6m powercat

[northerncat]

im looking at building a 6m (2.3m beam)ply powercat based on the thundercats
http://goldcoastthundercats.com.au/gallery2/main.php?g2_itemId=5273
they race around in the surf here in aus, i have a 60 hp merc bigfoot in the garage and want to get out and do some fishing again as im a litlle bit over boatbuilding at the moment

http://easycat.50webs.com/easy_cruising_general_029.htm

i basically intend to scale one of these up to 6m, i was going to build it out of 6mm ply and have drawn up some rough sketches on paper, i am going to make assyemtric hulls that are basically box sections with one outward chine,
i have 3 questions

1)should i go with a 15 or a 25 degree deadrise on the hulls

2)if i scale the hull beam on these things i get around 600 does this sound reasonable or should i go skinnier

3) is a tunnel height of 300 too low
sean

woud

[cookiesa]

Hi,

I am doing a similar thing only with a view to a family fishing boat and shorter. I am almost set on a hull beam of 600mm, tunnel width of 600mm and bridgedeck clearance of 400mm from the keel line. I have gone with a lower clearance to allow the deck to sit approxiamtely 60cm lower than the sides for safety with kids on board. I thought of going higher but also wanted to keep the windage down as much as possible as she will already have reasonably big windage.

I'd be curious what you are estimating your draft at. I think I was dreaming at 20cm and expect it will probably be closer to the 300mm at rest, around 10cm on the plane.

I went with a 14 degree planing hull for moderate weather handling. I'm not chasing spped (which is why I assume your basing on the thundercats) But stability at anchor and ability to handle swell around the 1 1/2m mark. I'm basing mine more on the devilcat and twinfisher design

Id be interested in how you go.

[northerncat]

im actually thinking of having a 25 dgree chine for rough weather handling im not reall after speed as with my motor you only get about 30kn tops but because of the bigfoot gearbox you get a lot of torque, i like to go fishing when i go fishing not when the weather says so, i too have children and so i am going to have a relatively high side of about 700 above the bridgedeck(600 if i lift it to 400, i am looking at using assymetic hulls as these are supposedly better for single engine mounts(less disturbed water in the tunnel=cleaner for the motor
as for draft im working on about 250 -300 as im hoping itll be a light little boat
sean

[northerncat]

oh and the other reason im going with assymetric hulls is theyre easier to build

one other thing ill do is extend the hulls out 300 past the rear bulkhead to help support the hundred odd kilos of motor weight, this should also help it climb out on to the plane quicker
sean

[cookiesa]

Looks like were going down fairly similar paths! I have gone with the symmetrical hulls mainly as I have seen 2 cats operating around here (Twinfisher & Devilcat) and both of these have the symmetrical hulls although the devilcat uses a round hull.

I'm going 300mm from the "stern" with the hulls for the same reason. The other advantage is if you intend going with a single engine (which if I'm reading correctly you are or considering doing) is it helps to give the prop solid water. The main issue is water flow around the prop once planing. If you picture the wash coming off the stern of a monohull you effectively have that "wake" coming off of two hulls and the airated water then joinging roughly in the middle, right where you prop is sitting (this is why the assymetric hulls don't suffer as much)

So I believe (and welcome any input to this!) if you bring the engine "inboard" from the stern so it sit's in front of the expected V created between the hulls from the wash of each hull "joining" and mount the engine so the plate sits slightly below where you expect the waterline at planing to be this should give the prop clean water to run in. Of course the trade off is a small loss of efficiency at low speeds, increase in draft and of course some loss of manouverability as the engine/steering is no longer right at the stern.

On the Twinfisher they actually have a "leg" coming down in front of the outboard leg however they mount the engine right on the stern so I believe tht is done to overcome the issue.

[cookiesa]

Another theory I have heard (I think it is in relation to the whaler cats) is a lower tunnel with a wide opening/mouth and with rounded edges (I was planning on filleting this area to help with strength and reduce slamming) actually forces the air and water to "compress" in to the tunnel and help cushion the ride.

