Keel Modification

Discussion in 'Propulsion' started by Ol Gillnetter, Jan 24, 2013.

  1. johneck
    Joined: Nov 2011
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    johneck Senior Member

    What was the original power and RPM? I generally work with larger vessels, but the numbers here look worrisome. The tip speed of the prop is quite high and the blade loading is also very high. That indicates high vibration levels. When combined with low tip clearance and high shaft angles, that indicates a potential problem.

    It appears to me that you will not be able to get sufficient blade area using a four bladed propeller to avoid some thrust breakdown. Perhaps it would be a good idea to look into a five bladed prop. This could helpin several ways, it reduces the pressure pulse associated with each blade, it avoids having two blades in the worst part of the wake at the same time and it allows for better thickness/chord ratios of the blade sections to better control the cavitation.

    I would avoid tunnels unless you are willing to properly design and build them. Bad tunnels are probably worse than not having them at all.
     
  2. Ol Gillnetter
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    Ol Gillnetter Junior Member

    Sat up last night and went through angles and tunnel clearance and prop diameters, and estimates of fiberglassing for the various options. And came up with a couple things-

    1) A tunnel will be damn invasive, and if not done right will lead to either a compromise on running or dealing with a running issue, such as cavitations noise, or worse.

    2) You guys are right about getting as much prop as is needed, 26" should be the goal.

    3) Leaving the shaft angle as-is poses the least amount of work since I can simply use the existing hole as my guide when boring the larger tube diameter.

    4) Leaving the shaft angle as-is means the keel is too shallow to protect the prop, which means either that is the end result or I have to extend the keel to make a proper captured rudder and protected prop setup.

    5) Has anyone here ever seen or done an entire keel extension?, I have about 23' of flat after it leaves the curve of the bow, so an extension would be nothing at the bow and up to 6" give or take at the stern. And while this may seem like a lot of glass work, I have a feeling it would be easier than a tunnel. Maybe build it down with a fiber reinforced board like Coosa or similar???


    As far as old power, these boats(Roberts) came with everything from 180HP 6-71 naturals to 450HP turbo 6v92's, and some in twin screw versions (though not many) Mine had a trusty 216HP 6v53 and like most fish boats was not powered with enough to overcome the bow wake, the fishermen with big wallets bought the larger engines.... I'm not looking for more than a nice 16KT cruise with an engine that isn't running at the firewall, and if she goes better then that's cake topping.

    My Roberts has been gutted to the stringers, engine relocated and spun around with Vee drive (I ran the 6V53 this way for 5 years), new deck, new cabin, new systems, just about everything new, except the engine and now I'm tackling the engine.

    Picture of the old gal attached
     

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  3. midnitmike
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    midnitmike Senior Member

    Gillnetter,
    For now I'd suggest staying with the 4 blade configuration unless and until you encounter a circumstance that requires you to reconsider. A five blade will require less pitch then a comparable 4 blade, so there's no advantage in performance either during low speed manuvering or at the top end. I doubt there's any difference in tip clearance requirements, so again I see no advantage. Let's see what you come up with in your next mock up and go from there.

    MM
     
  4. afteryou
    Joined: May 2012
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    Location: Southeast Alaska

    afteryou Junior Member

    "5) Has anyone here ever seen or done an entire keel extension?, I have about 23' of flat after it leaves the curve of the bow, so an extension would be nothing at the bow and up to 6" give or take at the stern. And while this may seem like a lot of glass work, I have a feeling it would be easier than a tunnel. Maybe build it down with a fiber reinforced board like Coosa or similar???"

    Good idea, that would be a very easy job. As long as you were only adding six inches or so I don't think it would change your turning ability much. But I should stick to what I know and that's goop. :D
     
  5. afteryou
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    afteryou Junior Member

    I don't think we're talking a real tunnel here. Just deep enough to gain a few inches. I do agree with you 100% on this though.
     
  6. midnitmike
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    midnitmike Senior Member

    Hi Gillnetter,

    1) You're right building a tunnel is a pretty invasive procedure, but it is comparable to many of the other options you posted initially. On the plus side you're not removing massive sections of the keel either.

    2) Now that we've had a look at the old gal I think most will agree that a 26" wheel is entirely appropriate for this build.

    3) Leaving the shaft angle as is isn't neccesarily the least amount of work. By doing so you'll have to make your other alterations just that much more extreme in order to fit the larger prop. So long as you already plan on fitting in a new engine, shaft and shaft tube altering the shaft angle a few degrees is one of the cheapest, easiest ways to gain a few extra inches of clearance.

    4) Except for the tunnel every other proposed option will leave your wheel somewhat more exposed then it is now. It's up to you to decide how to go about providing the required protection.

    5) While I've seen a number of these alterations over the years I have yet to see one where the entire keel was modified to the extent you're proposing. They can quite easily be accomplished with a more modest extension as shown in your keel overlay 1 pdf using glass or a variation of that using mostly steel...sort of depends on the builders area of expertise.

    There is one thing about this option I like and that is you can build the keel extension using a plug and later attach it to the existing keel. This would eliminate a lot of overhead glassing during the layup process and keep your original keel intact. This technique is similar to how keels can be patched after a run in with a particularly stubborn rock.

