Feedback on my first hull design

Kmorin,

are you sitting at a computer? They're very good at doing repetitive calculations.

All that's needed is a bit of code (which you can write yourself, no hassle) then put in your measurements and let the computer do the hard work. It is worth doing if only to see if there is any correllation between strake position and deadrise or number of strakes with beam or the like.

Please note that if you plan to do this, use Fortran or C/C++ , not a spreadsheet! you can't follow spreadsheets six months after you write them. C/C++ have GUI developement environments available to let you do interfaces and plot lines and points etc. I have found using QT3 and GNU/MinGW compilers is a good solution. If you want a simple bit of code to start you off, e-mail me and I scribble a bit up when I have time.

Cheers,

Tim B.

Tim@MarineDesign.tk

 

kmorin - Very valuable post, Thankyou. I will have to read it about 3 more times to realise all your suggestions. I now understand non-trip chines for which I am greatful.

Tim B - I've done a bit of coding in my lifetime but calculating where to place strakes on a v hull of 19.6 degrees with 250hp (which is what I am considering now, after checking out the merc and volvo lines) and an all up weight of.. 3500lb?!?!

Question: Sterndrive setups - If I wanted to have a single sterndrive (like the bravo III) - where would I place it on the V hull of 19.6 degrees? Would the non-cavitation plate be level with the keel? slightly above?


My partner is away for 5 days so I have decided now is a good time to build up a 1:10 scale model. I have decided to undercut the design process a little and build it by eye. I will be doing a few things to keep it true to plan and true to my design intentions. I would just like to breifly outline the process I am planning.

I have sketched up the profile of the hull, as I would like to see it. I will be cutting out identical frames (10 or so) all with a 19.6 degree deadrise, and all at the maximum height of the hull. So if I was to stick them all together, My boat would be a box with a constant V bottom. To acheive the shape I want, I will be lifting the frames as they get closer to the front. Its hard to explain, maybe the picture attached will help explain. - Using this method I hope to keep the deadrise constant and it will also give me accurate curves to work with. I can also then layout the interior and plan the engine mounting (by building a little box, to represent the engine. I am considering the idea of having an engine mount which can be moved forward and aft by 3' so I can get everything honed in just perfect. Once my model is complete, I can take it appart and take measurements for full sized frame dimentions. - I know this is a very unconventional way of designing a boat, but I just cant afford the software to do it for me, nor the time to learn how to use it. If I stick to using a few mathematical formulas for curves etc, I can upscale the shapes very easily. again, not conventional but I am confident it will work.

I will most likely upscale to 1:2 scale model first, and then plan the stringers and strakes and everything like that. I may even stick in some power and take it for a spin, 40hp should do it. just to see how it behaves.

So the design process is going very well, and I am appreciating everybodies help very much! please feel free to comment productively.

 

Vince,

I was really suggesting that Kmorins fears of repetitive number-crunching were un-founded, and using the computer to analyse the measured data obtained from other designs... mind you, given a CFD package you could do the sums...

The straight-building method sounds ok, personally, I'd draw the hull in CAD (prob. Rhino v3 of which there is a free demo (fully featured but only 20 saves) e-mail me for a tutorial on drawing hard-chine hulls) then take offsets at each bulkhead. Cut the bulkheads to the offsets (remembering the plating thickness) and then build from there.

The problem I foresee in your method is that you'll have a hull up to the chines, and nothing above it (no sides). That's not a huge problem, but a little time invested now will make building the full-size boat a hell of a lot easier.

Cheers,

Tim B.

 

Thanks for the suggestion Tim. I may have to email you for that tutorial.
I've tride CAD and haven't been able to get anything useful out of it.

Part of the idea with the mockup method I am thinking of, is to get the hull shape (the hydrodynamic part of the hull) worked out, and then with the corofoam defining maximum dimentions (must fit in a 20 foot shipping container.) I can mess around to get the look I want. I'm buying enough corofoam to cut each peice 10 times, so I have a lot of room to play. All cuts will be made along mathamatically defined lines (so I can replicate them when scaled)

I think this is a great way to do it for someone who doesn't know CAD. I get to see it in 3d and make adjustments to my hearts content. Then I can pull it appart and scale each peice. Anybody want to tell me that it is not possible to do it like this? I think it is. Will post pics if I get around to it. The hull profile I will be working to looks spectacular from the side. Now I just need to see it in 3d. She will fit a 250hp without a worry. I was almost tempted to stick in 425hp, but thats just getting crazy. 250hp is already too much for a boat this size. I am leanding towards contra-rotating duoprop stern drive.

