How to dimension a sailing catamaran?

WOW

this is excellent

when i see you i will buy you lunch one day

 

Hello Terho,

Excellent job once again.
I wanted to point out tow possible miscalculations and or typo's that you may want to investigate.

In the spreadsheet, the EU design category designates a B rating at 32 knots. If I understand it correctly, I thought it had to be to Force 8 conditions which is at least 34 knots so it may be a typo.

In the PDF, under Heeling Moment, you mention that "Also, in category B, the design wind speed is lower, 25 knots". If I understand it correctly that should be the wind speed for "C" so it may be a typo also.

I hope that helps, unless I erred of course.

Thanks again for the excellent spread.

J

 

JCD,

Thanks for your notes.

In DIS version of standard ISO 12215-9 design wind speed in category A and B is 32 knots for boats under 18 m. It can be higher though...

You are right with the PDF. I'll send a corrected version here.

I was wondering if I made any fundamental error in my board area calculation. Normally it is defined as a few % of sail area, but I wanted something more precise. Any comments on that?

Terho

 

Hello Terho,

Don't mention it. Many hands makes for light work.

I'm either interpreting it incorrectly or need to update it. I have:
Category A = "conditions above Force 8"
Category B = "conditions up to and including Force 8"

Well there is a silver lining. Can the spread be made to accept any wind speed desired for design or is it locked into the 32 for A&B and 25 for C?

Oh brother. Lots can be said on this. Unless grossly designed in one direction or the other, I don't think anyone can make any errors with this. Lets agree that the faster the design the smaller the area required. With that being said, there are programs out there that will calculate the appendages to the square millimeter depending on displacement, speed and foil section.

Most agree on one thing. High aspect ratio. After that, it's anybody's pickle. High aspect is great until it stalls, then the low aspect is better. I have read that areas should be tabulated as a percent of the lateral area depending on the vessel, and I have read that they may be designed as a percent of the sail area. These extremes range from 4-15% of lateral area to 5-10% of the sail area in square inches and then you solve for your aspect ratio. Then you have appendage "volume" and "lever arm" etc.

I am going to call out the appendages for the TR27B design in her full displacement so I can keep the extra area and strength for her in other lighter displacements because the difference is minimal and as her draft rises, their areas is also reduced.

To me it appears that the areas are pretty normal. IMO, I say go as deep as you can possibly go but no less than 3 times the draft for the daggerboards, and for the rudders it should be twice as deep as the draft so the rudder is in clear water, but not too much more than that. Then solve for your chord for both according to the aspect ratio, foil section and forces the appendages will handle.

With the spread, the TR27B rudder is 2.09 times the draft and the dagger is 3.62 times the draft so both are very close or perfect to how I would like to design the appendages on the TR27B. Based on "my" thinking about designing those, I would say that you are very close if not right on.

I hope that helps.

Thanks
J

 

Rudders

Hello Terho

My take on rudders is that everyone makes them too small - I don't know why and then the boats can't tack anywhere near as well as monos or Farrier tris. My 38 footer tacks well with 2.5 aspect ratio 0012s about 1.1 metres deep. My friends boats with smaller rudders don't tack as well.

Many designers have a bit of a blind spot on rudders - they see their boats always going around at 7-10 knots and calculate accordingly. there are two major load conditions on rudders and you have to design for the worst.

1 - Broaching down a wave - Cats do broach - or at least they can want to but the rudders or more correctly the leeward rudder stops them. On a broad reach even my deep windward rudder becomes almost fully airborne in a fair swell. It is in a swell that you most need to steer and it is a swell that you will probably have one rudder fully aerated. So my advice is to calculate on one rudder - a big set of rudders gives a lot of rudder volume to resist the start of a broach.

2 - Tacking - Cats don't tack usually because their rudders don't work - they are too small when flow over them slows. All foils have a load they can accept at a certain speed. At low speeds and high angles of attack the load a rudder can accept is low. So you have to increase area to compensate.

My 38 footer can tack without the jib. My little 6 metre folder can tack without a jib and can bear away from a stall under main alone- just like a good mono. The rudders on the folder are about 80cm deep and 30 cm chord.

As for boards I would say go big too - big boards mean you can tack without sliding sideways. The 1.2m by 0.5 metre single board on the folder means we can short tack in the moorings without slipping after a tack - the foil doesn't stall.

I bragged about how well the folder tacked and then tried tacking with one rudder raised - she then tacked as badly as the other cats around.

Foils are our grip on the water - make them big.

cheers

Phil

 

High aspect

Hello again

While I am in a run - about high aspect - don't go there - bad news for cruising boats in rough water - be afraid.

Sailed a (then) new 32 ft cat with incredibly high aspect rudders and foils. The designer tried them out, using someone else's money, and I was testing the boat for a magazine. This thing looked fast but was untenable.

