how to get bow lift

Discussion in 'Powerboats' started by weehenry, Oct 6, 2008.

  1. weehenry
    Joined: Jun 2008
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    Location: maghery NI

    weehenry Junior Member

    i had a 17x26 merc cleaver 4 blade witch was too big by 400 rpm witch is a lot when ive only got 3300
    so i sent it off to bret @ bblades he thined it rounded it and cupped the tips witch gave more bow lift but same rpm.s and speed he worked it three times but it made no difference.
    my best results were from this chopper type prop 16.5x24 but i cant find anyone who will make a simlar prop as i only got a lend of this one
    my boat seems to like more blades as i tried a 3 blade merc cleaver it was a disaster compared to the four blade ive always wondered how a 5 blade would performe but if i can find a 4blade 17x23 or 24 it would be a start by getting those xtra rpm.s

    thanks for your reply DSC00275.JPG

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  2. Volare
    Joined: Jan 2008
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    Volare Non lo so

    You keep going back to the same thing. Poor propulsion design and a lot of attempts to overcome its shortfalls with cupped propellers, raked propellers, foils, etc.. As you have noticed, nothing seems to change.

    As I stated before, your problem is staring you in the face, you probably have tripped over it far to many times already.

    Is the boat really this slow that you want to spend all your efforts changing it for a marginal improvement or are your expectations of speed that great whereby you are seeing a huge improvement potential for all this work?
     
  3. brunello
    Joined: Jul 2008
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    brunello Junior Member


    Marshmat you are absolutely right, I am a partner of Flexitab and I guess I should apologize for the bad etiquette.
    I hope that, being one of the few writing declaring my own name, no fantasy username, you might well believe that there was no bad faith.
    I still think that a pair of Flexitab’s would help and, in any case, I would never run a boat, especially a single screw semi-submerged, without a pair of trim tabs.
     
  4. weehenry
    Joined: Jun 2008
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    weehenry Junior Member

    what im sayin is
    my boat ran 65.4 mph with the 4 blade chopper that i barrowed
    and cant find a simlar prop now i have a cleaver on that is pullin 57 max
    now that i know i can get into the mid 60s its so fustrating beign stuck at 57
     
  5. brunello
    Joined: Jul 2008
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    brunello Junior Member

    Volare, given the fantastic improvements with your english, we might avoid Italian I guess.

    Please qualify your viscious statement: do you know who first built and tested a flexible composite tab? Do you have access to qualified informations, whereby you can assume that Flexitab has EVER copied Arneson?

    If so, please be precise. Besides, should you be another one working for Arneson and be qualified for making such statements, why don't you sue Flexitab?

    Please also read my recent reply to Rik of Arneson, to further understand that we shall no longer tolerate these unfair statements thrown against us.

    Technically speaking, congratulations on your believing that a fin right aft could be used as a turning or skid fin. That just proves how solid your hydrodynamical grounds are.

    Best Regards,

    Brunello Acampora - Flexitab srl - high tech marine equipment
     
  6. Volare
    Joined: Jan 2008
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    Location: Italy

    Volare Non lo so

    Mr. Brunello, i do not work for anyone in the boat industry. If you are so upset at what others make why are you not suing?

    You are the one trying to sell something, composite or whatever. This is an english web site so I write in english.

    Back to the problem with this persons boat.

    I read that IMCO has a twin drive one engine setup. This will give you bow lift and spread the torque to two drives. Get rid of the surface drive and then your speed will go up faster than what you saw from a friends propeller.

    That or offer a lot of pounds for your mates propeller.
     
  7. brunello
    Joined: Jul 2008
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    Location: italy

    brunello Junior Member

    "Volare",

    I am not upset at what others make at all, you are raising a non existing problem.

    I am simply very sorry about people, like you, making false statements on Internet about our company.

    While we have never been litigious, it is also difficult to defend ourself from anonymous false statements.

    Should we read a comment like yours, but duly signed, where the author was fully identifying himself, exactly as I am doing, we would certainly defend our brand reputation and sue the author.

    So, the wiser thing you should do is to carry on with your thread and avoid attacking or offending serious people who is doing serious work.

    Regards,

    Brunello Acampora
     
  8. Eddy Johansen
    Joined: Dec 2008
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    Eddy Johansen Hydrosport Sportboats

    I will not pretende to be a some kind of a prop guru, but my understanding is that your need a prop with round blades (with cupping) and a lot of blade area - as in a 4 blade prop. Aren't there anybody around (like Ron Hill or something) that can make one for you? PT prop?

    As for the hydrodynamics there are a lot of things you can do to create more bow lift:

    Interceptors in front of CG
    Speedrails in front of CG (reckon they are patented by ocke.se, but if you use a regular aluminum T-profile you get about the same result and the probability to get sued is quite remote - after you try a T-profile you might even want to order the real thing!)

    Or if you are a real nut like me and like to mess around with fibreglass and polyester, you can add strakes in the front; give a negative angle to existing strakes in the front; remove strakes in the stern or even add a false step in front of the CG. A step will not only lift he bow, it will lift the whole boat.

    Have fun!
     
  9. Eddy Johansen
    Joined: Dec 2008
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    Location: Portugal

    Eddy Johansen Hydrosport Sportboats

    Can someone please tell me how to remove this post? :confused: Original post was duplicated, I do not know why.:)
     
    Last edited: Dec 8, 2008
  10. weehenry
    Joined: Jun 2008
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    Location: maghery NI

    weehenry Junior Member

    hi thanks for your reply
    i have always thought of adding like a wedge forward of the cg to where the hull starts to straighten out to creat lift and maby a little air
    what you think
     

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  11. Eddy Johansen
    Joined: Dec 2008
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    Location: Portugal

    Eddy Johansen Hydrosport Sportboats

    Yeah...

    Makes sense to me. Reckon noone in this world can tell you exactly the result on your hull, but there is nothing as fun as trying!

    Usually I work most with steps (single or double, depending of desired result), but anything that creates lift and mixes air/water under the running surface helps lifting the whole stuff out of the water.

    Actually we have had most improvement in handling in speeds under 50-57mph (5,5m to 8m hulls - naturally it is higher with bigger/heavier boats).

    Over those speeds the improvement in using steps is not so evident and as all high performance boatbuilders have experienced at some point (if they say they have never had any problem with handling of stepped hulls at high speeds, they are lying with all their teeth) - when it starts to go really fast (80+mph) you often get too much of it and it is difficult to control the whole package. So the advantages of the steps are somehow lost. You might get a couple extra mph, but you have a hard time controlling it unless you run on flat water. I am here considering hulls of up to 8m (26ft).

    Anyway, I seem to have gotten carried away again... basically I think you should try!
     

  12. weehenry
    Joined: Jun 2008
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    Location: maghery NI

    weehenry Junior Member

    i rally appeciate you taking the time to reply eddy
    im gona give the wedge a try asap and ill let you know how i get on
     
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