1st Boat - Maxcat 28" - lots of questions

[davidkemps]

Hi guys,

I'm building a Maxcat 28" cat, my first boat, from scratch as well out of plywood then fiberglass over that.
The building is going well, I'm looking to order hardware and have a lot of questions, or can you comment if you think my ideas aren't correct please.

I'd like to go 60-80kph, nothing too crazy. We will build 2 of them and do circuit races, just for fun.

1 - rudder - is 95mm about right?
2 - esc - was thinking 120A Seaking or Hobbyking equivalent? waterproof
3 - prefer to run 4s as I have batteries already, was thinking 1 on each side.
4 - brushless motor - Watercooled Brushless Inrunner 3660SL 3180kv from hobbykind - was thinking 4 x 3.7 = 47000rpm, some people said aim for under 50k rpm?
5 - propeller - I've seen 38-40mm mentioned, not sure what pitch etc
6 - drive - 3/16" flexible drive
7 - trim tabs, - any particular size?
8 - turn fins - any particular size?
9 - steering servo - around 8kg - waterproof

anything else required? any other comments around setup or component placement would be appreciated.

thanks

Dave

[Heath M]

Hi,
Rudder sounds about right.
ESC probably to small, aim for 150amp min.
That motor is far to much for the boat. you want to aim for 30k - 35k total RPM. 2200kv motor would be best suited.
M440 - M445 prop with suggested motor.
.150 flex with 3/16 stub is plenty. but no harm using 3/16 flex if u want.
Cats don't need trim tabs if set up right.
Cats don't need turn fins.
Servo sounds right.

Pics of your build would be good.

[davidkemps]

ok sounds good. I'll get both props and try them. I'll buy 150-180amp esc.

would this 2200kv motor be suitable?

http://www.hobbyking.com/hobbyking/store/__22407__Turnigy_AquaStar_3974_2200KV_Water_Cooled_Brushless_Motor.html

is there recommendations of where the shaft goes through the hull and can you give me any information on how far out the back and how long it should be mounted?

here is my boat. just about to start the hull.
IMG_0255.jpgIMG_0256.jpg

[davidkemps]

is this the flex shaft i would need?

http://www.offshoreelectrics.com/proddetail.php?prod=ose-187L

[Heath M]

Yep that flex will do.
I used short bracket speedmaster strut on my cats with rudder offset to the right.
Positon of stuffing tube through the hull eill be determined by the placement and angle of your motor mount.
Easy way to do it id put all the hardware on the boat. Then using your batteries and motor, adjust the placement to get the COG right.

[davidkemps]

ok is cog meant to be around 30%?

think I've ordered everything except servo now.

thanks

Dave

[boredom.is.me]

A 120A should be more than enough with 4s 2200kv.

30% measured form the transom is a good starting point.

[Beaux]

Nice work on the cat.

[Heath M]

What he said about COG. Make sure you have some play with it so u can adjust for water conditions.
Cat looks good, did u get it laser cut?

[boredom.is.me]

I missed the pictures. Nice work!

[davidkemps]

Yeah we have a laser cutter at my work. Makes things so much easier

I've got the 1.4m maxcat solidwork's models and this one at half scale if anyone wants it.