Are trampolines REALLY necessary?

[purvisgs]

To the OP: you have to evaluate each boat/design individually, there is no cutoff point above which bridgedeck clearance (or lack thereof) becomes an issue for going 'offshore'...Things like beam (width), boat weight, keels vs boards, and much more come into play. This will become evident when you've spent some time on the water. Get out on other peoples boats; even monohulls as needed, ex: crew wanted lists, racing crew, whatever it takes - before you put $ on your own boat.


Whether a cat has a front hard deck or nets/tramps up front is just one factor in a particular design's seaworthiness.

[Bent Bugle]

There's a school of thought that says a solid deck is safer as far as burying the bows into a wave..more buoyancy... cats with trampolines can bury one bow & trip over sideways...also solid decks give you more deck area to work on up forward..more storage...they have their advantages..

[fxykty]

Originally Posted by Bent Bugle

There's a school of thought that says a solid deck is safer as far as burying the bows into a wave..more buoyancy... cats with trampolines can bury one bow & trip over sideways...also solid decks give you more deck area to work on up forward..more storage...they have their advantages..

“cats with trampolines can bury one bow & trip over sideways” ??????????????

WTF? That’s an absolutely baseless comment that has no basis in truth. If a cat is about to trip sideways, or pitch pole over the leeward bow or both bows, the presence of a trampoline or of a solid foredeck will make no difference at all. And in fact, that solid foredeck will make things worse due to windage.

It seems evident that for inshore, coastal, and gentle offshore conditions a solid foredeck can have benefits for accommodation and storage. But there is certainly no performance benefit - you don’t see any racing multihulls with solid decks bows to sterns now do you?

However, for larger seas and stronger winds the presence of a solid foredeck is dangerous. First, more loading from breaking seas. Second, more area to get caught by wind. Neither are good for stability. Again, you don’t see solid bridgedecks on performance sailing multihulls.

Prouts, Geminis, and the other classic solid foredeck designs, and now Balis, can certainly handle the conditions that a typical cruiser will experience. But don’t mistake that ability for superior safety or sailing performance.

BTW, what the heck does “solid decks give you more deck area to work on up forward” mean? The whole point of putting trampoline netting over the open areas between the hulls is to create a working platform. Making it solid doesn’t suddenly create more area.

[Bent Bugle]

[QUOTE=fxykty;3651319]“cats with trampolines can bury one bow & trip over sideways” ??????????????


Gees I think I hit a nerve..I think the "school of thought" is referring to small to mid size cats, comparing these old girls to large racing multis is ridiculous...the size & much greater roll moment of inertia of the big cats make them inherently resistant to capsize...you have to compare apples to apples...Read John Kretchmer's review of the Snowgoose & his comparism to some of the modern makes..also what would the likes of Alan Butler (26ft H?Twin), David Lewis ( Prout), or the Swales in their 30ft cat (all solid deck cats who circumnavigated) make of the assertion that these boats can be "dangerous, & meant for gentle offshore conditions".. not to mention the 100 estimated Snowgooses that have gone around ..without any capsizes.Yes, slow & at times uncomfortable, but mostly back in the days of no modern nav aids, weather routing etc & they went thru some horrendous weather.
On a recent passage in 30+ kts & large swell I had occasion to fix recalcitrant furling drum & was glad I was on a solid deck & not a net strung across the bows...their called trampolines for a reason...

[catsketcher]

Gday Bugle - Prouts have lots of nice traits but I think you would be hard pressed to get many to agree with your points.

In racing multis, decks are designed to be wave piercing. This is done to allow the bows to be underwater and at a negative angle of attack and reduce their tendency to create drag. When I interviewed Nigel Irens I asked him about fore and aft stability and he said the Tornado was one of the first cats that could go bow down, put its bow under the surface and not trip over. This is because the drag of the immersed bow is low and it is hard to trip with low drag.

This is why performance multis have reverse bows and rounded decks. I used to fang along in broad reaches in my Twiggy trimaran offshore. Its fine rounded float decks pushed under the swells on a broad reach - all fine as the drag of the immersed hull was quite low.

The reason I feel you will have very few people agree on your point about solid decks is because such decks are the highest drag form of cat bow if they become immersed. The angle of the under deck could provide some lift but like many lift devices at the front of the boat they can still create negative lift on a wave face.

Consider you are sliding down a steep wave at 15 degrees. The back of the wave in front is sloped up at 15 degrees. If the wave is short enough the cat quickly has to rotate through 30 degrees to climb up the wave in front. If your solid foredeck is immersed it could certainly provide some lift as you slide down the wave but it will not assist when you stuff the bows into the wave ahead. Now the big solid deck will act as a big tripping point with lots of drag. This is the reason why foils in trimarans are never put in the bows - even though they can provide lift, they can not provide enough lift for the most dangerous situation when stuffing bows into waves.

