Cutting a changing bevel...

Hello, I am sorry to drop in on this forum with a question but I am in need of some help. I am helping a buddy restore a 74' Thompson Utility. The boat is lapstrake and has a trim piece we are trying to duplicate. I am uploading pics to show what the part looks like. We are struggling to figure out is how to cut a changing bevel. The part against the hull changes with the hull and the outer part is square. Any help would be greatly appreciated.

 

It is relatively straightforward. Make marks on the face about a foot apart or so and number them. Take the bevels with a bevel square, mark them on a strip of plywood and number them accordingly. Get the new piece of wood and make marks on the face at the same spacing as the original. Transfer the bevel to the new piece of wood. It will be a mark at varying distances from the edge. Join all the marks with a batten to make a fair line. Plane to the line.

 

Where I work this is done by changing the tilt of a bandsaw table and using bevel lines as described by Gonzo above. We have a bandsaw where the tilt is set by a wheel and the angle is read off by a stick that lines up with marks transfered (and numbered) to the saw. It's a two man job, the person feeding the wood reads out the numbers as they pass the blade and the person wheeling the table tries to change the bevel smoothly between the numbers.
Here is a photo of it by Kevin O'Farrell (Kevin O'Farrell Photographer https://www.facebook.com/KevinOFarrellPhotographer/photos/a.297722902071.148019.293313657071/10153754480872072/?type=3&theater)

Looking at your photos though, wouldn't it be easier to build up a section by laminating strips clamped to the hull? This would get the correct curve and bevel easily and then you could remove it and finish the outside edges by eye with a power planer?

 

I assumed he doesn't have a ship's saw. Nice photo, it brings back good memories.

 

It can also be done with a modified circular or jig saw. Both utilize a custom foot plate that permits the blade to follow a rolling bevel. If the piece is marked with a line defining both the outside and inside cut, you can rotate the blade, with some practice, quite accurately through the bevel. Of course, ultimately, you'll still have to go back and clean up/fine tune this bevel by hand.

 

Myster
Are you able to clamp your new gunwhale piece to the curve in the bow or is it too stiff? before it is bevelled
If you can easily curve the piece, I would recommend scribing the upper line on the piece then power planning it. let me know and if you can I will go through the process.

 

Kevin's photos are great, there is also one there of marking off the bevels on the saw.
I haven't tried it, but I don't see why this couldn't be done with a hobby type saw using a lever attached to the table for changing the bevel.

The poster asked about cutting a changing bevel but looking at the photos, bending the wood to the hull (laminate or steam) looks more appropriate? It's more efficient for material, then remove and finish the outer edges.

 

If you clamp say 2 pieces of 1 x 3 inch piece of wood to the gunnel, when you get to the front the gunnel strip will pull up above the line of the gunnel/sheer. ( if the sheer is flat) It has to be vertical to permit the curve to take place
Laminating in this way will not work

 

Clamping the piece to the hull, usually isn't practical, as clamping issues crop up to say the least. Even if you can clamp the piece in place, it needs to be well over size as you have at least two bevels to cut, normally three; the backside where it lands on the hull, the upper edge where it rolls to match the deck crown and the bottom edge, where a drip groove or edge is cut and it's bevel is complementary to the upper edge. This assumes the outer face is set at the appropriate angle. Clamping to the hull usually means you end up with an unfair curve too, unless you can clamp a few feet past the end of the stem, to maintain the fair curve of the sheer.

Cutting this particular piece requires some concentration to the upper edge shape as it flows forward. It's easy to get it out of fair while planning it down to match the deck crown. I typically use a marking gauge to pick up the deck crown, while maintaining the proper sheer sweep, then rough cutting it with a saw and fine tuning with a plane. A belt sander (used with some skill) can be helpful too. The underside of the rail should have a drip edge or treatment of some kind, to save the hull from streaks and drips that bodge up paint jobs. Even a slight "undercut" will do in most cases. Lastly the outer face is commonly perpendicular to the LWL, so it places a hefty hunk of rail against whatever it'll bash into. In the very forward sections it's often tucked under a bit, exposing the upper edge to the brunt of dents and damage, but a well sized half oval can help a lot in this regard.

In the end, it's an eyeball sort of thing. For example the forward and aft end of the rail are usually tapered in both height and width. 15% of the height and 5% of the height fore and aft respectively are common taper percentages, across the last 1/5th - 1/6th of the rail length are also common. This helps the eye follow the "flow" of the sheer line. Lastly, if not brightly finished, thickened epoxy can really help the fit. The rail can be lightly tacked in place, getting the look and angles about right, then the bevel gaps can be filled, filed and shaped to satisfy any inconsistencies. One trick I saw many years ago was a dam made from battens attached to the hull, one along the lower edge, where they wanted the rail to live and another attached perpendicular to it, which served as the backside face. This "L" shaped batten was back filled with thickened epoxy and when the battens were removed a plumb and pre-shaped shelf was "cast" to the hull shell, so the actual rub rail could land on it, without having to accurately bevel the back side of it to match the hull. This rail was finished bright, though the epoxy fillet behind it was painted to match the hull. This process allowed them to simply bend a piece of dimensional stock around the hull and only the upper edge needed to be treated with a deck crown bevel.

 

I would do it with thinner laminations, say 1/2 inch, in any case with enough flexibility to curve fairly in to the bow and make the strips wide enough so that if they can't be edge bent then the sheer can be cut out of them just as if you were replacing a plank.

In this case the photos show that the hull would be simple to clamp to but even if there were a deck, laminating in this way isn't difficult using temporary screws with toggles though I agree that if there is a bright finish the holes can be difficult to hide.

 

Wow, great responses! I am going over your suggestions with my buddy. I will let you know what we decide to try. We have managed to scribe the bevel onto a few test pieces but have not found a good way to cut it. We are leaning toward laminating.

 

if you decide to laminate, try a test piece that is extremely wide so you know that when it bends away from the gunnel due to the angle, that it will capture the top gunnel.

If you try a solid piece as Gonzo suggested, ie scribed, the bevel square has to be held perpendicular to the keel line of the boat. Ie not tangent to the gunnel curve. To be more precise, it should be held in line with the bend axis that the plywood is making,which changes as the plywood developed, ie the developable sheet, but you will get enough precision at 90 degrees to the keel

 

Alternatively, use 1/8 MDF or plywood cut in to wide strips and joined end to end to the length of the sheer. Wrap the joined strips around the hull and run a pen along the sheer to mark the sheer curve on the MDF then you will know the width of the laminating strips necessary or if indeed it is not practical to laminate.