Boat Design Forums  |  Boat Design Directory  |  Boat Design Gallery  |  Boat Design Book Store  |  Thanks to Our Site Sponsors

Go Back   Boat Design Forums > Design > Propulsion
Register FAQ Members List Calendar Search Today's Posts Mark Forums Read Most Recent Posts Gallery Images Search

Reply
 
Thread Tools Search this Thread Display Modes
  #1  
Old 02-20-2012, 03:27 PM
vaderverdi74 vaderverdi74 is offline
 
Join Date: Feb 2012
Rep: 10 Posts: 4
Location: Balcan
Electric outboard conversion

Hello there all,
I am converting a old johnson 25hp 74' outboard motor to electric outboard motor with etek 48v. I use it on boat which is semi displacement (WL around 4.2m) weight around 500kg (or more) with cabine.

I have made some testing and get a result as for 48v around 90 amps i got top speed very low as a 3 knot or around 5.5 km/h with original propeller.
I was wondering how can i improve speed, first thing was changing propeller but to what bigger diameter ok, bigger pitch since i think original it was designed to operate to 5000 rpm or more, and i with etek do around less than 3500 rpm.
So what is your suggestion to put two pulleys so to get more rpm on shaft or change to different propeller and which one.

I was thinking to make diy propeller just for testing not actual using so i can see which pitch suites me best, but i know very low on propellers, and i do know it is complicated.

I also work with polyester and sheet metal so i can form some but i dont have clue to which form.

At moment I have on boat 4.5 gasoline with real on shaft less then 2.5hp around 5.8 kn kouts.


Any suggestion idea, invision is more than welcome.
Thanks
Reply With Quote


  #2  
Old 02-21-2012, 03:46 AM
CDK's Avatar
CDK CDK is offline
retired engineer
 
Join Date: Aug 2007
Rep: 1761 Posts: 3,007
Location: Adriatic sea
You are generating 6.8 HP with the present setup. That should give enough push to reach hull speed plus some reserve. Because the hull is only 4.2 m. long you won't get much more than 3-4 knots no matter what you.
The prop was meant for 25 HP and planing speed, it will not be very effective at the speed you achieve now. Try a prop with a larger pitch, not to get more speed but to reduce the current so you can extend the range.
__________________
cogito, ergo sum (Descartes' credo)
Reply With Quote
  #3  
Old 02-21-2012, 06:25 AM
vaderverdi74 vaderverdi74 is offline
 
Join Date: Feb 2012
Rep: 10 Posts: 4
Location: Balcan
I am novice about all prop stuff, i was planing to go with pulleys and prop rated to 10 hp which is same like mine but has less surface area, because i get lots of thrust i think at current setup it is way too high for thrust.
I am not looking to improve speed a lot just a little bit, to max 5-6 knouts no more.
http://www.evalbum.com/492
This guy built same almost like me and did greater in speed than me, prehaps diffrent boat, i think it is almost flat bottom maybe.
Reply With Quote
  #4  
Old 02-21-2012, 12:49 PM
CDK's Avatar
CDK CDK is offline
retired engineer
 
Join Date: Aug 2007
Rep: 1761 Posts: 3,007
Location: Adriatic sea
I'm sorry, but you cannot reach that speed. The link you supplied mentions a 26 ft hull, your is only 14 ft. so at 6 kn it lifts the bow creating lots of drag.

You give your location as "Balkan". If you are near the coast, observe the Trajekt (ferry), doing 24 km/h in displacement mode, while small craft must plane to go that fast.

There are no belt drives or gearboxes without loss on this planet, so don't use any unless there is no other option.
__________________
cogito, ergo sum (Descartes' credo)
Reply With Quote
  #5  
Old 02-21-2012, 03:36 PM
vaderverdi74 vaderverdi74 is offline
 
Join Date: Feb 2012
Rep: 10 Posts: 4
Location: Balcan
Yes i am near coast, like you on adriatic sea,
well i tried and on this web page with 4.2 meter on water lenght with displacement of around 800 kg, i get hull speed of 9.28 knouts, and to go 5 knouts says around 4 hp.
Have i mistaked somewhere?
Please do help
Reply With Quote
  #6  
Old 02-22-2012, 03:59 AM
CDK's Avatar
CDK CDK is offline
retired engineer
 
Join Date: Aug 2007
Rep: 1761 Posts: 3,007
Location: Adriatic sea
This boat speed calculator, using Gerr's formula, predicts approx. 6 kn. with 6 HP input:
http://www.psychosnail.com/boatspeedcalculator.aspx

Bear in mind however that this is purely theoretical; there is a considerable error margin, especially for small hulls. But let's assume that your boat has the ideal hull and you can reach 6 kn. at full power.
You need 8 batteries 12V/100Ah to draw 90 Amps for one hour. The weight is approx. 200 kg. so your estimate of 800 kg. for the boat, batteries and yourself is probably on the mark. But at full throttle the range of 10 km. is a bit disappointing, at least that is how I would feel about it. So you need more batteries and/or a generator, increasing the gross weight to more than a ton and shaving one knot from the top speed.

