Who has owned a cat with daggerboards?

[chrisr]

Originally Posted by tcpbob

and chrisr, if you hang around McClean please tell Ty on Sahara bob on barebones said hi.

we were at maclean last week...doing some shopping...but now back in our favourite spot of iluka

seen sahara around but not for a little while. will certainly say g'day next time

cheers,

[rob denney]

Daggerboards on a boat capable of decent speeds are a disaster looking for a place to happen.
If an immovable object (big fish, whale, log, container, etc) just below the surface is hit one of 3 things happens:

1) the boat stops. The crew carry on until they hit something or go over the bow. Google car crash test videos at 25 mph to see the forces involved at 21 knots.
2) the board sheers off and the object hits the rudder, prop or saildrive, almost certainly ripping them out.
3) the case is damaged and the hull fills with water.

Both 2 and 3 will have an element of 1) depending on the strength in the fore and aft direction of the board (generally huge) and how much beefing up is around the case (generally a ridiculous amount).

A kick up board with a fuse is better. Either below the bridgedeck (if the clearance is high enough) or pivotted in a case in the hull. Both still leave the saildrive/prop and rudder vulnerable.

[NevilleCat]

Surely a board pivoted in a case in the hull is just a swing keel with the attendant issues of how much space the thing takes up in the internal layout, the engineering involved in athwart-ships leverage, and the hydrodynamic interference of the long empty case? These can be overcome to some extent, and it certainly would be safer in event of collision but I wonder how much of a problem it has actually proven to be in the last 20 or 30 years since swing boards fell from favour for the above reasons.

[fxykty]

Originally Posted by rob denney

Daggerboards on a boat capable of decent speeds are a disaster looking for a place to happen.
If an immovable object (big fish, whale, log, container, etc) just below the surface is hit one of 3 things happens:

1) the boat stops. The crew carry on until they hit something or go over the bow. Google car crash test videos at 25 mph to see the forces involved at 21 knots.
2) the board sheers off and the object hits the rudder, prop or saildrive, almost certainly ripping them out.
3) the case is damaged and the hull fills with water.

Both 2 and 3 will have an element of 1) depending on the strength in the fore and aft direction of the board (generally huge) and how much beefing up is around the case (generally a ridiculous amount).

A kick up board with a fuse is better. Either below the bridgedeck (if the clearance is high enough) or pivotted in a case in the hull. Both still leave the saildrive/prop and rudder vulnerable.

Alarmist crap. How is anything that you’ve described any different on a cat with mini keel that can also break off?

[rob denney]

Originally Posted by fxykty

Alarmist crap. How is anything that you’ve described any different on a cat with mini keel that can also break off?

It's not. I did not mention mini keels (which have far more negatives than positives on a cruising, sailing multi), and don't know of any that are "capable of decent speeds". If one was, it would be every bit as bad in a collision as a daggerboard.

"Alarmist crap" is not conducive to a reasonable discussion. Please tell us why you think what I wrote is incorrect. Stick to what I wrote, rather than what you think I meant.

[Chotu]

Originally Posted by rob denney

Daggerboards on a boat capable of decent speeds are a disaster looking for a place to happen.
If an immovable object (big fish, whale, log, container, etc) just below the surface is hit one of 3 things happens:

1) the boat stops. The crew carry on until they hit something or go over the bow. Google car crash test videos at 25 mph to see the forces involved at 21 knots.
2) the board sheers off and the object hits the rudder, prop or saildrive, almost certainly ripping them out.
3) the case is damaged and the hull fills with water.

Both 2 and 3 will have an element of 1) depending on the strength in the fore and aft direction of the board (generally huge) and how much beefing up is around the case (generally a ridiculous amount).

A kick up board with a fuse is better. Either below the bridgedeck (if the clearance is high enough) or pivotted in a case in the hull. Both still leave the saildrive/prop and rudder vulnerable.

