# Advice on 20ft beachcat design and build

Discussion in 'Multihulls' started by Philippe Lapere, Apr 29, 2021.

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### Philippe LapereJunior Member

The figure of six people is not a necessity, more of a wishlist.
I think sailing with 6 people will be possible on lighter days, but for real excursions including gear, we'll probably only be able to get fewer people on safely.
You raise a good point on the additional volume that I would require on the leeward hull, if you say we'll need at least twice the total displacement, this would limit the load capacity on "non-leisure" cruises.

I'm aiming for the boat to weigh in under 200kg fully rigged.

Total displacement for one hull is 896 litres (good guess, so at a "total displacement to load capacity" factor of 2, load capacity would be ((896-200)/2) = 350kg; at a factor of 3, load capacity would be 230kg.

I have no limitation on the length, just thought that 6 metres is a nice length, give that I currently have a Hobie 16 (5 metres) and would like the boat to remain fairly easily handleable on the beach and slipway.

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### fallguySenior Member

no way does a 20' beachcat offer 230kg loading-something is off in the math

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### Philippe LapereJunior Member

I think you're giving me too much credit by asking what the design draft is, I've benchmarked a number of existing cats and tweaked the parameters as much as I dare without ending up with a pontoon boat
My idea is to load the boat up in light conditions for leisure cruises, and see how much load capacity I can get in stronger conditions.

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### Philippe LapereJunior Member

I'm quite confident in the calculations, why would you say that's not possible, if a Hobie Getaway (16ft) can reportedly take 270kg load capacity?

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### Robert BieglerJunior Member

What you have drawn reminds me of the Supercat 19, shown here sailing with four people, though some of them quite light weight:
I am guessing the total crew weight is about 200kg, and the boat doesn't look like is has any problem with that. I couldn't find any information on boat weight.

Edit: I found that a version of the Supercat is still being built. The ARC-21 looks like what you drew up. If you were planning to build only because you couldn't find what you want, you might be able to buy instead: Aquarius Sail Inc. - ARC-21 https://www.aquarius-sail.com/catamarans/arc21/index.html

If that is too expensive, the hulls of a second-hand Supercat 19 might fit into a 20ft shipping container, and perhaps that would not be too expensive to ship to you. So if you can recruit someone to look over a boat, you may have to source only the mast, or get that shipped separately.

Handling on the beach and slip should depend more on weight than length. Length gives you more longitudinal stability than freeboard, so if you can find a hull shape that gives you more length for the same volume, you also get a better displacement to length ratio and less pitching. Volume in the middle doesn't do much for longitudinal stability. So you could imitate the Hobie in raising the trampoline and lowering the freeboard in the middle (provided the structure you need for that doesn't weigh more than the hull surface you save), but making it a fair bit longer. And then you can probably go a bit lower on the total reserve buoyancy when sailing on only one hull, so long as you have it in the ends, primarily the bow.

Last edited: May 7, 2021
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### fallguySenior Member

Well, what is most interesting is the same boat ( (Hobie) has a EU rating of 240kg and a US rating of 329kg.

But camping involves lotsa gear and 6 people is more than 329 kg, so despite my error, you have some things to reconcile. If I saw 6 people (adults) on a beachcat and was a warden; you'd be getting ticketed without a rating plate.

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### SolGatoSenior Member

The Hobie Getaway can handle 1000lbs of weight with it being most sensitive of course to weight at the stern where there is the least amount of volume in the hulls. Typically when I see one overloaded with passengers it’s dragging it stern pretty badly.

As I said, it’s an impressive design with regard to capacity. This was one of the main reasons I chose it as a platform for my build. When you start adding stuff like batteries, motors, solar panels, BBQ’s, anchors, etc.., that volume really comes in handy.

I suppose one could always tow a “trailer” full of gear behind in the water if it came down to it.

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### peterbikeJunior Member

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### fallguySenior Member

I have actually had to do this with my canoe. We pulled an inflatable with about 150 pounds on it..

