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  1. #51
    Join Date
    Apr 2011
    Boat- SoF

    Default Re: Jet Ski and boat collision - Calshot - afternoon Monday 7 May 2018 - 2:30pm ish

    The vid of the Miami Vice Wellcraft Scarab 38 shows stern suction .
    It’s exacerbated by narrow beam as you say more so at D speed when they reach max D speed .
    Infact at .50 secs when it starts to lift ( max lift from the shaded V area on the pic I already posted ^^ ) the whole thing starts to early plane
    You can fit pressure sensors which what Lindsay Lord did on models in a tank and real hulls ,
    Drill holes or open seacock s ( JFMs suggestion if more / less in the correct place ) - all a bit of a impractical phaff ,
    Or —- find a vid —— hey presto to illustrate.

    Your vid above is interesting in some ways ,
    It’s pancake flat river running hardly a sea F 4 ?
    To me they talking about the engine cooling water flow pressure , which I presume ( happy to be corrected ? ) like any other out drive comes from the leg .Otherwise the leg oil will boil .
    There may be additional entry being such is the size of the blocks .
    Interestingly they are limited ultimately by the weight distribution, too much lift too far Fwds or is that stern too low ?
    AoA is crucial for min drag .
    At those speeds the resistance is all abour reducing the wetted area ,hence the stepped hull to intro air to reduce the WA
    I suspect the main lift comes from the hydroplanes on the leg , they early on discuss a warping / bending while standing at the back looking at the legs .
    Interesting how they use a straight ruler to test the hull alignment.
    Water is 800 x more dense than air and at those speeds ( pancake lake speeds every mm means a lot .
    Hence shave a bit off under the mid section ,or recontour more like to lower the bow a fraction .

    It’s actually a fly over boat deliberately so .
    With regards to seaworthiness. ( which this isn,t ) the planing hull is designed in order that the bow bottom hardly comes out of water, its vertical motion amplitudes becomes small in waves. And there is little possibility of ‘fly over’ phenomenon. Therefore, if the real every day hull form often operates in rough water, the hull should be designed in order that the bow comes out of the waves as little as possible.
    That boat is at the opposite end of the spectrum.

    Ultra Speed and seaworthiness are antagonistic

    As said earlier above in one of my posts with the jet skis nozzle , like out drives you can trim them .
    At the risk of increasing drag at the higher speed ranges .
    Note no flaps as well like a jet ski .

    Deadrise looked poor too .
    This because it’s just a speed machine for pancake flat rivers / lakes .Its got some but it’s designed to lift right out by the legs not to bash down waves .
    Last edited by Portofino; 13-05-18 at 07:44.

  2. #52
    Join Date
    Mar 2002

    Default Re: Jet Ski and boat collision - Calshot - afternoon Monday 7 May 2018 - 2:30pm ish

    Quote Originally Posted by Portofino View Post
    Ultra Speed and seaworthiness are antagonistic
    Well, obviously we must, yet again, agree to disagree - completely!
    What you are saying is certainly true of drag boat racing, but the boat in the previous video is a full flagged offshore racing boat.
    And as I believe to have already said in the other thread, for seagoing boats, speed and seaworthiness are just the two sides of the very same coin.

    If you don't mind me asking, have you ever actually tried a Fountain or similar boat?
    The record breaking one in the previous video (albeit specifically tuned just for max speed, in that occasion) is based on their epic 42' hull, a machine akin to the FB patrol boat I posted in the other thread.
    Both of which can eat Itamas and the likes for breakfast in ANY sea conditions.

    Just check out the other video below, taken in SoF, in white foam sea conditions.
    Among other boats, the blue Fountain has essentially the same hull as the boat in the previous video.
    There's absolutely no way to keep going at those speeds, unless you've got a seaworthy and extremely stable hull.

    PS: nope, in these boats the cooling water pickup ain't in the leg. It's in fact the bit pointed by Ben Robertson in the previous video.
    And those surface outdrives are based on drive sump. But we are already more than enough o/t, I reckon...
    If you wish to delve into that technology, the web is your oyster.
    Just collect all you already know on outdrives, and forget it altogether!

  3. #53
    Join Date
    Apr 2011
    Boat- SoF

    Default Re: Jet Ski and boat collision - Calshot - afternoon Monday 7 May 2018 - 2:30pm ish

    They are bouncing about ( excess vertical acceleration) all over the place , guys knackered after only 11 laps !
    It’s in a sheltered bay too **** reason here below
    Skimming over not parting , for as said obvious reasons of drag reduction .
    This thread point ( sorry for the TD folks ) was about stern suction and the uneven distribution of lift on a planning hull .
    By “Ultra “ I was thinking around 140 mph + btw .
    But the expression “speed and seaworthiness “ lets take out the ultra and insert high speed are antagonist, because going air bourne means you come down too - up down ,up down , up down .

    Other thread the AMG Cigarrette 50 I disputed the ride at speed given the deadrise is more / less the same as the alternative s .- gave a pretty detailed explanation revolving around low kg,s and lower planing forces narrow beams leading to more inclination to get air under it leading to increased vertical acceleration which is there in every vid for all to see .

    As far a max speed is concerned , that’s not disputed it’s the comfort that’s compromised
    I mean the pic of the Guardia Finanza FB boat gonna come down with a bang a fast bang mind agreed ,ideally it should not be in a fly over situation for comfort and safety.

    As said I have raced jetskis - think of them as mini extreme mono hulls , had a few spills from airborne situations or sudden turns or both at speed ,over 40 knots hitting the sea is like concrete,I’ve been knocked unconscious a few times .
    Saved by the impact vest .So with a L of less than 2M and approaching 100 Hp ( early 90 ,s ) we were hitting 70 knots where the sea state permitted- pancake flat .
    Back injuries common .Busted discs . - that’s all see below

    Last edited by Portofino; 13-05-18 at 12:49.

  4. #54
    Join Date
    Mar 2002

    Default Re: Jet Ski and boat collision - Calshot - afternoon Monday 7 May 2018 - 2:30pm ish

    And now, what on earth has the tragic death of an old uni colleague of mine to see with this debate?

    PF, it strikes me that your reasoning are strictly theoretical.
    The only experience you insist to mention with jetskis is as comparable with a FB long range/all weather patrol boat as my 2.80m dinghy is with my 17m DP.
    Interestingly, the only other reply you got from someone else who mentions to have actually tried racing boats is the one from Den100 in the boat porn thread:
    "That’s rubbish they’re really good in the rough, have raced a few in terrible conditions."

    See, of course that FB boat would "come down with a bang" after taking off at 70+ kts. Any boat would.
    The difference between those boats and sport(ish) boats like yours is NOT that those boats bang while your doesn't.
    The true difference is that those boats bang and keep going, while yours (and mine obviously, as well as anyone else's boat I can think of here in the asylum), in the same conditions, would bang and fall apart.
    It's that simple.

    But if you're not willing to come to terms with this well known fact, well, I'm not going to lose any sleep over it.
    Did I already say that I'm happy to agree to disagree? I really am, no worries!
    Last edited by MapisM; 13-05-18 at 17:56.

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