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Old 16-11-2019, 19:31   #1
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Weight and balance

Hey guys,

Got a question, I recently got a mystic 30 cutter, Iím planning on a repower that will likely save me 100lbs, adding a drop in (ish) fridge and adding a heat pump, thatíll put the 100lbs back on, new radar thatíll take out 20lbs, new nav/comm which will take out say 10lbs, moving aboard which will be like 350lbs or so of stuff plus a 40lb fur missile, might get into scuba so thatís like 100lbs or stuff under the rear cockpit in the engine room, which was were I was going to store a good bit of stuff as itís a large dead space.


Question is for a boat this size how much weight will yield a feel able or sees able change in the boat? Also where should the CG be on this boat?

Though she will be a liveaboard, sheíll be sailing a good amont, both distance stuff as well as day sailing too since she small enough that taking her out for a day sail requires little effort.

Thanks!!
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Old 16-11-2019, 20:56   #2
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Re: Weight and balance

don't forget diesel and water tanks full will probably double that weight.
but from experince in a mono hull as long as the boat is not massively overloaded it will make little difference in a displacement hull.
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Old 16-11-2019, 21:23   #3
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Re: Weight and balance

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don't forget diesel and water tanks full will probably double that weight.
but from experince in a mono hull as long as the boat is not massively overloaded it will make little difference in a displacement hull.
What about semi displacement?
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Old 16-11-2019, 21:34   #4
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Re: Weight and balance

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What about semi displacement?
less weight= more speed in semi displacement planing hulls

any weight beyond the designed payload will slow the boat down and possibly stop it from being able to exceed hull speed and reduce its ability to plane
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Old 16-11-2019, 22:22   #5
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Re: Weight and balance

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Originally Posted by atoll View Post
less weight= more speed in semi displacement planing hulls

any weight beyond the designed payload will slow the boat down and possibly stop it from being able to exceed hull speed and reduce its ability to plane


Any idea how Iíd find that number for the boat or the target CG location?
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Old 17-11-2019, 01:05   #6
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Re: Weight and balance

Our empty weight on a 31ft yacht is 4670kgs. From lifts on a crane I know that with full fuel and water tanks, dinghy and 2.3 outboard etc we weigh 5100kgs, so 4-500kgs of extra stuff. If the weight is spread evenly it hardly makes much difference but that can be quite hard to achieve.

This time last year we took the dinghy and engine home for the winter and the liferaft went in for a service. Result was really noticeable and even the wife said we seem to be more bows down than normal. Actually it was the stern up rather than bows down. Lying in bed you could feel the boat sloping forward. When we got home after a weekend away I went through the boat and removed anything we didn't need. Next I moved the dive kit from under the v berths back to the saloon berths and finally got rid of the last of the heavy duty shorepower cable. When we bought her 12 years ago she came with about 180m of HD cable . Most was old so ditched but one 50m length was kept. Coiling it up your arms really hurt. Replaced with a 25m normal extension lead, much lighter and easier to manage.

The hard bit is balancing the boat so not too much weight in the stern, after all you are going to be siting there at sea. Equally even little changes like which side the outboard is stored on makes a difference on a smaller yacht. We tried the dinghy on the stern which makes launching and retrieving easy, but it's better folded away just forward of the mast even if it takes 10 minutes to pump up and launch. This helps to offset the A frame and solar panel on the stern, again much lighter than some you see on larger yachts to minimise the impact.

I think you are spot on to remove anything no longer working or not necessary, there will be lots of stuff you do need if it's to be your home. Keep an eye on food storage, that adds up really quickly. Also batteries need thinking about, not to many or too few, just enough to meet your needs.

If you had 50ft that probably weighs 15 tonnes then you can have lots of stuff, big dinghy and large outboard, bicyles etc. However, in our size of boat then you need to compromise. You can still have the toys just small and fewer. Also it will take time as your ideas change but it's an interesting on going project.

