Cat designs for scratch builders

[Gerwin Brommer]

Steve O :

Even better ! :

Both MHZ and Hanspeter Rotschi introduced a model of it
Both 185 cm in lenght.
Check out their sites !

[Ub Hauled]

I think you mean Simon O., Gerwin... ehehehehe

[Gerwin Brommer]

Oops, hahaha.

, Simon is the one I wanted to reply to .

[shockerman80]

i already have a 75% done mold of that hull off mystics website. Its alot of work to design.

[lomdel]

i already have a 75% done mold of that hull off mystics website. Its alot of work to design.

Any progress on this C5000 mold, Shockerman? What length?

[Meniscus]

Good call lomdel! I want to see as well!

[shockerman80]

its in the 34-36 inch range i dont know the exact length right now its not infront of me. the hull shape is all there it just needs the finish work. ahhh the time it takes.

[ODIE105]

hi i'm new to scratch building so i'd like some help. i would like to build a MYSTIC C5000 at 54 or 60 inch's and I don't now what wood to use. I plan on useing it to make a mold for my self and make a carbon fiber hull.

[befu]

Input

Wow, just found this thread, fantastic. I am kind of surprised it did not take off more than it did?!?!
I have been pondering this also as I look for a mystic style cat. I have a 15” sea rider and it is fun, but I want a new hull. I do not race, I am more interested in sport running and longer run times. I also have younger children and they are starting to run the boats with me, so it must be stable. I am also looking at a variety of water conditions. A small lake (40 acres?) without boats on it, but open to wind chop (1-2”) is primary spot. But also want to run the boat on larger lakes where a boat wake may be present, so blow overs are a possibility. With 4 kids, smaller boats are feasible as they do not take up much room and are exponentially cheaper!
On my sea rider hull which is 15.5” long and 5” wide, it is an unstepped hull, flat ride pad with a high dead rise beyond that. When up to speed, it runs well but really likes to bounce in between. It also does not handle rough water all that well, even for a 15” boat. I have blown it over a couple of times, but generally it starts bouncing and stuffs itself! This hull runs a 24x45-3000kv inrunner. With a 2s-2200 Lipo it will run full speed for about 9 minutes which means it is pulling 14-15 amps with an X431 prop. My 3s-1800’s are due in this week!
Hull selection So I am looking to buy a hull and use it, buy a hull and modify it, or build a hull from scratch. I have posted on here about the MHZ micro iceberg and the H&M micro drifter. I like the looks of the bottom of the iceberg more. It is a mid deadrise hull with strakes as opposed to the H&M which has the ride pad and higher deadrise portion. The iceberg is smaller and narrower (17.3” x 5”), and reports are that it likes to roll in the corners. The Drifter is 18.5” but 6.7” wide.
The more I research this, I am inclinded to start building a plug and scratch build them. With a 16 degree deadrise stepless mold, I could make inserts to mold different bottoms. Add steps for ventilation, add ride pads or attach strakes and spray rails. Other than my time, the hulls would be pretty cheap to make for experimenting. Being an analytical mechanical engineer also helps with this as the experimenting is fun!
Style; The iceberg has the bottom, but the hatch isn’t for me as well as the hump in the rear. The drifter has the look, but the bottom is all wrong.
Aspect ratio. This has been covered fairly well above I think, but would like to add in you need to define the scope of your operation. If you are going for oval racing, lap time is king which means turning at speed. If you are going for strait line speed, only speed matters. If you are sport running, you need to look at the water conditions where you will run. These all play a factor.
A real mystic is a 3.6:1 aspect ratio (43.5’ to 12’) wet hull. My mean machine is 2.7:1 (wide) which is close to the drifter at 2.76:1. My sea rider is 3.1:1 and the iceberg is 3.5:1, actually close to the scale mystic!
With other variables the same, wider should turn better due to stability. Wider can also be more stable at high speed as long as the extra lift is controlled. If you are looking for a scale look, you need a narrower boat. Also, if you are looking at running in rough water waves, I would think you would want a narrower beam, less sail area. Maybe not the 3.6:1 ratio, but in the 3:1 to 3.3:1
Tunnel height Again, this depends on you conditions. If you are building for flat water, shallower should turn better. If you are going for a true rough water experience go on the deeper side.
Steps If I build a mold, I think it will be without steps or vents. Not sure if it would be needed on a boat that small? Would also help make the layup easier. You can always make inserts and add them to the next boat.
Conclusion: So, after writing all of this, it seems that for sport running with a 2-3 cell battery, long run time and inexpensive to set up: I need.
A mystic cat with a OA hull of 19-20”, a wet hull of around 17-18”. It will have the deeper tunnel for rough water. 16 degree dead rise unstopped hull with spray rails and strakes added as needed. Aspect ratio of 3.1:1 would give a width of 5.5” to 5.75”.
Could go WOF construction easier, but if I build one, I am afraid my two boys will want their own. Plus it is a smaller mold. Decisions, decisions.

