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Old 04-11-2014, 14:17   #46
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Re: The bow of cats

Got ya Cheechako! But who knows whether the naval architects have the freedom of designing the bows that they want, or whether that is part of the outline given by the manufacturer. Even Robert Perry designed some boats with plum stems when he was requested to do so. Indeed, he is also critical of canoe sterns, at one point saying he never bought into the 'Moses parting the waves' theory. Still, some of his most famous and commercially successful designs were double-enders (think Valiant). I seem to recall that his design brief from the manufacturer of the Valiant was for a performance cruiser with a canoe stern.

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Old 04-11-2014, 14:28   #47
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Re: The bow of cats

Overhangs do not provide more reserve buoyancy than plumb bows. Just the opposite - plumb bows have more reserve buoyancy than overhanging bows. Think about it - overhanging bows have all the buoyancy cut out of them from the deck to the waterline, while plumb bows contain that extra volume.

Where you are getting confused is in adding extra LOA to a boat in order to get that "reserve buoyancy". For any given length of boat, there is more volume in a plumb bow than an overhanging one.

Any flare you want to conjecture for an overhanging bow can also be done with a plumb bow (as Catana does regularly). Even more, knuckles or chines can be used to good effect with plumb bows to provide buoyancy unobtainable with overhanging ones.

There is no practical extra deck space in an overhanging bow - particularly so in catamarans, where one rarely counts anything forward of the crossbeam as deck space.

Again, you must compare LOA here - LWL is meaningless. Particularly so with catamarans, as this thread is about.

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Old 04-11-2014, 14:49   #48
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Re: The bow of cats

Quote:
Originally Posted by colemj View Post
Overhangs do not provide more reserve buoyancy than plumb bows. Just the opposite - plumb bows have more reserve buoyancy than overhanging bows. Think about it - overhanging bows have all the buoyancy cut out of them from the deck to the waterline, while plumb bows contain that extra volume.

Where you are getting confused is in adding extra LOA to a boat in order to get that "reserve buoyancy". For any given length of boat, there is more volume in a plumb bow than an overhanging one.

Any flare you want to conjecture for an overhanging bow can also be done with a plumb bow (as Catana does regularly). Even more, knuckles or chines can be used to good effect with plumb bows to provide buoyancy unobtainable with overhanging ones.

There is no practical extra deck space in an overhanging bow - particularly so in catamarans, where one rarely counts anything forward of the crossbeam as deck space.

Again, you must compare LOA here - LWL is meaningless. Particularly so with catamarans, as this thread is about.

Mark
it's a good point. If you are going to have the deck length, then having the waterline the same as the deck is certainly more buoyancy. It's all kind of relevant.... an overhang beyond a stated waterline is reserve buoyancy, but no overhang on an extended water line is more!
There is certainly not much practical deck space on an overhanging bow on a cat. But most my monos had the whole anchoring gear there including chain storage!
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Old 04-11-2014, 14:53   #49
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Re: The bow of cats

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Originally Posted by colemj View Post
Overhangs do not provide more reserve buoyancy than plumb bows. Just the opposite - plumb bows have more reserve buoyancy than overhanging bows. Think about it - overhanging bows have all the buoyancy cut out of them from the deck to the waterline, while plumb bows contain that extra volume.

Where you are getting confused is in adding extra LOA to a boat in order to get that "reserve buoyancy". For any given length of boat, there is more volume in a plumb bow than an overhanging one.

Any flare you want to conjecture for an overhanging bow can also be done with a plumb bow (as Catana does regularly). Even more, knuckles or chines can be used to good effect with plumb bows to provide buoyancy unobtainable with overhanging ones.

There is no practical extra deck space in an overhanging bow - particularly so in catamarans, where one rarely counts anything forward of the crossbeam as deck space.

Again, you must compare LOA here - LWL is meaningless. Particularly so with catamarans, as this thread is about.

Mark
The problem is he's hung up on the idea that you should compare LWL, so when you point out a 35' plumb bow boat he wants to compare it to a 42' boat with overhangs. So while technically correct (the 42' boat will have more reserve bouyancy in the bow), he totally misses how the vast majority of people would compare that to a 42' plumb bow boat.
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Old 04-11-2014, 17:51   #50
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Re: The bow of cats

It seems that most of us see it as a fixed deck size, then varying the amount of LWL to compare boats. I guess if you stick with a fixed LWL and change out the deck you do get more reserve boyancy with overhangs... But then does a reverse bow result in a boat that has so little reserve boyancy it can only sail underwater?
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Old 05-11-2014, 02:05   #51
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Re: The bow of cats

