Help me design the AmphiCat

[Amphicat]

Ahoy, me hearties!
I've never been much of a pirate, but my days of being a scallywag, is (hopefully) soon over.

I need some help deciding on dimensions for my Fiberglass catamaran. And drawing the boat in a program, so I can print out some sheets to go after.

Here's a link with more information about the boat.
AmphiCat https://docs.google.com/document/d/1mB8INK94t50nFlxTLlNQF97Of6LFJWccGGG4ky6lK-U/edit?usp=sharing

The use of the boat
1. A hideout for my girlfriend and I. We need a place where we can be alone. A hotel room on water -or on land.
2. Sailing around the danish coasts (not Kattegat, I don't have a need for those big waves).

Mainly a houseboat.

Design capabilities.

1. Initial stability.
   The design needs enough initial stability to be comfortable enough to be used as a houseboat.
   
   
2. Lightweight
   Cheaper construction and transportation.
   
   
3. Easy to operate
   It has to be a motorboat. No sails.
   
   
4. Easy to build
   I'm going to vacuum infuse epoxy into some fiberglass. The flatter the surface, the easier to manage.
   
   
5. Comfortable.
   The cabin has to be big enough to be able to stand inside. Yes I'm scared of the wind. I want the cabin to be 2,05m tall if possible. Around 4*2,3*2,05 meters.
   
   
6. Practical
   Wheels underneath the boat, to eliminate the need for a crane, make for easier transportation, easier maintenance of the bottom of the boat and to use the boat as a cabin when waves are too high. Just ride it up the beach.

Power generation
I'm planning to add some solar panels and a small wind turbine on top. It could be nice to have a battery in each hull, to hopefully provide more stability.

Trying to get the dimensions.
I've tried to fill in a document but I'm not sure of the results and if I understood everything correctly.

I'm really hoping someone can help me
I do realize this is SOME task designing a boat. I am familiar with working with fiberglass and resins, but not boats. I'm also not familiar with 3d drawing software -I'm ready to learn.

Pictures
Here's some pictures to hopefully show kinda what I'm going for.
It's a mix of these boats. I'm thinking a wide version of the top picture, with a cabin/deck design resembling the bottom picture.

Hoping you guys can help me.
-Mathias

 

[TANSL]

Welcome to the forum.
Before someone tells you to forget everything you have thought, that anything else would be preferable to that, that on the web you will find much better things, that you are going to ruin, and a hundred more things that will try to discourage you, I would ask you, so that we can give your opinion with a little more foundation, how you want to combine the two pictures of your post. A sketch, even freehand, of what you intend, would be very enlightening, as well as an SOR, as detailed as possible.
Good luck with the forum ... and with your project.

 

[bajansailor]

Welcome to the Forum Amphicat.

Re the photo below in the link that you posted, it looks like it should come fairly close to meeting the detailed Statement of Requirements (SOR) that you are proposing - do you have any more information about this boat?
It looks like a reasonable compromise between a houseboat and a planing cat.

I think though that putting wheels on a cat like this (as per the wheels on the blue planing cat shown above) will be very expensive and complicated, and they will be high maintenance, especially if you use them in salt water.

 

[Amphicat]

I don't want a fast boat. I want one that's safe and comfortable.
Only coastal sailing in decent weather (below 10 m/s average wind speeds -20 kn). At around 5 km/h.

Here's the link to where I found that picture.
This boat is way bigger than what I'm trying to do, but I like the shape of the hull and the design language.
https://www.designboom.com/design/vik-boats-electric-boats-for-life-01-06-2019/

You're probably right about the wheels. But maybe someone here can think of a real low-tech solution

I'll reply with one more message later

 

[fallguy]

Forget the wheels on the vessel. A month in the water and they are all shot, full of critters. The drag they add requires more horsepower for a slower speed.

