New power cat - help please!

[foxfish]

Hi, I would like to build a small cat, about 5mts would be ideal for my use.
This will not be my first attempt http://www.boatdesign.net/forums/showthread.php?t=23139 but I would like to get it right this time!!
So I am looking for advice, building methods, plans or any tips ect. I will power the boat with two Honda 50hp motors but will be happy cruising at 20kns. I did train as a boat builder but that was 30 years ago - still I am happy to work with either glass or wood to a reasonable level.
Mart.

[Fanie]

Hi Mart, befor jumping in the deep and and start waving the brush around, do some research on hull shapes, types and collect as many pictures as you can. All these would give you a good idea of what is out there and which is considered the best. One also pick up lots of tricks and get some good ideas.

I would also look at displacement hulls as to planing hulls...

[northerncat]

go for a displaning type hull that way you get speed and economy/low power
something like this but scaled down to 5m probably wouldnt need more than 2 20hp's
http://www.lizardyachts.com.au/designs/lizard_7-2.html
http://i38.tinypic.com/2im8gsx.jpg
sean

[northerncat]

i should mention it cruises at 18 knots goes flat out at 23knots and burns 10l an hour at 18knots
sean

[foxfish]

Interesting! So a displacement catamaran can still get good speed?
I will be doing plenty revision before I start the build but does anyone know of any plans that I can study?

[Guest625101138]

There are some nice big cats around and the lizard cats look nice. I guess you need to get hold of plans for glass construction. Many of the designers are looking at efficient displacement hulls now. So a matter of having a good look around.

I think a boat that will punch through heavy chop at 18 knots is very capable. Probably average similar speed to faster boats that have to slow down once they get into heavy going. The thing about these cats is that they sit well at these speeds. They do not need to be doing 30 knots to level out.

Rick W.

[Fanie]

If I ever make myself another power boat, it will have a displacement hull, be longer (loa) and wider (boa), similar to sailing catamarans and trailerable.

Our last trip at Jozini we had to go out of the water due to the wind pushing up waves every day. We were there on my friend's bass boat, but even the small skiboats abandonned... similar to that one in Sean's picture.

Size does count - Godzilla.

[foxfish]

What I am interested in is the underwater shape, the transom shape & the distance between the hulls etc.
I have a 17' dory (Nab 17') in my shed & I am thinking of doing a similar conversion on this boat as I carried out on the aluminium dingy. I think I can build two hull underneath the dory using the same methods but I dont intend to start until I have more info on hull shape.
There are some pics of my conversion method on page three - http://www.boatdesign.net/forums/showthread.php?t=23139&page=3

[northerncat]

my next project will probably be one of these lizards
sean

[foxfish]

OK I am convinced I need a displacement design, I now need search for inspiration - I found this http://powercatamaran.typepad.com/homeport/custom_designs/

[Fanie]

You have to make up your own mind as to what will work best for you, our say-so is just to get you to look around a bit.

[foxfish]

Ok, well I guess that is one of the problems of an open, international forum!
What I meant was - thanks for the heads up on displacement hull catamarans, I think they would suit my needs very well.
What I now need is some basic info on how to design & build such a boat.
I would appreciate any basic info on how to start the build process? :) :confused:

[Fanie]

Do a google for catamaran build projects... many articles and pictures.

[foxfish]

Hmm I have been googleing for hours! Not many small power cats that I can find? However is would appear to me that a displacement cat requires quite a gap between the water line & the under deck?

[Fanie]

Only for hours ? You just skimming the surface. Every now and again you discover something new that will bugger up your preconceived ideas :rolleyes: ... and get the creative juices flowing anew.

[foxfish]

Well I have been looking around at various local boats for inspiration & I came across an "x racing 20' x 7' carbon fibre cat". The boat looks like a conventional power boat that has been cut in half, ie the inner side of the hulls are completely straight, in fact it is an American boat that was once powered by twin 200hp motors & had a top speed of 100mph.
My questions are - can I fit two 50hp motors & expect the boat to work OK (20 -30mph would be great) & can I use polyester resin on any mods I want to carry out?
I would like to convert the boat to suit my needs as a open deck centre console boat, I only consider this as the cat is for sale at a very good price (very cheap)

[Guest625101138]

Have you got a photo?

