Bill-I mean absolutly no disrespect-I have old magizines describing your transition from FD's to Contenders and I would love to sit down with you with a beer, but why not put out a 20' 8'6" cat?

A few years ago when the CFR 20 was still in development, the Chief designer put a few posts about it on the old forum. I think his name was Preston Carter or Carter Preston or Preston Tucker, or something like that. I told him he should go to a wider beam either 10 ft. or 12 ft. He replied that the 8.5 ft. beam was neccesary because he wanted to stick to a "KISS" design phlosophy. I told him why not make his 8.5 ft version and call the 12 ft. version the CFR 20 XL. I also told him when it blows, I'd beat the 8.5 version every time with an XL. The thing to do is get the CFR 20 hulls, boards an rig and get some 12 ft crossbars. By hot-rodding the CFR 20, you can make your own CFR 20 XL. Then you can have light weight and more righting moment.

David Ho
TheMightyHobie18 1067

Preston Blake was the chief producer of the CFR-20, with design input from Morelli and Melvin. This potentially fine racing instrument saw superb performance early on in light airs with/after tuning by Randy Smyth, then has, sadly, disappeared from public view. This featherweight and still pristine prototype lives quietly in a shed somewhere in Gainesville, Florida, perhaps at the former Ghostdance Marine manufacturing location. It has been carefully kept dry and out of the sunlight at our local Lake Santa Fe. The CFR-20 has been invisible for the past 2 years or so, as has Dr. Blake.

An ARC21 can be ordered in a 20' length 8'6" width. Basically the same boat but with 1' off the stern. Contact Aquarius for specifics.

thom

I know I am in way over my head here, but I have questions, and I can't help it.

Grob says:
I didn't misinterpret righting moment and healing moment I deliberately swapped one for the other. My understanding of these two is that while they have different meanings they are equal and opposite so one can't increase while the other stays the same.
Can you explain how RM and HM can be different.

And Andrew says:
Righting Moment (RM) is the force that tends to keep the boat upright. Heeling Moment (HM) is the force that tends to knock the boat over. Shifting the rig to weather will increase RM the same way shifting crew to weather does: The lever arm for the mass of the rig is increased. HM stays the same, because a rig of X height, having a center of effort Y feet above the deck, will ALWAYS have the same HM. Of course, as the weather hull flies, the HM increases faster than it does when the rig is centrally located, as it rises above the water (and the leeward hull) faster.

And Mary (that's me) cannot understand physics concepts unless she carries them to their ridiculous extremes. So she interprets Andrew as saying that it doesn't matter whether the mast is on the leeward hull, in the middle of the boat or on the windward hull -- that the heeling moment will remain the same (except that heeling moment will increase faster if the mast is more to the windward side of the boat.

Did I understand Andrew correctly -- and is he correct? If so, it challenges my limited store of common sense.

mary, I think you're interpreting them correctly.

My thought on the matter is this: Our cats are faster when it's just beginning to fly a hull because of a combination of maintaing the righting moment and reducing wetted surface area. The problem is that as we fly a hull, the mast and sails start to lean toward the water producing an increasing downward component to the power provided by the sails. The farther you heel the boat, the greater this downward component becomes. If you were to lean the mast to windward so that if you fly a hull the mast remains perpendicular to the water and you reduce wetted surface area how can this not be faster?

BTW...I'm pretty certain that the open 60 tris have windward tilting rigs.

What I believe he's trying to suggest is that the heeling moment stays the same, because it's dependent on rig size/height/etc;

When one moves the rig around, you are making changes in righting moment.

Do I understand you correctly, Andrew?

I ain't gonna comment on the correctness of the statement, but here's a link for a GREAT article by James Wharram, he's been designing cats since God was a baby:
http://www.wharram.com/catamaran_stability.shtml

When I getaminit, I'll read Wharram's article...

sea ya
tami

As you heel, your are decreasing the cross sectional area of the sail to the wind, but increasing things like your tramp area to the wind (hence mesh tramps). You are also decreasing the amount of weight that you have that is put towards righting the boat. Since your weight stays normal to the ground line as you heel, instead of trying to right the boat, you are now just pushing down (varys with angle of heel) on it. The more you heel, the less you are doing towards righting the boat.

wouter

Gareth,

>>I didn't misinterpret righting moment and healing moment I deliberatly swapped one for the other. My understanding of these two is that while they have different meanings they are equal and opposite so one can't increase while the other stays the same.

