Bow modification/redesign on sailboat

[Burnsy]

So I've got this old DB1 sailboat on my hands:
http://www.geocities.com/teamheatwave2003/heatwave2.jpg

I'm thinking about putting together a 'false bow' modification, made out of foam wrapped in glass or carbon or whatever... to drop the stem down straight and pick up some dynamic waterline within the current hull size.

Some have said why not add a sugar scoop to the back end instead - I don't want to change the LOA. Also, the main goals would be to get more dynamic waterline for top end speed, and get a much finer entry for our steep waves.

The current knuckle is under static waterline and a couple feet back from the bow. One thing I'm considering is creating a new knuckle below the bow just a couple inches above the static waterline (like a scow bow), with a shallow curved line extending a sharp new forefoot down to where the old knuckle is. This wouldn't change static waterline or wetted surface much, but would become easily immersed as soon as the boat picked up toward hull speeds - kinda like how they did it on Nicorette. Not too sure on this one though, could just take it all the way down and put it underwater again.

The current specs are 33' LOA, 26.5' LWL, and 11' beam. The bow has a very flat forefoot that comes right up to the knuckle, with somewhat pronounced 'cheekbones' running down each side.

One final mod could be adding a set of anti-diggers - lips or bullhorns or whatever you want to call them, as a precaution to keep the bow from burying too badly. Something like this: http://www.sailinganarchy.com/fringe/2004/op_bowlip.htm

For the experts out there - any tips, caveats, suggestions, what the hell are you thinking... this could end up being a next winter project.

 

[Milan]

Take a look at the Wobegon Daze modification:

http://www.sponbergyachtdesign.com/Repairs.htm

Milan

 

[Tactic]

I am planning the same type of mods to a 29 year old 20 ft trailer yacht.
Pluming up the bow and adding a scoop at the transom.Hoping to gain speed at higher froude numbers upwind.
Over the years the average size of boats in our fleet has grown, I need extra waterline length to keep up.
Thanks for the links,glad to see I am not the only one looking at this type of thing.
Cheers
Tactic

 

[Eric Sponberg]

Burnsy, Milan, and Tactic,

Milan, thanks for the plug to Wobegon Daze.

The reason we added the false bow onto Wobegon Daze was to change the sailing trim--she sailed bow-down and was hard to steer. By adding a new bow shape right onto the old one, as I say in the website, 440 lbs of structure, we gained 928 lbs of buoyancy. The trim changed dramatically by a few inches--from 2" bow down to 3/4" stern down. The whole boat sailed a lot better, pointed higher, and was a touch faster. We had a very good experience, and the boat is still sailing very well.

I first learned of this idea from Adlard Cole's book "Heavy Weather Sailing." He relates a story of adding a false bow to beneficial effect onto a CCA type of hull for a race from Bermuda to England.

For Wobegon Daze's construction, We ground off the gelcoat in the repair area and made transverse bulkheads with 1" thick Airex foam core and two layers of fiberglass either side and spaced on 18" centers. I think we used 1708 biax and 1808 0/90, one layer of each on each side. These were tabbed onto the hull with a couple of layers of the 1708 biax. Then we added wood ribands longitudinally which were about 1" x 2" pine. These acted as butt-blocks for the plywood panels that went on later. Everything was done with epoxy resin.

We installed some bilge pick-ups into what was going to be the new void space between the two hulls to take out any water should any get in at a later time. We also cut access openings in the original hull so that you could inspect the inside of the new hull surface as well as allow air circulation through the void spaces. We installed plastic covers in these access openings.

Then, beginning at the bottom on centerline and working up, we laid up 1/8" (3 mm) plywood panels longitudinally from riband to riband. As each one went on, we were able to coat the inside of the panels with epoxy to seal them off. We created fillets at all the joints as we went along. We also filled the void space aft where it was very narrow between the two hulls with high-density foam so that if you picked the boat up with a travelift in that area, it would not crush the new laminate. We gradually worked up to the sheer. After all the wood was on, we laid up a few layers of 34 oz. triax fiberglass over the whole, 3 layers on the underwater area, and two layers above that. Finally, the surface was sanded smooth, filled, faired, and the whole hull repainted.

As I mentioned in my website, I wrote an article for this project in Professional Boatbuilder magazine issue #45, Feb/Mar 97. If anyone cannot locate a copy, write to me privately and I can mail you a copy.

Overall, it was a very good project and it accomplished exactly what we set out to do. It modernized the look of the boat and made it sail better.

Eric

 

[Burnsy]

Exactly. I don't see the point of not using all available boat size and speed potential, especially since my particular design excels most at heavy air and upwind work. And, I'm not restricted by the IOR rule typeformings anymore, so why not get the maximum performance out of it.

I want to keep the LOA intact, because the cost of many things is based on it (entry fees, storage, dockage, etc.).

I don't know much about the nuances - merits or disadvantages - of having the bow knuckle just above water (essentially a long low front overhang) versus having the whole bow under the static waterline right to the plumb stem. I think having a slight rise to an above water knuckly may help planing by bringing all the non-flat bow out of the water at speed downwind with weight aft, may 'catch' waves less or more gradually upwind or on a reach, and may have slightly less wetted surface for light air work, which was my initial reasoning. Might give it a little more interesting look too. I don't know if it would sacrifice any performance.

Any ideas on what type of foam to use to construct/shape the new bow section and glass over it into place? This false bow may have some give on impact and protect the real hull's structural integrity if we hit something too, which is one reason the Vendee Globe around-alone boats use this protective system - lots of containers and icebergs and other nasty stuff out there.

 

[Burnsy]

Never mind, my eyes are broken. Apparently Divinycell is the material to use.

