Sueños Mojados

It really depends on what their SOR is and what, if any, compromises were made in order to satisfy their SOR. No hull runs at "optimal" at all speeds, thus what was their target?

Almost all of our cats run at one target speed, but below, they look just like those you've selected, in terms of not being ideal to cover all bases.

The hull should be designed to satisfy the SOR...that's it

 

First post

As a new member I sure don't understand what is going on here. Are we not free to ask for technical advice? If you don't want to do that, why not just ignore the post?

 

A final address to you Ad Hoc, since it actually is impossible for you to demonstrate how my model to its current levels from the sum of discussions in any previous topics that I have raised, and you clearly have very strong feelings about what you perceive to be leaching, I suggest that its high time you forward your concerns to the admin of this forum. As it is, if we continue this current approach that you have chosen, we would only be butting heads. I will follow my own earlier advice and ignore you from this point. I only request that you cease bringing the thread down with this pathetic rubbish

 

Guys I hear your concerns about the bottom lines and transom shape. The two approaches seem to be the slick and curved full displacement and the more square and flat planing. I am inclined to take the planing assist route to reduce wet area only for being easier to implement and I feel that would help towards a better cruise and top end ability. I am also trying to tune out the squatting and pitching motions

My usage will be traveling from reef to reef to stop and fish. I will be doing very little trolling at slow speeds but will occasionally troll at higher speeds between 6 and 10 knots

I really appreciate the discussions of the pros and cons of both approaches though. An advantage of using PE as the build material is that the ability to cut and weld to modify to test and choose from various ideas is retained. As is the ability to easily add to the waterline by playing with pods and such

Btw, the new model Richard Woods is working on, the Jazz 30 is looking really good. I really think its carrying the potential to be a superior fishing boat then the Silver 29

 

TANSL's advice is useful. After you have developed your work as much as you can, hire a professional yacht designer to review and even develop your design further. The fee paid to a professional for this work will be minor compared with material and labor cost to build.

What you see with Ad Hoc's post is that some successful professional naval architects would probably refuse to wok on a design you have started because you might have drawn in misconceptions or errors from the start. Working from a clean drawing board with only and exactly the specific data (SOR) he requests, he wouldn't have to go backwards to check everything to start a design. (This is my assumption based on his post.) Some naval architects refuse to even sell plans to a non-professional yard for fear that an inexperienced builder will poorly execute the design.

 

Its very difficult to justify hiring a naval architect for such a small boat. Its a 25ft boat, not much more to it than a sailing beach cat! Talking about pro design in this instance doesnt help. The bigger boat, he wishes to charter at a later date - well thats a different story entirely...

Im all for experimentation, just do your homework first...

 

We have one maritime officer here who is responsible for surveying watercraft for registration purpose for our district. I am on very good terms with her and I do not see any difficulties with enlisting her help scrutineering safety and structural aspects at the plan and construction stages. Lol especially after a couple of rum and colas and attacking some welds with sledge hammers like the NZ guys in their vids!

 

Depends upon the budget and how much advice is required.

Not far off the mark.

I don’t refuse to work on a design, but it gets somewhat frustrating when a client doesn’t wish to listen. They come to you for advice and a design, yet have all these crazy ideas – owing to a lack of technical knowledge – and generally won’t be steered away from them. They approach design with a fixed preconception of design.

Once it is challenged, as all NA must do to understand what is required, just like the OP, they tend to get very emotional, highly subjective, and just rant. Even make false acquisitions and what has not even been said . All because they wish to keep things “theirs”….so they can call it their own. It is simply an ego thing…seen so many times on this forum before.

Those are the most difficult clients, ones that are emotional overly enthusiastic and little patience to do things on a time scale not their liking. Yet, have ideas but based on little and yet constantly seek advice, but only when it agrees with their line of thinking….but oh sorry my mistake, it is not advice, it is just comments as they are designing it!

I shall let the OP vent his spleen on how he feels emotionally torture for merely suggesting he’s out of his depth and needs to learn the basics first. Such a major spiral out of control for simply suggesting he reads some books first or just the manual of the freeware he is using. QED on the emotional levels!

 

It would be useful to specify the overall budget for this smaller boat.

 

Hey saqa, let's all be "amigos", no need to take offence at whatever advice is proffered. This is Fiji, bula !

 

To be honest I am still building a budget. I have priced the motors and buying sheet PE by the tonne. They are in range. My small gun with nozzle is only suitable for small work so for serious welding a decent unit will be the first purchase. Just taking a relaxed approach to making the purchases of the period of build time buying as needed

 

Groper, I missed this one. When I bring the transom edge up the draft goes from 10 to almost 14". I think this might be introducing more wet area at the front of the model. And might make it even more bow up under power

Jeff, I was only showing I figured out how to draw a pod. I can do it stepped if I find the hulls as now not good enough. As drawn though just that pod is displacing 160lb at 10" draft

 

Brakes.

