Various propulsion questions (35ft steel hull)

Discussion in 'Propulsion' started by Popeye, Jan 11, 2007.

  1. Popeye
    Joined: Apr 2004
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    Popeye Junior Member

    Hi All

    I'm not to sure where to start? A picture probably, here.

    I just want to get it in the water as a hobby fishing boat to start with.
    In 4 or 5 years it will be transformed into a livable unit (not sure of the name here) to go see Australia! (when the kids leave home)

    I need to know how the propeller should be set up what size, pitch etc?
    What would a reasonable top speed be for this type of boat?
    would 150HP be enough to power it?

    I do realize that it will be approximate but that is much better than I know now.

    I also need some input on where it goes (I know underneath). :D
    I always though the prop and rudder went up the back not in the middle?
    Perhaps its a trawler thing so the net won't tangle?
    Perhaps a bit is missing?

    I have a 150HP mitsubishi deisel motor and gear box that can be fitted in the boat.
    What are the Pro's & Con's for using the truck gear box agaist finding and buying a marine one?

    Thats a start
  2. gonzo
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    gonzo Senior Member

    You need to know the total weight loaded. Then calculate the center of gravity of the boat. The shaft line should pass close to it depending on the bottom shape. For example, a hull that tends to squat need the shaft line under the center of gravity; a hull that runs bow down the opposite. This is the aft side of the shaft. The gear ratio, HP, expected speed, etc. are all part of the calculation for the diameter and pitch of the propeller.
  3. Popeye
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    Popeye Junior Member

    Thanks for the reply Gonzo

    The weight is difficult to measure, the guy I brought it from estimated around 6 tonne?

    Sorry I don't follow the center of gravity part of your post. I'm illiterate with nautical terminology!

    For the moment a simple guess at the speed to expect for this type of hull would be handy for me, I don't know if 5 knots or 50 is realistic?

    The Keel(?) ends 2.6 mtrs before the back of the boat. To me this seems odd, the bottom has a 20 to 30 degree "Vee" from front to back if this helps.

    As I live in the bush I have no boaty people to exchange thoughts and ideas with, SO any comments here would be great!

    Regards Popeye
  4. Popeye
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    Popeye Junior Member

  5. Gilbert
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    Gilbert Senior Member

    I would GUESS you might do well with a 2 to one reduction gear and a rather ordinary prop of about 22" diameter and 22" of pitch. And I am also guessing that you will be doing displacements speeds, not planing. If it is a more easily driven hull than I think it is you might get ten or twelve knots flat out. The more information you provide the more helpful and accurate someone can guess. By an ordinary prop I mean one with three blades and a rather standard blade area ratio of about 38 to 40 percent.
  6. Guest625101138

    Guest625101138 Previous Member

    I did an approximate model of the 35ft boat pictured and used Michlet to determine hull resistance for a displacement of 7 tonne. You could expect around 11kts with 150HP. The 22" by 22" prop would be OK with gearing to give 1200rpm at full power. Economic speed would be between 7 to 8 knots and require around 50HP.

    The thrust loads for a prop of this size using 150HP are not to be treated lightly. You have to handle around 1.5 tonne pushing and as much pulling. So you need a well-mounted thrust box to react these forces. Then you have to contend with the angle of the shaft. You need to look at marine applications.

    I would be surprised if the original boat design does not have a layout of the prop shaft and rudder. It is not unusual to have the shaft extend well past the shaft gland. The overhanging shaft is supported on a strut from the hull or skeg from the keel to the rudder. If you look through the design gallery on this site or Google drydock images you should get an idea.