Rob, what do you recommend as the best way to do a fuse on a rudder case/cassette?

I’ve experimented with them and they kept triggering too early. I ended up driving wedges in instead, but a replaceable fuse you can pop out to lift the rudder when not in use seems so much better.

[valhalla360]

Originally Posted by rob denney

It's not. I did not mention mini keels (which have far more negatives than positives on a cruising, sailing multi), and don't know of any that are "capable of decent speeds". If one was, it would be every bit as bad in a collision as a daggerboard.

"Alarmist crap" is not conducive to a reasonable discussion. Please tell us why you think what I wrote is incorrect. Stick to what I wrote, rather than what you think I meant.

Got to agree with fxtyx on this one.

Ram fixed keels into a reef at 10-15kts and you can expect lots of major damage and if they manage to slide over the object rather than being ripped clean off, the rudders and prop are next to take a hit.

At high speed, I wouldn't even trust kick up boards and rudders not to be damaged.

Just generally not a good idea to be ramming fixed objects regardless of keel type.

[Dave_S]

Originally Posted by rob denney

Daggerboards on a boat capable of decent speeds are a disaster looking for a place to happen.
If an immovable object (big fish, whale, log, container, etc) just below the surface is hit one of 3 things happens:

1) the boat stops. The crew carry on until they hit something or go over the bow. Google car crash test videos at 25 mph to see the forces involved at 21 knots.
2) the board sheers off and the object hits the rudder, prop or saildrive, almost certainly ripping them out.
3) the case is damaged and the hull fills with water.

Both 2 and 3 will have an element of 1) depending on the strength in the fore and aft direction of the board (generally huge) and how much beefing up is around the case (generally a ridiculous amount).

A kick up board with a fuse is better. Either below the bridgedeck (if the clearance is high enough) or pivotted in a case in the hull. Both still leave the saildrive/prop and rudder vulnerable.

Originally Posted by fxykty

Alarmist crap. How is anything that you’ve described any different on a cat with mini keel that can also break off?

I agree with fxykity.

Rob your comments are way off.

my dagger boards have a shear point designed to shear before damage to the boat (fuse)

Rudders are flip up with a strap designed to fail if it is impacted with enough force to damage them.

Outboards lift up into the hulls.

It is significantly safer than other boats.

You sound like you have some agenda ?

[Tupaia]

The daggerboard and rudders on my boat do mitigate some of the points Rob cited.

The daggerboard is winched down (it is buoyant) to below a steel pin across the back of the case. The downhaul is let loose and the uphaul and the backhaul, a stretchy 3 strand rope, are tensioned. The rear of the case is heavily reinforced and radiused. In the event of a collision the initial impact is absorbed by the springy backhaul as the board is levered from under the pin. The buoyancy of the board together with the tension on the uphaul serve to withdraw the board into the case. The board does not retract completely but the series of events greatly reduces and absorbs the forces.

https://www.youtube.com/watch?v=XZoZG_jCDZE

The single daggerboard in the port hull is canted outwards. When the board is to leeward, on starboard tack, the effect of the board is reduced quite rapidly as the windward hull rises allowing flow under it and reducing the grip, more so than if the board was vertical. On a port tack with the board to windward the board hooks into the water increasing the grip. The single board was by design but the hydrodynamic effect of canting the board may have been consequence of making more room internally.

https://sites.google.com/site/shuttl...25/home/m8.jpg

The rudders are kick up relaying on a sheer plate as a fuse. The 12mm plywood plate requires a considerable force to break it so the surrounding structure has to be very robust as do the components of the rudder and the stock.

https://www.youtube.com/watch?v=IBoMJFrZj4w&t=25s

Most of John's boats have outboards so collision is no more an issue than if it were on a dinghy if it was even deployed at all while sailing. My own boat has sail drives with folding props that are arranged so the bottom of the sail-drive does not to project below the rocker of the hull.