A pod could really just be an add on when & if you feel the back end is draggy, as you've mentioned some lures need to go slow... maybe with already fairly slippery hulls you might need brakes!

Jeff

 

Of course it does! If you reduce the transom immersion, the the total draft must increase, all else constant, to displace the same volume. It also means that wetted area also increases for the same displacement. This is why I chose to design my cat with a relatively deep transom, to minimise wetted area. When I say relatively deep, the draft on my boat is very small anyway, so the total transom depth is only 300mm. If the displacement was required to be higher, i would have designed flat rise buttock lines to reduce the transom immersion to a more moderate depth as 600mm would have been excessively draggy.

Now the trim.
If the buttock lines are constantly curved aft - like a cruising sailing cat, then you can expect the boat to trim now up under power. Richard woods has a photo of one of his sail boats which was towing a water skier at 22kts after they fitted 2x 90hp outboards for the hell of it. The bow up trim is extreme. To prevent this, high speed power catamarans have no curvature in the buttock lines. The buttock lines are often rising to the transom, but they do so as a dead flat upwards run. This results in a slight bow up running trim which is limited by the angle of the rise, and is generally not a problem, some even favour it's characteristic as it reduces the requirement for freeboard up forward.

Understand, curved surfaces running in a fluid, generate dynamic lift. So curved buttock lines actually suck a hull towards the seabed as the lift is downward. With this lift, also comes a pitching moment depending on the location of the center of lift or CoL. If the CoL is directly under the CoG, then there will be no pitch change, however this condition is rarely met in a boat. Flat surfaces can also generate lift, but they need an angle of attack to do so. As the boat trims bow up, you begin to get upwards lift and this is how a planing hull works. You will note that planing hulls have large flat surfaces, no curves in the buttock lines what so ever.

After trim, there is also sinkage, which is how much the boat draft increases when underway. Large ships must limit their speed in shallow waters or they risk hitting the bottom as their draft can increases by a couple of meters simply by increasing their speed. it's a well known phenomenon. With more sinkage, also comes more drag. You can reduce it in a boat like yours, by utilising some positive lift, but again, you will need flat buttocks and a low buttock line angle.

Building in sheet material, I don't know why you don't consider the typical warped bottom deep v forward hull series - sorry can't remember the proper professional term for this hull series. Easy to build and it's a good compromise for the medium to high speed regime- many patrol and naval vessels use it as they generally travel and loiter at around 10kts but are required to move at high speeds in pursuits etc

 

Groper, I have read a lot of that in your design thread for your boat. I have been reading that with a magnifying glass over the past few months and trying to understand what you were on about by searching more online and reading up on the hits

All that is the major reason why I am looking the planing assist mode with the power available from twin 40s

Have a look at the forward half of my model, see how the bottom plates do not taper to points in a long triangle but are narrowish and flat and have to warp vertical to meet the side plates. I'll take some more screen shots in 3D. This creates a very sharp cutting edge that is ground wide like a blade going back almost a third of the boat length. I took this approach with my small monohull as well and it slices chop better then any panga or skiff type. The 25' length of my model pontoon lets me fair a more slender and sharper line in this area then my monohull. Is this warping to almost vertical what you mean?

The back half is modeled a lot from what I picked up from your design thread as well as trying to bring some abilities from planing cats in. I have put only 3" of rocker sweeping up to the transom lower edge. I have found that this much rocker is enough to get an acceptable motion when fishing at rest so incorporated that. It still lets me keep close to max buoyancy there. The trade off I found is the squat that you mentioned. I level this out with the wing foils on the outboard leg on the current boat and hope will perform the same with the longer twin hulls

A major point of difference between how you have laid out the superstructure on your cat how I intend to do mine is our respective type of fishing. Your is more gamefishing bias with the back of the boat forming the fishing area and mine is casting based with front of the boat being the fishing zone. I am trying to bring down vertical accelerations here with the type of warped panels merging my bottom to the sides here hoping that the water will move up rather then moving the bow up. This will be augmented with foils under that area to create drag/braking in the vertical plane as well create lift in the horizontal plane. I am applying this front end through intuition, that I can apply from limited understanding of what I have read, logical reasoning looking at boats out there and how my Speedo hand fins act on water. I am tempted to remove the rocker in the back and make it flat to extract max planing performance. Would you recommend starting there and cutting and rewelding if required?