Small Displacement powercat

Wonder if anbody out their might be able to answer a question?
I'm looking at a project to build a small displacement cat (25'Lx11'B)
as light as possible with as small outboards as I can to do about 15Knots max
It would be used in protected waters as a day boat seat about 4-6 people
comfortably a little like a big 'Tinny'
sea conditions expected 1.5M sharp chop in relatively shallow estuary.

Currebtly envisage semi circular awthwartship sections but what should the longitudinal underwater profile be? there seems to be a range of opinions based on what I've seen on my web searches.

Also wonder what the best construction method might be Duflex, foam glass
stitch & glue ply?

Any thoughts


thanks

 

I would suggest using a circular arc profile; in other words, design with a slight rocker and with the bow slightly curved upwards.

 

Thanks for that 'Dingo" but I was seeking some further understanding.
Your suggestion would seem to follow the style used by Grainger(Raider) & Pescott to name a few but I have seen others like the power cats of Schionning & Kerr who show a deeper forefoot with a very flat keel line and I wondered if the difference was
1. size related - getting volume into the hull without compromising beam as these are relatively small boats
2. Ability to tack in the case of the 'Graingers'( obviously not an issue here but may)
3. Broaching or wave related issues
4. Something entirely different
I suspect that the shape you describe would be the most slippery as it looks like it would have less wetted area but I am going well beyond my undestanding here.


Baycat

 

Canoe shape

My logic was to form the two demihulls as close to a standard racing canoe shape as possible, since they are designed as optimized displacement vessels. Or pick a displacement sailboat (mono or cat) and see how the keel-line moves in an 'arc'. These vessels already have been designed with the goal of minimizing hull resistance at displacement speeds.

 

Phil Bolger designed a trimaran named the Bantam 16/20 for rivers and for about the speeds you want. See a version here: http://www.treasurecoasthouseboatrentals.com/

Bolger being Bolger, the hulls are flat bottomed. 15 Kts is twice hull speed for a 25 ft boat. If you really want to go that fast, I think you want a firm-bilged hull shape.

 

You ask for a very reasonnable boat.

Go to the site of the naval architect Kurt Hughes, you'll see 2 small power catamaran which give you ideas. These cats can be depowered. Hughes has also a good documentation about cylinder mold (see below); good plans are the best beginning for success.

For a power cat of only 15 knots speed I'll take hulls similar to a fast sail cat like the Tornado (you can get the plans at the Tornado association for a few bucks). The initial drag is very low, and the power needed will be small. The lone big difference is a higher freeboard at the bow.

Such hulls are very well suited for compounded plywood (the Gougeon Brothers book will tell you all about compounded plywood) or cylider mod, the Hughes' (and me) favourite method. It's very fast and pretty cheap compared to composites.

If the hulls do not remain in water and are used below 20 knots in protected waters, skin door (indonesian 3mm 3 equal plies meranti plywood) in two or three thichnesses (so 6 to 9mm) make a good core for a exterior fiberglassing.
Epoxy resin (which not smell...) is needed.

Raka has a fairly good and forgiving resin easy to sand (try to sand a very hard and/or clogging resin...) with very good prices for all the materials, but you are a bit far from the States.

Take samples of resin, heavily coat some wood, let it cure 2 days, wash it with water and soap and sand it with a #80. It must make without effort a fine powder and not clog the sand paper.

Irrigation aluminium pipes may make nice beams and the whole project can be kept dirt cheap but effective and nice with a good strength and duration.

Simple materials for a simple boat... Good luck.

 

Here is a Catamaran I designed for a shool project. The cat should be driven human power and cary 6 students. The cat is build with long foam stripps and fiberglas/polyester.

The hulls are basicly two canoe hulls

http://www.friedeldesign.se/katamaran.htm

 

Huges has a nice litle power cat

http://www.multihulldesigns.com/stock/21cat.html.

