Small Inboard Electric Lake Boat

[gonzo]

The chine forward would benefit from sweeping up starting about a third of the length from the bow.

 

[jdray]

Thanks! In layman’s terms, can you describe what that will do? Also, how much?

 

[gonzo]

Without getting into obscure hydrodynamics, it lowers resistance. Maybe I am not seeing the shape correctly ( a side view would help) but the sides look like the chine is horizontal all along the hull. That tends to make the bow "plow" which results in bow-steering. The bow digs in and you have to continuously correct the steering. Also, going downwind with waves, it may broach.

 

[jdray]

Thanks! When I get the new shape crafted, I’ll post a side view.

JD

 

[jdray]

The modeling software I'm using is not as capable as I'd like, and neither am I in using it. It only provides orthogonal views, so a straight-on side view isn't possible, but after dragging some corners around and not entirely destroying the original shape, I came up with this:

That's 114 mm of rise over 2000 mm (two-thirds of the boat's length). I know you said from "about a third of the way from the bow", @gonzo. I tried that, and (again, given my modeling skills), it essentially wrecked the entire thing. Any feedback on this setup? Do I need to go up more to avoid plowing?

Thanks.

JD

 

[gonzo]

That looks pretty good.

 

[messabout]

You are getting there JDray. Keep up the practical work by raising the after end of the bottom such that it ends, at the transom, slightly above the water surface. That is aft rocker which is of important advantage for a relatively slow speed boat.

If this was to be a planeing boat you would leave the aft end of the bottom as you have drawn it, flat in the profile view. You will probably not be able to provide enough electrical power, in such a small boat, to make it plane. Thus, do not fight reality, give it some aft rocker.

 

[jdray]

How much power would this boat, as drawn, need to plane?

 

[fredrosse]

I built a small all electric cruiser years ago, and believe your goals are easily met. I converted a 14 foot O'Day Javelin sailboat hull, a decent fiberglass hull, and small boats like this, with a broken mast or bad sails are very inexpensive. I bought the boat, and trailer, on Ebay for $107.00, and invested about $1000 and 100 hours labor for all the material and work to get going on the water. Basically just cut out the sailboat centerboard well with a 7 inch circular saw, and fiberglassed in a PVC pipe for the propeller shaft. 4 or 5 people is a comfortable load, 4 MPH speed, 4 hours duration at maximum speed. 20 hours at half speed. Ran this boat for several years with no trouble. I have presentations and can send if you wish.

Questions

• What is a good prop size/pitch/blade count to meet my 3-4 knot speed goal?
• ANSWER: You should use a larger, low RPM prop, 3 blade is typical. My Javelin turned a 14 inch three blade prop at 400 RPM, 17 inch pitch.
• Will that same prop size get this boat up on plane with sufficient power applied?
• ANSWER: Yes, I have had this boat planing at 15 MPH with a 13 horsepower gasoline engine (Harbor Freight $380), but doing that with electric would be far more expensive, and range would be abysmal if electric.
• Waitaminute, will this boat even get up on plane? Not a requirement.
• How much torque does the motor need?
• ANSWER: I used a 3/4 horsepower continuous duty PMDC motor, with belt reduction from 1750 RPM to 400 RPM output.
• How many RPM is required?
• ANSWER: For good propulsive efficiency, around 400 RPM is good.
• How many kW is required?
• ANSWER: The Javelin, with 4 adults aboard, at 4 MPH, used 25 AMPS @ 24 Volts, that is about 1/ 2 kilowatt. Motor is rated 3/4 HP @29 amps.

I realize that the torque, RPM, and kW are all interdependent, which is why I asked the prop size question first. Using my 190 mm (7.5”) prop size, sticking with three blades, pick a pitch, and that should (?) answer the rest of the questions, right? That seems right, but I don’t know how to calculate it all, nor how to pick the right pitch.

Another, possibly less important set of questions are:

• Is 2 mm aluminum sheet a reasonable hull material?
• ANSWER: OK
• How do I figure weight for the hull (sans motor, battery, controller)?
• ANSWER: Do the math. If you use a popular used sailboat hull, the weights are published.
• How do I figure out what the DWL of the boat is (again, not too concerned about small variances in speed; it’s a lake boat)?
• ANSWER: If you are designing the hull, use free hull software, like Deftship. But that is a long learning path, perhaps you would be better off using some existing plans, a used hull, etc.
• How seaworthy is this design?

 

[fredrosse]

Cruising with 4 adults - Electric Javelin

 

[jdray]

ANSWER: If you are designing the hull, use free hull software, like Deftship.

Delftship is (unfortunately) only a Windows program, and I'm running a Mac. I'll search and see if I can find an alternative.

 

[messabout]

How much power would this boat, as drawn, need to plane?

That depends largely on the gross weight of the boat with all of its contents. Let's say that the whole thing weighs 300 pounds. You could get the boat to plane with a 3 HP outboard. Not fast, but actually planing.
500 pounds...now you are going to need about twice that power, say a 6HP outboard.
900 pounds better think of 15 HP .

Now put the 15 horse outboard on your 500 pound boat and it will go scary fast. Note: These are ball park figures based on past experience.

