20+ Footer - Building in Hawaii

[Jim Donovan]

Took care of the parting line crud and fixed that lump at the tail:

 

[Jim Donovan]

Varnish crew in the shed past week and making dust has been off limits, so limits the 20 footer projects.

Got the parting line crud off the bulb and have cleaned the bottom with a shurform; leaves a slightly grooved surface that the fairing will grab onto.

Bagged the core into one half of the rudder. Used the same core pieces I used to take the molds from; kind of perfectly shaped to fit. Rebated in the area of the Uni's, made a relief cut in the top to adjust for the laminate thickness, and sliced the leading edge off so it would accommodate the leading edge join rebate.

 

[Irrational 14]

Once you join rudder halves, is there a layer of UNI in the sandwich or other idea here?

 

[Jim Donovan]

Once you join rudder halves, is there a layer of UNI in the sandwich or other idea here?

You can see the Unis that are included in the shell laminate; they're about 6" wide and extend 70% down the blade.

 

[.Q.]

Hey Jim. 

What do you find more satisfying... doing large projects like the AC or the T4, or working in your garage on you own projects?

 

[Jim Donovan]

Hey Jim. 

What do you find more satisfying... doing large projects like the AC or the T4, or working in your garage on you own projects?

It's all good; lucky my work is what I like to do

 

[orca99]

The keel weight for your design seems generous.......On my 19 footer (a VX One), the keel weight is 60 kg / 132 Lbs in comparison.

 

[JonRowe]

The keel weight for your design seems generous.......On my 19 footer (a VX One), the keel weight is 60 kg / 132 Lbs in comparison.

A VX One is a dinghy-esque sportsboat that can capsize and turtle, with a small rig compared to its stable mates, I raced against them in Oz, and whilst I have nothing against the form factor, they push into one specific corner of the design envelope, so most similarly sized boats have more "generous" keels.

 

[Jim Donovan]

The keel weight for your design seems generous.......On my 19 footer (a VX One), the keel weight is 60 kg / 132 Lbs in comparison.

Stability is horsepower. 

My 20 footer is a "keelboat", and the ballast is there to make the boat faster upwind and reaching, and allow for the stability necessary to race safely in rough conditions. After sailing Melges 24s for years, I'm tired of those wipeout's followed by prolonged periods of time waiting for the boat to come upright.

My 20 has a MUCH bigger rig than the VX; the spinnaker is 80% larger and 40% more upwind sail area; the upwind sail area will be doubled with a masthead jib for light airs upwind, and close reaching all the time.

The 20's bulb is almost as heavy as the Melges 24, and my expectation/hope is the boat ends up about 400# lighter than a M24. My keel fin is deeper than the M24, so the ballast will be even more effective. When I built the boat, I was expecting to line up against the M 24 fleet that had the best sailors racing in Kaneohe Bay. I wanted a three person boat that might have a chance of finishing in front of a M24; if we accomplished that we'd all be Heroes! Failing that, oh well, we're 4 feet shorter; I liked that equation . . .

I'm using a M 24 mast and have a M 24 spinnaker (that needs a slight area reduction).

What we've learned from the GP26 is the deep fin/heavy bulb makes a lot of things work really well. We can sail with 40 footers upwind. 2 sail reach faster than moderate 40s and faster than some "fast" 30 footers.Wipeouts are measured in seconds and don't take you out of a race. That and the boat is super safe for any sailor in even the nastiest conditions; not common in high performance sportboats.

The 20 and the GP 26 share similar hull forms; I worked hard to make sure the boat didn't change trim as it heeled, and would stay on a plane at higher heel angles. When the puff hits, the boat responds by heeling more; the bulb rotates to weather and all of a sudden, you feel the stability boost exactly when you need it. This allows you to keep the hammer down without backing off. We've gone to big winches to handle the spinnaker sheet loads on the 26; there's just a shit load of horsepower.

What has never made sense (for me) are boats with lead ballast that does not prevent capsize; what's the lead there for?

I've seen the worst situation at CRW where a Viper dumped it's crew, self-righted, and then sailed away from the crew/swimmers. Thankfully there was a RC chase boat to recover the crew and then chase down the crewless Viper. A short time later, another Viper turtled with the keel bulb just hanging out there above the boat doing nothing. Next to the bulb was a 5-year old kid standing on the bottom of the boat; got to think he was having a "WTF Dad" moment.

BTW there's a plan for a "performance" keel.

After some time in the boat, we'll figure out what that is.

 

[Floating Duck]

What we've learned from the GP26 is the deep fin/heavy bulb makes a lot of things work really well.

Amen. 

There is NO replacement for draft. More efficient aspect ratio, more efficient ballast. Unless you can't have draft due to shallow water, more is always better. 

