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Old 25-08-2008, 14:59   #31
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The jury is still out on how to define planing, so any discussion on whether a sailing cat is planing or not is a bit futile.

For a cruising cat to get enough speed, you would probably need to be reaching, so one hull will be more heavily loaded, and probably mainly in displacement mode, whereas the upwind hull will probably be close to or at least semi-planing.

If we analyse the power needed to increase speed, we know that in displacement mode, hydrodynamic resistance increases roughly with the cube of the speed. Wind energy increases with the square of the windspeed. So logically, I reckon we can claim to be planing when we get a linear function between boat speed increases and wind speed.

This was the whole concept behind Yves Parliers Hydroplaneur cat.

I used to have an Iroquois Chieftain cat, when I bought it, it had 2 x 60 hp outboards and would do around 18 knots flat out, at around 12-14 knots she would in my book plane. But I never got her to plane with the sails....

I replaced the 60's with 2 9.9 yammies soon after.


Regards

Alan
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Old 25-08-2008, 15:05   #32
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Payload and clearance

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Originally Posted by mikereed100 View Post
You might try the manufacturer for this info. Keeping in mind, of course, that bridgedeck clearance is just one aspect to consider in a cruising boat.

Mike
You'd want to ask in what load condition they were measuring the clearance, too. Light ship, half of stated payload, or full payload?
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Old 25-08-2008, 15:34   #33
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Sail area and horsepower

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I have always understood that the standard 1.34 ratio for displacement hulls does not apply to narrow displacement hulls. Hence the narrower the hull for a given beam the higher the hull speed. To drive a hull past its hulls peed requires lots and lots of energy. To have cat engines big enough would be wasteful but as wind is still free sail power can be used to provide a lot more thrust than the engines. Whilst I do not think that cats can plane I do think they can surf in certain conditions. I chose a largish boat because for just two of us it provided a balance of comfort with performance. Some may spend large sums of money making a boat lighter but it seems to me in many real world conditions such a boat can actually go slower. Its simpler to opt for a larger boat.
I think it is more practical to go with a larger boat that to make a boat much lighter than average, too.

Wind power can be predicted with Martin's Formula, which gives a thrust in pounds per square foot-wind velocity in statute miles per hour squared x .004 = pounds per square foot of force. To convert to knots, divide mph by 1.1515 to get knots so 17.25 mph is about 15 knots. 15 knots gives you 1.19 pounds per square foot of force. An engine will give you something like 20 pounds of force per horsepower, after factoring in the efficiency of your propeller, friction losses, etc. etc. So, very very roughly, you might say that at a wind speed of 15 knots, you get the same push from 100 horsepower as you would from 1680 square feet of sail.

I have never seen a conversion factor for different points of sail, so I don't know what efficiency factor to impute to sailing, so I have used 100%. It must be a bit less when running, and might be a somewhat more when beam reaching.

One rule of thumb has been that it takes 1 horsepower per 50 pounds to make a boat plane, but that is speaking of boats designed to plane. That hull shape would make a very poor one for a sailing catamaran, though I believe Hydroplaneur had such a hull shape. However, Hydroplaneur was a huge, specialized racing machine, not a cruising boat, and as such it had a huge sail area compared to its weight, and a very large crew to handle the sails meant to keep the boat sailing at that speed.
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Old 25-08-2008, 15:48   #34
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Originally Posted by Nordic cat View Post
The jury is still out on how to define planing, so any discussion on whether a sailing cat is planing or not is a bit futile.

For a cruising cat to get enough speed, you would probably need to be reaching, so one hull will be more heavily loaded, and probably mainly in displacement mode, whereas the upwind hull will probably be close to or at least semi-planing.

If we analyse the power needed to increase speed, we know that in displacement mode, hydrodynamic resistance increases roughly with the cube of the speed. Wind energy increases with the square of the windspeed. So logically, I reckon we can claim to be planing when we get a linear function between boat speed increases and wind speed.

