Skeg construction

[Stumble]

Originally Posted by minaret

Originally Posted by Stumble
In that case you need to hire a NA to design a new rudder and skeg. Figure you will need to remove 2 or 3 layers of glass within a foot or so of the new rudder post in order to bond the new skeg on, and likely reinforce the inside of the hull where the new skeg will be located.

The upside is you can likely go to a much thinner rudder post since the skeg will be carrying part of the load so you won't need the post to be as stiff.


Last time I glassed on a skeg, I did 14 plies in epoxy. This means you must first remove enough material for such a substantial laminate to be applied and still have a fair bottom. A few plies and one foot of back grind isn't even in the ballpark, IMHO.

There was supposed to be an 'at minimum 2-3 layers' in there somewhere.

After thinking about it my first instinct is to cut a hole about 2'x1' then the hull, then feather the edges to 12:1, and rebuild the entire hull with the skeg as part of the layup....


Though my real instinct is this is ridiculous.

[minaret]

Originally Posted by Stumble

There was supposed to be an 'at minimum 2-3 layers' in there somewhere.

After thinking about it my first instinct is to cut a hole about 2'x1' then the hull, then feather the edges to 12:1, and rebuild the entire hull with the skeg as part of the layup....


Though my real instinct is this is ridiculous.

Sure, agreed. I might for something this size make a skeg "core" from 1" or so flat stock and cut a slot in the hull, with the skeg designed to stick up inside between the stringers a good 10-12", then install flat stock box frame to stringers, glass all inside and out. Maybe a partial skeg so the rudder is stil semi balanced, i.e. cut out part of top of rudder.

[Cheechako]

"heck, i'd actually prefer an outboard rudder with a skeg. but, that would move the rudder too far aft and affect the balance of the boat too much... not that i haven't considered it." Actually I thought of that also. A well built outboard rudder (if you are tiller steering) is simple and easy to inspect and maintain. Build two, put one in the V Berth for redundancy and forget the skeg!
It should actually improve the steering and not be a balance issue.
But frankly, stop obsessing with the rudder design and use your boat for a couple years. You'll know more then what boat to take offshore if you ever do.

[Stumble]

Originally Posted by minaret

Sure, agreed. I might for something this size make a skeg "core" from 1" or so flat stock and cut a slot in the hull, with the skeg designed to stick up inside between the stringers a good 10-12", then install flat stock box frame to stringers, glass all inside and out. Maybe a partial skeg so the rudder is stil semi balanced, i.e. cut out part of top of rudder.

I like this idea more. And it resolves my concern about localized loads ripping the skeg off (and a good chunk of the hull too). Since you can spread the loads over the entire box frame grid. But I wouldn't use flat stock, I am thinking some carbon oval tubing as the core. It going to need something really stiff to keep from bending. Or maybe 1" thick flat stock.

[Jim Cate]

Minaret and Stumble: While I agree that the job is POSSIBLE, and that with good engineering and practice it could be successful, do you really think that this extremely low budget (self confessed) chap is likely to do such a job? Carbon oval tubing? Really?

When coupled with all the other things he dislikes about the Cal's design (whether realistic or not) it seems like a really bad idea to attempt this job.

Jim

[Cheechako]

Thinking out of the box: Another option... Dual outboard rudders with a tie bar. They can be shorter in the water = less stress from water pressure = less likely to break.


A spade rudder can take part of the hull out too. Saw this mentioned in a sailing mag recently. I think it was in regard to building spade rudders with larger stronger stock. A skeg has the stress distributed over a large area, IF the designer put decent size radii in the fairing to the hull even better.


I wonder if designers eliminate core around the rudder stock area and build it to distribute stress in any way on modern boats?

[minaret]

Originally Posted by Jim Cate

Minaret and Stumble: While I agree that the job is POSSIBLE, and that with good engineering and practice it could be successful, do you really think that this extremely low budget (self confessed) chap is likely to do such a job? Carbon oval tubing? Really?

When coupled with all the other things he dislikes about the Cal's design (whether realistic or not) it seems like a really bad idea to attempt this job.

Jim

I would just remind you; it's a 27' boat, a glorified dinghy in my eyes. We're talking about relatively small parts without astronomical loads.

[minaret]

Originally Posted by Stumble

I like this idea more. And it resolves my concern about localized loads ripping the skeg off (and a good chunk of the hull too). Since you can spread the loads over the entire box frame grid. But I wouldn't use flat stock, I am thinking some carbon oval tubing as the core. It going to need something really stiff to keep from bending. Or maybe 1" thick flat stock.

Yeah, at least 1" flat stock. If you add 1/2" of glass to either side when glassing it into the hull this gives a solid skeg 2" or so thick. Might need thicker flat stock, as the skeg must be as thick as the rudder with similar chord to the foil.

My home made flat stock isn't quite as strong as G10 but it's damn close and much cheaper. I wouldn't hesitate to epoxy bond together three layers of 1/2" flat stock to make a piece of 1 1/2", and then grind it to shape.

