Help understanding hull design and sea capabilities of new boat

Discussion in 'Boat Design' started by Damon7, Dec 27, 2022.

  1. Damon7
    Joined: Dec 2022
    Posts: 4
    Likes: 1, Points: 3
    Location: Maland QLD

    Damon7 New Member

    Hi everyone this is my first post here and I am seeking advice on a savage bluefin I am purchasing. It currently has a 50hp diesel Isuzu c240 in it and has a proposed 10 knots max cruising speed. ( I haven't yet picked up the boat as I am putting down a deposit and saving up the rest as the owner has kindly agreed to this) I want to know the sea capabilities of this boat and my concern is most boats with these slowing diesel engines seem to be used only inshore. Is this because they are too slow or is there other reasons why it wouldn't be good for offshore. The boat has a small keel and is a fixed shaft with a rudder. I have five children and we just want something we can get out to the the offshore reefs here in Far North Queensland I don't mind going slow as I like to troll anyhow but is there other problems like in a following sea. And if the boat being too slow is a problem my next question is would a larger motor let's say a 75hp diesel or maybe a outboard plane this hull and achieve the required speeds or is it a design only for displacement. Thank you in advance for all advice.
     

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  2. bajansailor
    Joined: Oct 2007
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    Location: Barbados

    bajansailor Marine Surveyor

    Welcome to the Forum Damon,

    Savage boats seem to be very popular in Australia, including your Bluefin - here is one for sale with an Isuzu on a shaft drive, but I also found an advert for one with an outdrive leg instead of a shaft drive.
    https://www.boatsales.com.au/boats/details/1983-savage-bluefin/SSE-AD-13252898/?Cr=22

    The advert notes a maximum capacity of 8 persons 'in calm waters only' - but this is for an 'average' person of 90 kg - I am assuming that your 5 kids are all much less than this?
    Nonetheless, 6 of you in the cockpit might get a wee bit crowded?

    Re how you want to get out to the offshore reefs - how far away are they typically from your home port?

    Re putting a larger engine in her - don't even think about it. Use the boat with what you have, get some experience with her, and have fun, and then you will be in a better position to decide if you really do need to go faster or not.
    And if you really do then need to go faster (how much faster?), it would probably be easier in the long run to sell this boat and look for another boat that can comfortably cruise at your desired speed.

    You mention a max cruising speed of 10 knots - I am thinking this is more likely a maximum flat out speed, and a more realistic cruising speed would be about 6 or 7 knots (?) - but this is a good trolling speed, and you will be sipping diesel while trolling.
     
  3. Damon7
    Joined: Dec 2022
    Posts: 4
    Likes: 1, Points: 3
    Location: Maland QLD

    Damon7 New Member

    Thank you bajansailor. I have no intentions of going any faster or swapping the motor if I don't need too. It was more a question of safety in following sea's I thought maybe a speed fast then the waves might be required. We would be going about 50km around (30 nm) out but it's very sheltered by the great barrier Reef but can still get choppy at times.
     
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  4. kapnD
    Joined: Jan 2003
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    Location: hawaii, usa

    kapnD Senior Member

    The hull would definitely plane out with more power, but a bigger motor would probably require a totally revamped drivetrain, would be a costly upgrade, quite possibly worth more than the boat itself.
    This boat would be a good candidate for repower with an outboard.
    As for seakeeping abilities, it’s only a 20’ boat, so it will get kicked around pretty hard in any inclement conditions. The large spray rails indicate a wet boat.
    The low power would be a disadvantage if crossing an inlet with current and waves.
    The closed in bow and raised cuddly will deflect a head on boarding sea nicely, but expect a lot of spray to find the occupants in any crosswind with chop and or swells.
    Five kids makes for one very busy adult, but the boats small size may be an advantage there, as it will be easier to keep a headcount than it might be on a larger vessel.
     
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  5. Damon7
    Joined: Dec 2022
    Posts: 4
    Likes: 1, Points: 3
    Location: Maland QLD

    Damon7 New Member

    Thank you kapnD for your advice. I was worried that a larger motor might be a big job. I think I'm gonna leave it how it's is for now and maybe look at finding a turbo kit for the motor already in the boat if possible but I can't seem to find much info on this motor but slot on the larger version of it. It's a c240pw20 Isuzu but all I seem to find is c240pw28's which I believe push out a fair amount more power (75 HP). Would you call this hull style a semi displacement hull.
    Thank you Damon.
     
  6. kapnD
    Joined: Jan 2003
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    Location: hawaii, usa

    kapnD Senior Member

    No, it’s a planing hull as far as I can see, but due to power restrictions can only operate in a semi displacement mode.
    Hot rodding a marine diesel is seldom satisfactory, and may require drivetrain and propeller mods to utilize power increase.
     
    fallguy likes this.
  7. fallguy
    Joined: Dec 2016
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    Location: usa

    fallguy Senior Member

    Sure looks like a planing hull to me.

    If you plan to use her offshore, I'd not take her out without adding a kicker motor on alternate fuel system.

    Even old diesel can stir up in big seas and foul the engine, and that is a big crew for a 20km radio only range for hailing in an emergency if you run out for tuna far.
     

  8. Damon7
    Joined: Dec 2022
    Posts: 4
    Likes: 1, Points: 3
    Location: Maland QLD

    Damon7 New Member

    Thank you kapnd for the reply and to everyone else for the information and input.
    Damon.
     
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