Sueños Mojados

Yes, what I am saying is don't have curved buttock lines in the aft sections, keep the buttocks straight, and bring them to a suitably small immersed transom. How much transom immersion depends on your design speed. If you have zero immersion, you will have highest wetted surface area. If you have deep transom, you will have minimum wetted area but higher transom wave drag which is a bit of a hindrance at slower speeds. High speed will benefit from reduced wetted area and the transom hollow extends further aft whereby the wave created is less significant.

 

Thanks groper, good to know that I am on the right track

 

Thinking bridgedeck, the easiest solution is alu square extrusion 100mm x 100mm in 4mm wall thickness as main beams x 4

100mm x 100mm x 4m main beams x 4 = 65kg

These can slip into 4 slots in each hull. The slots will be part of internal frames inside the hulls. An inspection port will need to be fitted near the slot to allow hand and tool access inside the hull for bolting

I am planning to run 6 stringers over the 3 main beams, made from 25mm x 25mm square alu extrusion each 6m long and 3mm wall thickness. These will give around 12" spaced support under the deck cladding at a span about 5' beam to beam. These would be best welded to the beams

25mm x 25mm x 6m stringers x 10 = 42kg

Cladding to be in 6mm textured HDPE bolted to the stringers. I can use rivets but I feel bolts will be better

Cladding 120kg

Total bridgedeck weight 227kg minus bolts and such

Total boat weight so far comes in around 500kg

Twin 40hp at 100kg each

Thats 700kg boat and motors. Leaves me with 700kg to play around with for the console, batteries, fuel, electronics, crew and such to arrive at the light displacement of 1400kg. Bear in mind that the wet transom area in the model is at 800kg for just that one hull to work with a 1600kg displacement for the boat

What do you guys reckon, a good displacement figure there with 1400kg?

 

Hi Saqa,
The 6mm hdpe sounds thin & maybe springy.
The 25 x25 spanning 5'/1500 stringers sounds long & shallow in section, maybe get some & stand in the middle of 1500 span & jump like dynamic of waves, maybe 50 x 25.... ? or a C/channel section for bolting?
Maybe welding the stringers to cross beams/connective structure not the best joint choice? Apparently welding mid spar/beam alters temper & properties of material.
Maybe The most forward & aft beam could be deeper in section as these would offer most restraint of the hulls/platform. The fore & aft beams could have the stringers set with bottoms flush with stringers for neatness & intermediate beams set with tops flush the stringers, bracketing could be riveted...
Lots of maybe for me, I'm more familiar with composite glass foam & working to plans.
My Seawind cat did have hdpe deck before, but at about 18-20mm very heavy & springy, I took it off & have foam sandwich

Jeff.

 

Saqa, your on your own for scantlings as Noone has designed a boat like before that I'm aware of. So you can't compare to a known successful design. This means you would have to engineer from first principals and you don't have the required training to do it. I studied a bit about how to do it, but it far too complex a subject to cover in a forum thread...

To my dead reckoning, I'd say that bridge deck floor would be way to flexible. .. the stringers would probably need to be at least double that 25mm depth as Jeff pointed out, perhaps more. As to the hdpe , you can experiment with that yourself to get an acceptable spacing. But yes, 6mm would be pretty crappy and you would see the sstringers under it and it would cup between each one. I hope you can get it in white, because any other colour is too hot to walk on in bare feet...
My personal opinion is that your on the wrong track now Saqa. This boat is getting so much heavier than it could be if you chose a different material to work with..

 

You guys are not wrong about the weight. It is a bugger but is keeping within the project weight figures but there are some mistakes in my figures for the bridgedeck, the dimensions are incorrect and I need to chew on it more

 

The is a slightly larger molded glass cat with similar beams here, I need to go pay a visit

 

The average span of the beams is correct at 4m, it changes a bit with narrower span on the foremost and rearmost beams. The hard deck area is around 10' x 14' over the span of the last three beams. I made the mistake of spanning over all 4 beams

 

Groper, thanks for the heads up on the Prowler 330. I looked up quite a bit of info on that. Looks like their powerheads get to stay dry too!

 

Well guys, looks like that Tennant bloke knows what he is on about. I been playing with the CS cat hull type modelling for the last couple of days. Keeping displacement and such the same I kept running into increased draft and wetted area till my final two models

Keeping to my original model length of 25' and displacement of 800kg for the single hull model

New draft if 26cm and better then the original 30cm on page 1
Wetted area has also dropped by 0.12m^2
Using the resist/power cal in freeship 6.85 knot hull speed is with 200w less power then the original
20 knot is with 100w more power then the original
Transom flat lower edge is dry by just a touch
CS25 by jonny.toobad, on Flickr

Then I kept everything the same but extended to 28'
New draft of 24cm, 6cm less then the original
Wetted area increased by 0.14m^2
New hull speed of 7.17 knots needs 100w less then the original
20 knot needs 189w less then the original
Transom is even drier then the previous one
CS28 by jonny.toobad, on Flickr

 

Going by the renderings, it appears as if the whole flat section is in the air (just), and therefore not adding to your wetted surface areas if predicted by the software.

Perhaps try increasing the displacement until the whole flat area is touching the water for more realistic results. I could be wrong, but as it seems, any performance predictions are probably ignoring all that hull in the air. In the real world it will be in the water.

 

It will be a very noisy boat, and you will notice it the first night you decide to sleep on board...

 

So where do the outboards sit, with this new transom arrangement ? What is going to shield the legs, when they are sufficiently immersed to stay in the water, and how to deal with the interference to them, from the hull ?

 

NORTH_STAR__NORTH_WIND_(04) by jonny.toobad, on Flickr
The Custom 38 was called as less draggy transom but still some, when compared to the model I drew. From the numbers generated I can see that this last model has less resistance then the original at the same displacement. I tried to draw in lots of reserve buoyancy in the arse and a flat area that will squat, stop and cut clear water away under wide open throttle, as described by Malcolm

Its to be a strictly fishing boat without accommodations, the HDPE is a very dampening material. An available option is to move things around like fuel tanks to adjust the at rest trim so that the transom is wet if the noise gets too much over an idling motor and use motor trim and wing foils to bring it to the surface. The trim tab effect of that large flat area ought to do that anyway
The cav plate of the outboard will line up a bit under the transom. Its actually a long slender run to the apex of the skeg formed by the canoe stern on my model. The prop will line up behind and below that skeg. I need to find the distance from the cav plate to prop centre of a Bigfoot 40 to find out exactly where it will align

Mr E, the leg will be behind at bit of the transom plus all of the skeg. The only thing exposed would have to be the prop and the fin on the outboard.... Do you find this to be a bad thing?

Some inspirations

https://www.youtube.com/watch?v=EO3I7SOXz2k

image20 by jonny.toobad, on Flickr
DSCN006520 by jonny.toobad, on Flickr
Underway8 by jonny.toobad, on Flickr
https://www.flickr.com/photos/125790096@N04/16130708177/in/photostream/
Underway10 by jonny.toobad, on Flickr

These boats seem to be doing ok and I need to make a few decisions about whether I should go this route or a planning type hull to get the fast efficient cat. I have more pics of the new hull models, will upload them tonight

 

Dennis, the rendering are at a very high design draft. In the real world the cat will be 400kg lighter on most days

There is a planning pad there under the lowest part of the keel but not clear in that pic, that should have an effect too. Will post more pics later tonight. I have only shown that one pic so far as I was absolutely knackered from staring at the screen by the time I got to that point of the model so that was the only screenshot I took