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Thread: Power Cat 18MPG Interesting?

  1. #21
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    Default Re: Power Cat 18MPG Interesting?

    Quote Originally Posted by Woods Designs View Post
    Sorry, I'm still confused. Surely a boat's displacement has nothing to do with fuel consumption per hour? But engine size and horsepower does.
    is what I wrote and it seems you agree with me. I quite agree that heavier boats need bigger engines, which use more fuel. So a lighter boat can use a smaller engine and thus use less fuel. Or use a big engine at lower revs, which is (almost) the same thing.

    I was thinking of an engine in isolation, not necessarily even on a boat. Bigger engines use more fuel, so I meant it's the extra hp that uses more fuel, not extra boat weight. Just being pedantic I think

    My own Skoota 28 powercat uses twin 20hp outboards and cruises at 12 knots at 6mpg. It doesn't seem to make much difference whether we go at 10 knots or 12, the economy is much the same. So we tend to travel at 12 knots as we get there quicker.

    Since launching in March this year we've done about 1000 miles and lived on board for 2 months

    Richard Woods of Woods Designs

    www.sailingcatamarans.com

  2. #22
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    Default Re: Power Cat 18MPG Interesting?

    Quote Originally Posted by victor View Post
    Hi again Richard. Let me clear up the performance of my SeaWind under power. Under power I'm turning 3800 RPM doing 7 kn burning 1.1 GPH. Two 9.9 yamaha.
    I'm sorry but I don't understand when you say weight has nothing to do with fuel consumption. Please Follow my logic here, a vessel weighing 10000 lbs will do speed x a vessel weighing 20000lbs to go the same speed x will need more HP and more HP generally needs more fuel. Now if you have a 36 foot vessel that weighs say 4500lbs with two 38hp diesel cruising at 18 kn it's possible to get better fuel consumption than a 37.6 foot power cat weighing18900 lbs with 600 hp, which by the way gets1.2 MPG. Check Glacier Bay.
    Yes we also hope we will prove that the power marine industry has had it wrong by consistently perducing boats that are over weight and instead of using modern building materials to lessen weight they just keep increasing the HP. Reminds me of the American car industry and we know what happened to them.
    Richard thanks again for your input.
    1 liter per NM seems achievable. Bob Oram had a 40ft pod cat that did that at around 12 knots with outboards.

    I think Richard Chamberlain also built some cats around 36-40 ft that achieved something similar.

    You are hoping for something far better. Are you using modified Seawing 1000xl hulls and putting in diesels?

    I would be surprised if you can do much better than 1L per NM but good luck. Sounds interesting.

  3. #23

    Default Re: Power Cat 18MPG Interesting?

    Quote Originally Posted by downunder View Post
    1 liter per NM seems achievable. Bob Oram had a 40ft pod cat that did that at around 12 knots with outboards.

    I think Richard Chamberlain also built some cats around 36-40 ft that achieved something similar.

    You are hoping for something far better. Are you using modified Seawing 1000xl hulls and putting in diesels?

    I would be surprised if you can do much better than 1L per NM but good luck. Sounds interesting.
    You can achieve higher mpg at slower speeds for example a typical lightweight sailing cat running on 1 engine at 5 knts with a fuel burn of 2l/h is 2.5NM per litre.

    But to achieve 18mpg at 18 knots you have to be able to achieve 18 knots using only 1 gal this equates to about 20hp. Or 4 NM / litre

    Can't see how you would achieve your goal by installing 2 x 38hp

  4. #24

    Default Re: Power Cat 18MPG Interesting?

    [No we are not using SW HULLS this is a new designed power cat weighing 4500 lbs with two 38 hp diesels that we are planning to get 18 mpg at the cruising speed of 18kn

    You are hoping for something far better. Are you using modified Seawing 1000xl hulls and putting in diesels?

    I would be surprised if you can do much better than 1L per NM but good luck. Sounds interesting.[/QUOTE]

  5. #25

    Default Re: Power Cat 18MPG Interesting?

    Quote Originally Posted by Woods Designs View Post
    is what I wrote and it seems you agree with me. I quite agree that heavier boats need bigger engines, which use more fuel. So a lighter boat can use a smaller engine and thus use less fuel. Or use a big engine at lower revs, which is (almost) the same thing.

    I was thinking of an engine in isolation, not necessarily even on a boat. Bigger engines use more fuel, so I meant it's the extra hp that uses more fuel, not extra boat weight. Just being pedantic I think

    My own Skoota 28 powercat uses twin 20hp outboards and cruises at 12 knots at 6mpg. It doesn't seem to make much difference whether we go at 10 knots or 12, the economy is much the same. So we tend to travel at 12 knots as we get there quicker.

    Since launching in March this year we've done about 1000 miles and lived on board for 2 months

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
    Richard what is the weight of the skoota?