    MM
     
  7. JSL
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    JSL Senior Member

    If you are looking at any kind of speed (over 8 knots) keep protection to a minimum... a propeller net hoop at most. Net cages, beaver tails, etc. will kill perfomance.
     
  8. Ol Gillnetter
    Joined: Jan 2013
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    Ol Gillnetter Junior Member

    What would be considered an acceptable distance from the cutless bearing to the prop hub? My existing was damn tight, 2.5" at best, which seems way too close. I assume the farther away the better prop exposure to cleanwater, and of course the more reinforcing the shaft tube needs. Maybe 6"? Trying to finish up a mockup, though work has really been in the way of sketching progress.

    JSl- she doesn't fish any more, she did come to me with lots of hardware under there though, the stuff you mentioned, and still had fish scales on the deck, but I have never fished the boat, unless you count a guy with a pole and beer and a lawn chair. No lure of course, that might actually catch a fish and then you have to clean the fish and eat it and tell stories about it, lots of work..... better off just to let a friend do all the catching then invite him over to bbq his fish.
     
  9. JSL
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    JSL Senior Member

    The guide I use 0.5 to 1 shaft dia. [Other people may have guides that work. Your prop mfr. might be a good source.]
    If your shaft is 2.00" dia. the overhang (aft end bearing to for'd end hub) should be 1" to 2"(max). I suppose you could try more but this could put additional forces on the shaft and bearing eventually leading to premature wear or other problems.
    For better water flow, smoother fairing, strut rake, keel scallops, work best.
     
  10. midnitmike
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    midnitmike Senior Member

    I like to have enough room to install a shaft zinc plus a little so as not to obstruct water flow to the bearing.

    JSL is absolutely right on the points he brought up as well. It's the water flow going to the main blade area that you're concerned with...not so much the flow around the hub.

    MM
     
  11. Ol Gillnetter
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    Ol Gillnetter Junior Member

    10-4, just trying to calculate how much shaft tube to extend beyond the keel to get the prop away from the keel as far as possible, but realistic to fabricate.
     
  12. JSL
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    JSL Senior Member

    A correction perhaps (and my first attempt at adding an attachment) on this site
    The distance between prop and sterntube bearing here was about 1.5 to 2 diameters and worked fine for 25 years. But, it was a low load: on an 8.5 knot 38' cruiser. 200 hp with a 26 x 20 prop
     

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  13. Ol Gillnetter
    Joined: Jan 2013
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    Ol Gillnetter Junior Member

    Went through and basically sketched up an overlay starting at no shaft angle change through 3.25 degrees of change (increase). The prop of course gets pushed the furthest back with no change and the least with 3.25 deg of change. All result in needing more keel depth if a captured rudder is the end result, and the range is about 8" to 10.5" of added keel depth. Presently I settled on 1.75 degrees of increased shaft angle, which has a moderate, 16" estimated push back of the prop, no angle change is 30"+..... 3.25 degrees of angle increase is a still 9" push back. That all said, I need to increase the keel depth or end up with prop below the keel and a hanging rudder, which after recalling how many times I ran my crab pot line over last summer, seems like it would be inconvenient. So how to extend the keel depth; Maybe use a core material and extend down and shape as desired then make a plug to fit on the core material. The plug would not be full height, but full thickness on the bottom (1" or so) and probably 1/2 height and tapered so it can be glassed on to the existing keel sides and core material. Lots of work, but gets the end result. I'm open to ideas on the plug/core/fiberglassing, someone no doubt has a better method....
     

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  14. midnitmike
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    midnitmike Senior Member

    Gillnetter,
    I like the compromise you've reached between balancing shaft angle vs keel depth. I am however less enthusiastic about using Nidacore as a filler material between the keel and the plug. Because the plug will taper to 0 over some distance you'll also be required to taper the filler material at the same angle and using NidaCore won't make that an easy process. Might I suggest either using a suitable foam core 10lb and >, or better yet leaving the area hollow. In some cases where I've seen this done the builder may even drill drain holes into the hollow section allowing it to fill and drain freely.

    You might also consider extending the plugs inner most laminate high enough that you can use it to secure the plug to the keel thus facilitating the hollow construction method.

    MM
     

  15. Ol Gillnetter
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    Ol Gillnetter Junior Member

    Very true, research on the Nidacore shows it would indeed be difficult to taper, at least if left sandwiched flat versus vert, still difficult if vert. Maybe just pour it with foam, I found some 8# pourable polyurethane foam, Aeromarine. Or leave it empty and put a drain plug at the aft end. I was just thinking a core would add some strength to it, but in all reality, the keel is only 5" wide so glassing it to 1.25" wall thickness won't take much time and it will certianly be plenty stout. Got the bed for the melamine mold welded up this weekend, 25' long perfectly level end to end and side to side, will start forming the sides this week. Should have it ready for some resin by next weekend, if work doesn't get in the way.

    I also found a couple guys that ran into tip clearance noise and their solution was a slight wedge fiberglassed just aft of the keel with some through hulls just aft of the wedge, tie the through hulls to top side and they suck a little air down and make an air pillow and the cavitation noise goes away, so they say. I'll try and drum up a picture of it.
     
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