Help me design the AmphiCat

[fallguy]

You only count people engines and gear.

if you want to sleep four, skoota 24 is too small...??maybe

40 hp engine is 200 pounds, say 300 for helm, etc, leaves 800 pounds for food, water, people, the boat has decent ppi for overload

 

[fallguy]

Ppi is not stated, but say

20" wide hull
20' of hull at 20" is 20*20/12=33.33 sqft waterplane at one inch is 33.33/12 or 2.78@64 pounds x two hulls or about 356 pounds ppi estimate

So, the boat sinks an inch below her lines with 350 pounds extra big trip..

 

[Amphicat]

And you will need it to be seaworthy if you want to go along the coast of Denmark.

I'm not so sure I need that extra seaworthy-ness. I need initial stability more than anything. I the least amount of roll. I still really like the Skoota 20 though..

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.

Oh, I changed it up a little to make it cheaper to dock and then I added 4 cm to the length.

Tubular aluminium pontoon boats are very popular in the USA - do you have many like this in Denmark?

Nope, they look like a design for me though!

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.

Oh man. Well. I'll have to shape the hulls.
But I don't know what shape will do the best job and why.
Now we're coming to the real hard stuff.

Any help on this?

 

[Amphicat]

You only count people engines and gear.

Oh. People is around 170-450 kilos with all their clothes and stuff
Engine and gear is around 180 kilos total. Electrical engine(s).
So 350-630kg

if you want to sleep four, skoota 24 is too small...??maybe

I don't want to be able to have 4 people sleep on the boat. Just 4 people spending time on the boat, having fun Eating a meal, drinking some drinks.. a cozy day on the boat

40 hp engine is 200 pounds, say 300 for helm, etc, leaves 800 pounds for food, water, people, the boat has decent ppi for overload

This boat isn't supposed to be used for more than a weekend at a time usually. Or maybe 1 week. It's a hotelroom-ish boat So food and water won't weigh much.
Oh, how do I find out my boat's ppi?

 

[Mr Efficiency]

Relatively simple to have a small boat that can deploys wheels to be rolled up/down the beach, but much more difficult if driven wheels. You could have a winch connected to an anchor you bury.

 

[Amphicat]

I'm not going for driven wheels. That's too complicated.

You could have a winch connected to an anchor you bury.

I'm not sure I understand
For the wheels?

Otherwise yeah. I want to have a winch for my anchor, a small hans driven one

 

[Mr Efficiency]

I mean you can used a winch to roll it up the beach. I once winched a boat and trailer that got stuck in mud at a boat ramp, using a nearby light pole, it might have got embarrassing if it had pulled the pole over !

 

[TANSL]

I think you should treat the whole boat as a whole and, before deciding whether the Skota 20 meets your needs or not, clearly define what those needs are. The facilities that you have at your disposal, and your skills, to build the boat will also greatly condition the type of hull that interests you the most.
Since you don't want to reach high speeds, wave formation, which is the most important part of drag, doesn't matter much. The friction resistance in a catamaran will be greater than in a multihull but other considerations will lead you to decide on a catamaran. If the hulls are parallelepipedic or not, they don't have to increase drag much but they do increase wave formation. Since this is not important, you are free to choose your bow shapes.
Design a layout that allows you to carry everything you need on board, within the limitations that you want to impose in terms of overall length and maximum beam, and from there you can begin to define the boat. All the rest is an opinion for the sake of opinion without any real basis.
Decide if you want to build exactly the boat that interests you, the one that you like the most, or if, on the contrary, you have to adapt your needs to existing boats. For this decision you do not need to know dimensions, weights, or anything similar. Simply evaluate the prices and the satisfaction that each of these options can bring you.

 

[fallguy]

@TANSL remarks are good

keep finessing the requirements and the right boat will happen

the Skoota 20 is a really cool little boat that delivers big on seaworthiness, requires low horsepower, but would be the work to build two hulls and the folding beams and fancier trailer; the platform would be very stable for friends enjoying time at sea

 

[Amphicat]

I mean you can used a winch to roll it up the beach.

Ooooh! That makes so much sense!
I'll think about that!

...and your skills, to build the boat will also greatly condition the type of hull that interests you the most.

Oh you're so right. Easy does it. Even if it could be a little more effective, a quicker build is better. Find the best trade-off.

clearly define what those needs are

So I should make a floorplan of the deck I guess. Divide the deck in sections and distribute the weight evenly.
Then decide the hulls, right?

keep finessing the requirements and the right boat will happen

You're right. I'm really really far in the requirements of the boats. I just need some help drawing the design now I think.


Well. I'm trying to find a decent program so I can sketch the floor plan of the boat.
Anyone has any good free options?

 

[bajansailor]

Well. I'm trying to find a decent program so I can sketch the floor plan of the boat.
Anyone has any good free options?

The easiest, and (almost) free option is some sheets of A3 paper, a ruler, a couple of soft (at least 2B) pencils, an eraser and a standard set of French curves (you probably had a set at school?).
Try it. I think you will have fun drawing it out manually.

Mathias, please bear in mind that if you buy a set of plans for a boat, not only are you buying the plans (which will be comprehensive), you are also utilising the knowledge and experience of the designer, along with a guarantee that if you build the boat as per the drawings it will do the job that it was designed to do.

There is no shame in buying a set of plans - the cost might be 1% of the cost of building the boat.
You will learn a lot from having a set of plans to work with - and if you really want to change something, modify it because your idea is better, then you can do it (although it might be a good idea to run it past the folk on here first).

 

[TANSL]

So I should make a floorplan of the deck

A detailed SOR, an elevation and a plant of the boat, even by hand, will be enough to start.

 

[fallguy]

Do not design a boat. You will end up disappointed.

Buy a suitable design and then do the layup (on changeables) with paper and pencil and eraser.

 

[YoungGrumpy]

I do not want to express my opinion on the amateur design, but most certainly, studying what was done by the Pros would be very helpful. So, purchasing the study plans seems to me as a very good, and not that expensive way.
Did you look at the work of the Berndt Kohler?
POWERcats https://ikarus342000.com/POWERcats.htm
He has strong opinion on the topic of materials, but is well regarded in the field.

 

[bajansailor]

Here is a very neat and simple application of beaching wheels to a small coastal cruiser by @Tiny Turnip
Around Jura by half https://www.flickr.com/photos/tinyturnip/albums/72157719822032375/with/51422335952/

However they do not have the luxury of a waterborne caravan; rather, they pitch a tent ashore when they arrive at their destination (and wheel the boat up the beach).
Mathias, you could have a lot of fun with a little boat - it does not have to have sails, it could be just power (perhaps even with an electric motor?) .