T-Pain Is On A Boat: Re: [Electric Boats] Knots to Watts

Thursday, July 12, 2012

Re: [Electric Boats] Knots to Watts

One more idea
would be to move crew forward at low speeds to lift the transom stern out of water. They do this in 49er sailing dinghies, for instance. The 49er underwater hullform would be quite ideal for e-boats too, as both applications require low drag figures in a wide speed range!

redu

On 07/12/2012 11:47 AM, KEN wrote:

redu.. what you're describing has had me wondering a while back..

what "mud flaps" might do to reduce that kind of drag.

like full width flexible 12-16-18" long, looking like trim tabs,

kinda behaving like a paint brush at low speeds

--- On Thu, 7/12/12, redu <reino.urala@gmail.com> wrote:

From: redu <reino.urala@gmail.com>
Subject: Re: [Electric Boats] Knots to Watts
To: electricboats@yahoogroups.com
Cc: "Ned Farinholt" <nedfarinholt@comcast.net>
Date: Thursday, July 12, 2012, 2:07 AM

On 07/11/2012 09:32 PM, Ned Farinholt wrote:
Here are my numbers for a 1967 Starcraft aluminum utility, 15' x 5' [14.2' water line], 800 lb displacement, Torqeedo 4.0R outboard, Torqeedo high performance prop, 8 kwh LiFePO4 [16 x 160 Ah].

0.2 kw 3.0 kn

0.4 3.9

1.0 5.5

2.0 7.4

3.0 9.1

4.0 10.4

These are averages of multiple runs, no current, little wind during Sept. 2011. The boat was running in a semi-planing state. That is why you do not see the sharp knee at hull speed, 5.1kn.

Ned

Ned,
your hull with a sinked transom stern is poor at low boat speeds. This is why the hull speed kink is not seen in your data. Low speed kW data for a corresponding displacement hull with raised transom stern should be about:
0.055 kW 3.0 kn
0.15 kW 3.9 kn
At speeds lower than 3 kn speeds the sinked transom stern hull would show really bad drag figures in comparison with a not sinked transom hull. Ref. rowing boat hull small drag at low speeds. Well, propeller efficiency was not considered here, as the reference data were derived from a hull drag simulation software.

redu