This is a deceiving way of putting it. While your statement is correct is suggests something that is not correct.

By virtue of this relatio you conclude that while the heeling moment is unaltered by move the mast to the windward hull that the righting moment isn't changed either. BUT the maximum ACHIEVABLE righting moment is changes; this is increase to a crew is forced to depowered at a higher windspeed.

So yes there is always equilabrium but the windspeed at which extra effort needs to be taken (depowering the rig) is affected which in turn means that the overall speed of the platform is affected in depower conditions.

In the example the modified boat can sail faster in depower conditions because as a result of the increased MAXIMUM righting moment it needs to depower less to maintain equilibrium. Less depowering means more power in the sail and means faster sailing.

The analogy to trapezing or not trapezing is quite good. Also here the heeling moment is not directly changed but still teh boat with the trapezes is faster in depowering conditions than the one without.

>>Can you explain how RM and HM can be different.

It comes down to the fact that the relation to HM and RM is one of equality. It is more a master-slave relationship with two distinct windspeed ratios.

Light winds to fully powered up. : RM follows HM
Fully powered up to strong winds : HM follows RM

All the best

Wouter

Mary, Let me attempt to clear up the confusion I have caused.

First thing to remember is a moment is a force times a distance.

The righting moment (RM) is the total weight of the boat and crew multiplied by the distance between the centre of buoyancy (CoB) and centre of gravity (CoG) in the horizontal direction.

The heeling moment (HM) is the Force of the wind on the boat (mainly in the sail though) multiplied by the distance between the CoB and the centre of effort (CoE) of the wind in the vertical direction.

As with most things in engineering these two forces have to be opposite and equal otherwise the boat falls over.

With the HM the distance is more or less constant but as the wind changes the force increases and so the HM increases.

With the RM the weight is more or less constant but the distance increases as the crew moves to windward (CoG changes) and/or the boat heels (as the boat heels the CoB moves toward the leeward side).

One thing to remember here is that these two things change constantly but what we are interested in is the maximum HM or RM. With the HM there is no maximum, the wind can blow very hard, but there is a maximum RM the crew can only move so far to windward! That may be why most people talk about RM not HM.

So when there is no wind the HM force is 0 and so the crew sits on top of the boats CoB so the RM distance is also 0. As the wind starts to blow the HM force increases and so the boat heels and/or the crew move to windward just enough so that the HM and RM are again equal.

Lets say the Heeling moment distance is 4 and the RM weight is 200.
HM RM
Force x distance Force x distance
Stationary boat 0 x 4 = 200 x 0
Windy boat 100 x 4 = 200 x 2

Everything nice and equal.

This is a dynamic thing and both are constantly changing with the crew moving and reacting to each change in the wind, but over any period of time the two have to be equal and opposite and cannot exceed a maximum governed by the weight and width of the boat.

Some people think that by moving the mast to windward you are lengthening the HM distance as you are moving the CofE further away from the CofB.

And that is what I thought Mary thought and that was dumb dumb dumb as I have no Idea what Mary thought.

However moving the mast does not alter the HM because the heeling moment only takes into account distance in the vertical direction.

I think Wouter and others are saying it does increase RM because among other things it moves the CofG slightly further to windward.

I did not understand Wouters statement because I knew that HM and RM have to be equal and opposite and Wouter was implying they could be different. I don’t think that is what he really meant. What he meant to say was that maximum RM does increase. It’s a bit of a chicken and egg thing because then the HM also has to increase to be equal and opposite.

It’s a case of both of us not being clear enough. And that is partly why engineers argue all the time!

Is that Clear enough?

I have no idea what anybody is talking about because some people seem to think I was talking about tilting the mast one way or the other. I was not talking about that at all. I was just talking about moving the entire mast -- vertically, 90 degrees angle to the main beam -- moving the entire rig to windward. Grob seemed to originally say that doing this would increase righting moment and would also decrease heeling moment because the leeward hull would be able to help more with the mast farther away, just as with a wider boat.

Here's a drawing to go with this technical description by grob. This is one scenario where the windward hull just begins to become airborn. If I get a chance, I'll draw one with a tilting rig. The formulas for this (simplistic and 2 dimensional - it's not quite right because it doesn't take into effect the vectors of these forces - more on that later) are as follows.