(Designer Lock Crowther famously put foils on the bows of his A class cat. When racing one day he went down a wave and the foils would notl et his cat rotate and he cartwheeled over so fast a competitor quipped "Do that again I blinked and missed it". An offshore trimaran, the Kelsal designed VSD, also had bow foils fitted and capsized in waves because it could not rotate at the wave bottom. Fortunately it did so right next to a ship and the crew were rescued. Designers learnt in the 70s and early 80s, ensure that devices that can produce negative lift are removed from the bows. Even though a foil, or a deck, can produce a positive angle of attack relative to the water it flows through, a boat offshore sometimes needs to rotate at an angle much greater than the lift generated by the foil or deck, so the foil or deck is then producng negative lift. If the foil or deck resists faster pitching up, it can reduce a cats ability to conform to the wave profile.)

The same goes for powering along on a reach. If a thin bow with tramps is immersed then the cat will slow down only slightly as the drag is increased, causing only a slight increase in the torque of the rig/drag of the hull couple. But, if a solid deck, or large flat deck is immersed, then the hull drag is increased and the torque of the rig/hull couple increases leading maybe to a pitchpole. Modern cats try to reduce these effects by reducing immersed drag.

Prouts get around this situation by having an aft mast rig (allowing more volume in front of the rig CE), narrow hull spacing (producing a design where the sails are reefed because the sideways stability is less than the fore and aft stability) and reducing overall rig size (to reduce rig moment). But if a cat with solid decks and one with tramps were to stuff their bows into waves equally, the solid deck one would be expected to have greater resistance to pitching up and climbing the back of the wave in front. That is usually fine because a Prout is usually slower than a tramp cat and so is less likely to stuff bows into waves anyway. Also the problem of bow stuffing is usually only found in short seas. In long wavelength swells the foredeck will probably only ever stay dry and provide positive lift. It is in short seas when running fast that the foredeck or foils will become an issue.

Cheers

Phil

[Bullshooter]

One thing I don’t get about the description of rotational inertia is - wouldn’t weight on the aft tend to pull the bows up out of a wave? Not that you want the boat to be unbalanced and aft heavy but doesn’t the rotational inertia work both ways?

In other words, would moving weight to the center of the boat decrease the ability of the weight in the aft end of the boat to pull the bows out if they were stuffed?

Another question I had is - do the listed catamarans with solid foredecks have a record of pitchpoling and capsizing? Or is this more theoretical?

[valhalla360]

Originally Posted by fxykty

BTW, what the heck does “solid decks give you more deck area to work on up forward” mean? The whole point of putting trampoline netting over the open areas between the hulls is to create a working platform. Making it solid doesn’t suddenly create more area.

"Better" might be more accurate than "more".

Though many tramps stop a few feet from the end of the bow. That's less common on solid decks.

Depends on the specific tramp material but they tend to be bouncy and have holes that a small foot can slip thru or even with a large foot, you can catch a toe and trip. If you want to stay on solid footing, you need to stay on the sides over the hulls or sometimes, there is a narrow walkway in the middle.

A solid deck doesn't give and doesn't have holes, so it provides better footing.

In a calm anchorage, not a big issue but working forward in rough conditions, it's a bit of an advantage but not the end of the world.

The big issue is when you get to tennis court size cats, a solid bridge deck extending to the bow represents a much larger weight penalty compared to a smaller narrow beam cat.

[valhalla360]

Originally Posted by Bullshooter

One thing I don’t get about the description of rotational inertia is - wouldn’t weight on the aft tend to pull the bows up out of a wave? Not that you want the boat to be unbalanced and aft heavy but doesn’t the rotational inertia work both ways?

Roll Moment of Inertia (ie: rotational inertia) is about how far weight is away from the center of rotation.

Rotational inertia is less of an issue with pitching vs rolling. There's always lots of weight well forward & aft of the center when looking at pitching.

Even with narrow cats, rotational inertia is favorable to cats vs rolling side to side.

Where it becomes a problem is narrow beam monohulls with rolling. If you remove the mast, it significantly reduce the rolling rotational inertia because all the weight is within a few feet of rolling axis.

The issue with burring a bow is while the bow is burried, isn't really about Roll Moment of Inertia. When the bow is burried, the wave on the stern is still push the stern up, so weigh in the stern isn't going to help. The buoyancy of a solid bridge deck will help lift the bow and in many cases, ther is a dynamic effect of the large flat bridge deck hitting the oncoming wave surface stopping it from going under in the first place.

Keep in mind, the boats we are talking about are not high strung performance machines. Some of the big high performance cats need to be designed for wave piercing because they are simply going too fast ride up and down the waves. Also being larger, they are more prone to bridging across short steep wave vs riding up and over them.

[fxykty]

Originally Posted by valhalla360

Roll Moment of Inertia (ie: rotational inertia) is about how far weight is away from the center of rotation.



Rotational inertia is less of an issue with pitching vs rolling. There's always lots of weight well forward & aft of the center when looking at pitching.



Even with narrow cats, rotational inertia is favorable to cats vs rolling side to side.



Where it becomes a problem is narrow beam monohulls with rolling. If you remove the mast, it significantly reduce the rolling rotational inertia because all the weight is within a few feet of rolling axis.



