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Thread: Transmssion raito, shaft and prop sizing advice

  1. #1
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    Default Transmssion raito, shaft and prop sizing advice

    I am about to start building Spira's San Miguel this fall. I have selected my engine, Beta 38 with a PRM125D transmission. The engine is 38hp at 3600 rpm and 19hp continuous at 1400-1600 rpm. Shaft length, from my initial gesticulations, will be 5 to 6 feet. Spira says, "I designed this boat to be able to go 8-10 knots and burn in the 1 to 2 gallons per hour range. It'll plug along fine with a small sailboat auxiliary of about 30 hp." The quote is taken directly from his website. In an email to him, I asked about using a 16-inch prop and the Beta 38. He agreed and said it would be fine. A pic below will give more info about the boat size and weight.

    So, my questions are.

    1)What gear ratio for a 16-inch prop and why? Available forward ratios are 2.04:1, 2.50:1, 2.94:1 with a 2.50:1 reverse ratio.

    2) Shaft size and why? I would like to use a stainless steel shaft. I plan to use a flexible coupling, flexible engine mounts, and a rope cutter if that matters.

    3)Knowing the prop will be 16-inches, what pitch, what ratio, and why?

    I'm still research and studying this prop stuff and gear ratios, that's why I ask why so much. I want to learn this.

    The boat will be used to cruise the Great Loop for one to two years. Then, I plan to do some upgrades and trailer over to Washington state and do the Inside Passage up to Alaska. From there, who knows. I may make the 10,000+ mile trip from AK to AUS.


    Boat specs.
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  2. #2
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    Default Re: Transmssion raito, shaft and prop sizing advice

    I don’t know the answer to your transmission question, but I’m pretty skeptical about that 8-10 knots figure.
    -Jim

    Sucker for a pretty face.
    1934 27' Blanchard Cuiser ~ Amazon, Ex. Emalu
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  3. #3
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    Default Re: Transmssion raito, shaft and prop sizing advice

    Dave Gerr (who in addition to knowing lots of stuff, is a great writer) wrote "The Propeller Handbook" , essentially explaining the black art of prop selection, and containing the method to calculate your own. Specs of the boat are used to derive pitch, blade number and so on. Shaft thickness is selected based on torque of engine and length of shaft. In a displacement boat (I wonder a bit too about 10 knots) you essentially want to swing the biggest prop you can, pitched so that the engine can reach its intended max rpm. Transmission and prop pitch are parts of the same matching puzzle. The deeper reduction you get, the bigger prop you can swing. Usually there are hard limits to diameter (blades need clearance away from bottom of boat) .You could also get back to Spira to see if he's had any feedback about props from other builders?

    That (and Gerr's other books) is a great, worthwhile read.
    Last edited by Boatsbgood; 06-29-2019 at 10:10 PM.

  4. #4
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    Default Re: Transmssion raito, shaft and prop sizing advice

    Quote Originally Posted by jsjpd1 View Post
    I don’t know the answer to your transmission question, but I’m pretty skeptical about that 8-10 knots figure.

    Yeah, I am too but it's all I have to go on right now.


    Quote Originally Posted by Boatsbgood View Post
    Dave Gerr (who in addition to knowing lots of stuff, is a great writer) wrote "The Propeller Handbook" , essentially explaining the black art of prop selection, and containing the method to calculate your own. Specs of the boat are used to derive pitch, blade number and so on. Shaft thickness is selected based on torque of engine and length of shaft. In a displacement boat (I wonder a bit too about 10 knots) you essentially want to swing the biggest prop you can, pitched so that the engine can reach its intended max rpm. Transmission and prop pitch are parts of the same matching puzzle. The deeper reduction you get, the bigger prop you can swing. Usually there are hard limits to diameter (blades need clearance away from bottom of boat) .You could also get back to Spira to see if he's had any feedback about props from other builders?

    That (and Gerr's other books) is a great, worthwhile read.
    His prop book is on my short list. I'm hoping when I get the plans in hand, I'll be able to swing an 18 to 20-inch prop but I am doubtful.

    What you posted is what I have learn from research.

    I don't think any of his San Miguel plans have been completed if built. I hate to be the test mule but I'll have a good adventure while doing it.
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  5. #5
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    Default Re: Transmssion raito, shaft and prop sizing advice

    Well, our little Yanmar is 36 hp, we swing a 17” prop, and the waterline is 27 feet. Speed over ground usually runs between 5 and 7.5 mph depending on conditions. We briefly touched 8.9 once at wot and sustained 8.5. The waterline on your boat is shown at just shy of 24 feet, so all else being equal... but if you put enough fuel to it.

