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Thread: Why are inboard engine shafts angled down ?

  1. #1
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    Default Why are inboard engine shafts angled down ?

    I am fitting a small diesel in a 14' Poulsbo style launch.
    I 've built a small tunnel to be able to carry a 10 inch wheel (prop) instead of the old 6 " one. ( boat used to have a Briggs )
    The opportunity exists to install the engine almost flat, thus, the shaft almost flat.
    But I dunno I've never seen this done?
    Is it because the stern will squat too much/push the bow too high?
    The tunnel will eliminate about ,40 pounds of displavement/floatation from the stern anyway, so one does not want more freaking squatting.
    Thoughts? MMD,Nick,Ian,Kevin,Chris ?
    I think the downward angle must remain.
    Pics in a bit
    merci, b

  2. #2
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    Default

    Not an expert, just curious, but if the shaft is parallel to the water plane then the engine must be mounted well below the water plane. How the heck deep is this 14 foot boat?

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    Default Re: Why are inboard engine shafts angled down ?

    IIRC, those Atkin boys designed quite a few boats with horizontal shafts.

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    Default Re: Why are inboard engine shafts angled down ?

    Quote Originally Posted by wizbang 13 View Post
    I am fitting a small diesel in a 14' Poulsbo style launch.
    I 've built a small tunnel to be able to carry a 10 inch wheel (prop) instead of the old 6 " one. ( boat used to have a Briggs )
    The opportunity exists to install the engine almost flat, thus, the shaft almost flat.
    But I dunno I've never seen this done?
    Is it because the stern will squat too much/push the bow too high?
    The tunnel will eliminate about ,40 pounds of displavement/floatation from the stern anyway, so one does not want more freaking squatting.
    Thoughts? MMD,Nick,Ian,Kevin,Chris ?
    I think the downward angle must remain.
    Pics in a bit
    merci, b
    Is this little boat intended to work off a flat, sandy beach? Any other reason to restrict the draft?
    It really is quite difficult to build an ugly wooden boat.

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  5. #5
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    Default Re: Why are inboard engine shafts angled down ?

    I just installed an electric motor into my boat with the prop shaft dead horizontal. Expecting excellent performance.

    I think the only reason is necessity caused by the room (or lack of it) in the bilge.

  6. #6
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    Default Re: Why are inboard engine shafts angled down ?

    Quote Originally Posted by rgthom View Post
    Not an expert, just curious, but if the shaft is parallel to the water plane then the engine must be mounted well below the water plane. How the heck deep is this 14 foot boat?
    Not a correct assumption. The marine gearbox will be designed so that the shaft coupling is lower than the engine crankshaft. Some gearboxes are also angled - the engine stays horizontal and the shaft is lower down. A V Drive gearbox the shaft is mounted below the engine. See the pictures on this page from ZF Transmissions, showing a V drive and stepped low down shaft drive. https://www.zf.com/products/en/marin...cts_64501.html Tunnel drive with a parallel drive shaft can be very efficient Wizbang. Ideally you want to look at a prop diameter slightly smaller than the tunnel diameter, say 8 to 9 inches - a marine engineer or prop supplier should be able to spec it correctly.

  7. #7
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    Default Re: Why are inboard engine shafts angled down ?

    I spent a considerable amount of time and sailed a lot of miles on a 50 foot steel sloop in Australia that was made and designed by a farmer out of Port Lincoln. He avoided the use of a V-drive by using a tractor diesel engine that had a wide, high dollar racecar belt as the drive connection between the two shafts so that the propeller shaft was horizontal for max efficiency. We did the race from Hobart to Perth via Adelaide and Esperance and I got to see Dennis win the cup back.

  8. #8
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    Default Re: Why are inboard engine shafts angled down ?

    Tunnel drives pose their own issues. There's a lot of info about them, mostly empirical rather than theoretical so understanding why one set of tunnel proportions works in one boat but not another is tricky. I can't recall the name of that eccentric small boat genius who had been in the magazine but he has some cool tunnels. As does Dave Garr.

    In tunnel drives I've heard of, squat is countered more by a bit of tunnel "roof" curl down from the blades back with the blade time above the static waterline. The blade's turning will fill the tunnel with water.

    I believe that tunnel drives are not efficient in reverse but reverse efficiency is sacrificed in many installations.

