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Old 10-08-2009, 18:33   #1
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Keeping Level Fore 'n Aft

Hi all;

This weekend, we filled up the tanks and went cruising for the first time since we finished the install of our arch with solar panels. The arch is aluminum, so weighs perhaps 40-50 lbs. The panels are maybe that again as well.

Our fuel (20gal) and water tanks (30gal) are directly underneath the cockpit sole, behind the engine. The lazerette has stuff in it, maybe another 100 lbs total, plus an eight gallon hot water heater. There is also a fridge compressor, maybe 25 lbs.

The boat is now at least 1 inch, maybe almost two inches lower at the transom than the bow. I have plans for some other stuff back there, but now I am a bit hesitant to do anything more. I want to add more water capacity, my first choice would be under the setees. I could also add it up in the bow underneath the v-berth, but I don't want a pitching boat!!

Her design weight is 10k lbs. Just how much weight on top of the "bare" boat with can I expect to put in the bow/stern, before the motion is affected enough to be noticed, or a problem? She has a 4200 lbs fin keel, if that helps.

Chris
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Old 10-08-2009, 22:05   #2
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Do you have the anchors and chain on board? Sounds like you ae going to have to plan for significant storage up front.

Dry stores, sails, tools and hardware come to mind.

Obviously it is ideal to mass the weight in the middle of the boat but that's just not always practical.
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Old 10-08-2009, 23:13   #3
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In basic static stability, you are dealing with calculations based on the given design characteristics of your boat

TPCN (Tones per Centimeter Immersion)
CG…Centre of gravity
PP… Pivotal Point

Rather than getting into all of that, a practical and easy test is to use some large garbage buckets, measure or weigh them full of water and play with Changing your trim within your present condition by moving them to different realistic positions and refilling.

This gives you the amount of weight and optimum position; you need to move weight forward down below to improve the trim.

Make a spreadsheet when you measure it with full crew and full tanks simulated on board and then when you are close to even trim, make a note of “bodily sinkage” (Your new draft)

On your size of boat, stuff like this can change the boat’s trim and performance very easily, so putting extra water up forward or shifting house batteries more forward will help, but do simple tests first with water jugs under the v-berth first, go sailing in some weather to get a feel of what all this added weight does to handling, before committing to a permanent solution.


You will have changed the “dynamic” stability and while it may feel a bit wetter, it is probably a compromise you will learn to accept.

Good luck!
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Old 11-08-2009, 08:49   #4
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One thing I did not mention - we had maybe 40 feet of our 100' 5/6 chain out when I was in the water to notice the change. That may have had a significant effect? perhaps things are not as bad as I thought. The idea of using some moveable ballast to check thinks out makes sense.

Has anyone used one of those Plastimo or Vetus triangular shaped bladder tanks? The Plastimo can be had for ~$100. I am wondering out loud if the triangular tanks are any different than the others.

Chris
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