Are trampolines REALLY necessary?

[GordMay]

Originally Posted by ItDepends

You might seriously to look at where you are getting your blue water recommendations...
... Just to be clear: "blue water sailing" is crossing oceans on passages more than 500 miles from shore. Not sailing along a coast in the ocean. NOT sailing in the Bahamas, and NOT sailing in the Med ...

I don't disagree, but would liberalise your definition, to include any passage "out of cell phone range", salt, or fresh water.

[valhalla360]

Originally Posted by theBigKahuna

This is IMHO. But, the Gemini's we've seen used gas motors. My understanding is there are diesel now though.

But, on long passages, I, again IMHO, would avoid gas. Gasoline and plastic hulls are a poor combination.

On a monohull where the fuel tank and engine are buried down in the hull, this makes some sense. Any fuel leaks wind up with fuel in the bilge and gasoline fumes in the hull. Of course, with proper procedure, it's not hard to mitigate but it is something to be aware of.

On the Gemini and most other outboard powered small cats, the issues largely go away.
- The engine and it's fuel system are completely clear of the the hull so any leaks will be completely clear of the hull.
- The fuel tanks are on the bridge deck and bottom vented, so any leaks will be completely clear of the hull.

Think of it like a gasoline car. You don't worry about a fuel leak blowing the car up because any leak just drips onto the ground, evaporates and blows away harmlessly (not that you want leaks).

[catsketcher]

Originally Posted by valhalla360

On a monohull where the fuel tank and engine are buried down in the hull, this makes some sense. Any fuel leaks wind up with fuel in the bilge and gasoline fumes in the hull. Of course, with proper procedure, it's not hard to mitigate but it is something to be aware of.

Think of it like a gasoline car. You don't worry about a fuel leak blowing the car up because any leak just drips onto the ground, evaporates and blows away harmlessly (not that you want leaks).

Diesels are nice, but outboards on cats are perfectly safe because of what you say. Not so on a mono, but my cat uses outboards which vent straight down and into the sea. (My mate who has diesels on his new boat longingly talked of the good old days with his old boat and its easy to service and highly reliable outboards - modern 4 stroke outboards are incredibly good). My cat just never smells of fuel and the diesel smell from inboards does me in a bit offshore when sailing other boats.

I respect other people's opnions, but personally, and I repeat this is a personal reaction, I would steer clear of cats with no tramps. My 38ft cat can easily feel the difference with the weight of a person up front in a seaway. I pull the 22kg anchor off the bows when offshore because the boat rides better. Weight in the ends is usually a bad think for hobby horsing and the weight of a glass deck is many tens of kilos and lots more than a net. So you will hobby horse more than if you have nets.

Also the type of cat with a glass deck is going to be a slower cat and less sailing orientated design than a trampoline cat. So it is a bit of a giveaway about the design and build intent of the model with similar trade offs made throughout the design and build process. Not bad - but it lets you know the type of cat you are getting.

cheers

Phil

[valhalla360]

Originally Posted by catsketcher

I respect other people's opnions, but personally, and I repeat this is a personal reaction, I would steer clear of cats with no tramps. My 38ft cat can easily feel the difference with the weight of a person up front in a seaway. I pull the 22kg anchor off the bows when offshore because the boat rides better. Weight in the ends is usually a bad think for hobby horsing and the weight of a glass deck is many tens of kilos and lots more than a net. So you will hobby horse more than if you have nets.

Also the type of cat with a glass deck is going to be a slower cat and less sailing orientated design than a trampoline cat. So it is a bit of a giveaway about the design and build intent of the model with similar trade offs made throughout the design and build process. Not bad - but it lets you know the type of cat you are getting.

cheers

Phil

Particularly on small cats, high performance means weight is a huge issue. To eliminate weight means eliminating accommodations and comfort.

We are cruisers, so we carried 2 anchors on bow rollers ready to deploy. We also had a dingy on davits. We typically planned to travel at a brisk walking speed, though we have hit double digits (not surfing). Having to remove and stow the anchors every day trip would have been a non-starter. Never had any hobby horsing issues.