RM = CoG X D2
HM = WF X D1

If the boat is stable sailing like this (hull not going up or down) then RM=HM. If the hull is rising higher and higher then HM > RM (is greater than). If the hull is falling back towards the water (pointy side still up) RM > HM.

I was thinking in the same line you were, Mary. All I'm saying is that if you move a, say, 50-lb rig, from the center of the boat to the windward hull, you have accomplished the same thing as having a child crew moving the same way. Righting Moment is increased: clearly it will now take more force from the sails to fly that windward hull, because you now have the weight of the rig to lift. On the other hand, Heeling Moment, ALL ELSE EQUAL, is the same. For a given wind strength and angle, a given rig will always produce the same heeling moment and the same amount of drive (assuming everything else is the same of course.) In the same conditions in which the boat was just flying a hull, with the rig in the middle, it no longer is. Now, the equilibrium Heeling Moment, when the hull begins to fly, must be exactly the same as Righting Moment, but both forces will be greater than they were when the rig was in the center of the boat. Of course, don't try to tack!
hope this helps

No, it doesn't help.
Are you saying that if you have a 50-pound bag of sand in the middle of the boat, and you move that bag 4 feet to windward, it will increase the righting moment but it will not decrease the heeling moment?

Moving the bag, or moving the mast, increases the potential maximum RM, the RM and HM are always equal and opposite. Without the wind in the sails the boat simply heels until the centre of Gravity and Centre of Bouyancy line up agian.

Try this series of articles, http://woodsdesignssailingcatamarans.com/indexstart.html

It talks about HM and RM and why Hobie Cats can fly a hull safely yet narrow cruising catamarans cannot.

Should have added

Go down to the bottom of the page and click on the technical article "stability 1"

Mary, I hope you don't think that I was calling you dumb when I said "And that is what I thought Mary thought and that was dumb dumb dumb as I have no Idea what Mary thought.".

I was calling myself dumb for thinking that I knew what you were thinking.

No, Grob, I was basically saying that what you were saying made sense and that what Andrew was saying did not (at least to me).

Well, actually, I don't know anymore who said what except that I agreed with Luiz originally, and what HE said made sense -- that righting moment and heeling moment are essentially equal and opposite.

Hi Mary,
Moving the mast to windward, say 4ft, does increase the righting moment. Say the mast and rig and sails weigh 100 pounds. In the center of the boat they generate 400ftlbs of righting moment. Move them 4ft to windward of center and they generate 800ftlbs of righting moment. The people righting moment on this boat would be something like 11ft X 325lbs = 3575ftlbs. Moving the mast to windward would increase the righting moment roughly 400 ftlbs out of about 4000ft lbs or 10%. This sounds a somewhat interesting until we look at the included angle between the mast and the windward shroud. It the mast is stepped 8ft away from the leeward hull on an 8.5ft wide boat, what is our windward shroud angle relative to the mast?? Ans. 1.4degrees. I don't think that is enough to hold the mast up, Mary.
Bill

Wouter,
You abused what I said here and misused the information I supplied and I do not appreciate it.
I addressed hull weight only and scaled hull weight only with the LWL**3 relationship. My objective was to show interested sailors that the manufacturing method/process of the RC30 hull is outstandingly low in weight and by scaling a finished RC30 hull weight at 30ft down to 20ft and 18ft you arrive at hull weights equal to or better that anything else available anywhere in the world.
The rest of the RC30/27 parts like, beams, tramps, rigging wires, rudderheads, ropes, blocks, etc. are not outstanding in minimum weight because they are plain old aluminum and dacrom and 316 stainless and plastic etc. so they are not superlight as the hulls are. That is why these parts are not outstandingly low in weight. Well, 'why not', you might ask? COST!!! I still have appreciation for the customer. I'm sure at least 100 pounds could be taken out of the weight of a RC27/30. And guess what? IT WOULD DOUBLE THE PRICE OF THE BOAT!!! Tom Haberman and I don't think it is the thing to do. It is the same situation on the ARC22. It could be lighter also. The ARC27/30 and 22 hull weights are outstanding in construction method and weight; today's technology. The rest of the boat is 1960's technology as far as construction method and weights go. This is why what you did with the total boat weight scaling is misleading to other readers. You mixed apples and oranges and you didn't know you were doing that; but you did know that you wanted to make the RC hull weight numbers look highly questionable if not unbelieveable, "a bit much don't you think'?
Bill