Trampoline Vs. Solid Foredeck

[catsketcher]

The idea that a flat area up front would make a positive contribution to dynamic stability is not correct. It's a pretty dangerous way to get upwards pitching in a running downwind situation.

If you have a look at the all of the latest racing cats and tris they all have reverse bows. The idea behind the reverse bow is to decrease the deck area as deck area will contribute a massive increase in drag when the deck immerses. Adding greater volume up top is not what you want when a bow gets immersed.

But for our cruising cats we will rarely (if ever) immerse the bows. My 38ft cat with its forebeam, tramp nets, seagull striker and cleats does not have a bow that travels well through water when immersed. This is my biggest problem with reverse bows on cruisers, they don't work because of all the other draggy parts that are installed on a cruiser.

However putting a large flat area up the very front of the boat troubles me. Unless it is angled up at around 25 degrees it will not be able to make a positive contribution to fore and aft stability at the bottom of the wave. This problem is why foils on racing tris and cats are not put up the bows. It was done in the late 70 and early 80s but could lead to very fast pitchpoling in waves. You don't want anything unnecessary to act in a downward vector as the boat goes from nose down to nose up at the bottom of the wave.

My take on nets - my black cod ends have lasted 18 years and will be replaced soon but they look great and are still going strong - people love lounging on them and I can hold on in waves with my fingers and toes. They are light and that is important for pitch resistance. Best of all, I can just lie on them (on a bean bag) and watch my bows cut the water for hours and hours. How do you watch the bows when the deck is solid?

[contrail]

I don't know if I posted on the original post, probably did, but I am still here, too! And, I do know of power cats (with solid decks forward) that have had to slow down a lot when going upwind in the Caribbean (open seas, not between islands) because their decks caught lots of water. Not personal experience, but that of fairly reliable friends in the industry, so take it for what it is worth.

[Nelsons]

I was close to sign for a Bali 4.5 but I went for another brand with trampoline. I changed my mind after a conversation with a designer, who is very familiar with current cat design. They believe that in heavy seas the water pressure from the waves underneath might cause a risk for the stability. Big point as I will sail offshore. I think for charter in smooth waters it offers a nice comfort. But as owner you have to get to places first including rough seas.

[Captain Bill]

I did a quick measurement of the angle of the flat area under the foredeck on my Endeavourcat and it appears to be about 20 degrees, which is not too far from catsketcher's 25 degrees. I've taken green water over the bow, but only in shallow water in very steep short waves, so there was never any danger of pitch poling. In open ocean I have never managed to bury the bows. I suppose it's possible to build a wave that steep in open waters, but I'be never experienced it. I have surfed a couple of times on the foredeck in conditions where I've seen the bows go under in tramp equipped cats but I've not managed to bury the foredeck(yet). I've also noted that when the bows are buried that deep the boat seems to slow down or at least not accelerate much, probably due to the increased wetted surface.

I would like to be clear that I consider my boat a coastal cruiser, not an ocean crossing cat. The boat has too much windage and too high a center of gravity. I don't think the solid foredeck is as nearly a big an issue as those two things.

[Captain Bill]

I just looked at a youtube video of a Bali 4.5 at the Annapolis Boat show. No wonder You ran away from it nelsons, that is a scoop, not a solid foredeck. A solid deck like on my Endeavourcat is completely different. There is nothing on my deck that will hold water and the underdeck space provides a tremendous amount of buoyancy should it be submerged. I would be very concerned with the stability If I had a large water scoop across my foredeck.

[NPCampbell]

Originally Posted by catsketcher

However putting a large flat area up the very front of the boat troubles me. Unless it is angled up at around 25 degrees it will not be able to make a positive contribution to fore and aft stability at the bottom of the wave.

I was at the Annapolis boat show a couple weeks ago. The top of the foredeck on the Bali 4.5 is flat however, underneath starting with the leading edge of the foredeck there is around a 20-30 degree wedge underneath.

[NPCampbell]

Originally Posted by Captain Bill

I just looked at a youtube video of a Bali 4.5 at the Annapolis Boat show. No wonder You ran away from it nelsons, that is a scoop, not a solid foredeck.

The 4.5 didn't make it to the 2018 show Cyan re-started this thread because of my interest in the weight of the Bali 4.5 vs Lagoon 450s.

Originally Posted by Captain Bill

A solid deck like on my Endeavourcat is completely different. There is nothing on my deck that will hold water and the underdeck space provides a tremendous amount of buoyancy should it be submerged. I would be very concerned with the stability If I had a large water scoop across my foredeck.

I'm new to this but I'm assuming the problem here is running downwind and then picking up speed by surfing down a wave and crashing into the wave at the bottom of the trough?

