Electric Boat Design

Semicircular hulls have a beam/depth ratio of 2. These seem to work well as long as L/B is 10 or more.

 

Well, since no one else has posted about the construction yet...

It's really too bad that you don't have a garage or a shop that you can fiberglass in, because this is a natural project for one-off fiberglass construction. The hull shape is clean and without things like strakes or difficult chines which are the problem spots for first-time fiberglassers. The beam is narrow so it's an easy job for one person. And fiberglass is perfect for molding in the structure between the three. It's almost the perfect project to learn to fiberglass on. But it's not something you want to do in your house either (well I have, but....)

I really wish I knew more about working with aluminum, as mass-manufactured canoes look so simple and they're so durable. But I suspect this will be fairly expensive to do a custom design. I would be very curious though if anyone knows if this is feasible.

 

Check out
http://www.boatdesign.net/forums/showthread.php?threadid=298

 

also http://boatdesign.net/forums/showthread.php?threadid=299

 

I am also planning on building a lightweight fishing boat. I have decided on a pontoon style for stability reasons. I do not know much about boating but I would think that the center hull of your design would just add addition weight and resistance. I can't remember which company it was, but there was a pontoon boat web-site that showed and went into detail about running their boats with electric motors (both the motor you were talking about and a dual trolling motor set-up). They showed them attached to some pretty large boats, and claimed they worked quite well. There is also a site www.islandpontoonboats.com (might be the electric engine sight also?) that sells boat kits with aluminum pontoons. The Ercoa website sells pairs of pontoons in many different sizes, and if you start with a pair like theirs all you have to do is find a competent welder to fabricate and weld on pointed ends on the back. I do quite a bit of welding and know that this would not be too monumental of a task. Their pontoon design is that of a hexagon with the bottom being flat, on a still lake this design would probably take the least amount of motivation to get it going.

 

Okay, I am back...
I have been going to some boat shows here in Baltimore. I looked at many boats, fiberglass and aluminum. All the boats in the shows were planning hulls, no displacement hulls anywhere. I talk to a lot of manufacturers about my search for a suitable boat for battery-powered fishing. Many where aware of Ray Electric motors, there was one displayed in one of the booths. Many said they have outfitted one of their boats with an all-electric setup. However, all except the most largest mono hull or pontoon boats are incapable of carrying more than 350 pounds of passengers and gear after you put on the eight 6 volt deep cell golf cart batteries onboard. Many were telling me that I should not worry about exceeding the Coast guard rating on the hull, “the boat can hold twice that amount without sinking”. That may be true but I don’t want to take my two young children and my wife out on a fishing trip on a boat that is overloaded. I thought there might be a Whitehall hull or other type of displacement hull but none could be found at the shows. While the larger aluminum hulls offered by Lowe (17.5 ft) and Duracraft (18.0) had a suitable 1400 lbs capacity, they would hardly “slip” through water at very low speeds, more like “plow with a sea anchor out”. Their wide transoms (74 inches at the bottom beam) are design to get a “fast hole shot”, to get on to plane quickly. However, at displacement speeds the hull has to drag a huge volume of water behind it because of the wide transom.

I have decided to continue with my own design and hopefully get it built this year. What I plan to do is to post my progress on the design of the boat and encourage readers to reply with comments.

Restating the design goals:

1) Electric power - current motor identified is a Ray Electric Outboard, system 300 http://www.rayeo.com/motors.htm
a. eight 6 volt E-145 batteries in series, 48 VDC, 4 hp max
b. 679 lbs motor and battery weight
2) Primary purpose – fishing with four people, must be very stable, used on protected water, reservoirs
3) Sustain 6 MPH for eight hours , 48 miles per charge
4) Length overall – 12 ft minimum, 18 ft maximum, maximum length for any given hull is 16 ft. (getting it out of the basement problem)
5) Desire to have trailer width equal to or less than 6 ft wide

A previous post shows an early tri hull design. I have run several variants of this design and my now most favored layout is shown below. I increased the displacement to 2400 lbs, draft to 13 ¾ inches. Overall length is 18 ft., beam is now 9 ft 7 inches.

