Forefoot in or out of the water?

[kengrome]

I'm designing a paddle powered trimaran for nearshore fishing. The boat is inspired by the local 'banca' boats of the Philippines. It's almost 16 feet long and the main hull is about 2.5 feet wide. I've shown this boat with its forefoot out of the water, but others have questioned this feature.

Should I keep the forefoot out of the water on a slow paddle powered boat like this, or should I re-draw it to make the forefoot fully sibmerged? And why?

[Guest625101138]

Straight rocker is more easily driven at displacement speed. If there is potential for surfing then having some lift in the bow will help.

Having the outriggers in the water is energy wasting. If it is intended for operation in a bit of chop then you can place them higher than the waterline of the main hull. No point in having them bearing load until they need to in a roll. Aim is to have them do nothing most of the time. They are just there to prevent unintended swimming.

Rick W.

[kengrome]

Hi Rick, thanks for commenting ...

Straight rocker is more easily driven at displacement speed.

This seems counter-intuitive since most displacement speed boats have a lot more rocker than faster boats. If it's true the forefoot should definitely be in the water for easier paddling because that's the only way to straighten out the rocker without submerging the transom. But this leaves more rocker aft than forward, and I'm not so sure that's a good idea either ...

If there is potential for surfing then having some lift in the bow will help.

What do you mean by 'lift in the bow'? Do you mean leaving some rocker forward, or do you mean to adding some other form of lift in the bow?

Having the outriggers in the water is energy wasting.

I know, and I would do this if these were recreational boats, but they are fishing boats -- and I'm afraid that when the amas touch the water as the boat rolls from side to side they will slap and make noise that scares off the fish.

[Guest625101138]

Originally Posted by kengrome

Hi Rick, thanks for commenting ...

This seems counter-intuitive since most displacement speed boats have a lot more rocker than faster boats. If it's true the forefoot should definitely be in the water for easier paddling because that's the only way to straighten out the rocker without submerging the transom. But this leaves more rocker aft than forward, and I'm not so sure that's a good idea either ...

What do you mean by 'lift in the bow'? Do you mean leaving some rocker forward, or do you mean to adding some other form of lift in the bow?

I know, and I would do this if these were recreational boats, but they are fishing boats -- and I'm afraid that when the amas touch the water as the boat rolls from side to side they will slap and make noise that scares off the fish.

Rocker - Don't know where you have been observing. Look at boats designed to go fast with little power eg rowing sculls, racing kayaks, trimaran amas, fast cats. Not much rocker in any of these.

Lift - The flat section at the bow with the upward slope will provide lift if the hull is surfing down a wave. This is desirable compared with burying. My long slender hulls with little planning surface don't notice waves they just go straight through them. Not a good prospect on larger waves.

What evidence have you got that slapping of outriggers is an issue for chasing fish. In any event it is hardly a good reason as they will still slap unless you lean one way and load up one of them and completely unload the other. You then have something more like an outrigger when at rest. When under way you aim to ballance the outriggers clear of the water. The little hulls will contribute significantly to drag if they carry any sustained load. I even use torsion bars on mine now so they roll over waves rather than carve through them. In some chop going to windward they get a real workout.

Rick W.

[Fanie]

If you're going to fish from it one of the things I would suggest is make sure the ammas won't submerge if you both stand on them, the most probable place to get the fish out (unless you cut the line each time or catch only small ones ). You would probably have a trampoline between the beams to walk on. Very nice to have.

I agree with what Rick said. I think the bow on the waterline is ok, but if you have surf or breaking waves you may want the bow's deck heigh higher so a wave won't go over so easily. May wash your only fish off

You mention paddle ? Is it a 2hp Mercury ? I would strongly recommend one... in case the wind unexpectedly pics up and you're runnling low on vitamins and callories for the day.

I'm not critisizing... but I would make the boat as big as I think I can handle. It is always big on land but too small out there. Someone I know (he's a tough guy) bought himself a big paddle ski to fish from. It looked so big when he got it, but all that changed rapidly when he got it on the water. He didn't catch a single fish from it. Ended up surfing the waves with it... a lilo would have been cheaper Just my opinion.

One other question... why not consider having a little sail on it ? You don't have to participate in competitions for speed, but you can however sail up and down and trawl all day long if you want... save the legs some

[kengrome]

Rocker - Don't know where you have been observing. Look at boats designed to go fast with little power eg rowing sculls, racing kayaks, trimaran amas, fast cats. Not much rocker in any of these.

I was thinking about larger boats. My mistake, sorry about that. Okay, so all these boats have their forefoots in the water, maybe mine should too ...

Lift - The flat section at the bow with the upward slope will provide lift if the hull is surfing down a wave. This is desirable compared with burying.

So it's a 'good thing' to have the forefoot out of the water and to have all that rocker forward if the boat might experience surfing conditions. This was one of my two reasons for keeping the forefoot out of thee water in the first place, the other being easier turning.

What evidence have you got that slapping of outriggers is an issue for chasing fish.

Not chasing fish, just sitting at the fishing grounds once you get there, then trying to catch them without the amas splashing and driving them off. All the local banca boats have both amas in the water all the time, and I think the reason is to prevent them from splashing.

In any event it is hardly a good reason as they will still slap unless you lean one way and load up one of them and completely unload the other.

Sorry, you lost me here. How can they slap when they are both in the water?

I even use torsion bars on mine now so they roll over waves rather than carve through them. In some chop going to windward they get a real workout.

I thought of doing this, it probably means using only one aka instead of two, and shorter and smaller amas. What material do you use for your torsion bar akas? Do you think flat laminated wood akas might work okay as torsion bars?