This was the whole concept behind Yves Parliers Hydroplaneur cat.

I used to have an Iroquois Chieftain cat, when I bought it, it had 2 x 60 hp outboards and would do around 18 knots flat out, at around 12-14 knots she would in my book plane. But I never got her to plane with the sails....

I replaced the 60's with 2 9.9 yammies soon after.


Regards

Alan
Hi, Alan - I don't think the jury is out on how to define planing at all, though it may be difficult in some cases to say when a given boat is actually planing. Planing boats are rising up on their bow waves due to dynamic lift on the hull bottom (or on hydrofoils,) and it is usually said to be a condition that applies at 3 to 5 times the square root of the waterline length in feet, with the speed in knots.

I think planing hull design is fairly intuitively obvious, in that the bow sections are fairly sharp but very quickly become relatively horizontal well forward, with completely straight buttock lines aft of amidships and an immersed transom. To me, it looks like a shape that would plane if pushed hard enough. And one horsepower per fifty pounds of displacement is a pretty hard push. It's rather like picking out a rock to skip across the water-pretty obvious once someone has shown you how-flat on the bottom rather than rounded.
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Old 25-08-2008, 15:54   #35
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Great! I post a specific thread asking about Leopards and clearance and end up with 3 pages of debating general boat design! Any Leopard owners out there?
Thank you!
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Old 25-08-2008, 16:03   #36
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Try the user's group!

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Originally Posted by amelis View Post
Great! I post a specific thread asking about Leopards and clearance and end up with 3 pages of debating general boat design! Any Leopard owners out there?
Thank you!
Leopard Owners Group
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Old 25-08-2008, 16:05   #37
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OK I am also a Leopard owner so I will comment on the original question - I have a Leopard 38 and I have had it out in some pretty nasty stuff - it does not have terribly high underdeck clearance but I would have to say it slams less than I expected. You do get some good thumps but it does not seem to suffer from the constant thumping you get when bow waves combine under a too low bridge deck.

Now if I may take another shot at that other issue - I have sailed dingys that got up on a plane and you knew it right away by the change in sound and the speed jump. I have had My PDQ32 up over 13 knots and there was no planing involved. The windward hull would have been higher in the water but no where near a plane. The other hull gets pressed deeper to the point where you have top be carefull that it does not start to dig in too much.
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Old 25-08-2008, 16:23   #38
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Now if I may take another shot at that other issue - I have sailed dingys that got up on a plane and you knew it right away by the change in sound and the speed jump. I have had My PDQ32 up over 13 knots and there was no planing involved. The windward hull would have been higher in the water but no where near a plane. The other hull gets pressed deeper to the point where you have top be carefull that it does not start to dig in too much.

In the interest of pulling a few more railroad ties out and further derailing this train:

I have also had my Catalac up to these speeds. No planing at all. Just high speeds, a lot of "whooshing sounds" and a lot of foam behind the boat.

If it were to plane, the pattern in the water behind the boat would look less turbulent than it did.
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Old 25-08-2008, 16:29   #39
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Old 25-08-2008, 23:07   #40
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Trying to understand

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With a fully loaded D/L of 118, the St. Francis is actually quite a bit lighter than the Fastcat 425 is at 153.5, lighter relative to its size, that is.
Am I correct in assuming that "fully loaded" refers to DWL (lightship + payload)?
If so, why do my numbers come out different than yours? I come up with a D/L of 164 for the Fastcat using the calc utility you have previously referred to (and using the Multi Hull Maven specs). Did I not account for something?
But my calc came out the same as yours for the St. Francis 50.

Now the confusing part, as if I am not already confused! The St. Francis website lists the StF 50 with a payload rating of 9500 lbs, which is 1,676 pounds less than the Fastcat 435 spec as shown on MHM. Is this playing with numbers, am I missing something, or is there an accountable variable that comes into play?

