efficient 10m displacement powercat (build thread)

[Mr Efficiency]

No. I'm still in the pencil and paper mode as far as boat design goes, had a look at a couple of programs but can't see the point in going to the trouble of becoming proficient at it for my limited purposes. My design will certainly commit the sin of having flat panels, slab-sidedness doesn't bother me as long as it doesn't handicap the way it functions.

 

[groper]

Go for it mate, the software allows you to keep your design at your fingertips and query any measurement as you need it like a set of paper plans does - except you can change things with a click of the mouse if/when you need to...

Straight U sections (or vertical sides with square bottoms -) are very efficient hulls as it allows maximum displacement for minimum waterline beam - waterline beam directly influences wave making resistance, Less waterline beam per hull = less wave resistance. Often people are all hung up on minimum wetted area and thus round bilge, but for faster hulls the square sections can be a better option not to mention much easier/faster to build... For a given displacement, the square section will always have a narrower waterline beam than a round section.

the problem then becomes interior volume of skinny hulls... so you need either a decent amount of outward flare above the waterline or knuckles/chines or both... then keep the boat light and you cant really go wrong with respect to efficiency.

To keep things developable or single curved (derived from flat panels) best option is to twist the panels into your bow stem - i should have done this instead of mucking around with that direct female mold. Same goes for your topsides to get the flare in the bow. Every tape join is more work, more resin, more money, more weight and more time, better to use fold up panels where possible... but it needs to be designed with this in mind from the beginning as only certain shapes are possible this way - see Rob Dennys proas for the Gist on all this, seems he realized all this a long time ago...

 

[groper]

well the direct female molded bow section was slightly different in size compared to the folded aft sections so i had to do some gentle persuasion to get them to mesh together...

so ive taped the inside and now the 2 halves are joined,

i will roll it over next and fair out the bow section including the tape over the outside to finish it off, but first i have to go back to work for another week... when i get back i will infuse the other bow section and join it also, so i will then have both hull shoes done save paint.

Ive been keeping track of the panel weights and resin consumption during the infusions via a spreadsheet... so far everything looks good in that all panels are lighter than i orginally planned for which means i will hopefully have a boat thats lighter than i originially designed for... im sure there are things i have forgotton however, so these will eventually catch up on me, but the naked shell is looking like coming out at around 1000kgs... not too shabby for a 10.6m x 5m cat despite including alot of extra reinforcing in places - the entire bottom of the hulls laminate is 3000gsm triax around the turn of teh bilge for beaching and bumping coral etc, and 1500gsm triax sides below the waterline...

 

[Mr Efficiency]

This is a rough sketch to indicate where I'm tentatively heading, the length 29 feet is at the shorter end of maybe 29-32 feet, beam 12 feet is the minimum starting point depending on the requirements imposed by wave interference, will need advice on that, tunnel height may be too high, certainly would not go any more. The sponsons are simply flat-bottomed like a sharpie, but I would be radiusing the junction of topsides and bottom somewhat. The forward half of the bottom is convex lengthways, changing to concave rear half. As I said earlier, it will have to achieve 20 knots to handle bar entrance. Good ride comfort a priority.

 

[groper]

cool... looks like a very similar hull form to the BWseacat?

Hull separation and negative wave interference is only a problem at around 10-12kts for a cat in this size range... at higher speeds (anything above 15kts) negative interference is completely gone unless your hulls are extremely close together (under 2m)... so how much time will you spend cruising around 10-12kts? The extra drag from this interference is quite small anyways, and your very efficient at this modest speed anyway. You would most likely give much more influence in your decisions to the general layout and size you need from this boat.

As for tunnel clearance, i dont really know and it seems to be a bit of a black art... many variables with respect to riding on foamy water cushions, bridgedeck slamming, the pitching inertia of the vessel, how far forward the bridgedeck lowest point is carried etc etc... all very complicated... Alik has posted some good info on this subject in this forum somewhere, but i think most of its content is derived from planning cats - i cant remember now... SO, what most designers do for displacement cats, is to give as much clearance as possible without compromising the aesthetics of the vessel. - As the bridgdeck clearance gets higher, the entire boat gets higher and things can start to look out of proportion with excessive freeboard or tall cabin sides etc... then there is the extra weight / cost to consider when building everything larger/taller than it needs to be... this is why i say its black art, no easy answers on this one. Im using 750mm clearance at max displacement, more likely closer to 800mm when at typical average weight. This allowed the boat to look right to my eye, and its roughly what a few other similar boats are using so i figured it must be in the ballpark... i would have liked closer to 900mm but things started to look out of proportion on my design...