You could reckon required power would be roughly the cube of speed if it does not get heavier. If original with 400HP did 100mph then 100HP should give around 60mph.

If you are making big changes to the deck then you would need to determine what role the current deck plays in the structure. It could be critical.

If existing hull is carbon/epoxy then it is best to stick with these materials for any structural elements.

Rick W

[northerncat]

depends on the weight i would think although a carbon 20' stands a good chance of being a light boat
sean

[foxfish]

Here is the boat, I actually measured it at 21' 7'6''.

[Guest625101138]

Fox
That looks exactly like what you wanted. I would be reluctant to make radical changes to the deck.

I expect it will be light and it sits low for little windage so it will get along with 100HP. The predicted 60mph might not be far off.

Just need to make a careful inspection to ensure there is nothing cracking up. You should be able to get some history on the boat.

Rick W

[brian eiland]

Quite right Rick I do need a new design
Dear Foxfish, I've not had time to read your subject threads yet, but I do have an idea to pass before you...its a RIB with a unique hull...but you might not have to build it as a RIB initially.

Please send me a private email as I don't wish to post it on the forum at this time.

brian.eiland@gmail.com

[foxfish]

Still not really going my way! That boat has been for sale for two years but when I phoned the chap he said it has just been sold!!
Brain message sent.

[foxfish]

I have been looking at all the cats I can find in my area & it would seem that most of the commercial cats have the same design. The two most common cats, Cheater & Gemini are very similar & their sea keeping is legendary around these parts.
Do you think I could simply scale down this basic square sided shape & build a 5mt version?
http://www.geminiworkboats.com/STANDARD_GFO_7_8.pdf
I would however be using two motors.

[Guest625101138]

I have been looking at all the cats I can find in my area & it would seem that most of the commercial cats have the same design. The two most common cats, Cheater & Gemini are very similar & their sea keeping is legendary around these parts.
Do you think I could simply scale down this basic square sided shape & build a 5mt version?
http://www.geminiworkboats.com/STANDARD_GFO_7_8.pdf
I would however be using two motors.

These look good shapes for their size suited to moderate speed. Reducing the size by just scaling would result in a deep entry for the type of boat you want and weight further forward than needed.

I really liked the shape of the black one. If you were going to scale one for a smaller size then I believe this would be better for the speed of boat you want.

Maybe something in between the work boat and the racer.

These boats are well regarded in Oz:
http://www.markhammarine.com.au/cats_pleasure.html

Rick W.

[foxfish]

I see where you are comming from Rick but the square sides design is so basic & simple, I could easily build a one-off boat using basic frames & glassed over ply.
However if you could look at my purposed schematic & point me in the right direction I would really appreciate your help.
I only really need help with the below deck shape but as my two bottom pics show! however they are purely guess work! Any comments would be a help.
I will use two 50 hp motors because I have them but smooth ridding is more important that outright speed.

[Guest625101138]

I see where you are comming from Rick but the square sides design is so basic & simple, I could easily build a one-off boat using basic frames & glassed over ply.
However if you could look at my purposed schematic & point me in the right direction I would really appreciate your help.
I only really need help with the below deck shape but as my two bottom pics show! however they are purely guess work! Any comments would be a help.
I will use two 50 hp motors because I have them but smooth ridding is more important that outright speed.

I would go with a bit more "V" just to help with lift in the turns. Maybe even offset the V like the black hull.

Soft riding will be a combination of the V and the width of the hull - narrower is softer ride. If you go too narrow with the hull then it will be harder to get on the plane.

Narrow deep hulls push easily up to maybe 15kts but will be hard to plane. Above this speed for the length you have you want a planing hull.

You could have asymmetric hull like the black boat with a single bottom piece at 15 degrees and fit planing strake. This may be a simpler build than you currently have.

Best to make some cardboard models to get an idea of the hull
development.

You should shop around for plans of something that has been tested. The structure joining the two hulls needs proper design to counter the torsional loads it is subjected to.

Rick W.

