# How to calculate hull draft?



## Guest (Jul 24, 2018)

Google Archimedes principle, there are some good reads out there on how to use it. I just typed up a how to but after proof reading confused myself lol. I know how to use it just not how to put it in words lol!


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## Guest (Jul 24, 2018)

If you can’t come up with it, let me know and I’ll type something up that isn’t too confusing!


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## jmrodandgun (Sep 20, 2013)




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## Gatorgrizz27 (Apr 4, 2015)

They are correct in that weight of loaded hull = weight of volume of water displaced. Without getting into calculus and other advanced maths, it’s going to be an estimate and assume your hull also floats level.

You’ll need to break the hull into simple geometric shapes to calculate surface area, then multiply by the draft to calculate volume. For example, a hull that is 16’ long at the waterline and 48” wide at the chines with a V-nose for the first 4’ could be broken down into a 4’x4’ triangle and a 12’x4’ rectangle. You can solve for the inches of draft if you know the exact weight, but I find it easier to solve for the weight each inch of draft will support.

It should be obvious, but only the dimensions underwater count.

48”x48”x0.5 (formula for triangle) x6” draft = 6,912 cubic inches of water for the bow section
48”x144”x6” draft = 41,472 ci for the rest of the hull

48,384 ci divided by 1,728 (conversion for cubic inches to cubic feet) = 28.

28 x 7.48 (conversion for cubic feet to gallons) = 209.44 gallons of water displaced

209.44 x 8.56 (weight of a gallon of salt water) = 1,793 lbs at a 6” draft. You can then set this up against your known actual weight and cross multiply for your actual draft.

You can also skip the last 2 volume conversion steps given that saltwater weighs 64 lbs per cubic foot, 28x64=1,792 lbs, but gallons helps to show what’s actually happening.

If the hull isn’t a flat bottom, you can calculate the volume of the V portion using basic trig if you only know the deadrise angle, or if you have the actual dimensions just use the triangular area formula plus the amount of cHines submerged as a rectangle. Basically doing the same thing as the first steps except viewing the hull from the transom instead of the top.

It’s more complex than that if you figure in side flare, etc, but I prefer to do simple math and look for the worst case scenario, as people typically overload boats compared to what they think they will weigh.


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## No Bait / Lures Only (Apr 10, 2011)

Gatorgrizz27 said:


> They are correct in that weight of loaded hull = weight of volume of water displaced. Without getting into calculus and other advanced maths, it’s going to be an estimate and assume your hull also floats level.
> 
> You’ll need to break the hull into simple geometric shapes to calculate surface area, then multiply by the draft to calculate volume. For example, a hull that is 16’ long at the waterline and 48” wide at the chines with a V-nose for the first 4’ could be broken down into a 4’x4’ triangle and a 12’x4’ rectangle. You can solve for the inches of draft if you know the exact weight, but I find it easier to solve for the weight each inch of draft will support.
> 
> ...


I can calculate the volume n weight of hull, will post n let's see


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## No Bait / Lures Only (Apr 10, 2011)

Hull bottom dimensions are a flat bottom 5'wide × 18'long 77"wide @ gunnels 
1500# boat, gas, battery, misc.
Bow rake is like a flat boat


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## No Bait / Lures Only (Apr 10, 2011)

www.blueheronwings.com. Gavin Atkins software boat design
Software works well, fill in weight, length, width n hit calculate n Walla!


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## hoanghuy553 (Jul 24, 2018)

Is this still works ?


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## Pole Position (Apr 3, 2016)

I suppose you could do like 99% of the boat manufacturers---do the calculations and subtract 3".


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## State fish rob (Jan 25, 2017)

Whole lot of numbers flying around. Put it in the water and see what it takes to float it. Keep the math out of it. Draft on my boats change regularly with load


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## permitchaser (Aug 26, 2013)

Mine floats, its a "97 so thats a plus and thats all I care about


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## Guest (Jul 26, 2018)

State fish rob said:


> Whole lot of numbers flying around. Put it in the water and see what it takes to float it. Keep the math out of it. Draft on my boats change regularly with load


The point is to calculate pre build. He doesn’t have the boat to put in the water yet. The calculation for this is pretty accurate provided the estimated weight is accurate.