[cookiesa]

Just thinking it over perhaps the assymetric hulls with the hulls going further aft than the outboard would be sufficient to overcome the airated water issue anyway. Out of curiosity are you using 1 or 2 chines? and if so how wide have you made the "bottom" and the second chine board?

Also would be curious on your thoughts as to what HP your going to need.

Thanks.

[northerncat]

im actually going to chamfer the top corners of the tunnel(cheaper than filleting) with a small piece of ply at about a 45 degree to achieve the same thing, the other thing with assymetric hulls is that they are apparently sligthly better in the rough stuff and they will lean into a corner like a mono.

as for lower tunnels i am nearly finished the construction of a 40 ft ply sailing cat and have done extensive reading on bridgedeck clearance and have never heard that a lower bridgedeck clearance is good.

as for what horsepower i think my sixty will be plenty i lived in bamaga \(top of cape york for three years and they only use these motors up there, they can bolt these on a 6 m glass boat with a deep vee fit 2 dugongs (750k) each and three passengers and still get onto the plane easily, i had mine on a 5.25m allycraft and with 8 adults all fishing and diving gear on board it still flicked us up on the plane) i am a big believer in large ratio gearboxes now, they dont quite have the same speed as a normal sixty but they have heaps and heaps of torque which = getting up on the plane quickly. they have the lower gear unit of a 90 merc and they have the biggest block of any sixty. also they are start first pull and even my wife can pull it over.

what are you going to do about the apb issue?
sean

[northerncat]

i intend to have a multi chin bottom with a 100mm flat bottm going into a 25 degree bottom which will then go into a 65 degree chine which will then meet the side

i am however really stuck on the bridgedeck height and width option, i want my boat to be about 2.3m across but i dont want to make my hulls to wide
so im open to any suggestions

[northerncat]

the 75mm-100mm bottom gives me something nice to sit on and also should help the boat plane easier without being to wide to punch through a sea the next chine being my going through the rough stuff chine, with the last chine being for an extra spray dispersant chine to keep a dry wide, i ntend to overlap all the ply over the previous piece and then to fill in the gap with bog to creat a flat surface to force the water away from the hull

[cookiesa]

Sorry perhaps my post didn't read to well, that comment was made in relation to small power cats. Obviously a nice high clearance would be the best but on something the size we are looking at a large clearance is just not practicle. I took it to mean that sometimes the issue of clearance isn't that big on smaller cats as you can't get enough clearance to prevent it only reduce it and that sometimes there is too much of a compromise made so you end up with a big slamming tunnel rather than the smaller tunnel that also has the effect of rising the water level between the hulls and thus keeping the prop in solid water. They also commented that It has to go somewhere and the smaller tunnel forces it to "compress" to some degree helping to cushion the slam

This is interesting when you think about the tunnel hull concept versus a cat. They have what seem like very small tunnels and certainly in the offshore powerboats handle fairly rough weather

Interesting comments you make on the power, Thanks for the tip on the gearbox too.

You might have to expand on the apb issue, not sure what you mean.

My method for the hull V Bridgedeck width was after looking at a few in the same size most designers seem to have gone with about the same size for both.

Are you glassing over your ply? I'm thinking epoxy with 2 5 oz layers as suggested by another forum member (Tri-Star I Think)

[northerncat]

i read that post and am not sure of the equivalence but ill just use 200gsm per square maybe this is 10oz?? i have used the 200gsm on my big boat and have found nice and easy to handle and have done up to four layups with it going around things like the hull to bridgedeck join.

as for the apb, im talking about the australian builders plate which every new powerboat made after september 2006 needs to have
http://www.amif.asn.au/buildersplate.asp
part of the reason for me for building over 6m is to escape the bouyancy calculation issue, i will be epoxy sealing the inside of the hulls 3-4 coats and then foaming the bows and stern, the middle sections ill probably turn into lockers, ill also have about a 200 wide side to my boat that will also get foamed so that it will float upright for sitting on and mounting rods so that it will float upright however i do not wish to calculate this buoyancy so ill build mine to be 6.05m long.

when im finished ill take it down to the lake for a full imersion test to see how much weight itll support with water inside the boat, itll also be a self draining boat for ease of cleaning which means my bridgedeck will have a 50=100mm?? slant from front to back

i have looked at a few boats from 5m to 6m and they have beams from 2m to 2.3 i just havent been able to measure many to see what there bridge deck widths and heights are

[cookiesa]

Phew! I thought I had missed something in the design with the abp!

