michlet/rocker

Design

What principle do you apply to lift the bottom in this scenario ?


The outriggers have more strain to endure when you are on a wave face, broaching and the outrigger goes 'ama under'; this usually does not happen, but I think the construction would have to account for that.

I do not see how two outriggers would increase this risk either.


A temporary mold will then be the better option. We have not entirely decided on the legth of waterline yet; we may stay a little shorter than the theoretical optimum to gain a little control/manoever.

 

Eckhart
Given the apparent requirement to glide sideways I would have the mid section as follows:
WL beam around 12"
Sides down vertical to about 0.5" above the waterline
A chine panel around 3" across sloping 40 degrees to flat
A keel panel from the chine panel to the centreline that slopes 15 degrees to the flat

You would need to fiddle with this to blend along the hull. The chine panel disappears toward bow and stern.

After you have the new shape you check how it performs by exporting to Michlet. This then gives an idea of the cost of softening the chine.

Using a foam plug you would soften the corners and make slight curves instead of flat panels but basic shape would be as I describe.

There is no reason why you could not experiment with different shape amas on you existing boat. I would be interested to learn how the dart type perform in the ocean. They are simple to make.

Rick W.

 

It is just a lifting surface.

I have found that an edge at 45 degrees to the flat will dig a deeper hole. Around 40 degrees it starts to generate lift.

This is from experimentation rather than any soundly based theory.

Think of the side of the hull as a ski. As it slips sideways it needs to lift rather than dig a hole (low pressure) in the water and trip up.

I expect this makes it easier to turn as well.

Rick W.

 

Outrigger

The outrigger will likely be built first.
I am intrigued by its shape. I just have to find a good way to connect it to my existing boat.

I have two options at this point:

I entered the data of my current OC 1 into michlet, then opened it in Delftship. Here I added length, scaled it to the desired beam and draft.

I am hesitant to go too long for practical reasons and turning ability in the chop.

I now want to print out some stations and see what it takes to create a fiberglass one-off of the existing hull as male plugand adapt it to the stations/ new design.


Second option: as you described.


I have some problems interpreting the data: basically I am looking for the values under 'power' in the speed-to-hull file, comparing the values. Is that correct ?
The differences are small.

I have not solved the CAD question fully either.

 

Cad

moving ahead with the plotter software . . .

 

Yes. You just compare the power values. I normally graph them in Excel by importing that file. The key points of interest will be say 3.5m/s as this is the 1 hour speed in stable conditions and say 5m/s as this is the speed likely to be reached when on a wave. However the latter may not have the hull submerged anywhere near the normal waterline unless it is a big wave.

The CAD sections in the second post do not make much sense to me.

With the outriggers you need to fair the iakos as they get near the amas as they will be driving through waves. They need to be mounted at least 1ft high across the span or alternatively faired for full length.

Rick

 

Hull

This hull has numbers very close to a Godzilla hull at 6.75 m, 0.27, 0.11, displacement 0.107.
Godzilla narrows the beam slightly and lengthens the waterline but the numbers do not get better.

4 = 2.15
5 = 4.001
6 = 6.51

Next I will strengthen the harden chine and see what happens.

These dimensions seem reasonable to me.
The upper deck is a vague shape.

The wave piercing bow is much debated everywhere - it is diffiult to filter out what that really is. The key is to reduce pitching.

 

The numbers you posted are hundreds of watts.

i.e. 5m/s requires 400W

This is good. It is typical of a round section that has the volume in the right place.

I believe a long boat with flat rocker resists pitching. Some like flat sections in the bow and stern to increase damping.

I have attached the double chine version as I tried to describe earlier. This is the sort of section that will skid sideways. Its other feature is that it has better roll stability. With a good paddler you could possibly stay upright without an outrigger but not for long. It would be stable if you were laying flat.

Sadly it takes extra power to push it. Not a lot more but enough to be ordinary I expect.

Rick W.

 

Rocker

I understand 'rocker' strictly in terms of pressure relationships: if the flow accelerates over the longer surface than the lower pressure sucks the hull down.

This can actually not be the reason for better steering control. Thus it must be the reduction of the lateral plane that allows for rotation around a vertical axis.

The relationship between fore and aft rocker would then be important, a symmetrical distribution would pivot the boat around a central vertical axis, an asymmetrical rocker would let the bow resp. stern skid relative to the opposing end and describe a different arc.

The lateral sliding of the boat is mostly desired in very irregular chop that often comes from two opposing sides, one being the backwash. The back of the boat may be tossed towards an opposite direction comppared to the front.
The boat should therefore not sit too deep either.

Regarding the pitching - this is at least as confusing as the rocker issue. From flared ends to various volume distributions everything is being mentioned.
How flat surfaces reduce pitching is not easy to understand.

My best thought is that the boat has to be somewhat wet, like your ama, to avoid pitching most effectively.

The question would then be: if your bow is pearling but your boat keeps a good trim while doing it, will that be preferable in terms of foreward motion with the wave ?

 

A few comments:
With stern hung rudders, boats steer from the stern. To turn they must move the stern sideways. It does not matter if they pivot around the bow or further aft but the stern must be able to go sideways. I have tried a bow rudder on a couple of boats and it is not good because it induces roll-out in the turn.

