Engine size for Farmers' Piute

Discussion in 'Boat Design' started by Green65, Jan 22, 2021.

  1. Green65
    Joined: Sep 2011
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    Location: Whitehall, MI, USA

    Green65 Junior Member

    Most things I can figure out. Once in a while it helps to ask for help. To call in a Professional as it were...and while I could assume the answer, that does not always end well.

    Weston Farmer designed the Piute and in a very colorful way (he always seems to) described his choice of powerplant. If I am to build this beauty someday, I will need a powerplant, and I do not have, nor want anything weighting 650 lbs. ...and who wouldn't want to instantly subtract 500 lbs.

    If I am correct, he is calling for an engine large enough to turn a 16 inch prop that when idled down to 1000rpm, still makes at least 20 hp. It also looks like the 16 inch prop is mostly preference and not really necessary.

    So, my question is: could a 20 hp engine be used to turn a 9, 10, or even 12 inch propeller and if gearing is correct, still provide adequate power to drive this hull to 18 or 20 mph, or would a larger engine be recommended?

    BTW, as someone else mentioned, I think Semi displacement hulls should be more popular. I also really like the looks of this design because it looks just like a small PT boat to me



    "The hull is of semi-planing type. She will lope along all day at 18 to 20 miles, will not be insufferably wet when checked down in the harder chances, and is premised on weight, length, brawn, and slow turning prop—about 16-inch diameter at 1,000 rpm.

    Now this is fortunate. You can use the modern and utterly reliable -40 to 50 horsepower runabout fours with a reduction gear and net the prop kick you need. Without a reduction gear on today's motors in this boat you won't have that Cadillac ride. Modern motors without reduction do not have prop diameter enough to give a real horse kick to the business end.

    [​IMG]
    Fig 2
    So, to get this kick on a direct drive, we can revert to type in motors, too, and use the kind shown Fig 2- an older type such as the Red Wing AA, or Kermath Vanadium 20. This kind of motor never seems to wear out in normal use.

    Motor makers are always well-stocked with such engines they have taken in trade, reconditioned good as new, and can sell for $200 to $300. Gray model Z, Kermath, Palmer, Red Wing— all are available today. They will swing the wheel area wanted, and they have Percheron horses in their cylinders—not hysterical Shetland ponies.

    Any motor of about 4-inch bore by 4-inch stroke delivering 20 to 25 hp at 1000 to 1200 rpm and weighing in the neighborhood of 650 pounds will be ideal.

    I mention power at some length, with highlights, because you won't get the feel designed into Piute if you substitute a lightweight, high output direct drive mill. Large diameter props of low pitch, wound up at 3,000 or so, won't give you the lope and ease and range you need. So much for feel."
     
  2. Green65
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    Green65 Junior Member

    It's been a while, but I am still wondering. Does anyone know what size engine (HP) would move this hull about 18-20 mph using a 12 inch prop, hopefully in the 2800-3000 rpm range? Thanks.
     
  3. kerosene
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    kerosene Senior Member

    so original calls for 40-50 hp engine - just ancient and thus very heavy one.

    I would then look at 50-60hp diesel. But I am totally unequipped to do anything but gut feel guesstimate.
    That being said many old time plans and marketing materials are quite optimistic from what I understand. Thus err on the upper side.

    And no 50hp engine makes 20hp at idle I believe.
     
  4. kerosene
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    kerosene Senior Member

  5. Mr Efficiency
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    Mr Efficiency Senior Member

    You mean move as maximum speed ? There is no way any engine will propel that boat at 20 mph at 1000 rpm, the sort of engine that could produce the necessary output would be way too heavy for the boat.
     
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  6. gonzo
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    gonzo Senior Member

    I have a 3 cylinder Volvo Penta diesel of 28HP. The plans call for 20-25HP so it is in the proper range. It has an extra transmission too. I haven't run it in a while, but it was overhauled before storing it. I will check it if you are interested, PM me.
     
  7. comfisherman
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    comfisherman Senior Member

    Most comparable modern power in outboard form would be the 50-65 hp Bigfoot series outboards by Yamaha and mercury. These were quite popular on the smaller utility skiffs up here for many years, fairly common powerhead with a deeper case and a larger diameter prop.