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Old 17-11-2019, 08:53   #7
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Re: Weight and balance

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Originally Posted by Pete7 View Post
Our empty weight on a 31ft yacht is 4670kgs. From lifts on a crane I know that with full fuel and water tanks, dinghy and 2.3 outboard etc we weigh 5100kgs, so 4-500kgs of extra stuff. If the weight is spread evenly it hardly makes much difference but that can be quite hard to achieve.

This time last year we took the dinghy and engine home for the winter and the liferaft went in for a service. Result was really noticeable and even the wife said we seem to be more bows down than normal. Actually it was the stern up rather than bows down. Lying in bed you could feel the boat sloping forward. When we got home after a weekend away I went through the boat and removed anything we didn't need. Next I moved the dive kit from under the v berths back to the saloon berths and finally got rid of the last of the heavy duty shorepower cable. When we bought her 12 years ago she came with about 180m of HD cable . Most was old so ditched but one 50m length was kept. Coiling it up your arms really hurt. Replaced with a 25m normal extension lead, much lighter and easier to manage.

The hard bit is balancing the boat so not too much weight in the stern, after all you are going to be siting there at sea. Equally even little changes like which side the outboard is stored on makes a difference on a smaller yacht. We tried the dinghy on the stern which makes launching and retrieving easy, but it's better folded away just forward of the mast even if it takes 10 minutes to pump up and launch. This helps to offset the A frame and solar panel on the stern, again much lighter than some you see on larger yachts to minimise the impact.

I think you are spot on to remove anything no longer working or not necessary, there will be lots of stuff you do need if it's to be your home. Keep an eye on food storage, that adds up really quickly. Also batteries need thinking about, not to many or too few, just enough to meet your needs.

If you had 50ft that probably weighs 15 tonnes then you can have lots of stuff, big dinghy and large outboard, bicyles etc. However, in our size of boat then you need to compromise. You can still have the toys just small and fewer. Also it will take time as your ideas change but it's an interesting on going project.

Pete

So the 500kg made a notable difference eh?

Dingy wise, after looking into the fast roller zodiac with a 9.9 and others, I thinking the porta bote with a 3.5 merc, and outside from going distances I’d probably leave it at the dock or at work.

Is there any good rule of thumb for max gross weight and CG range for semi displacement, or ideally where I’d look it up for my boat? I know the arm x weight = moment stuff and would like to make sure I’m not too far aft of CG, seems my bow is possibly sitting a little high, and the boat does seem to list 1degree to starboard, but I don’t have any water in the tank which is in the bow right now ( bladder thank).

https://sailboatdata.com/sailboat/mystic-30
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Old 17-11-2019, 14:33   #8
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Re: Weight and balance

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Originally Posted by NorthernMac View Post
So the 500kg made a notable difference eh?
500kgs didn't make any noticeable difference until we removed some of it (dinghy and liferaft) then it showed up as we changed the balance of the boat.

I don't think you can work it out mathematically because it will always be changing. For example I can tell when the water tank is nearly empty because 130kgs missing from the starboard side means we list probably 5 deg to port. Strangely an empty diesel tank on the port side has less influence. Haven't really worked out why possibly because the tank is across the port side of the boat not length ways and at 90L that's enough to have less influence. Also people will move around the boat. A 100kg (220lbs) person moving from one side to the other will alter the boat instantly. Finally once you are underway it changes as the waves roll underneath you.

Good idea on the smaller dinghy, much easier to man handle at the expense of less carrying capacity.

If the boat is in the water and it is flat calm you could drop a tape measure from the bow to the waterline and the same at the stern. Record the measurements and as you add stuff like water note the difference. If you do it from the dinghy it won't be affected by your weight moving bow to stern.