A question was posted “Why three hulls?” I like this approach because the eight batteries can be placed in the center hull, and the wiring can be routed back to the motor mount. I plan to have the two outer hulls swing inwards toward the center hull when the boat is on a trailer. For the given length overall, a two hull craft with 18 ft hulls does have a lower total resistance, however, because of my 16ft max hull length, three staggered 16ft. hulls with an overall length of 18ft out performs two 16 ft hulls. Additionally, to be able to get the boat on a trailer and then get the width under 6 ft, it is much easier to do it mechanically with the three hulls I believe. The weight of the boat can be supported by the center hull and the deck supports, which would allow you to swing the two outer hulls while on the trailer.

In the image below, only the underwater portions are shown, bow pointing down. More info to come….

 

The site that talks about electric outboards is fiesta pontoons

 

looch

I have seen their pontoon boats. I have been doing quite at bit of internet seaching on pontoons, electric boats, small craft, RIB (ridgid inflatable boats) and all sorts of boat pages. At one point I favored a Duffy Sport Cat 18 ft boat. I called the manufacturer, they have one left, it is not in production. However, it is very expensive, 18K, plus transport from California to Maryland. Decided against it. The reservior I have a desire to fish has some crazy restrictions. They are listed in one of the earlier posts. I did find out they have changed some of their restrictions, kayaks will be allowed this year and pontoon boats are prohibited... don't ask me why. When I register my design boat I plan to list the type as a tri hull I do believe.

 

Boat weight estimate

In conceiving this design I have had to make some estimates and have been using some published equations. What I would like is to have some feedback on whether some of my estimates are reasonable and whether the equations I am using are valid for the problem I am solving for.

One of the design parameters that will ultimately affect the performance and hull form is the overall weight of the finished craft with passengers and gear. My known weights are the motor and 8 batteries weighing 679 (~680) pounds according to Ray Electric Outboard. The other is my desired gear and people weight, which I allowed 600 to 700 pounds. The hull design last posted actually has a displacement of 2300 lbs not 2400 lbs. So I have 2300 – 680 - 650 = 970 lbs for the weight of the boat itself. This is where I need a sanity check…

Question 1. Is 970 lbs a reasonable estimate for the boat weight?

I know this is going depend on what all is put on the boat and what kind of materials are used. To this point I have not received much in the way of construction methods. I am favoring cedar strip construction with layers of fiberglass and epoxy (WEST) on the outside and inside of the hulls. I have the book Kayak Craft by Ted Moores, which I have been using as a reference. The Kayak in the book has strength of various glass schedules and weights in oz/sq ft . Eventually I will be able to get better estimates as the design matures, however, I would like to cut down on how many iterations I have to do. The book references an Endeavor 17 ft kayak covered with 6-oz fiberglass cloth weighing 47 lbs. That seems very light but I guess that is the whole idea. Looking at the center hull and the two outer hulls, all of which are much narrower than the kayak but twice the draft, is 50lbs a piece reasonable? 150 lbs? Then comes the deck area. I am considering some sort of wood decking, mahogany or teak? $??. All support framing in white oak. Deck area approximately 8 x 15 feet. At least four fishing seats, cooler, anchor, fishing gear, railings and a T top or Bimini top. Those readers with the experience could probably provide a good guess. I looked at some production boats and noted their weights. An 18’6” aluminum deck boat weighs 1050lbs, a 17’ aluminum fishing boat 600 lbs, an 18’ fiberglass boat 1175 lbs.

 

I've never been good at building things light... I always tend to want to add that one more layer for durability. But I think your estimate is reasonable.

Let's see if I were building this, it would probably go as follows:

Center Hull: 100-150 lbs (using 150 for now)
Brackets/boxes/etc. for batteries: 40 lbs
Outer Hulls: 50-75 lbs x 2 = 150 lbs (using 75 for now)
Structure connecting hulls: 60 lbs
Deck: 100 lbs
Seats: 50 lbs
Railing/hardware: 50 lbs
Top: 50 lbs
Anchor: 15 lbs.
Lines/Misc: 25 lbs.
-------------
~700 lbs.

My assumption is that for trailering, etc. your hulls will be a bit heavier than a kayak, but I suppose this wouldn't necessarily have to be the case.

My numbers might be off, so feel free (anyone) to correct me.