If I ever build a boat after this one, I am very tempted to get a volvo penta IPS system. But at this stage I cant imagine myself building a boat large enough. whats that? equivilent to 800-1000hp? gorgeous system! It does everything right (except for the unprotected props, which worry me a little)

 

Duoprops handle like a dream. The ever-useful 4.3L V6 or 5.0L V8 would be the gas engines of choice in this range; there's also a few diesels to look at. Having the engine weights would be handy though, before you finalize the hull.

Foam-core mockups are great. I did one for my last boat even though it was from purchased plans, just to see how it fit. Excellent idea.

As for CAD stuff. If you don't like it, don't use it; boats were built for thousands of years without it. Likewise with FEA, CFD, all that nerdy engineer stuff. If you like it, great, they're good tools but not really necessary.

 

Marshmat - its great stuff isnt it?

Question still open to opinion: Sterndrive setups - If I wanted to have a single sterndrive (like the bravo III) - where would I place it on the V hull of 19.6 degrees? Would the non-cavitation plate be level with the keel? slightly above?

Thanks.

 

Hi Vince,
I've been following this thread with a great deal of interest. There are several aspects of it that I find appealing. First, it's kind of gutsy to be designing your first boat online with a large group of spectators who have a very wide range of experience and skill. I wonder how many of the spectators would have been so bold with their first design. I have in fact wondered if that might not make a very interesting new thread; folks sharing their very first design effort that was actually built.
Second, the type of boat you've chosen can be a real challenge to do a really good job on even though they are not all that large and do not have to be very complex.
And third, who among us hasn't dreamed of zipping along in a stylish, speedy little boat just for the fun of it on a gorgeous summer day.
But I have to confess I was actually more excited about your boat when it was going to have a shallower v-bottom and more akin to most of the classic runabouts.
While the deeper v-bottoms do provide a softer ride they also have some negative qualities. They typically do not manuver well at low speed, the wake is typically nasty for skiing and they use a lot more $3 a gallon gas/fuel.
But of course designing a boat is a matter of trade-offs.
Just for a point of interest, I'm attaching a wire frame image of the only runabout I've drawn. It's 18' 10" long and not very beamy although it looks beamy in this particular view. It has a monohedron bottom with 7 degrees of deadrise. The sections are for plywood or sheet construction. I wanted the lines to be understated, not extreme in any way, and so it may strike many as rather bland. Perhaps you can think of it as a sample of the road not taken.

Keep after it. You're doing well and getting lots of good input.
Gilbert

 

Thanks for the support gilbert.

The design I was after has morphed quite a bit since first starting this thread, and looking at the drawings I have on paper now, has turned a little more into a modernisation of the classic runabout. She sits lower than I intended, is designed for more power and has a much deeper vee. Her lines are sleeker and her functionality has been traded a little for looks. That said, as the "design process" advances, I like her more and more.

I originaly started with something very similar to what you have posted and I like it a lot. Its a very nice classic style hull. If you have developed the design to a stage where it can be modeled, I would suggest you have some fun and build a corofoam mockup of what your trying to do. A hull like that just oozez style. I was aiming for that originaly, but I veered to the trend of modern style. I congratulate you on acheiving the exact design you were looking for. Personaly I like to lower the deck as it goes aft, and I just love the barrel back! You could come up with a gorgeous boat just the way it is. I can imagine it could develop into something like the Costa-Brava 680 by Boesch. which I find to be an inspirational transition from classic runabout to modern speed boat. She has about 7 degrees deadrise and bugger me dead if the deck isn't about the same shape. Check it out if you can, a gorgeous boat. She's closer to 22 feet, but would look just as awsome scaled to 19'

I guess the trick to this forum is to not let the punks get you down.

