Figuring Max. Pay Load for Long Range Passage - SailNet Community
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post #1 of 28 Old 09-30-2013 Thread Starter
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Figuring Max. Pay Load for Long Range Passage

The Boat in Question -- 1970 Islander 37 Motor Sailor

Boat Wt. = 14,000

Draft = 5' 9 "

Ballast = 7,000 < forgot exact no#.

Beam - 10' 11"


1. I did a forum search and found some payload data on a 30 foot sailboat. I read the 3 pages of forum advice. I did not get a good solution to ... figuring max. payload.

2. I have figured my max. load wt. @ 6,000 pounds, 3 tons to the boat (empty wt.). I included everything added onto the I-37. The boat would be completely empty, then load the following: water (200 gal.), diesel fuel (250 gal.), 5 tanks of LP gas, dink w/ 3 hp. mtr., diesel repair parts, laptop computer, books, 75 days of provisions for two people, 5 sails, extra rigging lines & repair parts, emergency gear w/ provisions, 1st aid kit, etc. . The calculated weight would be @ a Max. Wt. of -- 6,000 pounds. perhaps as low 5,000 pounds.

3. I read about the water line (immersion) and design weight (max.) by the boat builder. I'm use to loading aircraft and max. weight allowed & center of gravity (C. G.) range limits. I ref. aircraft C. G. to the water lines markers on ocean type cargo vessels.

I thought that any boat would have a -- unlaiden weight and max. load weight. Sea conditions would be a 2nd factor; a sailboat heeled over... would be a 3rd factor, storm conditions.. adding a 4th factor to the sailboat max. wt. payload for all factored conditions in consideration.

3. The purpose of all the items covered (total wt.) in item 2. is to provide for a long passage legs (est'd.} @ 5,400 miles, about 60 to 75 days at time. I read in the forum about the 30 footer, 900 lbs. = 1 inch of water line. The I-37 would have a similar value... so, I'll guess @ easy figuring of 1,000 lbs. per inch; 6,000 pounds would be (roughly) 2 inches x 6 = 12 inches. I think that 12 inches would be excessive. I'm guessing that my max. payload would be around 3,000 pounds and 6inches max. water line depth.

4. I threw some numbers out there.. guessing... please advise.. and Thank You.


Avery

Last edited by HighFly_27; 09-30-2013 at 10:04 AM. Reason: mistakes
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post #2 of 28 Old 09-30-2013
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Re: Figuring Max. Pay Load for Long Range Passage

If you check the website below you will get an immersion value of 1126 lb/in, so 6000 pounds is something less than 6".

Could also comment on the trip you are contemplating. Where are you finding a leg that is 5400 miles? Also you are being very pessimistic about your speed (72 to 90 mi/d). If you are in trade wind areas it would be hard to imagine averaging less than 100 miles, especially since you have lots of fuel.

Since you have a motor sailor it would be a good idea to have appropriate light air sails. Also do you have a watermaker? You are carrying a lot of water. Even consider collecting water from rainfall. Could be weight saving here.

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Back home on Lake Ontario after something over 36,000 nm circumnavigator. Not surprisingly there is a lot of stuff I want to get done on Ainia both cosmetically and functionally. Getting an early start so it will be ready to go for next summer (Lake Superior?).
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Re: Figuring Max. Pay Load for Long Range Passage

My Irwin's calculated pounds per inch immersion is 1430, but then it's already on the heavy side of 'medium' in displacement. I figure I can load on 3k pounds - after that I've lost safety and I'm walking on stuff.

12 inches is extra immersion is indeed excessive, I'll call 4 inches a maximum - and I've seen cruisers pass with 4-6 inches of dry bottom paint.
Trying to distribute 6k pounds evenly through the boat and actually securing stowing it is another issue entirely. You do not want the ends of the boat heavy, nor do you want anywhere near even a fraction of that weight above the water line.
Needless to say any all of the published/calculated safety factors for the Islander 37 are pretty much null and void when you load up like that.
You can save a lot of weight (but not money) by offloading the water and and installing a water maker that will run on solar.

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Re: Figuring Max. Pay Load for Long Range Passage

A Valiant 40 has a lbs. per inch immers of 1,350 lbs. so I think you would be closer to
1,200 lbs.