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## Guest (Jul 26, 2018)

With a little cypherin, I’m gonna say 5”-6” depending on load and load balance. That will obviously change with the motor hanging on the transom! Let us know!


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## firecat1981 (Nov 27, 2007)

This is how I estimated my last boat, and it was pretty close.


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## Gatorgrizz27 (Apr 4, 2015)

firecat1981 said:


> View attachment 34884
> 
> 
> This is how I estimated my last boat, and it was pretty close.


Yep. That’s a great sketch of the pile of numbers I attempted to explain


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## Vertigo (Jun 3, 2012)

Calculating draft is only as accurate as the data used in the calculations. Does the OP know the all up weight to within a pound or two? Probably not. How about center of flotation? No? How about hull shape and how displacement changes as immersed? 

Without a complete identical hull that can be tested in the real world, calculations are not much better than an educated guess. The best answer can probably be had by asking the builder what his previous hulls have drafted.


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## Guest (Jul 27, 2018)

Vertigo said:


> Calculating draft is only as accurate as the data used in the calculations. Does the OP know the all up weight to within a pound or two? Probably not. How about center of flotation? No? How about hull shape and how displacement changes as immersed?
> 
> Without a complete identical hull that can be tested in the real world, calculations are not much better than an educated guess. The best answer can probably be had by asking the builder what his previous hulls have drafted.


While I agree with you, the op did say it’s a flat bottom hull. I’ll bet ya a beer my estimate is within an inch provided op gave good numbers lol!


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## No Bait / Lures Only (Apr 10, 2011)

Will post the legit depth upon hull arrival.


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## State fish rob (Jan 25, 2017)

Only way to know for sure. Keep the math out of it


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## permitchaser (Aug 26, 2013)

My boat is 181/2' and 90" beam. For my calculations its fat and needs to go on a diet. But it floats in 8 -9 or 10" I'm not good at math


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## No Bait / Lures Only (Apr 10, 2011)

permitchaser said:


> My boat is 181/2' and 90" beam. For my calculations its fat and needs to go on a diet. But it floats in 8 -9 or 10" I'm not good at math


Need to add the weights for miscellaneous n engine n gas n battery then use the calculatior in above post


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## firecat1981 (Nov 27, 2007)

For the calculations to work you need to know the width at the chines, not gunnels, and where the boat tapers. Can you post a pic?


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## permitchaser (Aug 26, 2013)

firecat1981 said:


> For the calculations to work you need to know the width at the chines, not gunnels, and where the boat tapers. Can you post a pic?


go to my post " On My Way" in General and shes there


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## Pole Position (Apr 3, 2016)

This thread reminds me of the annual guesstimate of the highly educated meteorologists who proclaim there will be "X" amount of named storms each year. Hell----w/ far less expertise, I know I can come just as close as those PHDs. That being said, we will have 16 named storms this year. And your actual draft will be @ 10" ( give or take an inch ; )

Oh, btw, by simply joining this forum you automatically lose 3" of draft, become capable of getting on plane in spit, and stay dry going thru 6' chop.


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## firecat1981 (Nov 27, 2007)

Pole, I think that's fairly insulting to those of us who can actually do the math given the proper parameters. There's no real guess work here, mother nature doesn't play a role. The calculations are straight foward.


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## firecat1981 (Nov 27, 2007)

permitchaser said:


> go to my post " On My Way" in General and shes there


What does your boat have to do with No Bait's? He said his was a flat bottomed skiff


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## Guest (Jul 29, 2018)

firecat1981 said:


> Pole, I think that's fairly insulting to those of us who can actually do the math given the proper parameters. There's no real guess work here, mother nature doesn't play a role. The calculations are straight foward.


