# Weight distribution in design of boats

Discussion in 'Stability' started by JohnMarc, Oct 29, 2020.

1. Joined: Feb 2018
Posts: 128
Likes: 7, Points: 18
Location: Knysna South Africa

### JohnMarc<--- My ultimate goal

Right I see, yes you understood the question many thanks, so does that mean the distance between the centre of the demi hull to the centre line is 320.... erupt another way the distance between the two demi hulls centres is 640?

2. Joined: Apr 2015
Posts: 363
Likes: 44, Points: 28, Legacy Rep: 37
Location: Berlin, Germany

### HeimfriedSenior Member

Yes

3. Joined: Feb 2018
Posts: 128
Likes: 7, Points: 18
Location: Knysna South Africa

### JohnMarc<--- My ultimate goal

Okay so as a newbie to line drawings would this be correct.... before I make a fool of myself and post the entire line drawing

4. Joined: Sep 2011
Posts: 6,317
Likes: 318, Points: 93, Legacy Rep: 300
Location: Spain

### TANSLSenior Member

It looks very good. I think you can continue, sections of the stern, longitudinal profile and a plan of the deck and, if you want, the water lines although with all the above you can already calculate the center of buoyancy and the transverse stability curves.

5. Joined: Feb 2018
Posts: 128
Likes: 7, Points: 18
Location: Knysna South Africa

### JohnMarc<--- My ultimate goal

Thanks Tansl.... once again dipping my toe in the water

6. Joined: Oct 2010
Posts: 9,337
Likes: 710, Points: 113, Legacy Rep: 702
Location: Australia

### Mr EfficiencySenior Member

how is it powered ?

7. Joined: Sep 2011
Posts: 6,317
Likes: 318, Points: 93, Legacy Rep: 300
Location: Spain

### TANSLSenior Member

Very well. Now, we go step by step, calculate the longitudinal position, for the estimated displacement, of the center of buoyancy (geometric center of the submerged volume), which should be aligned with the CoG.
If that does not happen, you will have to change the arrangement of the weights (working in the spreadsheet), until you achieve it, or admit that the boat, at rest, will have a certain trim.

8. Joined: Oct 2008
Posts: 6,712
Likes: 691, Points: 113, Legacy Rep: 2488
Location: Japan

So, now we come to the operational side of the boat. What speeds do you want/expect from the boat?..and how do you anticipate the boat to be powered?

Speed...this influences the shape of the bow and the lines accordingly. Currently, as drawn, it would suggest you are not interested in speed, other than just a hull to produce buoyancy for your house boat.
However, if speed is an input...i.e. you may like to motor over to a different location and if you wish to do that in a reasonable amount of time, then the bow region needs to be changed. For example, here:

This is too bluff...or too blunt. The water flow is like hitting a brick wall.
Thus, if speed is a consideration, then the lines need more shape to help "promote" the flow of the water aft - rather than - as it currently is - stopping it at the bow, so to speak, like so, for example:

And this then falls into the means of powering...if it is a simple outboard that is easy enough, you just hang it off the transom...
However, if you decide on an inboard motor, of whatever type, then promoting the flow of the water into the prop, assist sin this regard, like so:

You slowly raise the keel line, so the prop can fit under the hull, but also if the prop extends below the keel line, you then need to think of the easiest/best way to protect the prop.

So, in short, great start to your lines.
BUT...other than providing buoyancy to carry you weights... you need to consider how the shape will be influenced by the speed you hope to get, if speed is an input.
As this can greatly influence the shape of the hull, and hence the Lines.

There is no point doing anything else, unless you decide whether speed shall be an input to your final design.

So looking at the hull lines referenced by Heimfried, you can see the difference:

The lines above, are clearly designed for a speed.

9. Joined: Feb 2018
Posts: 128
Likes: 7, Points: 18
Location: Knysna South Africa

### JohnMarc<--- My ultimate goal

90 Hp outboard

10. Joined: Feb 2018
Posts: 128
Likes: 7, Points: 18
Location: Knysna South Africa

### JohnMarc<--- My ultimate goal

Wonderful, Thanks Ad Hoc, lots of things to consider, really grateful for the input I am getting, allowing me to have a far better understanding of where I am going and what I want to achieve..... Many thanks will do a redraw after rethinking my priorities... ie. speed or weight carrying capacity

11. Joined: Feb 2018
Posts: 128
Likes: 7, Points: 18
Location: Knysna South Africa

### JohnMarc<--- My ultimate goal

Gentleman may I ask a question while in contemplation of your earlier comments regarding my hull, which of the two shapes are better for my pontoon boat and reasons why......

12. Joined: Sep 2011
Posts: 6,317
Likes: 318, Points: 93, Legacy Rep: 300
Location: Spain

### TANSLSenior Member

Both will be very similar from the point of view of stability, drag and maneuverability. So I would choose the one that is easiest to build, according to the mechanical and human means you have.

13. Joined: Feb 2018
Posts: 128
Likes: 7, Points: 18
Location: Knysna South Africa

### JohnMarc<--- My ultimate goal

Thanks...yes easiest and the extra 20 Kg volume I get from the rounded shape seem to be the way to go.

14. Joined: Aug 2002
Posts: 14,777
Likes: 767, Points: 123, Legacy Rep: 2031
Location: Milwaukee, WI

### gonzoSenior Member

There is a fundamental mistake in the diagram if both boats weigh the same.

15. Joined: Sep 2011
Posts: 6,317
Likes: 318, Points: 93, Legacy Rep: 300
Location: Spain

### TANSLSenior Member

@JohnMarc, the polygonal section can be much easier to build, in the fore and aft areas, than the rounded one. Analyze several cross sections and some longitudinal before deciding on one or the other.
Don't worry about 20 kg more or less, the mistake you (or anyone) will make when calculating the weight is going to be greater than that number.

Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.