[minaret]

If you use tubing instead of flat stock you are subject to buckling loads due to hollow skeg. This is generally avoided with hollow molded siege by pouring the cavity full of something so it's no longer hollow. This approach has a number of drawbacks, including inflexibility/cracking of the pour, and the rocker/shape of the bottom making it impossible to pour the skeg cavity any higher than the leading edge of the sump due to gravity. I have seen some builders attempt to alleviate this by stuffing the skeg cavity with glass scraps and resin. Much better to just go solid out the gate when retrofitting. This has the added benefit of allowing you to have the skeg protrude I to the vessel a ways and then frame around it, tying in to stringers and bulkheads. Much stronger solution.

Just glassed up a skeg interior today!

[robert sailor]

When Lisa and Andy Copeland sailed around the world in their 80's built Beneteau first 38 one thing few followers knew was that the boat was a bit squirrely steering under a servo pendulum vane. He added a stainless skeg in front of the spade rudder thru bolted and swears that it helped settle the boat down under windvane. I viewed his boat and saw the addition he made at the time.
He may have recently changed vanes to a Hydrovane which would also add to his stability with the large steering rudder on this design.

[Stumble]

Originally Posted by Jim Cate

Minaret and Stumble: While I agree that the job is POSSIBLE, and that with good engineering and practice it could be successful, do you really think that this extremely low budget (self confessed) chap is likely to do such a job? Carbon oval tubing? Really?

When coupled with all the other things he dislikes about the Cal's design (whether realistic or not) it seems like a really bad idea to attempt this job.

Jim

Frankly I think the whole idea is absurd from the get go. Much like trying to turn a Cessna 150 into a bywing plane. I guess it's possible, but it's a really sillyidea that will cost far more than it could ever possibly be worth. Heck it will likely destroy any residual value of the boat as it sits.

Even IF you had VPLP do the design, and a full structural study of the existing hull, and the boatwright for the US Olympic sailing team do the modification with no budget limit I don't think it would come out very well. Doing it on a budget, with an inexperienced glass guy, working from what looks right without proper engineering... I wouldn't go sailing on it without a chase boat and a photographer to film the resultant sinking.

What's worse in this case is that let's assume the new skeg is structurally sound, and hydrostatically reasonable. At best a skeg generates more drag, more helm preassure, and hampers turning compared to a spade. And provides NO additional strength.

A skeg designed to handle 1,000lbs of load will break at 1001, a spade designed for 1,000lbs of load will also break at 1,001lbs. So which is stronger? NEITHER they are the same strength. The skeg is just a different way to achieve the same result.

Onthe other hand I enjoy running thru themental exercise of 'i it wasn't silly how would i do it.' Sometimes the process results in other good ideas.

[first wind]

Originally Posted by minaret

Sure, agreed. I might for something this size make a skeg "core" from 1" or so flat stock and cut a slot in the hull, with the skeg designed to stick up inside between the stringers a good 10-12", then install flat stock box frame to stringers, glass all inside and out. Maybe a partial skeg so the rudder is stil semi balanced, i.e. cut out part of top of rudder.

that's an interesting idea. basically giving the skeg some 'bury' and the structure to support it. i can see that.

[jrau18]

Why not just build a "proper" backup rudder? The cals all have a stern that shouldn't be too hard to adapt to a cassette hung rudder and tiller.

Break your spade rudder off, just throw ok your full size emergency rudder. Done right, it would restore steering to "as good as new" in 5 minutes of install time until you get home. It should even be easy enough to connect it to wind steering.

If you do pursue adding a skeg, include your results and process. Should be interesting to follow along.

[first wind]

Originally Posted by Cheechako

"heck, i'd actually prefer an outboard rudder with a skeg. but, that would move the rudder too far aft and affect the balance of the boat too much... not that i haven't considered it." Actually I thought of that also. A well built outboard rudder (if you are tiller steering) is simple and easy to inspect and maintain. Build two, put one in the V Berth for redundancy and forget the skeg!
It should actually improve the steering and not be a balance issue.
But frankly, stop obsessing with the rudder design and use your boat for a couple years. You'll know more then what boat to take offshore if you ever do.

really? so, you don't think that's a bad idea? i, personally, like outboard rudders. i have designed and built a few for smaller boats. i might have to give that some further thought.

like i said, previously, i am information gathering. i am not pushing to do any rudder change at this haul out, necessarily. there are other more important issues ahead of that. it all depends on the information i get and what suits my needs best.

[first wind]

Originally Posted by jrau18

Why not just build a "proper" backup rudder? The cals all have a stern that shouldn't be too hard to adapt to a cassette hung rudder and tiller.

Break your spade rudder off, just throw ok your full size emergency rudder. Done right, it would restore steering to "as good as new" in 5 minutes of install time until you get home. It should even be easy enough to connect it to wind steering.

If you do pursue adding a skeg, include your results and process. Should be interesting to follow along.

that's a good idea. as long as the rudder stock does break instead of prying open the hull, it would probably be the easiest solution to accomplish.