Feedback on my first hull design

Ok thats it, I've decided to bite the big fat ugly bullet. Where can I find all the relevent formulas?

 

I've just found a website www.dolvik.com which is a real inspiration to the design I am aiming for. These guys are getting 55-60knt on 33footers.

I've been doing some more research on engines. I was battling between the mercruiser D350 MAG MPI - 300hp and the D7.4L MPI - 310hp.
Originaly I said in this thread I wouldnt go above 250hp, but looking at all other boats of this similar design, you need power to be a player, and with the increased dead rise, i am more confident being a player. So stepping it up to 300hp seemed like a good move. But then I notice something.

The 300hp has these specs:
5.7L/350
V-8
length - 37"
width - 29"
height - 22"
weight with bravo 3 - 1046 lbs.

the next step up in engines is the D7.4L MPI with only 10hp more, at 310hp BUT....
7.4L/454
V-8
length - 39"
width - 32"
height - 22"
weight with bravo 3 - 1179 lbs.

The size dimentions for the 7.4 are consistant all the way up to the 8.2L, 415hp engine on offer. So if I decided I needed more power, having already installed the 310hp engine, I would have everything ready to accept a 95hp booster. I dont even have to plan for the extra weight as it is a negligable 30lbs. This is all according to the specs I pulled from http://www.brownmarine.com/mercstern.htm - It might cost a hell of a lot more, but in the long run will be worth it.

 

Yep,

Sound policy is to make the engine room (or compartment) big enough for a larger engine in the range, as you suggest. The immediate benefit is that there is more space around the engine (always good) and secondarily (assuming you put in suitable bearers) you can change the power later (just make sure that the engine is easy to lift out.

I have just done an arrangement for a 47' Power Cat based on twin 350Hp engines, and I'm now going to drop the engine rating to twin 220Hp, as the design speed ended up 15kts in excess of the target speed. Plenty of engine room for those who want economy, and it'll actually fit upto two 420Hp which will push speeds to more than 40 kts.

Kmorin, I am completely aware of the "repetitive" nature of design, as I tend to go around the loop myself quite regularly. I do tend to have a rule, though. Before I get anywhere near the computer, I'll go down to a marina and have a look around. Just to see what's there and how everyone else is doing it. Having looked, and done the odd sketch, and sighted a measurement or two, you are then in a position to enter the design cycle knowing that you should not have to make too many changes to the initial design.

Software is helpful, even at this point, as it may let you visualise the results, and thus pick up trends more easily (I also do a day's searching on the web to find displacement and critical dimension trends and the like). The next step, for me, is not to meticulously copy the sketches I've made onto computer (did that and got an ultra-sexy 30 foot powerboat though ) but to set up some key points (or boxes, it depends on the job) which I know must be either on, or inside the hull. These are all measured relative to a guessed keel-line (which is below 0,0,0) which is on the waterline, amidships. The keel line is taken from the extrapolated data collected earlier or has slightly less draught than the design breif... etc. etc. etc. and so the hull design begins. Once a basic hull is obtained, the questions asked may be, does it float (and at correct trim and waterline) and is it buildable. The answer to the latter is that most things are buildable, it is just a question of how (and you can do some VERY smart stuff with thin plywood).

Getting the initial design to float at the right WL and right attitude, though is a question of repetitive calculations. and a lot of them. I have recently been writing a hydrostatics plugin for LMH which has been using 2000 immersed cells to do basic bouyancy calcs on. That's a hell of a lot of hand calculator work!!! so, considering that each calculation takes at least 2 minutes, that's nearly 2 weeks hand calculator work for each point on the displacement graph. so as you can see, the advent of a computer really speeds things up a lot... it now takes less than a second!!! and that includes calculating all the points first!

You might be interested to have a look at the info about LMH that's posted around this forum and on my website.

As for the exact formulas, it's hard to say. Savitsky published a well-known paper for power prediction of planing craft. the ITTC 57 skin friction line is used for viscous (excluding wave drag) drag component and is 0.075/(LOG10(Re-2)^2), where Re is the reynolds number, sometimes based on 0.9*Lwl. For balance calculations, it is just a question of taking moments. Bouyancy is the summation of all the elemental immersed volumes (or the double-integration of the immersed hull surface) and centers of bouyancy are found by taking moments of all the elemental volumes.

Kmorin, as far as "installing C++" goes, or rather, installing the compiler FOR C++. I didn't have any trouble downloading the MinGW (Win 32) compiler and setting it up on Windows XP. It took me about an hour, and most of that time was downloading packages from CVS through a dial-up connection. You don't, of course, have to learn the whole ANSI C/C++ definition, that would take way too long, instead, like anything, you learn the bits you need. As long as you always write clear code (and comment a bit, or supply an explanatory manual wih source (as I do)) then you should have no difficulty learning the language. Of course, there are other languages except C/C++, it just happens that that is what I'm using at the moment.

I Hope some of this helps, and may your code never be cryptic or give the stupid answers that mine is at the moment

Tim B.

 

I read you on criptic code. horrible stuff. comment comment comment!

As for the engines - I had another think. I'm getting many different lists of the mercruiser range. They all include differnt engines.

http://www.miamimarina.com has a rather extensive list, but then the other website I posted earlier http://www.brownmarine.com/mercstern.htm has a different set. And then the mercury marine website has yet another set. Its really getting quite confusing. the only thing I've really settled on is the contra-rotating props. And I think above 250hp is called for. Then again, is it? in a 20' boat, 250hp sounds like a lot, but 300hp sounds like even more.

Back to hull design - If i have a constant 20 (or 19.8 or whatever it is) degree deadrise from the middle of the boat to the rear, and then increase the deadrise at the bow to (got knows how much, pick a number) is there going to be any real problems with this? On a plane she should be on the aft third anyway, and if she hits the crest of a wave with only 20 degrees deadrise, she could be quite awkward. Let me illustrate what i mean in hull design... give me a minuite.