If you are racing with minimal stuff on board, I can see the push to get a stripped down boat to go fast. Not so much for cruising but that gets into personal purpose for buying a boat.

[smj]

Originally Posted by valhalla360

Particularly on small cats, high performance means weight is a huge issue. To eliminate weight means eliminating accommodations and comfort.



We are cruisers, so we carried 2 anchors on bow rollers ready to deploy. We also had a dingy on davits. We typically planned to travel at a brisk walking speed, though we have hit double digits (not surfing). Having to remove and stow the anchors every day trip would have been a non-starter. Never had any hobby horsing issues.



If you are racing with minimal stuff on board, I can see the push to get a stripped down boat to go fast. Not so much for cruising but that gets into personal purpose for buying a boat.

The Gemini is very sensitive to weight and in my opinion would have been a great candidate for trampolines rather than the solid foredeck. The Prouts and Catalacs maybe not as much?

[catsketcher]

Gday Val

My boat is 4500kg loaded and 38ft long. I wouldn't call her small and I am an ex racer, so I have a background where performance matters.

Even on big 40ft monos racing, you can feel the effect of the bowman up the bow, changing headsails. It really makes the boat cranky and when on the helm you want the change to be quick so the crew can back nearer the rotational centre of the boat. It is strange that such a small proportion of the boats weight can make a discernable change. But getting weight out of the ends reduces rotational momemtum and reduces how hard the bows sticks into the wave when going to windward.

As an ex-racer I a bit religious about this - I remove my prodder sailing to windward, built the boat with water tanks in the middle, keep the ends of the boat as light as possible (we even have a light dinghy) and the boat sails very nicely to windward. I should have made it clear that I usually leave the anchor on the roller when coastal sailing but move it back - (like when we crossed Bass Strait), when we get further off the coast. It only takes a minute (and is one reason why I have an old fashioned "on top of the beam" bow roller) and makes a discernable difference to comfort.

Getting weight out of the ends of the boat is a huge benefit to performance and usually comfort too. By reducing the rotational inertia of a boat, it takes less bow being depressed to lift the bows over a wave. If there is more weight in the ends, then the bow is harder to lift and therefore requires more immersion to obtain a greater bouyant force.

(Think of it like holding two 5kg weights on a 2 metre pole. When the weights are close together near your hands it is easy to rotate the pole back and forth quickly. Slide the weights out to the ends and try to jiggle the pole - it resists the motion far more - its rotational inertia has increased.) Usually a boat designer wants to reduce rotational inertia when going to windward but if it gets too low the motion can be too fast - which is why large monos have a smoother motion to windward. But normally a boat with less rotational inertia is nicer offshore.

Each to their own but I love the feeling of a boat that lifts lightly to the waves. It makes me happy to sail to windward and the boat seems to like it too.

cheers

Phil

[Captain Bill]

My Endeavourcat 44 is a solid foredeck cat. I have had only one experience sailing down the face of waves where I was afraid I might bury the bow in large seas. The bridgedeck between my bows is essentially a flat slab angled at about 30 degrees. What I experienced is that the hulls buried down to the point that the angled slab hit the water and would go no further as the extra buoyancy from the bridge deck and the dynamic lift of the bridgedeck planing on the water prevented the bow from submerging any further. I have never buried the bow in large offshore waves. I find that very short period shallow water waves are much more of a problem for my boat. I have taken green water over the dodger when caught out in 7ft waves in 20 ft of water. That has never even come close to happening offshore. Waves above 2ft and below 4ft are the worst problem when it comes to wave slap.



One other thing is that I wouldn't rely on the evaluation of delivery skippers in evaluating cruising cats. They are almost invariably lightly loaded and don't reflect the reality of a cruising boat. As was said earlier Catanas are great boats. That is when they are lightly loaded. When you get a heavy load of cruising stores on board, not so much. I was once sailing south of Nassau headed for Highbourne Cay when Catana came out of a Marina about a half mile behind us. I remarked to the admiral that we would soon be seeing the stern of the Catana. I was quite wrong in that assessment. We were well over three miles ahead of her when we reached Highbourne. I later talked to the owner and apparently his cruising stores had much more effect on the thin hulled Catana than it did on my rather fat hulled Endeavourcat.