Brian

[BILL OXIDEAN]

Steps

I haven't read all replies, so excuse me if someone already touched on this, but the steps can be affective at as little as 35mph.

[befu]

Ah-Ha!!!!

I haven't read all replies, so excuse me if someone already touched on this, but the steps can be affective at as little as 35mph.

OK, honest question here. I actually find this to be interesting and was wondering what the parameters of that statement are. The reason being I wonder if it also applys to the small boat that I am looking at building (18") or is it on the 30" plus cats?

If so and it applys, that simplifies my building which is a good thing as I will be below 35mph. I just like to know the background information on the statement. Please do not take it as questioning you, I am an engineer I want to know the information that formed it so I and others understand better. If you have the time, please add some more information.

Also, anything on constant deadrise vs. flat ride pad?

Thank you for sharing,

Brian

[BILL OXIDEAN]

OK, honest question here. I actually find this to be interesting and was wondering what the parameters of that statement are. The reason being I wonder if it also applys to the small boat that I am looking at building (18") or is it on the 30" plus cats?

If so and it applys, that simplifies my building which is a good thing as I will be below 35mph. I just like to know the background information on the statement. Please do not take it as questioning you, I am an engineer I want to know the information that formed it so I and others understand better. If you have the time, please add some more information.

Also, anything on constant deadrise vs. flat ride pad?

Thank you for sharing,

Brian

Great point, My experience is with small cats 21 and under, so weight and boyancy could also be a factor on larger cats. My only big cat experience is watching them run, and watching big cat diaries on the discovery channel lol..

My BBY micro scat cat powered by a 19-turn brushed motor planes out splendid on 2s. I estimate its speed to be in the 32-35mph range with this setup. This small cat is capable of reaching over 50mph, and I feel that at those speeds both aero and hydrodynamics are affective.

If a cat/tunnel design is working properly, the boat is literally assisted by a cushion of air packed throughout the tunnel. I believe aerodynamics of the deck greatly affect the performance as well. I tend to modify my hulls by pulling the nose of the deck down, so its still piercing the air during a slightly nose high ride attitude.

Cats fascinate me a LOT, and I could go on forever lol..

[NativePaul]

Constant deadrise (of a reasonable angle, close to 0 degrees wont be better in rough water and close to 90 wont turn better) will handle rough water better and turn better, a flat (ish) ride pad will be faster and have a slightly better hole shot.

I'm just starting on steps really so don't take this as the definitive answer but at the moment I believe they should work right down until there not big enough to break the surface tension, and will work at any planing speed as the increased riding surface angle of attack is giving more lift at the same vessel angle of attack or conversely and more to the point, the same lift at a lower vessel angle of attack cutting aero drag and reducing the risk of blowover, also if you chose to run wet due to rough conditions a stepped hull running wet will have a lot less wetted area and hydro drag than an unstepped hull making it a heck of a lot faster/draw a heck of a lot less current.

