Electric Leisure Boat Design II

read Nature of Boats by Gerr.

It is not iverly technical or math heavy yet great 1st introduction of what factors impact boat design.

All kinds of silly things have been done, by companies and especially by modern youtube crowd who are after viewers more than accomplishing usable designs.

You will want something that is as long, as skinny, and as light as possible within the other design parameters. Length is a major factor for displacement boats. A person in a 12ft kayak will struggle to keep up with a 15ft one - even if fair bit fitter.

Forget RIB / rubber boat shapes. Canoes and classic rowboats should be close to the goal. Possibly with outriggers or twin hulls for stability.

 

How is that going to work in a rental business wanting to rent out boats for ten hours a day?

Regarding your posts in general, and sorry if I'm going to sound blunt again: I know there are people on this forum with 10x more knowledge on this subject than i have. I also know they will only start helping if you start asking the right questions.

 

Some lithium iron phosphate batteries can be charged very quickly. Maybe half an hour or so. It might also be possible to take out the discharged battery and put in a charged one.

If you have too much money, go ahead and buy the extra batteries and motors the little speedboat requires.

If it really has to look like a motorboat, you could go for a slightly shrunk Sneakeasy.
https://www.duckworksmagazine.com/04/s/projects/misser/m-1.jpg
It does run out straight to a submerged transom, but it's so skinny it's probably low drag anyway. The funny think is that, because the sides are straight, a Sneakeasy feels very stable.

Lots of old motorboats had relatively narrow hulls, which is just what's needed here.


Obviously, this one's a bit larger than needed, but it gives an idea. The tumblehome at the back might be hard to replicate in fiberglass, but maybe that part could be made in a separate mold. Someone who's kind of an artist and good at suggesting things without completely building them might be useful for this.

 

Ok, so out with rubber boats. Canoes and classic rowboats - the longer the boat, the lower it sinks in the water, and therefore the less resistance, less surface area and skin friction? I read about that somewhere.

I think a little explanation is in order: people here have been very nice and patient with a new person on the forum attempting to design a boat on paper. A paper boat? I have done lots of those. Very slow.

Here is my specific problem. I am sitting here 200 metres from beaches with protected lagoon and a maybe five water sports outfits. The Banana boat is not it. The Couch is pointless. The jetski they wont let me ride without training and it costs $60 per hour, but I don't see any point in Jetskis. Why is there not a motorboat I can rent? Are they not popular? In Florida, USA, for example, you can rent a car for $80 a day but a speedboat rental costs $80 an hour. Why is this? Why are boats so expensive? There is no sailing catamaran either.

BEST Fishing Boat Rentals | Find Fishing Boat Rentals Near Me | Boatsetter https://www.boatsetter.com/fishing-boat-rentals

A boat is not so complicated. When designing and constructing an aircraft, even a radio controlled one, there are so many factors to consider: wing loading, weight, control, power: if these are not right it will not even move. A boat seems so much simpler - the hull, steering wheel and outboard - no electrics even. I know, the kinds of boats on boat design are much more complicated that aircraft, but I am looking at the minimum. Why are they so expensive? I used boat will cost about $1000 as far as I can see, then there is maintenance and docking charges. If a kayak is $10 an hour why can't I rent a low-power electric troll motor boat for $20 an hour? Are these not popular - instead people want to be towed along on large inflated bananas and couches. What is going on here? Have tou seen the warning signs on jetskis that if you fall off and water enters your body at high speed...

So I set out to design a low-cost boat. See if it is possible. The best thing would be to buy a motorboat, rent it out to a water sports company and sell whatever is left after 1 year. I could even get it converted to electric and sell it all after a year.

Boat Without Motor For Sale - ZeBoats https://www.zeboats.com/used-boats/boat-without-motor-for-sale

Maybe even an inflatable. It's affordable.

Boat - Lankabuysell http://www.lankabuysell.com/Search/q=Boat&id=22

And close to home: about $1000

2021 Boat in Chilaw | ikman.lk https://ikman.lk/en/ad/for-sale-for-sale-puttalam-1489

Many of those involved in boating are the people with millions of years of experience and millions of dollars to spend. I am not really in that league at all. My next big purchase will be a car, not a boat.

So I will proceed with my design - it is just a design, do the power and endurance calculations, put up some more pictures and stop, only to take it up at first opportunity when I can get something together that floats.

So it is buy, DIY and sell the parts or buy, rent out and sell the parts . That way I can keep it affordable.
The end result maybe a newspaper article that stimulates interest in electric boats - in my part of the world - and a book about my experiences - that ought to be interesting given the outlandish (!) ideas I will try.

And the book - it is a good book but no Kindle edition. Any links to anything to read would be appreciated.