Case in point, my current 26ft boat has an 8ft carbon keel... and I wish it was 10ft! Maybe when you build my next boat Jim we can increase it  :lol:

 

[neuronz]

Hi Jim,

I have been toying around for years with the idea of replacing the steel keel fin in our boat with a carbon one. I already calculated how much additional lead needs to go into the end of the fin to maintain the RM. However, I was never quite sure what the internal structure should look like in a fin with a thickness of only 40mm, although I am quite sure I need some sort of shear web to support the 600kg bulb. Hence I would be very interested in what you are planning for the keel fin of the 20. And since pictures usually say more than 1000 words photos of the construction would be much appreciated.

 

[dolphinmaster]

Hi Jim,

I have been toying around for years with the idea of replacing the steel keel fin in our boat with a carbon one. I already calculated how much additional lead needs to go into the end of the fin to maintain the RM. However, I was never quite sure what the internal structure should look like in a fin with a thickness of only 40mm, although I am quite sure I need some sort of shear web to support the 600kg bulb. Hence I would be very interested in what you are planning for the keel fin of the 20. And since pictures usually say more than 1000 words photos of the construction would be much appreciated.

Jim,  how much for a custom keel design and engineering? ;-)

 

[Jim Donovan]

Hi Jim,

I have been toying around for years with the idea of replacing the steel keel fin in our boat with a carbon one. I already calculated how much additional lead needs to go into the end of the fin to maintain the RM. However, I was never quite sure what the internal structure should look like in a fin with a thickness of only 40mm, although I am quite sure I need some sort of shear web to support the 600kg bulb. Hence I would be very interested in what you are planning for the keel fin of the 20. And since pictures usually say more than 1000 words photos of the construction would be much appreciated.

I stay well away from supplying detailed specs on the parts I design and/or build, especially for keels and rudders which are "critical components" on a racing yacht. Add rigs, hulls, decks & framing to that list. For any and all clients, I go to an experienced racing yacht engineer to define those structures.

So much of a structure's success is in the hands of the builder; carbon is an especially "twitchy" material that requires an experienced builder.

Couple of comments:

40mm thickness sounds quite skinny for a fin carrying a 600kg bulb. That's thinner than my 20's fin.

Carbon is not as stiff a material as steel, so we tend to design for bending, which makes the strength of the fin quite conservative. That's a good idea for a keel fin that may take some serious abuse. The extra weight is quite small, and in an area where it doesn't hurt performance much.

On thin fins, it's often impossible to get the sidewalls thick enough to get the fin stiff enough; the laminate thickness is wandering toward the centroid and becoming less effective with every ply added.

Steel ends up being the clear winner for very thin fins. 

 

[neuronz]

I stay well away from supplying detailed specs on the parts I design and/or build, especially for keels and rudders which are "critical components" on a racing yacht. Add rigs, hulls, decks & framing to that list. For any and all clients, I go to an experienced racing yacht engineer to define those structures.

So much of a structure's success is in the hands of the builder; carbon is an especially "twitchy" material that requires an experienced builder.

Couple of comments:

40mm thickness sounds quite skinny for a fin carrying a 600kg bulb. That's thinner than my 20's fin.

Carbon is not as stiff a material as steel, so we tend to design for bending, which makes the strength of the fin quite conservative. That's a good idea for a keel fin that may take some serious abuse. The extra weight is quite small, and in an area where it doesn't hurt performance much.

On thin fins, it's often impossible to get the sidewalls thick enough to get the fin stiff enough; the laminate thickness is wandering toward the centroid and becoming less effective with every ply added.

Steel ends up being the clear winner for very thin fins. 

Thanks Jim, this much appreciated. To be clear I was not expecting you to share detailed engineering information, rather the structural concept as I have been looking into different options for this. Since I do not want to reinvent the wheel I am always interested in what others are doing in similar applications.

From a building perspective it would probably be the easiest to create a foam positive and wrap that in carbon. This could be combined with a carbon plate in the centerplane to create robust edges. The next level of escalation would be to add small ribs which could also aid with the shaping of the foam.

Another option would be to create a box girder and make the shape with non structural shells. However, with the limited thickness this would tend towards a solid carbon girder which makes it quite expensive and inefficient. But it would be easier to transfer the load to the retaining pin.

 

[Jim Donovan]

Thanks Jim, this much appreciated. To be clear I was not expecting you to share detailed engineering information, rather the structural concept as I have been looking into different options for this. Since I do not want to reinvent the wheel I am always interested in what others are doing in similar applications.

From a building perspective it would probably be the easiest to create a foam positive and wrap that in carbon. This could be combined with a carbon plate in the centerplane to create robust edges. The next level of escalation would be to add small ribs which could also aid with the shaping of the foam.

Another option would be to create a box girder and make the shape with non structural shells. However, with the limited thickness this would tend towards a solid carbon girder which makes it quite expensive and inefficient. But it would be easier to transfer the load to the retaining pin.

