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  #151  
Old 04-13-2013
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Re: Modern Hull forms and Motion Comfort

Have a great trip. It is always an exciting time in the life of a designer when a design goes from your mind to paper and even more exciting when it goes from paper to real life. Enjoy it.
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  #152  
Old 04-13-2013
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Re: Modern Hull forms and Motion Comfort

Damn it Wolfer!
You made me think.
I hate it when that happens.

Sure I coud do your boat with a fin keel and a nice skeg hung rudder or better yet a spade rudder.
It would look just like your boat,,,above the water. Under the water you would not recognize it. I would have to shave away all that deadrise and reduce displ while carving away some volume forward and reducing some hollows aft.

But in the end you and me would love it. It would be a WOLF in sheep's clothing like NIGHT RUNNER.

Thanks Jeff.
I expect I will be presented with a lot of questions to come up with answers for,,,on the spot. But I have a very good yard working with me and I think we are ironing out all the details in good order. I'll post some pics for you when I get back. Unless they kill me at the yard.

If you don't hear from me in ten days, send out a posse.
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  #153  
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Re: Modern Hull forms and Motion Comfort

Wolfy:
Next Wednesday post your hull lines again and a photo or two of your boat.
When I start thinking about a new boat I need to see it and I think it would be fun for all of us to see what your boat would look like with a more modern hull combined with that traditional Atkin look.

I have to do this to get it off my mind. I'm built like that.

Or, Jeff could do it. I know he could.
Or, Jeff and I could do it together.
He'd do the hard work and I'd do the pointing and gesturing.

Either way let's see if we can produce a boat that will get your juices flowing.

Now I think I'll go play my new Boz Scaggs lp MEMPHIS. It sounds great on vinyl.
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  #154  
Old 04-14-2013
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Re: Modern Hull forms and Motion Comfort

Back too the origins of the thread...
Firstly are we just asking how modern designed cruisng boats with roots in the new "gran prix" racing boats behave in offshore conditions or are we comparing them to the "tried and true" "blue water boats" of an older design. Though the thread's original question was to ask about motion in ocean conditions it seems to have evolved to a discussion about performance.
From a point of comparison you have to decide what type of sailor you are, a cruiser or a racer, what type of boat do you want, a cruising boat, a racing boat or a boat that is designed to emulate a racing boat but be used for cruising.
To be able to achieve hull-speed with less sail area is only important in racing. In really light winds the little bit of extra speed is only significant for a cruiser who is making a long passage. Keep in mind the extra weight necessary for extensive passages would affect the light weight boats in a seriously detrimental manner, while having little affect on a heavy displacement boat. If you are thinking of serious passage making not only do you need a boat that will not be impacted be the extra weight (some are actually designed for it) you want a boat designed for when the weather gets dicey, which could be one of the sources of this thread.
Not all "blue water boats" of old or new design are slow poor performers (though some are basically weekend apartments with sticks in them).
I have seen incredible differences in performance on the same hull with different rigs as well as significant differences in performance with very subtle tweaks to rigging.

OK maybe the Westsnail wasn't a good example for a comparison, other than alot of similarities in appearance, mostly above the waterline. My boat was a commission designed in 1936 as a single hander, accommodations were low on the priority list, it doesn't fit with most Atkin designs and especially is not similar to the newer boats with Atkin roots that have an emphasis spacious below decks and a strong Colin Archer influence. The lines of my boat are closer to a fishing schooner of New England heritage.

Last edited by wolfenzee; 04-14-2013 at 02:05 PM.
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  #155  
Old 04-14-2013
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Re: Modern Hull forms and Motion Comfort

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Originally Posted by wolfenzee View Post
(though the extra weight necessary would affect the light weight boats in a seriously detrimental manner). If you are thinking of serious passage making not only do you need a boat that will not be impacted be the extra weight.
This argument has always struck me as awkward at best. Such a boat, but built with lightweight, but stronger, materials, and with a lighter (but equally stronger) rig, which would then need less weight in the keel, would now suddenly be able to carry much, much more gear than before. So much so, that it once again becomes seriously detrimental to performance.

One could quite easily argue that much of the weight in old designs are excessive in todays world, and because of the different building methods and materials, they absolutely had to be able to carry that much weight. That, however, does not mean that the hull shape is suddenly able to perform well with even more weight aboard.

I simply don't get the argument that a heavy displacement boat is more likely to be able to carry a lot of gear, while still performing. And by "perform", I don't mean wallowing along, having to use the engine in anything less than 10 knots to get anywhere.
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  #156  
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Re: Modern Hull forms and Motion Comfort

Think of adding 1000lbs to a 5000lb boat (20% increase) compared to 1000lbs to a 15,000lb boat (6.6% increase), it is even more extreme when you figure inultra light weight and really heavy disp.
An not all heavy disp boat can't get out of there own way in anything short of a gale. I don't know what it takes to get my boat to hull speed, but main and working jib will do the trip in less than 10kts.
There is a formula somewhere that figures how much weight it takes to raise the waterline 1in, this is based on disp.

Last edited by wolfenzee; 04-14-2013 at 02:44 PM.
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  #157  
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Re: Modern Hull forms and Motion Comfort

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Originally Posted by wolfenzee View Post
Think of adding 1000lbs to a 5000lb boat (20%) increase compared to 1000lbs to a 15,000lb boat 6.6% increase, it is even more extreme when you figure inultra light weight and really heavy disp.
I am aware of that. But as I said, if the hull is already at the design max, it doesn't really matter at that point. I also used the example of an old design, but made from lighter and stronger materials, allowing it to carry more, and yet be well under the weight of the original. The point I was making is that a boat of any given weight can be designed to carry weight. In theory you can build a huge tub (literally) from carbon fiber and it will carry a lot. Now, if you let the sides of the tub bulge outwards from the designed waterline and up (flare, basically), you will have the most resistance to being emerged with extra weight.