As for performance, my boat has won 1st place in Class B multihulls four out of the five times entered in the Georgetown round the Island race. The only time I lost I had an equipment failure. In the in harbor race the only catamaran that has beaten me was another Endeavourcat 44. I have even passed many of the class A catamarans and they start 10 minutes ahead of class B. I only one time won by handicap and that was against two trimarans that finished ahead of me in real time.



I freely admit that I consider my boat to be a coastal cruiser and the highest seas I have her in were about 12-13 ft. I don't know how she would do in 25ft seas. I do know one person who did have an Endeavourcat 34 in 25ft seas and he said it did really well and he never buried the bows.


My 2 cents, change welcome.

[Tupaia]

Consider looking at Pat Patterson - twins's. These tough little boats ranging from 26ft (Heavenly Twins) 33ft (Ocean wins) and 38ft (Ocean Twins) have all circumnavigated and have solid foredecks. These like Prouts were designed for couples or families and were pioneers limited by materials tech at the time but have proven safety records.



Most of the cats with solid foredecks do not go fast enough to bury a bow even when surfing. Look at the sail power to weight ratio and although cats can exceed hull speed it is till a limiting factor on short cats.



I have once buried a cat up to the mast in the back of a wave, it had trampolines and we were sailing in the upper teens in 8m breaking seas (stupid really). Completely different aspects of design apply like being able to shed water quickly through the trampoline, rounded decks and having enough forward buoyancy like hull flare to surface quickly and a gradual increase in drag, not a sudden stop that would promote a pitch-pole.

[valhalla360]

Originally Posted by catsketcher

Gday Val

My boat is 4500kg loaded and 38ft long. I wouldn't call her small and I am an ex racer, so I have a background where performance matters.
...........

If you have a racing boat and are trying to achieve racing speeds on a regular basis...yeah, being obsessive about weight and where it sits makes sense.

For your average cruising catamaran, it's far less critical. I will readily admit, we sailed our Gemini a few times beginning of the season when it was near empty, and yes it was noticeably faster...but as cruisers we weren't going to cut the proverbial toothbrush handles off just to get an extra 1/2kt.

My experience is very similar to Capt Bill's. Mostly in moderate conditions with short steep waves where it's annoying but not a safety issue. Only place we ever took water over the bows was on the Mississippi River...A big 6x7 tow upriver was throwing a 10ft wake and there wasn't room to move further away without messing with a wing dam.

[catsketcher]

I wouldn't call your typical cruising Grainger, Schionning or Chamberlin a race boat. They sure have racing DNA in the genes but they can take a good load without at all being obsessive as shown by Fyxty and as our livaboard for years with a family of four and stores for months. This is why they make such a change from their earlier iterations like the Crowther Spindrifts . My point was that even a largish boat weighing about 4 to 5 tonnes will feel added weight on its ends because of the physics of rotational inertia. Reducing weight in the ends will have no effect on the waterline but substantial effect on the boat's movement out at sea.

It is a subtle effect that many people forget when building some boats, and they can put chain lockers and water tanks fore and aft to balance each other. This can lead to significant changes to the boat's dynamic behaviour with no change in waterline.

Adding to the issue is the fact that increased sideways rotational inertia is a good thing. Having the weight spread out from the rotational axis in beam seas, increases a catamaran's resistance to wave induced capsize when compared to trimarans. Even though tris are wider, they are more prone to wave induced capsize as they have most of their weight in the main hull, and have lower athwartships rotational inertia. This makes tris faster to rotate sodeways and more prone to sideways wave action.

So a designer should try to increase the sideways rotational inertia (for beam sea resistance) but decrease fore and aft rotational inertia (to decrease pitching). The best way to do this is to get the heavy items low down and in the middle of each hull, not centred in the bridgedeck. Then you get the best of both worlds.