[brian eiland]

I might suggest you have a look at this hull form for your experiments...shouldn't be too difficult to build:

...the 'Sea Knife' concept
http://www.yachtforums.com/forums/16028-post47.html
A boat capable of cutting through rough waves at high speed with astonishing stability has a hull provided with a flat planing surface, which in plan is the shape of a thin wedge or delta. The sides of the boat rise upwardly and outwardly with a simple concave curvature from the two edges leading from a knife edge bow at the point of the wedge. The slender wedge shape moving through the water at high speeds develops continuous spray sheets up its sides, which are intercepted by the outwardly curving portions of the hull sides. Spray rails or deflectors may also be utilized to intercept the spray sheet, such deflectors being inclined at a small angle to the bottom of the planing surface. The knife edge bow rises upwardly and forwardly with a concave curvature from the point of the wedge and eventually terminates in a forwardly sloped bow transom. A keel skeg minimizes side-slipping. A stern transom, which rises substantially perpendicular to the trailing edge of the delta or wedge may have a rearwardly extending bustle secured thereto for buoyancy roll stability at low weeds.

http://www.yachtforums.com/forums/16010-post46.html

http://www.yachtforums.com/forums/16033-post48.html (http://www.yachtforums.com/forums/16033-post48.html)

http://www.yachtforums.com/forums/24306-post65.html

Peter Payne was a very learned mathmatican who had a very real interest in boats as well as other subjects. His attempts to define planing and other hydrodynamic phenomena mathmatically is difficult reading for most of us, but he made very real test on actual prototype vessels to confirm some of his theory.

To quote a web reference,"Another type of Payne high speed boat achieves results in a different way. The SeaKnife hull form is a supercritical planing hull that cuts through waves instead of riding over them. This reduces the vertical pounding accelerations typical of conventional planing hulls by a factor of more than ten. There are a number of SeaKnife boats in operation and on the drawing boards. Designs include sport and racing boats, and military patrol boats. Another planing hull form is the WaveStrider, which has been produced as a 24-foot boat for the Navy, and is in production as a high speed ferry. (The stealth boat that appears in the new James Bond movie "Tomorrow Never Dies" is a WaveStrider.) He also developed the Air Lubricated Planing Hull (ALPH). A boat of this type is the Hydrotrac, noted as being the world's fastest production boat for a given power. All of his boat designs were tested in the Chesapeake Bay, near Annapolis, and the nearby Severn and Magothy Rivers. High speed planing hull theory receives a detailed presentation in Peter's book, Design of High Speed Boats, Volume 1: Planing. His computer program, BOAT3D, is used by many to model and design high speed boats."


I will post some pictures sometime near future when I receive something presentable. I happen to have been in the Chesapeake area when he was conducting some of his early test on SeaKnife and I was also working with the Navy at the time. I didn't pay much attention to his 'far out' hull designs at the time, but I recently sought to go back and look at his concepts again as a result of both Bladerunner's work and Payne's concept. Mr Payne passed away a few years ago and I am talking with his widow and his son.

[foxfish]

Thanks guys I appreciate your replies.
Brian there are some great pics amongst you links & I enjoyed reading about your findings but that design would be very difficult to copy without any actual plans or dimensions. I have built many boats & trained as a boat builder & although I might well be capable of building such an amazing boat, I dont really want to spend the next two years building complicated compound curves that require mutable formers & tiny strip planking.
Rick, I have searched the net for weeks now - if I could find suitable plans I would buy them.
I am confident with the "above water" design & have no worries about the structural integrity it is just the underwater hull shape that concerns me.
From my very limited experience of building power cats I can conclude that many shapes will in fact work however I would like one that works well.
My criteria -
A catamaran design to be very stable at rest & for beaching on small sandy bays, shallow draught & low free-board to avoid fast drifting.
Twin 50hp motors (not necessary to use full power but I have the motors)
4.9 x 2.1 is ideal because of mooring charges & plywood sheet size.
25 - 30 knots speed but able to coupe with a good chop!
Please persevere with me as I do really appreciate you support.:)
Fox.

[Guest625101138]

For the speeds you want to achieve you are into a planning hull. The biggest factor on power required will be weight. You can use this calculator to determine the power required:
http://illustrations.marin.ntnu.no/hydrodynamics/resistance/planing/index.html
You will need to do each hull as separate calculations. The power required is that on the hull so you need to multiply by say 1.4 to get actual boat power.

I would not go less than 15 degree dead rise for an ocean hull. I would offset to maximise the lift in a turn. Something like the black boat is my preferred.

