Changing a full Keel on my boat

Discussion in 'Boat Design' started by discovery, Aug 25, 2013.

  1. Mr Efficiency
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    Mr Efficiency Senior Member

  2. PAR
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    PAR Yacht Designer/Builder

  3. Mr Efficiency
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    Mr Efficiency Senior Member

    I think it is the same hull, "KB 30" (Keith Brown workboat). Happy to be corrected if not, though. Looking at it, it would be wet, wet, wet.....
     
  4. PAR
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    PAR Yacht Designer/Builder

    Okay, maybe I didn't understand the picture you referenced, as it could very well be the same hull. In any case, yep she looks like she'd be wetter than a *****, in the parking lot of a GM plant on payday.
     
  5. discovery
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    discovery Junior Member

    The boats came from the same manufacturer, but different molds. For all intents and purposes, they are the same. It is a wet boat, but you all have told me it is so now it must be even wetter :D.
    The intent is to 1, spray chines, 2, keel,3, re-evaluate prop.
     
  6. discovery
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    Not much advance on the skeg/keel since last post, But I have slipped it and removed the old spray rails and replaced them with a new pair made of pine. The new ones run about 6" below the old ones and I left the lower edge of them sharp, the old ones were very rounded along their whole length. They are working a lot better now than the older ones.

    While on the slip, an idea was put forward to me about going to a straight gearbox (currently 10 degree down) and putting the prop in a "pocket" or a tunnel. The thought was basically "if I'm cutting off the keel, for just a little more glassing, I could end up with virtually no prop below the bottom of the keel."

    I see a few advantages, and have been lurking and reading up a little on them, there seems to be very mixed ideas on their value on a craft like mine.

    I think I may pick up a few points on efficiency due to less protrusions and such in the water under the hull, mainly associated with drag, but may lose a little in the propulsive efficiency and reverse control.

    I found another with more power than mine,http://www.gumtree.com.au/s-ad/kiam...antastic-fishing-boat-with-mooring/1042789322
     
  7. PAR
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    I would strongly recommend you not cut the hull for a prop tunnel. Getting this just right for the professionals is "iffy" at best, so your chances are nil to none of getting it's shape, volume and arrangement right. Simply put, all hull shapes have good and bad things about them, along with limitations, some of which you've discovered. No prop in a tunnel is going to help much, on a boat with those set of shapes and variables, so save you money and effort, for things that will actually offer some return to your blood, sweat and multiple 100 dollar bills.
     
  8. discovery
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    Thanks for the common sense. I still want to remove the skeg , at least the wooden part of it, and reshape the chines to create a reverse chine shape scenario , and perhaps some lifting rails (see how busy I get). I am not trying to create a racing boat (bit hard when the hulls an inch thick) but this is something that I want to do right and I have to do it when myself and the boat aren't working. It will take some planning to minimize the downtime (for both me and the boat).

    I recently had to slip it for repairs and refoul but didn't get time to go any further than we did.
     
  9. PAR
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    With your spray knockers now working a good bit better, you might as well just leave well enough alone, as the addition of "lifting strakes" will not help your hull. As mentioned previously, their placement, shapes and goals will have very limited benefit on your boat. Maybe a couple forward to strip excess water from the hull as you power up, might be worth it, but "lifting" this hull with some strakes, well this isn't going to really happen. On a 2,000 pound bow rider, with 200 HP, yeah, they'll help a lot, but the power to weight ratio on your boat, just will not see much benefit from them other then knocking down spray and excess water. You'll need some form of skeg, though cutting down what's there will offer some drag decrease, as it is a bit big, to say the least. The current skeg is both a directional stability aid and prop/rudder protector. You can cut it down, knowing you'll loose some of these attributes (prop protection most likely), but gain a drag decrease.
     
  10. discovery
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    discovery Junior Member


    Thanks again, the last good plan I had was to remove the wooden part of the skeg, as well as the steel shoe, and replace it with a short upside down shark fin around the prop with a support for the rudder. Was going to do this in steel plate , mainly for protection of the prop and rudder.

    Since those photos you have here, I have made a new rudder ( the original broke off after hitting the only rock out off a sand beach), The new rudder having more area above the prop-shaft line than the old one. I have also put a new prop on, I sent the old one out for re-balancing and was told they couldn't balance it due to the pitting in the metal surface. Between being told the old one was useless and deciding to buy the new one, I bought a second hand 4 blade 21 x 24 with a high area ratio. I was led to believe I could run that prop, but found my ratio too high. Consequently I ended up having a new one made , 22 x 16, 4 blade 75% A/R. The vibration has changed a bit, now its a much higher frequency. I'm thinking now that the skeg blocking waterflow is the culprit. Its not vibrating too badly at the moment, and at cruise its easily liveable, but its still indicating I have issues with fairing.

    Chine flats are still on the radar as well. Now with the new spray knockers working, I can see the small ones previously fitted are working when at a higher cruise, so I think reworking them for full length of the boat and making them larger also will be worthwhile. There is some damage to the ones there presently so I will have to do something with them any way.

    Thanks again.
     
  11. PAR
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    You don't want the skeg too close to the prop or rudder. It should be about midship and don't bother with a shark fin thing, it's just not going to do anything. The skeg should be far enough forward so the water can "reassemble" behind it, before it reaches the prop.
     
  12. discovery
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    discovery Junior Member

    The shark fin thing is to protect the rudder and prop, If you look at the other photos, what came out of the mold is where I'm going back to. Obviously if they made multiple molds for this thing, it must have worked as a boat, and the mob that built my boat just added all that crap to it to suit their purposes. I simply don't see the sense in dragging all that crap around when it is costing me money in fuel used. The end of the fiberglass skeg is about 1 foot before the prop, that's how the mold is, and that's what I have to work with.

    "so your chances are nil to none of getting it's shape, volume and arrangement right." , thanks for the vote of confidence. I thought there was a reason I was viewing this forum, obviously its not for help and ideas or advice, I'll lift my rock and climb back under.:confused:
     
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    For any boat that any pretensions to speed (say 15 knots plus) that keel is a "Barry Crocker". I'd reckon you would pick up several hundred RPM of engine speed at the same throttle setting, without it's presence. As for plastering strakes and chine flats on to it, hoping it will bodily lift the boat and skim across the water, it is imo a waste of time, the boat is too heavy and the speed range too slow to make anything other than a small difference. I would have tapered that blunt ending to the keel, using epoxy and glass to maintain its integrity, to see where that got you. Not a major operation, but it may have made a significant difference to the flow of water into your prop, which currently is cr@p.
     
  14. discovery
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    I simply didn't have time to remove the keel this slipping. I didn't have time for much except a few basic repairs and re-foul . While it was on the slip, it was suggested to me about the prop pocket/ tunnel by someone who has done a few of them (retired boatbuilder). This is why I re-kindled this post and asked the question.

    I am not trying to build a racer, I simply would like a few more opinions from those that know.
     

  15. Mr Efficiency
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    Mr Efficiency Senior Member

    You don't have to remove the keel, just modify the trailing end of it, in situ, you might be surprised at the difference.
     
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