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Old 08-04-2008, 05:42   #1
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Rethinking Hobbyhorsing/Pitching

I'm having some weight distribution issues with my new boat.

Now, since we are talking hobbyhorsing or pitching, (bow up, stern down, etc...), the discussion is exactly the same as for a monohull, with the exception that I don't have that extra bit of weight at the fulcrum point of the pitching.

Here are the problems:

1) I have Yanmar YM30's in the engine rooms, with 6 Trojan batts even farther aft than those Yanmars and a whole lot of fuel tank back there as well. The boat sits stern-heavy before loaded with anything.

2) There is no weight at all in the bows. The shoot skyward pretty easily, and that's where the berths are. I was in a rough anchorage last night on purpose to see how the boat did. It pitched me up and down a lot while trying to sleep.

3) I have a little tad of moisture in the hull near the exhaust overboard discharges because they submerge when I'm underway at a good clip under power.

So, I have too much weight astern. But here is what I'm thinking.

There are, theoretically, two extremes to configure weight on the boat:

1) All weight in the middle of the hulls, including engines, water tanks, etc... This would make nearly 100% of the weight at the center (looking fore and aft). This would make it so the bows and sterns would adjust very quickly to the waves, having almost no moment of inertia around the fulcrum of the "see saw." I think this would make my comfort situaiton worse, but would make for increased seaworthiness, since you would have a hard time pitch-poling or stuffing a bow or burying the deck I have that runs to the ends of the bows.

2) All weight could be equally divided and stuffed in the bows and sterns. This would make it so the boat had its greatest moment of inertia around the balance point of the see saw. This would be like having two really fat kids on each end of the see saw. It would pitch/hobbyhorse *very* slowly compared to the first scenario. However, would this make me more apt to dig the bows in and run into trouble when it comes to seaworthiness?

This is where I'm stuck.

Which of the above is better?

I'd like the comfort of a huge moment of inertia, but fear the seaworthiness might go down.

Monos have an added moment of inertia to counteract pitching without any penalty in seaworthiness (as described in this thought experiment). They have the keel. It is *below* the boat, so its mass keeps the boat from pitching without putting weight in the ends (just like the weight of the mast does on both types of boats).

So... given my engines and gear in the aft are pretty heavy, how can I shuffle my personal stuff and tools so that I will have a comfortable ride and remain seaworthy?

Do I put weight in the bows to even the boat out, making up for the extra weight in the stern I have from the large engines, fuel and batteries? This would decrease the ptiching motion at anchor, and at sea but is that seaworthy?

Or... do I try to center all my weight in the hull, but move it just a little bit forward of center to coutneract the extra engine weight that is pulling my stern down too much? This would keep a high degree of pitching motion, but allow my bows to pop up quickly if stuffed.
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Old 08-04-2008, 06:35   #2
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As I believe you know, heavier ends make for a slower (low frequency) but more extreme amplitude motions (large pitch radius of gyration, or “gyradius”)
whereas lighter ends tend to create quicker (vertical acceleration), but lesser amplitude motions.
Likewise, lighter boats generally have a much quicker motion than heavier boats.

See:
Investigation into Wave Loads and Catamarans:
http://www.cmst.curtin.edu.au/publicat/99-23.pdf
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Old 08-04-2008, 06:50   #3
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Exaclty, Gord.

I am trying to determine which is best. Slow motions are more comfortable to me, while quicker motions are more seaworthy (I think). I am assuming they are more seaworthy since if some steep wall of water comes my way, my bows will not dig into it, but will instead rise up sharply.

This sharp rise is in direct opposition to my perception of comfort aboard.

BTW: Thanks for that PDF link. Looks good. I can't wait to sit down and read that tonight or tomorrow AM. Must weigh anchor and go at the moment....
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Old 08-04-2008, 06:54   #4
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The first rule of catamaran weight distribution is to keep weight off the ends of the boat. As Gord mentioned, lighter ends means a quicker motion which can be desirable. With the size of the cockpits we have, it's preferable to never be pooped.