Build a Woods Skoota 20 or 24. They will fold. A custom trailer and you are off to sea whenever and wherever.

the 24' is probably my all time favorite boat, but not the two cabin idea..that can be modified

 

[bajansailor]

@Amphicat I would agree with Fallguy re a Woods Skoota cat - I think that one of these should come close to what you have in mind re your thoughts (and photos) above.
Here is the intro to Skootas -
Sailing Catamarans - Skoota Power Cats Introduction (includes study pack) http://sailingcatamarans.com/index.php/designs-2/6-powercats/190-skoota-power-cats

Have a look also (for reference) at the other Skootas in the range -
Sailing Catamarans - Click here for Power Catamarans (8 designs) http://sailingcatamarans.com/index.php/designs-2/6-powercats

You could perhaps put a larger superstructure (houseboat style) on a pair of Skoota hulls, and then that would be similar to combining the two photos in your first post above.

 

[TANSL]

@Amphicat, If you want space, stability, moderate speed, think better of a pontoon type catamaran. Among many other advantages, it is easier to build.

 

[DogCavalry]

The wheels are fine, but theyneed to be lifted out of the water. Hinged mechanism. Normally up out of the water, swing down under their own weight, and locked with a pin or bolt. Individually hydraulically driven at low speed. Maybe a castering rear wheel, and individually driven front wheels for low speed steering.

 

[Amphicat]

#5
The wheels would not be in the water until needed. Retractable legs is the idea.

#5 and #6
I really really like the idea of a modified Skoota 20.
But it doesn't seem that... weight and stability oriented. Still. I REALLY like those boats. These boats are surely getting close!

#7
You're exactly right. A platoon type catamaran is probably just what I need.

Well... How do I calculate the dimensions of the hulls needed for the boat?
It has to be able to carry at least 850 kilos (to be on the safest sides of things), and hopefully around 4,94 meters long and 2,44 meters wide.

 

[TANSL]

It is essential to make an estimate, as detailed as possible, of the total weight of the boat. To do this, you have to start by drawing a General Arrangement plan where everything that the boat is going to carry is related and located. With this information, the dimensions of the two floats can be defined.
(What do you include in those 850 kg?)

 

[Amphicat]

#8
Hello there. Exactly my thought
Just need to think of a lightweight durable solution.
What you're proposing is just 2 axles on each side of the boat, that can be swung up and down? Seems like a good way to go about it.

Also I just want to thank everyone!
It's real nice to get so many good replies. I've been working hard studying boat designs, so this feels really really good. Thank you!

 

[Amphicat]

#10
I might need some help with that general arrangement plan, but I've written down the weight of almost everything I want to have on the boat. I want to change the max of 2 persons to 4 though.
I'll have to make a guess the weight of the hulls, but I can estimate the weight of the deck and cabin fairly well already now.

The equipment on the boat is gonna weigh IN TOTAL 180 kilos.
My girlfriend and myself weigh 135 kilos together, but let's just say 170 to be sure She's a baker, so you never know. Temptation will probably get us sometime

Now we're on 350 kilos.
+2 people. The heaviest guy I know weighs 120 kilos, so 2 of him equals.. 240 kilos.
350+240=590 kilos
So far so good. Now the deck, the cabin and the 2 hulls.

Deck weight
well... I still haven't 100% decided on the composite for the deck now.
Vacuum infused Fibreglass/Epoxy on bottom. Gelcoated for extra safety.
But some wood solution with a transparent coat on top could look amazing as the top layer, and be lighter probably...
It's just a big sheet rectangular piece of wood. Should be easy and cheap too!

Dimensions of deck.
4,94*2,44*0,008=0.0964288
0.0964288*the weight of the materials density kg/m3.
Maybe 8 mm is enough. Maybe I'll go up to 10.

Probably around 100 kilos depending on the materials I choose.
5 mm epoxy glass, with some lightweight 100% dried wood on top of the laminate -again coated waterproof.

690 kilos now.

Cabin weight
I should use more wood here, cause I need to attach stuff to the cabin walls. Less strength, but lighter. Windows will be made of PMMA, LSR or PC. All lightweight.

The sides of the cabin.
Let's say it will be made of 35% windows.
Assuming density of wood-composite to be 500kg/m3
4,4*2,4*0,01*(density of wood-composite*0,65)=34 kilos
4,4*2,4*0,004*(around 1200*0,35)=17,7 kg
~52kg

The roof of the cabin
Let's get some lightweight action now.
(4,4+a retractable part of 0,5 meters)*2,4*0,006*500=35 kg
35+2 for retractable system.
~37kg

The front of the cabin
Well guys, I'm just going to guess my way through for now. Design isn't even finished, so no use of making all these calculations too precise.
~30kg
30+37+52=
Rounded to 110.

