# Low speed planing hull theory

Discussion in 'Boat Design' started by Doh, Jan 5, 2020.

Tags:
1. Joined: Jan 2020
Posts: 16
Likes: 4, Points: 3
Location: Germany

### DohJunior Member

Hey there,
I'm reading and reading tons of threads here for a few month now I would like, if possible to have a bit of help on planing hull design/raw hydrodynamic theory.
I'm a technical guy, I know that what I ask isn't the clever thing all time, but more a try to understand the RAW theory in order to optimize a "legally restricted" design.
I would like to know what is, in theory only, the optimal design for a planing hull in terms off power/planing-speed/hull resistance for lightweight dinghy.
The reason for this is that I can't (legally)a hull longer than 2,5m and I'm also restricted in power (3,5kW).
Which means, very slow hull speed, very "short" power to plane that damned thing. "Perfect conditions"
My target is:
-planing with the lowest power possible
-planing at the lowest possible speed

I looked at comparison between cat-design inflatable compared with "normal" dinghy floor. Also tried to find some comparison between flat/semi-rigide/hard floors... etc
All of this without really being able to "quantifyingly"able to extract any basic-rule, except that weight/m² is the predominant factor with fixed hull length and available power.

As soon as I can't really count on hull speed, wouldn't a flat hull design the easiest way to quickly get enough lift to reach planing?
Or, having some kind of cat-hull would help me to reach a semi-displacement'ish state that would make the plane smoother?
This question is only theory oriented, feasibility, materials existence, handling, security....etc should be totally ommited as well as hydrofoils designs, need to be only Hull-design oriented.
Ps: I'm not an illuminated, actually pretty much the opposite. This is just an attempt to understand the basic of it. I fully know that there is tons of reality-check things that come on top of that when it comes to boat-design. And at the end, that the design chosen will be completely different than the "optimal one" for just one aspect. It's just to have the basic theory under the eye, when I want to optimise my little boat.
Greetings from Germany and happy New year!
H.

#### Attached Files:

• ###### images.png
File size:
3.5 KB
Views:
496
2. Joined: Oct 2010
Posts: 10,401
Likes: 1,033, Points: 113, Legacy Rep: 702
Location: Australia

### Mr EfficiencySenior Member

Your bodyweight, of all things, might just be the key factor here !

DogCavalry and Doh like this.
3. Joined: Jan 2020
Posts: 16
Likes: 4, Points: 3
Location: Germany

### DohJunior Member

☝️ As much as you are surely 100% right, that's the one thing I didn't want to read ^^
If we fix weight as parameter. What other aspect of the hull would help to get smoother/earlier plan? I didn't mention it already, but I do have (theorically) a bunch of thrust at low speed for a short time. Therefore my target to plan as early as possible using this extra power and then, reduce the power as soon as the dinghy plans

4. Joined: Oct 2010
Posts: 10,401
Likes: 1,033, Points: 113, Legacy Rep: 702
Location: Australia

### Mr EfficiencySenior Member

I don't see your weight mentioned ! It is possible to build ridiculously light, and still meet the strength requirements, but when the main weight element is the "driver", it becomes important to know that figure. To plane, you would opt for a flat bottom to have any hope, and preferably not a slender boat. But 8 feet gives you little to play with. What is 3,5kw, 5 HP ? What is the "bunch of thrust" you speak of ?

Doh likes this.
5. Joined: Jan 2020
Posts: 16
Likes: 4, Points: 3
Location: Germany

### DohJunior Member

The dinghy + motor+battery will be about 150kg (I weight 75-85kg depending on my girlfriend new diet). I'm building an electric motor for this. It's 3kW rated but will give about 4-5kW for a short period (10-20s) with between 500 and 800N thrust available at low speed (0-3knots) (depending then on the propeller pitch (8-12) I will use). If you want to compare with ice, it's about 10ps thrust equivalent, again, shortly, 10-20s or so, and it has then to be reduced to 300-400N (4-6 knots) (reducing with speed) if you don't want to burn anything.
1HP=0,75kW, but again, there is more to it because the torque expression at lower speed is way greater (I'm originally electric motor/generator development inclined).
I want to use this "boost" to get the dinghy to plane,and as soon as it's way more than what a 5hp would give, +lowered weight (motor +battery should weight about 15kg) it could also be a design having a steeper resistance until plane, if it means reduced resistance on plane (a bit like what hydrofoils do) just no hydrofoils, because that's a direction which will be 100% unpractical for me in the future.

