Classic runabout designs needed to electrify

Hi everyone,

We're interested in showcasing what our range of Mitek electric motors and batteries can do here in the UK and are looking for a nice looking boat for this.

My preference is for a small, classic Italian Lake looking boat.

I've come across designs from Glen-L Marine https://glen-l.com and Home https://danleeboatbuilding.co.uk/ for example that are broadly in the right area and need to find out more info such as load capacity etc.

Two quite different but intersting designs from Dan Lee:
Elouise - 17 1/2ft Electric River Launch https://danleeboatbuilding.co.uk/product/elouise-17-1-2ft-electric-river-launch/
and
Rocket MkII - 15ft Inboard Gentleman's Racer https://danleeboatbuilding.co.uk/product/rocket-mkii-boat-plans-wooden-15ft-stepped-hull-gentleman-race-boat/

are of interest for example - I need to understand more details to see what might be feasible for either of them.

Can anyone point me to some other sources for possible designs to scope out?

The use case is to scoot around the Solent area in the south of England (protected coastal waters) and trailer to inland waterways etc as a demo boat.

Something preferably 16ft and under (but possibly up to 20ft) for ease and speed of build and either inboard or outboard.

We're looking to probably use either our 20hp 13kw or 30hp 22kw outboards as they fall into the 48v category for ease but could go to the 40hp 30kw 96v motor.

For inboards we currently have 48v 17kw or 96v 27kw options in the right sort of power range.

We have access to a massive range of our LiFePo4 batteries so I need to get scoping out the weight capacity of various designs to see what might be feasible.

I am also looking out for anything suitable that we can just buy and convert but so far not really seeing anything of much interest.

All thoughts gratefully received.

 

I did a preliminary design here attached for a 16' runabout, inspired by classic ones like the Rascal, but not more developed neither built. But may be this approach with its figures (speed estimation versus the installed E power and the weight) could be helpful for your project.
Rascal : RASCAL - Small Boats Magazine (smallboatsmonthly.com)
Li'l Woodie Rascal Runabout (youtube.com)

 

Classic Motor Boats https://www.selway-fisher.com/McClassic.htm has a range of designs.

Talk to the designers about your wishes, have the entire propulsion package range weight and volume charts at hand when you do. You need a clear ideea about: desired speed, desired endurance, number of passengers.

 

Dolfiman, that is exactly the sort of thing! Great simple lines and your follow up work with the figures for the motors is very interesting.

It looks like you were using the Elco outboards in your calcs - I'd be very impressed if the EP10 could get it going at 18.6 knots as that is only 4.3kw 48v motor so a bit more like a 6hp and weighs around 30kg.

The Mitek 9.9hp is a 7.2kw 48v motor so more closely aligned with the expected petrol and weighs 27kg. The Mitek equivalent to the EP10 is the Mitek 6 which is a 4.3k2 48v motor and weighs much less at approx 24kg.

The Mitek 15 is a 48v 10.5kw motor weighing 49kg, the Mitek 20 is a 48v 13kw motor weighing around 56kg and the Mitek 30 is a 48v 22kw motor weighing around 72kg.

Anyway, assuming your figures are based around petrol equivalents then the Mitek 20 or 30 motors would potentially get it moving pretty nicely.

Your motor weight at 6okg is about right for the Mitek 20 (but under for the 30), but the battery weight I think is signficantly understating it.

At full chat, the Mitek 9.9 is pulling around 150 amps, the Mitek 15 around 225 amps, the Mitek 20 around 260 amps and the Mitek 30 around 400 amps!

Assuming you want to plane for an hour then the 9.9 will need one of our 150Ah 48v batteries that can put out 150 amps continuously and these weigh 63-68kg depending on which pack is used.

I expect even for that then that two packs would be required which at a pottering about power draw of less than 50 amps will give basically all day running or 2 hours flat out giving some serious range but weighing around 126-136kg so effectively two smallish people. I doubt the 9.9 will get it going overly well.

