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Thread: Setback theory

  1. #16
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    That boat looks good. I was picturing something with less freeboard

  2. #17
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    Hi!

    The freeboard is fine, but there are drain hulls through the transom who concerns me. They are mounted a little low, so when I have twins, the water tend to come into the splashwell. However, I hope with the 350 sci that the water stays outside. With the Promax 300 with 7,5 inch setback, the transom was 3 inches higher in the water. So I hope we will be somewhere in between. It would make my day a little easier

    Cheeers, Toffen
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  3. #18
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    Quote Originally Posted by Toffen View Post
    Hi!

    The freeboard is fine, but there are drain hulls through the transom who concerns me. They are mounted a little low, so when I have twins, the water tend to come into the splashwell. However, I hope with the 350 sci that the water stays outside. With the Promax 300 with 7,5 inch setback, the transom was 3 inches higher in the water. So I hope we will be somewhere in between. It would make my day a little easier

    Cheeers, Toffen
    I have the same issue when four to five people are in the back of my boat so I bought some ball scuppers and they helped a lot. I think the ones I bought were made by TH Marine.

  4. #19
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    Hi!

    Whats ball scuppers .... Sorry, I am from Norway ! Thanks

    Cheeers, ToffenG

  5. #20
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    This is a ball scupper its like a check valve lets lots of water out and very little back in. The steeper the angle of your transom is the better it works.

    Is the 350SCi a straight 6 too?

  6. #21
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    Hi!

    Aha Thanks!

    350 sci is an inline 6 engine as 300 Verado. Approx. same weight as well. Can be delivered in 20 inch midssection.

    Cheeers, Toffen

  7. #22
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    Quote Originally Posted by Toffen View Post
    Hi!

    Just a quick question. As we all know a jackplate gives you less drag and cleaner water, and a more optimum LCG (longitudinal Center of Gravity).

    I read back in time in BassAndWalleyBoatMag that if you added 1 inch setback it corresponded to 1/12 of the weight of the engine. So if you have a 300xs of 229 kilo and 7,5 inch setback, it corresponds roughly having an engine of 372 kilo (229/12*7,5 + 229) directly on the transom plus the weight of the jackplate itself (CMC SS 30 kg). I am unsure about this theory.

    The reason for asking is that I am to mount a Verado 350 sci on my Phantom 25, and I have to figure out the correct setback. I plan to use a HD Bobs of 4 inches. The Verado weight in 300 kg / 12*4 + 300 is roughly 400 kg plus the weight of the jackplate (16 kg).

    My two Merc 2,5 EFI SS are mounted directly, but have 2 inch setback built in. Plus steering, extra battery, pumps, etc etc. Thats is 350 kg / 12*2 + 350 is roughly 410 kilo plus all the extras (approx. 50 kilos)...

    Thanks!

    Cheeers, ToffenG
    wow i just started learning how to do this
    you dont learn this math in high school here in the US

    lol

    https://www.facebook.com/scott.steffe


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  8. #23
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    Toffen, there is no need to guess. Get a sling under the boat as it is rigged now and determine the current C of G when full of fuel. If you like the way the boat handles and lands then you can attempt to match the C of G or make the needed adjustments while on the sling.

  9. #24
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    Good lookin boat!

  10. #25
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    cool boat long and lean must cut through the rough stuff like a knife

  11. #26
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    Toffen I dont think your going to see the performance that you would see if you just mounted 2 300XS thats what we are seeing in the center console world. If your purely after speed that is. There is a guy on the hull truth with quad 350's who rumor has it has been trying to accomplish what another boat identical to his has been doing with quad 300xs. I want to say the quad 300xs boat has worked the lowers or anything. The 350 is going to kill the 300 in the hole shot but in top speed is going to fall short.

  12. #27
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    Hi!

    There are several boaters out there who have tested both; the 300xs and 350 sci. In the beginning with the 1st gen lowr unit, the 350 sci was 3-5 mph faster than the 300xs. With the 2nd gen lower unit it was only 1-2 mph faster, mainly due to the new gearcase being larger. The 2nd gen did not like to be jacked either. Now the 3rd gen gearcase is coming, and hopefully it will be a little faster than the 2nd gen. I agree with you that it will not be much faster with the 350 sci, but the Verado has 3 years warranty in Norway. The 300xs is now likely to drop to 1 year. And 300xs need 96 RON and 350 sci needs 95 RON in Europe.

    There is no question that the 350 sci is a strong motor. Most likely 375 hp. But the 2nd gen lower unit has made it slower than when it first was released. Mercury Racing will announce a new Poseidon gearcase in November, and I hope then to see if this gearcase can be jacked higher / being faster.

    Cheeers and thanks, Toffen
    Last edited by Toffen; 10-15-2010 at 08:43 AM.

  13. #28
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    Toffen - there are several things that affect performance with the use of a jack plate or set back arrangement. it sounds like you are referring to the application of a 'set-back' specifically.

    The reloaction of the engine further aftward does change the static longitudinal CofG of the hull. this does have an effect on performance, but not a significant one, in itself.

    More importantly, the aftward setback of the engine (and the location of the lower unit and propeller) affects the "dynamic balance" of the hull. This is the effect of all of the acting forces on the hull - hydrodynamic lift, hydrodynamic drag, aerodynamic lift & drag, and all the static weights (forces) of the hull and payloads.

    Of significance is that the Dynamic balance of the hull is different for each velocity of the speed range. So, changing the setback of the engine also affects performance differently at each velocity of the hull.

    There are a few articles on how you can do these calculations manually on my website, if you are interested in more detail.

  14. #29
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    Quote Originally Posted by Toffen View Post
    Hi!

    The freeboard is fine, there are drain hulls through the transom who concerns me. They are mounted a little low, , the water tend to come into the splashwell. )

    Cheeers, Toffen
    I owned a Glasstron GT150 (14.5 foot hull) with a 115 Johnson on it.
    the water came in and out of the drain hole a lot, I never worried about it.
    the hull weighed about 600 lbs and the motor + gas tank + battery weighed about 400 lbs. so it sat a little low in the water.
    the rub rail was submerged at the transom .
    allso when you let off the throttle suddenly the wave allways came completely over the motor, so the motor allways went under for a second or two.
    I actually had a mullet fish laying in the splash well at one time and a sand dollar on another occasion.

    Oh how I miss that boat........

    Harry
    Last edited by Old Sloppy; 10-14-2010 at 06:37 PM.
    1994 Baja 170 Islander with Mercury o/b

  15. #30
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    Quote Originally Posted by Greg G View Post
    Toffen, there is no need to guess. Get a sling under the boat as it is rigged now and determine the current C of G when full of fuel. If you like the way the boat handles and lands then you can attempt to match the C of G or make the needed adjustments while on the sling.
    I have done this before and it works great. Also what Jim Boat has said is the most note worthy. Theres very few people that have studied dynamics of hulls and power more than Jim on the planet. He has several books on this subject. I would call Jim to discuss this further to save yourself some setup time.
    She sounds like what the doctor ordered for a power plant. Your hull will take on a whole new dynamic in performance. My not be quite as fast as with twins but will be a pleasurable change in sound and torque!! Cant wait to hear how she goes.

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