i need help

i have built a couple of cedar strip and fiberglass canoes over the past couple of years. i even got the chance to build a wee lassie with Mac McCarthy. that's the only boat building knowledge i have. i'm really interested in building what i would call a electric fishing cruiser using the cedar and fiberglass method. what i need is a hull design that will be most efficient with electric motors. i understand there's something you guy's call maximum hull speed. i also understand i won't be able to get the hull to plane with an electric motor. so what type of hull can i use? i've looked at multi hull designs but they all seem to be huge sea worthy vessels. i would like the boat to be at least 17 foot long and 6 foot beam. it has to be able hold a lot of battery weight too? 600 lbs with my current motor selection. i have so many questions i don't know where to start. i know i can build it, i just don't know how to design it. what do i do?

 

I would say go a bit shorter if you want a 6' beam. Maybe even 13 ft.
You've got a very small amount of power, so going long isn't going to help you. A short and wide boat can have a more rounded bottom than a longer one, and be more efficient.
At 17 ft, 6 ft beam, the boat will have a nearly flat bottom. This will induce a lot of drag. Shortening it (and I'd suggest max 5 ft beam, even less if possible) will make for a shape easily driven with minimum power.
Hull type could be a double-ender, a peapod type on the wide side. There are some wider peapod designs (see WoodenBoat plans).
Figure 2 1/2 to 3 knots cruising and about a 4 1/2 kt top speed. That's about a 12 ft+ waterline length, 13-14 ft peapod,
Just a suggestion, of course.

Alan

 

thanks for the reply alan
i found something along the means of what i'm trying to do. if your board take a look at this:
http://www.budsin.com/lightningbug2003.html
this boat can achieve 6mph. i know they make much larger electric motors. some i've found can produce 4hp. i would like a hull that can be pushed to 12mph. another interesting hull i stumbled upon was this:
http://www.engin.umich.edu/dept/name/research/projects/solar/solar_boat.html
they were able to achieve 30mph. i'm not asking for that much. do you think i could stretch a hull like the lightngbug, put more power behind it and not just create a bigger wake?

 

Basic considerations are:
1. A long slender hull will be more easily driven. Something shaped like a canoe only bigger.
2. It seems you will have a lot of weight in batteries so if these are placed low you should be able to achieve good stability with a narrow hull.
3. You need to calculate the all-up mass. The hull, the deck, the passengers, the fishing gear, the batteries, the drive system, the rudder and other controls.
4. A high aspect 2-bladed prop will be more efficient than a smaller diameter prop. Is draft and issue?
5. Are there any real constraints on length - would 30ft be too long if it was only 3ft wide? If length is constrained then you might find a catamaran would be a better option to get performance.

There is a program called Michlet that has an optimising component called Godzilla that will produce you the lowest drag hull for your set constraints. I can run this for you if you set some of the above constraints. This will enable performance prediction.

Rick W.

 

700 watts will give you maybe 3/4 hp at the prop. That's 29 amp hours at 24 volts. A 60 AH battery will last two hours using all that 700 watts.
A battery weighs 40 lbs I think. Two batteries would give you four hours at 700 watts.
A human being can maintain (for a little while) 1/2 hp if he's in shape, but average would be maybe half that (180 watts or so). 700 watts from the battery is 3 times that applied to the prop (after losses), for some perspective.
6 kts is very good speed. 12 kts is going to require some real power. 4 hp is about 3kw. The motor isn't a problem, they make a lot of powerful electric motors (Briggs and Stratton ETEK, 15 hp, but very light and efficient).
It's the battery storage that is the bugaboo. Let's say you need 3 hp to go 12 kts. That will require perhaps 3 kw to deliver 2238 watts to the prop, guessing. That will draw 125 amps at 24 volts, so you will probably need a minimum of 2 12v batteries in series to last an hour. You can see how adding batteries adds time at top speed, but batteries weigh a lot, and must be charged (so you take the boat to the charger or the charger to the boat, and it has to be a big enough charger to replenish the whole bank).
The other thing is you can go perhaps 8-9 kts with half the power it might take to go 12. Food for thought.
By the way, that boat in the first link has a very peapoddy shape!

 

I did some comparitive numbers for a boat displacing 1800lb. This is based on the weight of batteries being about 1/3 total.

The lowest drag hull for this displacement to do 12mph is 48ft long and 20 inch beam. A bit ridiculous for a fishing boat. It would take 1.6kW at the hull to do the 12mph. This would translate to say 2.1kW at the motor.

If you reduce the hull to 24ft long and a minimum beam of 4ft then the power to do 12mph doubles.

Hope this gives an indication of what is possible.

Getting to 10knots in a short hull means you are approaching planing speed so there is a huge amount of wave resistance.

