Displacement designs for solar electric river cruiser, 20 - 25 feet

[Jfitzger]

This is still the main question, as keeps coming up. Paul Gartside thinks the boat needs some beam at the transom to support the solar, and that speeds of 4 to 5 kts are semi-displacement in any case. Tad Roberts says "Efficiency at very low speed requires fine ends, especially aft where things are apt to get draggy with blunt transom shapes." It is clear that modeling is not available to predict this accurately, and that boats need to be built and tested if maximum speed at low power is the main goal. It is on the wish list, but I am still concerned about the tradeoffs.

I am confused by Paul Gartside’s response to your questions. I’ve read quite a bit about how to design hulls to minimize wave making resistance. Principals of Yacht design has a chapter. Colin Archer developed a system over a hundred years ago. I’m sure there’s software out there that can do it.

Component waveform theory allows you to design a hull optimized to a particular speed. The cross section area of each hull station is determined to minimize wave making drag.
76CD87D6-1B5D-422A-9A97-3B46B77936A5.jpg
Please note that the the speed-length ratio above is metric and is equivalent to R=.938 and R=1.34. For a boat 22 LWL hull speed, which is R=1.34, is 6.2 kts and R.938 is 4.4. On a more or less consistent draft this would mean more beam and finer ends at slower speeds. Gartside’s design to my eye looks like something made to operate at R=2.5 or higher. Narrow with the displacement brought well back.

The idea of needing a wide stern to support the solar panels doesn’t make sense to me. If they were heavy they could push the stern down but that doesn’t seam to be the case. As far as rolling motion why not increase the beam amidships where it has more leverage as well as creating a hull shape with less resistance.

I find it helpful to think and read about the design process to help me pick a design or boat. I mean no disrespect to Paul Gartside in my questions. I present them in the hope we can all learn something from the discussion.

[johnw]

No pictures of this boat coming up in a search, I ordered a copy of the Chapelle book.

Page 205.

[John Meachen]

This is still the main question, as keeps coming up. Paul Gartside thinks the boat needs some beam at the transom to support the solar, and that speeds of 4 to 5 kts are semi-displacement in any case. Tad Roberts says "Efficiency at very low speed requires fine ends, especially aft where things are apt to get draggy with blunt transom shapes." It is clear that modeling is not available to predict this accurately, and that boats need to be built and tested if maximum speed at low power is the main goal. It is on the wish list, but I am still concerned about the tradeoffs.

I'm a long way from being an expert on the topic but I would think modelling to predict resistance ought to be available.Even the quite old version of Freeship I have on this computer will create a resistance curve.Whether it is likely to be 100% accurate might be a topic for debate but I have very little doubt that accurate modelling is achievable.

[rgthom]

After some more communication with Paul Gartside, I find he is really thinking of a faster boat. That makes sense for his sketch, we were not quite in sync on this. For my purposes it probably is not going to work, but we are still discussing.

[cstevens]

This article on another solar cruising boat from Sam Devlin was just published in 48 North:

https://48north.com/cruising/cruisin...ls-on-the-sun/

It's a much larger boat than we are talking about here, but I think still there is a lot of interesting and relevant info.

[JimD]

This article on another solar cruising boat from Sam Devlin was just published in 48 North:

https://48north.com/cruising/cruisin...ls-on-the-sun/

It's a much larger boat than we are talking about here, but I think still there is a lot of interesting and relevant info.

I tried to post a pic showing the beam of this boat, and hence the surface area of the wide cabin for mounting as much solar panel as possible, but couldn't get the pic to post. Nice looking boat.

[cstevens]

I tried to post a pic showing the beam of this boat, and hence the surface area of the wide cabin for mounting as much solar panel as possible, but couldn't get the pic to post. Nice looking boat.

Yes. Sam Devlin's Facebook page has a bunch of posts about it, including some shots from the bow showing the beam. Really illustrates how well the catamaran design works for solar once you get to a certain size.

