Motor advice for 45 inch Cat hull with twin outboards

[Terry Michael]

Hi guys, I am setting up a 45 inch cat hull which will run twin 3.5 cc brushless outboard legs, i am thinking of running 6s lipos for each motor. I am after some advice on what to look for in Kv motors to suit this application, i am after a fast and reliable setup. I come from a heli and car background, so i am use to working with pinions and spur gears to work out what Kv motor to use. So when it comes to choosing an appropriate Kv motors for my brushless outboard setup i am abit lost at what to look for or how to calculate the right setup. Any advice would be appreciated.

Cheers

 

[HTV Boats]

The total weight of your setup will determine performance. On 6S I would go with 1200-1500kv's. We have run a 1520-4082 size motor on a 3.5 lower with a .150 cable and it worked but that may be your weak link if you power up 3000-4000W peak motors. A 1515/1.5Y Neu would work at 1500kv but might get a little spendy. I have a 1250kv 4082 Leopard on 6S that runs low 50s on a 9lb total weight tunnel pushing a M445-M545-M447.I do prefer .187 cable .45 lowers on larger outboards above 4074 size. A 4020 0r 4030 Scorpion outrunner is an option also. No need for a water cooling jacket, and a very low profile.

If you don't counter rotate you will have a better prop selection and not need a custom reverse wound derive cable. Prop walk probably not an issue here anyway

How are you going to mount the lowers? On a cat I would put the props behind the sponsons rather than in the tunnel portion. That way the flow of water is more consistant as the sponsons will depress the water before it hits the lowers. In the tunnel you have a big surge of water level as soon as you turn. Keep us posted on your build.

Mic

 

[Terry Michael]

Thanks Mic for you reply, i appreciate you taking the time to help me out on this project. I will be running the outboards on the sponsons, i did this because that's how they are mounted on full size outboard class cats. Now that you have explained why they mount the outboards on the sponsons it makes sense and good to know. I am not going to run counter rotating props for now, see how it runs and if I need to down the track then I will. This project is for a semi scale look to the hull, so I need to try keep the scale look of the outboards as best I can. The bloke who designed and built the hull has done some modification for me when he built my hull, the transom has been strengthened to accommodate the twin outboards and the power. My main concern is trying to find an outboard leg that has the shortest measurement between the centre of the prop drive and the bottom of the mounting bracket. There isn't a lot of room to play with on each sponson in adjusting the outboard up and down to get the prop in the sweet spot ( I hope that makes sense ). The K&B lower looks like the best bet so far and has a better look to it for a scale outboard, i will be getting some custom motor cowls made up for the outboards. The hull is on its way to me now and I should have it in my hands on Monday.

Cheers

 

[Tim_Duggan]

My main concern is trying to find an outboard leg that has the shortest measurement between the centre of the prop drive and the bottom of the mounting bracket. There isn't a lot of room to play with on each sponson in adjusting the outboard up and down to get the prop in the sweet spot ( I hope that makes sense ). The K&B lower looks like the best bet so far and has a better look to it for a scale outboard, i will be getting some custom motor cowls made up for the outboards.

The K&B 3.5 or the REK lowers have the shortest pivot pin to drive dog length.

 

[Terry Michael]

Got my EVO C - 45 hull today,the quality and design of this hull is outstanding,