# How do I decide the draft value?!

Discussion in 'Boat Design' started by amispurs, Nov 16, 2013.

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### amispursJunior Member

Hey! I'm a beginner at boat design.

I'm trying to design a catamaran boat with dimensions 12ft length and 6 ft beam.
I'm confused as to how to calculate my draft value. I'm still not sure of the payload. How is it done??

Also, what do I make of the different colors in the gaussian/developabilty view?
And is it better to make the number of control points to as low as possible? I was thinking of usin 10 longitudinal and 5 vertical.

Amith

Last edited: Nov 16, 2013
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### Richard WoodsWoods Designs

I don't think you mean 6ft draft. 6 inches maybe, or maybe 6ft over all beam?

I assume you use metric units. In which case the displacement in cu.m will be the same in fresh water as the weight of the boat in T

Richard Woods of Woods Designs

www.sailingcatamarans.com

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### amispursJunior Member

Yes, i meant to say 6 ft of overall beam.

I know the mathematical equation. But i don't know the wegiht of the boat yet. So hw do i decide on draft already?!

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### Richard WoodsWoods Designs

It's been mentioned many times on this forum. It's called a design spiral (use the search feature). You make a guess, then draw a boat using that guess. Throw it away and try again. And so on

Richard Woods

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### Wayne GrabowSenior Member

If you are not ready to make a ball-park estimate of the weight of the boat & payload, then you are not ready to proceed with the design. You need a more accurate vision of what you are trying to accomplish. And still it will be a "design spiral" to finalize the sometimes conflicting requirements in a compromise that leads to your final design.

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### rwatsonSenior Member

Just hoping to clarify the professional help offered previously, to a beginner.

Add up all the weights that you expect to have to carry - people, supplies, hull weight ( really big guess ) rigging etc etc.

Say it comes to 500 kilos for example. Multiply by a safety factor eg 3 (huge)

Now, you need to design a hull that will support 500 x 3 (1500) kilos, or 1.5 cubic metres of water.

You can do the calculations manually eg. -
4 metre long hull x 2, and say .3 metres wide per hull. = 2.4 square metres in plan area. ( not allowing for pointy ends)

To support 1500 kilos, (sunk to the deck), the hulls would need to be .7 metres deep. (1.68 cubic metres )

Naturally, you need to finesse the calculations to allow for tapering bows and sterns etc, but these are just the basic steps.

The design 'cycle' is when you adjust the shape and depth of the hulls, and have to recheck the displacement, boat weight etc until you get your perfect shape.

Hint - look at similar designs and base your figures on proven hull shapes.

Hope that helps a bit.

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### HampusJunior Member

Hello there.

There is no escaping the design spiral. however, there are two methods to help you ballpark your initial draft. The first one is quite straight forward: Find a boat similar to the one you have in your mind and look at the data.

For the second method. Choose a Displacement/Length Ratio http://en.wikipedia.org/wiki/Displacement–length_ratio that you think fit the type of boat you want to design. Calculating backwards, if you know the length, you can get the displacement, from there on the estimated draft using the above formulas posted by rwatson. You can make your estimate even more accurate by trying to determine prismatic coefficient and estimating midsection area beforehand.

BRG

/Hampus

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### amispursJunior Member

Guys, that helps a lot! Thank you all.
I did think of the 'design spiral' method. Only i called it trial and error. I just didnt know if that was the way to do it. I'm a bit confused with all the terms I'm having to learn.

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### Richard WoodsWoods Designs

Actually they are different. Trial and error implies you don't learn from your mistakes. The design spiral means you are narrowing down your design ideas to create the perfect boat for your SOR. With more experience you can start further down the design spiral

Richard Woods

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### lewisboatsObsessed Member

From your initial post I am going to extrapolate that a) you are using some form of FreeShip/DelftShip and b) you have a drawing that you have done. If you have a waterline on the hull then you have assigned some kind of draft to the project in the project settings. If not then look at your hull and figure out where the waterline should sit so the hull will flow properly... like your entry and exits. You should have a clean exit with no transom dragging in the water, and perhaps a minimal amount of stem in the water at rest. Once you have a ballpark figure you can assign a draft amount in decimals of a foot, then you can tweak the results to where you want it. Now you have to go into calculations and design hydrostatics and see how much the boat displaces. This is given in long tons under the imperial measurements. Multiply the displacement number by 2240 to give the displacement in pounds. This is how much weight is supported by the water at the draft you have chosen. If it isn't enough for your estimate of how much the boat will weight fully loaded up and sailing then you will have to change something. Beam of the pontoons, draft of the boat, length of the boat etc. You keep going around the spiral until things come together with the correct draft for the displacement in a hullform that you can live with. That doesn't mean the boat will perform well with the shape you have come up with but it will support your target weight weight properly.

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### DCockeySenior Member

It will support the target weight properly if the center of gravity of the boat and the center of buoyancy are aligned vertically, and the metacenter is above the CG. But getting the immersed volume right is a start.

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### amispursJunior Member

Hello again. Thanks for all the help so far! Btw, the hull I'm designing is going to be used as a USV.

@ lewisboats : yes I'm using a free version of Delftship.

So I tried the design spiral method. And then I found this boat to use as a template for my own boat. It had a 14 ft long hull, a 6ft overall beam and depth of 15". The two hulls are joined by a central platform between the hulls.

This is my initial delftship model for a single hull. Length=14 ft. Beam = 2ft (Too less?!). Draft = 15"

We'll be moulding using FRP. Is this model feasible?

How do I attach a transom?

Hope this isnt bugging u guys!

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