Blackrock 24

Discussion in 'Boat Design' started by LP, Mar 12, 2013.

  1. LP
    Joined: Jul 2005
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    Location: 26 36.9 N, 82 07.3 W

    LP Flying Boatman

    I thought I'd take a moment to introduce my latest design. The Blackrock 24. It's a 24' trailerable coastal cruiser. I've tried to keep an emphasis on trailerable. Some of the design requirements are trailerable(again), stores in the garage (7' overhead door), sleeps a family of four (or five if they a small), rigs easily, spars stow within the length of the boat, basic accomodations, and an inboard mounted outboard.

    The drawing is still in the rough with some "residuals" still lurking. The hull is pretty much fixed. The sailplan and keel still need a little bit of attention, but are developed enough that I feel I could start the build today if I wanted.

    BlackRock24TSSailplan02.jpg BlackRock24TSMidship.JPG

    I've opted for a balance lug rig divided between two masts to keep the center of effort of the sails as low as possible. It also affords an uninterupted interior plan. I like the simplicity of the balance lug and the lack of hardward required for the rig. The longest mast will only be 20' and I am anticipating stepping both masts single-handedly if I half to. The sails will stay bent to their yards when stowed and should rigged and hoisted fairly rapidly. I have to admit to spending a bit of time on Michael Storer's website studying all he and his contributors have to say about balance lugs.

    Accomodations will be minimal. Head room is just shy of 5' and might get pushed to 5' because of a recent insight. 5' might sound uncomfortable, but I have devise a telescoping hatch that opens the entire length of the saloon area. So, in this regard, it has standing head when it not raining. This is very important as the galley area is located forward so that the quarter berths could extent under the cockpit seats.

    The hull construction will be a dry layed, strip built hull around plywood frames and bulkheads. I have studied some dry lay techniques and have done some preliminary test panels for filling the plank seams. The planks will be 5/8" thick by 1 1/4" wide. I plan to use a pneumatic nailer to drive the 2 1/2" edge and frame nails. Each plank will be gapped with a wooden toothpick when nailed. These spacers will be withdrawn prior to filling. The framing will be radiused to provide only a point contact at the planks that will allow me to wet and fillet the area after the planking is complete.

    The seam filler will be a wet mixture wood flour and glass or phenolic spheres. The wetness of the mixture will be extremely important as the dry planks will want to soak up a portion of the epoxy in the mixture. I've done some test mixing but haven't settled on anything, yet. I'm looking for a mixture that will be close in strenght to the cross-grain strenght of the planks in the hopes that it's hardness will also be reduced.

    This has been about a three year project to design and I am hoping to start the build in a couple of months. I have resently started acquiring hull materials and have started the rough cutting it and stacking it to air dry before the build.

    A few numbers:

    Sail Area 290 sq ft
    LOA 24'4"
    LWL 22'
    Max Beam 7' 10"
    BWL 6' 10.33"
    Draft 19"
    Displacement 1.4 tonnes/3136 lbs.
    Ballast 500 lbs keel mounted lead. (for now)
    SAD 21.5
    D/L 122
    SA to Wetted-Surface 2.0 (This ratio is low, but a light air mizzen mule will bring the value up to 3.0.)

    That's all for now. Hope you like it.
    Last edited: Mar 15, 2013
    1 person likes this.
  2. tom28571
    Joined: Dec 2001
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    tom28571 Senior Member

    Nice looking boat and a nice effort. I have some serious reservations about the stability and a few other things. 290 sq ft with only 500# ballast on shallow draft? Compare that to a very common Catalina 22 with 205 sq ft and 550# drop keel with 3 1/2' draft. Going to windward will not be a strong suit nor will standing up to a blow. I also don't get the reverse angle on the cabin. It makes a water catcher on the forward end and makes going forward on the side deck a real dance. There is a good reason that these are usually sloped inward on sailboats.
  3. LP
    Joined: Jul 2005
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    LP Flying Boatman

    Thanks and thanks for taking the time to put your thoughts down.

    I'm looking to spend time on the gulf coast which is known for light winds. I may be focusing too much on this fact and may have skoshed the square footage up to accomodate light winds. I also lose efficiency on the mizzen as compared to using all of the area in a sloop rig. I'm only counting 75% of the mizzen in my heel calcualation. The Dellenbaugh Angle is about 12.5 degrees on the calculated 260 sq ft area. I would prefer it to be down at about 10 degrees and may work to those ends. First reef drops the DA to less than 9% on about 209 sq ft. The sailplan is certainly not set in stone and I am still evaluating. Reducing it and flying light wind sails is certainly in the realm of possiblities. Windward will not be a strong suit and I will utilize aux power under conditions that warrant it. I am considering more than the standard 9.9 up to a 25 hp auxillary engine.

    The reverse/flairing cabin is to provide additional room downstairs. Mostly for sitting headroom. My original plan had the two sides meeting each other at a point on the forward deck. Subsequently it was shortened and a forward wall was added. The flaired cabin does complicate passage to the forward deck. