Changing a full Keel on my boat

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

  1. discovery
    Joined: Aug 2013
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    discovery Junior Member

    My boat (Discovery) is a small production model and so little information is available about modifications.
    It is 30' long and 9' wide. Currently powered by a 250 hp Cummins I get a pretty good 17 - 18 knots flat out, and a high cruise at around 14.
    It has a full keel that is 6" wide and made of timber (solid) with a steel keel shoe full length. One thing I am contemplating is to remove the keel and either replace with a fabricated steel one of much thinner design, or remove it altogether. I am less happy with the latter as I like the idea of the full keel (particularly with the shallow reefs around my area).
    My aims with this mod is to make the boat a bit faster, and more efficient. I also don't want to upset the current handling as I like the way it steers and handles generally. Another side issue is weight. I think a steel keel would end up lighter than waterlogged timber, and this would have to help a lot with speed and handling. (the only handling issue I wouldn't mind getting rid of is a "drunken wander" it gets when loaded up heavy. I think this comes from having a lot of weight (near overweight) and wave action as it is at its worst when quartering with a sea)

    Another side issue is the keel thickness. At 6" thick and the prop only 22" diameter, you can see there is a lot of disturbed water in front of the prop and at the thickest part of the prop disk. A steel keel (thinner and tapered) would have to be better for water flow to the prop.

    The new Keel I was thinking of would be roughly triangular in cross section, with approx 2" pipe as the bottom and each side made out of approx 6mm (1/4") plate and a 12mm (1/2") plate to serve as the part to mate to the hull.

    The underside of the hull and keel is similar to lobsterboats I have seen on the net, excepting the hard chines on my boat.

    I am looking forward to any considerations I may have missed. Thanks all, Trev
     

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  2. Mr Efficiency
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    Mr Efficiency Senior Member

    Who was that bloke who said if it ain't broke, don't fix it ? I guess the degree of what you call water disturbance is a function of two different things, the shape of the keel and how it tapers (thins down from that 6" you speak of) ahead of the prop, and secondly, having all that that surface area dragging water forward through friction, that the prop has to work harder to propel rearward, than it would if no keel was there. Even with a slim keel you won't get rid of that. Also, not irrelevant to calculations is the density of the timber in that keel, it may have neutral or near to neutral buoyancy, so you may not really gain anything in that department. I think a lot depends on just how well tapered that keel is, if it ends abruptly, and is creating a substantial eddy behind it, it is making life difficult for efficient propellor function.
     
  3. discovery
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    discovery Junior Member

    Removing the keel probably isn't an option , the weight lower in the water may be part of the stability equation. One thing that I have seen is one of these boats with a sterndrive and much less power going a lot quicker, but I don't know how tender it was in a sea. I do a lot of my boating in less than ideal conditions.
    The 2 core things I'm trying accomplish are better cruising speed, and to get rid of the vibrations from the badly failed prop aperture . To properly fair the current keel probably isn't going to be an easy task, and still won't be as good as a much thinner keel, and this still won't help the cutting edge of the keel which now is a 6 inch piece of flat bar.
    The prop is a big part in this but I need to get the boat right first. Engine wise the prop is spot on, but I've used a sounder with a paddle wheel for speed to measure the props water flow speed and moved it around the trransom to get a rough idea of the water flow characteristics . My rough measuring has showed that from idle to approx 1400 rpm prop flow is less than boat speed( or the prop plume hasn't reached the sender), 1400 to approx 1850 numbers roughly match gps speeds (8 through to approx12 kts ), 1850 up to approx2600, the prop plume speed blows out to approx25,26 knots. This indicates to me that the hull has too much drag and the dragging thing under there is the keel, hence my quest.
    I am confident i can get a lot of improvement but I think that the current keel probably needs more work than its worth. The knowledge that it is waterlogged (due to a previous worm infestation that has been repaired with epoxy) doesn't impress me either. The other part of my little measuring experiment showed little change in boat speed from 2200 up to 2600 rpm even though the plume sped up. That change in rpm only gained me from1 to a1.5 knots, whereas in my little head, i would have thought i would have got more.

    All up i like the boats handling and manners, and the keel probably will save me from the reef one day, but it bugs me knowing that there's more performance to find.
     
  4. upchurchmr
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    upchurchmr Senior Member

    I know very little about power boats, but .... (isn't that typical - I'm still going to give you an uninformed opinion).

    From the picture the immersed stern is about as deep as the prop is wide. Looks like a lot of water being dragged. Is there anything to be done there, like adding a stern extension to smooth out the flow of water? Seems like more potential drag than the width of the keel. it could take the form of a "swim platform" so to speak.
    Of course fairing the keel just in front of the prop should be good.
     
  5. JSL
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    JSL Senior Member

    You have been given some good advice.
    Yikes! From what I can see the trailing edge of the skeg/keel is a bluff 6" wide and has not been faired. This is not good and should be corrected, especially if you need speeds over 4 knots.
    You may be able to 'shave' the skeg, or add a fairing piece, or a bit of both. Important that you keep generous clearances forward of the prop.
    But, regardless of fairing etc., this is very poor type of keel for a faster (over 8 knots) boat. A cut-away keel with shaft and "I" strut would be the way to go but could be a major job.
     
