Understanding hull design

[flanagaj]

Hi,

Currently in the process of gaining knowledge of all apsects involved in boat building. Whilst I have my eye on a few starter projects, I would really like to understand the intracacies of hull design. This will then hopefully enable me to take an off the shelf plan and change the hull shape as I see fit.

Some of the retro speed boats are great, but they are so flat bottomed with virtually no V and would be terrible for UK south coast conditions, but with minor modifications to the hull I suspect you could get it to work.

Thanks

[Tim B]

There are loads of books on the subject, and they've been quoted on this forum so many times that I'm sure some quick searches would find a whole raft of them. From your other post (here Good GRP hull book ? ), I'm not sure whether you're undecided on the boat you wish to build, or are trying to learn about everything to do with hull-design. Either of these are fine, but power and sail are rather different disciplines.

The basic hydrostatics are pretty simple and remain the same for any floating body, again, there is plenty of reference material (Biran, "Ship hydrostatics and stability" is a good reference). However, the hydrodynamics of yacht and power-boat hulls is wildly different. Consequently, the structure is different in order to support the differing loads.

Retro speed boats are a bit of a special case, because as you've guessed they work in flat water, but not in chop. Unfortunately, the lack of deadrise aft also means that their directional stability is not as good as a "modern" hull-form, and that is significantly more dangerous than excessive slamming. Yes you can change an existing hullform, but you're changing the first thing that's drawn in the design process. Consequently everything else flows from that, how much detail you then need to re-check is then down to the boat and the modifications; However, as a minimum it will be the LCG and the running trim, and that could mean moving the engine etc. etc.

A good way to get a feel for overall hull-design and structure is to build a few model boats, but use full-size techniques. You can try to build for minimum weight, and that will give you a quick indication of which structures work and which don't at relatively low cost. You can also test the boats and find out which ones are good for handling, seakeeping etc.

Have a look at the link below, which contains all the photos for this year's works model powerboat build. It is 590mm long and (over) powered by a Speed 600 on 6 NiMH cells. Build cost was about £130 on the water, of which the hull was about £50. £40 of that was the incredibly thin (0.4mm) ply used. Hence on page 4 you can see the structure quite clearly through the ply. Next year's boat will be the same construction ('cause it's super light), but a different hullform.

http://openpilot.engineering.selfip....dir=RST_Racing

Cheers,

Tim B.

[flanagaj]

Thanks for the reply Tim. I can see the benefit of making a model hull. I bet the large boat builders do just that before committing the time and expense to making a full size version.

I will also have a look at the books you mention.

[Eric Sponberg]

Flanagaj,

Take a look at my website where I did what you suggest on a Chris Craft Cobra.

http://sponbergyachtdesign.com/ChrisCraftCobra.htm.

You do have to know what you are doing, so you will be studying quite a bit. Model building certainly will help. The owner of the new Cobra wanted a transom deadrise of 15 degrees, whereas I usually do deadrise at 20 degrees. This particular boat, as you will read, had a bit of a porpoising problem, which the owner easily cured. All my other designs have been quite successful at 20-deg deadrise.

The other factors that are important are the degree of warp (or not) in the bottom shape; the shape of the keel rocker (or not) and stem; the shapes of the forward sections, and the shape and position of the chines/strakes. On small boats, I do not use intermediate lifting strakes as they are very troublesome, both to build and when running if placed inappropriately with respect to the propeller(s).

Good luck.

Eric

[flanagaj]

Hey Eric,

That is a wonderful looking boat you have there. I have a lot to learn as I struggled to understand a great deal of terminology that you used

Thanks

[gonzo]

Carolina Sportfishermen handle rough water at 40kt with 14 degrees of deadrise at the stern. However they have a very deep Vee at the bow