Plastic centreboard for a dinghy?

Kelpie

Kelpie

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Looking for a 'fit and forget' centreboard solution for a sailing dinghy which lives on a mooring. Usual material is obviously plywood but this will eventually need attention as part of the board is always immersed. Another option would be a grp board, built up over a base of perhaps plywood, foam, or a plastic. And third option is to just buy a sheet of industrial plastic and cut it to shape.
The last option I find very intruiging- it could be a very easy way to make a new board which would never, ever, need any maintenance. The sorts of plastic available in the right sizes at reasonable cost are: polypropylene, HDPE, PVC, and 'RG1000' (which appears to be a type of HDPE, maybe higher spec). All of them are supposed to be tough, abrasion resistant, etc, but I wonder if a 20mm thick board over 4ft long is going to take my weight when I need to right the boat after a capsize.
In the back of my mind I do wonder that if it were this easy, then everybody would have a plastic board, and we wouldn't have to sand/paint them every winter.
 
I doubt that you could get plastic which would withstand the sideways stresses.

However maybe you know more than me.

You do get plastic fins on windsurfers, but even back in my windsurfing days the good fins were carbon fibre.
 
Yes, strength and stiffness are the worries. I'm pinning my hopes on stuff called 'RG1000' which is a slightly lower-grade form of 'UHWMPE' which is itself the stuff thaat dyneema and spectra are made of. So maybe it would be strong enough but perhaps too flexible. I'm waiting to hear back from the manfucaturer but was wondering if anybody had already tried this out on a boat.
 
Finn

Finn Dingy also uses an aluminum centre board . Might be a bit expensive and need some work but would work. Depends how wide the slot is. If its 25mm + you might as well build a new one out of cedar and glass sheath it after. Will be far stronger and if you go in you can stand on it to right the boat. What aout a SH Laser Board . You maybe able to get one to fit.
 
If it is a pivoting centreboard loads of "cruising" dinghies used galvanised steel plate. Plenty strong enough for righting when capsized and the extra weight will increase stability.

If using plastic, it will be as thick as a wooden one - can it be easily sanded or ground to a suitable aerofoil shape?
 
Well the supplier has got back to me suggesting HDPE and they reckon it is plenty strong enough. If this is the case I'm a bit surprised that not all dinghy foils are made of this stuff as it is zero-maintenance and needs no finishing.
I need to look further into machining the stuff- at £100 for a piece the size of a centreboard this is potentially an expensive gamble...
 
Ply/epoxy will probably end up cheaper but would be a lot more work, and I doubt that I'm good enough at working with epoxy/glass to get a good enough finish. Plus it's not really 'zero maintenance' because at some point water will get inside.
If ply/glass/epoxy is an easier, cheaper route, then that's what I'll go for, of course.
 
There's a data sheet for HDPE here.

Is there enough information here for an engineer to work out of it's strong enough? I guess the maximum stress would be someone standing on the end of the board to right the boat. So 100kg on a 1m centreboard 20mm thick. What's the front to back dimension (cord?) ? How much would it bend?

Lots of engineers on here. I wouldn't have a clue!
 
I assume this centreboard is for your Wayfarer,and not for racing in class. Sheathing a slightly undersize marine ply board should give you plenty of strength and minimal maintenance,after all it is going to be in its casing most of the time on the mooring; also you can use standard hard anti fouling paint that can be abraded for the matt finish preferred for racing performance. The part that I would be most concerned with would be the leading edge of the board.
I seem to remember recommending a brass strip be fitted to this leading edge in previous posts.
An aluminium board would need only to be 5/8th" thick for the leverage to raise a capsized boat but you then have to bush the pivot bolt in the casing to take up the 3/8" difference in thickness.
I guess you remove the rudder anyway.

ianat182
 
Yes, it's for the wayfarer and, no, I don't mind taking it outside of class rules. Low/zero maintenance is the real issue here. I recently got a bigger boat and the dinghy is now an occasional-use plaything for those days when I just want to spin around the harbour, most likely single-handed.
Epoxy/glass-sheathed-ply is in some ways the obvious answer for this and I'm sure it would do a great job. However to buy one would be over £300 and I'm not really sure I have the skills to make one from scratch. Maybe it's not as hard as I think? But surely it would be harder than taking a slab of HDPE and running a bandsaw around it...
 
It can't be hard, I have done a few times over the years.

