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  #1101  
Old 07-12-2013
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Re: Bob Perry's take on Wolfenzee's dream boat

Out:
We don't have "bads" here. It's always a good day when you learn something. I almost always learn something when I try to formulate answers to questions. I think chines work on high performance boats because they help keep the buttocks flat and push volume outboard where it provides stability gains. On lesser perormance boats chines can push volume aft and low eher it can be used for acomodations. i.e. quarter berth width.

I think narrow is always faster upwind or down. That's where a good part of the magic is in the big AC cats. The downside to narrow i a monohull is the form stability loss and this has to compensated for with deep draft and a low VCG.

A good indication for how much a boat is going to squat is the curvature of the rocker.
Sometimes this is called "buttock slope". Heavy boats have more rocker, high angle of buttock slope, for a given DWL. ULDB's have very little rocker, low angle of buttock slope and therefore very flat buttocks. They don't pull up a big stern wave.

If you were to se a Little Harbor model motoring alongside a similar DWL ULDB you would see a big stern wave on the Little Harbor and almost no stern wave on the ULDB.
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Re: Bob Perry's take on Wolfenzee's dream boat

Thanks Bob - Have read the usual books for neophytes on design and never fully understood this. Still unclear why a narrow boat would be faster down wind. Think I understand upwind but not downwind. Is it simply less wetted surface so less parasitic drag in combo with less frontal plane so less resistance.
Last weekend saw a motor launch at the Bristol museum. It was 40+ feet long and maybe 6feet maximum beam. It had fins like you would see on a 1950's car except horizontal just above the static waterline put all the way aft. Wish I took a picture to post. Looked real cool but wonder why they were there ( had nothing to do with the steering I could see). ?Early attempt to prevent squatting.
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Re: Bob Perry's take on Wolfenzee's dream boat

Out:
Forget upwind and downwind. Just imagine shoving a shape through the water.
If you fix LOA and let beam grow the boat will not only get beamier but heavier also.
Wetted surface increases, frontal area increases and displ increases.

When I draw a new boat one of the very first things I do is figure out just how narrow I can make the boat. Narrow boats behave nicely and have less assymetry to the waterlines when heeled.

If you look at any of the rating rules going back to the CCA you will find beam on the "slow" side of the equasion, beam and displ make a boat slow. Think of all the famous downwind flyers: Santa Cruz 70, RAGTIME, PYEWACKIT, Andrews 70 etc, they are all narrow boats. The only downwind flyer I can think of that wasn't narrow was WINDWARD PASSAGE and it's L/B was 3.8 and that's far from beamy.
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Re: Bob Perry's take on Wolfenzee's dream boat

To me this is really a trade off. Bob may correct me on this, but downwind its all about cutting drag by reducing wetted surface and wave generation. Stability does not matter much. Long light, narrow boats with rounded sections have comparatively low wetted surface (although they pay a wetted area price for their length) and make smaller waves. Its the same reasons catamarrans are fast.

But upwind, and reaching, stability becomes important to be able to stand up to enough sail area to go fast. To me, designs that sail well in a broad range of conditions and which are forgiving in changing conditions cannot be too narrow since its hard to get enough stability without adding weight (in ballast) and they cannot be too wide since on a boat that gets too wide, its hard to control wetted surface and wave making without doing the kind of ballancing acts that modern 'open class' style boats employ.

I think when it comes to cruising boats, that there needs to be some moderation when it comes to displacement as well. I also think that there is a L/D practical range in which speed can be accomplished while still having enough surplus carrying capacity to be useful as a cruising boat. My sense is that range is an L/D somewhere between 150 and 180 at least with our current technology. Designers are capable of designing cruisible boats with an L/D below 150 but these are somewhat like the space capsules. They either need to be very long to carry enough to supplies to be comfortable, or else one must be extremely disciplined about what the weight that is brought aboard. Race boats have naked looking interiors for a reason.

When I raced and cruised my Laser 28 (L/D somewhere around 125 if I remember right) I had 2 milk crates full of race gear and 4 milk crates full of cruising gear. When I went racing I would switch them out. I kept them in the trunk of my car which led my brother to ask upon seeing my trunk one day, "I know architects aren't paid very well but can't you afford a home?"

