Making Keel Fin Box ?

I would like to make a keel fin box to be same shape as the keel fin profile, the fin is straight 85 mm wide with gelcoat finish.

How would you recommend I mould the fin box to shape of the fin head:

1: Wax the fin and wrap fibreglass around it ?
2. Tape the fin with plastic packing and fibreglass ?
3. Other ideas ?

I’m worried that once I make fibreglass box around the fin head, that will not be able to separate the box from the fin.

Cheers Alan

Hi Alan,

Although I’ve not made fin boxes, when I made the ballast plug for ‘Enterprise’, the fin area of the hull was covered in packing tape and was then glassed with 2 layers of 165gm2 glass cloth. Once the resin had cured the moulding was then removed from the plug, packing tape removed and then replaced with fresh tape. The light moulding was then replaced and then further layers of csm was added to bulk up the moulding giving additional strength. Once finally cured the completed moulding easily removed from the plug. It’s worth noting that although the fin width is considerable thicker than the fins you’re contemplating building fin boxes for, the lower levels were almost parallel sided. As an added insurance it might be worth applying pva to the tape as well.

Hope that eases any concerns you may have.

Good to see you had a productive time in Italy & Nice last week - you must surely have a sore backside after all that driving!!



Hi Row,

Good thinking using light moulding to start with … will start this way, thanks a million !

Cheers Alan

P.S 4 weeks sitting aeroplane seats followed by 2,630 kms driving leaves few flat spots in the butt for sure and was great week, especailly the hands on boat design lessons from Claudio going into the early hours of the morning while in Nice :cool:

I have always done it by waxing the fin, wrapping with waxed paper, then wrap with glass. The waxed paper gives a slight gap which makes it all go smoother.

alan. my firs keel box I made with the paking tape method I was not abel to get the box off with out destroying it. so my next effor I laid a some terry cloth shop rags. flded ofer a few times.

then I laid a layer of saran wrap. then I laid up a few layer of glass with epoxy, then another layer of saran wrap, then the keel, another layer of saran wrap, more layers of glass/epoxy, another layer of saran wrap, more towels, and then a sandbag.

sand bag ackl like a vaccum bag to maintain pressure.

this makes a two peice fin box.

post #55

I used a couple of thick coats of PVA, then laid up a couple layers each side with 4oz glass. after removing the layup, added another layer to bond it with the already molded, mast box.

Hi Alan,
the Fin Box construction is always a small problem! If it is of the trapezoidal shape and too precise it may cause problem for the Fin extraction to.
The rectangular shape is the most easy to make is spite of the extra weight.
My experience with “wrapping” was rather messy all the times and non nice to see after.
I told my self why to make the life complicated if ,for few grams, can be simple ?
I tried various methods including half molding, at the end nothing was better then a previously laminated plywood Fin Box.
I have also tried to prepare a carbon fiber sheet of 1mm thickness and supported by 2mm plywood.
The best results are obtained with glass or carbon previously laminated plywood. After epoxy bonding a wrapped glass layer to ensure the joints. The walls are much less flexible giving the impression of solidity !
Another consideration is the amount of weight budget allocation for that, I’m sure that 10g more spent for a sturdy box will not ruin the stability of the boat.
Here below a series of various Fin Box pictures.
I will stick to the wooden one !


Hi All,

Thanks for all the feedback, has been helpful to think about best option to consider, strength without flex and being lightweight is always a tough recipe.

Last night I made a plastic sleeve that fits snugly and will hopefully allow for easy fin removal after curing.

I used unidirection heat shrink plastic film but after making the sleeve decided not to apply heat to the film as did not want to add complication of trying to remove it from the mould when the plastic was shrank to the fin.

Over the weekend I will put it in Gutenberg foam press with 2 layers of 245 gsm carbon using peel-ply outer wrap and let you know it comes out.