I know a good boat builder who has said he will help me out. He can issue them once I have finalised the details he'll have a look at it for me. (He's offered to help out more but this is a project for me and he mainly deals in yachts and tug refurbishments/building.)

What cat are you building? (My background is sailing and ocean racing in mono's but I always had a soft spot for the Simpson 11. A kiwi was building one in a yard in Darwin, cedar strip with epoxy. Beautiful boat inside and out!

I intend to make the cockpit self draining too. That's why I was curious about the draught your expecting.

Are you doing much in the way of a cabin or centre console? I'm planning on just a front with windshield similar to most of the 5m mono's. Not sure yet on rocket launcher with soft bimini top or rocket launcher and lightweight roof with clears????

Only problem I have with the hardtop is height could become an issue down the track with storage. Soft top is quicker and easier to drop but here you either have it on for shade or rain protection so it's almost always in use.

I'm just having a step up to the windshield and a side opening middle window for reaching out to raise/lower the anchor.

How hard an angle do the outside of your hulls need to be bent in to with your hull design? How far back from the bow before your hulls are at maximum width? Have you tapered the hulls towards the stern at all?

[northerncat]

i have been working on an easy 37(peter snell design) part time since oct 05 and will hopefully finish this in the next couple of months or so, im up to the exterior painting stage then its tack on the deck gear and launch

im actually thinking of having a hard casting deck up front as i love chasing pelagics, i am favouring a completely open boat at the moment with a big bimini

i wasnt really thinking of narrowing the hulls if i did it would just be for aesthetics as for thew front i plan to carry the hulls at full width up to the 4.5 m mark and then to curve them up and in from here

i take it then you are goin to foam most places
too
im not sure what you mean by the angle hull question

oh and i am a kiwi too
sean

[cookiesa]

Very nice! I like the nice soft lines on the Easy cats. You have done a good job 2 years part time!

No disrespect meant, I got on well with the whole family (I was good friends with their daughter through High School.) and I use the term Kiwi with affection. I have seen a few "home built" Kiwi boats and it must be in your blood because they all looked professionaly done. The called their cat (please excuse the spelling!) Terrimoana (going back about 8 years now!)

Yep I am thinking other than room for perhaps two 45litre ish tanks all under the floor will be foam. All under the "step" at the bow will be foam also.

What I was getting at with the hull angle question was more along the lines of how hard is forming the curves goping to be? Sounds like there will not be any real sharp curves by your description anyway.

One of the reasons i'm thinking of going with symetric hulls is I'm thinking of carrying the full width almost to the bows then bringing them in reasonably sharply with an almost vertical bow to get as much bouyancy up front to help reduce slamming and also to assist with running with the waves (hopefully lift the bow a little earlier.)

On the models I have built the angles are hard but achievable, just! My skills aren't good enough to strip plank the bow area and as I'm epoxying over them anyway I don't want the hassle of trying to fair it all either. Also to me kind of defeats the purpose of working with sheets! (May as well bite the bullet and go cedar planking, which I can't afford!)

I'm still not sure exactly what the reason to go with 2 layers of epoxy over 1 is but believe it is to increase the amount of epoxy resin and thus strength?

Did you find a good supplier of epoxy for your cat?

[northerncat]

i use fgi r 180 resin 304$ for 20l and really like this as for the curves coming to the front of the boa ill do these with 2 sheets of 3mm ply bend one and laminate the second over the top of them, then ill butt block them to the rest of the outer hull

bugger strip planking!!!

i think it would be possible to bend the 6mm but easier but slower with 2 3s, as for my angle coming up for the bows i am still tossing up whether to run the hulls full width to the last metre(not as pretty) but like you say better for bouyancy or start them at the 4.5m mark to get a nicer curve

the first layer of epoxy will literally dissapear into the wood the second and third seal it off, you need to add glass to give it strength the epoxy adds minimal strength without the glass but seals the ply nicely

sean

[Buildboats]

Have you thought to add a foil to your tunnel Sean?

