dory catamaran hull

My 2nd attempt at an Electric Catamaran

Hi Guys,
I found this thread after already starting a new one with a similar name in the Boat Design section. I have a very similar desire to find the "Ultimate Hull Design" for my update to my old 18' Electric Catamaran that I built 18 years ago. I was able to accomplish a great deal with this simple craft and other Pedal Powered boats of my past. I would like help with my new hull design, which in fact is a flat sectioned, hard chined design. I have attached a DXF file that you might understand showing my new intent and some of my ideas. Please consider my request for a "Pushing the Limits" on a 20' hull Utility Catamaran.

If I'm stepping on your Thread in a wrong way, please lead me to a more suitable one. You can see the one that I just started in the Boat Design area.

Also know that I have not yet figured how to post pictures, as everything I've tried either gets error messages, or the pics are miniature. Any help here would also be appreciated.
Regards,
John Vitsur

Thanks in advance for any help.
Regards,
John Vitsur

 

I refer to my boats as stabilised monohulls. They are quite different to a sailing trimaran. I was making the comparison between my main hull and the amas on BMW Oracle. If you took a section through my main hull and an ama on BMO it would be very similar shape.

So my aim is to operate with the stabilisers rarely touching the water.

On a sailing tri the ama becomes the main working hull once the boat is powered up and it is desirable to have it long and slender with a flat bottom as this will give the lowest drag over the widest range of conditions. Whether you have a canoe stern or a transom will depend on the speed potential.

Rick W

 

There are not many who do comprehensive performance comparison on different boats using their own engine. I have many accumulated hours on the boats with the speed versus my heart rate. I literally have a feel for the performance difference like very few others.

There was an interesting BBC show that aired on the ABC recently where 80 cyclists powered a house for a day. The occupants knew they were involved in an experiment but did not know what it involved. There is an extract on YouTube here:
http://www.youtube.com/watch?v=C93cL_zDVIM
It gives insight into how easily power can be taken for granted.

Rick W

 

John
If you attach an image it will show as a thumbnail on the bottom of the post. Clicking on it gives the full image in another window. It is the best way to show photos as ones in the text slow down the page loading.

The hull looks quite good. It has many of the features that have been discussed in other threads on efficient hulls.

To determine the lowest drag hull/s you need to nominate the intended displacement and the target speed or normal cruising power. An idea of the intended operating conditions would be helpful as well.

Something to be aware of is that a stabilised monohull will require about 40% less power than a catamaran for any given displacement and speed if you do not have a length constraint. The cat configuration gets more attractive if there is a length constraint.


Rick W

 

To Rich W

Greetings Rick,
I have indeed read hundreds (I Believe) of your posts, and I can't agree with you more about Long Monohulls. I too have found interesting ways of measuring power and performance. My decision to go Catamaran is simply based on Absolute Stability and large usable deckspace in the smallest length possible. My past boats have been so stable that you can walk out near to the end of either hull with minimum tilt. This easily allows more than one person abord without crowding and of course trailering and fitting into a garage. My most important parameter is my absolute passion for Multihulls and their gracefull yet formidable appearance and function.

My hopes are to keep this boat at around 20', eventually with a new bridge deck, go full trailerable width, low in the water for minimum windage, able to carry up to 4 average adults (but designed more for 2), and be rugged enough to handle choppy waters in Bay areas as my past boats have been. the hulls are sealed floats that allow sea water to fall off of them as sailing Catamarans.

My challange is naturally the weight working against the BLR of the hulls, and hoping that by shorter hulls farther apart, there will be little interaction with their wakes.

This leaves me with Hull shape questions and Efficient but simple propulsion. I have been very surprised at what a Trolling motor can do, and hope as you mentioned in my thread, that more advanced propellers can be used. I know of the advantage of the Flex-shafts benefitting the aperature drag, but I hope to fair the Trolling motor body into the aft section of the Hulls.

Some Questions I have are things like Partially submerged Transoms on fast moving Narrow hulls to break from the wave. Knowing that the Drive unit causes some Slipstream in it's area, can this be used to an advantage in the hull's shape? Though I tried this to some extent in a later design, can air be drawn into the aft area of the hulls to help with drag? My theory on this was as the water closes up around the hull after the Midship section, the air bubbles would allow the water to break free of the hull and go where it wants to at a given speed, and not be so determined by the final shape of the hull's Lines. Also, I know how Rocker works in Hull Lines and in turning, but with twin motors with reversing capabilities, how much is needed, or should I bury the entire hull and reduce the BLR?

I have many more questions that I have pondered, but I'll likely get to those later.

Thanks again for the input, and I hope you can tell me what you know about catamarans.
Regards,
John

 

John
I have made some specific comment back on the thread you started regarding the hull.

It only takes a few minutes to determine the lowest drag hull shape for your requirements and constraints.

Rick W

 

Thanks Rick,

I'll try to put together my parameters so that you can help with the energy calcs. Still, any help with the Hull Shape benefits would be great as that is where I am presently in this updated design project. Please advise.
John