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Old 05-12-2017, 21:00   #1
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How to calculate load capability?

I'm trying to figure out how to calculate how much gear I can safely pack in my boat. I've found a couple formulas but it seems there's a lot of debate over it.

My boat is a Grampian 23'. The owners manual only gives me the displacement of 3200lbs and the ballast of 1150lbs. There aren't any exact specifications on the boat other than she is 23.3' with a 3.1' keel.

I need to pack myself and then know what I can get away with. I'm not overly concerned about speed as I'm not racing however feel free to give whatever advice. I know speed can reduce need of resources but I'm mostly looking at safety. I don't want to overload too much and run into problems.
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Old 06-12-2017, 09:48   #2
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Re: How to calculate load capability?

Is this your boat? Do you have the swing keel or the fixed?

GRAMPIAN¬* 23
GRAMPIAN 23 sailboat specifications and details on sailboatdata.com

The quick and dirty answer is: not much. The sales brochure mentions a crew of 5, I'm guessing local trailered daysailing and beach cruising. IIrc you want to do the Great Lakes solo?
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Old 06-12-2017, 10:53   #3
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Re: How to calculate load capability?

Quote:
Originally Posted by Tidjian View Post
I'm trying to figure out how to calculate how much gear I can safely pack in my boat. I've found a couple formulas but it seems there's a lot of debate over it.
Because a formula that works for a boat with a deep, full keel -- a Dana 24, for example -- won't work for a boat with a relatively flat bottom and a fin keel sticking down -- like yours.

The bottom line, as already mentioned, is "not much." You have no bilge area, low down in the boat, where you can put heavy items. So, basically, everything you put into the boat is going to tend to raise the center of gravity. For that reason, instead of thinking "what is the most that I can put into this boat?" what you should be thinking is "what is the least that I can take with me, and still get by?"
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Old 06-12-2017, 17:25   #4
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Re: How to calculate load capability?

Thank you for the link to that information!

I have a fixed keel on my model. I know it is not an ode vessel for long cruising however it's what I have for the time being. I'm not sure selling and investing in another model is an option right now. More importantly is transportation of the boat from marina to home or wherever. I don't feel like upgrading to a diesel truck at the moment so I can lug a larger boat around.

You're right the selling brochure did say a crew of 5. Probably accurate in optimal weather for a day or weekend trip on the lakes. For this voyage I will be traveling alone and only looking for a safe estimation so I know what gear can be packed.

Right now she has no added weight or customizations. I am looking to outfit a mini fridge into her galley just so I can have some cold food storage. Otherwise mostly everything will be dried, canned and selected for light weight. Minor electrical gear such as laptop, nav, sat phone, propane hibachi, flex solar panels etc. Nothing super heavy. The only other relatively heavy gear would be her batteries and obviously water storage tank and possibly a ditch raft.

There is some storage in the boat that is under benches and what not like any boat. I get what you're saying about not having deep storage to add weight in the keel region. But I would obviously try to store the heaviest stuff in the lowest possible storage compartments.
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Old 06-12-2017, 18:25   #5
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Re: How to calculate load capability?

This is a difficult question to give an easy general answer to.

The best 'rule' I can point to for you is "A cockpit sole at least 2% LWL above the waterline". For you that would indicate the cockpit sole should be 5" above the waterline as the most you should sink the boat (I believe your lwl is 20' 11").

If you have cockpit scuppers, you can measure this by just sticking a tape measure or wire down one and seeing how far the waterline is below the cockpit sole. If you don't you have to measure on the outside of the hull and translate it to where the cockpit sole it. Neither is all that difficult.

According to this calculator , your lbs/in immersion is 600 - eg every 600lbs you add, the boat sinks an inch. (note: that sounds high to me, just off the cuff - might be worth checking the data/calculation there).

With those two things you can calculate a maximum loading.

The other way to look at this would be calculating a minimum stability - but that would be rather more complicated.
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Old 06-12-2017, 18:42   #6
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Re: How to calculate load capability?

Evans

Thanks for that rule of thumb. For our big boat it works out to about 9”. I’ll have to measure what our actual distance is.

BUT that brings to my mind center cockpit boats. Our boat is pretty low compared to most, I see a few center cockpit boats where the cockpit appears quite elevated. Would that not skew your rule?

Not that I want to pack more stuff, we hve half a Home Depot as it is. Sometimes I need to work for sitting space.
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Old 06-12-2017, 19:00   #7
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Re: How to calculate load capability?

^^ the rule is from the racing safety regulations.

It is paired with stability minimums in those regulations, so while a higher center cockpit would (probably) get 'extra credit' here, it would (probably) be knocked on stability if it is too tall.

It brings to mind a 24' British boat that was crossing the pacific the same time as our first time (with a very fun young couple on board). Their boat was so loaded, their cockpit sole was actually below waterline, and they had to put big corks in their scuppers to prevent water coming up. That was not super safe, if they had ever gotten into really bad weather . . . but they did in fact make it across the pacific . . . . so, as always, a good seaman can (quite often) get away with some pretty dumb stuff.

It is a sensible rule, but certainly not hard and fast.
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Old 06-12-2017, 20:43   #8
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Re: How to calculate load capability?

Thanks again for that help.

So if I'm doing this right I'm calculating the beam which is 8' by the lwl which is 20.93 by the maximum draft which is 5.33?

