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Go Back   SailNet Community > On Board > Gear & Maintenance
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  #11  
Old 08-19-2008
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Imagination is funny!
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  #12  
Old 08-20-2008
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One thing that might be possible is that the added weight has induced a "harmonic"; possible that if you added more weight at the bow or stern your rotational moment of inertia would increase a bit more and the oscillation frequency would move away from that harmonic frequency (or you are not fully harmonic yet and it would then get worse).
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Old 08-20-2008
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Yes. My boat is almost exactly the same displacement and length. Any weight at either extreme ends has a very noticeable affect on "hobby horse" tenancies. My boat actually prefers weight forward versus weight aft, but either way she will hobby horse more. My suggestion is to move all the heavy non-location specific gear to as close to center as possible. I used my tool boxes, spares and canned goods to trim the boat. I also added a 10 gallon flexible water tank to a strategic position to help influence the boats trim and list. The smaller the boat the more critical trim is...
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Old 08-20-2008
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Quote:
I recently added a Lewmar windlass (Pro 1000H) and replaced the 20lb CQR with a 33lb Racnor. Is it possible that the additional weight at the bow has increased the boat's tendency to hobby horse? It seems that way. Pokey displaces 12.6k, so I figured this amount of additional weight up front would have a negligible impact. At this point, I still have the CQR in the anchor locker, so I've actually added a little over 50lbs.
Try moving the beer to the aft lazarette...
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Old 08-20-2008
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Surely the boat must have a strong inate tendency to hobby-horse if 50lbs makes a difference? That means that if you're sailing along and a crew-member goes to bed in the forepeak, the boat characteristics change?!?!

I have a 600 litre water tank under my V berth bunks. The hobby-horse characteristics hardly change at all from full to empty and that represents 600kgs or 1320 lbs or 40 Rocna 33lb anchors . The only way I notice any change is that the water line rises in the bow. OK my boat weighs in at 25000 lbs but still . . . . .

I support Cam in the belief that imagination is a wonderful thing.

Sorry, don't want to sound disparaging but if my boat was that sensitive I'd be seriously evaluating continued ownership.

Andre
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Old 08-20-2008
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The human body is a strange thing in that it senses vibrations or oscillations at some rates more than others. If you move items to the extreme ends of a boat; usually the frequency gets lower; not higher because the moment of inertia is higher. Think about it. If you stand on one foot with your arms at your side it's difficult; with your arms extended easier; with a 12' pole and practice you can walk a high wire. It's due to increased moment of inertia. If you move mass out to the ends on a boat the boat might "hobby horse" at a slower and more noticible speed; not that it was not bobbing before, just that you notice it more now. In an extreme case; a boat that carried the majority of its weight at the ends, pitch stability would be maximized.

I know it sounds counter to what we consider the cause of hobby horsing; but the basic principle of moment of inertia and pitch stability should apply here. Of course; a motorcycle pitch dynamic may still yet to be fully solved/understood so it's also quite possible that a boat hull is a non-linear engineering problem also...

I don't think 50-60 lbs should make much difference on a 12k pound boat; but it -may- induce a harmonic mode in either the boat (or the sailor ) Sounds like a job for the Mythbusters!
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Old 08-20-2008
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At this rate, every time someone (who weighs more than 60#) goes forward during our travels, the whole boat's performance is going to go to hell and we're going to start hobby-horsing. I think not. If the boat is that tender then there is something else afoot to be considered.
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Old 08-20-2008
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Thanks, all. The admiral definitely agrees that I have an active imagination, which is a good thing many times! And that's probably the case here. We only brought the anchor on board last Sunday and sailed so configured once. Not a large sample. Some one had mentioned the possibility of increased hobby horsing, so I was looking for it. Based on all of what you good folks have had to say, it seems extremely doubtful.

But to answer the various questions:

Tankage is as built. I replaced the two head doors with curtains, removing a bit of centrally located weight, and added two batteries aft (one on each side, but otherwise haven't done much to change weight distribution. She does have a sprit on which the anchor rests on its roller. I will probably move the ununsed CQR off the bow to a more centrally located locker. And I vow to move the beer aft more earnestly next time out!

And I'll focus on more tangible issues.

Thanks again!
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Old 08-20-2008
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Try putting that weight on the end of the bowsprit Andre
Obviously the height above the water line makes a difference.....and yes even on your boat if a crew person goes to bed in the forepeak the sailing characteristic will change Not much, but it will.

Quote:
Originally Posted by Omatako View Post
Surely the boat must have a strong inate tendency to hobby-horse if 50lbs makes a difference? That means that if you're sailing along and a crew-member goes to bed in the forepeak, the boat characteristics change?!?!

I have a 600 litre water tank under my V berth bunks. The hobby-horse characteristics hardly change at all from full to empty and that represents 600kgs or 1320 lbs or 40 Rocna 33lb anchors . The only way I notice any change is that the water line rises in the bow. OK my boat weighs in at 25000 lbs but still . . . . .

I support Cam in the belief that imagination is a wonderful thing.

Sorry, don't want to sound disparaging but if my boat was that sensitive I'd be seriously evaluating continued ownership.

Andre
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Old 08-20-2008
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And if after all of the weight adjustments and you are still not satisfied, then rig a steadying sail aft to weather-**** your vessel. This will normally settle her down to just a little horsing.
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