Built a few fins, all different ways. More than half the time you finish the job and think, “Not going to do it that way again!”.  It’s just a lot of work, especially if you can’t afford beautiful CNC cut molds.

I like the internal spar concept for fins; the leading and trailing edges don’t contribute much to the strength/stiffness, and the T.E. is quite challenging to get structurally sound while keeping it thin. An internal spar makes the leading and trailing edges non-structural. Most of the damage to fins occurs in those areas, so they help protect the structure and are far easier to repair.

I built a redesigned Humboldt 30 rudder 16 years ago with an internal spar. In 2019 it finally succumbed to a hard hit on the reef off Waikiki. The foam tip below the spar busted off, and the rudder almost ripped the transom off the boat. A suspicious crack at the upper gudgeon, combined with 15 years of hard use, sent that rudder to the dumpster. Old, damaged carbon parts NEED to be trashed! Before throwing it in the dumpster, I added the tip back on the old blade and took a mold from it. Built the new HB 30 transom-hung rudder and took the Waarschipp rudder from the same mold.

Your 40mm thick fin must have a short chord length? That doesn’t leave much to hang a 600kg bulb on.

I’d initially look at what laminate thickness is required, again, looking at bending rather than strength. Thickening the laminate just adds material closer to the centroid, so every ply becomes less effective.

 

[orca99]

Stability is horsepower. 

My 20 footer is a "keelboat", and the ballast is there to make the boat faster upwind and reaching, and allow for the stability necessary to race safely in rough conditions. After sailing Melges 24s for years, I'm tired of those wipeout's followed by prolonged periods of time waiting for the boat to come upright.

My 20 has a MUCH bigger rig than the VX; the spinnaker is 80% larger and 40% more upwind sail area; the upwind sail area will be doubled with a masthead jib for light airs upwind, and close reaching all the time.

The 20's bulb is almost as heavy as the Melges 24, and my expectation/hope is the boat ends up about 400# lighter than a M24. My keel fin is deeper than the M24, so the ballast will be even more effective. When I built the boat, I was expecting to line up against the M 24 fleet that had the best sailors racing in Kaneohe Bay. I wanted a three person boat that might have a chance of finishing in front of a M24; if we accomplished that we'd all be Heroes! Failing that, oh well, we're 4 feet shorter; I liked that equation . . .

I'm using a M 24 mast and have a M 24 spinnaker (that needs a slight area reduction).

What we've learned from the GP26 is the deep fin/heavy bulb makes a lot of things work really well. We can sail with 40 footers upwind. 2 sail reach faster than moderate 40s and faster than some "fast" 30 footers.Wipeouts are measured in seconds and don't take you out of a race. That and the boat is super safe for any sailor in even the nastiest conditions; not common in high performance sportboats.

The 20 and the GP 26 share similar hull forms; I worked hard to make sure the boat didn't change trim as it heeled, and would stay on a plane at higher heel angles. When the puff hits, the boat responds by heeling more; the bulb rotates to weather and all of a sudden, you feel the stability boost exactly when you need it. This allows you to keep the hammer down without backing off. We've gone to big winches to handle the spinnaker sheet loads on the 26; there's just a shit load of horsepower.

What has never made sense (for me) are boats with lead ballast that does not prevent capsize; what's the lead there for?

I've seen the worst situation at CRW where a Viper dumped it's crew, self-righted, and then sailed away from the crew/swimmers. Thankfully there was a RC chase boat to recover the crew and then chase down the crewless Viper. A short time later, another Viper turtled with the keel bulb just hanging out there above the boat doing nothing. Next to the bulb was a 5-year old kid standing on the bottom of the boat; got to think he was having a "WTF Dad" moment.

BTW there's a plan for a "performance" keel.

After some time in the boat, we'll figure out what that is.

View attachment 442727

Thanks for the clarification, and by the way - i love your GP26 design.

The SA comparison with the VX put you close to the SA of a SB20 - right? (46 m2 Spi, 27-28 m2 main/jib)

Also roughly same lenght and keelweight and your hull should be about 100 kg lighter.

You are certainly not at risk of being kicked out of the SA sportsboat forum with those specs, even if you call it a keelboat

I guess there are two ways to nirvana; a boat which will recover easily after a broach, or a design, which has small sails and very little tendencies to broach, at least if it is helmed by a person with a dinghy background. 

Will your 20 footer be fitted with lifelines? or foot straps?

If you sail with footstraps (like my VX), I estimate that 1 kg of (hard hiking) crew weight generate as much righting moment as about 3-4 kg of bulb weight. (we sail with with about 10 degree lean both up and downwind).

I guess with a heavier keel, i could carry marginally larger sails, but would it make the boat faster in average in a windy location?

The same question apply in reverse for your boat; would it be faster with smaller sails and less bulb weight?