Going up to the Mitek 20 then a minimum of two 150Ah atteries will be required for the peak power requirements (or three of our 100Ah batteries that weigh around 45kg each) and this will give an hour and bit flat out but realistically three 150's weighing 189-204kg would be better giving an hour and half flat out and should be planing.

Lastly the Mitek 30 pulling 400 amps will require three 150's as a minimum so 189-204kg so two decent sized guys and again that will only give just over an hour flat out but should be moving along pretty well.

I think I will knock up some draft hull shapes in freeship to get an idea of the waterlines to help understand a little more around the weight limitations and see what might be within the bounds of possibilities in this size of boat.

We'll have some usfeul data shortly as soon as the weather clears a bit as will be out with the Mitek 15 and 20 on our eGuardian 420 boat:


The image is not showing for some so this is a link separately to it:
image

This shows it with the Mitek 9.9hp with temporary bench seat and offset console and this weighs around 205kg for the hull inc the bench seat and console.

The main version of it has a central console with integrated battery storage for 2 150's in the base design with either the 9.9 or the 15 as they are mainly used at slow speed for safety boats pottering about but can pull a string of boats no problem and the 15 moves nicely - we will get GPS data to show all the capabilities.

Thanks again for your input - it was really interesting and useful.

 

Hi Rumars,

Thanks for this - there are a couple of useful options on there.

 

A boat that comes to mind for me -- outside your size parameters but light, cheap, easy to build and designed for low power -- is Phil Bolger's Sneakeasy. Both the original sharpie version and the later stepped chine version (see pics) can look very elegant if finished that way. The original can do about 20 mph on 20 hp, the step chine model can take a little more power and go faster or just handle a little better at the same speed. There is also Tennessee, a similar hull but set up as a cruiser, that might also be fun as a demonstrator zipping along silently with almost no wake.

 

I rather like the look of that as well - I think it would get in quite a bit of bother in the Solent which is not reknowned for being the easiest of waters!

The shape however is pretty cool - long, thin and looks the part.

 

I have absolutely no experience in the Solent, but I will say that a flat-bottom, sharpie hull like this one can be better-behaved in some conditions than one might expect. The long hull bridges the crests from one wave to another in chop and so gives a much smoother ride than seems possible in the right conditions. They are not, however, the boats for big water or swells, full stop.

 

Actually I used EP for Electric Power in the proposed formulation, without making specifically reference to Elco products.
The formulation to estimate the peak speed (on calm water, no head wind) versus the installed power was initially established for « petrol » power P and is :
V = 502,56 * (0,087-0,0015 Beta)^0,5 * (P/m)^0,5
V = Speed in Knots , Beta = Aft deadrise angle (°), P = Power installed in Kw , m = weight in kg

In my above approach, I changed the coefficient 502,56 for 615,5 through assuming that in average 1 kw « electric » is equivalent to 1,5 kW « petrol » (so 615,5 comes from 502,56 * (1,5)^0,5). But from your Mitek catalog, it seems that it is not that simple and moreover variable from one to another model. So I recomputed the speed prediction using directly the initial coefficient 502,56 and the Mitek data given for "equivalent petrol power" , + also the engine weight taken into account for each outboard models (while keeping 80 kg fixed for the batteries).
>>> So here attached this revised version V1 with the various Mitek model from 6 hp to 40 hp, by hoping this is more relevant.

 

Ha! Thanks, I just assumed EP was related to Elco as that is their motor model designation.

The Mitek motors relate more closely to petrol HP equivalents to try to be more open and transparent for people looking to electrify but unfortunately it is not a simple linear relationship with the power delivery being quite different still due to the torque chatacteristics of the electric motors.

It is simply a case of trying to be more realistic than most manufacturers are to help people get as close as possible to the equivalent to their known petrol counterparts to not be horribly disappointed by sub par performance when told something is a 5hp equivalent etc as a lot of people have unfortunately seen...

Anyway I've re-created your formula in Google Sheets and using the parameters you describe I get the following:

so 14.1 knots with the figures in B3:B6, so I am some way out for some reason? I'm not sure what I am missing but am doing this while talking on the phone so not really concentrating.

