35' cat concept for the inside passage.

This might help you, my friends cat has 800mm bridgedeck clearance, this begins to taper up to the deck at a point located 3.5m from the bow stem. I was worried this was too far forward, but he assured me that it doesnt slam in rough seas... obviously there would be point where it would, but i guess he was shrewd enough in his seamanship to avoid such conditions...

The new drawing looks good, however the skinny hulls are only going to work if you can pull this build off with minimum weight... so i guess now would be a good time to discuss all the things you need/want to carry in this boat... you really need to consider EVERYTHING, from the number of people and fuel/water tank capacity, right down to the types and even brands of electrical equipment, desalinator, power generation etc so you can weight everything and its location to determine the center gravity and how much displacement its really going to need when its heavy ship.

In my design, ive gone the lightweight high performance spiral... therefore the power generation is all solar with top up from outboard alternators, all high efficiency electrical equipment such as LED lighting and efficient refrigeration with no air conditioning as its to be used in a warm climate - no heating either... so no genset, non electrical powered cooking, no TV`s and crap like that... so im saving lots of weight like this... The boat will be very fast and highly efficient which means low fuel consumption, so i dont need huge fuel tanks to get decent range, and the engines only need to be small and lightweight outboard motors - which also means no rudders and crap hanging underneath either to slow me down... the only time i plan on being below deck is to **** and sleep so i dont mind the hulls being skinny - so you can see the sprial ive chosen...

i suspect yours will go the other way however, and youll end up with much fatter hulls... at least this means you can have some decent volume inside them - which i dont... reason i say this, is that you wanna show off classy timberwork, fireplaces, cold climate etc your gonna wanna carry lots of gear

 

As you plan to cruise the Inside passage to Alaska and will only be travelling at displacement speeds (under 10 knots) you won't be leaping from wavetop to wave top as an open ocean fast fishing boat does.

So having a bridgedeck clearance similar to a sailing catamaran should be OK. So say 20in, not 40 in. That will reduce freeboard, hence weight and windage. So you can fit smaller more fuel efficient engines, again reducing weight. So your design spiral goes in the right direction

Having said that I still think you would be sensible to look at a 6000kg loaded displacement. That assumes a 8-10 knot cruising speed is still OK for you

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

Hi Richard, that 40" inches was an error on my part, its more like 36. But I'd like to be blue water capable in case I get an urge to really travel at some point. The previous design was at 31.5" so I'd be comfortable lowering it a bit.

the front elevation on the left is a 35' version and the one on the right is the 40' version. The windows are different cause I wanted to see what they would look like trawler style and flat.

I think I'm sneaking up on something

Groper
thats crazy

If you scale both those dimensions off the drawings I've done my clearance on the original drawing was 31.5 and and the tapir starts at 10' back. Or almost exactly what your friend has done. Guess I wasn't doing so badly after all eh.

 

I would try a broken sheerline, I think.

 

I come up with a target weight of 12,160 lbs or about 6 tons. at 35' and 20,500 at 40'. If you look there's a big difference in how wide the submerged sections are on the right hand profile drawing which I'm thinking represents a 40' version. Although I'd really like to stick with the 35' for now. I kept the 35' version at about 6 tons as per an earlier suggestion. I'm starting to really like the 39' option with the big nice deck on the back. I'd aim for 8 tons on that one. I'm hoping for 8 knots on 30~40 hp max of 12. Although the M 300D engines only has about 80 hp so top speed is what it is.



The latest rendition with 6" taken off the bottom and a few more details drawn in. I'm leaving off a fly bridge for now because of weight and money concerns.

Oh and I'm opting out of solar panels, there damn expensive, pretty heavy, delicate, and they wear out over time. So you get to replace them every ten years or so. They also don't work at least half the time, more like 2/3. A wind turbine works 24/7, doesn't cost squat, is light, can be collapsible and I can fix it. Might be less aesthetically pleasing but its cheap, and so am I.

 

Hi Boston,
Thanks for the link, I will critique / comment when I have read it all... Is this to be your live aboard "escape machine" and what sort of range are you seeking?

 

You should do 8 knots with 40hp but you'll never do 12 unless you make the boat half the weight. Speed/necessary power ratio is roughly cubic

I have always found windchargers to be heavier than solar panels, noisier, require more maintenance and are more dangerous. I know solar panels don't work in the dark, but in summer in the PNW you can get 15 hours of sunshine for days on end and no wind.