  6. discovery
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    discovery Junior Member

    Sounds like some of your thoughts are more " fix the keel you have" rather than " get rid of it and start again" . It has a copper sterntube through the keel timber and I'm really not interested in pulling the shaft again, unless it's for a much better result.
    I done a roughly 100 nm trip as a showdown and used over 500lts of fuel, which isn't good. ( mind you, my offsider was trying to drive it like a skiboat for 4 hours, which wouldn't ha've helped the fuel use).
    With a repair of my current keel in mind, what could be dome to rectify the blunt leading edge ?
     
  7. JSL
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    JSL Senior Member

    On looking at the photo again it 'appears' that there is some attempt at some fairing (beveled 45 degrees???) but some closer photos and a sketch showing the cross sections in plan view would help. The critical area is the outer 2/3rd of the prop.
    But as before, this is a slow boat keel on a fast(er) boat so losses at speed are high.

    ps: If the prop manganese bronze, I hope the paint isn't copper. Bad idea
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    The trailing edge seems more likely to me to be a source of drag than the leading edge, which might be flat and wide, but has a shallow angle of attack and even giving a little bit of dynamic lift underway. Reference was made to the submerged transom, but I doubt that is a problem at the cruise speed of 14 knots, water will be breaking away cleanly at that speed. Is there significant stagnation behind the keel because it has a blunt ending, if so the propellor disc is partially working in forward moving water, which is like pushing the proverbial uphill. Of course any streamlining of the keel aft will weaken it, which may have dire implications, considering all the thrust is being transmitted through the area. Without seeing what shape that keel ends in, it is impossible to gauge what effect it may be having. Were it to be altered, strength would need to be preserved, possibly by sheathing in high strength glass/epoxy.
     
  9. discovery
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    discovery Junior Member

    The whole Keel was repaired and coated with tri-axial glass and epoxy when the worm repair was done. Any mods I do will be at least to the same strength. I also intend on going to a 4 blade prop to increase the bite and hopefully get a better midrange cruise speed to rpm ratio. I believe blade area is the main culprit here, but its no use doing that without fixing, or at least besting the drag issue.

    As said somewhere in the previous posts, its a slow boat keel on a faster boat. the bare hull shape seems quite reasonable and I think that with a couple of lifting strakes and keel mods, it might end up with a reasonable turn of speed.

    To think a little more out aloud, to remove the keel completely, and fabricate a smaller, thinner, more hydro-dynamic keel for only perhaps 1/2 the length of the present one ( out of plate steel for example) would still protect the prop, help the directional stability, and allow good waterflow to the prop, remove some of the weight. Am I missing anything??
     

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  10. Mr Efficiency
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    Mr Efficiency Senior Member

    Seems to me you have a double whammy here, a drag inducing unstreamlined end to a rather large keel ( is there a bar you are crossing that needs that ? ) and a propellor that has the misfortune of being behind it, chewing stagnant water over some of the disc. I would be surprised if it is faithful to the original design plans, you should perhaps consult the designers to see what the original specification was, it just looks to me like it has been altered by someone more used to heavy displacement trawlers, you really don't want to be carting around a slab of iron plate along the keel on any boat this size that has pretensions to speed.
     
  11. discovery
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    discovery Junior Member

    They were still building them this way 4 years ago. My boat is 1988 model.
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    What is the make/model ?
     
  13. discovery
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    discovery Junior Member

    http://www.miboat.com/images/ASC_StartBoat/ASC_StartBoat.html

    Copy and paste this into a browser window and check this one out.
    Mine may not look as well faired as the one in the piccys, but it has had a lot more use and rot in this area. It doesn't look like 6" timber in the photos, but I can assure you it is.
     

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  14. Mr Efficiency
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    Mr Efficiency Senior Member

    I note that a couple of these I spotted on the web have 80 and 85 hp diesels, that is a lot different to your 250 hp. I wonder what they cruise at, presumably somewhat less than your 14 knots. It is hard to imagine how this would be an efficient set-up, a planing hull running at hump speeds.
     

  15. discovery
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    discovery Junior Member

    With a smaller diesel, they probably get their efficiency from being slower and a small diesel, they couldn't compete with a carvel hull style (or similar) in terms of outright efficiency but looking at the hull, they were more than likely looking for the stability of a hard chined boat for numbers of people on one side for the start of a race.
    To give an idea, my boat idles around the marina at 5 knots, and jumps as you select the gear, not really needing throttle to park at the wharf. When I nail my boat, I have a transom clean of backwash within 2.5 to 3 boatlengths.

    By all indications, the bulk of these were river trawlers (hence the keel) fitted with Lees diesels of around 120 - 150 hp.

    I still believe the basic hull is a planing hull, and am leaning towards removing the current keel and putting a "shorty" keel back on to support the rudder, sterntube and protect the prop. I have just never done this sort of thing before and was hoping someone here may have, or someone here could possibly run their design software and sort out what to expect when doing this mod.
     
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