  6. #26
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    Default Re: Power Cat 18MPG Interesting?

    The Coppercoat antifouling line is 40mm above the WL of 2300kgs. So we are a bit lighter than that, say 2100 in cruising trim and with half full tanks

    As a rough rule of thumb I have found that a 35ft cruising cat needs about another 10hp for every extra knot over 10 knots. Speed is expensive! And by implication (as in cars) the slower you go the better fuel economy.

    Richard Woods

  7. #27
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    Default Re: Power Cat 18MPG Interesting?

    Quote Originally Posted by downunder View Post
    1 liter per NM seems achievable. Bob Oram had a 40ft pod cat that did that at around 12 knots with outboards.
    I was probably not quite clear here. The 12 knots cruising gave 1L/NM.
    The vessel was capable of some 18 to 20 knots at a higher fuel burn.

    I think Dereck Kelsall has done something similar.

    18knots at 1L/NM for 36ft vessel would be fantastic.

  8. #28
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    Default Re: Power Cat 18MPG Interesting?

    Quote Originally Posted by downunder View Post
    18knots at 1L/NM for 36ft vessel would be fantastic.
    Agreed. Many designers of lightweight powercats have designs that do at least 1L/NM when motoring in the mid-teens. However Victor is saying his boat will do 1L/4NM

    ie more than 4 times better than anyone else

    Richard Woods

  9. #29
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    Default Re: Power Cat 18MPG Interesting?

    Richard,

    that's why his proposal is interesting.

    Certainly a massive breakthrough.

    Cheers

  10. #30

    Default Re: Power Cat 18MPG Interesting?

    Quote Originally Posted by downunder View Post
    Richard,

    that's why his proposal is interesting.

    Certainly a massive breakthrough.

    Cheers
    here's a Roger Hill 10mtr Cat sith twin 60hp ob's designed for economy:http://www.powercatsnz.com/public/in...wdesign&ID=162

    Nowhere near 18mpg.......more like 3mpg! ( click 3rd thumbnail of magazine review).

  11. #31

    Default Rustybarge

    I always assumed that Rustybarge was from the film "Breakfast at Tiffany's" and then I found out that the character was Rusty Trawler. Is he your brother?

  12. #32
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    Default Re: Power Cat 18MPG Interesting?

    Quote Originally Posted by Woods Designs View Post
    Interesting:

    Maybe others with 27-45hp diesels can comment on whether they use less than 1/2 gal hr at cruising revs. I know the Kubota diesels on the boat I was sailing for the last few weeks are not that economic

    What will be your range under power? I assume you will carry at least 50gal fuel - which implies a 1000 mile range - so it would be easy to make it a true ocean crosser

    Richard Woods
    I have a 50hp yanmar in a monohull which probably weighs in at 25000lbs loaded (20000 light). If I run at 1500-1700rpm I get around 1.5-1-7L per hour at 5K. So the engine in question is bigger than you asked about and the fuel consumption is less than 1/2G per hour. I think at low speed where no real waves are being made a mono may be more efficient than a cat due to less wetted surface area?

    I never really ran it at much more RPM than that to save fuel. I am sure the HP at 1500rpm required to turn the prop is very small, maybe around 6hp or so? My entire pacific crossing averaged at around 1.7L per hour. But I only put a few hundred hours on the engine. Most people who are coastal cruising would probably be moving at more revs and speed than the figures I have given though. But it shows you can get good economy with a powerful engine at low revs.

    HTH Cheers.
    Last edited by dennisail; 27th December 2013 at 01:25 AM.

  13. #33

    Default Re: Power Cat 18MPG Interesting?

    Quote Originally Posted by dennisail View Post
    I have a 50hp yanmar in a monohull which probably weighs in at 25000lbs loaded (20000 light). If I run at 1500-1700rpm I get around 1.5-1-7L per hour at 5K. So the engine in question is bigger than you asked about and the fuel consumption is less than 1/2G per hour. I think at low speed where no real waves are being made a mono may be more efficient than a cat due to less wetted surface area?

    I never really ran it at much more RPM than that to save fuel. I am sure the HP at 1500rpm required to turn the prop is very small, maybe around 6hp or so? My entire pacific crossing averaged at around 1.7L per hour. But I only put a few hundred hours on the engine. Most people who are coastal cruising would probably be moving at more revs and speed than the figures I have given though. But it shows you can get good economy with a powerful engine at low revs.

    HTH Cheers.
    Your comments are probably valid but only at very slow speeds. To push a boat beyond hull speed requires a huge increase in power and fuel.

    Your few hundred hours on you Pacific crossing sounds like a lot, did it include motor sailing? Is the 5k GPS SOG or speed through the water?