 

So let's assume I bought the boat in question. $1102. Adding a steering wheel, 4 lead - acid batteries and a motor will cost another $400 and another $500 for a golf cart motor. There was a project on boatdesign using a golf cart motor:

Golf cart motor for small boat. https://www.boatdesign.net/threads/golf-cart-motor-for-small-boat.30268/

Weights will be 40 kg per battery, that is 160kg, and the boat will run as a 2-seater only. Speed control will be somewhat unconventional consisting of 4 switches to switch on 1 battery or 2, 3 and 4 batteries in series to make up a total of 48V.

For endurance and range I will do some reading and calculations and get back...

 

No. Boat's bow makes a wave and that wave will grow with speed eventually making the boat go "uphill". The length of the boat shifts this situation too higher speeds and narrow and light boat make the wave smaller so the impact is smaller. This is why multihulls can often cruise with low power above their hull speed - which is not a literal limit but a "rule of thumb'ish" point ast which the resistance starts to grow fast.

The other resistance factor is skin friction so it doesn't make sense to make a 10m kayak. But read a book. not many of us bother re-typing these things to every new comer time after time.

 

Found it!

@SolGato made an awesome solar electric catamaran:



Slender monohulls as suggested by Kerosene and Mlock can also be very efficient but at smaller scales might end up a little too unstable for the rental market.

 

That SolGato looks very nice. I wonder just how much something like that would cost in commercial production, though. I wonder how much range he gets?

I wonder if a single hull, with concessions to stability, would have more or less wetted surface than a catamaran with optimized hulls of the same total displacement? The Hobie hulls in the SolGato would NOT be the optimum for a powered boat, but they'd be pretty good.

The Sneakeasy feels VERY stable, but I'm sure it has gobs more wetted surface than a more optimized hull. OTOH, 160kg of batteries down low would make almost anything feel more stable.

I could see buying two boats like the one shown in post 20, cutting off the back 1/3, and sticking them back together to make a boat that wasn't totally pathetic at displacement speeds. ;-)

 

Is there a good book in Kindle? ( I left out boats for dummies and inappropriate for my age group)


How to Design a Boat: Sail and Power (Sailmate)
by John Teale | Sold by: Amazon.com Services LLC | Jul 19, 2013


A Boat Builder's Guide to Hull Design and Construction - A Collection of Historical Articles on the Form and Function of Various Hull Types
by Various Authors | Sold by: Amazon.com Services LLC | Sep 6, 2016
8

Sorensen's Guide to Powerboats, 2/E
by Eric Sorensen | Sold by: Amazon.com Services LLC | Jun 4, 2008
12

 

The Nature of Boats is $4.05 in used condition:

https://www.amazon.com/Nature-Boats-Insights-Esoterica-Nautically/dp/007024233X

In case of a language barrier:

esoterica:

highly specialized subjects or publications.

 

The Sneakeasy is an incredibly beautiful design, in my opinion.

There are several old boats on that could be refurbished and recycled with an electric motor. I have seen one or two around here, here is a picture of one.

Don't worry, I am reading this:

potential buy of restore boat https://forums.iboats.com/threads/potential-buy-of-restore-boat.614683/

 

Yes

Shipping (ironically - shipping) costs much more than the book, but I should be able to manage that. Thanks.

 

From the description (useful):


SolGato is a converted Hobie Getaway featuring a pair of Caroute 1.5kW brushless 24V electric motors whose forward and reverse independent thrust are used to maneuver and steer the vessel via a handheld wired remote, allowing for piloting from any position on deck. For power, SolGato also uses a pair of 12V 100ah LifePo4 batteries in Series with a pair of Victron MPPT 75/15 solar chargers and 600 Watts of SunPower solar panels which provide autonomous charging and slow speed cruising at about 4MPH on sun power only. Top speed with a pair of 2-Blade props is 8MPH.

 

I guess in the case of the Solgato, assuming the motors are rated correctly and so on, 3kW will get us 8 mph, or about 7 knots. We only need 6. The power required, assuming that wavemaking isn't big enough to be a large source of drag, will be approximately proportional to the cube of the speed. (6/7)^3 x 3,000 = 1,889 W
If it turns out that 3kW is the output power at the shaft for the 2 motors together, and that motor efficiency is 75 percent (higher efficiencies can be seen with motors with rare earth magnets where everything is matched up right, 80 or even 90 percent). Anyway, if it's 75 percent, then the requirement is 2,518 W. Might as well say 2,500 W.
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Just for fun, I looked at what I thought might be possible if everything was done right on a trimaran with 3 hulls comparable to the single scull I used to row. I guesstimated somewhere between 100 to 150 W for me to go 6 knots. All very loosy goosy, order of magnitude stuff. So, say, 125 W for each hull, for a total of 375 W. If the prop or props are 60 percent efficient, we're up to 625 W. If the motors are 75 percent efficient, then we're up to 833 W. However, I suspect the boat's air drag will be much more than for me on a single scull. Let's guess that the frontal area of the passengers, awning, cupholders, seats etc. all add up to 25 square feet, or 2.3 square meters. (As I recall, the frontal area of a bicyclist is sometimes given as 5 square feet). 6 knots is 3.1 meters per second. The dynamic pressure of air at that speed is 5.8 N/m^2 . Watts out to cover that would have to be 5.8 X 3.1 = 18 W . Accounting for the efficiencies of the prop and motor puts that up to 40 W . Not so bad, huh? However, if we also have a 12 knot headwind, the airspeed will be 3 times as high, the resistance will be 9 times as much, and now we're up to 360 W, just for the air resistance. 12 knots blowing from behind should gain us 40 W . So in still air, we're looking at a total of 833+40 is 873 W. In a 12 knot headwind, 1,193 W. Downwind, 797 W.

