Waterline for planing boat

[Raggi_Thor]

I think we are getting along quite well...

Tender means (probably, my guess) that the boat is (initially at least) unstable at rest. That is, it feels tender when you walk on it.
This is a common problem for so called "deep V"s.
Water ballast in a tunnel that drain when you plane and fills when you slow down is a good solution.

I am not experiened with high speed planing boats, so I cant give a qualified answer to the question about the ideal draft. If the chine is not a big flat one, I would go for a waterline (at rest) just at the chine at the loaded (normal number of passengers etc) displacement.

I think this is an intersting topic, afterall.
For a given displacement and length and Cp, you have to play around with draft and waterline beam to find an acceptable compromise between soft ride and stability.

[gpaladin]

A way you educate yourself depends on your age. Since you are 19 you have probably finished high school. Maybe you go to university now. If you study something that is not related to naval architecture or to engineering than you have to educate yourself through books, internet, by finding some naval architect who would like to teach you. There are some internet courses that you might find interesting, like westlawn school.
So what is your current occupation?

[gpaladin]

Thank you thor.

Than the stability at rest is one issue.

[T1MMY]

I am currently a qualified plumber/heating engineer (innhouses etc...)

to be honest i am not trying to educate myself deeply in the desing or making of boats, simply trying to gain a better understanding of them etc... i searched on google for powerboat forums, and ame across this place, being a design forum i thought that it would be a good place to post a question regarding my current problem, but havent had any replies also thought i would look around to see what other types of topics there are on here, hoping there would be an answer to my problem in sumeone elses thread somewhere.

but not having alot of luck im afraid

[Raggi_Thor]

Normal problem, displacement for given dimensions and parameters is higher than we wish...

[Raggi_Thor]

Originally Posted by T1MMY

could you please explain what the 'chine' is and what 'station 4' is, thanx.

Timmy!
You did get an answer!

[T1MMY]

lol, i know dude, thats not the reason i joined the forum, that was just from reading the original post on here and not understanding those terms, so thought id ask lol, and i do now understand them thankyou for that.

the question i talk of that i have recieved no advice/answer for is on a totally seperate thread, hang on, ill find a link

EDIT: My original thread/question i speak of

[Willallison]

Right - I see that Raggi has set you straight on your other question Timmy (though I strongly disagree with his 1st suggestion - fillling with foam. This will only create a much bigger problem for you in the not too distant future. Find the hole and fix it. Go to a fibreglass shop, they'll give you all the advice you need and sell you the stuff as well)

Now - back to the somewhat odd discussion at hand
The reason we occaisionally give someone a light-hearted bollocking for asking (what appear to us) to be very simple questions, is that it takes time to answer them all. Of course we are happy to help - I've learned an enormous amount from the guys here. But when people ask questions that are very easily found by just doing a little searching, it tends to smack of laziness - "oh I'll just ask these blokes - that'll be easier than looking for mysel"...

And now - finally! - back to the original question(s)
The traditional way of finding out where the waterline should go is twofold.
1. Best Guesstimate, based on experience and other similar vessels
then 2. moving the waterline up and down (on your drawing) until displacement meets your design parameters. Note that the DWL is just that - the "design waterline length". It is a somehwt arbitary figure as the at rest wl length will obviously vary according to load.

If drawing by hand this can be a long and involved process. CAD makes life a LOT simpler for this kind of stuff.

Tender does indeed mean the boat's intial stability is low - it will tend to roll a fair bit as you walk from side to side. The noise comes from waves and water slapping on the underside of the chine.
Hope that clears it up fo you - and don't be afraid to ask more questions - we don't bite (or at least not with sharp teeth! )

[design.ho]

Hello;

Why dont you go for Froude's criteria?

You may consider Beam Froud No: in SI units greater than 1. i.e.

Speed(m/s) divided by
Sqr Root of (9.8 m/s/s x Beam(m))

And then go for Savitsky's standrd Beam/Length Ratio.

Or by a rule of thump you can take V(knots)
divided by Sqr Root of Length (feet)

value to be greater than 3.

Bye
Design.ho

Originally Posted by gpaladin

Is there a rule by wich waterline for planing craft could be determined when the craft is at rest.

Let say that vee hull ends with flat chine. Where is the waterline, at the chine or higher.

I am talking about 14 metres in length and 30-40 kn of speed.

I am aware that waterline is result of geometry and weight (mass) but I am interested in position of the waterline that should be appropriate for certain speed/length/displacement ratios.


Thank you for your comments.

[bhabanism]

Yup there is - when the craft is at rest, whether planning or of any kind, the weight is supported by the water displaced (remember eureka of Archimedies). So find the underwater volume from the offset table and multiple the water density - that must be equal to the weight of the craft.

[marshmat]

bhabanism, that's a basic principle that I'm pretty sure everyone on this thread knows inside and out (it ain't complicated)... the questions being asked here are about appropriate shapes and parameters for hulls of a particular type, weight and speed range.
Back to the original question though... generally a planing boat will be optimized for its cruise speed, and so might not be as agreeable going slowly (many have terrible tracking and rock like crazy when doing 3 knots). But in general, for common V-hull forms, the chine tends to meet the waterline somewhere between 40% and 75% of LOA aft of the bow when at rest. Not an authoritative rule, but it's just how most of them appear when docked.

[design.ho]

You can design the boat only in freely floating condition.

I got your question. It is not so easy to answer in one simple email. Still let me try.

This question/confusion might have arisen from your mind because you are thinking about the design of this type of boat with the same approach as that of a fully displacement hull - I guess So. I am wrong, you may contact me in the Tel No: 0091-44-2855 3040,42 Ext 432 (ARAVIND.G).

In conventional hull, we fix up the dimensions of the vessel keeping in mind the dimension restrictions, appreciable Cb parent ship data etc; and directly go for propulsion and powering after fixing all dimensions, ratios and coefficients.

Where as in semi displacement/planing hull design as per my view the procedure is more implicit. After fixing main dimensions we fix up powering and then come back to hull. You have to keep the beam Froude No: more than one (in SI unit) and Length Froude No: (in FPS unit) more than 3.Then from applicable B/L ratio take B or L as applicable and take your draft (owners requirement or from draft restriction). Cb can vary from 0.29 to 0.42. Depend on Parent ship analysis or methodical series charts for this.

Hydrodynamic parameters depend upon your beam Froude No:, Chine, Dead rise, LCG, Displacement etc. You can't control them during initial design. Main Dimensions is totally dependent on Buoyancy and Archemede’s Principle.Use Savitsky's method for Chine, Dead rise, LCG etc.

In other words, without knowing how much the boat comes out of water, how will you know the waterline during fully lifted condition.

Best Regards
Aravind.G

[gpaladin]

Thank you Will for your answers, and to all other guys that posted.

English is not my first language. Sometimes I do not know the meaning of the word. Like tender .... I came across that word mainly in cook-books until now .