Need help with tech drawing

The more I learn the less i know. As a retired engineer there have been few formula involving boats that have given me trouble but although I did O level technical drawing a very long time ago I'm useless when it comes to boats. Given that I decide on a length draft and beam, at what point on the length should the maximum beam be. How wide should the transom be if there is one. How do you decide on the radius for a round bilge hull. I could go on and on. It takes me longer to work out how to work a CAD package than a draftsman would take on a drawing. I understand tech drawing but just can't apply the little i know to drawing boats. This has forced me to think of copying others more than I would normally. please advise me where to start.
mik the stick

 

Hi Mik,
None would be able to help you in a forum It would be a long story!. Boat design is a kind of science especially if you look at sail boats. Lot of experience is needed before engineering a hull.
Keep in mind first that there is a world of difference between sail and motor boat!
I would suggest to read diligently some books:
Simple, basic one (sailcrafts) is "Yacht Design Explained" by Steve Killing
Comprehensive one is "Principals of Yacht Design" by Larsson & Eliasson
Interesting to start with (at least was to me) the old book "Yacht Designing & Planning" by Howard Chapelle.
Many other are available.
Hope I do not discurage you, for sure you need to have lot of passion to achieve a good design level!
All the best
Adriano

 

To follow up what Adriano has said, the questions you pose suggest you're a long way off from considering designing a boat yourself. To put in perspective, you're asking about designing your own airplane and would like to know where to place the wings on the fuselage, how long they should be, how big to make the fin and rudder, etc. Your have to have a reasonable grasp of the hydrodynamics involved as well as an engineering approach that can achieve the goals established (light enough, strong and stiff enough for the loads anticipated, etc.). As an engineer, you already should know the answer is research, meaning you have some studying to do. Your best bet would be to buy a set of plans, knowing the hydro end of it is already worked through, then add your styling touches, staying within a reasonable weight range for the application. The result will float where it should, yet look like the full up custom suited for your needs, that you desire.

 

Arrowhead first attempt

I will look for these book at amazon, thanks. Par I'm sure you realize I'm researching all the time. I did most of the maths (stability etc) for "Black Magic a cruiser racer but I could not draw it. So I decided to try what I would buy if I was in the market for a boat and had the required funds.

1 Not being a sailor* a double-ended lifeboat seemed appropriate. I think they are beautiful as well as seaworthy.
2* A pilothouse is a must.
3* An outside rudder makes an emergency steering position easy, the tiller arm is normally stowed on the aft deck.
4 Due to the expense of sails and rigging I prefer the Ketch (Marconi sail) rig. this is chosen to keep sails small with lower CE.

At 42ft a completely double-ender's bow seemed too blunt. therefore I drew an arrowhead shape thus the boats name. I have done no calculations yet but plank and frame hull core sheathed inside and out comes in at ~3700lbs.

I have yet to draw a pilot house but include this poor drawing, it gives you an idea of my limited skill.
mik the stick

 

Check over at the Atkins and company site for double ender lines (atkinboatplans.com).

 

Thanks Par, I bought Yacht Designing and Planning for my new Kindle I think it could help a bit. I like the book anyway it's appendix was worth the price of the book.

 

For Adriano

I have studied the the book by Chapelle I am still learning to use the information, but I think my drawing is improving. I can now calculate what the displacement has to be for the LWL of 3 feet.My guess of 25-30,000lbs gives a PC of about 0.42 ( too low I think) I now intend to estimate displacement by adding everything in the boat. and check this against the displacement for a LWL of three feet. A re-draw may be required but thanks to to you'r recommendation I feel I could do that.
thanks mik

 

My first impressions from your drawing is that big sort of barn door rudder you have along with the short tiller means you will be thrown around trying to steer that thing as set up.

 

Well there are several issues I can see, but this is the initial "napkin" sketch, so these details can be worked out.

 

Depending on how adventurous you feel, software like Freeship, or preferable Delftship, is an inexpensive way to do hull 'what-ifs'. They feature all the calculations with weight tables, varying drafts etc. with terrific accuracy

Practicing with the software will give you some valuable tools for future projects too

 

Tryed to download delt ship to try it without success. My lack of skill with 2DDesign is driving me nuts. I will try and draw with paper and pencil tomorrow.

 

The advantage of software, even the free or low cost stuff (which are limited) is the calculation process and more importantly the ability to make subtle changes and the resulting evaluation of these changes, comparatively. A good example would be developing a hull for a specific speed range. Software will quickly let you make adjustments for say a more appropriate Cp (for example) where a hand drawn hull will require tedious calculator effort and time with an eraser shield.

 

I would have replied sooner but trying to download delftship added nag programs on my computer. spent all day removing them. By luck or good gudgement I managed to get freeship in the few seconds I have tryed to use it I am very impressed. Thank you. It may be some time before I post anything in the future as I have just had very bad news. Thanks to all of you for you'r help and advice.
Mik the Stick.