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### Tiny TurnipSenior Member

FWIW here's my pedal boat using Dart 18 hulls and beams. The lack of rig will be replaced weight wise with the heavy deck and beams. 5 up, mixed age and weight, well down in the water (see the white waterline in the underwater shots) and as has been indicated, down in the stern:

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### SolGatoSenior Member

Cool build Tiny Turnip!

Looks like a relaxing way to get on the water.

You could always save some weight by replacing the decking material with something like Thermolite.

Also the HDPE tops of folding and portable tables are a great lightweight, strong UV resistant material if you want to add a table or top, etc..

I used the two halves of an 8’ folding table as seat backs on my boat. They make all different size tables and benches now, so lots of different shapes and sizes to play with.

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### guzzis3Senior Member

Regarding carrying capacity, I don't have a displacement for the caper cat but the 14' version would take 4 adults easily. The 18' version would carry 6 and gear no problem. The hulls were "chunky" ie built for cruising not racing, but the 18 had a big rig and could fly a hull in enough wind, and sailed by people braver/madder/more skilled than me. These boats had no boards and 8' beam. It's definitely doable.

But if it were me I'd look hard at the windrush 6000. 20' x 8', small berths in hulls, plenty of space leeboards and VERY fast. They have a real cult following here and I reckon you could sell such a boat at a profit. The value perception of having actual cabins over lockers really changes things.

Anyway...

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### Philippe LapereJunior Member

Hi all, update on the boat build plan: I've decided to first test/prove the planned manufacturing concept by building a simpler boat, namely a Patina Vela. I'm going to use 10 pre-laminated PVC bulkheads, drawn in CAD and CNC'd (for accuracy). The bulkheads will be attached to a strongback, and the hull skin will be made of 16mm PVC, pre-laminated on the inside. The Patina Vela has quite a simple hull shape with minimal curve, so flat hull sheets suit the manufacturing process.
A pic of the patina vela boat (from the net):

A pic of the bulkheads to be CNC'd:

The 20ft beachcat in the original post has a lot of compound shapes in the hull and I'm worried that using pre-laminated sheets will cause havoc when trying to fix the strips to the bulkheads. Hence I'm considering Biglar's advice for the 20fter, to use unlaminated PVC foam and (additional) temporary bulkheads, and fit, shape and laminate the outside, then remove all bulkheads and laminate the inside with a continuous layer of fibreglass as recommended.
I've ordered the materials, and the bulkheads should be fabricated in the next few days, I'll keep this post updated with progress - comments and advice welcome, cheers!

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### rturbettSenior Member

You'll need to go light on the hulls to be able to make it manageable to right without assistance.
Shark catamarans are 20 by 10 , tough to get more than 4 people on. (although the record is 16- from what I am told.....)

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### Philippe LapereJunior Member

Hi all, another update on my build. I'm now building a Patina Vela using the building method I plan to use for the 20ft beachcat. Let me know if I should create a different post for the Patina Vela build, I think it's relevant and part of the larger plan to build the 20footer?

I had the bulkhead stencils printed full scale, cut them out, marked it onto the 16mm (80kg/m^3 density) PVC which has been laminated on both sides with 160gsm woven fibreglass and epoxy resin.

Then cut out the bulkheads with a jigsaw and mounted them to the strongback (the CNC machine I had planned to have them cut out with broke down).

(I sanded that bulkhead above smooth before proceeding, for the perfectionistic of you out there )

I lined the bulkheads up using two pieces of fishing line (it's surprisingly difficult to mount them straight with only the centre line so the overhead line was necessary)

I then laminated a sheet of PVC on the one side with 160gsm glass, and cut it into 10cm strips, and bonded them onto the bulkheads (with the laminated side inwards) using bonding paste to bond to the bulkheads, and fairing compound inbetween the strips.

Next steps are to:
- plank the entire bottom hull,
- fill gaps with fairing compound,
- sand the hull smooth,
- laminate 3 layers of 160gsm on the hull
- sand smooth etc, flip over, do same on the top, repeat above for the 2nd hull, build crossbeams, etc - at least I don't need to build rudders for this boat

Progress has been a bit slow due to lack of time to spend on the project, and hesitancy to experiment with new materials like PVC and bonding paste, but I'm happy with the progress so far let me know any thoughts, comments, suggestions? Thanks!

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