efficient 10m displacement powercat

[Squidly-Diddly]

how about an 12 meter cat hull form?

Click my handle and see my gallery.

As you can see, I'm just using very simple Solidworks geometry for the underwater hull shape, just to give a general idea of which is the pointy end, displacement, etc.

I'm playing with the "fits inside a 40' container box" thingy, so I'm making my hull's "hard" shape about 38'-39' because I might want a kicked up stern drive to fit inside 40'.

 

[groper]

Ok ive abandoned the permit trailerable width of 3.5m and taken it out to a 5m beam, therefore it will naturally live permanently in the water but allows for much more boat for very little extra shell material and build cost. Length is now 10.6m or 35ft DWL. The shell surface areas have increased from ~150m^2 upto ~190m^2.

This enabled the following GA to be decided upon;

The arrangement is flexible and i have several options, but im already set on the saloon arrangement with galley upstairs and bathroom directly below in the hull for ease of plumbing and electrical - the kitchen and bathroom sinks, head and deckwash all lie within close proximity to the pumps, water tank and electrical system. The master stateroom lies up front with private access whilst the guests occupy the other hull. Kids bunks are an option or simply a king single forward, with a double berth aft, as shown above. This bedding arrangment has quite some scope for change however, i will decide when im building it what will work best once im standing inside the shell

I remain with the low cost, low maintenence, high efficiency design goal, therefore no genset is to be included (although there is room for one), energy is provided entirely by solar and LPG gas for cooking. Im estimating 500watts of panels and 400amp/hour battery bank should be enough to sustain the vessel for reasonably long periods -i say this in the sense of game fishing vacations of around 1-2weeks on the great barrier reef (where i live). The boats purpose is not intended for extended cruising for lengthy periods and its smaller size kinda precludes from this larger class of cruising yacht which i would save for 40ft or more...

Propulsion is to be via the new yamaha F70 outboards for their high power to weight ratio, 996cc engine each weighing 118kgs. Im also considering the suzuki DF90`s for an added weight penalty - 150+kgs each and a few extra knots of speed. Alternator charging increases from 17amps to 25amps also. Production designs similar to this have already proven top speeds in the order of 22-23kts from the DF90 outboards, with great efficiency cruising around 16-17kts - This is with a rather stout 4500kgs dry weight. The full foam epoxy construction im using should be much lighter than the aforementioned Polyester/gelcoat molded boats and i expect the same performance from the 70hp motors considering an estimated 1-1.5tonne lesser displacement.

In the above drawing, the main connective beam construction shown in green, is an integral part of the shell. 2 main box beams are planned with a central bulkhead beam at the rear of the saloon. The stateroom queen berth lies inside the center of the main box beam which is formed by 2 bulkhead beams as webs, and the bridgedeck and topdeck as top/bottom tension/compression flanges. The dimensions of this space, means i effectively have a box beam 1.2x1.4x5m internal, and should provide exceptional stiffness and strength. The laminate in this area will include extensive use of unidirectional fiber. The rear beam is typical of most modern backbeam designs although a larger than normal arc is provided to increase the rear deck area. It could be formed into a seat etc, however im going to keep it simple and simply maximize the fishing space.

Bridgedeck stiffness is enhanced by the structural furniture located central, the rear saloon bulkhead frame, and a single longitudinal glass stringer running between the 2 main beams along the underside of the bridgedeck (which isnt shown in the drawing)

I have considered small diesels such as the cummins 1.9L TDI @ 75kw etc. The weight is around 200kgs each, higher initial cost, more difficult maintenence and engineering means its hard to justify for a small increase in fuel efficiency... Im not even going to attempt to work out how many sea miles id have to cover just to recover the higher initial cost alone, suffice to say it would be several circumnavigations of the globe...
Heres an unfinished rendering showing the lines;

Does anyone have any suggestions? im selling my car to pay for materials to build this, and i expect to start building shortly...

 

[sabahcat]

Looks suspiciously like

http://www.boatdesign.net/forums/boat-building/my-little-piece-peace-25962-127.html#post510685

 

[FAST FRED]

"400amp/hour battery bank "

Does this assume the batt bank will be only discharged by 50% for long service life?

Cruisers seldom get to 100% full SOC , so most will size a batt bank to be sufficient between 50% and 85%.

FF

 

[groper]

hmm, good call on the electrical system, i had not put much thought into this yet as i figured id work it out later and size the batteries accordingly... in the latitude i live we get 6.1 hours of the full rated solar capacity installed. Therefore, with 500watts of installed panels, i should be able to generate 3 kw/h per day assuming clear weather + whatever charge is added by the outboard alternators - which id estimate running for around 3hrs per day at cruising speed whilst at sea, some days would see upto 8hrs per day when trolling for pelagic fish etc. The F70 outboard alternators are rated @ 17amps @ WOT each but i have no idea what is the actual charge current generated on an average daily usage basis?

The main consumer of electricity will be refrigeration/freezer. Everything else has a negligible power consumption such as the LED lights, and GPS etc... the depth sounder would be the next biggest consumer, but i only run that whilst the engines are running typically so im going to ignore this for now. There will be no air conditioning and for hot water i plan on using instantaneous LPG system - which is also used for the hotplate burners and BBQ. So, the planned freezer space is 150L and the food/drink fridge will be around 60-80L in capacity. There will also be large underfloor insulated boxs for whole fish etc and large amounts of beer will be kept in here prechilled with block ice in PET bottles before departure. From experience i can get around 5 days from this before the beer has gone too warm to be enjoyable and needs to find a fridge etc and the frozen bottles become drinking/cooking water... same goes for the freezer, fill it with frozen things first... the idea is to take as much stored energy with you as possible before unpluggin the shore power...