Thing is, if they sold boats by waterline length, (almost) every boat would have plumb bows anyway. Who's going to market their 60 footer as a 45 footer?
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Old 05-11-2014, 06:54   #52
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Re: The bow of cats

Again, we can agree to disagree. Valhalla, where did I say I wanted to compare a 35 foot boat with a plum bow with a 42 foot boat with a 7 foot front overhang? A 7 foot front overhang on a 35 foot cat would be ridiculous. And 44CC, I certainly wasn't talking about a 45 foot cat with a 15 foot front overhang! Nor was I talking about how manufacturers should market their boats. Quite to the contrary, for those considering boats I was and continue to suggest that they should forget how boats are being marketed and understand that typically, LOA is less relevant than LWL. Again, others can disagree.

What I was suggesting was that, all else being equal, a 40 foot cat with plum bows would have less reserve forward bouyancy than, let us say, a 42 1/2 foot cat with a 2 1/2 foot front overhang. Why? Because, unless one designed a plum bow so that it was the equivalent of taking a chain saw and cutting off a large V down to the waterline, resulting in a flat forward V at the bow, it would have less bouyancy at the end of the waterline than the cat with an same waterline, but a 2 1/2 foot overhang. Remember, the deck on the cat with a plum bow will typically taper to a point at end of the waterline, whereas the boat with an overhang will still have some deck width at the end of the waterline as it continues to taper to a point at the end of the ovehang. Keep in mind, this is to say nothing of the additional reserve bouyancy in the overhang itself!

Can a designer make up for the lost bouyancy and reserve bouyancy. Yes, as I pointed out, knuckles are and can be used to increase reserve bouyancy. The bow can also be somewhat flattend forward (rare, but yes, utilized on some Catanas) to increase flare towards the bow. However, these are design elements that are not present on all cats with plum bows. Furthermore (and to be fair) they can be also utilized on a cat with some front overhang in order to further increase forward bouyancy.

One must also consider the lever principle - bouyancy that is further away from the Cg of the vessel will have a greater effect in resisting submersion of the bows and in lifting them back up, once submerged. Again, since the 42 1/2 foot cat with a 2 1/2 foot overhang has more bouyancy at the end of the waterline than the 40 foot cat with plum bows (admitttedly, unless the latter boat has a flat bow), the former has an advantage in the lever effect both at the end of the waterline (more force from increased bouyancy there) as well as the reserve bouyancy in the overhang that is even further forward.

This, of course, can be compensated for by moving the Cg of the vessel further back from the bows. Indeed, this is why I commented on how the bridgedeck on a Gunboat is further back from the bows than on most cruising cats, pushing the Cg further back and increasing the lever effect for the bows.

By discussing the interplay of these various elements I was hoping to cause people to assess all of them when considering the bows of a cat (or trimaran or monohull, for that matter). I indicated that I would purchae a cat with plum bows if I was satisfied that various design elements had been used to ensure that there was adequate bouyancy and reserve bouyancy forward (e.g. chines, a bulbous bow, bridgedeck further back from the bows, etc.). Chines, of course, have the additional advantage of reducing foredeck spray - still important IMO on a cat, as I suggested, if you fly a symmetrical spinnaker or ever have to deploy a sea/para-anchor.

The costs for a manufacturer associated with putting a 2 1/2 foot overhang on the bows of a cat would be marginal. As I have pointed out, since the bows of a cat do not require a chainplate for a forestay (nor typically, a bow roller for ground tackle) the extension above the waterline can be built quite lightly. Some manufacturers still do have slight front overhangs - consider the Switch 51 which I complimented earlier in the thread. If you were in the market for a cat of that approximate size, would you consider it comparable to a 48 foot cat with no overhangs? Or would you be a slave to LOA?

Others here believe that plum bows are just plain better regardless of any of these other factors. They believe that it is not a complicated issue. Of necessity, they must also believe that the naval architects who continue to use some front overhang are stupid as there are no valid reasons to do so. Indeed, since the advantages of plum bows are so incredibly obvious (and since they have been around since the 1800's, if not earlier), all naval architects who specifed some overhang on the bows of cats in the past were also complete idiots.

I beg to differ. While I believe that there is much to commend plum bows on a cat (and as I have already said, I actually prefer the look), it is only so long as various potential issues such as forrward bouyancy, both static and reserve, and foredeck spray have been addressed in the overall design.