[smj]

Originally Posted by catsketcher

I wouldn't call your typical cruising Grainger, Schionning or Chamberlin a race boat. They sure have racing DNA in the genes but they can take a good load without at all being obsessive as shown by Fyxty and as our livaboard for years with a family of four and stores for months. This is why they make such a change from their earlier iterations like the Crowther Spindrifts . My point was that even a largish boat weighing about 4 to 5 tonnes will feel added weight on its ends because of the physics of rotational inertia. Reducing weight in the ends will have no effect on the waterline but substantial effect on the boat's movement out at sea.

It is a subtle effect that many people forget when building some boats, and they can put chain lockers and water tanks fore and aft to balance each other. This can lead to significant changes to the boat's dynamic behaviour with no change in waterline.

Adding to the issue is the fact that increased sideways rotational inertia is a good thing. Having the weight spread out from the rotational axis in beam seas, increases a catamaran's resistance to wave induced capsize when compared to trimarans. Even though tris are wider, they are more prone to wave induced capsize as they have most of their weight in the main hull, and have lower athwartships rotational inertia. This makes tris faster to rotate sodeways and more prone to sideways wave action.

So a designer should try to increase the sideways rotational inertia (for beam sea resistance) but decrease fore and aft rotational inertia (to decrease pitching). The best way to do this is to get the heavy items low down and in the middle of each hull, not centred in the bridgedeck. Then you get the best of both worlds.

I’m guessing having a rig with reduced weight (carbon fiber spar and dyneema rigging) would also reduce motion significantly and also reduce risk of capsize?

[Dave_S]

Originally Posted by catsketcher

I wouldn't call your typical cruising Grainger, Schionning or Chamberlin a race boat. They sure have racing DNA in the genes but they can take a good load without at all being obsessive as shown by Fyxty and as our livaboard for years with a family of four and stores for months. This is why they make such a change from their earlier iterations like the Crowther Spindrifts . My point was that even a largish boat weighing about 4 to 5 tonnes will feel added weight on its ends because of the physics of rotational inertia. Reducing weight in the ends will have no effect on the waterline but substantial effect on the boat's movement out at sea.

It is a subtle effect that many people forget when building some boats, and they can put chain lockers and water tanks fore and aft to balance each other. This can lead to significant changes to the boat's dynamic behaviour with no change in waterline.

Adding to the issue is the fact that increased sideways rotational inertia is a good thing. Having the weight spread out from the rotational axis in beam seas, increases a catamaran's resistance to wave induced capsize when compared to trimarans. Even though tris are wider, they are more prone to wave induced capsize as they have most of their weight in the main hull, and have lower athwartships rotational inertia. This makes tris faster to rotate sodeways and more prone to sideways wave action.

So a designer should try to increase the sideways rotational inertia (for beam sea resistance) but decrease fore and aft rotational inertia (to decrease pitching). The best way to do this is to get the heavy items low down and in the middle of each hull, not centred in the bridgedeck. Then you get the best of both worlds.

I'm not sure I get it.

If you add weight at the ends of the hulls you increase the the amount of pitching but reduce the harshness of the boats action. If you increase the weight laterally wouldn't the same apply and increase the amount of roll which would increase the likelihood of the hull digging in ?

[catsketcher]

Gday Smj

Caveat first - I have only designed two cats (built 3 and one tri), but did learn lots during the long processes of design and building. (and redesigning and more building.....)

Reducing mast weight is a great way to reduce BOTH axes of rotational inertia. So having a shorter mast. or a lighter rig will definitely reduce pitch moment. One of Australia's good sailmakers (and world champion crew in Solings) used to mouse every wire halyard with venetian blind cord on long windward races like the Hobart. He found it improved the speed and motion to windward. Just the halyards.

Mono design has changed because of lighter rigs. Now boats can have finer bows because the rigs are so much lighter that you don't need as much voume up front to get the bows over (or through) waves. So absolutely, if your boat is pitching, drop the Code 0 and reduce weight by using light halyards and lighter staying material like Dux.