Asymmetric hulls are OK but I would not have odd kinks in the side of the hull. Just nice curves. I would have a shallow for foot to avoid the bow digging in off a wave.

So the first thing is to work out what the all-up weight is going to be. This will be iterative. It will depend on the type of building material and what load you want to carry. How many people, what gear, 2X 50HP outboards, fuel, basic hull, controls etc.

You will then be able to determine what speed you will get with the planing hull. I expect it will be max around 25kts. It might be slower if you have a lot of weight to carry. A first approximation for the hull weight would be 500kg.

Rick W.

[foxfish]

Thanks Rick, I am going to the Southampton boat show next month (a huge show covering several sq miles) I hope to study some different designs.
I have found, on the net, quite a few designers offering custom plans for any size boat but it seems to me a tryed & tested design rather than a calculated guess is the way to go? However I enjoy the challenge & I am going to build something one way or another.

[Guest625101138]

Thanks Rick, I am going to the Southampton boat show next month (a huge show covering several sq miles) I hope to study some different designs.
I have found, on the net, quite a few designers offering custom plans for any size boat but it seems to me a tryed & tested design rather than a calculated guess is the way to go? However I enjoy the challenge & I am going to build something one way or another.

There is a lot of detail in designing any boat. Hopefully a good designer/builder has sorted most of the problems so there is advantage in getting good plans.

Thinking about designing a boat should make you a more astute observer so it is not wasted effort to do the homework.

You should try to get test rides in particular boats as well. Often it is cheaper simply to buy the finished product than all the trouble of building.

Rick W.

[Seagem]

I might suggest you have a look at this hull form for your experiments...shouldn't be too difficult to build:

...the 'Sea Knife' concept
http://www.yachtforums.com/forums/16028-post47.html
A boat capable of cutting through rough waves at high speed with astonishing stability has a hull provided with a flat planing surface, which in plan is the shape of a thin wedge or delta. The sides of the boat rise upwardly and outwardly with a simple concave curvature from the two edges leading from a knife edge bow at the point of the wedge. The slender wedge shape moving through the water at high speeds develops continuous spray sheets up its sides, which are intercepted by the outwardly curving portions of the hull sides. Spray rails or deflectors may also be utilized to intercept the spray sheet, such deflectors being inclined at a small angle to the bottom of the planing surface. The knife edge bow rises upwardly and forwardly with a concave curvature from the point of the wedge and eventually terminates in a forwardly sloped bow transom. A keel skeg minimizes side-slipping. A stern transom, which rises substantially perpendicular to the trailing edge of the delta or wedge may have a rearwardly extending bustle secured thereto for buoyancy roll stability at low weeds.

http://www.yachtforums.com/forums/16010-post46.html

http://www.yachtforums.com/forums/16033-post48.html (http://www.yachtforums.com/forums/16033-post48.html)

http://www.yachtforums.com/forums/24306-post65.html

Peter Payne was a very learned mathmatican who had a very real interest in boats as well as other subjects. His attempts to define planing and other hydrodynamic phenomena mathmatically is difficult reading for most of us, but he made very real test on actual prototype vessels to confirm some of his theory.

To quote a web reference,"Another type of Payne high speed boat achieves results in a different way. The SeaKnife hull form is a supercritical planing hull that cuts through waves instead of riding over them. This reduces the vertical pounding accelerations typical of conventional planing hulls by a factor of more than ten. There are a number of SeaKnife boats in operation and on the drawing boards. Designs include sport and racing boats, and military patrol boats. Another planing hull form is the WaveStrider, which has been produced as a 24-foot boat for the Navy, and is in production as a high speed ferry. (The stealth boat that appears in the new James Bond movie "Tomorrow Never Dies" is a WaveStrider.) He also developed the Air Lubricated Planing Hull (ALPH). A boat of this type is the Hydrotrac, noted as being the world's fastest production boat for a given power. All of his boat designs were tested in the Chesapeake Bay, near Annapolis, and the nearby Severn and Magothy Rivers. High speed planing hull theory receives a detailed presentation in Peter's book, Design of High Speed Boats, Volume 1: Planing. His computer program, BOAT3D, is used by many to model and design high speed boats."