Something about 30ft long and 3ft wide might be a good compromise.

Rick W.

 

Yes, Rick, and that's reaching a high efficiency at the highest speed. A boat that long would waste power at low speeds due to hull friction. This is also true of kayaks and canoes. The target would be the highest efficiency at the average speed (which lowers the high speed potential).
Because an electric boat is severely limited in terms of how much net total amp hours it can carry, a good comprimise would be between distance and speed. The hull would reflect this by being just long enough to make reasonable speed with a small amount of power. Put a gerbil in a treadmill hooked to a prop, and a bath tub would make better speed than an olympic rowing shell.

 

Alan
The lowest drag displacement hull ends up having about 10% of the energy going into wave making. So the optimum hull length is strongly linked to the desired top speed through the phase velocity of the hull wave - the well known "hull speed" although it is not as precise as usually given in texts and is dependent on hull shape to some degree.

I determined the range for various boats all displacing 1800lb and fitted with a 3000Wh battery.

24ft long 3ft beam @ 4kts goes 101nm
31ft long 3ft beam @ 5kts goes 61nm
31ft long 2ft beam @ 5kts goes 62nm
29ft long 4ft beam @ 5kts goes 55nm
17ft long 4ft beam @ 5kts goes 24nm
17ft long 4ft beam @ 4kts goes 75nm
17ft long 4ft beam @ 3kts goes 225nm

Note each of these boats have optimised hulls for the set constraints.

So if you are happy to do 3kts and go a long way then make a boat 17ft long.

If you want to do 5kts and go as far as you can then make a 31ft long boat with a waterline beam of 2ft. Although this does not do much better than a 29ft long boat with 4ft beam. Remember a boat with a WL beam of 4ft could have a beam of over 5ft at the deck.

If you decide that you want to do 5knts but need something shorter than the 29ft then you can reduce displacement to say 1200lb by halving the batteries and making everything a bit lighter. The boat becomes:
25ft long 4ft beam @ 5kts goes 30nm.

Better than the 17ft boat with twice the amount of batteries and almost three times the required power level.

Hopefully this gives some idea on the compromises available.

To sum up, if you want to go a long way then it should be relatively short and travel slow. If you want to go a bit faster like 5kts then you need something around 30ft to maximise the range.

Matt- hope this gives you some clue as to what you might be looking at. I can refine the numbers and give reasonable prediction of the range if you want to start homing in on a design.

Rick W.

 

Great research, Rick. Puts the choices into perspective.

 

From a purely practical point of view, try to forget the 12 knot figure. Electrics are a real joy to use, but you must be satisfied with modest speeds. If you really need the 5 minute per mile speed then figure on a gas engine.

Check out the Florida gulf coast traditional small craft association site. Fgctsca.org for pictures of "Chelsea" She is a 17 foot whitehall that has been converted to electric power. It uses a modest sized motor and two Wal-Mart batteries. With an all up weight of about 1600 pounds (including four husky guys) it does 4 knots. The builder has fiddled with prop selection and gotten the speed up a little better than 4. Chelseas father is Dave Lucas. He can be reached at Skipjack@Tampabay.rr.com. Dave is a good guy who will surely be pleased to furnish information and exchange notes with you. He is also the owner of Lucas Boat Shop so he knows boat building.

Canoe and Kayak Magazine has plans for an 18 foot power canoe called Cinnamon. Plans are cheap and fairly complete. They swear it will do 8 knots with a 2 HP Honda outboard. I'm skeptical but maybe so. In any case this would make a very competant fishing/camp cruising boat. It is strip built and is said to weigh 100 pounds. No doubt a good candidate for electric power. Incidentally the plans have the outboard hidden in a stern box. The little Honda four stroke is quiet, and enclosed in the box, it will be even less audible.

Check out the book: Electric Boats by Douglas Little ISBN 0-07-038104-6 There is a lot of useful information in the book but not a great source for ultimate boat designs. Ultimate designs are in John Gardners; Building Classic Small Craft. ISBN 0-07-142797-X. This is the definitive book for boats of the type that you will probably focus upon.