6. Joined: Oct 2010
Posts: 10,401
Likes: 1,033, Points: 113, Legacy Rep: 702
Location: Australia

### Mr EfficiencySenior Member

Why do you need to plane ?

Doh and fallguy like this.
7. Joined: Jan 2020
Posts: 16
Likes: 4, Points: 3
Location: Germany

### DohJunior Member

4knots max hull speed. Is way too slow

8. Joined: Oct 2010
Posts: 10,401
Likes: 1,033, Points: 113, Legacy Rep: 702
Location: Australia

### Mr EfficiencySenior Member

That depends on how far you are venturing, and how much battery endurance you have ?

Doh likes this.
9. Joined: Oct 2010
Posts: 10,401
Likes: 1,033, Points: 113, Legacy Rep: 702
Location: Australia

### Mr EfficiencySenior Member

I think a catamaran is your best bet. But you will have to avoid placing your weight at either end ! And that won't be planing, it won't need to be.

Doh likes this.
10. Joined: Jan 2020
Posts: 16
Likes: 4, Points: 3
Location: Germany

### DohJunior Member

Thanks. If I go the displacement hull multihull way, even if I optimise the weight/hull speed, I will get some speed range in the 4-5Knots (if my calculations + scepticism lowering factor are right).
My target is definitively to get faster than this. I have about 4-5Kwh at my disposal. (5-6Kwh battery) that should be about 2h of 2,5kw propulsion. which is in my plan totally ok.
I would love to use a 5m long displacement multihull and a smaller motor. but that's legally not an option at this point. and, I do admit having a bit of fun planing on my eboat. even if its mean reduced range and loading weight

11. Joined: Oct 2010
Posts: 10,401
Likes: 1,033, Points: 113, Legacy Rep: 702
Location: Australia

### Mr EfficiencySenior Member

I hope you don't have the idea of more than just yourself aboard ?

Doh likes this.
12. Joined: Jan 2020
Posts: 16
Likes: 4, Points: 3
Location: Germany

### DohJunior Member

haha no, again, not illuminated there. ground physic are accepted I'm currently repairing a small 20kg zodiac c240. and as soon as I have to re-make the whole floor. I was considering options

13. Joined: Nov 2008
Posts: 390
Likes: 82, Points: 28, Legacy Rep: 288
Location: 59 45 51 N 019 02 15 E

### HJSMember

Very interesting project.

I made an initial calculation based on 150 kilos total weight based on my experience. A bottom width of about 0.8 meters is required to obtain a sufficient planing surface.
With three horsepower you should be able to get up to about 14 knots. By equipping the bottom with an aftship interceptor can the drag be further reduced.

A few weeks ago Keyhaven Skiff was tested. It showed that the calculations were correct as expected.
http://sassdesign.net/Keyhaven Skiff, a semiplaning lightweight skiff.pdf

JS

Doh likes this.
14. Joined: May 2008
Posts: 2,514
Likes: 297, Points: 83, Legacy Rep: 1082
Location: Beaufort, SC and H'ville, NC

### philSweetSenior Member

It's doable, but versatility will suffer with the electric setup. Where do you use it - what conditions - what do you use it for - what types of dinghies are you familiar with and comfortable with in terms of handling and stability?

You won't really plane at that weight, but you can semiplane two-up and fully plane one-up. My 9 1/2 foot dink ran an Evinrude 4 hp 2 cylinder 2 stroke from the '70s, and my GF and I and gear, call it 500# payload, could spend 12 hours touring around islands in the Bahamas in it.

PS, use the same prop that an OB of the same size would use. Something like a 7.5 X 5.5. Don't try to go bigger. If you have the torque, you can up the pitch a bit and slow it down some.

A monohull would probably be best at the one design point you mentioned, but a cat dink might be a better all rounder with a higher speed potential running light.

"Or, having some kind of cat-hull would help me to reach a semi-displacement'ish state that would make the plane smoother?" That's my take on it.

Links to some pics of my dink -

Last edited: Jan 5, 2020
Doh likes this.

15. Joined: Aug 2002
Posts: 15,768
Likes: 1,195, Points: 123, Legacy Rep: 2031
Location: Milwaukee, WI

### gonzoSenior Member

Jimboat likes this.
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.