I will post some pictures sometime near future when I receive something presentable. I happen to have been in the Chesapeake area when he was conducting some of his early test on SeaKnife and I was also working with the Navy at the time. I didn't pay much attention to his 'far out' hull designs at the time, but I recently sought to go back and look at his concepts again as a result of both Bladerunner's work and Payne's concept. Mr Payne passed away a few years ago and I am talking with his widow and his son.

I am in the process of putting together a design for a pocket catamaran cruiser able to range the entire Caribbean and the Sea Knife concept might work well, since fuel efficiency at 22/25 knots cruising speed is of paramount importance...

http://www.boatdesign.net/forums/multihulls/discussion-pocket-cruising-power-catamaran-design-recession-times-25620.html

I'd like to know more about it and your views as to its suitability for a catamaran...

My last sailing boat was a catamaran I sailed with wife and 10 years old daughter from France to the Cayman Islands: while the Atlantic crossing was reasonably fast in 14.5 days compared to previous passages in a monohull, it was very tiring without an experienced crew to rely on: my next boat will be a medium displacement 45' monohull with centreboard, unstayed mast and a modified Aerorig or Dyna rig...

[foxfish]

Hi guys I am back with more questions :)
I have been given a phantom 21' power boat, the boat only has a 5'6'' beam.
I was thinking about cutting the boat in half right down the length & adding a center piece to in effect turn the mono hull into a catamaran!
Is this feasible?
Thanks Fox.

[rasorinc]

How DEEP is the hull? you will be raising the sole way up and will need to raise the sides. you need a tunnel at least 18" above the water. Sounds like a lot of work to do on an inexpensive hull.

[foxfish]

I am not concerned about the work involve as this is my hobby (building unusual boats) I was inspired by this boat http://www.boatdesign.net/forums/attachments/multihulls/24140d1218469872-new-power-cat-help-
The hull looks so similar. There are a few more pics on page two of this thread.

[rasorinc]

How do you plan on holding the 2 half hulls together? What materials?

[f250]

This is a nice design that you would like i think.
We saw this at the Belgian boat show yesterday.
Resque cat that can lower the front mid section for boarding .

Greetz Alain.



Sorry for the bad English.

[Frosty]

You dont need to build a boat yourself in this fiscal climate. There must some good deals all over.

Build your own and you could very well end up with a dog,--a worthless dog too.

Buy at least a professional hull and it will sell when you want to. keep to specs and you will know for sure what performance you are going to get.

Cats are worse, they can be a disaster.

[f250]

Maybe you can use som surplus tubes from henshaw to be modified to be used in your project?

There is some good software to analyse cat designs from aero marine research.

Greetz Alain.



Sorry about the bad english.

[foxfish]

Frosty, you may well be right but I have now cut the phantom in half, straight down the length with a motor powered 9'' diamond blade disc cutter!
I have cut away the transom too so I now have two pieces 19' long, they are being delivered to my house later this week.
I have previously built two cats neither of them worked very well but I have learnt a lot from their builds, this is my hobby I just cant wait to get stuck in.

[f250]

Foxfish wat will de beam be if you ad a midsection of 1m?

[foxfish]

The phantom hull measured 5'6'' at the transom, I will make a 2' tunnel so the finished boat will be 7' 6'' x 19'.
I will post some pics very soon.

[f250]

Still no pics or progress ?

[Jimboat]

foxfish - looks like a fun project! you may want to check out "Tunnel Boat Design (http://www.aeromarineresearch.com/stbd2.html)" material here.

[foxfish]

Well yes in fact I finally had the two halves delivered yesterday!
I have been starring at them all day!
After revising & thinking about how to build the center tunnel I still haven't made up my mind, there seems to be a few options like glassing over ply or making a mold - going to give it plenty of thought...
Pics tomorrow.

[foxfish]

Here are a few pics with a bit of artistic licence ;)
I have cut away all the top deck structure & 900mm off the rear, this now gives me a length of 5.4mt x 1.8mt beam. I intend to add a 600mm tunnel to ultimately give a 5.4 x 2.4 finished boat?

[f250]

Very nice.
I think that most of the tunnels are wider and the sponsons narrower.
you can verify at the test papers on jimboat site of a cougar and a stv.
Just a thought.

[foxfish]

I am not expecting to design the ultimate performance cat just a functional boat, however I really appreciate any input especially at this point!
I plan to power the boat with two 50 hp motors & would be happy with 25mph top speed?
I guess this project to take a year to complete so it is quite important to at least believe it will work?