But for monos there is an issue - back in the 1930s, the Americas Cup boats used to have to cross the Atlantic on their own bottom. Thomas Lipton's Shamrocks were towed across with no racing mast up. But they would roll horribly, so the crew would have a stub mast and winch a concrete weight up the stub to INCREASE the rotational inertia and slow the roll. Anyone who has been dismasted on a mono knows this - all of a sudden the boat jiggles about like crazy.

But in a cat, our boats have so much righting moment at low angles of heel that there is little change in sideways motion with reduced mast weight (or no mast at all - witness cat power cruisers). So unlike monos, I feel that there is little to be gained by keeping some weight in the rig for motion's sake. So get rid of rig weight, don't have a massive mast if not tyoyu reef all the time, drop the Code 0 when heading into the wind, and move the anchor locker aft if possible.

[catsketcher]

Originally Posted by Dave_S

I'm not sure I get it.

If you add weight at the ends of the hulls you increase the the amount of pitching but reduce the harshness of the boats action. If you increase the weight laterally wouldn't the same apply and increase the amount of roll which would increase the likelihood of the hull digging in ?

Gday Dave

Same caveat as above.

Athwartships and fore and aft stability are different things. Cats have HUGE athwartships compared to fore and aft stability. Witness the visible change in trim when you put a crowd of people down the back of a cat - say when we have a crowd on the boat to go watch fireworks and they sit in the cockpit - I can feel the bum sink down.

But when they all go to one side of the boat to watch the fireworks there is much less change in angle. Cats are far more resistant to changes in heel than changes in trim - winch a heavy dinghy up the side and you can't see much but do the same out the back and your bums get immersed. The athwartships stability is huge compared to fore and aft stability. Sideways the righting moment at low heel is massive but fore and aft it is much like (or even lower than) a monohull.

So because cats are so stable for and aft (because of their wide beam) this easily compensates for the increase in athwartships rotational inertia (but you are right - a cat with most of its weight in the centre - a pod cat - would be quicker in its sideways motion). I think the gains in beam sea capsize resistance are worth the slight increase or delay in responding to beam seas.

cheers

Phil

[smj]

Originally Posted by catsketcher

Gday Smj



Caveat first - I have only designed two cats (built 3 and one tri), but did learn lots during the long processes of design and building. (and redesigning and more building.....)



Reducing mast weight is a great way to reduce BOTH axes of rotational inertia. So having a shorter mast. or a lighter rig will definitely reduce pitch moment. One of Australia's good sailmakers (and world champion crew in Solings) used to mouse every wire halyard with venetian blind cord on long windward races like the Hobart. He found it improved the speed and motion to windward. Just the halyards.



Mono design has changed because of lighter rigs. Now boats can have finer bows because the rigs are so much lighter that you don't need as much voume up front to get the bows over (or through) waves. So absolutely, if your boat is pitching, drop the Code 0 and reduce weight by using light halyards and lighter staying material like Dux.



But for monos there is an issue - back in the 1930s, the Americas Cup boats used to have to cross the Atlantic on their own bottom. Thomas Lipton's Shamrocks were towed across with no racing mast up. But they would roll horribly, so the crew would have a stub mast and winch a concrete weight up the stub to INCREASE the rotational inertia and slow the roll. Anyone who has been dismasted on a mono knows this - all of a sudden the boat jiggles about like crazy.



But in a cat, our boats have so much righting moment at low angles of heel that there is little change in sideways motion with reduced mast weight (or no mast at all - witness cat power cruisers). So unlike monos, I feel that there is little to be gained by keeping some weight in the rig for motion's sake. So get rid of rig weight, don't have a massive mast if not tyoyu reef all the time, drop the Code 0 when heading into the wind, and move the anchor locker aft if possible.

Thanks Catsketcher,
The reason I bring this up is we have owned numerous cruising catamarans, but none that have had the gentle motion of the one we own now, both rolling/beam to beam motion and pitching motion. I put this of on partially the design of boat, but also the greatly reduced weight aloft.