Just looking at this from an engineering standpoint, I am ballparking that a 1 sq ft foil traveling at 20 kts can generate greater than 1000 pounds of lift? Correct me if I am wrong ...

Taking this into account, the front of the Bali must have at least 150 sq ft of giant 20-30 degree wedge starting with the leading edge of the foredeck ... just like your Endeavorcat. It would seem that the hull dynamics would generate a massive upward force which would completely dominate all other forces until the speed of the boat slowed down at which point buoyancy would take over. Like your Endeavorcat, it probably has a sh**ton of buoyancy upfront due to that massive wedge compared to a catamaran with just tramps.

I'm sure someone will blow holes in my theory though but that's what I'm here for

[danielamartindm]

Originally Posted by Southern Star

The initial post inquired soley about the relative merits of solid foredecks versus trampolines, but of course 44'Cruisingcat is correct - in order to improve performance and reduce 'hobbyhorsing', you need to keep weight out of both ends.

An even more important consideration on smaller cats, which are most prone to hobby-horsing in the first place.

[valhalla360]

Originally Posted by thinwater

Having sailed on both, I think I find tramps more fun. It doesn't hurt if you fall. They are more comfortable for lounging and that really is important. It can be absolutely zen to hang you head over the front and watch the water go by. I believe you are less likely to wash off, because there is no crown and the water is pushing you down. We left a boat hooks up there, unrestrained, with green water many times, and never lost one, on two different cats. Yes, the footing is "different," but not alarming to a multihull sailor. With the quick motion of a cat in rough sees, I like it better.

Of course, I've not fallen on our solid bridge deck cats ever but have both times I was out on a friends tramp I did.

I've lay down on the bow and stuck my head over the edge to watch the water and anchored, it's a great place to put out cushions or even lawn chairs to watch fire works.

Not that I would stick my nose up if offered a cat with tramps but I don't see them as any more comfortable or convenient.

Slamming is an issue in the right conditions but annoying...never felt the boat was in any danger.

[Sos]

I have not had experience of running down a wave and burying the bows but I have had large waves over the front quarter on a 40' Wharram. The waves eventually broke the front laminated cross beam that the netting and walkway were fitted too. I dread to think what it would have been like with a solid foredeck. I am more than happy with netting and would not have a solid deck if going bluewater.
Prouts however have solid foredecks and they are good solid, albeit, slowish boats.

[catsketcher]

Originally Posted by NPCampbell

The 4.5 didn't make it to the 2018 show Cyan re-started this thread because of my interest in the weight of the Bali 4.5 vs Lagoon 450s.



I'm new to this but I'm assuming the problem here is running downwind and then picking up speed by surfing down a wave and crashing into the wave at the bottom of the trough?

Just looking at this from an engineering standpoint, I am ballparking that a 1 sq ft foil traveling at 20 kts can generate greater than 1000 pounds of lift? Correct me if I am wrong ...

Taking this into account, the front of the Bali must have at least 150 sq ft of giant 20-30 degree wedge starting with the leading edge of the foredeck ... just like your Endeavorcat. It would seem that the hull dynamics would generate a massive upward force which would completely dominate all other forces until the speed of the boat slowed down at which point buoyancy would take over. Like your Endeavorcat, it probably has a sh**ton of buoyancy upfront due to that massive wedge compared to a catamaran with just tramps.

I'm sure someone will blow holes in my theory though but that's what I'm here for

Yeah the theory does not stack up. The problem is pitchpoling. You don't want to "stub the toe " and trip over the bows and this is a tricky problem.

You don't really want to get extra dynamic lift from the front deck hitting the water. This means the bows are too low already and if the situation is changing quickly then any lift generated may be too late to stop a rotation stern over bows. You want extra stability to start way earlier at lesser levels of bow down angle.

Then as soon as you get any water onto the deck top you are in a very bad way. Any drag up the front of a cat is bad news once the bows are under. That is why racers have crowned decks, reverse bows and no forebeam. So most cruisers go for nets to easily reduce drag with bows under, amongst many other advantages.

I have had a look at the pics on the Bali 4.5 and once the front deck goes under it would have the same problems as any solid deck. The shape of the underside would be of no issue but it may be helpful when heavily pressed.

When cats were designed to primarily sail well no one would have a solid deck. They could cause problems with the boat feelling skittish when going to windward (a sort of high pressure zone caused by the lack of anywhere for the contained air to go), allowing waves to move through the netting but weight of a deck is a killer for me. Even if you let me build the deck out of nomex and carbon I could not get it anywhere near the light weight of a net. And weight at the ends of a boat, especially the bows, makes the boat hobbyhorse much more badly. So a solid deck would compromise the handling characteristics to windward as well as the safety in waves.