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

Cruising @ 12 so probably 15-16 on the fly.
I would think it will have squatting issues in the bum and perhaps a reverse of rocker to a flat and a squaring up of those edges would be in order.

add: I am also using a bastardised Simpson Hullform from the Slipstream 15 http://www.2hulls.com/usedcatamaran-2005/Slipstream1.html

Click for larger view

 

Sabah, he doesnt know speed it will do, he is only guessing and his current build is the first one that doesnt have sails... i suspect you are correct however and i would be surprised if it went 16kts @ full speed heavy ship - he is gonna load it up with HEAPS of gear so its gonna be heavy...

Another powercat i walked thru recently, was built from full foam core construction, around the same size @ 46ft and another bastardised sailing cat hull- not sure the designer tho... Cost the guy nearly a million dollars to have it built down the gold coast... The installed power was 2x 140-160hp diesels (cant remember exactly) - its was full of gear and held some huge amount of fuel (again i cant remember exactly but i think it was 1200-1600litres per hull, he estimated the displacement around 10 tonnes and top speed was 20kts but again cruised @ 12kts or his fuel bill went sky high...

 

Hi Richard.
The M 300D motor puts out about 80hp, which seems like it works out perfect for running it well within its power range and not stressing the engine. Plus being able to give me a little extra boost from time to time.

I'm not sure what kind of wind generators your used to but the helical vertical wind turbines are silent, have no spinning blade but a rotating tube instead, and they can be DIY'd for a few hundred dollars, a fraction of what just one solar panel costs.. And yes the PNW has, like other regions a fairly calm season as well as a windy one. I've been meaning to build one anyway so I'll be able to test the theory before I commit to applying it to the boat. Kinda like I did with the diesel engine that runs on WMO. Also the intensity of the sun is significantly reduced the further north you go so again solar panels even the glow of an arctic summer may not work at there prescribed power output.

I very much appreciate your participation in the thread by the way.

speaking of which I always found the cubic relationship of power to speed a very interesting fact. I believe its related to the squared cube law. Or something like that.

 

And thats the thing
Speak to people who have actually done ocean passages and sea state usually dictates a 10 knot speed.
Fuel usage and range needed certainly does.

It then makes sense to me to build for this speed

Smaller engines = Lower speed
Lower speed = Less fuel required
Less fuel required = Less load carrying ability needed
Less load carrying ability needed = less strength and structure needed to carry load and cope with higher speed wave impacts.
All which in my opinion makes it a more efficient and more affordable boat which ultimately makes it a better boat.

 

How heavy will a pair of these be, marinised with gearboxes attached?
Will the hullform you propose be able to carry the weight effectively?

Add: 300D is the car
The engine is a http://en.wikipedia.org/wiki/Mercedes-Benz_OM617_engine

 

Sabahcat: He plans to use just one of these engines.

I briefly thought about doing the same thing he's doing. I have a Sprinter with the same engine (but the newer one with turbo and common rail). He is going to split the power to two props.

It's one hell of an engine, that's for sure. Pretty crazy. I'm in Florida. Just hit the old cruise control and keep feeing it diesel and I arrive in Canada in comfort and with ease. These engines don't miss a beat. Good choice of engine, Bos.

 

BTW, just as a reference data point, ive modelled the 35ft hulls ive designed in both michlet and maxsurf hullspeed, the results were almost identical in both... with a modest waterline, the displacement is 3.1 tonnes but would comfortably carry upto 4 tonnes with an increased draft... The predicted total power required for;
25kts = 79.9hp per hull
20kts = 41.4hp per hull
15kts = 19.9hp per hull
@ 3100kgs displacement, DWL = 10.5m, L/B ratio = 15:1, waterline beam = 700mm

This assumes a 70% drivetrain efficiency - a 30% loss in motive force. Does anyone know what the typical efficiency/losses of a midsized outboard motor is, and to keep it on topic for boston, a typical total efficiency the drivline losses of a v-drive, gearbox, shaft and propeller combo sized in the 50hp-100hp range?

 

70% is too high. The is efficiency of propeller alone is 0.70; then add hull and drivetrain efficiency. For planing and semi-planing speeds we take 0.65 or less for overall propulsive efficiency, for shaft drives. This also assumes that resistance of appendages and air drag are included; in Your case they are not.

We usually take 0.55 for outboard engines and sterndrives.