electric engine set up

I'm building a Vosper RTTL from an old plan. its 34in long and the plan states that its suitable for up to a 3.5cc diesel. my back ground is in aircraft modelling where I would usually use a brushless motor with 3 or 4 cell lipo battery. This is my first venture into RC boat modelling. my question is a brushless motor suitable for this type of boat? if so do you need cooling for either the motor or speed controller? 

Im thinking of using a just engine 28-30-1280KV Brushless Motor. would this be ok? if so what size prop shaft and propeller should be used?

Any help would be appreciated

Pete

Generally for a boat like this an out runner motor would be needed, in runners are to much rpm for this project along with not able to provide the torque needed to throw the prop correctly with out blowing out the esc...

In this case I’ve built a “few” boats of this size and found that a 1500kv motor is better than an 11ookv ( or similar) as they have the added bonus of getting out of trouble speed!

My go to brand for a 1500 kv our runner is overlander they also run on a 30 amp esc ( which is easy to find a water cooled esc in that size!, other sizes struggle to find water cooling until you hit the 120-180amp sizes ( designed for in runners for the mega fast speed boats)

For a 34” model a M4 prop shaft is more than suitable ( just check your length from motor to prop and remove 1” for the coupling (huco couplings work fine!) size of the prop is a little hit and miss depending if u want accuracy or scale speed... I don’t go for either generally as it’s a waste of time, so a PLAIN 40 (40mm plain) prop is more than enough to give you on a 3s Lipo about 17,000 rpm at the prop minus load... so about 15000rpm 

That will give the boat a good top speed and a little more if u have the speed devil inside wanting to get out, given it’s a Vosper then U want that speed straight out ( if it’s a twin prop boat the twin 1200kv ca n do the job too!) 

So sum up

M4 prop shaft..
40 plain prop ( use a X40 if u want even more speed but unlikely with above suggestions) a plain 45 is also a possibility however your upping the motor load with bigger props but I think going of the boats hull design and prop shaft exit distance a 40mm maybe the max u can fit anyway.
1500 kv OUT runner no need to water cool an outrunner just the esc!
30 amp water cooled esc ( seaking) 
3s 35c lipo

Remember boats much like aircraft don’t react the same way to the elements, but boats fortunately you can build stronger and heavier with little change to overall performance other than minor speed decrees.. 

Have a look through the vintage boat and build blog sections to see some of my boats with this same setup!