Cheers Alan

Sorry to pollute your fine building standards with my expedience, but this was designed for a kids’ Footy project so they can easily make a yacht themselves. Have used it successfully in my RG65 shown.
Simply, a piece of corrugated sign board - Corflute- Corroplast… 3mm for a footy, 4mm for my RG65 helicopter fin, 5mm would be very sturdy and still light-ish.
Slit the flutes to accommodate the fin. The little I beams thus formed grip the fin tightly and give adequate lateral support. Glued in with cheap contact cement, including inside fin slot where it meets the hull.
The forward slots give infinite graduations of mast positions. I use swing rigs for simplicity, especially for the kids so wire mast drops straight into slots. Once you have determined a few useful mast locations the others can be covered with something sticky or more permanent. and a bush placed in the bottom of the slot to protect the hull.
Takes away all the drama of where to put the mast for the non designers and allows for many types of rigs.

Gidday Rusty,

Great idea for footy projects, wish it could be so simple for AC 120 with 3 kilo bulb & 79 dm2 rig :slight_smile:

How are the footy projects going with Kiwi kids ? can imagine there be a lot of interest.

Cheers Alan

I prefer the rectangle box(like Claudios) because it leaves you with a little wiggle room. If you use the form fit method and get it slightly off when you glue it in the boat there is no way of fixing it. The rectangle has room to rotate the fin slightly so with a little sandpaper and shimms you can get better fore and aft allignment.

Hi Don,

I like the rectangular fin box too ! I’ve used it on all my hulls to-date, for all the reasons you’ve stated, but you need to have a collar at top and bottom of the fin head.

On this next boat I would like to be able to adjust the fin length, hence cannot use collar this time.

Cheers Alan

Hi Don,
How is the life ?

I take the opportunity of your message to add another point about little unavoidable play inside the box where the fin may “turn” by about 2.2° in one sense or in the other when the boat start heeling.
All depend on where the bulb CG is positioned with respect to the fin attachment. It can produce a fin positive angle of attack or a negative one.
It is sufficient to insert a small piece of soft rubber at the fin tail to reestablish the central position of the Fin inside the box.
see this sketch :


Could not wait until the weekend so laid up the fin box last evening & will leave it to cure for 48 hrs, ambient temp has dropped into low 20’s

Interesting point on low density foam, firstly I tested the compression of the foam using only the fin & was surprised how it makes a very nice female mould impression of the fin into the foam.

Thought, I later try and make a reproduction of one of my fins by coating foam sheet with epoxy and laying fibrglass layer directly onto the foam surface, then layer of plastic film over, then lay fin on plastic and clamp the platens together under pressure so the foam/fibreglass forms to the fin shape, let it cure then you have two perfect halfs cured into the foam.

Then later, peel the foam off the fibreglass laminate and build up the female mould with several layers until you have complete ready half moulds … a simple holistic approach to making high quality fins !

Cheers Alan


I like the LDF. I used my terry cloth towels and sandbag to mimic your press…great Idea…

Has anyone experiment with non woven fabrics…I have some 2 and 4 ounce nonwoven polypropylene…was looking at it the other day…and got the grey matter kicking…

Ok it’s only 24 hrs after laminating but I wanted see if I would have any problem taking the fin out of the lamination. Parted the platen & took off the peel-ply & trimmed off excess carbon.

At first could not slide the sleeve off the fin by hand force and gave fin a few taps with hammer to break that plastic film grip on the fin and walaa !!! and no damage to the fin :cool:

Made it long enough to have two boxes and they slide freely over the fins without any wiggle, weight 10 grams.

Thanks everyone for your ideas and guidance.

Cheers Alan

Hi Alan
so if I have understood well, with the press it is possible also on a relative “large surface” to squeeze it !

Hi Claudio, the platen (board) used was 13 x 29 cm and the low density foam is the stuff used for heating insulation of homes here in Germany, it compresses very easily with finger pressure.

The fin is 7 mm at maximum chord so not too much pressure was required to have form the foam to the shape of the fin, the larger the surface area the more pressure will obviously be needed … a 500 mm long fin would be no problem.

Cheers Alan

Claudio, your comment and schematic on post #13 had my imgination going again, do you think it maybe possible to influence a “positive” angle of attack to reduce lee-way, by using something like rubber blocks inside the fin box ? example: having fixed but pivoting point on the leading edge of the fin with softer rubber block at the tail edge, inside the keel box.

Cheers Alan