    On the inboard front not much exists in the gas marinized or easily marinizable market under the big block general motors platforms. Reality outboards and planing won the ideology war vs semi planning and inboard, the purist have to pay for small diesels. In both price upfront as well as weight penalty.

    In practical application 20 hp turning a 9 inch prop will be much inferior to the same 20 hp through a reduction turning a 16 as long as there is sufficient clearance for both. Those inefficiency introduced each revolution stack up. They can be really noticeable especially on the slower side of things.

    I'm very familiar with 2x boats that were from the same mold, had very minimal differences and identical power packages. One opted for a deeper keel and swung a 31 inch wheel the other did not and opted for a shallow ratio and a 28 inch wheel. Both weighed in at weights that were so close it was insignificant. The top end speed differences was a bit shocking. Both hit similar hulls speed efficiency but the smaller wheel boat was almost 5 knots slower on the top end. Now it was justified as it made the boat 5 to 6 inches shallower overall and in that particular fishery can equate to some rather significant advantage. It was a bit shocking to see the results.... as the owner of the 28 inch wheel boat.....
     
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  8. Mr Efficiency
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    Mr Efficiency Senior Member

    Good post I think, It also occurred to me that the high-thrust 60 hp outboard would be about what power would be required to get acceptable performance, you might get 14-15 mph cruise, and close to 20 mph flat out. But of course it would require modification to the boat, it is reliant on the deep skeg to avoid broaching. It is a slender boat, and a diesel inboard with enough power might be a little too much weight to carry.
     
  9. wet feet
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    wet feet Senior Member

    Post #7 is a great real life example.From my memory of reading Weston Farmer's thoughts on the matter,he was a great believer in using a good size propellor and I vaguely remember him quoting a formula for the size of the disc relative to the midship section.Taken together,these things would suggest that the prospective builder would do well to investigate the modest size diesels of 25-40 Hp that have their origins in small tractors or forklifts as even with a reduction gearbox that would permit a large (by present standards) propellor to be used,they may actually weigh about the same as the old iron.
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    It just won't be going 20 mph, that's all.
     
  11. wet feet
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    wet feet Senior Member

    Please enlarge on your comment as I would expect an engine that weighed about as much as the original to perform about the same as the original.I would also expect any diesel to be able to generate a lot more torque with which to spin a propellor.I do get the point about direct drive,but haven't seen a direct drive engine in decades and it was ancient then.
     
  12. kerosene
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    kerosene Senior Member

    Torque is not the appropriate measure but power in this context. Diesel’s low rpm torque is of no real consequence here.

    I too don’t quite get why modern inboard would be heavy, aren’t they much lighter than the engines of the old days?
     
  13. comfisherman
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    comfisherman Senior Member

    Modern diesel inboards aren't all that bad. A yanmar in about that 60 hp range will be under 500 pounds and will require a gear that isn't all that heavy.

    The shock will come with the price. Emission controls have meant the dropping of many smaller propulsion diesels from the lineups of even major diesel brands. Whats left are still neat little engines they just come at a price.

    Good inboard engines are long stroke big bore units, like referenced in the article. Sadly that's the opposite of what the auto industry has done to compensate for emissions for the entirety of my life. It decreases the availability of possible donor engines from the car world.

    This is the harsh reality of the change of time, we have boat plans that were a great idea, but 50 years later a whole set of circumstances make it less so.
     
  14. DCockey
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    DCockey Senior Member

    Optimum propeller diameter and pitch depend on resistance characteristics of the boat and the resulting the speed through the water. With higher drag and lower speed a larger propeller is more efficient. With lower drag and higher speed a smaller propeller more efficient. If smaller than optimum then energy is lost due to the water passing through the propeller having to be accelerated more than required by a larger propeller to produce the same thrust, which makes the propeller less efficient. If larger than optimum the drag of the blades through the water uses more energy than the energy saved by accelerating the water passing the the propeller less, which makes the propeller less efficient.
     

  15. comfisherman
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    comfisherman Senior Member

    And in a relatively heavy for length narrow semi planing runabout..... big slow wheel wins over small fast egg beater....
     
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