First boat design

Hello all,

I am new to this forum. Recently I've been playing around with FreeShip, and reading plenty of books on boat design and boat building.

I have designed my first boat (hull) and thought I would post it to get some feedback.

My plan was to design a small dinghy in the style of the old norwegian racing dinghies, but that would plane.

This boat is 3m (9.8') loa and 1.2m (4') max beam. Depending on the material, I would guess it could be built to weigh less than 50kg.

Given a suitable weight and sail area (and other appendages), will this boat plane?

Any other comments would be great!

 

Wouldn't it be a bit heavy in the bow?

Kelly Cook

 

Hi Barney, The shape looks like it would plane in theory BUT!! Why so much rocker in the bow? The waterline length will be in the vicinity of 2.3 meters; very short for an adult sailor. However if the sailor weighs less than 40 kg then it should plane fine in 15 knots of wind. For an adult sailor make it larger and reduce the bow rocker. Why dont you look at the fireball dinghy to get an idea of proportions for a similar shape hull. Check this web site:
http://www.fireball-international.com/php/gallery.php?gallery_id=5

 

I agree, you are using up too much waterline length (speed) trying to get the nose up in the air. Your weight aft will shift the CG aft and lift the nose until it gets up on plane anyways. Reduce the rocker and width forward (easy on the width) until the bottom of the front transom is about 15-18cm above the waterline WITHOUT anyone in the boat. When you get in it will trim aft and raise the bow. Rake the forward transom more and it will deflect any water hitting the transom downwards and reduce the chance of being significantly slowed when it hits a wave (within reason). I can't see the scale properly but it looks like the bottom of the front transom is almost .5 m above the waterline! Lean down your upper chine and shear a bit too, you won't need so much buoyancy forward, especially if the boat has a deck (which is should).

Steve
PS: Here is a rowing hull I did up a couple of years ago...Straighten the run aft and keep the width from max beam aft from reducing too much and this is about the shape I am talking about.

 

Something vaguely along these lines...

Steve
"Pictures are worth a 1000 words"

 

Lewisboats,
All you need to do now is put a rig on it, with a red ball on the sail, call it a Fireball, and you'll have a winner
Steve (yeah, another Steve...)

 

Given the same criteria... like minded folks sometimes come up with the same or similar thing Notice: I did modify my OWN design to come up with the hull, and it is on the order of 16 ft long. Fireball looks to be 12-14 ft or so (4.928m) (1.359m Beam)(disp 79.4 kg ). Not sure on the # of chines on it either. Had to look up the #s to respond to this. Independant thought CAN come to the same conclusion on occasion.

Love the name BTW...Surly must be Noble!

Steve L.

 

OK..., so 4.928 m IS 16 ft...I still came up with the damn hull by myself!

Steve L.
A pox on the Metric System!!!

 

Thanks for all the suggestions! The bow rocker is purely for styling, but I guess it is a little severe. I will have a close look at the fireball for some ideas, then have another go.

What is the relationship between length and speed for a planing hull? I guessed that is was totally different than with a displacement hull, and more length would equal more wetted surface area, in turn creating more drag? That is why I wasn't too concerned with having a fairly short waterline length for the boat.

Thanks again for the suggestions!

 

OK, I've reduced the bow rocker which has lengthened the waterline and brought the CG back a little bit. I also decreased the width forward just slightly and raked the front transom a little bit more. I also increased the width of the rear transom a little more.

As predicted, the result isn't too dissimilar to a fireball, but it is just over half the length, and has a few more chines.

Here she is in here second incarnation. Further comments?

 

Barney, the second version is a significant improvement BUT: why only 3 meters long? You havent said what the anticipated all up sailing weight including crew will likely be. This will have a huge bearing on the desirable waterline length. Please let us know this "minor" detail

 

Hi Frosh, Thanks again for your feedback. I suppose the decision to make the boat 3 metres was a combination of reasons - and perhaps it isn't possible to have everything how I want it.

I was first inspired to make a little pram when I saw a salty old sea dog out on the harbour in his nutshell pram having an absolute ball - I was inspired by the simplicity of such a small simple boat. He didn't need a 50 foot sloop or power cruiser to enjoy himself, just a vessel to get out and enjoy the wind and the spray.

Hence the length - small, simple, light, easy to store and transport.

I then thought to myself that I already have a small sailing boat (12 ft), so maybe the pram would be more fun if it could plane, and maybe (just maybe) if the new boat was light enough, I might even be able to use the same rig.

So, the expected total weight of the vessel with one crew member would be hopefully around about 130kg - that is 90kg for me, and 40kg for the boat and rigging.

I have based by estimate of the weight of the boat on nothing more than my experience with my current boat. The current boat weighs approximately 65kg, and is fairly heavily built from 6mm and 9mm exterior plywood, and plenty of epoxy and fibreglass. Its designed weight is 45kg with okuome.

I would plan to build this dinghy from 4mm gaboon/okoume ply, which is considerably lighter than marine/exterior ply, to attain the light weight.

My existing rig is approximately 5 square metres and is a gaff rig, although I might be kidding myself to think that this will be big or efficient enough to get this boat to plane - but we will see.

So in summary, my wishlist is this:

classic looking pram dinghy that planes
small enough to handle and store easily
total weight of 40kg
able to plane using 5sq metre gaff rig

So, this should fill in a few of the gaps.

 

Hi Barney, seems you have your reasons to keep the overall length to 3 metres. That is OK and this hull will float you fine, even sail reasonably. Planing is a whole different ball game. Even disregarding your existing rig and putting on an efficient larger rig would still leave me with doubts it would ever plane at an all up weight of 130kg. With the 5 metre gaff rig the chances are very slim. I might be wrong however, but I think not, unless you want to sail in moderate gales. On another point, why the extra building complexity of triple chines? If you want to avoid the boxy look of a simple hard chine shape, go for double chines.

 

Hi Frosh,

Thanks again for your input - I really appreciate it.

I guess my question is why won't it, and how to make it plane? Does it need more length and less weight? I read somewhere about a SA/displacement ratio of 37 or more to make something plane, but I got lost somewhere in the middle of the equation

On the triple chines, I am going for a pseudo clinker look, so would be prepared to deal with the extra building complexity for styling reasons.

 

Barriers to planing

Barney, my reasoning for the inability to plane is like this. For every Kg of weight the boat will displace approx. the same number of litres of water. Your computer program can probably give you the waterline position of the hull with crew aboard. Drag forces on conventional dinghys just at the point before planing occurs are very large. i.e. the hull has to get over a large drag hump just before planing, then drag forces significantly reduce due to hydrodynamic lift, therefore less hull immersion. This leads to a reduction in skin friction in the water due to less wetted surface, also more importantly the wave making resistance of a fairly deeply immersed hull is greatly reduced.
From this you will understand that you need to have a drag hump just prior to planing that is not too great and enough sail power to provide the driving force to get over the hump. I believe that your design will fail on both counts. You cant just keep on increasing sail size on a very short boat as it will be unbalanced and uncontrollable. The shortness of waterline length also leads to a loaded hull that is deeply immersed at rest.
The changes you need to make to ensure planing in medium winds are: more waterline length to reduce the immersed depth, probably 3 to 3.5 metres, (waterline) and more sail area which can be controlled on the longer hull. I suggest 7 to 8 sq. metres, but depends on your local prevailing wind conditions. There are other subtleties also in hull shape but for the moment the above factors are the main things you need to consider. Regards, and good luck