How do you know that old hull is not already overweight when it's empty? Most "old school" boats are overbuilt to the extreme, and they're old, so they probably have soaked a lot water since then. Of course, that's even without considering how much weight all that extra surface there is on an old hull, compared to a modern fin keeler or the like, also adding to the weight.


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An not all heavy disp boat can't get out of there own way in anything short of a gale. I don't know what it takes to get my boat to hull speed, but main and working jib will do the trip in less than 10kts.
Hull speed is funny thing: More modern designs surpass the hull speed of older designs at the same length. Having more weight and more surface area under water (drag, in other words), also makes it more resistant to light breezes. You can design a lightweight boat, with a more modern hull form (and keel and rudder etc) that can carry at least as much weight as your old thing, and since the all-up weight will still be lower than the old design, it will perform better.

The same hull design of that heavy displacement cruiser would be a much better performer if it were lighter overall. It would be able to carry more, and still come in lighter than it does now.

I'm not saying that all old boats are slow, but if one of the most important criteria is that it should be considered a cargo vessel and carry 1 or 2 tons of extra gear, obviously the design will reflect that and as a consequence tend to be rather slow in light winds. Not to mention that a lot of cruisers don't like to do too much "tweaking", so they tend to go with a conservative sail area, making it even worse.
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  #158  
Old 04-14-2013
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Re: Modern Hull forms and Motion Comfort

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Originally Posted by One View Post
...

I simply don't get the argument that a heavy displacement boat is more likely to be able to carry a lot of gear, while still performing. And by "perform", I don't mean wallowing along, having to use the engine in anything less than 10 knots to get anywhere.
Well, they don't perform very well in first place. Put a lot of weight on them and the difference will not be much because they were already very heavy without the additional weight. A bit more will not make much difference: their performance was never good by modern standards and they were already heavy without any added weight.

Bob Perry explained already that the loading capacity It's a function of WPA and not displacement.

Regarding modern boats everybody knows that weight is detrimental to performance, specially the few cruising boats that are designed to plan. But even a modern cruising boat that can plan easily in minimum charge condition will sail much better with its Max load than an heavy old design, specially with light and medium winds.

The difference of speed between the old full keeler empty or loaded will be much less than the difference in speed on a planning hull while light or full loaded but even so the performance of the light modern boat will be far superior.

Anyway most modern cruising boats are not designed to be truly planning boats so that difference will not be as big but even so they can reach and maintain hull speed (and in some conditions sail over it) with much more ease and less wind than an old heavy full keel design. The difference in performance in light and medium winds and upwind sailing will be the points were a bigger difference in performance will be bigger.

Regards

Paulo
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Last edited by PCP; 04-14-2013 at 03:13 PM.
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Re: Modern Hull forms and Motion Comfort

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Originally Posted by PCP View Post
Well, they don't perform very well in first place. Put a lot of weight on them and the difference will not be much because they were already very heavy without the additional weight. A bit more will not make much difference: their performance was never good by modern standards and they were already heavy without any added weight.
Yes, very true.

Quote:
Bob Perry explained already that the loading capacity It's a function of WPA and not displacement.
I forgot I read that. I think I missed it because it's as if people have a hard time realising that modern design is more efficient.

Quote:
Regarding modern boats everybody knows that weight is detrimental to performance, specially the few cruising boats that are designed to plan. But even a modern cruising boat that can plan easily in minimum charge condition will sail much better with its Max load than an heavy old design, specially with light and medium winds.
I agree.

Quote:
The difference of speed between the old full keeler empty or loaded will be much less than the difference in speed on a planning hull while light or full loaded but even so the performance of the light modern boat will be far superior.
Hehe, and agree once again.

Quote:
Anyway most modern cruising boats are not designed to be truly planning boats so that difference will not be as big but even so they can reach and maintain hull speed (and in some conditions sail over it) with much more ease and less wind than an old heavy full keel design. The difference in performance in light and medium winds and upwind sailing will be the points were a bigger difference in performance will be bigger.

Regards

Paulo

Well put.
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  #160  
Old 04-14-2013
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Re: Modern Hull forms and Motion Comfort

Wolfer has it pretty much correct as long as we grant each other some degree of generalization.

The amount of weight it takes to sink ANY BOAT 1" has NOTHING to do with displacement. NOTHING. Say it three times.

"lbs. per inch immersion" is all about waterplane or the boat's "footprint" in the water.
Two 40'er of grossly different displs can have very similar water plane areas so they will have very similar lbs. per inch immersion.

No sailing vessel short of a Mac 26 or a radical trapeze dinghy is designed to be a " truly planing boat". If they were they would look like power boats that ARE designed to be truly planing boats. While many modern, high performance boats can plane when the conditions are right I have yet to see a cruising boat of any flavor do this. Just think about sailing upwind. The typical, even high performance ballsted boat can't even do hull speed up wind so the demands of displacement performance have to be addressed in the design if only for upwind and light air sailing. If you knew you would only be sailing downwind in 30 knots for the rest of your life you could probably get along fine with a true planing hull. But that is not the real world.

The fact is that it takes so much effort and skill to make a sailing vessel sustain a plane that this kind of performance is way beyond the crew of the average cruising vessel regardless of design. Maybe if you cruise with a crew of twelve 20 year old guys, preferably in their off season from rugby, you could do it. But that does not sound like my idea of cruising.
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Last edited by bobperry; 04-14-2013 at 04:22 PM.
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