[JimD]

[johnw]

No pictures of this boat coming up in a search, I ordered a copy of the Chapelle book.

Here's a picture. You can get full-sized plans for any boat in that book from the Smithsonian.


gillnet skiff.jpg

[rgthom]

This article on another solar cruising boat from Sam Devlin was just published in 48 North:

https://48north.com/cruising/cruisin...ls-on-the-sun/

It's a much larger boat than we are talking about here, but I think still there is a lot of interesting and relevant info.

Thanks, that would be a great boat but far beyond my budget. Much higher power, so they went with a cat to maximize the solar and huge battery banks. Might well be applicable to electrifying Skookum Maru, though. Two Devlin quotes stand out regarding my smaller project:

“a monohull would be more forgiving to different degrees of loading.”

Devlin noted that generally, “solar electrics have to be much more like a sailing hull — in other words, they can’t drag their ass around.” It required discerning design when it came to things like exit lines on the hull.

[rgthom]

Some things I like about this solar boat from Croatian company Agena Marin: the solar roof over a covered cockpit with side seating, and a long narrow hull with fine lines at the transom. Make it outboard powered, with just a windshield and a lowering roof, then we are getting close.
https://www.facebook.com/16875823781...54358743147872

Apparently I do not know how to embed FB video....

[kjl38]

Have you seen this book Electric Boats on the Thames 1889-1914 https://www.abebooks.com/servlet/Boo...-srp1-_-image1

[pez_leon]

Rick, I don’t know if this is at all relevant, but it made me think of your project.

https://flic.kr/p/2mThwd7

I happened to turn it up in an architectural history of Colusa by Jane Carter.

Happy holidays!

James

[rgthom]

Have you seen this book Electric Boats on the Thames 1889-1914 https://www.abebooks.com/servlet/Boo...-srp1-_-image1

Thanks, looks interesting. I've ordered a used copy.

[rgthom]

Rick, I don’t know if this is at all relevant, but it made me think of your project.

https://flic.kr/p/2mThwd7

I happened to turn it up in an architectural history of Colusa by Jane Carter.

Happy holidays!

James

Nice! An old low power design, long and lean. It's in the right place!

[heimlaga]

I finally got an opportunity to photograph the boat I mentioned which I think would fulfill your requirements.

An easily driven 19 foot square sterned boat that was powered by a Penta U2 outboard rated at 4hp though in modern money it was probably around 5hp. My grandfather built the boat by eye around 1955 for use on a muddy river and in the rather protected archipelago outside the river mouth. Therefore the hull is rather shallow to fit the river yet he tried to make it reasonably seaworthy. The transom is largely above water to make the boat easily driven.
It was taken out of use in the mid 1970-ies and put away in an old hay shed and is still there.

Everybody who has used the boat agree that grandfather made one misstake with the shape. As he based the design on a much older pleasure boat he made the bow "krokstamna" that is the stem has an even curve and ends almost vertical at the top. He should have made it "rätstamna" that is made the above water part of the stem straight and made it lean outwards which would have enabled him to give the bows a lot more flare above the waterline. In the style of a modern motor powered fishingboat at the time. The added reserve buyancy would have made the boat dryer and more seaworthy in bad weather.
In all other respects the shape and build of this boat was concluded to be a sucsess.
If there is interrest in using her shape as starting point for a new build I rekon my uncle and I could take the lines of her at a very reasonable cost. She was built largely by eye using some measurements taken from a much older rather similar boat so no orginal drawings have ever existed.

[heimlaga]

A picture of the bow of a "rätstamna" boat for comparition. This is the sort of above water bow my grandfather should have given the boat had he thought of it.

fiskarbåt2.jpg

[rgthom]

I finally got an opportunity to photograph the boat I mentioned which I think would fulfill your requirements.

