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  #21  
Old 11-23-2004
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I want to start by saying that I am finding this to be a great discussion, and in many ways I think that, at least to me, this represents what is the best of what a discussion group should be. I do think that my comments were not as defined as they probably should have been and that has to the very interesting kind of grappling for a definition that seems to be taking place here.

I would like to start with my comment about plumb bows having more reserve buoyancy than a raked bow. Let me try to explain this with an example. I am suggesting that we visualize two boats with the identical deck plans, and the identical static buoyancy forward of the point of maximum beam, but one boat has a plumb bow and the other a ''destroyer bow'' (by which I mean the straight raked stems found on most IOR era boats). Both bows hit an identical wave. On the boat with the plumb stem, the forefoot of the stem will be in contact with the rising portion of the wave and will begin to feel upward forces causing it to start to rise at a point several feet before the stem at the deck is in contact with wave. The stem of the destroyer bow will only feel the lift of the wave several feet later, somewhere around the point that the waterline contacts the wave and it is only at that point that the bow starts to lift.

In the case of the plumb bow, as the boat''s momentum carries it into the wave, the buoyant volume represented by the flare of the topsides between waterline and the deck would represent the reserve buoyancy and that volume is beginning to lift the bow several feet before the volume of the topsides on the destroyer bow would come in contact with the wave. It is that difference in location of the reserve volume that gives the plumb bow its increased reserve buoyancy.

What complicates this discussion are a number of other factors which I think Jack alluded to. Modern race boats often have not only plumb stems, but also have minimal flare to their topsides. When you have a plumb stem and a plumb topsides there is less reserve bouyancy than is ideal. When you further couple that with the very fine bows on the more extreme racing types, you end up with a bow that is very intolerant of weight forward. That said, old IOR boats were far less tolerant of weight forward than any other type of boat that I have ever sailed.

Which brings us to other pieces of the puzzle. If we go back to our original example, the plumb stem would tend to have a sharper entry angle. This means a much more gentle collision with waves because while the bow starts to feel the lift of the wave sooner, the actual upliftinf force is acting over a smaller surface area, so instead of slamming into each wave the plumb bow tends to start to slice into the wave. That feeling of slicing into the wave at first gives the feeling of ''submarining'' into the wave, but the deck is less likely to slice through the top of the wave because ultimately there is more reserve buoyancy to prevent that per the above. To some extent it is that softer impact that gives the newer plumb bowed boats a more comfortable ride.

Here is where I need to abandon the model described above because as has been noted, newer boats have their center of buoyancy further aft than was popular during the IOR. When combined with a plumb stem angle of rotation when passing through a wave is reduced. I know this sounds a bit counter intuitive if you think that a plumb bow has greater reserve buoyancy but I can explain it this way. I think that we can assume that all other things being equal, the bow of a boat with greater reserve buoyancy would rise further out of a wave than a boat with less reserve buoyancy. (This is somewhat mitigated in our discussion because the reduced impact of the finer stem imparts less impact energy to push the upward than the fuller bow of the destroyer bow.) So while the bow might rise higher, the angle of rotation would be measured to an axis of rotation that is further away and so the actual angle of rotation is reduced. And because of the greater damping of the longer waterline length (mentioned by Phil I think), there is less of a tendancy for the pitch to continue as the pitching energy is disipated.

Anyway, those are my thoughts.

Jeff
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  #22  
Old 11-23-2004
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Jeff''s description matches our experience. One thing we have noticed with our fine bowed new boat is how much drier the cockpit is sailing upwind. The entry slices into the water instead of hammering into it like older boats with shorter waterlines and a lot of topsides flare. Waves will slop onto the foredeck but run off before they get far aft of the shrouds and there is much less spray flying around. So not only is the motion gentle but you stay drier.

