35' cat concept for the inside passage.

Just low enough to direct the surface piercing propellers from that 750hp engine (this boat is about 40 ft loa catamaran and weighs about 4~4500kg and does 150+mph) - man what a blast from the "rooster tail" over any fool that gets close to the stern....

On a more serious note, this is 33ft (10metres) has 2 double berths, a 'pilot berth and reasonable space inside uses a pair of 50hp engines and is very efficient...

 

Are you a sales rep for chamberlin or something masalai? Youve posted, pimped and plastered the same damn pics of the C10 in almost every thread on here... give it a rest will ya?

The C10 is a great example of a 10m powercat, i like it alot, but its not for everyone and ill gove it some critique right now...

Firstly, the interior layout in the saloon is not very appealing, too much of its volume is "boxed out" to allow for bigger spaces in the hulls.
Second, the exterior lines and aestheitcs i find quite ugly, the lines are not graceful and the forward bridgedeck up near the bow is very rounded/blunt which i not only find ugly, but because its carried so far forward with relatively low bridgedeck clearance, it would most likely prevent anything but very slow speeds in a headsea due to slamming.

Also, double beds that are oriented fore/aft with access from the side corridor, means someone has to climb over the other person in the middle of the night to get outa bed to pee, and you inevitably end up waking them trying to do so. Access from the foot of the bed is better.

Some access around the side of the cabin would be useful depending on the intended usage, C10 doesnt have it.
The biggest reason its so efficient, is because its very light, not so much due to the canoe stern... it still looks like it squats considerably underpower and so there is probably room for improvement in its hull shape... i see a considerable wake here;

 

As I said earlier, I should have bought it... But......... There is always a "But" is there not ?

 

Here some hull modelling data you may find interesting...

Firstly, ive constructed a 10.5m canoe stern hull model, as close as possible in form to the shape of the chamberlin 10m hull posted by masalai earlier.



The resistance results PER HULL are shown below;


Now lets take a look at another hull of identical displacement, identical length beam ratio, identical length etc. The only difference is the shape, and required draft to acheive the same displacement - so we are comparing apple to apples. Remember L/B is same, DWL is same, displacement is same;


OK, so why the difference? Well, the most stand out reason is because of the large difference in wetted surface area. This occurs due to the complex shape of the canoe stern which presents alot of area without contributing much displacement.

The wetted area of teh canoe stern hull is 13.339m^2 taken from the model i constructed.

Whereas the wetted area of the simple transom stern hull is only 11.336m^2, a difference of over 2m^2! This contributes significant extra viscous drag of in the order of almost 20%.

Back to you masalai...

 

Any sort of well submerged transom and you are gonna significantly increase the draft with the shaft drives. All a compromise.

 

Another thought, the shaft drives pretty well commits you to running both motors all the time, as single operation will be hampered by dragging the dead prop through the water. I know when I had power cats, I only trolled on one engine and tilted the other up. Certainly saved fuel, even at fast troll speeds.

 

It is all a matter of compromise and which takes first importance - on my boat I had to add minikeels to - a) - protect the propellers (saildrives) and - b) - to facilitate letting the boat sit on its own bottom when careening or the tide goes out... I do not have the instruments to accurately determine the cost of dragging an unused propeller - when running on one engine at 6 knots plus as the burn is pretty close to half with both running but a bit faster - not much though.... I will have to do some serious long distance runs to get some good figures... normal passage steering as well as following the shoreline to take advantage of tidal flows are easily handled by the autopilot... Negotiating bars and marinas I rely on both engines for greater accuracy and control...

 

There are options for having just one diesel engine running on a Cat, but have both props turning.

The Hybrid-Marine packages would allow you to do this - you could run either diesel, which turns that prop directly whilst generating power for the other side's integrated electric motor to be used. Take a good poke through their various pages/links - a lot of info there.
http://www.hybrid-marine.co.uk/

Basically they have electric motor in 10kW size, and can pair one or two of these to a gearbox that can be bolted onto a range of diesels. However, they do seem a bit expensive at this point......

But it does look quite an elegant solution to running just one diesel at low rpm while enjoying motor-assisted sailing, or displacement cat cruising.

Beta obviously like the idea, and they are marketing both 'seagoing' and 'broadbeam barge versions' which use differently cooled versions of their engine range.
http://www.betamarine.co.uk/seagoing/betahybrid/seagoing_hybrid.html

The Yanmar saildrive version (on Beta marine's site) looks quite slick.

