4.50m Sailing Yacht Concept

Hi,

I wonder if anyone could give any opinions on this concept for a low-budget small yacht, especially with regard to seaworthiness and its handling in rougher conditions.

There is just enough room for two 'bunks' side by side in the cabin and it gives just under 1.1m of height- enough for sitting headroom. It has a self-draining cockpit and the cabin roof is designed with water shedding in mind when in waves. The keel could also be made lifting to allow access to shallower waters or to make trailing/ launching easier. There would also be a lot of space beneath the cockpit floor for storage.

Specifics:
LOA: 4.50m
Max. beam: 1.50m
Draft: 1.30m
Keel: approx. 120kg
Displacement: 500kg
Upwind sail area: 12sq m
Construction: plywood/ fibreglass more cheaply or GRP with foam core

Any comments would be welcome.

 

Not good shapes (the chine) to be built in plywood.

 

Why no flare?
Is bulb keel really a smart option for low budget boat?
The rounded chines mentioned by Tansl - why? And how do you see them built?
How many hours have you sailed in your life?
How is one supposed to go to the bow in bad weather?

 

I own a Winklebrig, which is a 4.5m gaffer, displacement monohull, similar to a cornish shrimper, with 2 quarterberths, and a v berth, displacing 500kg bare, twin bilge keels, self draining cockpit. Its a fine little boat in relatively sheltered conditions, cute as a button, but its length limits the top speed to a scrape over 5 knots, and it is hard to tack in stronger conditions, as there just isn't enough momentum to carry it through the tack, worse if there's waves of any size. The point I'm making is that the length alone alone limits seaworthiness, unless you go to a different basic hull form. The water shedding cabin and self draining cockpit really aren't as significant as the overall length and hull form. My Solway dory trimaran is marginally longer, bermudan ketch rig, less sail area, (actually half the sail area) reef by rolling around the unstayed mast. it is an open boat, and only weighs 100kg. However, the skinny hull means it will do 11 knots, and is much easier to tack; the ketch rig helps with this. I consider the Solway Dory to be by far the more seaworthy boat.

 

Hi kerosene,

I have done a very large amount of sailing (admittedly in dinghy racing so I lack experience in keelboats) and hope to study Naval Architecture- I am 16. I don't have much experience in design which is why I posted on here to get feedback. I will explain the reasoning behind the design as best I can.

There is no flare because I intended to create the maximum waterline beam (for the overall beam) to give a high initial stability. I thought this would be beneficial for the boat's motion in swells and combine well with the relatively sharp corners to provide good damping to any roll.

The bulb keel is there to allow for improved performance over just a ballasted boat and superior stability. When I stipulated a low budget, this is really in the form of the boat's small size whilst I would still like the boat to handle and perform well.

The rounded chines I incorporated thinking that they would reduce resistance compared to a corner and they were also partly inspired by a boat designed by Sven Yrvind which uses the same sort of chine line. I don't know that much about construction but I see them being either strip planked if the boat is built in plywood or just foam and GRP for the whole boat.

I don't see any reason to go to the bow in bad weather (in any case it may be very dangerous on a boat this size). Making a flat deck to walk on would also decrease cabin size in an already cramped boat. I think that furling headsails would get around this problem.

Thanks for the interest.

 

What do you mean with "strip planked if the boat is built in plywood ", plywood strips?
Think that GRP construction requires a mold that could be more expensive than the whole boat, unless you want to make a serial construction of many hulls.

 

Hi JAL,
you have more sailing experience than I do so take my comments with a grain of salt.
-I was wondering the access as I imagined slightly more cruising purpose with the sit in space and sleeping accommodation. in such use you need to have access in case something goes wrong. If its a race boat then I guess its a different ball game. the cockpit at 1.5m looks small to me.

-the flat bottoms is not very ideal for resistance either. It does help with planing but with the amount of rocker it doesn't look like a good planing boat anyway.

-I don't think extreme initial stability is desirable. It means that the boat will act extremely stiff and when it starts to go over it does it without much warning.

I am an amateur hobbyist designer but as such I would recommend following books:

- Gerr, Nature of boats
- Uffa Fox, Sailing, Seamanship and Yacht Construction
- gougeon brothers on boat construction

after those (all easy and enjoyable to read) read more Uffa Fox when you can (they are fun books) and save money for the principles of yacht design by Larsson and Eliasson

Good luck.

 

TANSL,

The method of construction I refer to is this sort of thing: http://www.amateurboatbuilding.com/articles/howto/foam_sandwich/planking.html (this uses foam planks but a wood could be used also).

Kerosene,

Good points. For access, I would have said the roof is shallow enough to stand on with a gripped surface if you really had to but obviously not ideal for wandering around on- it depends on the intended use. I had not thought of your point on initial stability and this may be an issue, thanks.

 

Yes, a wood could be used too but, imo, not strips in plywood.
By the way, 500 kg is the weight, empty ship or with crew?.
I'm sure you already know that the weight of the hull in GRP or in strip planking will be totally different. You should not go ahead with your project without setting this feature.

 

500kg is quoted as a lightly loaded weight ie. one crew and supplies. The boat could take more however. The displacement per distance of ?'sinkage'? is very great because of the very flat hull form so adding a little extra weight does not make too much difference.

As to whether it would be GRP or plywood, although I don't think I will get to building the boat I think plywood would be more realistic; as you pointed out, GRP requires moulds. I will try to do some more accurate weight calculations with the structural plan I have in mind.
This leads me on to another question..
Obviously, I have little to no knowledge of structural design. What I have done so far is based on common sense in terms of analysing to find high stress areas and looking at other boats and what they use. Has anyone got any advice on improving on this?

 

You just need to design the structure and calculate scantlings. ISO 12215 is a very good reference for boats scantlings.
I made a calculation of your boat's hydrostatics (see attached file) and an additional weigth of 42 kg results in an increase in draft of 1cm.

 

TANSL,

Wow! Thanks very much for the hydrostatics! Very useful for me.. I had tried Delftship but got fed up trying to draw it so did the calculations by hand. These confirm my estimations very well. How did you manage it from a lines plan?

I will take a look at ISO 12215

JAL

 

From the images in your post # 1 I've drawn a lines plane, very rudimentary in AutoCAD. With an application developed by myself, I can calculate Hydrostatic, cross curves of stability, and DN, GZ curves for loading conditions, etc. If you need something else, just tell me. I can also calculate the mast and rigging and, of course, the scantlings of the structure.

 

That would be great if you don't mind. Could you please do a GZ curve? I would be interested to see the effect of the initial stability etc. I will start on rigging once I have finished developing the hull I think. Not sure what a DN curve is?
Again, thanks very much for your help.

 

I enclose the results and the stability curves for the load condition "Full Load with 2 people". The lightship weight and center of gravity are invented, since it is not yet known, but I think the results may be useful for a first impression.

DN is the dynamic stability (integral of GZ values for each angle of heel)