I would settle for a lower speed and try to optimize the propulsion.
You need an Ammeter, power controller, rpm indicator and a GPS, so you can reliably plot a graph. If the functions of rpm and speed are linear all the way (that means a straight line) the prop doesn't have enough pitch. With the right prop, the line should start to curve near the end.
Also check the relation between current drawn and rpm. That should also be more or less linear with an offset at the beginning of the line, showing the power lost in the transmission. Use a thinner oil in the lower gear case: the temperature without exhaust gases is much lower.
__________________
cogito, ergo sum (Descartes' credo)
Reply With Quote
  #7  
Old 02-22-2012, 07:41 AM
michael pierzga michael pierzga is offline
Senior Member
 
Join Date: Dec 2008
Rep: 1173 Posts: 4,758
Location: spain
I read that guys playing with electric boats...and your E TEK motor...often fabricate props from wood when considering props.

Perhaps useful reading here.

http://www.electricboats.co.uk/motor.html

http://www.electricboats.co.uk/docs/241.301.pdf



http://www.electricboats.co.uk/docs/241.304.pdf
Reply With Quote
  #8  
Old 02-22-2012, 11:01 AM
vaderverdi74 vaderverdi74 is offline
 
Join Date: Feb 2012
Rep: 10 Posts: 4
Location: Balcan
Postovani CDK,
i allready have 4 batteries 12/100ah and for 1 hour is 64amp's, for 48v at 64 amps gives me around 3kw or 4hp, for top speed is just like runway when needed, but my goal is to travel around 5 knouts for travel time of less than 1 hour. Boat is rated usually around 500-600kg, so batteriers 30x4=120, gears lets say around 60 kg, outboard maybe 35kg (didnt measure), 2 persons 160kg so in total lest say 1000-1100kg. And i have all items to measure (amp meter via shunt, volt meter, and android with gps:-), controller i am making in the moment)

I saw that page and especialy Terapin great project, i was thinking to make prop just for testing, with larger diameter(13"-14") with different pitch's, maybe it will improve to some point and i will do that with extend of shaft so prop could little bit higher than cavitation plate, and after i surround prop with aluminium ring to improve eff.

I have just found book of dave gerr reference of props, just started to reading it, cant wait.

thanks for helping me
Reply With Quote
  #9  
Old 02-03-2013, 07:22 AM
nevada14 nevada14 is offline
 
Join Date: Feb 2013
Rep: 10 Posts: 2
Location: France
Out board motor conversion

I have read with interest this conversion. I beg your pardon as i dont write english very good. I have a 5.20 m boat with a 6hp johnson converted with a 500w 24v electric motor.
With 27 A (648w) the boat run 5.5 Km/h.
Boat 500 Kg
Batteries 200 Kg
Crew 200Kg
With a 10 HP Johnson (Gasoline) we can run at 11 Km/h
I have try a 10 HP converted with a 2 HP electric motor. The boat run at near 7 Km/H with 24v 60A (1440w)
The good new is the boat can run at 3Km/h at 24v 5A (120W)
The bad new is he run at 4Km/h with 10A (240w) and 5.5 Km/h at 27A 24v (648w)
Reply With Quote


  #10  
Old 02-03-2013, 09:39 AM
nevada14 nevada14 is offline
 
Join Date: Feb 2013
Rep: 10 Posts: 2
Location: France
I think you can have a better speed with an other outboard motor with the same electric motor. The 25 HP johnson have a gear ratio of 2 between the motor and the propeller.
The 25 HP Johnson is frequently sel with a 11 or 12 inch pitch propeller. That's to hight for you boat. This pitch give the speed of*:
Calcul for the speed with pitch*:
pitch x 25,4 (to have mm) / gear ratio x RPM x 60 (for cm/h) x Coeff / 1000 000 (to have km/h)
For this hull, coeff is 0,60
11 x 25,4 /2 x 3000 x 60 x 0,6 / 100 000 = 15,08
If you have a 7 inch pitch you have a speed of 9,6 Km/h with less current than 90A, or you can have more speed for less current.
Unfortunately, it is very difficult (perhaps impossible) to find a 7 inch pitch for 25 hp johnson.
You can use the 10 HP Johnson who have a gear ration of 2,42. and you can find propeller from 5 inch pitch (Pirahna propeller). A good pitch with this base will be 9 inch (easy to find) an who give 10,2 Km/h or 8 inch who give 9,07 Km/h
You can also use a 50HP johnson (or Evinrude) who have a 2,42 gear ratio with 8 or 9 inch pitch propeller or the 55HP Johnson or Evinrude who have a gear ratio of 2,67.
I have read on internet*:
For the hull speed*: 2500w/ton
For 90*% hull speed*: 1000w/ton
For 80*% hull speed*: 700w/ton
For more the hull speed it is very expansive of power if the hull cannot fly on the water.....
If you want double the speed on the water, you must use x 8 power. The speed is very expansive.
Reply With Quote
Reply



Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes


Similar Threads
Thread Thread Starter Forum Replies Last Post
Propeller for electric conversion of 25ft (Albin 25) Electric Props 2 07-19-2010 12:49 PM
Electric Jet Ski Conversion glines Propulsion 44 07-17-2010 09:41 AM
Electric Row Boat conversion update... sparky_wap Hybrid 9 07-12-2008 11:04 PM
conversion diesel to electric maitlot Open Discussion: All Things Boats & Boating 18 03-09-2008 10:29 PM
Diesel to electric conversion kach22i Propulsion 3 09-06-2007 08:47 AM


All times are GMT -4. The time now is 11:51 AM.


Powered by: vBulletin Copyright ©2000 - 2014, Jelsoft Enterprises Ltd.
Web Site Design and Content Copyright ©1999 - 2014 Boat Design Net