Think of it this way. The area of the bottom is X square inches. The boat must be coaxed or forced to move fast enough to generate some dynamic pressure on the bottom. Now if the bottom has ....say 1000 square inches of surface exposed to an amount of lift or upward force, the 500-pond boat with its 1000 square inch lifting surface will need a dynamic pressure of one half pound per square inch. A thousand pound boat with the same bottom area will need one pound per square inch.

OK that is the general concept. But it gets a lot more complicated than that. The pressure must push upwards and so the boat will need to present some angle or other to the oncoming water. ...lets say perhaps a five degree angle of incidence. Now we can do some trig. We can also insert the density of water, something the smart guys call rho. Then we will have to consider the velocity of impingement of the water on the angled surface. That involves an exponent in your basic exploratory equation. V square over two......etcetera

This is just the most elementary physics and math that you must do to arrive at a reasonable estimate of the power needed to cause the boat to plane. The wild "ball park" numbers that I have suggested above are just that...Ball park or as sometimes stated SWAG which is :Scientific Wild *** Guesses. Those Swags Are based on experience not math. but you could actually do it with some well structured math.

Here is one more thing that plays into the complex mess. That is deadrise angle. The more deadrise the more power needed to achieve a plane. Your initial scheme had a 15 degree angle which will increase the power demand by a considerable margin. A flat bottom will need the least power to get you into the planing regime.

I will, apologetically, allow myself to be politely critical. Why do you think that you can design a boat that has some degree of efficiency? Are you relying on a computer program to draw your boat. I urge you to take note of the GIGA concept. The computer does not know how to design a boat and you have to give it informed instructions if it is to deliver good result.

I apologize once more for questioning your methods. Design your boat to a suitable scale with a pencil and a large sheet of paper. You can get the feel of it far better while using that method.

 

[jdray]

I will, apologetically, allow myself to be politely critical. Why do you think that you can design a boat that has some degree of efficiency? Are you relying on a computer program to draw your boat. I urge you to take note of the GIGA concept. The computer does not know how to design a boat and you have to give it informed instructions if it is to deliver good result.

I apologize once more for questioning your methods. Design your boat to a suitable scale with a pencil and a large sheet of paper. You can get the feel of it far better while using that method.

Oh, criticize away; no sweat off my brow. I do appreciate constructive criticism, which you and a few others have provided.

My overall goal is not to create a new hull design, but to come up with a scalable design for a simple, affordable, maintainable boat with an inboard electric motor. So I dove in and started to learn. I’m still learning (hell, I’m just starting).

I’m a systems analyst by trade, so approach things from (probably) a different angle than many people, though I don’t know anything at all about how boat designers approach their craft (no pun intended). It appears to me, through study of other craft, that boats with inboard drives either have the entire prop and rudder below the bottom of the hull or they have some sort of tunnel. A jet boat is the exception. I did some research and essentially chose a 15 degree deadrise angle arbitrarily (or nearly so) based on a few inputs and a lot of guesswork. Hearing what you say about flattening the bottom and reducing deadrise angle is helpful. It’s not going to change the draft any, and should (if I understand this part correctly) increase displacement.

Regarding power, with electrics (you’re probably aware) it’s straightforward to have far more shaft horsepower available than is needed for most use cases. The difference in size and weight between a 3 kW (4 h.p.) electric motor and a 12 kW (16 h.p.) is not significant (a few pounds). Yes, the latter will drink four times the power at full draw, but I feel like it’s worth the few extra pounds to have the extra headroom and have the power when you want it.

I based the design on a 5 kWh lithium iron phosphate (LiFePo) golf cart battery. If the half-kilowatt estimate at 4 mph SOG above is correct, then that’s ten hours of cruising at low speeds. Even at twice that draw (heavier boat, less efficient prop), five hours of cruising around at that speed would be great. If the boat could get up on plane at… five times that draw?? That would be fine, even if it only lasted a few minutes.

Oh, and the GIGA principal? I’m very familiar with it.

Thank you for your input. It is greatly appreciated.

JD

 

[gonzo]

Boat designers look at successful designs as starting points. Even a revolutionary design will get compared to previous successful design to decide if it is better, cheaper or in any other way superior. I assume that if someone asks you to design a computer system you don't start from scratch either.

 

[jdray]

Heh. Yes.

I really didn’t start from scratch. I took a lot of inspiration from a the layout of a 3 m boat that looked simple, but it (being simple) was designed for an outboard. So I started looking at other boats, but finding hull designs for 3 m/10 ft power boats with an inboard didn’t meet with much beyond what appeared to be converted rowboats with full-displacement hulls. XShore is producing larger boats that are almost exactly what I’m aiming for except… larger (22’ and 27’ I think). And I don’t know of anywhere I can look below the waterline for their designs.

So I started to cobble together something and put it here for feedback. You’ve provided some good advice, as have a few others, and I appreciate it. Progress is being made. I don’t have the right tooling, and honestly if someone said “oh, here’s a hull that looks just the way you were aiming to have yours look”, I’d be on it in a minute. Not trying to reinvent the wheel here. Or something like that.

Cheers.

JD