There is one further advantage that daggerbaords give that is the lack of buoyancy amidships due to not having minikeels this more evenly distributes the buoyancy and significantly reduces the potential for hobby horsing.

Rob has gone a long way to mitigating the issues associated with proas, but not completely otherwise we would all be sailing them.

[rob denney]

Originally Posted by Chotu

Rob, what do you recommend as the best way to do a fuse on a rudder case/cassette?

I’ve experimented with them and they kept triggering too early. I ended up driving wedges in instead, but a replaceable fuse you can pop out to lift the rudder when not in use seems so much better.

The rudder fuses I have used have been 10-12mm dia soft wood pins with rope wrapped around them and held in a notch. Works on small boats, probably not reliable enough for a 50'ter. We have also used friction brakes and adjustable spring loaded detente pins, which work well on rudders mounted in horizontal axis rotating drums, could perhaps be made to work on your rudders. Send me some pics (harryproa@gmail.com) if you want to discuss it.

On the 24m/80' prototype cargo proa Cargo Ferry Prototype – Harryproa we have 3m deep side mounted rudder blades (no daggerboards or other leeway reducers) in cassettes. Getting them to kick up (and work in both directions and be liftable) was a challenge. The solution was a lightly loaded secondary line which releases and causes the main hold down line to release.

There are a couple of videos showing the set up and the results with the lightly set fuse, but not the blade or several other yet to be settled details at https://www.facebook.com/robert.rasmussen.319 Something like this might work for you, depending on your arrangement and space.

Valhalla,
We agree that it is dangerous to have non kick up appendages below the water on performance boats. How dangerous depends on how many heavy potential impacts are in the area you sail and how fast your boat is. Reefs are avoidable. Whales, logs etc not so much.
One at high speed is more than enough for me.

Dave S,
Could you describe the "shear point", please? ie what it is and where it is located.
I'm pretty sure the load required to fore and aft shear your board in a collision, while being strong enough for normal use sideways would result in a rapid deceleration, with the result I described.
How does the shearing work when the board is raised so the sheer off point is inside the case, but some board still sticks out?
A sheared off board is not trivial. What would a replacement cost?

My "agenda" is to get people thinking about and discussing safer, cheaper, easier to sail boats. It works best when the personal stuff is excluded.

Tupaia,
The Shuttleworth design shows it can be done, although only if the board is fully down (under the pin), which it won't be on most high speed points of sail? If it impacts a sold object at speed, above the bottom of the board, I cannot see it floating up until the boat has stopped or the board broken.
His rudder system should be on all cats and tris.

People sailing proas is nothing to do with my attempts to alleviate problems. It is more deep seated than that. Getting them to only install one daggerboard in a cat is a first step. ;-)

[Tupaia]

Originally Posted by rob denney

Tupaia,
The Shuttleworth design shows it can be done, although only if the board is fully down (under the pin), which it won't be on most high speed points of sail? If it impacts a sold object at speed, above the bottom of the board, I cannot see it floating up until the boat has stopped or the board broken. His rudder system should be on all cats and tris.

I have often thought of a system with multiple pins (3) at intervals up the case and a notch, overhang at the top of the board that engages with a pin. Since the back haul will already be slack after the board rotates from under the first, or second pin, there should be no force making it engage with the second or third pin as it rises in the case, especially given the forward inertia of the boat the board should be pushed against the the roller at the front of the case.

Originally Posted by rob denney

People sailing proas is nothing to do with my attempts to alleviate problems.

Just teasing

[tcpbob]

Daggerboards on a boat capable of decent speeds are a disaster looking for a place to happen.
If an immovable object (big fish, whale, log, container, etc) just below the surface is hit one of 3 things happens:

1) the boat stops. The crew carry on until they hit something or go over the bow. Google car crash test videos at 25 mph to see the forces involved at 21 knots.
2) the board sheers off and the object hits the rudder, prop or saildrive, almost certainly ripping them out.
3) the case is damaged and the hull fills with water.

rob, i hate to say it but that is alarmist. it is obvious you have a barrow to push here but don't let your personal, perhaps financial interests get in the way of common sense.



your array of unsupported assertions are embarrassing. build your boat anyway you want but respect others who differ, who are about a million to your one.