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    Default Re: Why are inboard engine shafts angled down ?

    Hey, Bruce... not an NA as we all know. I have studied this a bit and talked to many NA's and boat designers.

    Shaft angle helps lift the stern, as you suspect. This is more important for planing craft and not so important for displacement speed boats.
    There was a production builder-Penn Yan--that marketed " tunnel drive." This was deep shaft tunnels and near horizontal shafts. Now, designers ( as I understand it) often specced certain shaft angle and certain loss of bouyancy aft in order to keep the bow up. A little up angle is a good thing, especially in a following sea. May not matter to lake boaters. However, the boats squatted a bit more than " normal" boats and threw quite a rooster tail for a slow(ish) planing boat.

    They also didn't handle that well dockside, the props and rudders being buried; single engine models especially. Basically, little useable reverse thrust.

    Shallow draft protection was an advantage, along with supposed better efficiency from the flat shaft angle/ better thrust vector.

    Some pics from the net.

    Screen Shot 2022-03-03 at 9.34.37 AM.jpg



    Screen Shot 2022-03-03 at 9.41.16 AM.jpg

    Kevin

    Edit: None of this applies to modern "Tunnel Hull" performance boats ( Essentially Cats with really close hull spacing) that run ventilating propsand are 100-plus mph boats; nor the "Texas Sled" boats, which are oversized Jon boats, outboard powered, with a tunnel and a raised helm for specifically fishing extensive miles of shallows as found off Texas.
    Last edited by Breakaway; 03-03-2022 at 10:02 AM.
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  10. #10
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    Default Re: Why are inboard engine shafts angled down ?

    Both of those Penn Yans have hydraulic trim tabs, presumably to bring the bow down. Did they come standard?

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    Default Re: Why are inboard engine shafts angled down ?

    Quote Originally Posted by Gordon Bartlett View Post
    Both of those Penn Yans have hydraulic trim tabs, presumably to bring the bow down. Did they come standard?

    I don't know if they were standard. They are an easy retrofit. Either way, tabs can bring a boat's bow down. .
    I think it's important to note that tabs cannot bring the bow up.

    Kevin
    There are two kinds of boaters: those who have run aground, and those who lie about it.

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    Post Re: Why are inboard engine shafts angled down ?

    Penn Yan introduced their tunnel drive boats in 1969 without trim tabs as standard equipment. See the attached catalog pages for more details.

    Benson



    Penn-Yan-1969a_Page_01.jpgPenn-Yan-1969a_Page_02.jpgPenn-Yan-1969a_Page_03.jpgPenn-Yan-1969a_Page_04.jpgPenn-Yan-1969a_Page_05.jpg

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    Default Re: Why are inboard engine shafts angled down ?

    Quote Originally Posted by Ian McColgin View Post
    Tunnel drives pose their own issues. There's a lot of info about them, mostly empirical rather than theoretical so understanding why one set of tunnel proportions works in one boat but not another is tricky. I can't recall the name of that eccentric small boat genius who had been in the magazine but he has some cool tunnels. As does Dave Garr.

    In tunnel drives I've heard of, squat is countered more by a bit of tunnel "roof" curl down from the blades back with the blade time above the static waterline. The blade's turning will fill the tunnel with water.

    I believe that tunnel drives are not efficient in reverse but reverse efficiency is sacrificed in many installations.

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  14. #14
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    Default Re: Why are inboard engine shafts angled down ?

    An interesting side bar to trim tabs is something I learned yesterday. The captain of this boat told me the computer-controlled vertical tabs on this one (when activated) add two knots to the cruising and top speeds.
    274118727_5155208954531047_4079575859411996533_n.jpg
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    Default Re: Why are inboard engine shafts angled down ?

    Interference plates are a little bit different than traditional trim tabs though, aren't they. One of those things that is not exactly intuitive .

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    Default Re: Why are inboard engine shafts angled down ?