But in some cats, they would not compromise the boat as much.

cheers

Phil

[cyan]

Originally Posted by Captain Bill

I just looked at a youtube video of a Bali 4.5 at the Annapolis Boat show. No wonder You ran away from it nelsons, that is a scoop, not a solid foredeck. A solid deck like on my Endeavourcat is completely different. There is nothing on my deck that will hold water and the underdeck space provides a tremendous amount of buoyancy should it be submerged. I would be very concerned with the stability If I had a large water scoop across my foredeck.

Also, I believe the beam on a Bali 4.5 is almost 6 feet wider than the beam on an Endevour Sailcat 44, yet both with a similar length. Simply put, the solid foredeck on one boat has massively more surface area than the other. Maybe there is a design tradeoff: heavy weather vs heavy drinking?

[NPCampbell]

Originally Posted by catsketcher

And weight at the ends of a boat, especially the bows, makes the boat hobbyhorse much more badly. So a solid deck would compromise the handling characteristics to windward as well as the safety in waves.

I'm a technical guy so I am having problems understanding how adding weight to the ends of a boat can cause hobbyhorsing. I don't know if this is myth or there is good science behind this to explain it.

My take on it is:

Every boat will have center of gravity (CG) and a center of buoyancy (CB). The CB is below the CG. If both centers don't align vertically then you will experience a righting moment based on the angle between the CG and CB and the displacement of the boat.

If I add 1000 pounds on the CG of a boat or 500x2 pounds on either side of the CG at the same distance the CG is the same for both cases and the CB is the same for both. If the CG and CB doesn't change how can the righting moment change? If righting moment doesn't change then what would be causing the hobbyhorsing?

Or looking at this from a simple harmonic standpoint, the angular frequency of a simple pendulum or metronome is basically SQRT(g/l) where l is the distance of the bob from the fulcrum or pivot point. Increasing the distance of the bob LOWERS angular frequency in the same way that a spinning skater extending her arms slows down the rate of rotation.

What gives?

[fxykty]

I can’t speak to the physics, but all racers and some cruisers have experienced the increased hobby horsing when there is weight in the ends of the boat. The CG and CB move a fraction then stabilise, but when moving in waves the boat with a 35kg anchor on the tip of the bow will pitch a greater distance and more slowly than the same boat with the anchor in the bilge behind the mast. Is it the greater inertia of the weight further from the CB?

For the same reason sailors want to reduce weight aloft - not only for improved righting moment but also to decrease the distance of the rolling. Witness that the solution to extreme rolling is to move weight from the ends to the middle - eg move people from the flybridge to the cockpit or into the cabin.

[cyan]

Originally Posted by NPCampbell

I'm a technical guy so I am having problems understanding how adding weight to the ends of a boat can cause hobbyhorsing. I don't know if this is myth or there is good science behind this to explain it.

My take on it is:

Every boat will have center of gravity (CG) and a center of buoyancy (CB). The CB is below the CG. If both centers don't align vertically then you will experience a righting moment based on the angle between the CG and CB and the displacement of the boat.

If I add 1000 pounds on the CG of a boat or 500x2 pounds on either side of the CG at the same distance the CG is the same for both cases and the CB is the same for both. If the CG and CB doesn't change how can the righting moment change? If righting moment doesn't change then what would be causing the hobbyhorsing?

Or looking at this from a simple harmonic standpoint, the angular frequency of a simple pendulum or metronome is basically SQRT(g/l) where l is the distance of the bob from the fulcrum or pivot point. Increasing the distance of the bob LOWERS angular frequency in the same way that a spinning skater extending her arms slows down the rate of rotation.

What gives?

This is a repeated debate here and in other forums, but unlike anchors and guns, there is always a winner: All things being equal, more weight in the bow and stern will hobbyhorse MORE than when the same weight is closer to the center. Almost everyone that tests it in the real world on their own boat arrives at this conclusion, it seems. The theoretical debate is always fun, because it does seem a bit counterintuitive.

I think the harmonic equation argument for a pendulum (or seesaw) misses a key point- a pendulum is powered by the steady force of gravity. Conversely, a bow or stern has rhythmic (non-steady) forces acting upon it. Simply, entering a rising wave will cause pitching in either a heavy bow or a light bow. While the heavier bow might not pitch as much at first, it will keep going up with MORE MOMENTUM than the lighter bow would. Yes, the harmonic frequency is lower, but that just means that the bow is now rising later in the wave cycle, and thus has farther to fall into the trough, with mass and momentum finally going downward but later (and farther). Add the stern to this effect, with the right wavelength, and the hull pitch can oscillate to the point where it seems comically exaggerated. Hobbyhorse. Reduce the weight at the bow/stern, and reduce this effect.

Disclaimer- that's all just the musings of a bored engineer playing around with bow weight on a lazy afternoon's sail...