Cat designs for scratch builders

**DISAR** · 02-17-2009, 04:50 AM

Very nice subject.

I can only comment on the beam. I had two cats about same lentgth but one has bigger beam. The bigger beam provides better stability and safer turns especially at high speeds and I strongly prefer hulls with bigger beam. When I was looking for a cat (Aeromarine or H&M) I saw that this ratio varies from 2.57 to 3.50. The aeromarine hulls have bigger beam than the H&M.

**Meniscus** · 02-17-2009, 08:47 AM

Simon, I'd start by converting the dimensions off of the H&M site for their various versions. Then send them an email for more info, especially related to tunnel depth.

Good thread, I look forward to keeping track of all the input.

**Simon.O.** · 02-21-2009, 03:46 AM

Not excactly screaming along with input here so I will fire in a few specific questions.

Length
I see a lot of references to a Cat that is so many inches long.
Is this measured from the bow to the rudder or to the rear of the sponson or so other mythical location.

Some cats have a hull overhang aft of the sponson rear and some do not.
Where do we all measure to ???

By now some of you will have figured out that I am after a big cat.
I am initially loking at a single XL can on 4S but may go for twin L's or XL's
We are looking at 29-34" to the sponson rear on handmade stinger type drives.

How are cats measured ??

**Jesse J** · 02-21-2009, 11:45 AM

I agree, Simon, length should be bow to back of sponson. Overhang can be added, but length of wet surface is what counts.

I am curious how important steps and strakes are for micros. I know you are working at the larger scale and they are probably more help than not, but at the 14" scale, they probably do no help?

I can't offer much experience with driving my cats (probably not more than 1-2 hrs total), but can suggest what variables should be discussed since you are talking scaling issues and how to design around them.

I think for your "engineering design" questions, there should be some discussion about air and water viscosity at the scale you are talking 1/10-1/14. Speed is also a critical factor that must be considered when discussing air and water viscosity. Water conditions as well, I have seen that many full scale cats have no strakes, more offshore than others? this would also factor into your deadrise question, I would guess.

As for your length to width ratio, like the MM vs the plan I use from LoneWolf which has a very narrow ratio, is related to more scale off shore racer vs. our smaller rc scale which make much tighter turns and go WAAYYYYY faster than the real ones.

As for tunnel dimensions, I still think we need to compare graphs of air density/viscosity vs speed for real scale boats and our rc boats. In the rc calculation, will wind speed be a much bigger variable than in the offshore guys?

Strut depth I would bet is intimately related with all the other dimensions of the boat listed above PLUS water conditions, prop type/size/pitch and other variables like race, saw, vs sport use.

This is a very interesting and R&D kind of thread. It will be cool to see how it develops. I am sure there are mechanical engineers and others with design talent.

**Simon.O.** · 02-21-2009, 05:56 PM

Some more specific questions.
Steps
Some cats have what I shall call long steps and some have what I call vents.
The sketch below demonstrates my point.
Which is prefered and why.
Are they really needed on a 30" cat under 50mph

Sponson bottoms
These seem to have many different features.
My existing 23"(wet) cat has the flat wedge as a ride surface and seem to work.
I see some have a spray rail as well
Some have none of these and are featureless.
Sketches are attached to show what I mean
Which is the prefered sponson profile and why?

I will design and build another cat, I want to know why I am including features.
It will be long and narrow as it will be for speed on calm water.

Attached Files

**Meniscus** · 02-21-2009, 07:20 PM

Simon, maybe you can get Fluid to weigh in on this one.

**robby76** · 02-22-2009, 12:12 AM

contribution. hope your project goes very well and you can mold it and put into production.

Attached Files

**NativePaul** · 02-22-2009, 05:18 AM

Good luck with the project, its always nice to see a new OD cat.

Length to beam Varies for the intended use a wider hull puts the CoG at a lower angle from the ride surfaces which makes it more stable and able to corner faster, a narrower one has less frontal area and thus lower aerodynamic drag for a given drag coefficient, I run round an oval and am concerned by the lap time not the speed down the straight and I favour hulls in the 2-2.5:1 range, if you an oval but are concerned by the speed down the straight and don't care about the laptimes then a hull in the 3-3.5:1 range will be better for you.

Deadrise A higher deadrise angle has less lift so it cuts through waves better and can handle rougher water, but because it has less lift you need wetted area for a given effective boat weight and speed which slows you down.

Ride pads Ride pads try to avoid the deadrise compromise by having a small low deadrise area which is efficient to plane on at high speed but giving a high deadrise area to cut through waves which will drop into the water if you slow down for rough water.

Steps I don't know enough about steps to comment really, I have only made 1 stepped cat, I gave it "long steps" which seem to seem to have affected its cornering ability compared to my ustepped hulls, ones I'm building at the moment still have the long steps but at a much lower angle, 2deg instead of 5deg, it remains to be seen if this helps, I have seen some hulls that look like the "vented" type but when you put them on a straight edge have a very small amount of the "long step" as well, I don't foresee see plain "vented" types helping the top speed any over unstepped sponsons and unstepped sponsons should go round corners best.