Then, if I up the weight for the batteries to 3 x 150Ah I get the following:


I will have some GPS data shortly from our eGuardian 420 shortly for the Mitek 6, 9.9, 15 and 20 and will post the results for comparison - it is significantly shorter at 4.2m and a rotomoulded hull but we will gather some useful info I think.

I've very quickly started lashing up a rough hull outline shape on FreeShip using 4.75m LOA and 4.1m LOW and max beam at deck of 1.7m, BOW around 1.61m, deadrise 14 degrees:


I've bodged the hull weight to be 670kg to very roughly emulate the MO20 with the 3 x 150 batteries and 2 crew @ 150kg and the waterline is at 22cm so bow freeboard around 38cm.

The lines are clearly utterly **** but it is close enough to get some idea of viability - this approx size can take the sort of weight required if built well enough and light enough.

So seeing how a hull would perform with the various spec motors and batteries is key.

We desperatewly need a weather window to get out testing the eGuardian 420 as soon as we can which will really help - we need this testing to close out the design asap as we have sailing clubs queuing up for them but most won't be buying until end of this year ready for next season.

Separately then we are electrifying a venerable Rigiflex 360 as we have a lot of enquiries around them - we are getting on with working out mouldings for combined battery / seating solutions for them as there are thousands of them around sailing clubs that could be fairly readily converted.

Anyway, that is totally separate - thanks everyone for the input so far.

 

Where we are is known for it's choppy conditions and can make navigating quite tricky to deal with. The tides can be quite challenging around the narrows between the Isle Of Wight and Hurst Point with fast moving water and massive changes in depth.

In some areas there are 6 different currents flowing around against each other and we get some comedy standing waves and turbulence so generally a bit of stability is a good plan.

It would be interesting to do something like this once we get some larger premises and have more storage as currently have zero space - hence looking at something smaller.

 

Oupsss ...., I did a stupide mistake in my revised computation : I simply forgot to use 502,56 instead of 615,5 in the column C ! , so the right value in column C is 129,1 (=502,56 * (0,087-0,0015 Deadrise)^0,5 when Aft deadrise = 14°) ) >>> and the right revised document is here attached.

An additional thought about the center of gravity longitudinal position, the weight of the batteries being an important % of the total. From good authors, they proposed guidelines for a CG centering which could a priori both avoid porpoising occurence (when CG is too far aft) and less planing speed than expected (when CG is too far forward). In short :
** some proposed : 1 < LCG/Bc < 1,25
** some others proposed : 30% Loa < LCG < 40%
In my example : Loa = 4,88 m , LCG = 1,80 m, Bc = 1,60m >> LCG/Bc = 1,13 and LCG/Loa = 37%

 

In your formula, it is …… (E10/G10)^0,5, not …..(D10/G10)^0,5, this is the explanation of the difference you mentioned.

 

Teach me to try to multi task!

FYI I've updated the spreadsheet with the actual Mitek kW ratings as follows:


and with some massive battery power:


I desperately need to get on with some testing in the eGuardian first once this next storm has passed and the weather hopefully clears and I will post the results based on the info I have for that hull.

In the meantime I will carry on looking at some possible suitable designs and look into how the battery placement could work in them as this will clearly be key to success.

 

Actually, in column E should be just the conversion in kW of the equivalent petrol power hp put in column D.
And the column C, being now 502,56 * (0,087-0,0015 Deadrise)^0,5 , should give 129,1, no longer 158,1 (see my quote #12). (I think you don’t use it presently in your computation of V)
And then V in column H is : C*(E/G)^0,5

I actually started from the historical Crouch formula, calibrated the coefficient with available data for the Rascal with various outboard hp, then tinkered a coefficient formulation to take also into account the aft deadrise (Rascal one being 7°).
https://cs.brown.edu/people/jhughes...l#:~:text=Note that it doesn't,C/((DISP/HP)**.