An easily driven 19 foot square sterned boat that was powered by a Penta U2 outboard rated at 4hp though in modern money it was probably around 5hp. My grandfather built the boat by eye around 1955 for use on a muddy river and in the rather protected archipelago outside the river mouth. Therefore the hull is rather shallow to fit the river yet he tried to make it reasonably seaworthy. The transom is largely above water to make the boat easily driven.
It was taken out of use in the mid 1970-ies and put away in an old hay shed and is still there.

Everybody who has used the boat agree that grandfather made one misstake with the shape. As he based the design on a much older pleasure boat he made the bow "krokstamna" that is the stem has an even curve and ends almost vertical at the top. He should have made it "rätstamna" that is made the above water part of the stem straight and made it lean outwards which would have enabled him to give the bows a lot more flare above the waterline. In the style of a modern motor powered fishingboat at the time. The added reserve buyancy would have made the boat dryer and more seaworthy in bad weather.
In all other respects the shape and build of this boat was concluded to be a sucsess.
If there is interrest in using her shape as starting point for a new build I rekon my uncle and I could take the lines of her at a very reasonable cost. She was built largely by eye using some measurements taken from a much older rather similar boat so no orginal drawings have ever existed.

Thank you for taking the effort to photograph your grandfather's boat. That was very impressive to build by eye. It does look like a hull that would work for low power electric, and I appreciate the offer to take the lines, but for my amateur skills I think that I will continue the search for an existing plan. I also like the simple side steering wheel, I bet it worked just fine.

[heimfried]

Hi rgthom,
in Germany are 3 Bernd Kohler designs under construction ("ECO 62") with some differences in the lenght and the cabin configuration. The first one hit the water just for some minutes because she has to be moved in the boat hall.
https://www.dabbeljuh.de/media/Galad...-Geniessen.jpg
https://www.boote-forum.de/showthrea...3&#post5210093

http://uploads.tapatalk-cdn.com/202...754f3db673.jpg

https://www.boote-forum.de/attachment.fphp?attachmentid=890281&d=1595872895

Plenty of roof space for solar panels, low driving power needed for going slow because nice katamaran hulls, standing head room in the cabin ...
While your wife will probably not like the boxy shape, may be she will like a separated toilet room which most womans do.

[Alan H]

I have to say that Handy Billy looks awfully good for the delta. So 2 of the new 1500W motors would be like having twin 2 hp outboards on the back of the boat...like a 4 hp. That will drive Handy Billy at 3-4 knots I would think. I don't think I'd try to head straight upwind from Berkeley to the Golden Gate in August with that, but to go for 4-5-6 hours in the Delta, then set the hook or tied up at a Marina for the night, sure. There might be a few times when you'd opt to stay anchored and pick a travel period when the wind was less, but who's in a hurry? It's quiet and has a fraction of the windage of most of the big delta cruisers. And honestly, Handy Billy is just a really good looking boat, though it looks like you might have moved past her in your design search.

You know, it's entirely possible to plan to tie up at a dock with an electrical outlet, say every third night or so, just to top off the batteries. You could also add a wind generator, if you can find a quiet one, to run at night when at anchor. You'd have to put it up on some sort of easily fold-able mast, but it might mean you can get away with a bit smaller solar array.

They're a touch pricey, though.
https://www.homedepot.com/p/Coleman-...8400/204233177

[Alan H]

I just erased my post, and am dubiously looking at the previous one, since everything I came up with has already been contributed in the thread.

GREAT thread, BTW.

[rgthom]

A quick update on the project. After a lot more discussion with Paul Gartside, we are now looking at a canopied sampan style. Initial lines drawing below, with Paul's permission:



Features of this concept as a home built, low power, Sac Delta cruiser include:

1. A lean, easily driven displacement hull. Beam is only 6 feet and waterline beam less.
2. Clean wetted lines, transom is above the waterline.
3. Simple plywood construction for quick build.
4. Light weight, minimal structure.
5. Low windage, important for the windy Delta.
6. Pram bow to add some deck space forward.
7. Draining anchor well at the bow, for the muddy ground tackle.
8. Double berth aft under the canopy, with roll down curtains.

