efficient 10m displacement powercat

[masalai]

Why this interest in an "axbow bow"? - That does not add to displacement efficiency apart from making the LWL a little longer than the LOD? WTF for?

Chamberlin C10 uses a pair of 50HP diesels, a horizontal shaft, gives better than 15 knots and at that speed is somewhere around 1 NMile / litre... It has done extended passages and is one I should have bought (everyone is wise in hindsight)...

 

[Mr Efficiency]

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...

It's a shame they don't make them for 25" in the smaller sizes, however at one stage you could get factory extension kits for some motors in that 50-70 hp range, and maybe still can. Someone may have experience with that, but motor drowning is an issue with cats, made more problematical by the motor sitting right out there on the corners, combined with the skinny sponsons.

 

[groper]

I can already see where this is heading... im going to have to put 90hp outboards on it...

This isnt such a bad thing tho really, the engines wont be working as hard for a given cruise speed and i doubt the fuel economy would change much, if at all, using the larger engines as they would probably end up in a more efficient rpm range for my desired 20kt cruise speed. The only penalty is 50kgs per engine and a couple thousand dollars on the purchase price, im happy with that...

 

[Mr Efficiency]

I can already see where this is heading... im going to have to put 90hp outboards on it...

This isnt such a bad thing tho really, the engines wont be working as hard for a given cruise speed and i doubt the fuel economy would change much, if at all, using the larger engines as they would probably end up in a more efficient rpm range for my desired 20kt cruise speed. The only penalty is 50kgs per engine and a couple thousand dollars on the purchase price, im happy with that...

20 knots cruise sounds a bit optimistic, I think too, you have to be mindful of the gear ratio and the higher reduction ratios and bigger prop diameters are preferable for your likely cruise speeds. Varies a bit between engine makes.

 

[Richard Woods]

20 knots with twin 90's should be quite feasible.

Here is a photo of my 35ft Banshee catamaran towing a waterskier at 22 knots with twin 90hp. We had earlier sailed this boat at 19 knots. Sorry for the quality, it was taken over 20 years ago.

This is a 26 year old design and obviously is primarily a sailing boat. So it has a sailboat hull, hence the extreme trim. Removing the rig, keels and rudders would increase speed some. Speed would increase significantly with a hull designed as a powerboat only

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

[groper]

HAHA, thats gold richard! i love it... can imagine all the bewildered looks you received from people you passed by that day... classic...

I think 20kts is perfectly realisitc aswell, ive been known for being a bit of a risk taker, and ive not published by plannned laminate shedule as yet... the empty weight of the boat splashed including engines should come in well under 2.5tonnes... using the 190m ^2 of panel area in the shell and structural furniture and the correct densities of each panel, im only upto 1500kgs...

Iill likely use carbon in key structural areas, ie beams... using twin 90hp instead of the original planned 60-70hp, i should have a displacement catamaran with one of the highest power to weight ratios in recent times i would think?

 

[groper]

One more thing, ive been thinking about the electrical system some more... the weight of all those panels and batteries and accociated fixings/wiring etc adds up to quite some weight... Im thinking a little honda 2Kva genny, mounted below deck with some sort of ventilation might be a better answer. I have unleaded fuel in the tanks anyway, run a fuel line directly to it... So only need the 2 semi deep cycle batteries for starting engines and running the fridges overnight. End up much lighter and cheaper... anyone done this before?

 

[Mr Efficiency]

Another thing to consider is whether your tilted outboards clear the water when the boat is moored. Not desirable to be growing barnacles inside the water intakes.

 

[masrapido]

The more I'm looking at the hull, the less I am convinced it is a good choice of shape ... Way too much surface, square end submerged and not a planning shape to bring the rear end out of the water, not even close to the surface... It'll create a huge hole behind it, and a huge drag. It will plough through the water.

 

[groper]

The shape is based on a minimum resistance high speed displacement hull form described in a patent from 1978. Relevant points i have extracted below;

 

[Mr Efficiency]

Groper, for your information, there is a high thrust 4 stroke 60hp Yamaha with XL shaft according to 2011 catalogue.

 

[groper]

Thx Mr E, but I cant find it, do you have a link?

 

[Mr Efficiency]

That was from a printed catalogue of Yamaha 2011 series outboards, engine model FT60DETX. Transom height is quoted as 25.4". Seems it is still current.......

http://www.yamaha-motor.com.au/products/marine-outboard/4-stroke-mid-power/10-ft60d
click on "Specifications" to see the two shaft length options.

 

[groper]

well geesus, wouldnt ya know it... it also lists the F70A with XL shaft and 2.33:1 gear ratio now too... looks like im back to 70hp outboards again

 

[groper]

Been doing some seakeeping analysis as i was worried about bridgedeck clearance with the fine bows im using... seems it should be ok. Heres a simulation @ 20kts boat speed of a randomized wave spectra on a 4m swell. Heading is 45deg from head seas to show roll characteristics aswell.