    Quote Originally Posted by mmd View Post
    Then came prop tunnels, and the game became more complicated. The principle is simple - lift the prop into a recess in the hull and shallower shaft angles could be realized (good for engines), props could be situated at an angle closer to horizontal (good for speed), and a happy by-product was that the boat's draft aft was reduced. The complication is that the hydrodynamics of water flow around a propeller, over the hull surfaces, and through a prop tunnel became significantly more important and complicated. A badly-designed tunnel actually reduces the net thrust of the propeller and the boat goes slower than its identical-but-for-the-tunnel twin. Up until the advent of reasonably accessible computational fluid dynamics in recent years, the design of prop tunnels was one of the 'black arts' of boat design. Many variants have been developed (as illustrated in the posts above), and most are pretty good, but in an awful lot of cases (most, IMHO) development was a series of trial-and-error experiments based on informed guesswork. The bottom line is that prop sizing, prop tip clearances within the prop tunnel, and the shape and size of the prop tunnel are all important, interconnected, and subtlety is the buzzword in their design. Get any part a bit wrong and the installation will not operate at the peak of its potential performance. It is kind of like the old adage that anybody can build a boat that will float, but it takes a lot of training, education, and experience to build a boat that will float and perform to a previously predicted level at the first iteration that hits the water..
    Which is probably why tunnels are only used on slow speed "displacement" hulls like old time lifeboats and boats that work off of a shallow flat beach. For faster craft, everyone now uses jet drive.


    It really is quite difficult to build an ugly wooden boat.

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  17. #17
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    Default Re: Why are inboard engine shafts angled down ?





  18. #18
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    Default Re: Why are inboard engine shafts angled down ?



  19. #19
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    Default Re: Why are inboard engine shafts angled down ?

    Looking great and it will be fine boat. I have a friend with a Poulsbo boat rigged for sail and another friend who put a steam engine in one.

    This diesel Poulsbo will be a great addition
    What's not on a boat costs nothing, weighs nothing, and can't break

  20. #20
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    Default Re: Why are inboard engine shafts angled down ?

    We're shooting for a 10 inch outboard prop , with one inch clearance.
    If it sings too much...we'll jiggle it s'more.
    She only ever had a tiny 6 inch wheel,home made direct drive or rube goldberg clutch.
    Boat has been in the family 70 years, my third time helping out since 1980. First time I ruined her by sawing out the cotton and filling the seams with youknowwhat.
    Second time, about 25 years ago, we got around to lamming a few floors and dynel sheathing.
    She goes aground twice a day, moored in front of the family home in an oyster/barnacle filled lagoon, thus the big bilge keels and tunnel.

  21. #21
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    Default Re: Why are inboard engine shafts angled down ?

    To stop the water coming in, water always flows downhill

    Quote Originally Posted by wizbang 13 View Post
    I am fitting a small diesel in a 14' Poulsbo style launch.
    I 've built a small tunnel to be able to carry a 10 inch wheel (prop) instead of the old 6 " one. ( boat used to have a Briggs )
    The opportunity exists to install the engine almost flat, thus, the shaft almost flat.
    But I dunno I've never seen this done?
    Is it because the stern will squat too much/push the bow too high?
    The tunnel will eliminate about ,40 pounds of displavement/floatation from the stern anyway, so one does not want more freaking squatting.
    Thoughts? MMD,Nick,Ian,Kevin,Chris ?
    I think the downward angle must remain.
    Pics in a bit
    merci, b

  22. #22
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    Default Re: Why are inboard engine shafts angled down ?

    hmm, it has been my experience that a singing prop was cured by using a smaller wheel. you recon the problem was fixed because the smaller wheel had a different trailingedgeprofile ?
    what is "prop disc"?
    We pretty much have the position locked in and know it's a crapshoot.
    lil putt putt never made more'n a few knots and was never short on viberrattinin.

  23. #23
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    Default Re: Why are inboard engine shafts angled down ?

    Quote Originally Posted by wizbang 13 View Post
    hmm, it has been my experience that a singing prop was cured by using a smaller wheel. you recon the problem was fixed because the smaller wheel had a different trailingedgeprofile ?
    what is "prop disc"?
    We pretty much have the position locked in and know it's a crapshoot.
    lil putt putt never made more'n a few knots and was never short on viberrattinin.
    Well, changing the propeller will change the profile of the blade's trailing edge, yes? Unless you are referring to the rubble from cavitation?
    If a big ship had a singing propeller, the dry dock would send a fitter down with a big file to take a swipe or two off of the trailing edge of each blade.
    The prop disk is the "disk" swept out by the blades, in your case 10-inch diameter disk swept by the blade tips.
    It really is quite difficult to build an ugly wooden boat.