Tunnel depth If a wave hits the tunnel roof it will create a large amount of lift that the boat is not designed for which will lead to unpredictable(depending on where it hits) and potentially disastrous results so you want the tunnel clear of any waves that you are liable to run in, I have cats with high deadrise and high tunnels for rough water and low tunnels and low deadrise for smooth water, best to have it as low/shallow as you can for the water you will run in as a higher tunnel will raise the CoG and lower the potential cornering speed and also make the boat taller than it needs to be, giving it more frontal area and increasing its aerodynamic drag for a given drag coefficient slowing you down on the straight.

Strut depth All my cats have ended up with the bottom of the strut a little above the bottom of the hull, higher up unloads the prop giving more RPM and a little more speed and can allow you to pitch up the prop getting your load back and a substantial speed increase, too high can result in excess cavitation, excess propwalk and even hooking.

Length Measure from the sponson tips to where the sponsons leave contact with the water whilst on the plane at the stern. The extension at the back is just to close the air behind the boat cleanly, it lowers the aerodynamic drag coefficient reducing the drag and increasing the speed for a given frontal area, I don't know what they call it in the marine world but in the context of bullets it would be called a semi boat tail,

Bottoms I believe that any medium to high deadrise area needs at least a spray rail at the top of it to stop the water climbing the freeboard, if you fit spray rails to an area that is too low in deadrise it may touch the water surface increasing the drag. I fitted turned down spray rails years ago to a glass hull that barely had them it ran so much cleaner it looked like it had slowed down but the lap times decreased. At the moment my bottoms are looking something like this:-

NB: The aerodynamic compromises are skewed for us in comparison to full size boats, halving the aero drag would make a LOT less proportional difference to our top speed at 60mph than it would to a turbine offshore at 200mph.

PS: Robbie, what hull is that in the drawings? You don't have the top view too do you?

**Flying Scotsman** · 02-22-2009, 01:59 PM

Simon, a newbie question.
Is it possible for you to build a prototype which will allow you to make minor adjustments back and forth to some of the issues you are discussing and explore advantages and disadvantages?

Douggie

**robby76** · 02-22-2009, 07:06 PM

"PS: Robbie, what hull is that in the drawings? You don't have the top view too do you?"

this might be it. have alot of drawings.

the curves aren't there but close enough.

ok found it. hope it helps.

ok too small let me work on it. save the file then open it with paint. is supposed to be big enough.

Attached Files

**Simon.O.** · 02-23-2009, 02:54 AM

Paul.
That is the most comprehinsive answer I have EVER seen to one of my questions.
Thank you big time for the effort that you put in there, it is pretty much all the info I am after. I will copy and print that answer and put it in my tech folder.

Robby
Those pics you have posted are just the sort of inspiration that I need.
All the technical knowledge is no good if I can not put it to use in a work of floating art. That cat that you have some pics of is a work of art.

Douggie.
How many hulls to you want me to make to test all this data.

My plan is to amass enough knowlegde to design a hull and know why it is that way. The steps, strakes, and spray rails on some of the production cats do look very nice and I can put them in a build. Why ??????
It appears now that we have a lot of the answers to that question.

Back to a prototype.
I have at present a low deadrise cat with flat ride pads, I also have a ABS tunnel that honks well.
I will draw up a larger cat with moderate deadrise only.
I can then add 0 deg ride pads and spray rails later as needed, I think that is what Douggie meant.

This will be a cat that needs to go fast on the straight and acceleration is important too, I know there is sometimes a trade off here.

I will keep filling my folder that I can read while sitting and having a coffee and take in all this info.

Now it is beer time !!!

**DISAR** · 02-23-2009, 04:18 AM

Maybe you already know this hull Sprintcat 40, it has an interesting bottom design with lots of steps. As heard from others it is a very stable boat
http://www.offshoreelectrics.com/pro...rod=Aero-sc-40

**Gerwin Brommer** · 02-23-2009, 11:38 AM

Drawings from the cat without the manifold boxes : Mystic C5000

**Simon.O.** · 02-23-2009, 03:05 PM

Originally posted by DISAR

Maybe you already know this hull Sprintcat 40, it has an interesting bottom design with lots of steps. As heard from others it is a very stable boat
http://www.offshoreelectrics.com/pro...rod=Aero-sc-40

Dimitrios
That is a very complex bottom shape, it has all of the possible features we are discussing here.
I can see a flat ride pad, a central strake (spray rail) and an outer spray rail. Then the whole thing is stepped fore to aft with the short vent type steps.
I question whether the stability is a function of the beam to length ratio or of the complex bottom shape.

Gerwin
The Mystic C5000 ?? Do you have any photos or drawings of this hull please.

Cat designs for scratch builders

Cat designs for scratch builders

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