This is a complex issue though as mentioned. Weight forward makes a boat wet. I raced on a Hood 38 that was set up for serious cruising. They carried 300''+ feet of 3/8" chain in the anchor locker. That boat pulverized every wave it met and they would end up in the cockpit. In a chop though you could stand below without holding on, the motion was so easy. Getting back to the earlier point, a Hood 38 is a good example of a boat that was designed in the IOR era that exhibits few of the trademark characteristics such as pinched ends, distorted run and bumping at measurement points.
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  #23  
Old 11-24-2004
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Really great discussion. Another reason to come here more often.

Jeff, that was a terrific explanation, thank you. Very worthwhile and instructive. I can see now how a plumb bow boat can have reserve bouyancy.

That and Jon''s point lead me to conclude that the overall design of the boat also contributes to her bouyancy and dry ride. I would agree that in either case bow flare is important in giving either boat greater reserve bouyancy and be able to produce a drier ride. Also, as Jon points out, balancing the boat is cruicial.

I keep my boat well balanced and do not carry as much weight in the two bow lockers as the one Jon was on (do you recall the name?...we are a small community). I don''t find much pounding or get water in my cockpit. I do love the ride on this boat (another thread perhaps). I have heard from a couple who cruise a Passport 37 (you probably know them, very nice people) and report that they are sailing somewhat bow down due to carrying a tremendoud amount of ground tackle forward. Still, they love the ride they get with that boat and its performance.

I can certainly see the value in plumb bow boats now. Although...I have seen a few models that do not seem to have much flare forward and look to be wet. Clearly, I need to sail more of these boats...any offer? Will to trade stints at the helm.

Not to detract from the true IOR boat discussion (and apologize if this is a hijack), your points regarding that design are well taken. I think it useful to discuss "IOR Era" boat designs that were influenced by the IOR rule, but departed from it....as these boats tend (IMHO) to be the majority of well sized boats being sold at reasonable prices. Perhaps another useful discussion in this thread is the difference and contrast between true IOR rule boats and those that were competitive under the rule but departed from it.

All the above said, it still appears to me that boats with overhangs at least still have a very good amount of reserve bouyancy forward...I am not sure which design would have more...to me at least I am not certain we can state which has more. I can see the point about the plumb bow boat with adequate flare meeting the wave and being carried up. But...the same would be true of a boat with moderate overhangs and adequate flare (like my Hood and your Farr 11.6). As I think about the bow of these boats meeting a wave, I still see the overhang touching the wave face and providing life and or bouyancy as the bow entry at the static waterline approaches and then rides the wave face up on its bouyancy. I still see a very good if not high amount of reserve bouyancy in this design.

As for pounding when meeting the wave, I am convinced by your model of a finer entry cutting through and thus reducing the counter force of the elastic collision. That does make sense.

But still on the question of reserve bouyancy, I guess I still want to throw into the mix the fact that again, modern deep sea vessals such as warships have overhangs...I see this perhaps producing some pounding (having been there) but also greating reserve bouyancy on those boats and helping with a drier ride. Add to this that AC boats have overhangs and the overhang design of the bow of Alinghi was said to contribute to her speed.

Nice to chat with all of you.

My best

John
s/v Invictus
Hood 38
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Old 11-24-2004
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Hello John. The Hood 38 I''ve sailed and raced on was "Ghostrider". Last I heard the boat was in Southern California somewhere. Recall two complete sets of heavy ground tackle (all chain) in the bow. I was impressed how the boat could lug all that weight without being seriously bow down. They also had the full kit on the stern such as wind pilot, wind generator on a pole etc. so it probably helped balance things out. With lots of weight in the ends the boat would not rise easily to a sea but smash through. A lighter boat would have been stopped hitting waves that hard but a lighter boat with weight more amidships would slide over waves without banging into them.

