# Determining the draft of the hull (trimaran sailboat)

Discussion in 'Multihulls' started by prabs, Jun 2, 2021.

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1. Joined: Mar 2020
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### prabsJunior Member

How to determine the draft of a trimaran sailboat? Length: 25 feet, Beam: 8 feet.

It has 6 stations(longitudinal) in the section view and 3 panels(vertical) in the front view. (Freeship software).

What value should I input in the draft field?

What is the ratio between draft to length or draft to beam? What is the optimum? Is there a formula for an optimum draft?

What is the draft of the ama?

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### Robert BieglerJunior Member

Is that the overall beam, or that of the main hull. If overall, that is very narrow. You have to go back to the 1960 to find a high proportions of trimarans with beam as little as 50% of the length, and I think that limit was because of how strong a structure a home builder could be expected to build, given the design knowledge of the time. These days, overall beam is typically in the range of 70% to 100% of length. You are aiming for 32%. You would have to expect wave interference between the hulls, very limited stability, and a relatively large angle of heel at maximum righting moment.

If the reason for the narrow beam is that you only want to take the rig down for trailering, I think a catamaran would be a better choice. Have a look at Duckworks - Slider Cat https://duckworksmagazine.com/09/designs/slider/index.htm. If you design a rig no higher than that, you keep the boat as safe as Slider is.

If the beam is for the main hull, that is extremely wide for a trimaran.

Depends on what you want to optimise. If you want low wetted surface at rest, a draft of about half the beam at waterline should be close. How much that draft and beam is relative to length depends on the displacement you need. If the hull needs some minimum beam for you and crew to be comfortable inside it, and that is wider than mandated by displacement, reduce draft to compensate. If you want a soft ride in waves, make the hull narrower and deeper.

You have similar trade-offs. To avoid tripping over the ama when pushed sideways by a wave, desired volume is about 150% of maximum displacement. Race boats might go up to 200% to increase longitudinal stability. Choose your volume, length, and how full you want the shape to be. That gives you the cross section at the widest point. Then trade off beam against depth. Less beam gives you greater heel angle at maximum immersion, a softer ride, and a greater hull surface area, and therefore greater weight.

Or did you mean draft at rest? If you want to optimise sailing alone, then zero. But then if you tie the boat to a pontoon, waves might lift the ama over it. So then you would want just enough draft to prevent that. How much that is would depend partly on the beam of the boat.

Sardine Run 23 is close in size to what you want. It might give you some ideas: Sardine Run 23 https://www.sardineboats.com/voiliers/sardine-run-23/

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### zstineJunior Member

Freeship will output the displaced volume and displacement for a given draft of your boat design, or you can find the draft if you know the weight (displacement) of the boat. The 'calculator' looking button turns on key hydrostatic properties, like what you see in the profile view below, or you can get more under the calculations -> hydrostatics button, which allows you to input a range of drafts and will output various hydrostatics over that range. At max righting moment, the ama will have 100% vessel displacement (the main hull will be just clear of the water). The max righting moment is the lateral distance from the ama's center of buoyancy to the C.G. of the boat. Rigging design is typically sized based on max righting moment times a Factor of Safety... Not sure if that helps. It's not clear exactly what you are trying to do. Is your issue understanding the Freeship software or is it a question of naval archy/engineering principles? If you have a shape for the boat already, you need to know your boat's weight to determine the draft.

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### gonzoSenior Member

You can input anything you want. Do you have a draft restriction? Also, it depends on the speed of the boat. A slow sailing cat will benefit from a circular section, where a fast planing cat will be better with a vee section.

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### prabsJunior Member

Thanks people for your responses. I have only read the Freeship manual in English. There is not much to learn from the tutorials I watched on youtube. Can I be able to access the table of offsets (generated??) after the templates are created.

I want to know how I can proceed towards lofting. I understand that table of offsets are needed to loft full size of a boat. Does Freeship generate templates that can be used to print out patterns for frames, transom, etc. How to obtain full sized patterns in Freeship?

I do not understand how it all works in Freeship. Anyone recommend a good tutorial on Freeship?

I want to be able to build a small liveaboard multihull cruiser. (trimaran/catamaran).

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### DCockeySenior Member

There is much, much more to designing a boat than just creating a 3D model of the hull. A major part of design is working back and forth between different aspects of the design until they are in agreement. The major dimensions of a hull are tightly linked to the displacement.

prabs - Is this a student project?

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### prabsJunior Member

This is not a college assignment or a school project. I have been reading all things about boat building over years. I would like to build a boat myself and just do not want to read only sail blogs written by others. I want to connect boat design*(theory) to lofting(practical). I am trying to find out the missing bits and pieces.

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### gonzoSenior Member

Is the primary goal building or designing a boat?

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### prabsJunior Member

Hi gonzo. I would like to design as well as build it.

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### Robert BieglerJunior Member

The very first boat you design is unlikely to be good enough that it is worth as much effort as a 25ft boat will take. I suggest building either a a few models, or else roughly canoe-sized boats, scaled so that your weight corresponds to the payload you want for the full scale boat. So if you want the payload of the 25ft boat to be eight times your body weight, you build the test boat at half scale. Perhaps even build a comparison boat closely modelled on an established design. Compare them, see what you like about each, work out what design features produce the traits you like.

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