# Calculating draft on a boat



## firecat1981 (Nov 27, 2007)

I've been working on my new design and how I want to set it up and rig it out. One thing that keeps coming into my head is draft and weight. I started working on some calculations on how to roughly figure draft with different set ups and loads. After calculating out my last few boats I think it's fairly accurate enough to give you a half way decent idea of what's going on.

Ok so heres the basic example I wrote up. It assumes a Flat hull bottom and a hard chine. If your boat has and v shape to it or rounded chines it will be harder to calculate as those will increase draft. 
I did the figures on a water contact area of just 13ft because alot of skiffs even though they may be 14-15ft long after you take away the bow and transom tapers the actual water contact area is much smaller. I also set the width at 48" as I see that as a standard on alot of designs. 

As far as the motor weights go, they are both 4-stroke motors. The 20hp motor is 120lbs, which is what my yamaha was with electric start. The 25hp is set at 200lbs, which is what I'm finding out alot of them weigh once you add TnT. I estimated everything, even the fishermen, to be fairly heavy to give a large margin of error. 

You can easily adjust the numbers to come up with a rough estimate for your own skiff. It assumes you have a blanced load, if you have an unbalanced load (too much weight in the rear) the numbers won't jive. 

You will see that with a properly balanced load, the extra weight of the larger motor doesn't effect the draft by very much. Let me know if I missed anything cause I did this on the fly.


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## [email protected] (Feb 28, 2010)

very intresting!! gonna try it on my boat. good job


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## Brett (Jul 16, 2008)

That's one way to go about calculating displacement from volume.
I use an old school method computing volume from the averaged end area of sections.
First I draw or build my model, from which I obtain my dimensions.
Then divide the model into cross sections, 1 foot intervals, the length of the hull.
I then plot the cross sections on graph paper at 1/4 inch equals 1 inch scale.
Don't have graph paper, but do have a computer and printer?
CNET Download.com has free software that will allow a Windows computer to print
all the graph paper you need, in whatever increment base you want.
I plot only the half hull dimensions, as the hull is symmetrical about the centerline.
Then calculate the area at the desired draft either by computing triangles and rectangles
or counting the squares inside the hull outline under the draft line, for each station.
Then average the area between 2 successive stations and multiply by 12
for the volume in cubic inches between each set of stations.
Add the volumes together and you have the half hull draft volume.
So when I'm done I just multiply my total calculation by 2 for the entire hull draft volume in cubic inches.
An example is linked below...

http://picasaweb.google.com/bdefalco/VolumeCalculation#


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## Pranay Mohite (Jan 9, 2020)

firecat1981 can u post formula to calculate draft of flat boat


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## SomaliPirate (Feb 5, 2016)

Thread back from the dead!








I just pole until I start scraping and figure that's what I draft.


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## lemaymiami (Feb 9, 2007)

aand... for the hull to get in as skinny water as possible it has to float perfectly level with whatever load is actually in it (and that includes us.... plus fuel, ice, gear - you get the idea...).

Years and years ago when many were rigging skiffs themselves (for a variety of purposes -since there just weren't many good production shallow water skiffs being offered...).... We'd take a new or restored hull to the ramp - then add stuff to it - battery, ice chest (half fill it with water on site for an honest read, fuel tanks with fuel (if the skiff was going to have portable tanks) along with whatever was going to be on that skiff when it was getting used.... Then you had the great fun of moving various components around to see what was actually needed to help that skiff float absolutely as level as possible while making notes about where the heaviest items needed to be for final rigging results... 

Hope this helps.... aren't boats fun?


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## Big Fish (Dec 20, 2019)

If this is how boat builders come up with their draft numbers than I know its wrong lol


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## Guest (Jan 10, 2020)

When your draft exceeds the waters depth, you are most assuredly aground! Benjamin Linsey 1772
Draft calculated, nuff said!


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## Smackdaddy53 (Dec 31, 2012)

Big Fish said:


> If this is how boat builders come up with their draft numbers than I know its wrong lol


It can be done before the boat is ever built. Math


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## Guest (Jan 10, 2020)

Smackdaddy53 said:


> It can be done before the boat is ever built. Math


Basic math!


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## lemaymiami (Feb 9, 2007)

Years ago it was also very common for production boat builders to fudge about hull weights and outright lie about the draft on their skiffs...


