18ft planing boat, how much hp?

Discussion in 'Boat Design' started by Ozparker72, Apr 18, 2015.

  1. Ozparker72
    Joined: Mar 2014
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    Ozparker72 Junior Member

    Hi All
    Wondering if anyone can help me with some advice?

    I've just aquired an 18ft planing hulled, fishing/day boat.
    Heavily built GRP, my guess would be 1980's?

    She's currently got a Volvo penta leg, that I've removed. And she was powered by a v6 ford Essex inboard. Which has also been removed.

    So, Ive been given the boat and would like to use her as a launch/light workboat to my Tug. And occasional fishing boat.

    I plan to glass up the hole in the transom, and fabricate an outboard bracket to bolt on and hang outboard on.

    My question is, how much hp I would need to get her on the plane?

    I've been offered a 70hp 2 stroke, but I'm not sure if it has enough power

    Length 18ft
    Beam 8ft
    Deep v planing hull

    I would post a pic, but I can't work out how to?

    Thanks in advance
    oz
     
  2. Ozparker72
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    Ozparker72 Junior Member

    Here's a pic
     

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  3. Ozparker72
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    Ozparker72 Junior Member

    And another
     

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  4. Ozparker72
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    Ozparker72 Junior Member

    Transom
     

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  5. NavalSArtichoke
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    NavalSArtichoke Senior Member

    You sure you're not in Oz? Yer bote's upside down!
     
  6. Ozparker72
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    Ozparker72 Junior Member

    sorry about that, I'm doing this on an iPhone, Badly!
     
  7. rasorinc
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    rasorinc Senior Member

    A 70hp should make it plane unless it is rock heavy.................Can you lift up the bow?
     
  8. Ozparker72
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    Ozparker72 Junior Member

    Hi Razorinc
    It's pretty heavy, I'm quite strong and I can't lift the bow at all.
     
  9. PAR
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    PAR Yacht Designer/Builder

    Assuming you have a heavy boat, in the 5,000 pound range (that's a fat asss 18'), 70 HP will struggle to get you up on plane in certain (rough) conditions, mostly because of the shape of that boat, but this said, you'll manage the low to mid 20's once she's up on plane and scooting along good. If she's a 4,000 pound boat, you'll have the reserve capacity to punch up on plane fairly well and top speed will be in the mid 20's. A typical 18' boat like that will be in the 3,000 pound range and with a 70 HP, she'll run in the high 20's, maybe slightly over 30 in flat, smooth water.
     
  10. yofish

    yofish Previous Member

    You're wasting time with less than a 115.
     
  11. Ozparker72
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    Ozparker72 Junior Member

    Thanks PAR
    That's really useful, it sounds like 70hp may not be enough power, so better wait till something bigger comes along.
    Rather than buy an engine that may or may not plane the boat.
    I was hoping to go for something not too big, due to the fuel costs.
    Which in the UK are insanely high.

    Maybe I'll look for a 90ho?
     
  12. PAR
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    PAR Yacht Designer/Builder

    It all depends on the weight. Drag it to the local weigh station, which can be found in a number places. Everything from concrete block manufactures to port authorities will have a set of truck scales. Take the boat and the trailer and weigh them. Then launch or remove the boat and weigh just the trailer/tow vehicle and do some simple math. A typical 18' boat doesn't weigh much and it'll plane off easily with the 70 HP. The cabin does add some weight over the usual bow rider or fish/ski, but not all that much, so get a fairly accurate weight and see where you are, before plunking down cash for an engine you might not need.
     
  13. yofish

    yofish Previous Member

    "It all depends on the weight." Only partially true. Degrees V in given beam + lifting strakes = drag. Also, the more V the more HP to get over the hump. The world is full of dead outboards that spent their life pegged; "Move to the bow uncle Harry!" I've done it myself more than once.
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    A quick look at the specs shows that Ford V6 was worth about 130hp, which renders your 70 marginal at best. I'd say anything under 90hp would not be advisable, you will not save fuel by flogging a 70 near its max just to get it on plane, and keep it there. You may even find with a full load of fuel and people/gear on board, it won't even get on to the plane. But, by all means follow PAR's advice and weigh it, I would say if the bare hull weighs more than 2000 lbs, a 70 is not going to do it.
     

  15. PAR
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    PAR Yacht Designer/Builder

    Assuming a 2,000 pound hull and a 1,000 load, you're running in the high 20's to low 30's on a 70 HP, which isn't anything to complain about. It seems Americans and Australians like to place as big an outboard as possible on the butt of stuff. This is fine, but not necessary. A 50 HP would likely get the boat up, though little reserve and the thing would be screaming through it's short life. A 70 HP would get her done, though heavy weather and serious chop would hamper her a bit. A 90 HP would be nice and a 110 getting close the what she once had (agreed, likely about 130 HP). It's all about the weight and yes there are other factors, but there's no chine flats on that boat, nor an extreme amount of deadrise, so . . .
     
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