 

I am not changing engines now, - and I feel Boston still likes the single engine on his mono and apair diesel engines as an option unless he reverts to steam... He reviewed engine options some time ago?

 

aaarrrggg
long evening out

OK so whats the deal
I notice we've morphed into a conversation over hull forms and efficiency
OK so whats going to be my best bet on a cat for fuel efficiency
I'm about to go spend a butt load of time figuring out the center of mass on this cuddy and if the hulls aren't going to be symmetrically balanced then now is the time mention it.

 

Mas, the point is you could add these hybrid units to your existing engines. If you wantd to. But the cost is a bit prohibitive, you will be better saving/allocating those dollars to mast/sails etc.

I suspect the larger 'PRM 500' gearbox they have could fit Boston's single diesel plans.

The way I'd use this hybrid system would be to have different sized diesels in each cat hull. The larger one for 'high speed cruise' and a small one on the other side (with a battery bank to balance weight distribution) for 'economical cruising'. For max. speed you would run both diesels together, and not use the hybrid capability. So the smaller diesel can't be just a genny. You still have the great redundancy of two diesel's as well as driving both props.

But prop selection could be tricky. Obviously as large as can be fitted, but it might be desirable to have a continuously variable transmission instead of fixed gear ratio in order to get maximum economy and flexibility.


OK Bos, your thread hope the last few posts are too much of a distraction. as well as further discussion on it (carve to a new thread?) I'm also interested in the hull forms posts a bit before that. I thought the Tennant/Maine Cat canoe-stern arrangement would perform better...

 

Well within the limitations of computer modelling, i think bostons original hull form were pretty close to ideal (surprisingly) from a resistance point of view. Why? Because the shape provides maximum displacement for a given water line beam and L/B ratio. Therefore by default, it will have the minimum wetted area when contrained by length/beam ratio and waterline length. The only change i would make, is to incorporate a very gradual flat run from around midships aft whilst maintaining the section fullness/squareness. This amount of taper would be have to be determined taking into account the center of bouyancy requirements based on bostons general arrangement and resulting ship Center of Gravity, and also the need to provide clearance for his props and rudders etc.

Theres other considerations like interior volume and structural stiffness which need to be considered however, so a slight deviation from vertical sides and flat bottoms will help in both respects with very little penalty to hull resistance in a multihull...

 

hmmmmmm so are you suggesting I loose the multiple chine form hull and replace it with the relatively flat bottom Bolger style?

I though you were happy with the rocker for an aft as well as the multiple chines. Seemed like they would help "slip" off beam seas better.

Richard, Kite surfing the PNW sounds cold. I'd use the kite when I was going across from the coast to Hawaii and while I was there some. I've looked into the sailing options in PNW and not only are the winds light but they are primarily running downhill most of the time. I was looking at information to see if I wanted wind a generator or panels and the wind charts are pretty lame for up there.

 

I dont beleive i ever commented on the chines... what i liked was you gave the sides a little outward flair so that you had some usable volume in the hulls for accommodation spaces...

If was building in plywood (which i was considering at one point) i would use multiple layers of 4mm ply bent into a flat bottomed U shape in the bilges of the hull. Once your above the tight radius`s, go for the 3/8 ply topsides which you should be able to bend into a slight arc ending in vertical at the sheer line. If you keep all the panels dead flat, you will have stiffness problem thats not easily solved without paying a significant weight penalty in the form of lots of stringers and frames...

Where you need to fair the midbody into the bows, gradually reduce the U shape section so there is less and less flat bottom going forward, eventually it will become round bilge before you taper it upto and form the prow. The careful shaping of this is why i would choose 3 layers of 4mm ply in the bilge that can be easily bent and laminated into this shape. The topsides have very little compound curvature (mostly londitudinal to form the bow) so the rest is easy.

 

Chines or round bilge the differences are NEGLIGABLE... Go with what pleases your eye and is cheapest and easiest to build... By now you should have a pretty good understanding of the merits or otherwise for what you need... I still figure that a 21 x 40 ft cat less than 6500 kg or thereabouts fully loaded ready to go is near ideal but I do not know the weather and sea state constraints in your cruising area... A cat, being light, and built using epoxy based glues etc., may be a fire risk structure so be careful of fire sources... At all other emergencies stay with the boat, with a fire, evacuate at the last second, or drown ??? I still like the look of your mono dream............