That gives me a displacement of 4760lbs in salt water. The boats bare displacement is 3200. So if I did this right that should say i can get away with about 1500lbs give or take? Obviously I'm part of that weight.

That's also figuring by your 600lbs per inch since o have a minimum draft of 2.36'. 1560 vs 1800 not far off.

So myself and water being about 166lbs @ 20 gallons. I think I'll be all right. That leaves me a little over 1000lbs to work with. I'll obviously have to invest in a desalination system to replenish water since 20 gallons won't get me across the Pacific.
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Old 07-12-2017, 07:46   #9
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Re: How to calculate load capability?

Heaviest gear I think I'll have besides water and food will be a sea anchor and ditch raft. Everything else as I mentioned will be relatively light. Miscelanious tools, electronics, clothing, etc. I'll try not to load her to the max if I don't have to.

Calculations are telling me 42 days at 6 knots but that's obviously counting on perfect conditions. I estimate 8 weeks myself. Mostly because there will be stills, possible squals (I pray to Poseidon there aren't) and I'll be reefing at night for safety. I also need to figure on possible course drift since I'll be sailing solo with no auto pilot. I'm guessing even tieing off the rudder may still result in current drift when I'm not at the helm. I wish I had a fancier more expensive boat with AP but we work with the tools we are given. Any additional advice is well appreciated!
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Old 07-12-2017, 09:11   #10
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Re: How to calculate load capability?

Tidjian,

I’m reminded monthly, by the credit card bill, that those little numbers add up fast. Good luck on your journey.

Evans,

Thanks for the entertaining story and background. I think I’ve still got a couple of tons to go.


Another consideration is your engine exhaust riser. If you are lowering the boat too much you MAY be lowering your cooling loop to the point where it could get some siphoning. Or your toilet. Just think through ALL the ramifications.
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Old 07-12-2017, 15:28   #11
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Re: How to calculate load capability?

Quote:
Originally Posted by Tidjian View Post
Heaviest gear I think I'll have besides water and food will be a sea anchor and ditch raft. Everything else as I mentioned will be relatively light. Miscelanious tools, electronics, clothing, etc. I'll try not to load her to the max if I don't have to.

Calculations are telling me 42 days at 6 knots but that's obviously counting on perfect conditions. I estimate 8 weeks myself. Mostly because there will be stills, possible squals (I pray to Poseidon there aren't) and I'll be reefing at night for safety. I also need to figure on possible course drift since I'll be sailing solo with no auto pilot. I'm guessing even tieing off the rudder may still result in current drift when I'm not at the helm. I wish I had a fancier more expensive boat with AP but we work with the tools we are given. Any additional advice is well appreciated!
Once you get to the other side you will probably need to drop an anchor. You'll find the weight of the anchor and chain (+ spare) will cut into a lot of your weight allowance. Getting to shore may require an inflatable kayak or dinghy. Spare fuel for stove and engine. You'll probably be averaging less than 6 kts, 4 1/2 would be closer. Spare parts for your engine? Spare sails?
There will be plenty of hours of boredom, you'll need things to keep you occupied; fishing gear, guitar, books, etc.
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Old 07-12-2017, 21:47   #12
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Re: How to calculate load capability?

While your calculated hull speed is about 6 knots, your average speed will be much less. I kind of doubt you're going to be able to count on 100 n. mi. days.

What I am basing this on is a trip from SF to HI in a Yankee 30 with a 25 ft. waterline. It is approximately 2045 n. mi., gt circle route. Our off wind leg, we averaged 127.8, which included one day being chased by a tropical storm, where the days' run was 160 n. m., under storm jib alone. The trip back to SF, on the wind, took 21 days, for a leg average of 97.4. So, for the whole trip, the boat averaged 112.6. You'll note that the 160 mi. day is an average of 6.6, compared to a calculated hull speed of 6.7. It was the fastest that boat ever went, she surfed. That makes the average 4.7 for the whole journey.

Now, there are two kickers to consider. The first is that the navigation was done by sextant, plotted using noon fixes and three star fixes, and plotted on paper charts. Such fixes do introduce some suspicion of the true length of each days run, it's close, ball park, but not exact. It may be that we actually sailed more miles through the water than my quick calculations reflect, which would improve our average daily runs somewhat. But for planning purposes, a conservative shot at it would be to take that percentage difference between the waterline length hypotheses of hull speed to true speed, and apply it to your boat, then see what those days runs are more likely to be ON AVERAGE. Until you know that, you will not know how much water you're going to need (and you can get by on 1/2 gal per day, if you do all washing in salt water), and calculate the food provisions based on the number of days for the passage, plus 50% more. You need the same safety margin for your water--it's even more important to your body.

It is the feel for the above realities that are making some people suggest that you may not be able to rtw in this boat, plus the questions of its sturdiness and righting moment.

Even the wind vane adds weight. And it would be a good solution to the a/p issue. Jim and I have done on the order of 40,000 miles with the one he built steering. Not the Yankee 30, but a Standfast 36. As long as you pay attention, the course average is fine.

Ann
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Old 10-12-2017, 02:31   #13
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Re: How to calculate load capability?

6 knots average is never going to happen. On my previous 25' foot boat, we would passage plan at 4.5 knots and on my current 27 footer, we passage plan at 5 knots.
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