This from the IOOD 50 site: "Divinycell H80 is used in the hull with H130 used in forward slamming areas. The water ballast tanks utilise H80 foam core while Superlite balsa is used in the other watertight Bulkheads. Superlite balsa is used in the deck construction with H200 foam used in high load areas." http://www.mcintyremarine.com.au/Iood50.html

Apparently they used H80 for the 'lips' on Ocean Planet.

H130 sounds like the material to use for the bow addition, with perhaps H80 for the lips.

 

[Eric Sponberg]

We specifically did not use a solid foam form to create the new bow on Wobegon Daze because I think that would have added too much weight, even at 2 lbs./cu.ft., which is the typically foam used in most boat construction. This is the stuff that you buy in two parts at boat building supply shops. You can get spray-on mix from other venders and foaming companies, and by altering the ratios of the two parts, can get some pretty high densities. If you get to a density of 8-10 lbs./cu.ft., it's like wood--very dense and strong. It depends on your design and how much additional hull thickness you are going to add. Obviously, the denser the foam, the more weight you are adding forward, and the added weight will affect the final trim of the boat. Again, that's why we went with a wood/FRP skin structure--to minimize weight and maximize buoyancy.

If you want to make a solid foam bow, I would go denser than 2 lbs/cu.ft because it isn't very strong, yet 8 lbs./cu.ft seems like overkill on both weight and strength. I'd probably use 5-6 lbs/cu.ft and see what it did for added weight vs. added buoyancy for your particular design.

Eric

 

[Burnsy]

Ok, the H80 is rated at 5.0 lbs/cu.ft. The H130 is up at 8.1 lbs/cu.ft. The H80 sounds like the right density range.

The boat is not very heavy, spec'd at 7275 lbs displacement. This should minimize the loads. We don't have an existing problem with bow-down attitude or excess weight forward. If anything we can adjust our trim on the fly because we usually sail with 5-8 monkeys onboard that will sit where needed - and we can shift other weight such as completely unloading the bow of sails and other equipment. I believe the saying goes, 'the only thing you put in the bow is an echo'.

I was mainly considering the foam idea for to ease of working and shaping, and once done resulting a solid form to glass over in place - makes things easier. This will be a DIY project, so major surgery and reconstruction to the existing structure is not really desirable.

 

[Paul B]

This type of mod has been done on quite a few old IOR warhorses. You need to start the mod back around station 3 or 4 to get things to fair up properly.

The Custom Davidson 45 Pendragon had this type of mod a couple of years ago and it really changed the boat a lot.

http://www.bravurayachts.com/pendragon_refit.htm

 

[Tactic]

I still had the original plans to my boat,so I have input the table of offsets into AutoCad and drawn the lines plan to see the effect of my proposed changes.

The original hull has a displacment of 713kg with a waterline length of 5080mm
the lcb is at 54% of the wl aft of station 0.
the pc is 0.516

My proposed change extended the waterline to 5310mm increased displacment to 723kg the lcb is 55% aft from the "new" station 0 and pc has decreased to 0.503

The original waterline had a slight hollow forward. I have made the new waterline almost straight to sation3. The new waterline has a finer entry angle.

Should this proposed change make this hull faster in displacment mode?

cheers
Tactic

 

[Eric Sponberg]

Tactic,

Just on speed-length ratio alone, you would pick up 0.12 knots (less than 1/8th of a knot) for having lengthened your waterline. However, you started with a relatively low prismatic coefficient (ideal likely would be closer to 0.53 to 0.54) and you are going to an even lower prismatic coefficient, so you might lose any speed advantage because of the shift in prismatic.

This is a small boat, so the changes in speed will be small. Shifting the crew weight, fore and aft or port and starboard, will likely have a much greater effect on speed than the changes you make to the bow shape.

Eric

 

[Tactic]

Thanks Eric,
You can assume that crew weight is always in the right place on this yacht modified or unmodified.
1/8 knot would make a big difference in the fleet I sail in.Thats roughly 200m in a one hour beat,well worth the mod.

I will study the changes in prismatic and try to find a better solution.
Thanks for the advice

Tactic

I have just performed another series of calcs.
Increased the wl to 5472mm (bow is alomost straight up and down now)
displ increased to 749kg and managed to get the pc up to .51 (original .516)
A bigger rig will also be going on the boat so increase in w/surface is not a major issue I hope.

 

[Tactic]

Anyone else have any thoughts on these types of modifications?
I am not sure if I should proceed or not.
Are you any further ahead Burnsy?

Cheers all
Tactic

 

[asathor]

Looking at the picture and the discription of the boats already good balance you might need to replace your monkees with gorillas to keep the stern afloat.

In other words, to improve the boat you will have to extend both ends. Some of the maxies use the trick of extending the hull some distance with a initial rise almost parrallel to the waterline - when at rest in the harbor for measurements it floats on it short lines - when at sea with the monkees (gorillas) and somewhat heeled it sets itself in a lower and longer groove as it nears hull speed and encounters wave action.

I am not sure the benefits will outweigh the costs unless you are already able to sail the boat overcanvassed so that you have the necessary power to lug the extra flow resistance and weight around in light air.

Since you are not sailing as a class boat look at some of the other options - mast profile for example - rudder mods etc. That could help you sail higher AND lower which could just as easily translate to faster than a hull mod. Take a look at this http://www.biol.rug.nl/itieleman/OutsideJEB4.pdf and think keel and rudder. It works, and it is a lot easier to install, change or remove. I did my rudder and all I have to do is pull out a beltsander to go back to stock.

 

[Tactic]

Attached is an image of my proposed mods.
comments welcome
Cheers
Tactic