can make more gains.

Now what is he shifts alot of burden towards his father and go easy on his mother (very wide beams) will he get even further still. NO, because now the patience ofhis farther is more limiting than that of his mother. He still reaches one single limit before the other. Best ratio is when both reaches their limit at the same time than you have maxed out how far you can go given the kids stuff resistance potential of both parents.

Boats are much the same. And I've found this happens indeed. For example the 13,5 ft by 7 ft wide Hobie Dragoon is imposibble to trapeze of as an adult. as soon as you do that the noose goes down and you can't get aft enough to get it back up. It sprays like crazy and goes slow. Slower than than hangin out like on a monohull and keeping his noose up.

Now different course have different ratios that is true but each course has a ration beyond which extra width is waste,

Wouter

Hi Wouter…I think there is a flaw in your analogy…Bill has already stated

“PS. I put a SC20 together once that was 20ft wide and it sailed fine. To windward it was a rocket. Reaching don't trapeze. Downwind same as a normal SC20, watch the bows.”

Therefore, based on his testing, (even when taking it to the extreme of beam=Loa) I would come to the conclusion, that when factoring in all points of sail , the wider boat may not have a increase in usable speed down wind…but it will not be at an disadvantage either. While on the upwind and reaching legs it would have a clear advantage.

If you can advance performance on many points of sail, and keep performance of your slowest point of sail equal…I believe that is increasing the performance of the boat overall.

Maybe this would be a better analogy:

When Son goes to Dad for date Money…Dad gives him $20.00

Then Son goes to Mom for date Money…Mom gives him $20.00

Son tells Dad he has a date with the most beautiful girl in school and asks for more money….Dad says “that’s my boy”…gives him an extra $20.00 (beam increased)

Son tells Mom he has a date with the most beautiful girl in school asks for more money…Mom says I gave you enough already...no more money……$0.00 (length stays the same)

So originally the Son had $40.00 (original performance with original beam) but after approaching his parents again he picks up an extra $20.00 from Dad (increase in beam) he now has $60.00 to spend instead of $40.00.

$60.00 is greater than $40.00 no mater how you spend it…

Bob