Removing the IOR bump

[The Dark Knight 348]

Are there many old IOR boats that have had the infamous IOR bump removed and how was it done?

I was just looking on the "Police car" FB page https://www.facebook.com/police.car.sailing/ and someone had posted photos of work they had done to remove the bump from a Davidson 39

It's not structural work, so probably not so "expensive" to do. I wonder if the change has tamed the boat's down wind manners.    

 

 

 

[SloopJonB 4,854]

It would have to help some but that bustle and pinch was only a contributor to the "Death Roll".

The real problem was the boats were full displacement hulls and the racers would run them up to the hull speed wall and keep piling on sail - tall, skinny spinnakers in 25 or 30 knots of wind are going to broach any boat that can't break out of the displacement wave train. It just digs a bigger & bigger hole and rolls around in it.

People just didn't do that sort of shit on Westsails or Taiwan Teakies or they'd roll the decks under too.

[Grande Mastere Dreade 934]

2 hours ago, The Dark Knight said:

It's not structural work, so probably not so "expensive" to do. I wonder if the change has tamed the boat's down wind manners.    

 

 

 

 

 

yeah, you're right,  that doesn't look expensive...    to some billionaire

[The Dark Knight 348]

1 minute ago, Grande Mastere Dreade said:

 

yeah, you're right,  that doesn't look expensive...    to some billionaire

It will be cheaper to put on a false bottom to smooth out the bump than it would to make a structural change to achieve the same goal. 

[MiddayGun 49]

Quite a few old half tonner & quarter tonners have been 'IRC Optimised' here in the UK. Which does sometimes involve removing the bump. 
Some good info & pics here: http://www.corby-yachts.com/refits.html

I've no idea how much it makes a difference because its usually combined with new keel, new rudder & new rig. 
I can't imagine it being a cheap job though.  

[The Dark Knight 348]

Interesting looking at some of the mods done by Corby.

They take the same approach as in the photos I posted earlier. Add some external ribs and fill with hard foam, then put a skin over.

[Zonker 1,044]

Wonder why all the ribs? Probably just to get the shape right. But I'd treat the existing hull as the structural/watertight hull, and just add a foam layer + very light glass exterior skin.

If you wanted to it right you would laser scan the hull (< $2k), bring it into a CAD program, add the foam layer in 3D, and then CNC cut foam blocks that would perfectly create the shape you want. No upside down glassing of ribs and fairing. It would all just be very fair from the start.

Police car ribs look like plywood (shudder)

[El Boracho 571]

Would add considerable flotation, no? What is the result? Insignificant?

[12 metre 126]

The wood ribs are used for fairing in the Davidson crease on a Davidson 39 - not Police Car, which was an alloy Dubois.  The Davidson mod photos got mixed in with the Police Car posting.  The Davidson crease is fairly evident in the photo below of Waverider

Not really removing the bump (crease) on the Davo per se.  I suspect more extending/fairing it to increase the sailing length.

[Bruno 87]

Apples and oranges. Those photos show an egg crated bump removal, not added surface area, thereby shoaling the hull. Most bumps, as in the Dubois example cited, are molded in, cutting out part of the hull and adding a new piece ain't easy. Neither is adding foam and glassing upside down and achieving a fair result. Nb the keel stub showing after the ribs are removed. Some IOR boats had U shaped bows, adding a bustle to such a boat might help. Or not, here's an intelligent article about shape.

http://www.oceannavigator.com/January-February-2003/Shaping-an-offshore-hull/

[Zonker 1,044]

Then there were the IOR local measurement point bumps that looked like a grapefruit was stuck to the hull and glassed over. At least they would be easy to fix. 

[CaptainAhab 47]

Are you going remove lumps while you are doing the bumps? Our 1982 32’ IOR had all of the maxed out measuring points and duck tail. It was one of Brit Chance’s last IOR efforts. The mathematically optimised hull was a sight to behold out of the water. She was susceptible to broaching above 25 knots going downwind

[CaptainAhab 47]

Are you going to remove her lumps while you are doing the bumps? Will this be a full liposuction reduction?

Our 1982 32’ IOR had all of the maxed out measuring points and duck tail. It was one of Brit Chance’s last IOR efforts. The mathematically optimised hull was a sight to behold out of the water. She was susceptible to broaching above 25 knots going downwind.

[The Dark Knight 348]

18 hours ago, Zonker said:

Police car ribs look like plywood (shudder)

Not Police Car. It was photos from someone else's bump job mod that they shared on the PC page.

 

Police Car is getting the bottom paint finalised, bump and all, ready to get wet.

 

The owner is keen to sell but no one wants to buy a project on the hard, so he is trying to get it ready for the water to make it a more attractive proposition for buyers. 

 

 

Unfortunately it seem that the sellers budget will not cover the full on PC paint job or vinyl wrap

 

 

https://www.facebook.com/police.car.sailing/

 

[charisma94 130]

I wonder what the deck is like? It had holes in it 20 years ago! Was only 3mm plate to begin with...

[stumblingthunder 497]

Regardless (N/M 1 ton) version, had the stern IOR whoopties evened out and the bow reshaped in the early 90s.   OH Rodgers engineered that project.   I was part of the sanding/fart-rock crew that faired the hull.

As far as I know, the boat is still up in the Great Lakes.

- Stumbling

[Brain-O 0]

Fix these bumps cheap

[Carrera 23]

Back to the original question, adding an external fairing to fill in the IOR bustle (leaving the structural hull intact underneath), while laborious, is not that expensive. We did it to a Farr 37 about 15-20 years ago. We actually connected with the Farr design office to see what shape they recommended and they gave us some good drawings and specs to work with. From there it was just grinding the section of hull down to glass, building up the foam core as recommended by Farr, shaping the foam, covering with multiple layers of glass and fairing compound, a lot of sanding, barrier coat, and finally paint.

It took the better part of every weekend for 3 months but we got it done. I'd say it definitely added some stability and speed (from the increased LWL) off the wind.

[George Hackett 27]

So how is the rudder dealt with?  A third rudder bearing and longer rudder shaft?