    We burn about a half a gallon an hour.
    -Jim

    Sucker for a pretty face.
    1934 27' Blanchard Cuiser ~ Amazon, Ex. Emalu
    19'6" Caledonia Yawl ~ Sparrow

    Getting into trouble one board at a time.

  6. #6
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    Default Re: Transmssion raito, shaft and prop sizing advice

    Quote Originally Posted by jsjpd1 View Post
    Well, our little Yanmar is 36 hp, we swing a 17” prop, and the waterline is 27 feet. Speed over ground usually runs between 5 and 7.5 mph depending on conditions. We briefly touched 8.9 once at wot and sustained 8.5. The waterline on your boat is shown at just shy of 24 feet, so all else being equal... but if you put enough fuel to it.

    We burn about a half a gallon an hour.
    That would make me happy if I could turn 6kts at .05gph. I have my doubts though.
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  7. #7
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    Default Re: Transmssion raito, shaft and prop sizing advice

    One of the big variables missing is competed weight....That seems to be an unknown but I wouldn't expect anything like 16,000 lbs without a lot of it being lead ballast. So I'll guess 8500 lbs. This is the wrong hull form for 8-10 knots in this length and my guessed weight. This boat will run nicely up to ("hull speed") 6.5 knots, if you need something faster choose a different hull with a bigger stern and more transom immersion. The 36 HP is plenty of power for the maximum 6.5 knot speed, which (at 8500 lbs) will require 12.4 HP at the propeller (about .74 usg/hr). This is a low speed hull and you need a low shaft speed for maximum efficiency. One old rule of thumb was 100 rpm for each knot, but commonly available gearboxes don't go that low these days, nor do designers make the prop aperture big enough. So we do the best we can. The 3:1 gear gives a max shaft speed of 1200 RPM which will turn a 19" x 14" or 18" x 16" prop. The larger diameter will push more water and provide more thrust at slightly lower speed. 1" ss shaft is the smallest easily available these days, I would go with that or perhaps 30mm, assuming a cutlass bearing at the aft end and a standard packing box at the forward end of the shaft log.
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  8. #8
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    Default Re: Transmssion raito, shaft and prop sizing advice

    I agree with Tad. The results of my 'quick & dirty' calculations differed very slightly from Tad's numbers in prop size (possibly from our selecting different numbers of blades), but not enough to quibble over.
    Hope for the best, but plan for the worst.

  9. #9
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    Default Re: Transmssion raito, shaft and prop sizing advice

    There's no boat that goes 8-10 knots that is 23 feet long.
    It either goes 5-6 or passes 8-10 on the way to planing at 15 ish. and up.
    You need a sleek and light 45 'er to do 8-10 knots with a 38 hp diesel.

  10. #10
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    Default Re: Transmssion raito, shaft and prop sizing advice

    Quote Originally Posted by wizbang 13 View Post
    There's no boat that goes 8-10 knots that is 23 feet long.
    A Hampton Boat will, one of those built by Dick Pulsifer will cruise at 10 knots with a 29 HP Yanmar. But no cabin, no interior beyond some seats and a canvas cover.
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  11. #11
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    Default Re: Transmssion raito, shaft and prop sizing advice

    I stand corrected !

  12. #12
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    Default Re: Transmssion raito, shaft and prop sizing advice

    Thanks, guys for the input. I'm not building a speed machine. I'll be happy at around 6kts. More speed would be nice but not needed. When I set out, I won't have a set schedule.


    Quote Originally Posted by TR View Post
    One of the big variables missing is competed weight....That seems to be an unknown but I wouldn't expect anything like 16,000 lbs without a lot of it being lead ballast. So I'll guess 8500 lbs. This is the wrong hull form for 8-10 knots in this length and my guessed weight. This boat will run nicely up to ("hull speed") 6.5 knots, if you need something faster choose a different hull with a bigger stern and more transom immersion. The 36 HP is plenty of power for the maximum 6.5 knot speed, which (at 8500 lbs) will require 12.4 HP at the propeller (about .74 usg/hr). This is a low speed hull and you need a low shaft speed for maximum efficiency. One old rule of thumb was 100 rpm for each knot, but commonly available gearboxes don't go that low these days, nor do designers make the prop aperture big enough. So we do the best we can. The 3:1 gear gives a max shaft speed of 1200 RPM which will turn a 19" x 14" or 18" x 16" prop. The larger diameter will push more water and provide more thrust at slightly lower speed. 1" ss shaft is the smallest easily available these days, I would go with that or perhaps 30mm, assuming a cutlass bearing at the aft end and a standard packing box at the forward end of the shaft log.
    Thank you. That is helpful. I am hopeful that there is room to spin a larger prop than 16". Our finished and loaded weight guestimation is very close. I was thinking 9500lbs loaded and 8500lbs dry on the trailer ready for tanks to be filled and launch.
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