 

[Mr Efficiency]

Well that is a lot if you go to 900mm, but your's is what, 35 feet ? 750 sounds about right to me. It would be easy enough to plonk a wavebreaker in there for a little extra insurance I guess. Lots of planing cats do suffer from tunnel slap underway, the notion that dynamic lift raises it out of the water so you can get away with lower clearances is not borne out by experience as I have seen it. You have to think though, that looking where the transom ledges are relative to the tunnel height in the 29 footer, they are well 'south' of that, and the idea of needing much more clearance is hard to imagine.

 

[groper]

well i setup the infusion for the second bow of the second hull shoe today, didnt quite get it all done before sundown so ive left it overnight and will have to do the infusion tomorro;

Ive already joined the first hull shoe and its sitting upside down ready for the final fairing of the bow section and join. more pics over the weekend...

 

[Mr Efficiency]

You seem to be a can-do character, Groper, looking forward to seeing it take shape. When do you expect to be hitting the water ? Did you notice that 35ft displacement cat on the Suzuki-Haines site with the twin 90's ? Seemed a bit of a slug on the speed figure given.

 

[groper]

You mean the Argus E35? Its a varient of the roger hill design built here in australia... the kiwis have another couple of varients being built across the tasman... Argus boats have their own website with heaps of info here -> http://www.argusboats.com

Theres a youtube video of the original kiwi version;


I used many of the design clues for my boat; main difference is the weight with mine being mostly infused foam epoxy whereas these production boats are handlayed polyester/vinylester, not sure what cores they are using...

 

[Mr Efficiency]

Yes, that was the one, thanks for that info I'll check the site. The Suzuki 4cyl 70-90 hp seems a reasonable choice given it has high cc's, and a nice reduction (2.59 to 1) which suits the target speeds.

 

[groper]

The roger hill design shown above, also uses the traditional deep forefoot with rather full transom... its powered by 60hp hondas in the video, gets along alright and certainly fast enough in anything but very calm conditions, a bit more speed would be nice for when its glassed out. The argus version built on the gold coast, seems to be loaded down with heaps of extra weight (4500kgs lightship) in the fitout as the kiwi version is much lighter, and looks much more simplistic in the interior. In some of the blurbs on the kiwi prototype, it mentioned the 60hp outboards delivered 21kts...

Yep, i think the 90hp suzi will fit the bill nicely for both the 25amp alternators and the 2.59:1 gearbox... im looking for the largest diameter 4 blade props i can find, and make a guess to the pitch with a top speed in the high 20`s...

Infused the second bow section today, everything seemed to go fine... applied the first coat of fairing bog on teh other hull, will have to bust out the dreaded torture board tomorro, first time since the start of the build

 

[groper]

the second bow section came out today, gave it a quick sand and getting it lined up at present so i can tape it to the aft section.

The first hull had its first dose of long boarding aswell, it wasnt as hard work as i imagined... only spent about 30mins doing the front half, probably needs a touch more bog in a few places before its fair enough to give it the final skim coat.

Not quite sure what this part weighs, but its 4.1m long with 1500gsm triax outsides, 3000gsm triax bottom, and 750gsm inside, 16mm corecell.

And yes, im itchy tonight...

 

[hambamble]

Looking good!

Looks great Groper!

Are you going to fair the fore and aft hull sections individually, or join them up to fair it as one piece? Also, might be best to fair the hull after the framing goes in, that way the hull wont move as much and lose its fairness. Mind you, a foam core is normally quite stiff.

Also, where abouts in Oz are you?

 

[groper]

Heres where im upto;

Got the bottom of the hull shoes prepped for antifoul and flipped them upright.

Set the hull shoes up level and straight in 3 dimensions, used a plastic tube with water in it clamped to all 4 corners whilst we moved everything around until we got there, spent the best part of a couple hours mucking around getting things true.

Dropped the main beams into place with a few bulkheads aswell, propped them all up plumb whilst the epoxy cures.

Need to make another full width beam and laminate the 2nd side of the stem bulkhead, before i drop these in place next. Then i can make the inside chamfer panels, twist them into position and lift up the bridgedeck after that.

 

[groper]

A little more progress and can see the sheer line developing now...

The rear cabin bulkhead, stems and transoms are in. Made the transoms from coosa board which is a synthetic plywood substitute for a completely timber free build. Also added all but 1 of the front beam web stiffeners which somehow didnt get laminated yet? Next to go in will be the hulls inside chamfer panels, was going to infuse them today but my infusion table has developed leaks and needs to be fixed