I''ve raced a Saga 43 which has a fine entry but lots of flare to provide the reserve bouyancy you mention. Its nice having the deck space forward to work on but the flare seemed counter-productive in terms of keeping spray down. Intuitively you''d think flare forward (and overhangs) would make a boat drier. I''ve started thinking they make the foredeck drier but the cockpit wetter. Boats with fine entries, short overhangs and not much flare have wet foredecks but don''t generate a lot of spray to end up in the cockpit when sailing upwind.

As to AC boats having overhang, that''s all about rating rules. They measure the length just above the waterline. They are trying to get unmeasured sailing length. Look at "box rule" boats like the TP 52''s where there are no constraints other than length, width, beam, displacement etc. They all have virtually no overhangs.

Have a great Thanksgiving.
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  #25  
Old 11-25-2004
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JohnDrake

I would be honored and delighted have you sail with me on Synergy. I must say for the record that Synergy is 25 year old design and so does not have a plumb bow. Boats have really evolved a lot since she was designed. Synergy (a Farr 11.6 for those tuning in late) is a kind of missing link in the evolution towards the IMS typeform that has replaced the IOR typeform.

While she has a very fine bow for her era and her center of buoyancy is quite far aft, she also has a destroyer stem and a fair amount of flair. She has extremely low freeboard. Similar to the descriptions offered above, she does not collide with a chop but knifes through. When I first bought her and was sailing in a steep chop, I would find myself bracing for the impact with each wave only to be surprised at the gentleness. She does not go bow down when heeled as was typical of IOR era boats but when she is at high speed beating into a chop she will slice off the top of the wave and send it flowing aft. Slowing down a little stops that problem (dropping from 8 knots to somewhere in the mid-7''s).
Similar to the description above, while she takes some water over the deck, it does not make it aft of the shrouds. I think that she would be improved by slightly more freeboard, a more vertical stem and less buoyancy aft.

The following is a link to a picture of Synergy heeled over which pretty much shows how she sits on her lines at a compartatively high heel angle....

http://cruisersforum.com/photopost//showphoto.php?photo=441&password=&sort=7&thecat=99 8

The following is a link to another picture that shows her heeled from astern:
http://cruisersforum.com/photopost//showphoto.php?photo=438&password=&sort=1&thecat=50 0

So while she does not represent the best of modern design, she certainly reflects some of the better attributes of a modern thinking. I would be honored to sail with you when convenient. Please email me at jhhalpern@hotmail.com so that we can set something up.

Best wishes,
Jeff
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Old 11-29-2004
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Jeff

You are very kind and I have always appreciated your words and advice here and elsewhere. I would love to take you up on your offer and sail aboard that fine boat. I know I would enjoy the sail, company and conversation greatly.

I would like to offer you a place at the helm of Invictus as well. She is being hauled this week, so next season certainly. I think you would find she sails much the same, moderate, dry and sea kindly in most respects. As you know, I got her as much for her cabin as her hullform. I hope you can come by to see her sometime.

Let''s make it a point to get together. I will email directly. Are you sailing through the winter? I will be spending 3 weeks in FLA helping a friend double hand his HC 38 from StA to the Keys. Other than that, will be around and working on the boat.

Jon - you have some nice friends with some very nice boats. I knew about the box rule, it wasn''t that, it was something in a comment Gary Jobson made after the race that there was something about Alignhi''s bow that made her knife through the waves faster. Not sure what it was...

Best to all, hope all has a good holiday.

John
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Old 11-29-2004
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Hi John:

The knuckle bow exploits the way that rated length is taken at 200mm above the measurement waterline in the Americas Cup class. The knuckled stem profile is designed so that a rated length equal to that of a conventional bow provides a 200- to 300-mm longer effective sailing length. If you type "Alinghi knuckle bow" into Google you can learn more about this than you probably care to know. My point about the box rule is that when rating games such as described above are removed, plumb bows result.

Plumb bows have their own issues, like raising the anchor without banging it into the stem. The Saga we''ve sailed on solves this with a substantial bowsprit / anchor handling fabrication. Had a nice holiday weekend - spent it on the boat.


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