Re: [electricboats] Pocket Trawler Electrification

Sorry my post is a little rambling but that is my mind.

You have done quite a lot using standard techniques. However all this stuff was developed for hose and buggy days.

In water what matters is boat drag (in all its glorious ways) and thrust from the prop plus some effects from wind and waves.

Your hull size, boat displacement will set primary hull speed. That is physics and there is no going round that. Having a poor surface finish will reduce theoretical and practical through water speed. The closer water speed gets to hull speed the more motor torque you will need. Spinning the prop faster just makes bubbles and eventually cavitation. Trust goes up in a square law or higher. Unless you get to a plane you are stuck. I read in many older books on sailing that "people are always amazed" at how little power is need to push along a big ocean yacht at 2-3 knots. Street talks of using a single Seagull to push his 38' ocean boat. Adding another absolutely does it. My 2 tonne Norwalk Island Sharpie appears to be able to accelerate to 4 knots in 10 seconds with a thrust of 35 newton. That is pretty fast for me.

But you can be more efficient. Bigger better props get conversion from shaft toque to trust better. Yes having toothed belt drive will help efficiency and if sized correctly improve reliability. You should be better able to tune the down ratio too.

Have you measured hull drag at various speeds? Tow the boat and have a scale measuring force. You can estimate wind load using standard formulas and assume boat is an ovoid.

Prop trust is given by some manufacturers but not many so need to use one of the estimator apps. Prop efficiency (prop torque to thrust is hard to estimate. I gather good props correctly positioned have efficiencies of 50% or so. My guess is a bigger nicer prop, spinning more slowly wins the conversion rate.

Now looking at sailing boats with conversions, I see that 2-3 knots for a 30' displacement hull runs at 500w or so. A light 21' trailer sailer with an e propulsion does 100-200 w at the same speed. At max boost 750w it might get to 3.5 knots. The bigger boats typically 30' displacement weighing three to four tonnes gets 4kw at 4.5 knots. So your 10-12kw or 20kw or 30 kw+ still won't get you to 6 knots. Your batteries will last minutes not hours. You will be current limited by BMS and battery cells. You will need massive copper cables. Your motors will need high flow water cooling.

Best Regards

Lee@vombatus.com.au
0427874796

On 28 Oct 2025, at 3:18 am, maaseidvaag via groups.io <maaseidvaag=yahoo.com@groups.io> wrote:

Greetings, new to the group and it looks like I've found just the right place to get some ideas on my project.  I came here from the link at Thunderstruck.

 

My boat is a 27' trawler designed by Ted Brewer.  The design is the Blue Hill and the specs can be found here http://www.tedbrewer.com/power/bluehill.htm

 

The boat is powered by a Yanmar 4JH3E which is way overpowered for displacement speeds (what I do) and underpowered to push the boat on a plane.  My normal cruise is 5-5.5 knots at 1800 rpm, which is really too slow for the motor to be spinning.  The transmission is a 1:2.67 Kanzaki reduction, into a Walter v-drive (no reduction) to a 17x12 three bladed prop.  The transmission is making a clanging noise at low forward rpms which disappears at the rpms increase.  There is visible movement of the output shaft and I can grab it and fell it clunk as I pull up and down.  Forum wisdom is that that transmission isn't worth rebuilding even if the parts are obtainable, and a ZF or TwinDisc replacement is the way to go.  So, since I'm already lined up for a $4k+ bill...

 

It would appear that 10-12kw will be enough power to get me to near hull speed of 6.5 knots or so, and easily at my target 5.5kn.  My cruising grounds are the Great Lakes, North Channel, Trent-Severn waterway, etc.  I do have a NextGen 3.5kw generator onboard, and have experience putting together my own DIY LiFePO4 batteries.

 

Options I'm considering: 10-12kw Thunderstruck kit/motor with the belt reduction, which because of the very low placement of my prop shaft would still require that I use the energy sapping Walter v-drive.  Or, two ePropulsion pod 6.0, for a total of 12kw between the two pods.  I like the redundancy of two pods, and removing the weight of the v-drive and prop shaft (1.5", 7 feet long).  That extra weight alone would allow me to add another 48v, 314ah battery.

 

Thoughts on the pods?  Thoughts on the inboard motor direction?  I'm sure I'm missing plenty.  Thanks in advance.  Data from vicprop.com for the pods is included in the image below

 

Lars

 

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