In your situation, move the batteries forward in the engine compartments right up on the aft cabin bulkhead. Relocating 600 pounds of batteries sitting almost dead in the stern of the boat will make a big difference. This might be all you need do. You might also give some consideration in relocating tankage and fuel storage slightly forward as well.
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Old 08-04-2008, 08:03   #5
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Can you get that weight under the lounge that would usualy be at the mast BH?

Need some bigger Batt cables and extra fuel line, but that is where you want the weight IMHO

I have yet to see a production boat that wouldnt benifit from a step job as well.

more bouyancy aft, easier access on and off and extra waterline length, its all good.

To give you an idea Hull Extension , peice of piss. (-;

Dave
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Old 08-04-2008, 09:09   #6
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Thanks for the input.

As I suspected, all said moving weight to the center would be best. I'm assuming you are all talking about safety over comfort in saying that.

Point well taken.

I'm on a 1700 mile delivery. I'm not moving batteries right now.

But... moving my stuff around...

Can I take all the stuff that is currently centered (tools, food, etc...) and move it all (as a group of mass) foreward a few feet to offset the heavy engines and batts? That would mean a more balanced boat, with weight slightly off of the middle of the boat, not not totally in the ends.

I'm kind of looking for a "running fix" for the delivery. I can futz around witih the big stuff after the charter season is over
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Old 08-04-2008, 09:09   #7
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PS: The intracoastal is soooo boring!! Ha ha. I'm online while underway. Bad, I know, but YAWN.....
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Old 08-04-2008, 09:16   #8
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Move your stuff around, get a few(enough to make a difference) 5 gallon water jugs and play around with those, just experiment.

Or, when you charter, play around with your new "ballast"...

"EVERYONE TO THE FRONT OF THE BOAT!"
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Old 08-04-2008, 09:26   #9
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PS: The intracoastal is soooo boring!! Ha ha. I'm online while underway. Bad, I know, but YAWN.....
Just keep it on the wet part

Sean, You can move those battery boxes if you get motivated
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Old 08-04-2008, 10:52   #10
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Sean, You can move those battery boxes if you get motivated

Single hand a boat from the Keys to Maine and tell me if you would be motivated... ha ha.

I'm not sure you are following the amount of effort the delivery takes.
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Old 08-04-2008, 12:42   #11
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Single hand a boat from the Keys to Maine and tell me if you would be motivated... ha ha.

I'm not sure you are following the amount of effort the delivery takes.
You're right, I don't. But gimme your lat/long corordinates and I'll bring the tools.... block and tackle and the beer.....
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Old 08-04-2008, 12:51   #12
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You're right, I don't. But gimme your lat/long corordinates and I'll bring the tools.... block and tackle and the beer.....
I can't believe the offer!

You must *really* like working on boats. Next time I have a refit, I'm flying you up!!
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Old 08-04-2008, 14:20   #13
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I can't believe the offer!

You must *really* like working on boats. Next time I have a refit, I'm flying you up!!

Actually ... you're right..I love boat projects .. I'm in the process of creating blueprints on that last suggestion you made. Well, to be perfectly candid, my admiral is now insisting.

I think it's my second or third rule of boating...... never suggest boat improvements to the admiral.
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Old 08-04-2008, 15:06   #14
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PS: The intracoastal is soooo boring!! Ha ha. I'm online while underway. Bad, I know, but YAWN.....
You haven't seen boring yet, you're still in Florida with lots to look at. Wait til you get to GA.
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Old 08-04-2008, 15:36   #15
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You haven't seen boring yet, you're still in Florida with lots to look at. Wait til you get to GA.
I know! ha ha

Everyone says GA is the worst part because of the twisty, turning ICW there.

I was thinking of going outside once I'm in GA as much as possible. GA appears to be a series of snake-winding rivers, so my theory (until I can check this out on some charts) is that I can sail outside, and tuck into a river to anchor at night, then go back outside the next day, etc...
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