And, they have worked for many builders in the past! Also, like mentioned... the calculations are only as good as the parameters used in the calculation, but it is pretty darn accurate!


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## permitchaser (Aug 26, 2013)

permitchaser said:


> go to my post " On My Way" in General and shes there


sorry i was wrong its in Bragging


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## DuckNut (Apr 3, 2009)

No Bait / Lures Only said:


> Need information to calculate draft for new hull having built?


The builder does not have this nor able to calculate it for you?

Isn't this something you would like to have known before purchasing?


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## No Bait / Lures Only (Apr 10, 2011)

Custom built aluminum plate flat boat with a flat skiff deck in South Louisiana, not a normal build for the builder, following my design.


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## firecat1981 (Nov 27, 2007)

Is he already building it, or are you in the design phase still?


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## Pole Position (Apr 3, 2016)

firecat1981 said:


> Pole, I think that's fairly insulting to those of us who can actually do the math given the proper parameters. There's no real guess work here, mother nature doesn't play a role. The calculations are straight foward.


I can assure you I ( or anyone w/ a second grade education ) have no problem doing the math; but assuming you're not Nostradamus, neither I, or you, or anyone else can predict such variables as number / weight of the crew, where they are standing, weight of gear, fuel, supplies,etc---the list of variables are endless. So yea, there is considerable guesswork to determine the exact draft of any vessel. .....apologies that you felt insulted ( insert rolling eyes emoji ) ...


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## No Bait / Lures Only (Apr 10, 2011)

I gave the builder my sketch/details of my specs, aka spear glades style cap with a pad bottom. Three months wait in line for starting fabrication. 
Should start within the month.
Upon starting this conversation, probably should have researched hull calculators more. Seems like my questions po'd some, that was not my intention.


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## firecat1981 (Nov 27, 2007)

Pole Position said:


> I can assure you I ( or anyone w/ a second grade education ) have no problem doing the math; but assuming you're not Nostradamus, neither I, or you, or anyone else can predict such variables as number / weight of the crew, where they are standing, weight of gear, fuel, supplies,etc---the list of variables are endless. So yea, there is considerable guesswork to determine the exact draft of any vessel. .....apologies that you felt insulted ( insert rolling eyes emoji ) ...


You can easily calculate all those variables, go look at some of my layout threads. Fuel loads, livewell size, batteries, coolers, supplies, outboards, and yes, where people are positioned along with estimated weight, can all be acounted for to get approximate draft numbers along with a center of gravity calculation. Most builders start with this info at the design stage. I guess your grade school forgot to mention that.


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## Guest (Jul 30, 2018)

It’s funny that a manufacturer can post what they’re boat drafts static and no one challenges it. Yet, a member on this forum asks a simple question and people get pissy! It’s obvious to most that the draft will change with load and balance! While the OP’s question might have been open ended, I guess those of us that offered something constructive assumed he was refering to static draft! The calculation method still works, even if ya need to know draft each outing! Just add up the weight of gear, ice, fuel, plug in the numbers!


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## Smackdaddy53 (Dec 31, 2012)

Lots of butthurt here...


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## Guest (Jul 30, 2018)

Smack, I don’t think anyone is butthurt. Some of us have used these calculations with success and know that they work. Some have not and only trust physical proof. It is a calculation and an estimate dependent upon the data entered. I am still gonna say that OP’s boat is gonna draft 5-6” depending on load and balance.


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## firecat1981 (Nov 27, 2007)

I agree with BB. I doubt any of us really put much stock into what someone says on an Internet forum. Let's be honest, this is one of the more tame threads to come up lately.
No bait, can you post the sketch? What did the builder estimate was going to be the bare hull weight? What options will it have?


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## No Bait / Lures Only (Apr 10, 2011)

Builder estimated 750-850 +/-, fabrication starts in 2 weeks.


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