determining waterline height

Discussion in 'Boat Design' started by sidthecoolguy, Sep 16, 2007.

1. Joined: Sep 2007
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Location: Arlington, Tx

sidthecoolguyJunior Member

hi

i am trying to figure out the boat size i need to float a load of 50lbs (it includes the weight of the boat, engines, propellers, battery etc basically everything.)

i calculated that to keep it floating the volume of the submerged part of the boat has to be .81 cubic feet.

Now i am trying to figure out what is the optimum waterline height needed if the hull i use is a catamaran as well as V hull.
WHat would be a relation between the volume of the boat to the volume of submerged part of the boat.

Can anybody please help me in finding out the dimensions of the Catamaran and a V Hull that will be able to float with a weight of 50lbs with the optimum waterline length

2. Guest625101138Previous Member

The reserve bouyancy does not need to be very much if the boat is sealed up so water cannot get below. For a displacement of 50lb a reserve buoyancy of 50lb would be ample.

If the boat is open then you need to consider what sea conditions it will be required to handle and then take the sides high enough to avoid it being swamped. Even then it is wise to have enough solid buoyancy or sealed chambers so it will not sink if swamped.

You should provide more information on what you want to do if you want meaningful answers. You started out with a payload of 50lb and now you are talking about an all-up displacement of 50lb. There is a lot of difference.

It would help if you set out the requirements for the boat in some detail. Will it be operated in a swimming pool or the ocean? Will the load be top heavy or compact and low down? Is the 10mph the required speed or your estimate of a "slow" speed? We know it will be electric power. I assume with a battery rather than solar. What range is required? What turning radius? How precisely will it need to be positioned?

Rick W.

3. Joined: Sep 2007
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Location: Arlington, Tx

sidthecoolguyJunior Member

Thanx again for answering my questions patiently

There are the requirements for the boat:
- The boat has to carry a payload of 20lbs+ carry some electronic equipments of about 20lbs + the various parts of the boat and the hull itself about 10lbs = the total of 50lbs of load

- I have to design a boat which will give maximum manuverability (small turning radius)and max stabiliy.

- It has to operated in a fresh water lake

- I narrowed down my choices of hulls to either a Vee hull or a catamaran

- It will be battery powered.

- The 10mph speed limit is a requirment and not a choice

- As of now I am trying to find the dimensions of the boat(made of fibreglass) I need to get which will give me optimum staibilty and manuevring

- Also i need to write a report on it so i need equations and maths to prove that this would be an optimum design. If you could help me with them.

- It would also be great if u can tell me how to calculate manverability, stability and turning radius of the boat that i will be choosing

4. Guest625101138Previous Member

The software I use is based on metric units so I will give you some numbers but they will be metric. The 50lb becomes 23kg and the 10mph becomes 4.4m/s.

Minimum Power Monohull
The minimum power monohull has WL length of 3.5m, WL beam of 160mm and draft of 71mm. This has a power requirement of 104W at the hull so if you have a propulsion system of 50% efficiency the motor will need to be 200W.

This is clearly a long slender hull. It will have a lot of turning resistance so will need a large rudder to turn quickly. The slow speed steering could be improved with bow thrusters and/or steering nozzle.

This type of hull is quite easy to analyse accurately because it works in displacement mode. It also has very little wave drag because the hull is long and slender.

Miniumum Power Catamaran
The minimum power catamaran requires 132W at the hulls. So say 260W total power. Each hull is 2.2m long, 115mm beam and 62mm draft. The hulls would be spaced so overall beam is 1.11m.

These are shorter hulls but there are two hulls. Turning resistance will be similar to the monohull. The advantage is that you can have a very stable boat and if there is a motor in both hulls you can get very good steering at slow speed.

Typical Planing Hull
As you go to shorter, wider hulls you will have a planing boat at 4.4m/s. As stated before I estimate drag as 1/8th of weight for a planing hull as a first approximation. You may do better than this with wide flat aft sections and it will be worse with deep "V" hull. Working on this value you need 125W at the hull so say 250W installed power. It migh take somewhat more power than this to get it onto the plane. A short planing hull has much less resistance to turning than a long displacement hull so it will turn well at speed. Low speed turning is still an issue unless you have thrusters or a wide hull with two props.

So these are some of the possible options. If you home in on what you like then I can certainly help with the detail on the displacement hulls.

The catanaran will be most stable and will turn in its own length at slow speed.

It would be possible to fit small outrigger hulls on the monohull to improve stability. This is then a trimaran.

Rick W.

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Location: Arlington, Tx

sidthecoolguyJunior Member

Hi,
so can i conclude that catamaran will give the most stability and manuverability at low speed??

Also, is there a mathematical way or a proof something besides expert opinion.

Can you also tell me which software did u use to get these numbers??

6. Guest625101138Previous Member

Yes
Yes- GMT is 10.05m, turning moment a function of propulsion units
Michlet from:

Rick W.

7. Joined: Sep 2007
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Location: Arlington, Tx

sidthecoolguyJunior Member

hi

Hi,
I want to know if there is any specific ratio of Lenght Vs Width Vs Height for a Deep V Hull

Also is there any standard ratio of the same for each of the catamaran???
Also is there any equation for calculation the distance between the 2 hulls of the catamaran

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