On Apr 25, 2017, at 12:11 PM, Hannu Venermo gcode.fi@gmail.com [electricboats] <electricboats@yahoogroups.com> wrote:
EXCELLENT POST BY James Sizemore !
Imho.
He used a boat of 13 tons, vs the == 8 tons for the SeaRay - but very similar otherwise, a planing deep-v go-fast powerboat, of its era.
His data is 7800 W, 7.8 kW, for 5 knots.
15.6 kW for 6 knots.
DOUBLE load from 5 kn to 6 kn.
So, for 8 tons vs 13, for a similar length/type boat, => 4-5 kW, at 5 kn, +/-.
So the SeaRay 11 m, might need approx 4-5 kW total, only, for 5 kn.
A similar length similar-type boat (go-fast sea-ray, chris craft, etc.) of 8 tons vs 13 tons, would use maybe about 30-40% less power at displacement.
This is quite close to what I predicted for the SeaRay OP, at the lower end in power for perhaps 5 knots, and at the mid-high mid perhaps for 6 knots.
As James said, he needs 34 kW for hull speed displacement ...
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James...
Your data is extremely valuable.
Have You tried to plane / does your current system permit trying ?
I suspect Your props / driveline or perhaps motors may be limiting, and after an initial 20 secs surge, perhaps 13-15-18-20 kW would probably keep her on plane.
This is just commentary.
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The newest updates on the web are 2015, and you said the motors were overloaded on a comment on the blog.
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My point (If there is one).
Electric motors are very good - due to high torque at low rpm.
Big ice motors on old big heavy displacement boats are very good - due to high torque at low rpm
A friend changed the main motor on a 70 metric tons displacement, 24 m length, 80 feet, lake cruiser, to an old 100 hp engine.
The engine, == 2 tons, came from a train locomotive.
It was vastly overpowered, usually running at 10% or so, around 5-8% or less, of power output.
The prop was far too small, for the torque of the new engine.
Engine would free-wheel or overpower the prop very very easily.
So, proven, a 100 hp engine is much too big for 70 tons of boat with a conventional ICE prop for a 70 ton boat.
Old 200-300 tons, metric, trading vessels, early 1900s, were 150 hp or so.
Certainly their props, gears, driveline, were not too optimised or excellent.
Length in these trading vessels 20-35 m.
Similar to the fleet of christopher columbus.
Commentary:
Old rule of thumb for marine naval architects was 1/2 hp per metric ton of displacement.
So, an 8 ton SeaRay, => 4 hp = 3 kW.
And they had poor drivelines and props.
Example:
I have moved a 28 tons Baia Azzurra, 24 m deep-v 60 knot boat, mutliple times by hand.
2 x 1300 hp.
My power was less than 70 W, at low/zero wind, and speed == 0.5 kn.
At some wind, my 70-100W max power is inadequte to hold it.
My 70 W can move the 70 tons cruiser at == 0.5-1knot, fairly easily.
No way, 1.5 knots or more.
On 25/04/2017 02:05, James Sizemore james@deny.org [electricboats] wrote:
I converted a 41 foot Chris Craft Commander, at 13 tons displacement I would say your Sea Ray would be lighter, below is a blog post showing what my Semi-displacement hall needs to make it move. 34kw or 46hp gets my hull to hull speed. She will never plan again, but I sure do like all the extra space as a classy live aboard.
On Apr 24, 2017, at 6:40 AM, cliff_sadler@yahoo.com [electricboats] <electricboats@yahoogroups.com> wrote:
As the OP about the Sea Ray, thank you for the insight I was scanning for. I have no intention of trying to get on plane if I were to do the conversion. I understand the requirements for that would be prohibitive in cost, size, and complexity. What I am looking to do is more like the e-Tolly project, but in an updated platform, and retaining my creature comforts of air conditioning, 120v power, etc via an onboard generator. I am lucky enough to live in an area where nearly everything we wish to do on a boat is well within an hour's reach at 6-7 knots. I needlessly carry around 800 hp that I rarely utilize. That is the whole point for me.
https://www.youtube.com/user/WhoIsHayley
-- -hanermo (cnc designs)
Posted by: James Sizemore <james@deny.org>
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