I couldn't keep it on a straight line and couldn't tack it, the builder couldn't tack it and the owner coudn't tack it. We had to reef it to tack with me backing the jib to windward. We were so slow in this go fast boat we were got flogged by the slowest roomaran.

High aspect stalls easily and you should never stall a cruisert- I would stay normal aspect - 2:1 or similar for good performance at stall and in rough water. Don't get seduced by lab figures for a normal rough water cruising boat.

When I think foils I think tread on tyres. A cruiser is like a 4 wheel drive - you need lots of grip for when the bad things happen - racers use slicks. Tread is like more area.

cheers

Phil

 

Hi Phil, I like your thinking.

I was looking at some pics on a cat with the boards and rudders stuck out underneath and I was wondering if they are going to work. It looks like this massive aeroplane and these small slim wings on the side which would only work if you have looots of driving power.

Mostly the weather would be fair to mild and I think that's what one should cater for.

 

Hello Phil,

I would first ask what is considered high aspect and then would agree that extreme high aspect is not good. I like to see an AR of 3 on the daggers and probably the same on the rudders and think it's a good compromise for a wide range of designs and Lwl's, but not really less than 2.5. Again, this is IMO and I have a weird enough way to design their sizes anyway.

As far as stalling high aspect boards...I agree 100%. No motion, big trouble. However, many designers today have sort of walked away from simple little things like giving the daggers 2 degrees toe in to generate extra lift at slower speeds and greater angles to the wind and instead they just place the daggers inline with the center line.

A lot of trouble to design and build an entire vessel and then it's too much trouble to design and build 2 degrees toe in? Go figure.

Thanks
J

 

Terho you have helped and educated me lot - THANKS

What allowances are made for the bridge deck in this program, is it included as part of the general picture for a bridge deck cat?

The bridge (wet) deck clearance is clearly stated as 6% Lwl which is good because there is a figure that can be worked with, which ends up being 500mm to 650mm anyway.

Usually the bridge deck is approx 2m high and 3m wide and 4 maybe 5m long on average for small cats = 30 / 35 ft

Could the design wind speed "Vawk" also somehow include a factor to indicate that your bridge deck could be too big / high for your cat, especially if the cat had to go smaller than 28ft per example.

FBI and BAS are clearly defined on your drawing, i was hoping you could include something "there" for the cabin
as we already have the 6% LwL

would be nice????

 

I agree with Phil about board sizes and aspect ratios. What works in theory usually means it only works in flat water and at optimum speeds. In waves you go slower and the boat, and thus board, gets thrown around thus making it less efficient and easier to stall.

I have experimented (with my money on my own boats) with high aspect foils and they aren't very successful in real sailing conditions. Having a board stall out on you is not good news.

I have also tried trim tabs on boards, it's quite fun to be able to sail sideways!

Aspect ratios are always less than ideal. Say you have an 8m boat. Board might be 900mm deep. A 3:1 AR means it is 300mm wide, and by implication 25mm thick. I'd be surprised if you could make one that thin strong enough, remember the lever will also increase with higher AR.

On an 8m boat I'd use boards say 900mm long, 500mm wide and 35mm thick.

As a guide use 3-5% of sail area for the boards, about half that for the rudders

I hope this helps the discussion

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

Thanks Phil and Richard,

I changed the daggerboard/rudder area ratio to be 2:1 as Richard suggested. My original thinking of the ratio 3:1 was from an analysis of some succesfull designs. The total area is quite near, what Richard says.

The spreadsheet doesn't fix aspect ratio, user can choose it free. All the yellow areas are for user to change.


Manie B,

Have to leave something for the designer to decide...


Terho

 

Wing loading

In my rant yesterday I was trying to come up with the term aircraft designers use - it is wing loading.

What they do is calculate the load the wing has to lift - the weight of the plane and then use figures on the lift characteristics of a foil and then alter the wing area to suit.

Howard Hughes had his Hughes racer with two sets of wings. One set was larger for a long distance record and he had a small set for speed attempts.

One other thing is that people assume that larger foils have more drag - this aint necessarily so. A foil has a drag profile so a foil 30% bigger might have less drag than a smaller foil at a higher angle of attack. Drag is also due to a number of factors - induced drag (from the amount of lift) is also part of the equation - not just skin friction.

cheers

Phil Thompson

 

Hello Terho,

I'm no expert but, I think that your initial study based on the AR of 3 may have been accurate as you anticipated and as I believed from what I also have read. Because I'm no expert, I used a NASA foil calculator that I can access to run some numbers and get some results. It may or may not mean anything in the real world but I feel very confident that at least they are accurate. This is not to say that what Richard and Phil said is not accurate, it is just on a different level and experience of cruising and for the designs they have experienced.