Brad
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Old 05-11-2014, 07:21   #53
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Re: The bow of cats

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Originally Posted by Southern Star View Post
What I was suggesting was that, all else being equal, a 40 foot cat with plum bows would have less reserve forward bouyancy than, let us say, a 42 1/2 foot cat with a 2 1/2 foot front overhang. Why? Because, unless one designed a plum bow so that it was the equivalent of taking a chain saw and cutting off a large V down to the waterline, resulting in a flat forward V at the bow, it would have less bouyancy at the end of the waterline than the cat with an same waterline, but a 2 1/2 foot overhang. Remember, the deck on the cat with a plum bow will typically taper to a point at end of the waterline, whereas the boat with an overhang will still have some deck width at the end of the waterline as it continues to taper to a point at the end of the ovehang. Keep in mind, this is to say nothing of the additional reserve bouyancy in the overhang itself!
.

One must also consider the lever principle - bouyancy that is further away from the Cg of the vessel will have a greater effect in resisting submersion of the bows and in lifting them back up, once submerged. Again, since the 42 1/2 foot cat with a 2 1/2 foot overhang has more bouyancy at the end of the waterline than the 40 foot cat with plum bows (admitttedly, unless the latter boat has a flat bow), the former has an advantage in the lever effect both at the end of the waterline (more force from increased bouyancy there) as well as the reserve bouyancy in the overhang that is even further forward.
No, this is not the case. A 42' LOA with plumb bows will have more static and reserve buoyancy and more lever effect than a 42' LOA with a 2.5' overhang. This is the primary reason one does not see new designs (or designs within the past 20yrs) with overhanging bows. It is not a marketing thing - it is real.

For hobby-horsing, the overhanging bows will be much worse for three reasons: 1) the hull design itself requires more rocker in it to accommodate overhangs, 2) the boat will dip faster than a plumb bow because the reserve buoyancy comes into play much later and much more gradually than a plumb bow - then, when going back up, it will dip sooner again because it will lose its buoyancy faster. 3) there is more weight forward of the waterline - increasing the lever causing hobby-horsing.

The waterline profile argument makes no sense. I have never seen an overhanging bow that had a thick, boxed profile at the waterline. They all have a narrow entry profile - just like plumb bows. What overhanging bows do not have is all that extra buoyancy above the waterline, since they cut it away to get the overhang.

The argument for reserve buoyancy of overhanging bows only works if one continually lengthens the deck beyond the plumb bow LOA, while calling it the same LOA. This is ridiculous - no designer, builder or buyer thinks of boats this way, and for good reason.

Do you really look at a 40' LWL boat with a 2.5' overhang and think to yourself "that is a 40' boat"? I can tell you the marinas, ad-measurers, documentation people, etc do not see them this way. A builder would certainly not market it this way, and a customer would not buy them this way.

Because they are different boats. One can desire overhanging bows for many reasons, but it is delusional to think that they provide additional buoyancy over an equivalent plumb bow boat.

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Old 05-11-2014, 07:26   #54
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Re: The bow of cats

The original point was large overhangs on mono's were the result of racing rules. A 42' mono with a 35' waterline is not an extreme example if the overhang is split fore and aft (as was typical at one point due to racing rules).

You are the one arguing overhangs make sense on cats.

For your cat example, you are still making the same assumption that is not consistent with how people design market and buy boats.
- Yes, technically a 42.5' cat with a 40' LWL would have more bouyancy than a 40' cat with plumb bows but pretty much no one would consider them otherwise equivilent size boats.

You are still looking at it backwards. The overhang isn't just free (or cheap bouyancy). You could just as easily (and cheaply) tack on 2.5' of plumb bow on the 40' boat and achieve the 42.5' LOA while also gaining the extra LWL. This is exactly what a lot of the high performance cruising cats do. They basically design accomodations based on a 40' boat and extend the hulls to 50'.

For a simplle and less extreme example: Our Gemini 3400 has modest overhangs, when they updated it to the 105, they didn't lop off 1.5' from the LOA to sacrafice reserve bouyancy, the length remained the same and they pulled the bow forward to be plumb and retain a similar reserve bouyancy while increasing the LWL.
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Old 05-11-2014, 07:38   #55
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Re: The bow of cats

If they pulled the waterline out keeping the LOA the same, then they increased reserve buoyancy - not retained similar.

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Old 05-11-2014, 07:53   #56
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Re: The bow of cats

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If they pulled the waterline out keeping the LOA the same, then they increased reserve buoyancy - not retained similar.

Mark
There is a complication as the older boats bows flared out a bit but the newer boats bows don't flare very much much, so I'm guessing a bit but overall it's probably pretty similar if you are talking about the ultimate reserve bouyancy before submerging a bow.

