# rough DC motor sizing

Discussion in 'Boat Design' started by ENG Student, Mar 18, 2010.

1. Joined: Oct 2009
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### ENG StudentJunior Member

Hi guys
im no expert but this just doesnt seem right to me,
Using CAD ive designed a basic hull outline which ive then imported into a CFD package to optimise the drag values.
The Boat is a catamaran design the hull is roughly 9m LOA 1m Beam 1.5m depth and weighs approx 1800kg, without batteries or motors.
From CFD i found the drag to be approx 300N using single phase flow (just the submerged part of the hull)

Now using the instructions on this website
http://www.solarnavigator.net/hull_drag.htm

calculated the required HP required to 2.75HP @ 7knots which relates to just over 2kW power.

Now ive always liked the lynch electric motors so i chose a 3kW motor...which is startlingly small for the size of the boat

Am i just being paranoid or are my calculations completely wrong

PS this is a university assignment im not actually making a boat..just create and analyse the system

2. ### Guest625101138Previous Member

I have not checked your drag calculation but, if correct, the 2kW will be close. You may do better with a purpose made propeller.

Rick W

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### gonzoSenior Member

My 25' Coronado would get to about 5.5kt with a 3.5 outboard. I never optimized the prop which had a bit too much pitch. The displacment was about 4500lbs (2100 kg). With a strong head wind it dropped to about 1kt. I think that 3kw will give the adequate reserve power.

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### jonrSenior Member

High winds can easily require several times more thrust just to stand still than moving in normal conditions. I'd either overbuild or make sure you are ok with leeway or anchoring and waiting it out.

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