I'm going to assume that your 24x45 300kv inrunner is a typo and it is actually 3000kv if so when your 3s pack gets to you

that simplifies my building which is a good thing as I will be below 35mph

I hope that this statement won't be correct. My last mini cat (not the one I am running now which is a real screamer)was 18"x9" (yes I like ovals) with constant deadrise of about 18degrees and no steps, it ran on a Mega 16/15/3 which is a 28x36 motor on 3s and an x431p cut down to 28mm because the pack was only 10c at the time, even on 28mm I was only getting about 9v and any bigger was no faster after a quick burst due to voltage sag. Here is a clip of it running in a small sheltered bay on a bigger lake probably not winding up to full speed but close, it was time at records weekend a week or 2 later at 36.4mph average with modern LiPos I would expect higher speed but also you may have to prop down or fit a bigger ESC, I can't find the graph but I think I was drawing 20-25A constant and i remember the peaks were right on 30A.
It sounds like you are designing a faster boat too so wouldn't be surprised if you were around 40MPH if you keep it reasonably light.

[befu]

20" cat

Paul, I find your results encouraging. First, thankyou for pointing out the kv error, I have fixed that.

From what I have been reading on other's post, it seems the cats with constant deadrise have problems cornering and want to roll. Maybe it was the hulls that had constant deadrise were narrower, you have to take everything into consideration.

Maybe it is also the pad has more tendency to slide some, and also gives more lift so the outside sponson doesn't want to dig in, it slips. Still corners well, but if you over rudder it, it slips vs rolling. On the constant deadrise sponson, the contact point is a little further inboard also increasing roll, and it will ride a little deeper helping to cut down on sliding, maybe this is the reason for the rolling? Not that it turns all that worse, but it is much less forgiving on rudder input as it rolls vs slides.

As far as speeds go, yeah your probably right. As my kids will be driving some, my single with the 2445-3000kv in it will run on 2 or 3 cells. With a x430 or x431 prop, this should give me low-mid 20's on 2s, mid 30's on 3cells. Adjust the speed for the driver with a 2s or 3s battery.

I will end up making a twin. Not sure with what , but I already have CR X430 props from years ago, so why not build a boat around them! With 3000kv motors on 3x, mid to upper 30's woudl do, this is only going to be a <18" boat wet hull. 380's with water jackets will fit into the sponsons also.

I will go with around a 16 degree dead rise. I can always pad it later if I want to. Also, the pads on my searider seem like they are too small. The right one runs really wet due to torque of the prop. Has water climbing all the way up the side of the hull. Of course, 3s might help that also, we will have to see.

Now all I need is to get through Christmas and start building. This year the kids are asking for a complete Wii setup, 4 controllers, light sabers, guns, games everything. Daddy is asking for a HD 1080P projector plus screen and 5.1 sound systems! If that works out and that red suited fat bastard doesn't bring doll houses, riding toys and such, Christmas break might be a bit easier this year.

Brian

[Jesse J]

It looks like I should have posted all my dimensions here too... oh well. Anyway, my 22" scratch cat (dimensions on two other posts) has a dead rise of 12.5deg. With the scale ride pads in the plans, she had moderately ok turning abilities (its been a while and this was my FIRST scratch cat build almost two years ago). Since I added the 1/8" x 1 1/4" ride pads to the existing ones, she corners great - you can see the 3-4 deg resulting deadrise; much more sliding and less flipping. I would even venture to say speeds are greater and overall stability greater. The torque roll vanished too. oooo, I may have to add these pads to my micro 14" cat....

Here is a set of bulkheads and the stringer I digitized from the shockerman plans. all dimensions are 1:1. I could send someone a pdf of this - it was too big and had to render to make it upload. these plans can be scaled to any size!! Even 1/2 scale and we all go for a ride!!