 

hull speeds

Thanks everybody, i've been reading and researching for months. joined the forums last night and already i've learned more from you two in a couple hours what would of taken me weeks.
a little disapointing though. as much as i would like to build a 30 foot, i'm going to be restricted to 18 feet long, unless my friend wants to put an extension on his garage.
so where do i go from here?

rick- you mentioned a catamaran maybe being more efficient. is there catamaran designs out there that are between 15 and 18 foot long, that would serve my purpose?

alan- you mentioned the eteck 15 hp motor. i currently have a 107lb thrust 36v motor that moves me around really well in my 15 foot jon boat. so lets say i found a way to strap a 15hp motor with 48v to that same boat. i'm guessing i would just make a bigger wake? what kinds of boats are people putting these powerful motors on that make it worth it?

thanks for the help guy's, matt

 

I would guess that more powerful moters are going to be limited to boats that can generate their own electricity, and feed that electricity directly to the motor, allowing the engine to be located wherever desired.
The ETEK is an amazing motor/generator, extremely efficient and light. Such a motor could be charged by the prop being drawn through the water by a sailboat, charging batteries for brief powered navigation such as entering a harbor or keeping headway while dealing with sails.
You won't be able to take advantage of such power unless you have a semi-planing hull, which makes for a somewhat slow dispacement hull.
Here's the art of it: About 400 watts consumption as a cruising target would be the same length and dimensions of boat that would make for a fast human-powered vessel.
It is then only a matter of choosing a very efficient rowboat design, one that is fastest in the hands of a good powerful rower.
The whitehall comes to mind. Or there are others. Maybe 17'x 3' 6" or wider, but there's a penalty in terms of speed potential the wider you go (in which case a 200 watt output would be fine--- slower but very efficient in a shorter wider hull (like a peapod). I can't overemphasize the importance of efficiency unless you keep going back to shore to recharge.
At 200 watts, you should be able to do 3 knots continuously, drawing 8 1/3 amps from two 12v batteries. If they are 60 ah batteries, that's maybe 10 hours on a charge. A whole weekend of poking around and covering 90 miles of distance. Not bad for fishing needs.
Lengthen the boat, make it narrower, like a long (20 ft) whithall, and you will perhaps go 45 miles on the same charge (albeit at a faster rate, maybe 6 knots).
A lot of fishermen use flat-bottomed wide boats and trolling motors have a tough time making them go. The peapod, maybe a 13 footer with a 4-4 1/2 ft max beam and a waterline beam of maybe 3-3 1/2 ft has the right shape for 200 watt consumption, I think.

 

alan,
I think i need to give you a better idea of what type of water the boat will be in. I live in Missouri where there is countless small lakes that don't allow gas powered engines. Currently I fish a lake that is only 15 acres, in a 15 foot aluminum john boat with a motorguide 36v 107lb. this works well, i can fish four full days of fishing without a charge.
My initial idea was to just build a wood strip john boat with two 36v 107lbs thrust motors. then i found the min kota 202lb thrust motors and wanted to strap two of them on my wood strip boat giving me 404lbs of thrust but 480lbs of batteries and maximum hull speed is holding me back.
i don't mind having to charge over night to run all day. I would never have to travel as far as your talking about. I would rather sacrifice speed than running time.
i looked at the peapod and was also told to look at the 17' Chelsea. i like the size of them but can i push them faster?
thanks for the help.

 

Matt
I did the numbers for a cat.

I will not convert all the numbers to imperial as I work with metric. I looked at an optimised catamaran that is 5.5m (say 18ft) long and 2m overall beam. The displacement used was 800kg (1760lb).

The 48V motors you refer to are rated at 2HP. This is close to 1500W. So fitting two motors gives total power of 3000W. This will get the cat to 9.3kts. So goes 9.3nm on a single charge for a 3300Wh battery (allow 10% motor loss).

If you look at an economic speed of 4kts then the power required drops to 500W. So range is now 24nm.

An 18ft monohull with 4ft WL beam and same displacement will get to 7.4kts with the two motors. So at full throttle the range is 7.4nm. Drop it back to 4kts and it has the same range as the cat - 24nm.

The monohull starts to do better below 4kts. At 3kts the cat will go 36nm while the monohull will go 42nm.

There is one more option worth considering if range is the priority. An optimum 18ft designed for 4kts will go 86nm but the top speed is limited to 5.7kts.

The cat hulls are very simple shapes. They could be made with a hard chine. At 9.3kts they are almost on the plane. The performance might be improved a bit by going wider overall but you need to work out if 800kg is the right target displacement for your needs.

I can provide the optimum hull shape if you work out your target displacement and decide that the cat is a good concept to pursue.

Normally I would go for a monohull for efficiency but the length constraint and desired performance make the cat a better option in your case.

Also note that the above figures are for the boat at the design displacement. The displacement would include about 4 people so you could lighten the load and improve range to suit the circumstances.

I am not sure if the standard props would achieve the nominated performance but it is easy to make efficient props for low power ratings.

Rick

 

Rowcat

If you are looking for a skiff design that is easily driven by a small electric motor checkout the 15 ft rowcat at www.boatplans.dk. It´s a small stitch and glue catamaran. The beam is only 4 ft but it should be possible to increase it.

Minnkota produces some reliable electric outboards about 4 hp. When fishing the silence of electrical power is definitely an advantage. Go catch some trout!!!!