[kengrome]

By the way, in addition to 'torsion bar akas' I have also considered using self-steering amas ...

My concept is to locate a vertical aka attachment tube at about 1/3 of the ama's length from the front. The aka's vertical pipe section would fit into this tube and allow the ama to swivel on the vertical axis to adjust to the varying angles at which they hit the waves and currents.

Any thoughts on the viability of this concept?

Edit: At one point I considered using this swiveling connection and locating short amas aft so they could function both as amas and as rudders in a similar engine powered boat.

[Guest625101138]

How do they slap? You assume the boat will be sitting flat. If there is any waves the boat will roll about. With waves a couple of inches high the amas will start to flop. Normal drift is side on. Possibly different if bow on but in this case even if the amas are above the water they will both always be in water contact in anything other than dead calm.

A single torsion arm is fine. I use 25mm square aluminium tube that is 1.2mm thick. My current outriggers displace 15l so the force is limited. You would need something heavier. Drag is low so the risk of bending the tube is slight. My tube is triangulated with support braces as well but this is to do with folding system not really strength. But it does stiffen them up in vertical and horizontal planes; not in rotation though.

Rick W

[kengrome]

If you're going to fish from it one of the things I would suggest is make sure the ammas won't submerge if you both stand on them.

Local fishermen don't stand on their amas or cut their lines, so I don't think this will be one of their concerns.

I agree with what Rick said. I think the bow on the waterline is ok, but if you have surf or breaking waves you may want the bow's deck heigh higher so a wave won't go over so easily. May wash your only fish off

I'll deck the forward part of the hull to keep the water out but I don't want the bows to get too tall, I'm trying to focus on low windage instead.

You mention paddle ? Is it a 2hp Mercury ?

No motors, these boats are for manual propulsion only, which is why I'm designing them to be low profile and light weight and as small as practical for two fishermen.

I'm not critisizing... but I would make the boat as big as I think I can handle.

My concept for this one is to keep it shorter than two lengths of plywood to monimize production costs -- and to make everything else as light and inexpensive as possible too -- within the practical realm of safety and performance of course.

One other question... why not consider having a little sail on it?

I'll modify the design to create a sailing version later. Right now I'm trying to focus on designing the best possible 16 footer for 1-2 men to take out fishing using only their arms for power ... so sailing is in my mind but it's not a feature of this particular boat.

[Guest625101138]

Originally Posted by kengrome

..

Any thoughts on the viability of this concept?

Edit: At one point I considered using this swiveling connection and locating short amas aft so they could function both as amas and as rudders in a similar engine powered boat.

The approach is inconsistent with keeping the amas out of the water unless they are preventing unstable roll. Take look at the fast tris:
http://www.graingerdesigns.com.au/st...detail-145.php
Aim to keep both amas out of the water if possible.

My OC1 has adjustable ama height so I can set the degree of twitchiness.

With ballanced outriggers I can set them a little higher when there is chop to take advantage of just clipping the wave crests. On the same setting they would allow flop in calm water. I also set my amas to just skim the diagonal wave off the bow so they get a free ride using energy I have already put into the water.

Hulls are about is inefficient as you can get as a foil so they do not make good rudders. The aft paddler will be most effective at turning the boat unless you fit a rudder.

Rick W.

[kengrome]

... even if the amas are above the water they will both always be in water contact in anything other than dead calm.

My concept was to insure that the amas never come out of the water. The local fishermen don't seem too concerned by the extra effort required to paddle their existing boats which have all three hulls in the water all time time, so that's what I was shooting for.

If the main hull is stable enough, a better solution might be to have amas that can be lifted while in transit then lowered while drifting at the fishing grounds. I don't want this boat to get too complicated though, so if I do this the mechanism would have to be cheap, simple and reliable. Maybe it would be better to just remove the amas from one slot and slide them into another rather than trying to design some kind of complicated mechanism to allow them to rotate ...

A single torsion arm is fine. I use 25mm square aluminium tube that is 1.2mm thick. My current outriggers displace 15l so the force is limited. You would need something heavier.

I have a source for thick walled aluminum tubing, I'll have to check with them to see if they have some that's big enough for my estimated needs -- which won't be known until after I re-design the amas so they work something like yours.

Is there a particularly good ama shape you might suggest for use as torsion bar amas?

[Guest625101138]

Originally Posted by kengrome

....

Is there a particularly good ama shape you might suggest for use as torsion bar amas?

You can see three versions in this video:
http://www.youtube.com/watch?v=ckWqIgmVM4Y

The PVC pipe is not mine but I used the same for a fast build and it was copied here. They do not displace enough. In this case they are also always loaded because the boat is stern heavy.

The orange ones are end-on-end cones mounted offset so they present a straight rocker. These are good in rigid set up. They work very nicely in chop.

The black ones are intended to plane. They have raked entry and flat run through middle and aft. The aft is tapered so it leaves clean at low speed. These have more distributed buoyancy than the conical ones and work well on the torsion arms. The lift quickly on a wave pitching up then down as they roll over a wave. Gives slightly less drag than if they were rigidly supported. I have had concern about fatigue when you see them working hard into chop but I think it would take a lot of miles.

Rick

[kengrome]

The black ones are intended to plane. They have raked entry and flat run through middle and aft. The aft is tapered so it leaves clean at low speed. These have more distributed buoyancy than the conical ones and work well on the torsion arms.

Mine are closest to the black ones but with uniform shallow rocker from end to end. If I put them on a torsion arm I will probably make them shorter because they measure 12 feet long as is. I'm thinking maybe 8 feet is long enough for a torsion arm installation