    Some smaller monohull trawlers (circa 40ft) can achieve 1 L/NM but only at limited speeds 6 maybe 6.5k beyond this even if power is available fuel consumption is exponential. They tend to have slow revving engines with large propellers that are not compatible with multihulls.

  14. #34

    Default Re: Power Cat 18MPG Interesting?

    Quote Originally Posted by Peter View Post
    Your comments are probably valid but only at very slow speeds. To push a boat beyond hull speed requires a huge increase in power and fuel.
    Unless it's a cat. (Or a very skinny mono). Hulls with a waterline length:beam ratio of over around 8:1 can exceed hullspeed without dramatically increasing power requirements.

  15. #35

    Default Re: Power Cat 18MPG Interesting?

    Quote Originally Posted by 44C View Post
    Unless it's a cat. (Or a very skinny mono). Hulls with a waterline length:beam ratio of over around 8:1 can exceed hullspeed without dramatically increasing power requirements.
    Yes, I agree. Should have stated mono instead of boat.

    But the statement about the wetted surface of a cat may also be true and at very slow speeds a cat may well be less efficient than a mono hull.

  16. #36
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    Default Re: Power Cat 18MPG Interesting?

    Quote Originally Posted by Peter View Post
    Your comments are probably valid but only at very slow speeds. To push a boat beyond hull speed requires a huge increase in power and fuel.

    Your few hundred hours on you Pacific crossing sounds like a lot, did it include motor sailing? Is the 5k GPS SOG or speed through the water?

    Some smaller monohull trawlers (circa 40ft) can achieve 1 L/NM but only at limited speeds 6 maybe 6.5k beyond this even if power is available fuel consumption is exponential. They tend to have slow revving engines with large propellers that are not compatible with multihulls.
    Well Richard asked the question about how much fuel a 25-45hp engine uses at cruising revs and I answered. And the answer is a yanmar 50 uses 1.5-1.7L per hour at around 5K (GPS) on a 40 foot mono hull. Sometimes motor sailing sometimes full throttle against waves, this was our average. Exceeding hull speed is not cruising revs. I seriously doubt anyone here with a cat usually cruises at above hull speed under engine either. I know this thread is about a fast power cat but I answered Richards question.

    I think we done more like 150H or so, I can remember. But either way you don't know where we went, how long for, when our lightwind sails we destroyed, or how many days we had with no wind, so your assessment of "sounds like a lot" is pretty much invalid.
    Last edited by dennisail; 11th January 2014 at 03:01 AM.

  17. #37

    Default Re: Power Cat 18MPG Interesting?

    Quote Originally Posted by dennisail View Post
    Well Richard asked the question about how much fuel a 25-45hp engine uses at cruising revs and I answered. And the answer is a yanmar 50 uses 1.5-1.7L per hour at around 5K (GPS) on a 40 foot mono hull. Sometimes motor sailing sometimes full throttle against waves, this was our average. Exceeding hull speed is not cruising revs. I seriously doubt anyone here with a cat usually cruises at above hull speed under engine either. I know this thread is about a fast power cat but I answered Richards question.

    I think we done more like 150H or so, I can remember. But either way you don't know where we went, how long for, when our lightwind sails we destroyed, or how many days we had with no wind, so your assessment of "sounds like a lot" is pretty much invalid.
    I wasn't trying to be critical or disparaging. The implication that you motored for 200h (8 days) on a single Pacific crossing of maybe 15-20 days did sound like a lot. The type of cruising and long crossings often distort average consumption figures.

    I am very interested in the fuel burn and performance of different vessels. There are very few displacement speed catamarans and Richard is pioneering a new breed with the Skoota but there is still a lot to learn especially if the 18MPG as put forward here can be achieved.

    Your comments about wetted area are often missed because catamarans are so easily pushed through the water but it actually means that in order to be successful from an economy point of view a power cat should probably need to cruise at around 8-12 knots, where the drag from the increased wetted surface becomes less important compared to the wave making forces that monohulls are limited by.

    For example I burn 1.8 - 2.2 l/h at a speeds of between 4.5 - 5.5kts using 1 x 30hp engine on a 43ft catamaran that is approximately 2/3rds of the weight of your mono.

  18. #38
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    Default Re: Power Cat 18MPG Interesting?

    Kurt Hughes has some great info on displacement power multis on his website.

    http://multihulldesigns.com/power.html

    Your boat seems to use more fuel for the same speed even though its lighter. One could argue that makes it less efficient, but I am sure you have much more space than on my boat so if you compare that as well it changes things.

    I think its a given that a cat which weighs the same or similar to a mono will use more fuel at typical cruising speeds IE 5K. Now if you compare say a 44 foot 12ton mono to a 44 foot 6ton cat like Alans, then its probable the cat will use less fuel in all conditions besides perhaps beating into very strong winds. Cats have more wetted surface for the same weight, but if the cat is light (as they should be!) then this may not apply. IMO

    When I say pacific crossing I mean from the day I purchased the boat in California to the day I cruised in to my home port in Brisbane AU 10 months later including all the cruising in between. I would like to see what sort of cruising boat crosses from CA to AU in 15 days!