 

See what I mean? Its going to want to fly off anything it can. I have the strongest feeling that I have to increase the deadrise, as per the other attachment. correct me if I'm wrong ofcourse, but almost every hull I've looked at has the deadrise increase towards the bow.

 

Generally, the increased dead-rise towards the bow reduces slamming loads (and discomfort) at the small price of getting on the plane a bit later. My own experience ('cos I'm really a yacht designer!) says the small dead-rise for'd (eg. Rondar 420 (olympic class)) tends to hit waves and stop, whereas lots of deadrise (eg. the LARK (same size)) generally keeps going through waves without much hassle. It is, however, a trend that should hold true for any boat. It has been noted though, that once the 420 pushes over the tremendous "hump" it does plane at a lower speed than the LARK, although this is more to do with the bottom sections and trim than the forward dead-rise, in my opinion.

I'll post a lines-plan of a 30ft powerboat later on, when I've finished all the work I have to do.

Cheers,

Tim B.

 

I think with the amount of power behind it the delay to getting no plane wont be much of a problem, and with the amount of power it would be very much worth adding the deadrise increase, considering my 2foot chop design spec. the increase in deadrise I originally made (from 7 or so to 20degrees) would be worthless without increasing the forward deadrise. agreed?

 

well, I said I post a lines plan, and while I had the computer re-booted into windows I thought I'd do some nice piccys too.

This is a 9meter powerboat which I drew just because I had an idea that I thought looked nice. It is a sports boat, rather than a floating caravan, but there is some accomodation provided by the full-size double V berth up front.

This is not a run-about by any means (or if it is it's a very modern one) but it illustrates nicely the use of a varying dead-rise deep V hull.

If anyone is interested in this boat, e-mail me and I'll finish the drawings for you for slightly more than the cost of a stock plan (to cover my time).

Cheers,

Tim B.

Ps. a table of dead-rise angles is supplied in CSV format. It should be usable with almost any spreadsheet.

 

For those that are interested, I got on my bike today and went to look at power-boats. Most large boats (32ft+) use about 18 to 22 degrees deadrise at the stern and 35 degrees or more at the bow. A few are constant-deadrise in the 22-26 degree region. Most smaller boats are constant-deadriseat about 20 to 25 degrees. Strakes where fitted seem to fall roughly into a few positions which tends to be:

position = kc* i * 1 / (n+1)

where kc is the distance from keel to chine, i is the number of the strake you're interested in, n being the total number

Thus for two strakes we get: 1 * kc * 1/3 and 2 * kc * 1/3.

These are based on observations by eye. but I don't think they're far out. if someone wants to pay me for real results I'll get my protractor and tape measure and do it again.

Cheers, have a good weekend,

Tim B.

 

Very useful information, thankyou.

 

Ok so I decided to increase the deadrise to 35 degrees at the bow.

The method I used to calculate the deadrise while maintaining profile view was rather interesting but turned out very well.

I am considering drawing up 1:2 scale plans quite soon. The whole boat is now designed by formula so it can be easily scaled to any size.

 

Excellent, if you post some pics on here, we'll give you some feedback.

What formulae are you using to design with?

Tim B.

 

Well all the lines, every curve, every angle can be attained by doing some simple maths.

For instance, the barrel back curve is basically a 1/4 circle between two points, On the 1:10th scale drawings it might be something like this.

Width of deck = 20cm
Height of barrel back = 6cm
Flat area at the centre is 4cm (2cm each side of centreline)
So then I have a curve which is 6cm high and 8cm wide.

On 1:1 scale these dimentions would change

Width of deck = 200cm
Height of barrel back = 60cm
Flat area at the centre is 40cm (20cm each side of centreline)
So then I have a curve which is 60cm high and 80cm wide.

There is no change in shape at all.

When calculating the deadrise:
For width of 20cm, floor to chine vert. height will be 4.4recurring cm.
Increase the deadrise to 35 degrees and the hieght for a width of 20cm is 7.7recurring cm. Its a simple step to then get the height of the chine at each frame inbetween. Infact its possible to get the height at any given point.

Scale to 1:1 - For width of 200cm, floor to chine vert. height will be 44.4recurring cm.
Increase the deadrise to 35 degrees and the hieght for a width of 200cm is 77.7recurring cm.

Even the profile lines of the boat can be drawn this way.
I'm half considering writing a book on this method called "doing it right, the wrong way" hehe.

Anyway, its working wonderfully and I know I can re-create the design at any scale.
There is a lot of work to go.

 

In fact, if you describe the dead-rise as a function or x/LOA and do the same for the side panels, you've got a lsurface definition for the whole hull. It's basically the same thing as I'm doing, except that I'm using (nearly) algebraicly defined surfaces to do it. I say nearly, because the NURBS basis function I use is calculated numerically.

You might like to have a look at B-Splines, which are completely algebraic and very easy to use. There is plenty of info on the web. I have a B-Spline routine I can e-mail you if you want. However, I wouldn't bother stopping to write a GUI interface for it. I've been working on mine since June, and it's still not finished.

Tim B.

 

Well, its a motor boat and to me all motor boats are horrible so I am probably not the best person to make constructive critism. However, I see that you have a lot of twist in some of the panels and based on comments on another forum a year or two back I would question whether the free hull designer program will give an accurate panel development where there is a lot of twist - Some people believe that this program uses an approximate method for developing the panels, perhaps the writer of the program may be able to comment.

My guess is that the people who suggest that this boat would pound in a rough sea are probably right, but it could still be a nice boat for a lake and you can always slow down if it gets uncomfortable.

John