Then, if one uses the StF payload spec instead of the greater FC payload spec, the D/L calculation becomes a bit closer; FC=133 vs StF=118. And logically (at least to my simple and uneducated mind), one would think that a 43 ft cat would have less of a payload spec (all other things equal) than a 50 ft cat. For the sake of simplicity, is one further reduces the payload by 7%, the D/L gets even closer (129), but not equal to the larger StF. Even to my uneducated mind I'm certain there are a load of parameters that come into play when calculating payload. But for these two boats, as an example, what might be coming into play that you can actually put a finger on with regards to published specs that might account for the greater payload spec of the FC?

At any rate, I thank you for educating me a bit. Your persistence in posting calculations and naval architecture now-how is at least helping me understand what I am ignorant to.
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Old 25-08-2008, 23:23   #41
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Am I correct in assuming that "fully loaded" refers to DWL (lightship + payload)?
If so, why do my numbers come out different than yours? I come up with a D/L of 164 for the Fastcat using the calc utility you have previously referred to (and using the Multi Hull Maven specs). Did I not account for something?
But my calc came out the same as yours for the St. Francis 50.

Now the confusing part, as if I am not already confused! The St. Francis website lists the StF 50 with a payload rating of 9500 lbs, which is 1,676 pounds less than the Fastcat 435 spec as shown on MHM. Is this playing with numbers, am I missing something, or is there an accountable variable that comes into play?

Then, if one uses the StF payload spec instead of the greater FC payload spec, the D/L calculation becomes a bit closer; FC=133 vs StF=118. And logically (at least to my simple and uneducated mind), one would think that a 43 ft cat would have less of a payload spec (all other things equal) than a 50 ft cat. For the sake of simplicity, is one further reduces the payload by 7%, the D/L gets even closer (129), but not equal to the larger StF. Even to my uneducated mind I'm certain there are a load of parameters that come into play when calculating payload. But for these two boats, as an example, what might be coming into play that you can actually put a finger on with regards to published specs that might account for the greater payload spec of the FC?

At any rate, I thank you for educating me a bit. Your persistence in posting calculations and naval architecture now-how is at least helping me understand what I am ignorant to.
Well, whatever the designers assume the payload to be will probably not govern what the owners choose to put in the boat. There are two ways to look at payload- one is what the designer intended, and the other is what the owners actually put on board. (A designer must make certain assumptions to optimize the boat's hull shape, primarily with regard to wetted surface.)

Some things you put in a boat will (or should, at least,) scale up with the boat, such as anchors, chain, rigging, etc. Many more will scale to the people aboard, and their preferences. I think it is reasonable to compare boats in the same general size range with the same payload.

We probably used different sources for boats displacements and payloads if we came up with slightly different answers. I get the displacement from whatever internet source comes most readily to hand, often Multihullmaven, and I use a payload from the same source if it is given, or apply the same payload if I am directly comparing two or more boats. You will note that even with slightly different assumptions, we both came to the conclusion that the Fastcat 435 cannot reasonably be considered a light boat when used as intended-that is used for ocean voyaging.
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Old 25-08-2008, 23:42   #42
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Okay, I understand - there are some un-written variables that can come into play. As a pilot (PPL) I guess I was assuming there were clear and defineable specifications that an owner could not exceed. Much like weights and balances in an aircraft. Obvioulsy there would also be a figure where the design of a given cat becomes unsafe, but from your explanation it appears to be much more of a gray area than with something like an aircraft.

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You will note that even with slightly different assumptions, we both came to the conclusion that the Fastcat 435 cannot reasonably be considered a light boat when used as intended-that is used for ocean voyaging.
Frankly, I can't conclude that yet, as these two boats are the first two I have measured-up in terms of D/L (I'm learning). However, I'm fairly certain you would be proved correct, at least in terms of D/L specifications. I will say that it is interesting that for all the talk of lightweight materials being used, that the StF weighs only 3,500 lbs more than the high tech, lightweight FC 435 (lightship weights), yet is 7 feet longer and has a two foot wider beam (and would think is cavernous compared to the FC) .
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Old 26-08-2008, 09:50   #43
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D/L ratio