    Quote Originally Posted by Peter View Post
    I wasn't trying to be critical or disparaging. The implication that you motored for 200h (8 days) on a single Pacific crossing of maybe 15-20 days did sound like a lot. The type of cruising and long crossings often distort average consumption figures.

    I am very interested in the fuel burn and performance of different vessels. There are very few displacement speed catamarans and Richard is pioneering a new breed with the Skoota but there is still a lot to learn especially if the 18MPG as put forward here can be achieved.

    Your comments about wetted area are often missed because catamarans are so easily pushed through the water but it actually means that in order to be successful from an economy point of view a power cat should probably need to cruise at around 8-12 knots, where the drag from the increased wetted surface becomes less important compared to the wave making forces that monohulls are limited by.

    For example I burn 1.8 - 2.2 l/h at a speeds of between 4.5 - 5.5kts using 1 x 30hp engine on a 43ft catamaran that is approximately 2/3rds of the weight of your mono.

  19. #39

    Default Re: Power Cat 18MPG Interesting?

    Quote Originally Posted by dennisail View Post
    Kurt Hughes has some great info on displacement power multis on his website.

    http://multihulldesigns.com/power.html!
    Yes I like Kurt's approach but he has nothing that comes close to what is being discussed here.

    Quote Originally Posted by dennisail View Post
    Your boat seems to use more fuel for the same speed even though its lighter. One could argue that makes it less efficient, but I am sure you have much more space than on my boat so if you compare that as well it changes things.
    It would appear that way but I suspect this is more a function of the propeller. I also do very little motoring so it is difficult to assess consumption accurately especially when you factor in wind and current and restrict it to actual progress through the water, ignoring marina work and anchoring. Hence my question about Speed through the water and GPS (SOG), 1 knot of current at boat speed of 5 knots is 20%

    Quote Originally Posted by dennisail View Post
    I think its a given that a cat which weighs the same or similar to a mono will use more fuel at typical cruising speeds IE 5K. Now if you compare say a 44 foot 12ton mono to a 44 foot 6ton cat like Alans, then its probable the cat will use less fuel in all conditions besides perhaps beating into very strong winds. Cats have more wetted surface for the same weight, but if the cat is light (as they should be!) then this may not apply. IMO
    What you are essentially saying is that for the same wetted surface the cat needs to be lighter. So the real issue here is being able to compare like with like.

    There is not much difference between a sailing cat and a power cat, but there is a huge difference between a sailing mono (heavy keel) and a power mono (no keel). So we need to compare monohull PB's with catamaran PB's. The only benefits come when you increase the speed. A mono will be restricted by it WLL whereas a cat will not and the big question is can you do this while using less fuel.

    It is possible for a big heavy fishing boat maybe 40 tons (mono) to achieve 1 l/NM by using a slow revving diesel and a very big propeller but only at speeds between 5-6 knots. We also know that a sailing cat under power can achieve 0.4 l/NM at roughly the same speed but it's weight and hence wetted surface is also low.

    The big economy to be gained by power cats, as I see it is not in the fuel consumed but in the savings in sail gear, sails, masts and rigging and still exceed monohull displacement speeds.

    Quote Originally Posted by dennisail View Post
    When I say pacific crossing I mean from the day I purchased the boat in California to the day I cruised in to my home port in Brisbane AU 10 months later including all the cruising in between. I would like to see what sort of cruising boat crosses from CA to AU in 15 days!
    Sorry, but how was I to know that. A Pacific crossing usually refers to Panama (or Galapagos) to Marquesas, typically 15-20 days not an extended cruise.

  20. #40

    Default Re: Power Cat 18MPG Interesting?

    Quote Originally Posted by dennisail View Post
    Well Richard asked the question about how much fuel a 25-45hp engine uses at cruising revs and I answered. And the answer is a yanmar 50 uses 1.5-1.7L per hour at around 5K (GPS) on a 40 foot mono hull. Sometimes motor sailing sometimes full throttle against waves, this was our average.
    Really, to know how much fuel you use at cruising revs, you need to motor at your cruising speed in no wind or current, and measure fuel consumption, either with a fuel flow meter or by measuring fuel used from a full tank over a time.

    If you include motorsailing, or motoring into strong wind, (or anchoring etc etc) you're not getting an accurate figure. ie. how much motorsailing, how much into heavy wind?

    With cats it gets even more tricky. Most sailing cats are more fuel efficient using one engine, especially if they have folding props. Power cats may not have folding props, and their efficiency on one engine can suffer because of that.

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