    Attached Files:

  7. hartley
    Joined: Feb 2006
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    hartley Junior Member

    Popeye ,,,,this looks like a worthwhile project .A couple of questions??
    Did any plans come with the hull?.if not you will have to fly by the seat of your pants ,You will have to ascertain the engine position,shaft angle rudder placement etc ,etc.see if you can obtain the original plans ,it appears to be a professional job
    1..this hull is a typical Aussie medium sized displacement trawler hull
    2 The shaft will extend well past the keel and be supported by a ''V'' or ''Y'' bracket welded to the hull ,this bracket will of course be part of the shaft bearing (Cutlass bearing)
    3 Is the rudder tube in place?
    4 Now to the engine 150 HP will be adequate ....but....the truck gearbox is a no no ,this topic has been done to death a thousand times ,it is just no good
    now I understand that as a ''Bushie'' you will probably be able to make it work
    But iif you can make that work you should be able to mate a proper marine gear to the truck engine ,it is not that difficult ,especially if you use a Borg Warner ''Velvet Drive''
    5 regarding the propeller,the Castle marine site is ok I don't understand the
    figures you came up with ,I will run them through and see what I come up with,also selection of the correct propeller is vital to the success of this project,do not take any notice of people that say a propeller of XxY ''should do the job''
    6 Lastly .if you cannot come up with the original plans,go to your library ,they should have books on these subject.
    By the way were are you ? in Qld maybe?
    Let us know the outcome.....cheers Hartley
  8. Popeye
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    Popeye Junior Member

    Thank you all (very much) for the input.

    I hope there is still more to come, it is very enlightening.

    Yes it was professionally built but NO plans came with it, unfortunately.
    I brought it from a guy in Sydney (Nov 2006).
    He said he got it from a liquidation sale of some boat builder (Sydney) some 10 years ago?? and did nothing with it in all that time.

    It has an engine compartment and two more below floor spaces (fish tanks perhaps?) built in.
    A "room" up front (galley perhaps) just needs fitting out?

    NO rudder fittings or prop tubes are visible, so I'd say they never got that far!

    Thanks for clearing up my perception of the keel, funny what you expect when you have never seen the real thing.
    I had a look around the net and I now picture something like this (one prop though).


    Some links on the gearbox would be appreciated, so I can learn why not!

    I understand the prop is important, and probably the last thing I need to worry about.
    But it seems to me, I at least need to have some idea of size etc to go ahead with the rest.

    I'm in Bombala, my boat is in Buxton, 500km away (my brothers place) if that helps. :)

    The figures on the web site are my guesstimate (+ or - a few cm's) of the length at water line etc.
    I redid the propcalc thing, get different results each time, strange?

    (NOT because of my seamanship, but "POP EYE's" when at school.) :)
  9. hartley
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    hartley Junior Member

    Popeye....Re propeller ,firstly a couple of things don't seem right .you say 26 ft on the waterline on a 35 ft boat,if thats correct it is not the type of hull I thought it was .however before I noticed that I put some figures in a couple of prop calculators.I had to make a few guesses here ,not knowing anything about the engine ,and doubts about the waterline length.
    Displacement hull ....I used 8 ton ....30 ft waterline ....140 HP ...max RPM 2000.....cruising RPM 1500 ....gear ratio 2.1 .This gives 7.5 knots ...shaft HP req 36...2 gals per hr ...propeller 26in diam by20in pitch . this gives a diam/pitch ratio of 1.3 which is good

    Semi displacement hull ...max performance gives you 11knots ...shaft HP of118 at 1900 RPM...7 gals per hr.....which is not good

    Cruising performance of this hull will give you 8 knots ..shaft HP 57 at 1500RPM 3 gals per hr ....which is good

    As you see propeller diam and pitch is the same in all 3 examples ,one thing to remember ,on slow speed hulls ,the slower the shaft RPM and the larger
    the prop diameter ,the more efficient the propeller will be .

    Regarding shaft diam ,it is usual to divide the prop diam by 14 ,thiis will give you 2in , you might get away with 1/12 if it is not too long
    More on gear later if you wish
  10. nautical
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    nautical Junior Member

    Do you have to enter prop diameter in meters of in inches into that program. IT say the shaft is revolution is not good for the boat. what does that mean.

  11. Popeye
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    Popeye Junior Member

    Hi Nautical

    It seems all prop sizes are in inches, so I assume it is inches.
    I'll have to get back to my boat soon, new job etc diverted attention, bugger!

    Thanks for the replies to my question, I will have more soon.

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