 

thanks for your input everyone its appreciated - Hans & Iian voyager , yes I had seen the Kurt Hughes site and thought the slightly larger 28' craft was closer to my concept(in fact I found a good site which showed construction).
I used to own a tornado in the mid 70's so am familiar with this shape also
(also remember burying the bows in 25knots - but thats another story!) . I have been using the free download program 'Hulls' to look at shapes and displacements - if you put in enough chines it approximates a round bilge demi hull. The displacements I have been looking at are around 1800lbs (crew fuel motors etc) this seems to leave about 4-500lbs for basic hulls does anybody have a feel if this is achievable based on 'cylinder mould' technique or similiar? I also wonder if their is a view on placement of outboards - I have seen some located inboard of hulls and forward of transome on nacels which might alleviate cavitation perhaps?

cheers

 

I drew this little beast a few years back. Should oughta go about 20 knots with a 50hp diesel in each hull

 

My Power Cat is a bit short - but the project is really practical - my site is www.davesflatcat.com

If you power each hull - and are not looking for the absolute shoal draft - two 4-stroke forties or fifties will zip you along nicely.

 

Baycat, keep it simple...

For a project of week end boat as yours keep it simple and cheap.

Two 50 HP four stroke outboards (Yamaha or Honda are equally good) which are silent, simple to maintain and can be found at good price (for far less than one 50 HP diesel...) A 50 HP FS outboard weights about 110 kg (240 pounds) and needs a simple fuel tank. Bolt them on the hulls transoms and do not worry about cavitation, slim displacement hulls at 15/20 knots do no make big waves, but it's good to match correctly the propellers.

With 100 HP you'll have plenty of power, and probably you'll be cruising at 1/2 throttle. Well maintained the outboards will give you long years of service.

Basically your cat is a platform bolted on 2 hulls and keep it that way. The platform must be high above water so everybody will stay dry. The most common defect on power cats is a too low platform which hits the waves, stopping the boat and splahing everywhere. 45 to 60 cm between water and the bottom of the platform is a good measure. The platform must not extend too far forward or it will slam at the smallest chop. The platform may be structural and act as the beam. In this case the cat may be dismountable; the platform is bolted on the hulls.

Keep the platform flat, so water won't stay and with hulls well divided by watertight compartments the whole boat is almost unsinkable.

A such cat is multitasks, it can be used with several seats, or for sport fishing, or as dive boat or anything you can imagine, as simply you can bolt what you need on the platform.

Some tech precisions: for 15/20 knots hulls of 1/10 ratio, Prismatic coefficient of about 0.57 to 0.60, LCG at 60% in full displacement, transoms of about 75 to 80 % of the max hull widths and you'll be good. Total width of about 40 to 45 % of the total length. It gives a very stable boat which can be driven with only the engines throttles.

That gives for a 28 feet cat;

Length 8.54m. 0.85 m wide hulls at LWL. LWL about 8 to 8.54m. Transoms 0.68 cm wide (just the width of the outboard). Forward freeboard about 0.85m at least.

Total width 3.41m. If you trailer it may be a problem as generally it's limited to 2.44 m.

A very fast calculation with semi-round max underwater section gives a total possible displacement of 2.6 metric tons. So the weight is not a great concern.
6 persons adult 480kg
2 engines 250 kg
Gas 100 kg
Seats etc 300 kg
Misc (anchors etc) 150 kg
It remains more than 1300 kg for the hulls and platform...

Correctly built you can go under 700 kg, keeping it strong.

I throw the ciphers while calculating on a paper with a pencil. I may be wrong...

Scantlings: 9mm 7 plies marine plywood, 6 oz glass is enough for your program for the hull decks and top sides. At the bottom a 12 to 15 mm light wood with biaxial glass is more than confortable. It could be built far lighter but with more internal structure and more concern about dings and scratches.

That must be checked and rechecked before building !!!!.

A last counsel, follow the MISS (Make It Simple and Stupid) principle... do not go in complications and gadgets.

Good luck, you have a nice simple project very rewarding while finished.

A profile like Jet Services would be a good one. I join a pic of this very fast cat.