# Weight distribution in design of boats

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

1. Joined: Feb 2018
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### JohnMarc<--- My ultimate goal

Difficult question to ask because I am not too sure if I can explain myself clearly, but I am sure with a wee bit of tolerance from you guys I can get my point.
It is regarding weight distribution across a pontoon (what I think they call a party boat) boat that I am in the final stages of "design"
I am trying to understand weight distribution and just how much to allow for passengers (if at all)..... By that I don't mean the average weights etc. but wether the rear of the boat (say 30%) should be heavier by design to accommodate the weight of the passengers.... evening the distribution of weight across the whole boat. But and this is the but (in my ignorance) at times we will only be two on the boat but at other times we could be up to seven or eight.
How do I allow for this.
Or another way to put this is I have options in positioning the water tank and batteries etc. which could distribute the weights evenly or unevenly ..... allowing for passengers food etc.
I will attach a drawing(s) which I sincerely hope you can make sense of.

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### TANSLSenior Member

An uneven distribution of weights could lead to problems in the longitudinal strength of the ship-girder but, in a ship of these characteristics, this should not be a problem. On the other hand, whether you are carrying 2 or 7 passengers, which is + -500 kgs, is not very important either, since, probably, the total weight of the boat fully loaded will be much higher. What is important is that the center of gravity of the assembly is in the centerline plane and that its longitudinal position coincides with that of the center of buoyancy.
In any case, having more information in the form of General Arrangement plan or lines plan, as well as your estimate of weights and CoG, would be of great help.

3. Joined: Feb 2018
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### JohnMarc<--- My ultimate goal

In the image of the three colours I have put in the weights.....maybe an overkill but they are all my estimated weights in Kg's ..... I will "google" the Centre of gravity and try and answer that question. The second image I linked to my post my estimation of the different fitting/equipment although the image with the three colours includes the whole boat.

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### TANSLSenior Member

In the images one cannot see much but I seem to observe that the weights of the structure, the engines, ... and other quite important items do not appear there.
To get a first approximation of the ship's weights and CoG it is very useful to use a spreadsheet. On the other hand you have to determine the position of the C of buoyancy.

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It is a relatively simple procedure. But you need the hull form and hence the hydrostatics in order to answer this simple question.

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### JohnMarc<--- My ultimate goal

Okay Maybe some more elaborate images are called for..... I did all on spreadsheet....... will try and give a better description of my plans

#### Attached Files:

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• ###### boat equipment.pdf
File size:
115.5 KB
Views:
241
7. Joined: Feb 2018
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### JohnMarc<--- My ultimate goal

It's a pontoon boat, or is that a completely silly answer......

8. Joined: May 2017
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### BlueBell. . . _ _ _ . . . _ _ _

Let's start at the bottom.
Are the pontoons round in cross section?
They appear square in the graphic.

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### gonzoSenior Member

Passengers are not static weights. You need to calculate the worst possible situation, where the passengers all move to one side.

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10. Joined: Feb 2018
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### JohnMarc<--- My ultimate goal

cross section ..... length being 6 metres

11. Joined: Feb 2018
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### JohnMarc<--- My ultimate goal

Don't know of this makes sense but at waterline (or one I would like) at 250mm I have 0,856 M3 per pontoon (x 2) so can I assume that a total weight up to 1712 Kg's will give me a draft (I think you call it) of 250mm

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### TANSLSenior Member

Boat width is 2.4 m so, something is missing in your transversal section.
Lon g. profile would also be necessary.
The calculation of the weights and their position can be described as "a relatively simple procedure" but the truth is that it is very difficult to do it well.
I do not know if you have calculated the weight of the pontoons, I cannot see it in your graph of the weights, which, on the other hand, is difficult to interpret and, I think, does not allow knowing how high each item is on the baseline .

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### JohnMarc<--- My ultimate goal

entire size is 6 metres by 2,4 metres and yes in the spreadsheet layout are the entire weights .... ie. the furniture, fire extinguishers, batteries, all fibreglassing structural elements, (pontoons deck beams etc.) so the total weight at these initial stages is, 1109,8 Kg's

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### TANSLSenior Member

I am enclosing the weight estimate of a boat similar to yours.
1109.8 kgs seems to be a low weight.

#### Attached Files:

• ###### SWB Weights Estimation.xlsx
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15. Joined: Feb 2018
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### JohnMarc<--- My ultimate goal

Many thanks for that Tansl, so appreciate this forum at times like this, I will follow the same spreadsheet.
I have made (vacuum infusion) up a few panels of the 20mm and 10mm glass/epoxy lycell panels and weighed each one individually and based my calculations on them. I will not be using any aluminium in my boat as stiffeners or otherwise. I have built a few items such as the lounger and the weights seem to check out. I have no doubt there are bits and pieces I have left out. My original question was not about the weights etc. but rather about the distribution of the weights across the area of the boat, is the idea to get it as balanced as possible?

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