[rasorinc]

You are going to need to come down from the underside of the tunnel to the cut edge of the sponson at almost 90* to narrow up the sponsons or you will be bobbing like a cork in the water. I would make the beam at almost maximum for trailering to help in stability.

[Fanie]

Why did you cut 900mm off'n it ?

[rasorinc]

Good question........did you keep the 3' you wacked off the rear and do a neat job of cutting it off?

[foxfish]

I could try to make the tunnel a bit wider but as the boat will be kept on a marina mooring the beam & length makes a big difference to the square footage cost, not cheap in our marinas!
I live on a very small island with very narrow roads, towing boats is not a very practical option & a police escort is required for any boat over 2.7 wide.
I have been looking at a couple of cats, one has a 350mm x 1000 tunnel the other only 300mm x 800.
I was thinking about making my tunnel 400mm deep x 600 but I could go 400 x 700? the other reason to keep the tunnel at 600 was because the plastic coated board I was going to use for the mold is 600mm wide.

[Jimboat]

a tunnel of 400mm (15") deep X 600mm (23") wide for chord length of 5.4m (18ft) is very narrow, and extremely low aspect ratio. results will likely be marginal.

sizing a hull by simple scaling sometimes gets unexpected results.

[Fanie]

Have you thoughjt about making it wider and to make it trailable you can make it to fold ?

You could also try and find someone close to the water to leave the boat at so you won't need a police escort, pay them an affordable amount instead, almost like you hire a little piece of their land.

Also ask your authorities if they have a patch of land or an area they cannot do anything with and leave the boat here.

[Fanie]

I must say that I don't approve personally to your's cutting that boat up. Oh, it's your's, you can do with it what you want, but I have doubts about the new boat's hull in the making. It would have been much better and safer to make new hulls from scratch. I haven't ever seen it work right structurally for what you are about to do - unless you plan to make a mould off'n those two halves.

[foxfish]

OK plenty of negative replies but how about helping me work out a good ratio for the tunnels width & depth?
What are the disadvantages of a narrow tunnel? would I get cavitation or aeration on the props?
I respect you guys advice & recommendations but I don't understand the problems, are you saying the boat wont plane, wont turn, or what?

[f250]

I think tunnel 900mm sponsons each 750mm en 250 to 300 mm deep .
this wood improve stability.
How deeper the tunnel how better it wil thake ruf water without slaming on the roof of the tunnel.

[Fanie]

As f250 said, the higher the bridgedeck the better.
The water that gets pushed sideways by the hull also accumilates between the hulls, thus pushing it up a bit (it has to go somewhere). The further the hulls are apart the less the effect, but the better the chance of a lump of water to go between the hulls. Imo the further apart the more deck space you would have and the higher the bridgedeck the more pleasant the boat should be in choppy water.

I wouldn't worry too much about not enough draft, you will find plenty of weighty stuff you will want to pickle along when you go out. Boats are like females :rolleyes: They gain weight quickly after that inital shallow draft launch.

OK plenty of negative replies but how about helping me
I respect you guys advice & recommendations but I don't understand the problems
The intent is not to give you negative replies, but to give the right information. I am really worried about your joining the hulls the way it looks like you are going to do. The hull of a powerboat takes a hell of a bashing while you are underway at speed. In roughish water, go out with someone else and jump into the water a couple of times when speeding. You will get the idea. Even though a hull has a shape, the water doesn't just flow around it or underneath it. A lump of water that you crash through is similar to riding into a wall with your car. If the hull is not made seamless out of a one piece integrated form, you may encounter serious problems. The forces at speed due to the hulls encountering water at different angles are huge huge.

You can also hold your hand on the inside of a hull when it planes. I think you are going to be shocked at the force of these little bumps it makes.

I see Frosty made a comment on buying a hull. If you do, you will still have plenty of work to do on it (trust me) but you will have a proper hull that is strong enough IF you do the structures inside it right. If you don't, and the hull twists you have a good chance of things breaking loose inside it.

I'm not sure why people think a boat is just an overpriced piece of fiberglass they can just slap together themselves for cheap. I bet if it was an aeroplane you are building you would think differently. Not much difference between them, you can fall just as hard with a boat and drown, together with passengers whom you as skipper is responsible for.