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## redchaser (Aug 24, 2015)

lemaymiami said:


> Years ago it was also very common for production boat builders to fudge about hull weights and outright lie about the draft on their skiffs...


Has that really changed much?


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## jonterr (Mar 24, 2013)

redchaser said:


> Has that really changed much?


I don’t know about other boat builders, but, Carolina Skiff list their draft, then in small letters it says , no motor


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## Zika (Aug 6, 2015)

There is really no industry standard. Some builders list as hull only or engines up, while others claim draft with engines down. And those are based on dry weights, no gas, ice, gear or passengers. So there's no apples to apples comparison. Best bet is to break out the yardstick while on a test ride and measure yourself.


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## Big Fish (Dec 20, 2019)

Smackdaddy53 said:


> It can be done before the boat is ever built. Math


Math is nothing more than an approximation of reality. Once applied to the real world it is fraught with flaws as exact mathematical forms do not exist in the physical world.

Too bad more people don't realize this as they would stop claiming that their 24 foot pathfinder drafts 12" because the website says so lol


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## firecat1981 (Nov 27, 2007)

Pranay Mohite said:


> firecat1981 can u post formula to calculate draft of flat boat


Wholly dead thread batman!

I'll see if I can look up the pictures later and repost them. Photobucket is holding my old ones hostage. 

I will say that with all the speculation and doubts..... my estimates were damn close. But I also made sure to estimate the load balances too.


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## jimsmicro (Oct 29, 2013)

If you own the boat you go till it doesn't float anymore and measure how deep the water is. If you're a manufacturer you take the motor off the transom, empty the gas tank, and measure the boat with nobody in it. Then subtract an inch or so and post the number on your website where everyone will believe it as gospel.


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## SomaliPirate (Feb 5, 2016)

jimsmicro said:


> If you own the boat you go till it doesn't float anymore and measure how deep the water is. If you're a manufacturer you take the motor off the transom, empty the gas tank, and measure the boat with nobody in it. Then subtract an inch or so and post the number on your website where everyone will believe it as gospel.


Don't forget to pick it up by the bow and post on Instagram, and trash other builders!


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## ebr (Sep 6, 2019)

Big Fish said:


> Math is nothing more than an approximation of reality. Once applied to the real world it is fraught with flaws as exact mathematical forms do not exist in the physical world.


I would argue that Math is absolute and any flaws in a calculation stem from the omission (or incorrect assumptions) of some set of variables .

Still, to your point, the only 100% accurate way for any particular instance (assuming you cannot know all the variables) is with a ruler.


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## firecat1981 (Nov 27, 2007)

Here is some of the workup I did on my last boat. I think I was within a 1/4" of the calculations. Keep in mind, my boat was mostly flat bottomed, with a slight v up front. And the estimates are for the water contact area only.


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## Guest (Jan 19, 2020)

lemaymiami said:


> aand... for the hull to get in as skinny water as possible it has to float perfectly level with whatever load is actually in it (and that includes us.... plus fuel, ice, gear - you get the idea...).
> 
> Years and years ago when many were rigging skiffs themselves (for a variety of purposes -since there just weren't many good production shallow water skiffs being offered...).... We'd take a new or restored hull to the ramp - then add stuff to it - battery, ice chest (half fill it with water on site for an honest read, fuel tanks with fuel (if the skiff was going to have portable tanks) along with whatever was going to be on that skiff when it was getting used.... Then you had the great fun of moving various components around to see what was actually needed to help that skiff float absolutely as level as possible while making notes about where the heaviest items needed to be for final rigging results...
> 
> Hope this helps.... aren't boats fun?


Would we call "old school"?


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## permitchaser (Aug 26, 2013)

I don't care what my boat draws. Its been plenty skinny for where i have fish. And what am i going to do if i did care. I already took out a rotten 35 gallon gas tank and replaced with a 21


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## Snakesurf (Jun 18, 2019)

I don't see any part of the calculation that takes into account the water salinity or temperature.


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## firecat1981 (Nov 27, 2007)

Well the difference between fresh and salt water is .05lbs per gallon, so it will have a drastic effect on draft. Like 0.002"! Oh and don't get me started on temp. When Tampa bay freezes each year draft goes way down, but when it boils in the summer your boat will sink at launch.


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## Snakesurf (Jun 18, 2019)

Yep, you get an awesome draft when the water is frozen.


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