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  1. #61
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    Default Re: Gludy

    Thanks for the comments.

    Jets in smaller boats of the type you have mentioned are really a different aniumal to thge jets I am talking about in cruising boats.

    In fact, the slow speed agilty of these boats is outstanding but yes, I imagine that they would use more fuel at such slow speed but this is of no significance compared to the overall use. Do not forget that the Cara Marine claim higher fule consumption at mid range speeds - but if I had a boat that can cruise at 28 knots more efficiently than a prop job, I would not cruise at 15 knots!

    You know now matter what energy you deilver to your props, the fact that they are not aligned in the direction of travel loses a lot of energy just pushing water at right angles and not contributing at all to the forward motion of the boat.

    What I would like to see explained is how a semi-displacemnt hull can achive so much. I would also like to see contributions about the claim that these larger cruising boats really do not plane - they just lift their bows a little higher but do not properly plane. Therefore we should not be using planing hulls for boats over 30 foot!!!

    My first boat did surprise people with its size and everyone raises eyebrows about my next boat - but I am com,pletely bonkers about the whole subject and no matter what I do I try to learn and learn. If that means challanging long held myths- so be it. I cannot lose, I always win because I always learn. I do not give a damn about being right or wrong.

    I did my first berthing alongside a wall in a small harbour the other day - about 10 foot away from the shops and with public 3 foot away. It was a tight berth and I handled the boat OK. When the public spoke to me, I explained I was an absolute beginner and someone on my boat questioned me why I had said that wehn no one knew and it was such a good bit of berthing ... I answered because it was the truth and I am not ashamed of it. I also know, that next time, I would just as likely hit something.

    I am pleased about the change in the moide of the forum as it keeps threads that deserve to be alive alive and lets the ones that are exhausted die away. This thread, which is already in three parts, would now be pages down the list and postive contributions such as that just made by tcm would be missed



    Paul
    Paul
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    http://www.multihulls4us.com

  2. #62
    tcm is offline Registered User
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    Default case adjourned!

    All agreed.

    Actually, i do think it a bit poor that uk boats seem very unadventurous with regard to lots of developments. Seems you do too. Lots of 20-year old items are very much the same on modern boats.

    I didn't mean to be nasty in using the word "boring" - as again i think you know! This dateslipping means that the post is on top again, as it was on thursday. With the old method, it wd still be on the first page, but not at the top. It's a "known thread" now, with all others having seen it and just a few contributing. IMHO it should be a bit further down, but still "alive", as before. It should predominate over "today's new threads" as it is doing.


  3. #63
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    Default Boatbuilders wanted, prev exp making V tablets req

    Gludy:
    Hi. I think I'll refrain from an engineering/thermodynamics type debate with you - you flipped massively back to semantics above!

    And I agree overall mpg is the best yardstick.

    But if the Cara boat has massive mpg (all other things being equal) is it becos of the jets, or something else like the hull shape? What other info is there on this Cara boat? Very little on their website, odd they don't publish data if they have launched 2 and are building #3? Do they claim it has a wonderfully efficient hull or issit all down to the jets?

    Do they have a demo boat? I guess not, if all built to order. I'll be in Carrigaline for 5 days across the 2nd w/e August so might drop by! If they are trialling one I'll drive alongside and GPS clock their speed!! Carrigaline is an odd place for luxury cruiser to be built, BTW. Not making any specific comments about Cara but before lashing 1/2 million on anything made there I would be very careful to examine it to check they have got the joinery/shipwright skills needed. It isn't Viareggio, and finding cabinet makers in a town whose only other big output (apart from farming) is Viagra, must be quite a challenge for Cara.


  4. #64
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    Default Re: case adjourned!

    "I didn't mean to be nasty in using the word "boring" - as again i think you know! This dateslipping means that the post is on top again, as it was on thursday. With the old method, it wd still be on the first page, but not at the top. It's a "known thread" now, with all others having seen it and just a few contributing. IMHO it should be a bit further down, but still "alive", as before. It should predominate over "today's new threads" as it is doing.
    "
    Not true - the third jet thread would be well off the page by now. The first one would be many pages back.

    Once a thread passes off the page, it is treated as dead and many who would have contributed do not.

    When I log on now, I see which threads I have not seen post on by the number in red brackets - its so easy and enhances the depth of debate on the forum.

    Say the jet thread drops to the next page say 3 pages down - then I have some real verified data on the Cara boat - I post it and it would remain buried in the old system - in this system this new real data would be there for everyone who has followed the thread to see.

    The old system promoted shallow depth topics and cut deeper threads off early.

