# Question About how to calculate Floatation of my design

Discussion in 'Boat Design' started by AdamOMahony, Sep 11, 2017.

1. Joined: Sep 2017
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Location: Ireland

Hello there!

My first time posting but long time reading! I've undertaken the mission to build a small Canal boat for Ireland (Yes I'm over the big pond) I made the design's based off a Cat/ Double Chime hull and am currently cutting the plywood rib template's out. The final boat will be a ply and wood structure that's fiberglass on the outer skin.

My question is about floatation and bow shape, I've managed to make the main design on Sketch Up and began cutting wood last week! I've been a little lost for thought on how much weight my boat will be able to carry as currently it's looking like a jigsaw puzzel missing a main piece.

I'll attach some picture's here to help give an idea of the boat it self.

It's designed in meters and since the majority of posts here are in feet and inches I'll do my best to provide information in both formats.

So the length of the vessel will be 6.5 meters or 21.325 Feet (21' and 4" roughly) and it will have a beam of 2.4 meters (7 feet and 10 + 1/2 inches) the reason for the small width is the canal system in Ireland has very specific width's and Air drafts.

Basically anything with 1 meter of draft is going to have trouble getting along without touching the ground and anything above 3 meters air draft may hit a bridge. (think hundreds of years old stone bridges and not the most up to date canal system)

I would like a draft of .5 meter's on the boat this would mean it would displace 2.76 m3 of water, (97 cubic feet)

Although it probably will more likely sit at .6 m which would displace roughly 4.21 m3 of water, (148 Cubic feet)

My main question is how much is a safe weight to float on this structure? as the house part is tiny only 2 meters by 5 meters as it will just be for my wife and myself.

Sorry for the long sh-peel but I am curious as to if anyone has suggestions for the bow design and if I need to move the house structure more central?

Thanks for reading appreciate any suggestions or assistance, can attach some snap shots of the plywood cut out's if needed.

Here's the look inside of the Aft ( or Stern) to 5 meter's of the boat length with the last 1.5 meter's undecided as for bow design.

2. Joined: Sep 2011
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Location: Spain

### TANSLSenior Member

The data are not many and therefore I can be very wrong but I have made some calculations and I get a displacement of 5.4 m3 for the draft of 0.6 m and a displacement of 3.9 m3 for the draft of 0.5 m. Are you sure of your calculations?
As for the freeboard, look at the normal freeboard in other similar ships but, since there will be no waves of significant height, a freeboard of 0.5 m, or even 0.4 m, could be sufficient.
Have you considered the possibility of making a mono hull?

3. Joined: Sep 2017
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Location: Ireland

Thanks for the reply, and yes It actually will be one solid hull just with the double chime shape, it's more of a mono hull with two main keels than a Cat really.

the information on displacement was purely by using the Sketch up software to gain the volume of the area below where I would like the water line to be, I was sort of hoping to gather what sort of ratio to total displacement of the vessel to leave as a safety region like if it ended up being 5 tonnes displacement to leave at least 1 tonne untouched or along those lines.

It's my first attempt at boat building although so far it's very addictive haha

If you want more dimensions I can provide anything since the drawing is all to scale on the software.

again thanking you kindly for the response, much appreciated.

4. Joined: Sep 2017
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Location: Ireland

I was originally thinking of a U shaped hull then went to deep V and finally settled on this design which from what I gathered would be more stable with minimal draft.

That's great news on the Freeboard, the total height of the drawing from the bottom points left and right is 1 meter so at .5 draft, the freeboard would be .5 and .6 and .4 for the other scenario.

5. Joined: Sep 2017
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Location: Ireland

For the Bow I am thinking of a shape like this, I will probably have to shape the inner parts by hand to get the middle of the boat fitting right but this is what I am thinking of going ahead with.

6. Joined: Sep 2011
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### TANSLSenior Member

If you have a body lines plan, with transversal sections and the longitudinal profile, it would be helpful to perform the detailed calculations.
A "U" shaped hull, with the same breadth, would give you less draft for the same displacement, not to mention a simpler construction.

Last edited: Sep 14, 2017
7. Joined: Sep 2017
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Location: Ireland

Thanks for the reply, unfortunately Ody Line's I do not have, hmm so the U shape wasn't so bad after all, I was afraid of rolling due to shifting weights inside the cabin area and thus went away from that.

Construction has already begun as such I seem to have a more complex build ahead of me so!

We've already cut 7 of these out with another two more to go at that size. Lateral and Cross bracing to go in sometime after all these have been cut.

8. Joined: Sep 2017
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Location: Ireland

Thanks for the help though, A rough estimation was all that I was hoping for, really appreciate your time and effort! Is there a place on the forum to post how the build is going? As I intend to document it completely.

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### BlueBellAhhhhh...

10. Joined: May 2011
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### Nick.KSenior Member

It's difficult to tell from the photos, but it looks as if the ply you are using is the same that's sold here. We have pieces literally fall apart after a few nights outside. If the ply is to remain inside the structure, I recommend you put a lot of thought in to how to keep it dry and how to vent all the closed spaces. Condensation may be an issue and will cause the ply to rot rapidly

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### gonzoSenior Member

In general, you would finish designing before cutting any wood. If you have a restricted draft, the vee bottom is not ideal. A flat or rounded shape would be better. In a canal, where there won't be any major wave action, you could have a scow bow, which will provide more buoyancy for the length.

12. Joined: Jul 2017
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Location: Cork, Ireland

### NamuNew Member

Where do you find materials for building your project ? I too live in Ireland (Cork) and have yet to find a place to get fiberglass and epoxy in reasonable prices (I'd rather avoid shipping costs if I can).

13. Joined: May 2011
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Location: Ireland

### Nick.KSenior Member

Ireland is expensive except for our rain which is free and plentiful...
GRS in Middleton are probably your best bet. The only way that I know to get cheaper without paying delivery is to buy in barrel and roll quantities. If you have a boat building yard close, it may be worth asking if they will sell materials.
For import, check out the products and prices here: Home - (EN) R&G Faserverbundwerkstoffe GmbH https://www.r-g.de/en/home.html but yes, when you pay the delivery, the savings aren't significant.

14. Joined: Sep 2017
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Location: Ireland

Woah, Sorry I had taken some time away and had not replied, really appreciate all the comments here.

I am supplying my timber from a local shop and it is water resistant but not marine ply, their price's were fair and I believe its the next step up from general use ply wood but not marine standard.

As for Fibreglass I'm looking at Fibreglassdirect.co.uk they seem to be very helpful with my emails, the shipping cost is a downside but found their prices better than the place in cork.

As a further note, as for the ody line's I don't have but i do have a hull front and side profile for those interested.

15. Joined: Sep 2017
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Location: Ireland

the design I am settled upon other than the bow but I believe I am going with this and just bringing the middle section up in a similar manner or at an angle, I'll have a look at how the plywood will shape from the middle shallow V to 20 cm below the front deck.

The piece's cut so far are just the middle section which I was sure of, I left cutting anything for bow until I've settled on it and think this is the way I'm going.

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