 

Ok the specs of my hull (as I will model it in corofoam) are:

Maximum beam at chine: 2000mm
Maximum beam goes from 3000mm aft to the transom (constant width from the middle of the boat back, basically)
Maximum beam at the deck: 2300mm
Total length: 6000mm
Hull deadrise: 19.6 degrees constant.
Maximum total hull height: 1300mm
Maximum height at transom: (eigther 1000mm or 800mm, I forget which I decided on)

The other dimentions I cant pull from the top of my head, I guess I will have to get off my butt and scan the sketches.

 

Hi Vince,

I don’t know where you are living but watch max beam. I made the mistake making my design 2800mm. In most of Europe it isn’t trailerable! You have an exceptional transport in that case! So I had to redesign! As you can’t simply scale it down.

Just a hint

Greetz,

 

US max beam is something like 8', right?

 

Max beam

In belgium where I live it's 2550mm

 

In all of North America, maximum trailerable beam is 8'6" or 2.59 m. Oversize load permits are needed for anything wider. The fully-loaded vessel and trailer must together weigh less than 4600 kg (10,140 lb) in most areas, or else a Class-A or equivalent trucker's licence is needed to tow it. And bridge clearance, when on the trailer, on most roads must be less than 4.2m.

So if you have a 3000mm beam, your vessel is not trailerable in North America.

The propshaft of a Bravo may be mounted anywhere from 4" to 8" below the keel. Typical is in the 7" range. Higher mountings are only for high-speed racers and are tricky to handle. The antiventilation plate should be entirely submerged when on plane and at operating trim, at the speeds you are looking at.

 

number crunching during design

Vince & TimB, I must not have made clear why the iterative math can be time consuming, in my opinion. The reason isn't that there is confusion about the use of PC, nor that the algebra requires an original application to perform.


The inference I'd hoped to express was more about the design cycle's process of drawing-then-calculating, then redrawing-and-recalculating, not that repetitive calculations were such a burdensome exercise. I was trying, apparently without much success, to express to Vince, TimB, the one reason that designing is a non-trivial exercise is the repetitive or cyclic nature of the process. One cycles between the algebra and the hand (or CAD) drawing process adjusting the visual to reflect the calculations and once the lines have been altered you need to refigure the displacement which influences the CG, CB and around and around.

Since Vince is coming up to a loop within the overall decision tree in the design process; his decisions about the rails seem best served by having a table of values to look at and decide between, say, "max or min beam of the inner most rail at station 5, realizing that the beam has a vertical component that varies in relation to the V of the body plan at that station".

I suggested a spreadsheet to assist in creating the 'lookup table' to reduce the number of times he had to REDRAW, not the number of times he had to use his calculator. This method (still) seems easier since the interface is automatically a table, just like the millions of tables used in engineering texts of all types including marine.

Most of all, I was trying to encourage Vince to spend the time to define the relationship between the angle of deadrise at a given station with the beam max-min of the strakes AND the resulting vertical component of that horizontal distance. After all, to complete his boat plans he'll need to locate and size these components, and their orientation along with their taper, by trial and correction; so learning C++ might be a bit of a distraction? I suggest that the time to even install, run, and begin to use the application, after its purchase, an obstacle to his design completion(?)

TimB, I'm glad that you've already invested the time to learn to use these more robust programming environments, like C++, not everyone has, so I'd offer that simpler more commonly installed PC tools were more utilitarian for most designers.

Cheers,

 

I think I've confused a few people with my terminology

"Maximum beam at chine: 2000mm
Maximum beam goes from 3000mm aft to the transom (constant width from the middle of the boat back, basically)
Maximum beam at the deck: 2300mm"

What I am saying here is, the beam gets wider and wider as it gets further back, to a maximum of 2000mm chine, about in the middle (3000mm from the front and 3000mm from the back) of the boat. So my chine has a maximum beam of 2000mm. My deck has a maximum beam of 2300mm. Total maximum beam is 2300mm. This is not only trailerable, but containerable for intercontinental shipping.

As for writing applications to do the calculations, I am proficient in many programming languages, however I dont know the formulas which I would use to define these applications. Give me a PABX log and tell me to write a billing system for it, no worries. Calculating the location of the strakes on a 19.6degree hull? I wouldn't know where to begin.

Right now I'm battling with the idea of increasing the deadrise slowly at the front half of the hull. 19.6degree deadrise slamming into a wave is going to hurt. 25-30 degrees might not be so bad.