I used Richard Woods call out for daggers because I didn't have anything else I can compare to the TR27B, but he called out 7% of chord and I used 12% because the TR27B will be 12% of chord. If you like I can run them side by side and provide that information for review.

This should be used as a comparison only and I am very certain that Richard or Phil would be able to design low AR daggers that will be maximized for their designs given enough time. I think that they and I agree that large areas is important and our disagreement is more on whether it should be deep or wide and then by how much.


Richard Woods Recommendation for 8 Meter @ 3 degrees Weather Helm
Speed = 10 mph ,
Aspect 1.754
Camber = 0.0 % chord , Thickness = 12.0 % chord ,
Chord = 1.67 ft , Span = 2.93 ft ,
Surface Area = 4.893 sq ft ,
Angle of attack = 3.0 degrees ,
Under Water
Altitude = 0 ft , Density = 1.94slug/cu ft
Pressure = 14.694lb/sq in, Temperature = 60F,
Lift = 373 lbs

Speed = 5 mph ,
Camber = 0.0 % chord , Thickness = 12.0 % chord ,
Chord = 1.67 ft , Span = 2.93 ft ,
Surface Area = 4.893 sq ft ,
Angle of attack = 3.0 degrees ,
Under Water
Altitude = 0 ft , Density = 1.94slug/cu ft
Pressure = 14.694lb/sq in, Temperature = 60F,
Lift = 93 lbs


TR27B Foil comparison in same conditions @ 3 degrees Weather Helm
Speed = 10 mph ,
Aspect 2.99
Camber = 0.0 % chord , Thickness = 12.0 % chord ,
Chord = 1.5 ft , Span = 4.49 ft ,
Surface Area = 6.735 sq ft ,
Angle of attack = 3.0 degrees ,
Under Water
Altitude = 0 ft , Density = 1.94slug/cu ft
Pressure = 14.694lb/sq in, Temperature = 60F,
Lift = 514 lbs

Speed = 5 mph ,
Camber = 0.0 % chord , Thickness = 12.0 % chord ,
Chord = 1.5 ft , Span = 4.49 ft ,
Surface Area = 6.735 sq ft ,
Angle of attack = 3.0 degrees ,
Under Water
Altitude = 0 ft , Density = 1.94slug/cu ft
Pressure = 14.694lb/sq in, Temperature = 60F,
Lift = 128 lbs

As you can see from the results, the high AR out performs the low aspect at even the slowest of speeds which in this case are way lower than any 8 meter cat would be expected to perform. Most vessels will have some weather helm designed into them and that is about 3 degrees, so I figured the skipper would be cruising with that at either speed. I can calculate any other angle of attack if you would like to review those.

As I stated before, "extreme" AR's are a definite no no, but moderate to high AR's have been proven to perform well within a broad speed range when engineered correctly and not overly done. Will a skipper want good area to put on the brakes or maintain way in a crowded anchorage? Absolutely. Does he need that level of drag while cruising? I don't believe so.

For the TR27B the daggers are 1.5% of sail area of 460ft^2, 27% of the lateral area and 3.07 times the draft in length, if that means or is worth anything. I hope it helps.

Thanks
J

 

Not the full story

Hello JCD

The data from the foils is not so compelling. If you work out the load per unit area you get exactly the same figures for each rudder. The reason you get more lift from the high aspect one is that it is bigger.

Lift per unit area comes out at 76lbs/ft for both foils at 10mph and 19lbs/ft at 5 mph. I think the software may be doing some shortcuts - it doesn't seem right that the figures would be exactly the same.

My personal take is that 5 mph is too high for foil selection. I like sailing multis with lowly loaded foils because they are nice to sail. Coming out of a tack with some multis is terrible because their poor foils are way too highly loaded. Remember that the critical times that foils need to work are at the very low and very high speeds. No a cruiser skin drag should be a very low priority at cruising speeds.

Why not work out the sail loading on the rig, main only. Work out how far aft the CE is and the moment to the LCB of the hulls. This will give you a torque. Then with your knowledge of the distance to the rudder from the LCB you know that the rudder torque must equal the rig torque (actually exceed it)
Then make the rudder area-lift combination greater than the rig torque at a nice low tacking speed of 2 mph. You will then be able to tack your boat wherever you want.

Design for the worst case scenario - not for typical smooth water stuff. The consequences of not tacking on a lee shore are disastrous, the penalty for carrying an extra 20% more area - negligible.

cheers

Phil Thompson

 

Oops

There was a bit of gooboldegook in the last message

No a cruiser skin drag should be a very low priority at cruising speeds

should have read - For a cruiser skin drag should be a very low priority at cruising speeds

Phil