If I knew this is what you are going to do my response from the start would have been different.

Reconsider what you are going to do !

[Fanie]

Furthermore, I would suggest you first learn a bit more about hulls and cat hull shapes since that is where you're interest is. Here is something to get you started.

http://www.multihullsmag.com/magazine/articles/cathulls/cat%20hulls.htm

[foxfish]

O sorry Fanie, I dont want to cause any hassle on your site, I just kinder thought some of your members might want to follow me building my boat?
I haven't started any building as yet because I want to make a good & proper job.
I appreciate all the answers so far but if you think it is inappropriate to continue posting about my idea of course I will stop?

However I still think I can utilize the hull sides & get a wider tunnel, it will mean cutting off more fiberglass down the length of each half but I think I can still keep the beam at 2.4?
I was hoping that by glassing in the tunnel right across the width of the hull & glassing in multiple bulk heads plus the deck would give a pretty secure structure?
I dont know if my idea will ever work out work but I feel very confident about the integrity & quality of my work.

[f250]

This is a cool project keep on posting !

[Fanie]

O sorry Fanie, I dont want to cause any hassle on your site, I just kinder thought some of your members might want to follow me building my boat?
I haven't started any building as yet because I want to make a good & proper job.
I appreciate all the answers so far but if you think it is inappropriate to continue posting about my idea of course I will stop?

I'm not trying to put you off from your project. I want you to realize that you could be running into problems. There are things that will work, and there are things that can be potentially hazardous. The reasons is the same why one cannot take certain suspension arms on a car, cut them and weld them together again, no matter how sporty or nice the outcome would be. In this case it would be best to weld the arms to what it should be, then have it made out of a one piece of the right material so the required strenth is achieved.

Wrt your project - I would suggest that you assemble your hulls with the tunnel in it. Once you're happy it is as you want it, finish it and use it as a mould. This way the whole hull would be one integrated piece of glass and would be of required strength.

You can easily do a test. Make two layups with glass. In the one you make a layup, let it dry out then add another layer halfway off it. The second layup you do the same, but do both at the same time so they go wet on wet.
Afterwards you would be able to pull the first layer off the second (delaminating it), while the second that went wet on wet you would tear the glass with great force, but it would not delaminate.

If you plan on doing a good and proper job then there are some things you will have to take into consideration. The glass types and orientation of the glass is also an important consideration, and also the amount of layers. It will determine if the hull is going to be stiff and rigid as it should be.

This is what makes building a boat alone so difficult. Boat builders either have a team of people that do the layups from start to finish, or they use vacuum infusion where one or two people do the final infusion. If you plan on doing this alone then I suggest you look at infusion. You can place all the layers and do the prep at your own time, then when you're ready you can infuse the whole hull in a shorter while. It would also be stronger and lighter than hand layups.

[foxfish]

Do you think it would be of any benefit to either make an angle on the tunnel sides or step in the tunnel sides, will this stop water creeping up the tunnel?
Will a vertical side to the tunnel work just as well? (much easier to construct)

[Fanie]

A few steps in the tunnel side have a buffering effect in you plow through a wave, while at the same time at speed it provide lift like a foil. You can check it in the article I posted a link for previous. It will also add to stifness in the hull.

[northerncat]

this is an idea i have considered often i reckon it would be very doable, i would however make sure that i had good stringers installed and possibly put ring frames into the hull before i split it
sean

[foxfish]

Northercat, I made numerous templates & took plenty of measurements before I cut the boat in half however the two halves are surprisingly rigid!
The original Phantom power boat was often fitted with 225hp motors but the hull is amazingly thin, I guess the boat was built on the limit of strength v weight.
The few stringers fitted have a sort of compressed cardboard form rather than the usual plastic pipe or hose pipe often used 20 years ago. The hull shows no signs of fatigue or osmosis even though is was kept in the water.
I am quite familiar with fiberglass as I have worked with the product for 30+ years, in fact I completed a 5 year boat building apprenticeship 29 years ago!
I haven't been a professional boatbuilder for the last twenty years so I am definitely not up to speed with the latest methods however I still work with composites & resins to this day.
Guernsey, my home is only 9 x 3 miles so the sea & boats play a huge part of all the residents lives, I have been boating, diving, surfing & building all the related equipment all my life.
I, like yourself, have often wondered about completing such a project as this one - so here we go! I am going to do it!! :)

[foxfish]

Next issue for me is how to construct the tunnel, seems like two main options - make a frame & cover it with plastic coated board or formica to be used as a mould & then removed.
Make a ply tunnel & glass it in place & then encapsulate the ply from underneath with epoxy & fiberglass woving?