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  24. #24
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    Default Re: Why are inboard engine shafts angled down ?

    thanks chaps....I may go back to the bigger/tighter fitting prop later then ...after a lick with a file. (this on one of my own boats)

  25. #25

    Default Re: Why are inboard engine shafts angled down ?

    often the lower the weight the better as ballast for seaworthiness, and the denser the water, so less cavitation for displacement hulls

  26. #26

    Default Re: Why are inboard engine shafts angled down ?


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    Default Re: Why are inboard engine shafts angled down ?

    Looking at this purely from a thrust vectoring perspective, I see a level engine, straight/level shaft as optimal. Sure, fluid dynamics come into it but you're asking about angled shafts here and it seems to me that a level driveline is ideal. I've always seen angled prop shafts as a necessary evil. With small craft, most of your boat is above the water, most of your propulsion device spends much of it's time below the water. The answer, angle the shaft but that has always been a compromise. Also, before fully pressurized oil systems in engines, they wanted to be flat and level to provide the best lubrication possible when first started up. Under normal use, the most wear to engines occurs at start up due to lower lubrication function. Tilting engines starved upper cylinders of oil so from that perspective also, flat engines were the best.

    Trim tabs and attitude correctors have been mentioned here. If an undesired force is acting upon a hull and affecting it's attitude you can trim it to correct it. The result of that is you have one desirable force acting against an undesirable force and none of that force is being used to propel the craft forward. If you can run a straight level drive line with no need for trimming, I would say go for it.

  28. #28
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    Default Re: Why are inboard engine shafts angled down ?

    We wound up having the inboard end of the shaft at about the same place and raising the aft end 2.5 inches, thus flattenning the shaft angle a bit.


  29. #29
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    Default Re: Why are inboard engine shafts angled down ?

    I wanna use an outboard prop, just for the availability and economy of playing around with different wheels, 20 bux vs.400.
    The bearing is graphite and 404 with WEST resin...we'll see. home made hippie chit


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  30. #30
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    Default Re: Why are inboard engine shafts angled down ?

    It'll be interesting to see if the very different prop RPM can be made to work well with the OB prop. I assume it is a prop from a more powerful OB than the yanmar?

    My prop shaft is level btw, certainly not in a tunnel though.

    Love the hippy chit.

  31. #31
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    Default Re: Why are inboard engine shafts angled down ?

    The bearing is graphite and 404 with WEST resin...we'll see.
    New wave babbit...I love a good experiment! Thanks for sharing this one -

  32. #32
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    Default Re: Why are inboard engine shafts angled down ?

    As for prop-shaft down angle in motor-cruisers :

    My 60 yr. old 30 ft wooden motor-cruiser is powered by a pair of 4-cylinder-turbo diesels.

    Fitting said diesels contributed to some 300-350 kilos (660-770 lbs) of extra mass in the engine room - the OEM petrol V-8 engines were discarded - which saw the boat burying itīs transom as the throttle levers were pushed forward.

    Came a day when the underwater struts needed replacement and during the latter refit, the motor mounts were moved forward about 3 inches and the shaft down-angle increased modestly to "make things fit".

    Back in the water on engaging the throttles, the transom stopped digging-in and the hull seems to plane at a faster pace, effortlessly it would appear (vertical component of thrust vector).
    Last edited by carioca1232001; 04-15-2022 at 03:48 PM.

  33. #33
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    Default Re: Why are inboard engine shafts angled down ?

    If the shaft is angled down and we would like to keep the engine level , is their a reason we cannot use a u-joint?

  34. #34
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    Default Re: Why are inboard engine shafts angled down ?

    Quote Originally Posted by Pete H View Post
    If the shaft is angled down and we would like to keep the engine level , is their a reason we cannot use a u-joint?
    You can, but... You will need a thrust bearing between the U Joint & prop, as a U Joint can't handle (for long) the pressure the prop puts on the shaft (the pressure needed to propel the boat). Also, a U Joint will not last - particularly if a steep angle - the more angle, the faster the U Joint will wear.

    U Joints have been used, particularly on boats where the engine has to be placed a long ways from the prop, but they do add complexity. Is there a reason you want the engine level?
    "If it ain't broke, you're not trying." - Red Green

  35. #35
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    Default Re: Why are inboard engine shafts angled down ?

    herez ware we r at jusnow.... fine tuning the inside


    a lil picture i made of her about 20 years ago

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