The exact amount of weight you can safely carry will depend on where that weight goes.
If you can keep it low I would say 4,800 lbs., sinking the boat 4" would be a reasonable estimate.
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post #5 of 28 Old 09-30-2013 Thread Starter
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Re: Figuring Max. Pay Load for Long Range Passage

Yes, I'm looking into a Water Maker and would lessen my Load to below 5,000 pounds. I need to fit add-in diesel tanks that are placed next to the existing fuel tank. Add-in tanks = 200 gal. of diesel @ 1,600 pounds.

I understand that I'm making less than 100 NM miles a day. I figured worst case situation and under sail or motor 20 hours, not 24 hour. I figured the days progress... lower than expected to have a extra 15 days or so of reserve.. to ensure that I was Not Out of Fuel, Water, or Provisions.

Avery
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Re: Figuring Max. Pay Load for Long Range Passage

200 gals of diesel?

I thought you said this was a sailboat.
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post #7 of 28 Old 09-30-2013
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Re: Figuring Max. Pay Load for Long Range Passage

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Originally Posted by bobperry View Post
200 gals of diesel?

I thought you said this was a sailboat.
You beat me to it, Bob - I'm scratching my head over that one, as well :-)

5,400 mile passages of 60-75 days duration? Not too many of those out there, unless he's thinking of doing a Reid Stowe sort of thing... Most sailing routes in the world of that length involve the Southern Ocean, certainly not much shortage of wind down there...

Maybe he's thinking of the Northwest Passage, perhaps?
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Re: Figuring Max. Pay Load for Long Range Passage

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Originally Posted by HighFly_27 View Post
Yes, I'm looking into a Water Maker and would lessen my Load to below 5,000 pounds. I need to fit add-in diesel tanks that are placed next to the existing fuel tank. Add-in tanks = 200 gal. of diesel @ 1,600 pounds.

I understand that I'm making less than 100 NM miles a day. I figured worst case situation and under sail or motor 20 hours, not 24 hour. I figured the days progress... lower than expected to have a extra 15 days or so of reserve.. to ensure that I was Not Out of Fuel, Water, or Provisions.

Avery
You are adding in another 200 gal of fuel? What will that make the total?

Some strange assumptions - why assume 20 hours rather than 24? The worst case situation does not happen for the entire trip so you do not need as much stuff as you are saying. You are taking a worst case situation, that is hard to imagine then adding on another 15 days. Add that to the 5400 mile assumption and it does not make much sense to me.

Back home on Lake Ontario after something over 36,000 nm circumnavigator. Not surprisingly there is a lot of stuff I want to get done on Ainia both cosmetically and functionally. Getting an early start so it will be ready to go for next summer (Lake Superior?).
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Re: Figuring Max. Pay Load for Long Range Passage

It's a I-37 Motor Sailor.... you guys were joking me but it ain't that fast.

I wanted to know my maximum weight (6000 pay load, with everything) and how to figure the weight and what immersion value rate I should use.. suggested @ 1 inch / 1200 lbs. I think 5000 pounds is close to the right figure; would the I-37 do 6 K and not be overloaded for most sailing conditions.

I don't think I need all that diesel fuel and the trouble of making up aux. floor tanks. However, if I did needed that much diesel fuel (250 Gal. total) then, could I haul it anyway ?!?

Diesel Usage Figure -- .8 gal. / hr., 5400 NM mile leg. , 5400 NM / 05 knots = 1080 hours / 20 hour day = 54 days @ 20 hours / day. With the worst winds... it should work out to 1/2 of 54 days = 27 days @ 20 hours / day or even less. If I bought the diesel at a low price... I'd already have it onboard and avoid higher $$ cost diesel and more unnecessary stopping along the way.

The water maker is almost a must buy item @ $ 3K or so... but what if it breaks.

My hull speed (top) is 7.2 K. I figured 5.0 to 5.5 knots as (low) average speed, particularly when sailing against the Trade winds. Again, I wanted to figure conservative and have a 15 day reserve.

Avery
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Re: Figuring Max. Pay Load for Long Range Passage

The 20 hours is based on Not Sailing on a Continuous (24 hrs.) Basis, the range may go up to 6000 NM but not what I'm looking for here in this post, it's (nearly) a Singled Handed situation.

** My I-37 MS question is geared toward boat loading of a max. weight of 6000 pounds. I'm working numbers (only) and trying to get the Immersion Rate for 1000 pounds of wt. / ____ X? on the water line in graduations & what is too much weight for the I-37 MS.
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