Now we're at 800 kilos.

Another 50 for the hulls seems a bit low when I think about it, but...

that's how I came up with 850

Let's make it 1100. Then I can feel safe.
******* what a post...

 

[bajansailor]

I really really like the idea of a modified Skoota 20.
But it doesn't seem that... weight and stability oriented.

Mathias, you do not have to worry about stability on a Skoota 20 cat unless you decide to modify it drastically (such as add a second deck above) - the stability should be such that it could easily cope if you wanted to have a full height cabin to stand up in.
However it will look pretty ugly if you do this.
Everything in boat design is a compromise - nothing is perfect.

Tansl is correct, re how a cat with simple pontoon hulls should be easier to build than a Skoota cat - but the Skoota will be much more seaworthy. And you will need it to be seaworthy if you want to go along the coast of Denmark.
A simple pontoon hull would literally be a rectangular box - it can be improved by adding some rake at the bow.

Well... How do I calculate the dimensions of the hulls needed for the boat?
It has to be able to carry at least 850 kilos (to be on the safest sides of things), and hopefully around 4,94 meters long and 2,44 meters wide.

I am intrigued Mathias as to how you arrived at these dimensions of 4.94 m x 2.44 m?
I see that the hull length in the Excel spread sheet in your first post is 4.90 m. and the overall beam is 2.49 m.
But you will need to have a much bigger cat if you want it to carry 850 kg.

How to calculate the dimensions of the hulls?
As Tansl said, you need to start off with your general arrangement plan, and then do some initial weight calculations of the weight of materials that will go into the hulls, deck and cabin, and the weight of all the things that you want to load on board (including people).
If we use lengths in metres, then for one hull (assuming that it is a simple rectangular box) -
Length x Breadth x draft = Z cubic metres.
Fresh water has a density of 1,000 kg / cubic metre, so the displacement of your cat with two hulls will be 2 x Z x 1,000 kg. = 2,000Z kg.

Calculating the displacement of a hull form like a Skoota is a bit more complicated, because it is not a rectangular box.

Tubular aluminium pontoon boats are very popular in the USA - do you have many like this in Denmark?
Here is a link to a builder in the USA showing their range -
Harris Pontoon Boats | Luxury Family Pontoon Boats https://www.harrisboats.com/
You could build a houseboat on a set of aluminium pontoons like these.
But tubular aluminium pontoons are not so easily constructed by a home builder.

Which brings us back to the Skoota cats again......
Have a look at all the free study plans relating to Skootas on Richard's website - they will tell you a lot, but not enough for you to design one yourself.

Edit - after seeing post #12 above (we overlapped slightly when writing), re your displacement of 1,100 kg - if your length is 5 metres and the overall beam is 2.5 metres, lets assume that you have simple rectangular block shaped hulls, and that the hull beam is say 0.75 m, giving 1.0 m. spacing between the hulls.

So we have 5 x 0.75 x draft x 2 = 1.1

This gives a draft of 0.13 m. for this displacement.
But if you want to add some shape to the hulls, rather than have rectangular boxes, then the block coefficient will decrease, and the draft will increase.

 

[fallguy]

#5
The wheels would not be in the water until needed. Retractable legs is the idea.

#5 and #6
I really really like the idea of a modified Skoota 20.
But it doesn't seem that... weight and stability oriented. Still. I REALLY like those boats. These boats are surely getting close!

#7
You're exactly right. A platoon type catamaran is probably just what I need.

Well... How do I calculate the dimensions of the hulls needed for the boat?
It has to be able to carry at least 850 kilos (to be on the safest sides of things), and hopefully around 4,94 meters long and 2,44 meters wide.

850 kilos is a lot,,,what needs to be accomodated?

Skoota 24 allows 1100 pounds of gear. That is generous. I will go back and read your 850.

 

[Amphicat]

850 kilos is a lot,,,what needs to be accomodated?

It was for the while boat, with all equipment and people onboard
I'm pretty sure that's not enough though. 1100 should be safe.