    I did not mind a damn about the thread being called 'boring" I have enjoyed making you pay for the word all morning (must get some work done here)



    Paul
    Paul
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    http://www.multihulls4us.com

  5. #65
    tcm is offline Registered User
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    Default Re: mpg and urban cycle

    ah , careful

    For the sake of an example, lets say that thye prop boat does 2.5 mpg at 18knots, 1mpg at 28. Let's say that the jet boat does 2.0-2.2 mpg at18knots (10% -20% worse than prop) and 1.1 or 1.2mpg at 28knots (10%-20% better than prop).

    Now, it remains the case that the pleasure user out for a day may not alway blam along at 90% of top speed. On a nice day, in easy conditions, he'll throtle back to have a comfier ride, and a more extended ride, for half the fuel money. He won't say"But I have to stay at high speed! - because under those conditions I am more efficient than a prop boat!" - cos he's now in a jetboat. Throtlling back will still save money, but not as much. Only inveterate thrashers for whom nothing but 28knots will suffice , will find better mpg. (and they won't be out in F8 waves, I hope)

    Would this seem to answer why not so many jets on boats other than mpg-non-critical small speedboats, OR where a ferry operator does indeed have to make the trip at 28knots? Perhaps?

    What would be more interesting would be an "urban cycle" mpg figures. You'll only achieve savings if you blast along, but at easy crusing of 20 knots, you'll lose. There might be equal mpg at say 24 knots.


    and and, the thread wd still be on the page under the old system, aswell you know! Using the pub analogy, we would rightly be the old gits in the corner rabbiting away, but we'd still be in the pub and not have been thrown out (ie on "today's page") cos the thread has changed today. However, with dateslipping, this thread is front and centre , the main event: Tonite - same pub quiz as the previous five evenings continues!

    oh, and obviously a very possible option for rtw hmm? Or at leaast, driving thru fishfarms...








  6. #66
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    Default Re: mpg and urban cycle

    The urban cycle .... There is no big disaster trhottling back to 15 knots and being 10% less efficient. I accept that tat time folks do that. On the other hand the winds are 4 to 8 much more time than 1 to 4. So, in practice, to be out there there will be waves - those curvy things on top of the water. The result is that any any reasonable sea the engines with a prop job will be varying their load all the time - whilst the nice, whispering jet will always have a nice constant load on the engine at 15 knots, 5 knots or 34 knots.

    Then there is the big question of pottering along hitting floating fidges, nets, fish farms etc.

    I want a boat that I can take of to the Med for three months - I want to reduce the targets I can hit and have a safe boat in any seas that I might find myself in. Back home I want to crew a 60 footer with just two of us (hence slow speed agility) and a boat I can take out to get to places on nice sunny days that might still say have a force 5 like on the last time out. Hence the type of boat I am looking for has to give me a smooth ride in a force 5.

    In addition I want a boat that is very easy to maintain - having just spend over 1k to replace two little 3 inch long stainles steel pipes, I want accesability not just for the engine etc but for all the equipment.

    Yes the Cara Marine boat excites me because if the claims are true it meets my requirements. I will be able to visit places and dry out. Need to look at the hull - I can dry out.

    I will no longer join the Channel Islands list of boats from Swansea which seems to hit something on every trip. I will be safe, even in a force 8 and have a boat buyilt for it ( I would never want to go in a f8 - but in case I am caught I have a boat that can go in such seas every working day of its life.

    I have a fast boat and a boat that is economical at cruising speed and can turn on a sixpence in the marina.

    Question?
    How may forum members have hit something or touched their props in the last two years?

    Question?
    How many of our sleek, stylish boats we all like have accesability to every compenent without the need for dismatling half the boat?

    I first of all want a boat - secondly, but still important, I want luxury and space.
    However, my preference is to make sure above all it is a boat and that the functions of a boat are not compromised for a bit of curved style.





    Paul
    Paul
    St Francis 50 Cat
    http://www.multihulls4us.com

  7. #67
    tcm is offline Registered User
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    Default everything from one boat?

    Humm.

    To answer your questions, I'v not hit anything. Secondly, i've not had an occassion where needed extra manoevrability from a boat. Both these probl to do with decent weather, rather than awful western approaches like what you think is normal, but is in fact amongst most nasty in the world. Thirdly, engine etcaccess in 60 footer is always fairly fine - much better than any 35footer- so this isn't an issue.

    For a med boat, you need different things perhaps? Important kit is airconditionaing, and access from rear. Imprtant stuff is not ability to do 28knots in F8. Also, get rid of the greeen carpet in those piccies....


  8. #68
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    Default Re: everything from one boat?