You see surf skis with deep entries and flatter runs aft so they tend to steer more around the bow than mid section.

My V11A rides through waves as if they are not there. By comparison V7 (the yellow one) wants to ride over them. V11A is much faster in waves (chop). However in a following wave that has a wavelength just a bit longer than the hull I am able to drive down the wave and into the back of the next one. The hull would be angled at about 10 degrees bow down. I back off when the bow is completely submerged for more than 2m. I think I could push it further under if I keep powering. Not sure if the buoyancy would eventually take over. On bigger waves it would be hard to achieve the speed to get into the back of the next wave.

I do not have any experience with these long hulls in swells or confused chop. I can see potential to bury the bow so having some extra buoyancy and/or lifting surfaces possibly has merit. The bow in my original HC file is about what I think has reasonable balance between wave piercing and lifting but I am not an expert.

Hulls with little rocker and little buoyancy in the ends just glide. There is little tendency to pitch. On the other hand they need a following sea to be under them before they start to respond and pick up speed. I rarely get waves big enough to actually ride. Most of the time I am driving through them. I am able to hold about 14kph by running diagonally across a smaller wave that is moving at say 10kph.

Rick W.

 

Trim

We have roughly three types of waves:

- The smaller chop that your V11 does so well in.

- The confused chop that hits the boat mostly form the side with a trend to lets say 20 degree aft.

- The larger swells. The larger swells are usually quartered to gain speed.

In a downwind run the angle is determined by the deepest trough in front of you - that is where you are aiming the bow, you actually want to follow aiming at the bottom of the trough as much as possible. The angle here is probably 30 - 40 degree right and left from straigth down the wave face.

True 'quartering' is used to get over the wave and gain the speed on the back face, or when you do not see a deep trough you try to find the smaller waves runing off the larger swell at about 45 degrees and gain speed there.

Your V 11 design may then not do not so well in the confused stuff ?
To do well here the boat must sit high; can length related buoyancy do this ?

When going straight down a wave face ?

Why do you think you would not get enough speed to get on a larger wave ?
Usually we try to keep the hull speed up quartering the wave and then we drop in once we have sufficient speed and sufficient steepness in the wave.


The angle of Hawaiian waves would be steeper, 15 degrees, maybe up to 20.

I would not mind to have the bow in the back of a wave as long as the boat can follow that wave.

My thought is that I would try to allow piercing of the wave at a trim of up to 15 degree unrestrictedly and gradually hinder piercing at steeper angles.

How about keeping the firs 2 - 3 feet of the bow as piercing as possible and then start up building volume to lift the hull starting at 3 feet tot ake effect when the trim gets greateer 15 degrees or the bow gets too deep into the back of the wave ?

 

Last point first.
The idea of allowing the bow to bury to a point and then generating lift behing the bow might be good. My V6 boat had an exaggerated version of this. It carved through small waves well up to the point of the iakos hitting wavetops (see video). It was a true wave piercing design. The bow sat about 50mm below the surface when at rest. It was faster than V7 in most conditions and just a bit slower than V11A.

One other point.
My comment on the larger waves was related to driving into the back of the wave in front. The wavelength is related to the speed. In shallow water things change a bit but that is beside the point. My meaning was that if you are on a wave with say 50ft wavelength it will be moving to fast to drive down the face and into the back of the preceding wave. Rather you would be working just to ride the wave.

Unless you are riding waves they will slow you down. What I have found is that a boat that is fastest in calm water will still be fastest in rough water. However I cannot speak from experience in large waves.

Rick W.

 

Dc_oc 1

Rick,

you are right on the performance of the chined hull, the speed drops form 4.7 to 4.0 knots at 20 N.

I will look at your V 7, I do not remember seeing that video.


The wave troughs are not all that long, they get relatively longer when you quarter; also before you drop in only part of your boat is in the trough, the rest extends over the crest.
The key issue with my current boat is that it takes off too late.
I moved my position by several inches forward, that helped.
Up to 1/3 of the hull sticks out of the water in steep waves just before take off. This happens mostly when you have to adjust the direction of travel.

Ideally the boat would easily assume the right trim and do this early.

Eckhart


PS.: I used 'autogroup' for layers and plate development - it seperates all the layers correctly.

 

V 7

Very nice.


This bow behavior is exactly what I imagine for my OC 1.


The important part would be that this bow could display this behavior with a trim from - 10 to 15 degree or so.


Is the bow just a conus, like your outriggers ? Or is it a z bow under water ? Would a little more vertical volume change the behavior too much ?


One of the important features of your v& bow is that even when you have water on your deck it does not matter much, because the surface area is small enough to tolerate it.
I feel that this is the only way to substantially address pitching issues, isn't it.

 

V7 is the yellow Pahoa OC1. It has a high stern that sits out of the water at rest. It is not wide or flared but quite high - about 8". This part of the hull has a lot of leverage so submerging it gets the stern up quite quick.

If you want the boat to lift onto a wave then I expect that you need buoyancy right aft. You could try this on you existing hull by just adding some faired foam above and behind the existing stern. Just tape it in place with duct tape to see what it does for a brief trial.

Rick W.