The issue with burring a bow is while the bow is burried, isn't really about Roll Moment of Inertia. When the bow is burried, the wave on the stern is still push the stern up, so weigh in the stern isn't going to help. The buoyancy of a solid bridge deck will help lift the bow and in many cases, ther is a dynamic effect of the large flat bridge deck hitting the oncoming wave surface stopping it from going under in the first place.



Keep in mind, the boats we are talking about are not high strung performance machines. Some of the big high performance cats need to be designed for wave piercing because they are simply going too fast ride up and down the waves. Also being larger, they are more prone to bridging across short steep wave vs riding up and over them.

Hmmm, of the big cats designed in the last few decades for racing around the world, only Philipps was designed for wave piercing and it didn’t even make the start line before breaking up. All the rest had tall high volume bows.

Back to the world of cruising cats, the solid bridgedeck examples are all relatively small and relatively slow, likely incapable of even double digit speeds. Mr B has entertained us with his stories of his voyages in his Gemini, which certainly took care of him and was safe enough. I don’t think anyone is arguing that. But the attributes of solid bridgedecks are not generally applicable to faster cats.

At higher speeds it’s the dynamic lifting effects of the bow sections and entire hulls, and the minimisation of form drag, that keep the cat from tripping over its bows. A solid bridgedeck would slam the boat to a stop and that could cause the boat to pitch pole. Same as hitting a whale at speed, or a sand bank close inshore, or the back of a steep wave.

[catsketcher]

Originally Posted by valhalla360

"Better" might be more accurate than "more".

Though many tramps stop a few feet from the end of the bow. That's less common on solid decks.

Depends on the specific tramp material but they tend to be bouncy and have holes that a small foot can slip thru or even with a large foot, you can catch a toe and trip. If you want to stay on solid footing, you need to stay on the sides over the hulls or sometimes, there is a narrow walkway in the middle.

A solid deck doesn't give and doesn't have holes, so it provides better footing.

In a calm anchorage, not a big issue but working forward in rough conditions, it's a bit of an advantage but not the end of the world.

The big issue is when you get to tennis court size cats, a solid bridge deck extending to the bow represents a much larger weight penalty compared to a smaller narrow beam cat.

I would respectfully disagree, if you have nice nets. You would need a large hole net to let a foot slip through and I don't know any nets that would do that. I personally do not like the popular Ferarri style nets common to many cats. They can rip along their whole length and a crewman is unable to hold on with fingers and toes.

A nice 2-3cm cod end net will not tear open like a Ferarri net and will allow you to grip with fingers and toes. I love sitting on the nets when sailing and at anchor. I would really miss the feel and vibe of lounging on nets as they are so much nicer to lie on than a deck, watching the bows split the water as we slide along. We even sleep on the nets as they can be so comfy.

As for small cats not worrying about the weight in the ends, again I would ask people to do a simple check before aggreeing. Put 100kg of water in the bows and also at the sterns and go sailing in a chop. I would be almost certain the boat would perform worse with the weight in the ends, mine always have.

As for rotational inertia being useful in a pitchpole situation - I don't think so. Rotational inertia is really only useful in beam on situations as increasing the rotational inertia slows down the response to rotation. This is a good thing in beam seas but not usually in cases where the boat has to pitch. Moving weight aft is helpful to avoid nosediving - dinghy sailors do this all the time, but it doesn't change the rotational inertia - rather it changes the centre of gravity and moving this aft in big waves is probably helpful.

Val - I am not sure what you mean by roll moment being favourable and your other statement about being favourable to cats vs side to side. There are a few different roll moments of inertia (rotational momentum). The pitch roll moment should be reduced to reduce hobby horsing. This is done by keeping weight out of the ends and keeping the mast light (and short). However cats can do very little to reduce athwartships roll moment of inertia (roll). This is because two heavy hulls and all they carry are far apart. So this axis has a high roll moment which is fine. It resists wave induced capsize on the beam which is a good thing.

The other effect the increase in heavy hulls with wide spacing have is on yaw moment. This is also high, which is a bit of a bummer when tacking but nice when offshore and wanting to stay straight. So weight out of the ends helps reduce pitch roll moment but has no effect on roll moment or yaw moment.

Monos have the mast add huge amounts of roll moment athwartships (roll), which is where the mast's effect comes in. But cats have such huge amounts of righting moment sideways that the effect of the mast on damping sideays motion is much reduced compared to a mono, which is why sailing cats make such good lightweight motor cruisers with just a little volume added at the bum.

cheers

Phil

[Strygaldwir]

Safety, comfort and fun, these were/are the most important things to me when I am cruising. So often we get caught up in notions of speed, and heavy weather sailing. If you're cruising, you'll spend a lot of time avoiding heavy weather and learn speed isn't all that important. Pretty much guarantee I can get there faster by plane/jet.

Having opined, one of the things I love doing is keeping watch while laying on the trampolines. The autopilot keeps course far better than I and the remote lets me make helm corrections if I see something in the water. And when a pod of dolphins joins you for a bet of fun, THAT's the best!

Sigh, got to get back out to sea!

Keith