This is a change from the original V-berth under a deck. That was going to add windage or be very cramped, and impede access to the ground tackle. Now the berth is aft, under the canopy, but bigger and able to fold away.

I like this, maybe not as salty as some but more practical for the mission. It is a bit like Vivier's Seil enlarged, a proven river boat design.

[amish rob]

I think the words are Oh, and Wow. I might not be able to get both out in a row, though.

Dibs on painting the oculi.

[rgthom]

I think the words are Oh, and Wow. I might not be able to get both out in a row, though.

Dibs on painting the oculi.

Hmmm, like this?

[amish rob]

06CFC423-9C88-456D-89EC-0326B55F7354.jpg
Yar!

[rgthom]

OK Rob, you got eye duty!

The canopy has room for a kW of solar panels. My markup of Gartside's layout drawing below shows two ways of placing six of the 170 W Sunpower panels and one with just 5 panels.



The main issue going forward is we do not yet know how much electrical power is really needed to drive this boat, for both average cruise speed and as short term max motor output in urgent situations. My approach would be like this:

1. Build the hull without canopy.
2. Do some drive tests, by towing, using the small motor I have, and hopefully borrowing a bigger motor. This should determine electrical parameters.
3. Buy a motor with enough max thrust and decent cruise efficiency. Electric motors are usually pretty efficient even at reduced power, so this should not be hard. I am anticipating that a bigger motor from EP Carry might be released by the time I need it.
4. Buy batteries to provide 5 hours cruise speed.
5. Use for a season with shore charging.
6. Finally build the canopy with solar scaled for day trips of 30 to 40 miles.

[Dryfeet]

rgthom, I really, really, like where you and Paul are going with this. The pram style is a great one, both traditional and good looking to my eye. I'm less enthusiastic about having your bunk essentially 'out in the open'. Rain, fog, spray, etc could easily make your night's sleep a touch damp. That was the advantage of the cabin forward. With a canopy however, don't you basically have the windage anyway? Perhaps a birdwatcher type walk through cabin would make some sense...?

[rgthom]

rgthom, I really, really, like where you and Paul are going with this. The pram style is a great one, both traditional and good looking to my eye. I'm less enthusiastic about having your bunk essentially 'out in the open'. Rain, fog, spray, etc could easily make your night's sleep a touch damp. That was the advantage of the cabin forward. With a canopy however, don't you basically have the windage anyway? Perhaps a birdwatcher type walk through cabin would make some sense...?

I hear what you are saying, but consider these points: First, it's for the Sacramento Delta which in summer has almost no rain and rare fog. The canopy/roof is to be thin and almost flat, it should present little windage as I have found with the flat solar panel on the Walkabout. This keeps windage low while underway. The side curtains to enclose the berth are deployed only when anchored. I am thinking of something like the curtains on Chaser:



Paul prefers to keep it simple with the canopy at head clearance and curtains the full height. I think he has not spent a night on the windy Delta . I would prefer the added complexity of being able to lower the canopy roof to seating height, both underway if desired and at night. Then the curtains would be a lot shorter and the wind profile lower.

Managed to dig up a couple of pictures of boats with low aft cabins, both of these are hard cabins but of about the scale of the lowered canopy I am thinking of. Fred Shell's schooner:



Unknown design (is this Chris Cunningham's boat?):



In a rainy climate without constant wind a cabin like the boat above would be nice.

-Rick

[Clarkey]

The latest Gartside collaboration is just delightful. I have always loved the way the Vivier Seil sits on the water and this seems that it may have a similar quality. Tremendous.

[rgthom]

The latest Gartside collaboration is just delightful. I have always loved the way the Vivier Seil sits on the water and this seems that it may have a similar quality. Tremendous.