Dynamic Pressure https://www.engineeringtoolbox.com/dynamic-pressure-d_1037.html

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I found this research paper:
https://www.researchgate.net/publication/254825704_Mechanics_and_Energetics_of_Rowing/download

Using equation 2.6, the constants derived on page 33, and a total weight of 114kg, the "drag power" worked out to 134 W, which probably means my guess above was extraordinarily lucky. We only have to add (134-125)/(.75*.6) = 20 W !

We might be able to do a little better with one long hull taking almost all of the displacement, with little outriggers that are only in the water when the boat is stopped, particularly if they have tiny little hydrofoils to lift them. Looking back at the paper, it seems to mostly be about single sculls. However, we can guess that, at the very least, results will improve by the surface to volume ratio of hte hull. Given geometrically similar shapes, if we triple the displacement, the new surface area will be (3^0.33)^2 or 2.08 times the old area. I'm going to just say 2, because the Reynolds numbers will be somewhat more favorable. Presumably, that applies to the drag of the hull as well, though probably not to the air drag, unless we switch to tandem seating. Since we only have one hull, the drag of it will be only 2/3 of the previous 3. The new hull will be an even less workable 28 or 30 feet long, though. Anyway, now the power in still air will be something like 609 W, including air drag. This probably means that a single hull can have a shape with 50 percent more surface area than the carefully optimized scull, and still use no more power than the optimized triple hull. I'll admit I haven't looked at surface to volume ratios for a Sneakeasy vs a single scull.

Apparently, there is some interaction between the hulls when you have more than one, but I'm assuming that isn't very significant, which goes along with the assumption that wavemaking is minimal. If wavemaking isn't minimal, the long single hull still has an advantage.

---------------------------------

I think the above is really a best case scenario, and the SolGato is a real world scenario. We can assume that a real world scenario using a hull from a planing motorboat hull will be MUCH worse, except possibly upwind in a gale. With enough homework done, and a custom hull, it's probably possible to improve on the Solgato a bit, since the Solgato hulls are designed for sailing. I suspect those power units have less than optimal hydrodynamics, too. It might be worthwhile seeing what the human powered boat people do, though it may not be possible to adapt such tricks to survive in a rental boat environment.

This was so much more fun than what I probably ought to be doing.

 

BTW, I ran into a formula on line, attributed to David Gerr, for estimating a more realistic "hull speed" for light displacement boats. A user named David Gerr had a comment which had no criticism of the formula. Since I've also seen this formula in one or two other places, I'm going to assume it's right. Whether I copy it right might be another issue.

First off, in Imperial units, D/L is (disp in lbs/2240)/(water line length/100)^3
For instance, for a boat with a 20 foot waterline length, and a 1,000 lb displacement, that works out to about 56, which is very much on the light side.
Then, S/L = 8.26/(D/L)^.311 and speed = (S/L)*sqrt(waterline length)
For our hypothetical boat, that comes to 10.6 knots, compared to a traditional hull speed of 6 knots. It really pays for a hull to be long and skinny, if you want it to be fast.

Let's look at a boat of the same weight, with a waterline of only 15 feet. Then D/L is up to 133, still on the light side. Our hypothetical hull speed is now only 7 knot, which is till a bit higher than the traditional 1.34*sqrt(waterline) of 5.2 knots.

By this formula, if we have a boat of 800 lbs displacement, and we want a hull speed of 6 knots, then the waterline needs to be about 13 feet for a hull speed around 6 knots. But I bet it would be more efficient if the speed we wanted to operate at was well under hull speed, so maybe we'd be better off with a 14 or 15 foot waterline.

Waterlines, of course, change under power, especially if the boat has long, shallow overhangs, If the stem and stern are pointy and vertical, we may not need to worry about this. I was going to post a link for a picture of a sailboat with absurdly distorted lines for a reduced upright waterline and rating, but the internet intuited this and is hiding all the pictures. Just as well, because I don't think we're doing this.

I'm a bit fuzzy on this, but I think the idea is that the resistance goes up unreasonably when exceeding Gerr's substitute for the traditional hull speed. I'm sure planing or hydrofoils are a whole other thing. Otherwise, we couldn't have raised the speed of our old runabout by 5 or 10 mph just by scrubbing the gunk off the bottom. (Something like 30 mph up to 35 or 40. )