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Originally Posted by KGP View Post
Okay, I understand - there are some un-written variables that can come into play. As a pilot (PPL) I guess I was assuming there were clear and defineable specifications that an owner could not exceed. Much like weights and balances in an aircraft. Obvioulsy there would also be a figure where the design of a given cat becomes unsafe, but from your explanation it appears to be much more of a gray area than with something like an aircraft. Frankly, I can't conclude that yet, as these two boats are the first two I have measured-up in terms of D/L (I'm learning). However, I'm fairly certain you would be proved correct, at least in terms of D/L specifications. I will say that it is interesting that for all the talk of lightweight materials being used, that the StF weighs only 3,500 lbs more than the high tech, lightweight FC 435 (lightship weights), yet is 7 feet longer and has a two foot wider beam (and would think is cavernous compared to the FC) .
From Tedbrewer.com:

"DISPLACEMENT/LENGTH RATIO: The D/L ratio is a non-dimensional figure derived from the displacement in tons (of 2240 lbs) divided by .01 LWL cubed, or, Dt/(.01 LWL)3. It allows us to compare the displacement of boats of widely different LWLs. Some examples of various D/L ratios follow, but are generalities only as there is often a wide range within each type."



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Old 26-08-2008, 11:04   #44
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The Displacement / length ratio and hull length / beam

The Lagoon 44 has a lightship D/L ratio of 143, and the Leopard 46 has a D/L ratio lightship of 122. Loaded as recommended, The Lagoon 44 has a D/L of 164, and the Leopard 46 has a D/L of 186. If you take each boat's lightship and add 10,000 pounds, the Leopard has a D/L of 169 and the Lagoon 44 has a D/L of 204. The Leopard is a couple of feet longer, and that helps its figures.

When cruising on catamarans, the bigger the boat, the faster, if you want to carry a lot of fuel, water, equipment, personal property, and luxuries. Both boats look to me like they have very wide hull beams, which certainly won't help their speed. This is one place where the Fastcat 435 does excel, with a hull length / hull beam of 13. I'd say that the Leopard and the Lagoon above have a hull length / beam of between 7 and 8. The net effect is probably to give them less wetted surface than the Fastcat when heavily loaded. Wetted surface is your chief form of resistance when sailing in light airs, so if you are going to carry a lot of stuff on a relatively small boat, you may actually do better in light airs than a boat with wide hulls than in a boat with a narrower hull. The boat with the narrow hulls will do better when the wind pipes up, however. Obviously, it is easier to push something narrow through the water than it is to push something wider through the water.

Best of all is if you can have a large boat made by keeping the hulls the same beam, and stretching the boat. That will make your boat faster in both light and heavy winds, if you keep the accomodations the same rather than increasing them when you stretch the boat. That is the philosophy I followed when designing the BigCat 65, which has a D/L ratio of 61 lightship, and a D/L of 78.5 with a thousand gallons of fluids (half fuel and half water) on board. This is also the philosophy of, for example, the Outremer 64.

I wouldn't say that moderate overloading of a catamaran was dangerous. Obviously, it will slow the boat down, however.
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Old 26-08-2008, 11:54   #45
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two things......

1. In our years of cruising our AIKANE56 we ran next to Lepoards on a number of occasions. Yes, the light weight and waterline length of our cat allowed us to reach the destinations first but the Lepoards always arrived. They had just as much fun when they got there, and at least as many creature comforts as we did in a package 9' shorter. I many times wondered if it would have been a better idea to have gotten the smaller boat.

2. as to planing cats, I had the pleasure to run a couple of 55' daycharter cats in Hawaii that were equipped with rotating wing masts as well as twin 350hp Cummins diesels. Wide open under power with no passengers aboard theswe boats would motor at 25knots. When they did the bows would rise, but it did not seem that they were fully planing. Under sail they still reached the high teens on occasion even dragging four blade props.
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