Geometry failings

Hi,

you’d think after taking high school geometry twice I’d be better at it…,

I have worked my way to the bow of the boat. I am looking to raise the gunnels (shear?) significantly. If I follow the existing flair of the hull the increased height would make the bow almost 2’ wider. I suspect that this means I need to change the angle of the bow sides to match up to similar widths at the increased height? Let’s try a picture. It started with extremely low gunnels to reduce hoisting height of commercial catches.

 

If you want to raise the sheer, the smart thing to do is to extend the stem instead of trying to change the rake of the stem. By doing this the lines and forward stations stay correct, all you have to do is extend the frame heads to the new sheer line and add/spile a new sheer plank (which you would have had to do anyway). You can use the existing plank spiling or panel shape.

Spiling (boat building) - Wikipedia https://en.wikipedia.org/wiki/Spiling_(boat_building)#:~:text=Spiling%20is%20a%20technique%20used,boats%20that%20have%20complex%20shapes.

 

I think I follow you and I had planned on extending the stem as that seemed necessary for going up. The issue I am facing now and tried to show above is the sides, as they approach the stem are flared out sideways somewhat dramatically. The “old angle” I show in the picture is the existing side before it makes the turn towards the stem.

 

Can you maybe post some more photos please showing what you have built so far?
And / or maybe post a copy of the plan that you are working from?

 

I haven’t built anything in the bow yet, I am in the demo phase. Here are more pictures but it’s difficult to capture I think.

The only plans I have are in my brain, the one labeled Abby-something…

I went past the boat yard today and I think I have some better examples. Another local builder, Davis Boats, makes a hull similar to where I am going. His boats were based on my hull years and years ago when he started. The very bottom of the hull on the pretty boat is pretty similar to mine.

I don’t know how to explain it well but the top deck is 8’ wide. Starting from the chine(?) if you are going up 15” or 30” the angle changes dramatically. You can see the Rock Harbor boat has very vertical sides and mine doesn’t, my deck was super low to start with.
Here are some pictures:

 

I guess the question I have to ask is - WHY?

seems like a lot of work that will (in all likelihood) cost more than planned and reduce the value of the boat…..

 

Your original assessment is correct, as per the template you made, the angle must change to keep the beam under control.
The purpose of the increased bow flare at the top is to increase buoyancy as the bow submerges, so decreasing the angle will decrease its reserve buoyancy.
So by changing this, you’ll lose some safety factor to gain some cabin height.

 

It isn't really going to be wise to mess with the bow/hull to that degree. (fun pun)

If you want the gunwhale to be higher, have you considered returning it like a production cap?

 

My catamaran has a return as well. This allows you to vary the sheer more.

~a Richard Woods design, but I messed with the sheer to make the aft section of the cockpit bench heights, then added safety railing; the gunwhale goes from level on the left side of the picture and drops down and even makes a drop at the gate..cushions not in pic n stowed

At the bow, there are two ways to go if going up for a cuddy. You can keep the existing sheer and make a narrow walking deck and come up inside or you can return and make it walkable above. If you are making it just higher, but a cockpit; the return would be immediate and the angle steep so as not to lose space..etc



here shows the sheer changing a lot, sorry this comment is for the first picture; I can edit later as tired atm

 

Just as an aside does anyone else think maybe they went a little overboard on the bow deck construction ?
Almost 3/4” of solid glass top and bottom? I’m surprised this thing could float…

 

Not sure that is solid. But it can bump a dock; for sure.

 

That is a lot of chopped fiber, some unknown white filler, and some rotten plywood, but the outer laminate is probably quite strong.

 

Lot of the fisherman I knew with old radons docked by braille.... so that's probably about the right thickness.

It seems like a tremendous amount of work to fairly fundamentally alter hull characteristics.

 

I doubt the designer specified a laminate like that and it might have added enough weight to depress the bow and cause the problem that is now being addressed.

 

The older radon’s relied on their weight and lots of horsepower to smooth out the ride.
Here in the islands, we see more raised bows, most with a broken sheer.
Many of the older Radon’s were more of a ski boat style.