Raw water intake size Thru hull

Discussion in 'Powerboats' started by fpjeepy05, May 7, 2012.

  1. fpjeepy05
    Joined: Jan 2010
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    fpjeepy05 Senior Member

    I'm re-powering a boat that had gas engines and is now getting diesels. I was wondering if anyone knew if there is a general rule for intake size to hp ratio or such. I would like to use the watts high speed thru-hulls which are one inch and then run the sea strainers in the boat. The engines are Yanmar 6lp, 315hp. I would like to do this because the best place to put the intakes is in front of the props and I don't want to mess up the water by putting a scoop strainer or the other strainer there.

    http://www.freshwatersystems.com/p-...um=Feed&utm_campaign=Product&utm_term=THFZ402

    [​IMG]

    Thoughts?
     
  2. tunnels

    tunnels Previous Member

    :idea: Like is with superyachts and the like they have a really big inlet that serves everything thats likely to ever want water . one hole thats it ,so make it big !! Once and forever more !:p:D:p
     
  3. CDK
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    CDK retired engineer

    Any object smaller than 1" gets sucked in and lands in the strainer. If you use the type with slits, only tiny stuff can enter, so you don't need strainers at all.

    Instead of 1" (which is sufficient for your engine) I would recommend 1 1/2" because the intake velocity is much lower.
     

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  4. rxcomposite
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    rxcomposite Senior Member

    A seacock/valve must immediately follow the raw water intake for good design. If there is any trouble or blockage in the cooling system, you can shut off the seacock/valve and remove the hose.

    Also a good rule to follow is that the internal diameter of the engine cooling water intake must match the i.d. of the raw water intake to prevent restriction. Most probably you have a rubber hose connection and a pipe flange from the engine so your threaded raw water intake must have the same internal diameter. Use a bigger size as CDK has suggested and fit a reducer or a pipe flange to the seacock/raw water intake to fit the rubber hose.

    The best layout is raw water thru hull, seacock/valve, strainer, hose to engine. That way, if the strainer clogs, you can shut off the valve and clean the strainer.

    Large yachts have a redundant system. Two thru hull, cross coupled, with a clearing tube that goes all the way above the waterline. If one fails, the other side can be used while one is being cleared without closing the valve.
     
  5. CDK
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    CDK retired engineer

    In this case, the hose connects to the raw water pump, which will be only 1/2" or 3/4". The seacocks can be omitted if the strainers are above the waterline, but it is safer to install them anyway. I would use stainless ball valves with the handles removed and attached nearby with a tie wrap.
     
  6. rxcomposite
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    rxcomposite Senior Member

    Another consideration is that you don't put anything in front of the prop that will disturb the flow of water even if it is a flush or mushroom type. On the side maybe. You have a 315 hp. in there. Must be high speed.
     
  7. fpjeepy05
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    fpjeepy05 Senior Member

    It is relatively high speed, and that is the problem, there is no easy place to put the pickups that isn't in front of the props. The props are 24" SP and the stringers run just to the outside of the motors and the motors are only 26" wide. The only place is outside the engine compartment on the other side of the stringers which don't have good access in the case of an emergency.
     
  8. tunnels

    tunnels Previous Member

    While i worked in Korea we were making a fishing boat about 25 foot . The korean fishermen have a external water pick up system they always install and fits on the transom !!so no big holes in the hull it is fool proof has a screen on the bottom and the hose goes up the transom and inside to the motor . because theres so many plastic bags and lumps of alsorts of crap and things floating around strainers and bottom pick ups were a real hassle so they developed something that worked very well and never gave any probelms ever . Simple ,easy and everone uses them even on big boats just a bigger version of this and they never run out of water even on fast boats , the ones we were making did 42 knots full out !!:D:p
     

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  9. powerabout
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    powerabout Senior Member

    MerCrusier make one exactly like that is just not removable
     
  10. SheetWise
    Joined: Jul 2004
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    SheetWise All Beach -- No Water.

    I like that! Easy, and solves a lot of potential problems.
     
  11. tunnels

    tunnels Previous Member

    And no holes through the bottom !! I really dont like holes in the hull on any sized boat !! :):p:D:p
     
  12. tunnels

    tunnels Previous Member

    The stainless engineer that did all our work makes them for any one . becaus:pe of its design and where it is its forcing water up to the pump!, the faster you go the more water !
     
  13. sean-nós
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    sean-nós Senior Member

    I have one of those in your photo and want to run a 5.7 V8 do you think it will be fine without an inboard strainer also it may be a stupid question but what way do you face it, do you have the tapered end facing forward to help scoop up the water or backwards to help avoid any bits of debris.

    Thanks.

    [​IMG]
     
  14. Stumble
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    Stumble Senior Member

    Sean,

    Most engine manufacturers specifically recommend against using screens like that. If facing backwards the flowing water can actually drain water from the system, starving the engine for cooling water. If facing forward they can pressurize the water system and force water into the engine. Facing either way the slats make a great place for marine growth to get started and clog the water intake.

    For engines particularly the best option is an standard round hole that feeds into a sea strainer. These have large enough mesh to allow free flow of water, but small enough to catch junk in the water. Further sea strainers allow you to clean them out from inside the boat, while scoops require you to jump in the water every time a jellyfish gets jammed in it. These scoops are just a bad design idea from the get go in my opinion.

    See http://forums.sbo.sailboatowners.com/showthread.php?t=124964 for a nice write up.
     

  15. CDK
    Joined: Aug 2007
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    CDK retired engineer

    Sean, I installed mine with the slits facing the bow, hence the product name "water scoop". Not that it really matters, your pump will draw water anyway, but at speed there will be some dynamic pressure; I'm sure the pump impeller likes that.

    Today I did some last checks before the boat returns to the sea in a couple of days. I brought a wire brush to clean the water intakes but didn't need it because there was just some dry algae I removed with one finger.
    One engine was lifted out this winter, so I know the Johnson pump is clean; I guess the other one will be clean too.
    I installed the scoops 4 years ago and decided not to use strainers because the engine bay is pretty crowded and if one is clogged I need a dwarf to clean it. Until now, the need for a strainer didn't arise, although this is a very warm sea, full of life I don't fancy.
     
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