New power cat - help please!

I found a 21' phantom with a water line, they obviously sit stern down at rest as the water line on this one fitted with a 200hp two stroke is 400mm deep at the transom but only extends 17' down the boat.
I feel I might need to raise the height of the proposed tunnel to 500mm, I realise this will make the deck pretty high & I will need to add more freeboard but better safe than sorry?

 

It does'nt matter because very soon the British government will ban all offshore accounts so what you need is a fishing boat to catch food or leave the Island unless you grow tomatos.

 

Well thanks!

The boat will be used for fishing & I am not involved in finance anyway!!

If the banking shuts down then the thousands of non Guernsey workers will leave our Island & peace & tranquility will return, house prices will go back to acceptable levels & hopefully tourism will once again become important.

 

You mean you want more peace and tranquility?

Well there is only finance or turning round those little propellor air planes that land there, oh and tomatoes and bed and breakfast. Sorry I spelt tomatoes incorrectly in my last post but I guess you need to know more about power cats to get out of there. I stand aside,---please continue.

 

foxfish - perhaps you should calculate the tunnel dimensions that will work out best for your desired application?

 

Jimboat, Greatfull thanks

 

Jim, I am not looking for a high power boat, I would like to have a stable platform, 20 -25mph, & a flat self draining deck.
My hobby is spearfishing, cats seem to offer plenty of benifits for my proposed use - ease of getting in & out of the water & ease of drying out on the little sandy islets that surround our island.
We are free divers so no heavy tanks or tons of equipment.
Anyhow, I have built a mock up tunnel & the boat looks huge!
I feel I am getting closer to the dimension now = 800mm wide 500mm deep. This however, only leaves 250mm from top of deck to original gunwale height so I will have to build a pretty high combing to get 600mm free board.

 

foxfish - sounds like a fun project. hopefully you will have sufficient sponson pad lift to plane the hull at your desired speeds.

 

Hmm - Jim I really appreciate your input but I am not sure if I understand what you mean?
It is not easy to take pics from my little allocated corner of the garden but I will try & get a human in to add perspective next time.

 

it seemed that you were establishing your hull design bottoms by observing other production boat dimensions. My point was that it is usually best to set your design and dimensions, especially for running (Lift) surfaces, based on your hulls own specific needs. To achieve your desired "stable hull design capable of 20-25mph", your outboard sponsons (providing most of your lifting surfaces) will need to be sufficient to generate the required Lift to plane your boat efficiently, with stable CofG. This can be calculated for your specific boat's planned weight, hp, payload, velocity, etc.

 

There are obversely a few issues to contend with most of which I am only just grasping as I go along!
I am concerned with the tunnel height, the phantom hull drew 400mm at the transom but the cat will be heavier & shorter so the cat will draw more water.
But I thought I could move some weight forward to cantilever the bow a little however I can now see I will need to raise the tunnel even higher to get enough clearance under the tunnel.
However the higher the tunnel the higher the deck & gunnels the more the boat will weigh!
Of course the boat will lift when planning & there will be more clearance under the tunnel but will the boat ever plane because the sponsons will be to deep because of the increased draught
I know some of you guys were sceptical about my project & I can see why as we go along but never say die (not yet) any more thought from anyone will always we appreciated.

 

foxfish - i'm not sceptical at all, about your project! i am just partial to doing the engineering analysis so that i know how my design will perform before i start building or modifying.

your 'static' CofG will be the least of your areas of concern, although at-rest draft is a needed area of attention to be sure. Your dynamic CofG (much different) will determine how your boat performs. Your sponson design will determine whether you get the Lift that you need to generate, in order to attain the speed that you want.

 

So the sponson shape & size that is presently built into the hull may need to be modified - I guess that I could make them wider but it would be difficult to move their position? I guess that at least it would be possible to alter the sponsons via a course of trial & error?
http://www.boatdesign.net/forums/attachments/multihulls/29863d1236440529-new-power-cat-help-

 

foxfish - it's hard to know what you need as design without doing the calculations. i don't know what your all-up (gross) weight is. Your existing sponson template appears to have pads that are about 600mm wide at a 16 degree deadrise. When you estimate your all-up weight and planned HP, you can estimate the Lift required to plane your hull at your desired 25mph, and compare this to the calculated available hydrodynamic Lift from your sponson design. (the other approach is to build the full boat, then try it out, then take it apart to modify it to requirements).

 

Well being more practical than academic your second solution sounds more appealing
As I already own a newish Honda 50, I plan to buy another one so the boat will have 100hp.
Jim I realise that without any real details you can only help me to a certain degree but could you speculate just how small a tunnel might work or just give me the low down on why I need to have a wide tunnel?
I would be surprised if the boat did not need some mods after its sea trial & maybe a little disappointed if it did not!
PS I will try & get a revised & more detailed template up tomorrow.