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Thread: Minimum power for half cabin

  1. #16
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    Re: Minimum power for half cabin

    I'm going to go down a different route and point out that the intended application of the boat is clearly unorthodox (for a planning hull), and there is no reason an unorthodox motor choice won't suit that limited application... Really short answer is that a 40hp motor will definitely push the boat along comfortably at displacement speeds, especially if you get a suitably low pitch prop on it. At 1,000 rpm a '100 hp' motor is only going to be making 10-20 hp, or as another example, electric trolling motors are tiny and move bigger boats around no worries (slowly). So if you only ever want to idle around and not get on the plane (though you might with a light load, smooth water and plenty of revs) it'll be fine. As others have said though, the boat will NOT perform as the manufacturer intended with such a small motor on it. You will be very limited in where you can use it safely and likely devalue the whole boat package. Assuming the reason you are looking at small motors is to save money, worth considering the full value equation (enjoying your time on the water is worth something, being able to use the boat for more things is valuable, change to resale could be more than the cost of a bigger motor).

    As a side note, might be worth investigating if there are any legal/liability issues with have an 'underpowered' boat, should something go wrong as a result of the boat not performing as it was designed to.

  2. #17

    Re: Minimum power for half cabin

    Trastan i bought a 4.3m tinny with a 55hp on it i later replaced the engine with a 25hp mercury bigfoot its a high thrust model off a house boat my speed went from 47kph to 32-33kph it was a huge difference all though the boat still planed heaps easy and quickly it still left me wanting the power of the 55hp engine but the fuel economy of the mercury 25hp was unreal

    Going at 15kph u wont like it one bit you'll go out and after the 4th trip of sinking a few thousand bucks into a engine you'll wish u had those few extra ponies

  3. #18

    Re: Minimum power for half cabin

    Quote Originally Posted by trasten View Post
    Hi all,

    Thanks for the comments but most are implying I'm trying to get to a good speed. 95% of the time I'll be boating in the swan river in perth where a lot of the speed limit is 8 knots and even 5 knots further upstream. The remaining 5% is possibly ocean fishing not far off from the shore and I'm happy to not do this at all if a new motor is going to cost thousands extra.

    If the motor can deliver ~15 knots and or a bit slower if against current and wind without struggling too much I think that would be more than enough for what I need.

    Just browsing around to get data for my purpose and found these:
    25 ft sailboat with 13hp diesel gave 6.5 knots max: https://www.quora.com/What-size-of-m...a-17-foot-boat
    18.6ft half cabin with 135 hp gave 38 knots. One third the power should give well above one third the speed due to the drag equation : https://www.hitechmarine.com.au/chiv...0l-tks-engine/

    I saw a boat yesterday with very similar length and shape to mine but seemed to be powered by quite a smaller suzuki motor compared to mine. Don't know enough to determine model.

    Any other thoughts?

    Cheers.
    Well if money is that limited and the use that restricted why not sell the boat and get an open tinny - eg around 4.2m? Also you haven't said that anything is wrong with your 80 hp. If it's running OK why not just keep it for a while and save up for an eventual replacement. I agree that the cost of mid sized motors is getting out of hand, but you could get a 2nd hand one.

  4. #19

    Re: Minimum power for half cabin

    Hi.

    We used to have a 50hp on a 4.55m alloy runabout.
    1st day out I thought 50hp was heaps.

    Then after a hot day out, always had extra water.
    Another day, problem with anchor, always packed a spare, plus mooring ropes and fenders.
    Another day, after long trip against current used most of the tank, I always carried a bit of spare fuel.
    Another day, battery got low, so added a 2nd battery.
    Wife wanted shade, bimini added wind resistance.
    Fishing gear, eskies, emergency jackets, etc etc.

    It got to the point where with 2 people engine had to be revved hard to get anywhere and with 4 people engine had to be almost redlined.

    Another point, 80hp 2 stroke will have more torque (nm) than a 80hp 4 stroke.
    So to just match the 80hp 2 stroke, you would need a 90hp 4 stroke.

    tjotter

    Sent from my SM-T820 using Ausfish mobile app
    Transire vinus forticulus

  5. #20

    Re: Minimum power for half cabin

    Old memories of old half cab hire boats.
    Never had a hope of getting on the plane, (couldn't tell you if the hulls were even designed to) but from memory also didn't need a licence for that reason. No idea how long the motors lasted, but I reckon they were worked very hard every time they went out. For longevity, perhaps they were rev-limited even tho they were small motors? No idea.

    Filled a need for some slow lazy days for many.
    Frustrated the hell out of many others.. hah

    As an aside.... there is a safety concern to be considered.
    I once hired a little tinny with an old seagull motor up at Shute Harbour many many years ago.
    It couldn't compete with the wind and current and I couldn't get out of the path of a bloody big ferry. Wasn't sure the skipper had even seen me. Bloody scary for a very young novice back in the day.

    There is such a thing as "too underpowered" for certain locations.

    I know a small tinny and seagull engine is not comparable insofar as boat and motor size.. however I've no idea of boat traffic (commercial) or tidal current at the op's location. Just thought it might be worth mentioning.

    All the best with your decision

    Cheers
    Gurn

  6. #21

    Re: Minimum power for half cabin

    Just speaking for my self, me and my cousins and mates go out in any weather even on blistering 42c days when itshigh humidity and after 7 hours on the water you feel like absolute crap just the sun draws it out of ya its those days u dont wanna be doing 8kph for 10km run back to the air conditioned car it really drills u that last effort of the day just knocks u

    On dayslike this i know i would rather head back at atlease 50kph rather than 8kph chugging along

    Also i have owned some slow boats one night we pulled the pin to head back under powered we just made it under a bridge and it bucketted down we had a break in the weather made it to the ramp loaded the boat and took off we got around the corner 2mins drive pulled into a servo for fuel and it hailed golf balls all 3 of us would have ended up in emergency if we had stayed out any longer, so the quicker your boat goes you can avoid situationslike this

    You may not out run a tsunami or big storms but it may be the differenxe between water and hail on a bad day

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