Keel weight is not always a good thing. I still recall Jørgen Boisen Møller (olympic gold medalist) say that the Olympic Tempest sailed exactly like his Flying dutchman when the cockpit was filled with water. this is perhaps relevant as a modern sportsboat is faster than a Flying Dutchman dinghy in average, if the Portsmouth handicaps are to be believed.

Another poster believed a heavier, deeper keel is always faster/safer. If it was true, the VX with 60 kg keel and 1.3 m draft would have no chance against for example a M24. In reality they should be quite even both per Portsmouth and IRC handicaps.

In the hypothetical case that a sports boat could exceed the waterline limited speed even upwind, I am fairly certain that the optimum keel bulb weight is as low as possible, meaning whatever minimum weight is required to get you into the keelboat events in your area, and what you feel safe sailing with.

 

[neuronz]

Thanks for the clarification, and by the way - i love your GP26 design.

The SA comparison with the VX put you close to the SA of a SB20 - right? (46 m2 Spi, 27-28 m2 main/jib)

Also roughly same lenght and keelweight and your hull should be about 100 kg lighter.

You are certainly not at risk of being kicked out of the SA sportsboat forum with those specs, even if you call it a keelboat

I guess there are two ways to nirvana; a boat which will recover easily after a broach, or a design, which has small sails and very little tendencies to broach, at least if it is helmed by a person with a dinghy background. 

Will your 20 footer be fitted with lifelines? or foot straps?

If you sail with footstraps (like my VX), I estimate that 1 kg of (hard hiking) crew weight generate as much righting moment as about 3-4 kg of bulb weight. (we sail with with about 10 degree lean both up and downwind).

I guess with a heavier keel, i could carry marginally larger sails, but would it make the boat faster in average in a windy location?

The same question apply in reverse for your boat; would it be faster with smaller sails and less bulb weight?

Keel weight is not always a good thing. I still recall Jørgen Boisen Møller (olympic gold medalist) say that the Olympic Tempest sailed exactly like his Flying dutchman when the cockpit was filled with water. this is perhaps relevant as a modern sportsboat is faster than a Flying Dutchman dinghy in average, if the Portsmouth handicaps are to be believed.

Another poster believed a heavier, deeper keel is always faster/safer. If it was true, the VX with 60 kg keel and 1.3 m draft would have no chance against for example a M24. In reality they should be quite even both per Portsmouth and IRC handicaps.

In the hypothetical case that a sports boat could exceed the waterline limited speed even upwind, I am fairly certain that the optimum keel bulb weight is as low as possible, meaning whatever minimum weight is required to get you into the keelboat events in your area, and what you feel safe sailing with.

I have two comments to your post.

First of all not every design decision is driven by maximum straight line speed. For example if you want to participate in distance racing you will be able to extract a higher portion of your performance with a heavier bulb which can easily transfer into higher average speed as your performance decreases less significantly when you take a break from hiking.

If you think your point all the way to the end you get rid of the bulb entirely and sail a catamaran.

 

[Chris in Santa Cruz CA]

Hi Jim,

I have been toying around for years with the idea of replacing the steel keel fin in our boat with a carbon one. I already calculated how much additional lead needs to go into the end of the fin to maintain the RM. However, I was never quite sure what the internal structure should look like in a fin with a thickness of only 40mm, although I am quite sure I need some sort of shear web to support the 600kg bulb. Hence I would be very interested in what you are planning for the keel fin of the 20. And since pictures usually say more than 1000 words photos of the construction would be much appreciated.

One thing to consider here is that we found  that even though the first C30 keel spar was strong enough being 100% prepreg carbon fiber and had an optimized foil shape designed and built to ABS standards, it was too flexible torsionally and the bulb would swim and oscillate. We had to add more carbon to get it stiff enough to stop the swimming. Ended up overbuilt to ABS. It was interesting at the time to learn how ABS really did not address loads occurring at higher speeds of the modern planing hulls. Made me wonder how far into the safety factor some rudders and keep foils were actually going

 

[neuronz]

One thing to consider here is that we found  that even though the first C30 keel spar was strong enough being 100% prepreg carbon fiber and had an optimized foil shape designed and built to ABS standards, it was too flexible torsionally and the bulb would swim and oscillate. We had to add more carbon to get it stiff enough to stop the swimming. Ended up overbuilt to ABS. It was interesting at the time to learn how ABS really did not address loads occurring at higher speeds of the modern planing hulls. Made me wonder how far into the safety factor some rudders and keep foils were actually going

Thanks for the comment. What kind of cross section shape was the spar? Torsion is actually the reason why I wanted to build a box girder for a structural spar as it is a closed cross section compared to options like T-girders. This might also be an point in favor of the foam core wrapped in carbon approach.

 

[Irrational 14]

Most C30's scrapped the carbon fin and replaced with iron. Harmonic issue gone.