Yes, this has taken a turn in a good direction I think. I have a problem though. Since my retirement is delayed I will not be ready to start for 2 to 3 years, then a year to build the hull and another year to determine the best solar setup. That is fine with me, but Paul is chomping at the bit and wants a full design ready for an upcoming Watercraft issue. I have tried to explain, first gently then more clearly, that I have only built the small solar system on Walkabout and am in no way expert enough to specify the full electrical and solar propulsion system ready for design release on this boat. If anyone has this expertise please reply here or contact Paul directly.

For the motor it should be straightforward to use one of the two predominant suppliers in the US, ePropulsion and Torqeedo. Both offer motors around 3 kW, which is 6 hp equivalent and should be a safe margin. Both offer remote scalable batteries, and remote steering and controls. Neither offers solar at a kW scale as far as I can find, just smaller re-charge scale solar. So that would be the main part to define and specify with confidence.

[seasnail]

Hi Rick,

Did you know you can rent a 21ft x 7ft Duffy electric boat in the estuary? The place is called Bae boats and is located at the Grand Marina in Alameda. Prices are a bit steep, but it might be worthwhile.

You also mentioned you're not sure how much power would be required. To determine the hydrodynamic drag in calm conditions is trivial - just model the hull in freeship, then export to michlet and simulate the drag. I've done it and the learning curve is not steep at all. I would imagine it will take you just a couple of hours. Just download freeship (2.6 is best?), start a model with the correct number of chines/stations, manually edit the offsets by clicking on each point (this is the tedious part), highlight the chines and "crease" them, check the displacement, then export to a file that michlet can use. As a bonus, you can export the flat panels as dxfs for CNC and avoid spiling process. Both pieces of software are free. If you model the drag you may find it is all skin friction and not wave making drag at practical solar speeds, which may have some implications for hull type, beam, etc.

Obviously you still need to compensate for motor and propeller efficiency, perhaps ep carry will give you this information if you ask nicely.

You can also get a good idea of the power generated by the panels at from the PVwatts caculator here: https://pvwatts.nrel.gov. I think this calculator is better than others because it compensates for weather using historical data as well as other inefficiencies.

Hope this helps (and I hope I am not repeating myself!)

-Nick

[rgthom]

Hi Rick,

Did you know you can rent a 21ft x 7ft Duffy electric boat in the estuary? The place is called Bae boats and is located at the Grand Marina in Alameda. Prices are a bit steep, but it might be worthwhile.

You also mentioned you're not sure how much power would be required. To determine the hydrodynamic drag in calm conditions is trivial - just model the hull in freeship, then export to michlet and simulate the drag. I've done it and the learning curve is not steep at all. I would imagine it will take you just a couple of hours. Just download freeship (2.6 is best?), start a model with the correct number of chines/stations, manually edit the offsets by clicking on each point (this is the tedious part), highlight the chines and "crease" them, check the displacement, then export to a file that michlet can use. As a bonus, you can export the flat panels as dxfs for CNC and avoid spiling process. Both pieces of software are free. If you model the drag you may find it is all skin friction and not wave making drag at practical solar speeds, which may have some implications for hull type, beam, etc.

Obviously you still need to compensate for motor and propeller efficiency, perhaps ep carry will give you this information if you ask nicely.

You can also get a good idea of the power generated by the panels at from the PVwatts caculator here: https://pvwatts.nrel.gov. I think this calculator is better than others because it compensates for weather using historical data as well as other inefficiencies.

Hope this helps (and I hope I am not repeating myself!)

-Nick

Thank you Nick, all this is very helpful. I have avoided dipping my toe in with the hull design programs so far. It always seems you can get into trouble by missing important details as an amateur designer of anything (speaking as a designer of laser systems), so I was leaving it to the experts. Maybe now is the time to at least poke around with some of this.