That said, whats a fair energy consumption figure of a 150L freezer and an 80L fridge, assuming its well insulated with 3inch foam? I have the current draw specs on the compressors but the actual consumption per day is too variable to make a calculation, id simply ask those with experience to chime in here... but at the end of the day, i can always add another battery or 2 if i find my energy needs are not being met - i cant stand the noise of wind generators and engine generators running however, i will put more panels and batteries in if i have to, whatever it takes to avoid that damn noise.... one of the best things i enjoy when going away is the peace and quiet of a secluded anchorage, leeward a rainforest covered island beach on the great barrier reef with the smells of the days catch cooking on the BBQ wofting across the deck with beer in hand...

I was under the impression AGM batteries could be run down as low as 30% of their capacity without damage?

 

[groper]

an updated GA and profile;

Everything has been moved aft 700mm as im worried about having enough bridgedeck clearance at the front of the tunnel due to the fine bows... does anyone know a good method for determining this clearance with respect to forward reserve bouyancy and speed?

Now everything has moved aft, its looking a little bum heavy... although the hulls have a CoB at 40%, the masses are still quite aft, i may have to extend the transoms to get more bouyancy at the stern...

 

[Richard Woods]

You may find it is better to have a more gradual slope to the forward part of the bridgedeck. Many multihulls are too vertical fronted.

If you really mean your CofB is 40% aft from the bow then it is almost certainly too far forward for any boat, but especially for a powerboat

I guess you saw these comments on the FAQs page of my website

"Recently "ram bows", or a reverse rake, have become popular on beach cats and of course now feature on the new America Cup boats. They are even seen on some cruising multihulls.

I'm not sure about that though, it seems impractical to me. I have often had times when I've been anchored in light winds and when the tide changed have had the anchor line catch under the stem. Obviously with a ram bow the warp will not release itself under the hull but rather ride up to deck level.

At the very least this will be noisy and disconcerting, especially if you are asleep at the time. And I'm not sure what happens when hitting flotsam or weed, while coming bows-on into a dock or to another boat is tricky if you cannot fend off easily and safely. (And any damage will be on the WL not high up)"

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

[groper]

No, its 40% from the stern, so 60% from the bow... probably not the way a NA is supposed to dimension things, but then again, im certainly no naval achitect...

Ive read your blurb on "ram bows" and i agree with nearly all of it... What i have been trying to acheive is basically a plumb bow, with a tiny bit of aft rake for aesthetics... there is only 200mm of rake from the forward vertical to deck level. According to the cross section areas, the bows should be virtually the same volume as a plumb bow of similar dimensions. The 200mm has been "added on" rather than subtracted, so there should actually be slightly more volume in them compared to say a 10.4m boat with plum bows etc... I think it would also be easier to build ram bows with a very fine entry on the waterline compared with other designs... i beleive this helps lower resistance with a very low entrance angle.

Still, im wondering how to best determine the forward bridgedeck transition location in order to maximize volume in the boat?

My best reckoning is putting it about 32% from the bow at 750mm above waterline assuming zero angle trim, hows that sound?

 

[groper]

BTW, the underwater hull shape ive designed is very similar to this one;

Its different to the usual displacement shapes, primarily designed for operating at high froude number... Im hoping i can cruise at a froude number around 1.0 without burning TOO much fuel...

 

[masrapido]

Hmmm... Am I the only one that cannot see the underside of the hull on the photo...?

 

[groper]

The INCAT shown above, is built from aluminium plates, and so has chines... but its similar in form to this with one key difference;

The fullness of the midbody aft does not taper off to a thinner and shallower section- because its a powerboat rather than a sailboat and we dont want it to squat. Sailboats have the opposite problem of course, so they remove some bouyancy from the stern. Its an interesting form whereby the front 1/3 bow section produces a wave that partially cancels the wake of the midbody and lowers the overall wave making resistance.

 

[groper]

This rendering shows the shape better...

And this is the 3D rendering of the boat as its currently dimensioned in the 2D autocad general arrangement i posted previously;

I think im about ready to start cutting some frames...

 

[Mr Efficiency]

So you've gone from a transom clear of the water to a deeply submerged transom ? This could restrict your efficient speed range, no ? Don't cut those frames just yet !

 

[Mr Efficiency]

Also, there is an issue with outboards on a deeply submerged cat transom as regards keeping the powerheads far enough out of the water, particularly the h.p. range you are talking, with 20" legs as long as you can get. At the other extreme, with a transom just kissing the waterline, you have the problem of running the cav plates well below the bottom to stay in solid water, with a marked increase in drag.

 

[groper]

Hmm, nicely spotted mate, thanks for the consideration - ill have to look into this more closely...

The designed displacement of this boat as it sits at this point in time is 3500kgs. At this displacement, the waterline is 400mm from the keel. If i sink it further down on its lines to 450mm, it displaces 4150kgs - which i would call its maximum displacement when overloaded as the bridgedeck clearance would then be reduced to 700mm.

According to the yamaha specs, they reccommend a transom height of 508mm, this puts the cav plate 1 inch below the transom bottom. http://www.yamaha-motor.eu/uk/binary/F70 uk_Leaflet_tcm230-414389.pdf

So as it sits at 3500kgs, the transom top where the outboard bracket bolts onto would be 108mm clear of the waterline at rest. Underway, backing down on a fish, i wouldnt be comforatable with this, probably drown the powerheads when reversing into the wind... damn, i might have to change things to look more like the americas cup hulls and see if i cant get another 100mm... or just bite the bullet and put 25in shaft 90hp 4 strks instead...