Twin stern tube propeller with only one engine

Hello friends.

I intend to build a 12X3 feet catamaran and I want to keep the rudder in the center of the hulls.
The idea is to use a 15 hp imboard engine to move the two propellers. Do I need to pay attention
to any details of this assembly?

(( Sorry for the sketch. I'm better with saws, grease and tools. ))

Tks. Valter

 

Hi Valter,
What is the intended usage of this catamaran?
And how did you come to the conclusion that a vessel like this will meet the usage requirements?
Do you have any drawings or sketches showing what the design of it will look like?
Re how it is 12' x 3', I presume that the 3' dimension is the width of each hull, and 12' is the length?
With the engine on the bridgedeck of the cat, that will be a complicated arrangement with your chain drive to connect to the shafts in the hulls.
And where will you sit on this boat?

 

Hi Valter.f, it looks fraught with problems. Real Hondas are very good machinery.
Copies (called Chondas), which appear identical are renown for parts from one not fitting another identical one due to poor quality control, and parts coming from many small factories according to cheapest price. The agricultural chain and sprockets will be very noisy and vibrate, and will need a take-up system to keep it sort of taught and counter its wear. A shroud will be needed around the chain for safety, and to contain flinging oil, as the chain will need lubrication of some sort. A metal frame will probably be needed to support the motor and your shaft bearings in fixed alignment positions. What supports the propeller shafts ends ? Twin skegs will add extra weight.
12 x 3 feet is very small, (is that 1 or 2 hulls), will it buoyantly support a lot of hardware and a person ? Is the engine designed to work at an angle regards sump oil pickup. Those short shafts will need a big angle, or hulls shaped to take them in a tunnel, meaning less buoyancy at your engine heavy stern. The fuel tank should be shifted to reduce chances of fire, especially when refueling.

What type of boating do you really want to do ? Show a photo or drawing of your proposed boat so better suggestions can be made. Do you really need 15Hp on a 12 foot boat ? Does this agricultural assembly come with a local supplier, good parts availability, and a lifetime warranty (about 6 months use I'd give it). The propellers look plastic, if so get some spares.

It would be much easier to go to twin rudders, and a single central prop, or small outboard motor even easier.
With twin rudders you have a spare ready to use if one breaks or is lost. With twin props you have added complexity, weight, and potential points of failure. Also double your chances of striking objects and the bottom with propellers.

I wouldn't do it, but if for instance you got everything for free, and have too much time, go for it.

 

A single center rudder on a catamaran with drives on each hull?
What advantage will that be?
Placing the rudder(s) in the prop wash will allow significantly smaller rudders with likely much better control.
I would like to see more details of your project.
Please sketch up your ideas, the rest of us cannot see inside your brain!

 

Hi Bajan
Really missing details of the boat. Sorry.
The use is for fishing in sheltered places. The measurements are correct (12x3 feet).
It is narrow because it is a kayak-type catamaran with its two hulls almost together.
In the attachment is the shape. The intention is to transport it on the roof of my car,
a mini van. Sorry for the sketches. They suck, I know. (||>=

 

Hi Seasquirt.
Perfect! I noticed that you managed, even with little information, to visualize the entire project in its smallest details.
I didn't anticipate the noise of the chains, the lubricant needed for them and really a 15 hp engine is a lot for what is practically a kayak, just a little wider. I thought about supersizing because
I would use two propellers, but I was already changing my mind, thinking of just one propeller and a maximum engine of 7hp.
Using the center of the stern I will hardly be able to get the kick-up system on the rudder and propeller to reach the beach with this small boat, but I don't see any other way out. I wouldn't want to
use the "rabetas" (sterndrives) we have here that require a stationary engine above the stern. I want it inside the boat, on the deck, but having the option of moving the set to dock on the beach, which is more
difficult. Should I opt for a fixed system. A feather.
(see new images above)

Thanks for the precious help.

 

Hello Kapnd!
Good news to know you can't see inside my brain. It would be embarrassing...
Really the information about the boat was little, sorry. I've attached more pictures and I really think I'm going to follow this idea of twin rudders with a central propeller.
I would just like to keep my engine on deck, have a kick-up system for its propeller so that I can dock on the beach. See the new images. Thank you friend.

 

Re the drawing that you have just added to your original post, this looks more like a trimaran shape with three 'hulls', rather than a catamaran?

Having an overall beam of only 3', with a high centre of gravity (re how you and the engine are sitting 'on' the deck, rather than 'in' the hull, like a kayak) suggests to me that this boat will be rather tender (ie prone to capsize), never mind that it is a catamaran (or trimaran).

 

Hey guys.

What do you think of this sketch where the engine would be below a seat,
with kick-up system and rudders in line with the hull.
These rudders would be submerged only four or six inches,
would the draft of this boat be made of styrofoam and fiberglass?

Tks.

 

It will be as tall as a kayak, 30/35 centimeters and 20% wider. Perhaps because it is made of styrofoam and fiber, lighter than these polymers, its draft can actually be smaller,
but we are talking about 20 kilos of engine plus about 100 between pilot and so on. I'm in doubt. This shape tends to be very safe.
I cut mold to actual size and to a template. Maybe it's better to build a good model first. What do you think?

 

Baja...
In time: Forgot to mention: Perhaps your perception of exaggerated height is because this pattern is in its natural size for a skiff using the "female" pattern.
In the case of this catamaran (or trimaran) in the design, the height of the side would be much reduced compared to what is there. the measurements would
be 12 feet x 3 feet x 1 foot on the side of the entire side, lowering the center a lot in relation to the image. I don't know if you understand?

 

I will repeat again my original questions :
What is the intended usage of this catamaran?
And how did you come to the conclusion that a vessel like this will meet the usage requirements?

Have you calculated how much buoyancy you have in the hull(s) at the intended draft?

Re stability, your craft would be more stable if you omitted the central section of immersed hull, and instead have it as a pure catamaran.
Even so, I would not want to have one of those petrol engines on deck (or even in the hull somehow).
Those plastic kayaks can still be easily capsized.

Have you already bought that 15 hp Honda engine?
If not, then it might be a safer investment if you purchase a 4 hp 'conventional' outboard motor instead - this will be more than enough power for your craft.

 

You might like to look at a website “mud skipper.com”.
They offer lightweight propulsion packages that are customizable to your needs.

 

Ok. Tks Kapnd.

 

Hello Bajan.

I hope now it's easier to understand.

The question of transporting on the roof of the car
makes me prefer the kayak, but the skiff will be
one option.
I can not opt for outboard motor, need
be imboard because the price difference between
both are 10x1 and this skiff needs to be
low cost. I still don't know how I'm going to install it,
but I will opt for one propeller shaft and two rudders.

I thank you and all my friends for answering my questions
about the number of propellers. Maybe I'll post a video on
YT building these two thumbnails and then one of the options
in full size. I'll keep you posted. Thank you very much.