For the solar I have a pretty good idea of the scale needed once the cruise power consumption is known, both from solar modeling and from the experience cruising the smaller boat. For a solar panel rated at 1.4 times the cruise electrical consumption of the motor I can cover 30 to 40 miles on a sunny summer day at 3.5 to 4 kts. So if the motor on the new boat uses 700 W at 4 kts cruise speed then a 1 kW solar roof should be right.

I did not know about BAE, but there is a Duffy operating as a tour boat on the Napa River. I race him sometimes (it's not very fast).

[Bill Perkins]

Still OK to kick design ideas around? Just for the deck layout . Here's a Weston Farmer take on an outboard boat with flush deck forward and canvas sleeping shelter aft. The cabin is for dry storage of clothes , bedding and gear . There's a small counter for food prep . Sitting headroom only , and you're sitting on the floorboards. If I squint, I can see solar panels on the foredeck and the flat supporting the tent; the anchor line led to the cockpit by fairleads on the perimeter. It's a planeing hull ,so not your case below the waterline; also shorter at 17 ft.7 inch .(6 foot beam) than your boat will be .

I like the way windage is low and concentrated aft, with the canvas faired into the windshield, minimizing resistance. I would think she'd be stable at anchor in a breeze. If it's warm a hinged windshield could be opened to the horizontal. Good underway and also an operable window for the tent. The side mounted wheel , or maybe a stick ,seems to make sense with the restricted beam and low speed .

[Woxbox]

OK Rob, you got eye duty!

The canopy has room for a kW of solar panels. My markup of Gartside's layout drawing below shows two ways of placing six of the 170 W Sunpower panels and one with just 5 panels.



The main issue going forward is we do not yet know how much electrical power is really needed to drive this boat, for both average cruise speed and as short term max motor output in urgent situations. My approach would be like this:

1. Build the hull without canopy.
2. Do some drive tests, by towing, using the small motor I have, and hopefully borrowing a bigger motor. This should determine electrical parameters.
3. Buy a motor with enough max thrust and decent cruise efficiency. Electric motors are usually pretty efficient even at reduced power, so this should not be hard. I am anticipating that a bigger motor from EP Carry might be released by the time I need it.
4. Buy batteries to provide 5 hours cruise speed.
5. Use for a season with shore charging.
6. Finally build the canopy with solar scaled for day trips of 30 to 40 miles.

That's a highly reasoned approach. Personally, I'd just cram on as many solar panels as I could fit under the notion that you can never have too much power. It's also best for the batteries not to run them totally flat.

That third photo of samples is Cunningham's boat. I believe his son built it. The key design feature, as you probably know, is that the house lifts up the height of those windows to create decent headroom at anchor. A recent Small Boats article had a photo of him doing this. He lies on his back inside and just pushes it up with his feet. So you have a boat with a low profile under way, and a taller one at anchor.

The sampan approach is really attractive. Are you thinking bright colors and all of that?

[johnw]

A quick update on the project. After a lot more discussion with Paul Gartside, we are now looking at a canopied sampan style. Initial lines drawing below, with Paul's permission:



Features of this concept as a home built, low power, Sac Delta cruiser include:

1. A lean, easily driven displacement hull. Beam is only 6 feet and waterline beam less.
2. Clean wetted lines, transom is above the waterline.
3. Simple plywood construction for quick build.
4. Light weight, minimal structure.
5. Low windage, important for the windy Delta.
6. Pram bow to add some deck space forward.
7. Draining anchor well at the bow, for the muddy ground tackle.
8. Double berth aft under the canopy, with roll down curtains.

This is a change from the original V-berth under a deck. That was going to add windage or be very cramped, and impede access to the ground tackle. Now the berth is aft, under the canopy, but bigger and able to fold away.

I like this, maybe not as salty as some but more practical for the mission. It is a bit like Vivier's Seil enlarged, a proven river boat design.

I like this one a lot.