[northerncat]

yes and no i have just been loonking into this but i have no idea how to go about it, ill probably build it first and then think about whether it needs it later

im still just wondering whether i can just build a boat using commonsense without using some sort of planning software and even whether im on the right track, i have an educated idea that the motor will do the job but am not sure whether my hull chines will be to steep( more power needed to get on the plane but better ride in the rough), whether my bdeck is to low ?? and at what draft it will float at , how far to extend the first 25 degree chine etc etc
this is why i posted to this forum in th hope that some of the gurus may be able to help, otherwise ill just build it and see what happens
sean

[Buildboats]

I did it with little effort to my 19 ft sciff and it is perfect http://www.buildboats.info/sportfishingmcatamaran.html

I'll be happy to share it with you Sean

[Buildboats]

I used oak 1x2 laminated and then wraped in glass was pretty basic stuff really

[northerncat]

did you give it a profile ie naca 12? did you v it and support it by the tunnel, did you put little foils on the back and if so where do you place them is there a formula related to boat length or did you just guess, did you give it an angle of attack?

thanks for any and all assistance
sean

[northerncat]

out of curiousity if i wanted to tiller steer your boat would i be able to see over the bow? is there any way to have a foiled boat with a more bow down attitude,
i only ask because my preference is for tiller steered boats due to the quick response you get in steering, when you are hooning around over reef sometimes you have to make lightning quick decsions to avoid a bommy sticking out?
plus i also want fairly high sides on my boat which would compound the problem even more so
sean

[Buildboats]

I gave the foil 2 degress of incidence against the line of the keel and I developed my own shape since I wanted a foil to generate lift between 10kts and 15 kts, so it would plane fast. I chose low sides for fishing and I had liitle money so it took less material to build that way. I am adding more to the topsides. I haven't added aft trim tabs yet as The engine has a dolphin I made, that works well. I am adding move able tabs this spring to help with weight changes due to passengers moving around. I built the boat 15ft long first then cut it in half and added 4 more ft.. you know its never big enough. If your running around corelheads... I'd think about adding nylon runners to the hull bottoms like a airboat use, then all you need to worry about is the prop. The foil runs right at the surface so that should be fine and yes you could use a tiller. I used a V foil with a strut in the tunnel. I would go strait next time. You could a kevlar to the bottom also.

[northerncat]

if you have a straight foil do you still reckon youd need a strut in the middle for support?
when you say you developed your own shape, does this mean that their is an accepted profile for foils on cats or that you chose foil that would give you large amounts of lift at low speeds(with the resultant small amount of drag at high speeds)
sean

[northerncat]

also if you place rear foils should these be placed right at the rear of the boat ? also how high up the hull should these be mounted at the bottom so that you are trying to get the most out of the foils lift to get maximum hull out of the water or slightly up from the bottom
sean

[Buildboats]

It depends on how wide the tunnel is and how thin the foil you build is. I have herd that winglets help to reduce vortex generated by symetric hulls. I have tried this yet buy will be this summer. Trim tabs should go low as possible.

[northerncat]

is a winglet like a rear foil or is that a trim tab, also sorry about the more questions than answers approach i just like to understand something before i do it

sean

[northerncat]

out of curiousity to simplify hull design can you just take a set of hull frames from a boat like the fao boats and split them down the middle to create your assymetric hulls or is there a little more to it, i have drawn up a set of plans but a friend suggested this approach to gain a bit of an inside track in terms of design, the fao boats have the beam that i desire to use when halved for my hulls width plus they look a little funkier than what i drew up. are there any major problems with this approach?
sean

[northerncat]

any input?

[cookiesa]

If they are around the dimensions you could always use the plans as a basis for looking at number of bulkheads, stringers etc anyway.

But can't see why not, effectively that is what an assymetric cat is.

[northerncat]

yes this is what i thought i was just hoping that someone with a bit of insight might chime in, hows you plans/boat coming along
sean