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Old 17-11-2019, 15:18   #9
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Re: Weight and balance

Quote:
Originally Posted by NorthernMac View Post
Any idea how Iíd find that number for the boat or the target CG location?
mystic 30,there is a pdf you can download as well

https://sailboatdata.com/sailboat/mystic-30
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Old 17-11-2019, 15:21   #10
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Re: Weight and balance

I can only let you know how weight goes on my 29' and it might be helpful. Your boat is considerably beamier, but I don't think it is at all a planing hull. Ideally the weight is as central and low as possible and under the mast on just about all displacement hulls. In my case the boat came with 3120# of ballast (similar to yours) and later they added 1000# apparently to stiffen it up for better windward performance and make it feel less tender. I have the 3120# of lead but I can tell if my water tank is full or not by its tenderness amazingly enough. Specific to your beamier boat I cannot say for sure at what point you'll start to notice the weight, but it sounds like what you have proposed is still far from being too detrimental to performance.
Your bow looks fine and 1 degree list is not really a list in my book.
BTW, I haven't seen one in person, but from the pics online, it looks like a really nice little boat!
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Old 17-11-2019, 15:27   #11
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Re: Weight and balance

Quote:
Originally Posted by NorthernMac View Post
Hey guys,

Got a question, I recently got a mystic 30 cutter, Iím planning on a repower that will likely save me 100lbs, adding a drop in (ish) fridge and adding a heat pump, thatíll put the 100lbs back on, new radar thatíll take out 20lbs, new nav/comm which will take out say 10lbs, moving aboard which will be like 350lbs or so of stuff plus a 40lb fur missile, might get into scuba so thatís like 100lbs or stuff under the rear cockpit in the engine room, which was were I was going to store a good bit of stuff as itís a large dead space.


Question is for a boat this size how much weight will yield a feel able or sees able change in the boat? Also where should the CG be on this boat?

Though she will be a liveaboard, sheíll be sailing a good amont, both distance stuff as well as day sailing too since she small enough that taking her out for a day sail requires little effort.

Thanks!!
I raced a Heritage One Ton, 37 feet, #15000 I recall. Extra weight was critical for racing but in reality, we are talking fractions of a knot. Keep weight central and low if possible. As others have said, donít massively over weight the thing. At 30 feet, weight matters. We kept a large dock box and removed stuff not needed on race days.
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Old 17-11-2019, 15:45   #12
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Re: Weight and balance

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Originally Posted by Don C L View Post
I can only let you know how weight goes on my 29' and it might be helpful. Your boat is considerably beamier, but I don't think it is at all a planing hull. Ideally the weight is as central and low as possible and under the mast on just about all displacement hulls. In my case the boat came with 3120# of ballast (similar to yours) and later they added 1000# apparently to stiffen it up for better windward performance and make it feel less tender. I have the 3120# of lead but I can tell if my water tank is full or not by its tenderness amazingly enough. Specific to your beamier boat I cannot say for sure at what point you'll start to notice the weight, but it sounds like what you have proposed is still far from being too detrimental to performance.
Your bow looks fine and 1 degree list is not really a list in my book.
BTW, I haven't seen one in person, but from the pics online, it looks like a really nice little boat!
Thanks!

Honestly I doubt all of my stuff and upgrades to the boat will be close to 1,000lbs, so thatís good, Iíll just try to keep the weight close to forward of the mast, think I can get the AC up there as well as getting a fridge with a remote compressor, forgot the name of that one though, and maybe put its compressor forward as well.

Hull wise I thought she was a displacement hull too, however I think her projected hull speed was shy of 7kts, and Iíve had her on GPS north of 7kts without the staysail, with not much of a current showing as a factor in forward push, ether way Ive been pleasantly surprised with her performance and just donít want to mess that up with the pretty stuff, electronics upgrades and creature comforts like a heat pump and drop in fridge.
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Old 17-11-2019, 16:59   #13
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Re: Weight and balance

Hereís a better view of the hull
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Old 17-11-2019, 20:23   #14
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Re: Weight and balance

Yeah my hull speed is supposed to be around 6 but I get 7 pretty easily outa mine too, on a reach anyway.
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Old 17-11-2019, 23:39   #15
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Re: Weight and balance

The CG should be above the keel centre.
You will like it to be as low as possible.
So - install everything as low, as clise to boat centerline and as close to the middle of the boat.
The above is usually very hard to achieve.
Anyway, I believe that you are not racing, so the ideal weight distribution is important but not a must.
Think about four crew members moving around on the deck - how their weight distribution is constantly changing.
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