[catsketcher]

Considering I have owned my cat for 21 years and never attained 21 knots and she is pretty fast I feel it would be imprudent to have any mention of this scenario in the specifications of a cruising boat.

A cruising cat will most often only attain high speeds when sailing fast with the waves - going deep downwind. As most daggerboard cats need no or very little daggerboard on a broad reach the scenario about having a deep daggerboard down and going very fast is very rare. Kick up cassette rudders can be a real boon when they work well and a pain if they kick up too early. Mine are held down with plastic pipe and it works very well.

D Flawless is an example of a cat that fell apart after hitting a whale but she was awfully lightly built and sailed very quickly. Her circumstances have little relevance to most daggerboard cats.

We should remember that cruising cats are usually sailed by couples and families. Blasting along can only be done in the the most rare of circumstances, otherwise the crew mutinies and threatens to get off (at least mine did). Also a reasonable skipper always looks after the crew and the boat and so we rarely go full, stress flat out. The great thing about a nice daggerboard cat is that we can slip along, at 10 knot average (hitting 15s), under autopilot taking pictures, reading books and making food. Scenarios involving full daggerboards down at 21 knots are factually incorrect for the vast majority of cruising crews.

Remember that forces multiply by the cube. The energy in any moving body is related to the velocity squared. KE = 1/2 mv^2. But the time taken to accelerate (or decelerate when you hit a log) is halved at twice the speed. So the Power (or energy divided by time) increases by the cube when we go fast. What this means is the catastrophic scenario that occurs at 21 knots has 9 times less force at 10.5 knots. So going super fast is going to cost you, but considering few cats spend much time moving over 15 knots, having a severe design compromise because of this does not make sense. Slow down a little and the loads decrease greatly.

If you want to go fast with some dagger down then do what Shawn Arber recommended to me. I cut the bottom 30cm off the board end and then glued it straight back on. It gives a break point for the end. If you hit the bottom when full down it will probably break off first. Or when you need some board on a reach, put both boards down a foot or so and you have two fused board ends. I built the boards an extra 30cm long and have the spare ends in my shed. I have never lost a tip.

[JustMurph]

Originally Posted by rob denney

It's not. I did not mention mini keels (which have far more negatives than positives on a cruising, sailing multi), and don't know of any that are "capable of decent speeds".

Pretty much anything designed by Eric LeRouge. Properly fast cruising cats that sail well, with mini keels.

[Chotu]

Thanks, Rob. I guess my fuses were just a little too fragile. I’ll try making them more robust.


I don’t worry about impacts with my boards.

Since I built the whole boat, I know there are over half a dozen nicely done wraps of 36oz triax around the bottom part of my dagger board trunks. These very robust trunks are then inserted through the hull - right through both fiberglass faces and the core so they protrude out the bottom.

Then the protruding part was ground down to match the hull curvature outside the boat.

From there, any little gap was filled with epoxy and countless layers of 6oz cloth were applied to the join between the inside of the trunk and the outside surface of the hull.

Then a similar process inside, making fillets and layers of triax on the fillets.

Then there is also an “under the cabin sole” low profile bulkhead holding the trunk in place inside the boat.

Contrast all that with what the board is: a huge piece of foam with a single skin of 36oz triax and you quickly realize it’s impossible to have a problem even on a 50 knot impact. The board breaks WAY before the trunk and hull. It’s a fuse, no different from the fuse holding the rudders down.

Note: the large faces of the board have multiple layers of triax strategically placed to take the side loading of the boat under sail, but these have nothing to do with the board’s strength on impact from an underwater object.