Jeff
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Re: Bob Perry's take on Wolfenzee's dream boat

The only thing I would add is your range of D/L's sooms a bit low if you consider offshore cruising boats.
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Re: Bob Perry's take on Wolfenzee's dream boat

Quote:
Originally Posted by bobperry View Post
Out:
Forget upwind and downwind. Just imagine shoving a shape through the water.
If you fix LOA and let beam grow the boat will not only get beamier but heavier also.
Wetted surface increases, frontal area increases and displ increases.

When I draw a new boat one of the very first things I do is figure out just how narrow I can make the boat. Narrow boats behave nicely and have less assymetry to the waterlines when heeled.

If you look at any of the rating rules going back to the CCA you will find beam on the "slow" side of the equasion, beam and displ make a boat slow. Think of all the famous downwind flyers: Santa Cruz 70, RAGTIME, PYEWACKIT, Andrews 70 etc, they are all narrow boats. The only downwind flyer I can think of that wasn't narrow was WINDWARD PASSAGE and it's L/B was 3.8 and that's far from beamy.
I hear ya there Bob. I got a tour of Rage a couple of years ago when she was tied up in Alemeda.Despite being 70' she seemed very simple and manageable. I would have loved to go out for a sail but no joy that day.
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Re: Bob Perry's take on Wolfenzee's dream boat

Bob/Jeff- Thanks for the explanation. It finally makes sense. Now I understand the very beamy round the world racers with smiling Frenchmen on them as being another critter. They are functioning as semi planning hulls nearly all the time so these rules don't apply. And they have nearly no weight to them so less boat in the water. I do understand why the sliver project should result in a very fast boat. What I don't understand is the current generation of French/German cruisers carrying their beam all the way aft and nothing in the last few years not having chines it seems. I would expect with more volume folks will throw more stuff under the berths aft. And in cruising mode they will rarely if ever be planning. Still, to my untrained eye it seems the evolution of "modern" cruising boats is to have beamy and beamier hulls.
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Re: Bob Perry's take on Wolfenzee's dream boat

Quote:
Originally Posted by bobperry View Post
The only thing I would add is your range of D/L's sooms a bit low if you consider offshore cruising boats.
Bob: I think that there are several things that I was considering when I wrote that. For better or worse, I was using the race boat convention of the big 'D', displacement in that calculation equalling the boat dead empty, while in reality I think that when designing offshore cruising boats the designers I have known use the half load, or partially loaded displacement when laying out the DWL. I think that in reality, most offshore cruising boats end up being 15-25% over the dead empty load, and in many cases with full tanks and lockers well over that.

I also perhaps was not as clear as I probably should have been when I wrote, "My sense is that range is an L/D somewhere between 150 and 180 at least [achieveable] with our current technology." My thinking is that kind of range is possible, but it requires careful engineering and a calculated use of exotic materials and design approaches such as vacuumed down matrixes, cored bulkheads and casework, deletion of liners, greater draft to allow less ballast for the same stability, maybe carbon spars and so on. My sense is that this level of weight attention is rarely practiced in offshore cruising boats, and frankly perhaps may not be appropriate in where high safety factors trump speed.

That said, my thinking was these kinds of L/D's would seem achievable. Outbound 46's published L/D is just outside that range.

Jeff
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Re: Bob Perry's take on Wolfenzee's dream boat

Out,

Some of the beamier cruise boats may be folks liking that look. OR thinking they they can do this same type of sailing per say with a retired couple sailing it across the ocean! When in reality, said retired couple probably can not even get said boat design to 1/2 or 3/4 throttle and handle the boat! So reality is, a slightly more moderate boat hull design would make sense. Beam not carried quite so far aft, chines can be a good or be in either sense in my eye/mind. I would probably go with some sort of chine if I were to design a boat for me.

Then I can see a DE being as fast or slow per say going down wind as a transom model, usings D/L's as Bob did, as boats that heavy, are not going to plane etc in either case. Even a local IOR war horse that still wins local races, the transom is soo far off the water, in reality it is a DE boat! And will not plane going down wind for very long, might surf a bit, as any boat can or should depending upon wave size etc. Comparing this boat at 40' to the Jeanneau 44ds at a recent YC even, the Jeanneau appears to have the ability, to sail faster, especially down wind with the correct amount of sail area etc vs shoot the moon!

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Re: Bob Perry's take on Wolfenzee's dream boat

Spot on Marty!
"in reality it is a DE boat!"
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