    " rather than awful western approaches like what you think is normal, but is in fact amongst most nasty in the world."
    Do you mean that? Are you telling me that this absolute novice boater last September has been learning in those conditions? Yes, I do think they are normal - I am in for a pleasant surprise evrywhere else? Thats good news!


    The access in the Cara is superb and not just to the engines but to every piece of equipemnt form air con units etc. etc.

    "For a med boat, you need different things perhaps? Important kit is airconditionaing, and access from rear. Imprtant stuff is not ability to do 28knots in F8. Also, get rid of the greeen carpet in those piccies....
    "
    All points accepted. There are three access points on the Cara inlcuding the rear. The air con is full environmental control inlcuding built in dehumififier throughout the whole boat - even a cabinet you hang your wet clothes in and the de-humifier dries them!

    There is no way the green carpet is staying but then I will be specifiying a lot of the interior - its not a production boat and so each one can be much more tailor made.

    The boat has to cover all conditions for me, both at home where it will be based and in the Med. Should be good at flashing over fish farms in the Med.

    I have by no means decided on the boat ... who knows what will happen between now and next May when I plan to order my new boat. I am just on a pilgrim's progress so that by the time I get there, I can purchase with full knowledge of the pro's and con's of whatever boat I choose. I will ensure that everyone suffers with me on the way

    I will tell you what though - should I purchase the Cara marine boat you will be amongst the first to get on her and be able to report back to the forum .... !!!! Deal?


    Paul
    Paul
    St Francis 50 Cat
    http://www.multihulls4us.com

  9. #69
    tcm is offline Registered User
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    Default the med is on the way to Damascus!

    Ooh. I have been looking at the site and....it looks fabulous! I am a total convert to the wisdom of a jet boat, especially a nice new one.

    And offer of crawling over halfbuilt boat in ireland or alternatively zamming about in med gratefully noted down, photocopied and stuck on my wall! Mind you, i think all boats are fab, really. specially if i don't have to pay.

    As regards swansea, yes, i really do think that those seas are amongth the most treacherous that are generally used by powerboats. ok, so cape horn can be rougher but not too many marinas...But seriously, monster tides, prevailing F4, tight marina, means that the med is (relatively) a total doddle.

    INcidentally, if you do plan med/uk useage (and even if you don't) then there are some spec things that could be jolly good. One ace thing I chose was a wooden floor, like a sailyboat. Then, nice and cool in summer in med, and no carpet cleaning. And so on. Also, get lightest (colour) wood to maek it all feel big, imho.




  10. #70

    Default Re: More questions than answers

    Fascinating thread

    Some thoughts occur to me:-

    Jet drive propels small mass of water at high speed whereas prop boat propels large mass of water at low speed. Since drag increases as square of speed I would expect jet to have greater losses due to drag for a given amount of power input.

    However as power output from a drive equates to mass x velocity squared I would expect a jet drive to give more power output for a given input since the higher velocity is squared as against a higher mass moved at same speed by propellor.

    Don't understand the constant load argument. Load on props varies with depth of immersion due to increasing hydrodynamic pressure with depth. Since props are commonly fastened to boat it follows that this only occurs when boat (or blunt end of it) is more deeply immersed. A jet drive is also fixed to boat so I would have expected the same arguement to apply.

    Water density doesn't apply - water is incompressable (within practical limits) so density will not vary with depth.

    Dont quite follow the planing argument either. If non planing boat is moving at several times hull speed (rootWL x1.4) then it must be moving enormous amount of water. The system(boat +engine+drive) would be so inefficient that I find it hard to believe that the type of drive would make a significant difference either way.

    Drag on surface of hull: This I think is most complex part of argument. I thought the idea of a planing boat was that the boundary layer of water close to the hull becomes aerated allowing the hull to pass over the water without actually displacing it. Hence the less wash from a really fast boat. (sorry!)

    If this is the case I would worry about a jet drive that did plane since there must be a good chance of air or aerated water being taken into the drive and applied to the impeller. This must be a bad thing causing considerable variation in load possible enough to cause engine damage.

    It might be worthwhile looking at engine wear rates and reliability as part of your study.

    The inlet to a jet drive will interupt the flow of water along the hull disrupting this boundary layer and causing excessive drag from the area of the inlet and some considerable distance aft of that point. I would expect that this would also create a downward force on the aft part of the hull which would in turn mean that a greater amount of water would have to be moved at any given speed. The sums are too hard for me but I sort of think this would be a square law relationship too so much worse as speed increases.

    I think your will have to buy one and try it - gone too far down the road to back out now. I wish you luck

    Me

    I'll stick to my bits of cloth and string - this is too complex for me.

    Thanks again for the thread best reading on here for ages.


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