 

Design Speed vs BHP

Another key design parameter is the design speed of the boat in the water. This parameter drastically affects the hull form. I see this when I change the design speed in the computer program Michlet 6.06 software (Leo Lazauskas), especially the relative positioning of the two outside hulls with respect to the center hull (wave cancellation). To determine the design speed I had to consider the motor I planned to use. Ray Electric claims the motor can produce a maximum of 4 hp. So as this as my upper limit, I set out to determine what speed can I expect to obtain. The reference I am using is John Teale’s book “How To Design A Boat”. He has published an equation for determining the break horsepower required to move a motorboat based on the Total Resistance (Rt in pounds) of the hull, at a given Speed (U) in feet per second. The Total Resistance is the combination of friction and wave-making resistance. This value, Rt, is one of the many output files generated by Michlet (in kilo-newtons) as a function of speed (meters per second). Making the conversion from kilo-newtons to lbs and meters to feet, I use these values in Teale’s equation…

BHP = (2 * Rt * U) / 550 equation 1

I have been using this equation for estimating the required horsepower to move the boat to any speed U for which I have generated data Rt. All speeds that I have been considering have been under 10 mph (14.667 ft/sec). I know this can only be used as an estimating tool because the ultimate speed is not only based on horsepower, but on prop pitch, efficiencies, wind resistance, etc. He did state that this equation is based on dead calm water and no wind resistance. This equation is for displacement hulls, no considerations for lift and reduced friction due to planning effects.

Question 2. Is this a valid equation for estimating the horsepower required to attain a specific speed of a multi-hull in displacement mode?

 

Tri Hull vs Di Hull

After posting my reply to looch I decided to verify what I said was true. I have been running many variations of the hulls and I was not sure if I was comparing similar parameters. I ran Michlet for a Di Hull with the same ship constraints. That is, the same displacement and speed optimization as the Tri Hull which is shown above. Each hull in the di hull configuration was limited to the same maximum length of 16 feet. The resulting overall length of the Di Hull therefore was also 16 ft. The image I have attached below is a plot comparing the two types of hull configurations. As you can see the Tri Hull barely out performs the Di Hull in the horsepower range which I consider the most critical, ~ 1.5 hp. This is the horsepower range at which you can run the Ray Electric outboard for ~6.5 hours continuous. You can see the Di Hull performance crossover is at ~4.5 hp. If each hull in the di hull configuration were allowed to go to the full 18 ft, it would definitely outperform the tri hull configuration.

This also points out how I am using the horsepower formula (posted question 2) to alter my design options.

 

I believe that you could easily build the boat you speak of to weigh less than 500lbs if you used aluminum. As far as hulls are concerned you could fabricate those out of 12 gauge aluminum which only weighs 1.397 lbs per square foot. The crossbraces could be either aluminum square tubing, or maybe angle. Remember aluminum is light and very strong. In fact, the heavy part of the boat would probably be the plywood decking and accessories. If you feel uncomfortable working with aluminum just remember that not much of it would actually need to be welded. It drills and cuts very easily. To cut pieces to length simply use a jigsaw with a bi-metal blade, and to attach crossmembers, etc. simply drill appropriate holes using a 3/8 or 1/2 inch drill. Use stainless bolts to attach everything and there you have it. The beauty of aluminum is the fact that even if you overbuild it (which will likely be the case) the added weight will not be that significant. The only area that I see you running into difficulties is in the building of the hulls, but you might find a competent metal man that will help you there. The boat that you speak of is very similar to what I plan to start building in a couple of weeks. I want mine to be able to hold about 700 lbs. also, but my deck area is only going to be 5'x10' (enough for 2 people and my hound). I have run some numbers and the frame and pontoons (without the deck or railings) should clock in at around 200 lbs. It may increase or decrease depending on how overboard I go, but not by leaps and bounds. Good luck

 

John:

Are you running the catamaran and the trimaran at the same displacement, or are you accounting for the fact that they may not be the same weight? Also, are they the same Bmax? And on the cat, are you locating the LCB 53.4% aft as I've recommended based on Larsson?

 

Stephen

I did run the the program with the same displacement. The tri hull has 26% more surface area in the hulls than the di hull. If I use the estimate of hull weights provided by Jeff, 300 lbs, I should reduce the displacement by 78 lbs for the di hull.

The maximum beam is slightly differrent, the di hull was 2 inches wider.

The LCB was approx. 50.3% aft.

As I stated before, if each hull in the di hull configuration were allowed to go to the full 18 ft, (LOA of the tri) it would definitely out perform the tri hull configuration. I do not wish to persue the di hull configuration.

I am sold on the tri hull configuration because I do believe it will be easier to design the swinging amas to reduce the max beam to less than 6 feet when on a trailer.

I was hoping you would comment on my posted question on estimating required horsepower to attain a specific speed when hull resistance (as a function of speed) is known. See question 2 above.

thanks