choosing designer / naval architect

[ViennaYachtworx]

We would like to build a 4,5-5m electric powered boat in GRP with a classic appearance. How do we find the right designer for the job? The underwater is bit more tricky as it may seam at first glance as we would like the boat to plane with merely 2kW.

[HJS]

To plane with five-meter boat with 2 kW electric is probably quite difficult.
A boat with a total weight including driver of 300 kg has a planing speed around 12 knots.
And it is impossible to achieve that with 2 kW, especially if the boat has a classic look.

I enclose a picture of a boat that I designed for a client. It has very good qualities thanks to its special bottom form adapted for small engines, electric power and rowing.

Length over all 5,5 m
Length in waterline 4,8 m
Beam over all 1,6 m
Beam in waterline 1,1 m
Displacement on datum waterline 0,33 m3
Weight 130 kg
Weight with el. engine and lithium batteries 165 kg
Weight with el. engine and lead batteries 210 kg
Range with lead batteries 25 nm at 6 knots
Range with lithium batteries 50 nm at 6 knots
Speed with 8 hp outboard engine, 1 pers. 16 knots
Speed with 3 kW electrical engine, 1 pers. 9 knots

[FAST FRED]

Until the batt set gets 10X to 50X the power density of what is aviliable today, most likely any "designer" you get to take this comission seriously will be a con artist.

FF

[ViennaYachtworx]

Dear Fast Fred. The battery set for a 2kW engine for a small lake boat is in LiFe under 20kg. reducing weight further is easier by putting the driver on a diet. ;-)

By classic look we meant above the waterline.

Can a 4kW outboard be enough for a 250-300kg boat (including driver) for planing?

Thanks for all the answers!

[HJS]

Can a 4kW outboard be enough for a 250-300kg boat (including driver) for planing?

With 4 kW outboard of proper design (most are not) and the right propeller it CAN be done.

js

[ViennaYachtworx]

Dear HJS,

What do you mean by "proper design" ? - We have a Torqeedo Cruise 4R in mind - is that proper enough for the task?

[HJS]

Originally Posted by ViennaYachtworx

Dear HJS,

What do you mean by "proper design" ? - We have a Torqeedo Cruise 4R in mind - is that proper enough for the task?

Just a sketch, there is a difference.
Also check the prop, is it suitable for 12-13 knots? I doubt.

js

Originally Posted by ViennaYachtworx

Dear HJS,

What do you mean by "proper design" ? - We have a Torqeedo Cruise 4R in mind - is that proper enough for the task?

NO,
the Torqueedo will not get any boat that size on plane.

I fear you will have either to give up planing speed or el. propulsion.

Regards
Richard

[ViennaYachtworx]

Dear HJs and apex,

Took a few days to receive the numbers from the engineers but nevertheless.

The Torqeedo Cruise 4.0 R develops roughly 350N at 12,5kn. A Testboat (from Torqeedo) with a total weight of 200some kg was measured at 27km/h. An Ultra highspeed Prop is also currently in development.

They do however recommend us to stay below 250kg total weight.

Originally Posted by ViennaYachtworx

Dear HJs and apex,

Took a few days to receive the numbers from the engineers but nevertheless.

The Torqeedo Cruise 4.0 R develops roughly 350N at 12,5kn. A Testboat (from Torqeedo) with a total weight of 200some kg was measured at 27km/h. An Ultra highspeed Prop is also currently in development.

They do however recommend us to stay below 250kg total weight.

No surprise,

planing speeds are NOT to achieve at these spec.s

sorry.
Richard

[ViennaYachtworx]

Dear Richard,

I'm not a naval architect but isn't 27km/h with a boat this size impossible in displacement mode? - therefore isn't it planning?

Isn't this boat powered by 1 Torqeedo Cruise 4.0 R planning: http://www.youtube.com/watch?v=JVFnI...eature=related

[HJS]

Isn't this boat powered by 1 Torqeedo Cruise 4.0 R planning: http://www.youtube.com/watch?v=JVFnI...eature=related

No, it is not planing. According to the second video is the boat traveling 15.9 km/h, that´s 8.5 knots. That is probable true with a 12" x !0" prop at 1250 rpm. That is the highest rpm according to the Torqeedo website. That gives a 17 % slip.

js

[daiquiri]

Originally Posted by ViennaYachtworx

The Torqeedo Cruise 4.0 R develops roughly 350N at 12,5kn. A Testboat (from Torqeedo) with a total weight of 200some kg was measured at 27km/h. An Ultra highspeed Prop is also currently in development.

350 N @ 12.5 kts = 2250 W effective power.
Not enough to push a 300+ kg planing hull at that speed. Besides, since the motor absorbs 4000 W, it means that the propulsive efficiency is roughly 0.55, not really a stunning value.
With a better designed prop the overall efficency could hopefully be pushed to 0.80, which would give 3200 W effective power. In that case you would have something to start with.

[Wavewacker]

Will a hydrofoil design work? (Just a thought, I have no idea!)

[daiquiri]

Originally Posted by Wavewacker

Will a hydrofoil design work? (Just a thought, I have no idea!)

A classic appearance is required.