To make a 3/4" thick dagger board for my cat dinghy, I cut two pieces of 3/8" to shape & glued them together with minimum horizontal grain for strength, then aerofoiled them roughly with a coarse sander, finishing off with a few progressively finer grades. Then a dozen coats of poxy resin, sanding each one down when dry. Or you could use a layer of tissue cloth on the first couple of coats to create a more resilient coating.

It takes time, but it ain't hard & is very satisfying when completed.
 
Ruffles said:
There's a data sheet for HDPE here.

Is there enough information here for an engineer to work out of it's strong enough? I guess the maximum stress would be someone standing on the end of the board to right the boat. So 100kg on a 1m centreboard 20mm thick. What's the front to back dimension (cord?) ? How much would it bend?

Lots of engineers on here. I wouldn't have a clue!
Click to expand...

Using those properties for HDPE and assuming 25mm thick with 400mm chord with 100kg at 1m from the hull the material will be just at its yield stress (i.e. the point where it won't straighten out after you get off it) and the deflection will be 530mm! It is much too soft to use as a centreboard!
 
DJE said:
Using those properties for HDPE and assuming 25mm thick with 400mm chord with 100kg at 1m from the hull the material will be just at its yield stress (i.e. the point where it won't straighten out after you get off it) and the deflection will be 530mm! It is much too soft to use as a centreboard!
Click to expand...

Please, PLEASE tell me how you worked that out. The Wikipedia entry defeated my maths; it goes back to first principles. I assume there must be a simplified formula?
 
Ruffles said:
Please, PLEASE tell me how you worked that out. The Wikipedia entry defeated my maths; it goes back to first principles. I assume there must be a simplified formula?
Click to expand...
OK:
The bending moment "M" is 100kg or approx. 1000 Newtons at 1000mm so 1,000,000 Nmm

The section modulus "z" is b*d*d/6 = 400*25*25/6 = 41670 mm3.
Hence the bending stress is M/z = 1,000,000/41670 = 24 N/mm2 = 24MPa. Yield stress is 25 MPa so this is already looking dodgy!

Deflection of a cantilever with a point load at the tip is P*L*L*L/(3*E*I)

P = 1000 Newtons
L = 1000 mm
E = 1200 MPa (Modulus of Elasticity in Flexure)
I = 2nd Moment of area = b*d^3/12 = 400*25*25*25/12 = 520,800 mm4

So deflection = 1000*1000^3/(3*1200*520800) = 533mm.

Easy (when you've been doing it for 25 years).

If you can manage to double the thickness at the root (you can tpaer it towards the tip) to 50mm then the stress reduces by a factor of 4 to 6MPa and the deflection by a factor of 8 to 67mm and all is well.
 
Last edited:
DJE said:
OK:
The bending moment "M" is 100kg or approx. 1000 Newtons at 1000mm so 1,000,000 Nmm

The section modulus "z" is b*d*d/6 = 400*25*25/6 = 41670 mm3.
Hence the bending stress is M/z = 1,000,000/41670 = 24 N/mm2 = 24MPa. Yield stress is 25 MPa so this is already looking dodgy!

Deflection of a cantilever with a point load at the tip is P*L*L*L/(3*E*I)

P = 1000 Newtons
L = 1000 mm
E = 1200 MPa (Modulus of Elasticity in Flexure)
I = 2nd Moment of area = b*d^3/12 = 400*25*25*25/12 = 520,800 mm4

So deflection = 1000*1000^3/(3*1200*520800) = 533mm.

Easy (when you've been doing it for 25 years).

If you can manage to double the thickness at the root (you can tpaer it towards the tip) to 50mm then the stress reduces by a factor of 4 to 6MPa and the deflection by a factor of 8 to 67mm and all is well.
Click to expand...

So what do yer reckon then -Plywood? ......Hat coat...........door!

ianat182
 
Thanks for all the input! So HDPE would be 'strong' enough but too floppy.
I will try to find numbers for 'RG1000' but it would have to be an incredible improvement over HDPE to be possible.
To be honest I doubt that many materials 20mm thick could withstand 100kg on the tip of a 4ft board... I always thought that when righting a capsize you stand on the board much closer to the hull and lean your weight out... plus I'm nowhere near 100kg anyway...

So, if not plastic, do I go down the road of simple plywood, or ply sheathed in glass, or something else (PU foam anyone?) sheated in glass? Hmmm...
 
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