In more moderate conditions (say 2' chop) the longer bows do seem to ride a bit more smoothly as they react sooner but with less initial bouyancy.

Realistically in really rough stuff the solid bridge deck is going to control reserve bouyancy but that's wandering onto a different subject.
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Old 05-11-2014, 10:32   #57
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Re: The bow of cats

Colem J, I compared a 42.5 foot cat with a 40 foot waterline and a 40 foot cat with a 40 foot waterline for the reasons I indicated. Anyway, I give up. Plum bows are indeed better in every way regardless of any other factor. The naval architects that are now using knuckles forward are doing so for no valid reason as there is no advantage to reserve bouyancy above the waterline. Similarly, all naval architects who specified some overhang at present, or in the past are also idiots because, again, there are clearly no advantages to having any reserve bouyancy. So too, the naval architects who design cats with a bulbous bow (Catana) are idiots for the same reason. In the same vein, naval architects need not consider the Cg of a cat and the distance of the bow from the same in assessing bouyancy in the bows. Also, spray on the foredeck is something that only an idiot like myself and some naval architects would consider as being relevant in assessing bows. No, you are right, no one should consider reserve bouyancy, or the lever principle, or spray in assessing the bows on a catamaran, or any other boat.

The sad thing is that the idiocy seems to be spreading. Various manufacturers are now going to knuckles or chines on the plum bows of their boats, even though they had not done so in the past. We know they can't be addressing the reserve bouyancy that was lost with the elimination of front overhang, so they must be doing it to make a fashion statement. Those stupid naval architects!

Anyway, I am out of here.

Cheers!

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Old 05-11-2014, 10:59   #58
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Re: The bow of cats

Again, front overhangs do not add reserve buoyancy to a bow. They add additional buoyancy only if one compares a longer LOA with overhangs to a shorter LOA without. Nobody categorizes boats in this manner, and they never have. Boats are always categorized by LOA. For the same LOA, plumb bows have more buoyancy.

Using your logic, boats should have ever increasing overhangs. It just makes no sense at all to claim an overhang has more reserve buoyancy without admitting that the reason is because there is more boat there and you are not comparing similar boats. A longer plumb bow boat also has more buoyancy than a shorter one.

Of course NA's access buoyancy, Cg, lever arms, spray, etc. That is why all designs in the past 20-30 years have been more and more toward plumb bows - they solve the problem much better. In a catamaran, overhanging bows are a bad thing - more weight over a lever arm with no buoyancy to support it, short waterline, a pronounced hull rocker and less prismatic to allow for the overhang, hobby-horsing until people puke, etc.

Let's consider the Catalac 9M, for example. Lots of overhang, AND a chine knuckle at the bow. Hobby horses like no tomorrow and not a speed demon.

Your argument about that is specious and hinges on your belief that overhangs have more static and reserve buoyancy than plumb bows. This is not true and even the currently designing NA's are trying to show you this with their designs.

Please point to a current design that has overhang long enough to make the case you are making (and not just a few inches of it for esthetic reasons). Are these NA's idiots that do not know what you know? Are they intentionally avoiding overhanging bows and adding a dog's breakfast of extreme and more expensive design just to get away without them?

Knuckles/chines are not an indication of insufficient buoyancy due to no overhang. They are a design component that works well with either design, except they are almost impossible to build in an overhang because of esthetic and hull rocker reasons. Even so, these features are nothing new - again, consider the Catalac.

Spray is easily solved with plumb bows and is not an inherent problem with them. A slight flare, a rub rail, etc deflects spray. Spray is also usually a problem with speeds in excess of 10-15kts. Most boats don't do that and, interestingly, many designs that do go at those speeds do not have overhangs and do not have spray issues.

Everything you say is true only if you do not compare like with like. Please draw a triangle from the deck of an overhanging bow down to the water and back to the waterline. Tell me what is missing?

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Old 05-11-2014, 14:26   #59
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So after reading this thread I've come to the conclusion that the new wave piercing bows are totally useless. They give a longer waterline but have no reserve buoyancy. Why wouldn't the designer just continue the deck to the point of it being plumb over the waterline and gain the buoyancy? New fad because it looks cool?
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Old 05-11-2014, 14:27   #60
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Re: The bow of cats

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........
What I was suggesting was that, all else being equal, a 40 foot cat with plum bows would have less reserve forward bouyancy than, let us say, a 42 1/2 foot cat with a 2 1/2 foot front overhang. ........Brad
OK, but the 42 1/2 foot cat with plumb bows would be better again...
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