[northerncat]

make some frames glue the 2 halves of the hull onto them and then glass and build the tunnel onto the frames
sean

[Fanie]

Do think about adding some permanent floatation, something like a foam. It makes construction easy and the double skin adds to stiffness. Just don't use PU foam, it's a heat insulator and absorbs water.

Closed cell foam and use the right type. I like PE foam but there are others that is aparently also good. And they are all expensive. It is a nice thought though that if worst comes to worst the boat cannot sink.

I would never - after a nasty storm I was in - build any boat without permanent floatation ever again.

[foxfish]

I have taken some templates from the back of the boat & made a full size template for the cats transom......what do you think?

[f250]

Nice you can screw this against the transom.
Maybe you schould make some steps in the tunnel like you said.
Damn guernsey is so far, or i would come over to help.
Cool project keep the pics coming.

[foxfish]

Ha Ha you would be welcome but wait for the summer first.
Yes I will make a step & radius the top corners.
I am still revising the final dimensions - I need to find where the water line was - the hull was recently spray painted so I cant work out the water level!
I might have to raise the tunnel height a bit as my finished cat will be quite a bit heaver than the original mono hull & I want 150mm clearance down the tunnel. (I think?)

[rasorinc]

Found this picture . might offer you an idea. That's all.
http://www.boatdesign.net/gallery/showphoto.php/photo/8916

[dimzik]

Does anybody know where i can get UIM rules for racing cat hulls?

[Jimboat]

you can get the UIM 2008 Complete Rules manual (http://www.aeromarineresearch.com/free_downloads.html) here.

[foxfish]

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?

[Frosty]

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.

[foxfish]

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.

[Frosty]

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.

[Jimboat]

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

[dimzik]

Jimboat, Greatfull thanks

[foxfish]

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

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.

[Jimboat]

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

[foxfish]

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.

[Jimboat]

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.

[foxfish]

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 :confused:
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.

[Jimboat]

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.

[foxfish]

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-

[Jimboat]

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).

[foxfish]

Well being more practical than academic your second solution sounds more appealing :D
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.

[Jimboat]

foxfish - thanks for PM. i really can't analyze performance without more info on proposed design, weights, etc. I'm sure that you'll have plenty of fun building and testing and modifying your boat. Let me know if i can help.

[foxfish]

The pic shows half the transom on a hardboard template, I think the hull angle is 17 degrees, what would be the effect if I changed the angle to say 14 degrees?
This would give me more buoyancy but what is the trade off?
(650 = total height)

[Jimboat]

Lesser deadrise generates more Lift, but is slightly less forgiving in heavier seas. Actual benefit in Lift depends on your velocity and width of pad, and angle of attack (trim).

[f250]

any evolution?

[foxfish]

Yes some, I have been doing a lot of sanding with the 9'' grinder, in fact the the boat must be half of its the original weight!
Of course I will be replacing all the material with fresh glass but at the moment its just more sanding!

[Ernie Travers]

Hello foxfish,
we built about 23 power catamarans from 8.2m to 13m under the brand Silver Tiger. Each one was a development of the previous boat. The most efficient was 9.75 loa with a flybridge. With two 250hp outboards it used 1L petrol/motor/NM at 20 knots
The same fuel consumption at 35knots that our previous cats got at 25knots.
3.5L/NM/motor at it's top speed of 48 knots.
With 50hp motors on a 5m boat, you need a planing bottom.
One of our cats was hit by a heavy boat on it's mooring and went ashore on rocks in a 100knot storm. My son pushed it off the next day in a 50knot wind and drove it for an hour and onto a boat ramp.
We were considering selling plans and have CAD sketches of rather stylish 6m and 7m boats. We could easly draw a 5m if you are interested.
If you supply your e-mail address, I will send some photos of some of the cats and the rescue off the rocks.