# Hull balance diagram questions

Discussion in 'Hydrodynamics and Aerodynamics' started by Sillouete, Mar 13, 2013.

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### SilloueteJunior Member

Hi,

Lately I've been trying to understand some of the dynamics of different hulls while turning or steering at variable speeds. I provided some templates below of a monohull and a catamaran, and I would appreciate if someone could provide me with force and momentum balance diagrams for each hull.

My next question is, which hull design turns better and faster, assuming calm waters, and and motor propelled?

Also, given that the turning capabilities of either hull is affected by velocity, is there a range of speeds in which one is better than the other, but another range where the one that wasn't that effective before, is better at the new range?
- For example, assuming that at 10 knots Hull 1 has a better turning capability than the other, if we speed up to 25 knots, what are the chances of the Hull 2 being better than the Hull 1 at the new velocity range?

My last question. Given that certain hull performs excellent while turning, are there are certain scales and sizes where that hull may not perform as good?
- For example, if a catamaran has a good turning capability given certain dimensions, let's say over 50 feet long and 20 feet wide, would a catamaran of much smaller dimensions such as 15 foot long and 5 foot wide, would still retain that grade of effectiveness while turning?

I would love to understand the math and physics behind all these explanations as I'm designing my very first ship, so if anybody could explain it to me or point me out to some references, it would be greatly appreciated!

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### PARYacht Designer/Builder

Welcome to the forum.

The logical course would be to learn about the fundamentals and principles of yacht design, Then move on to ships, don't you think? Simply put, yacht design is an engineering discipline, so how's you math skills and general engineering understanding? For most of us it take a few years of intensive study or a life time of casual study and as far as designing ships, well this is what a team of engineers and architects are for.

Maybe it would be more helpful if you told us what you're trying to do and offer an overview of your engineering and hydrodynamic skills.

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### SilloueteJunior Member

Thank you.

I am a Mechanical Engineering student and I do have a basic and general idea about these systems. I would like to learn more about hydrodynamics as I am designing a small electric boat. By small I mean no longer than 20 feet and no wider than 10 feet. The electric motor would be producing a power output of about 15 horsepower. I am looking for a total weight of no more than 500 pounds.

Basically my goal is to design a boat that isn't only stable and fast, but also very maneuverable. I am debating whether which hull is more efficient given the conditions I mentioned above. I am fairly new to ship design and I would like some pointers on this subject.

Also, thanks for pointing out yacht design, I will definitely look into that.

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### PARYacht Designer/Builder

You have much to absorb - try the book store here with "yacht design" in the search box.

A 15 HP electric motor will likely need to be surrounded with a huge chunk of foam, as this will be all the boat will be able to carry, given your 500 pound limit and the power supply. Maybe you should first look up the physical specifications for an average 11 KW motor, then enough batteries to run it for, say a single hour. This should get you into reality fairly quickly. Unless of course, you're looking to use a BLDC style motor, so you'll need a minimum of 200 amps at 48 VDC. If on the other hand, you're thinking permanent magnet motor, then the 150 pound range for a 11 KW is about right, plus batteries.

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### gonzoSenior Member

Being an engineer you understand the concept of not having enough significant information. There are many more variables than what you indicated that affect the behavior of a boat. Trim alone will change the turning characteristics.

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### tomasSenior Member

If he were so inclined, I would support PAR for any elected office.

Alas, people with PAR's mindset eventually loose because we the public, as a group, loathe reality and consequences. We prefer to support those that offer easy answers with no real plan, so we aren't challenged in any way.

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### gonzoSenior Member

The OP is asking questions to understand the math and physics involved. There are libraries written about the subject, so maybe he needs to narrow the subject. For example, is 500 lbs the loaded or empty displacement. If that is empty, three people with minimal gear will double the displacement.

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### PARYacht Designer/Builder

My premise and what seems fairly obvious to most here is a need for research, which is the hallmark of all good engineers. In this case hydro principles and concepts.

I've seen some relatively new 11 KW motors of surprisingly low weight, but there's still the storage issue, which will easily overwhelm the 500 pound limit, assuming it's not an RC concept. A gas/electric could solve this, but a 15 - 20 HP gas engine, spinning a 11 - 12 KW powerhead, just to turn a 10 - 11 KW motor is just forcing you to accept huge loses, over more conventional setups and you still have the gas engine to contend with.

If he's any kind of engineer, he'll pull the references necessary and some quick math will toss reality in his path in no time.

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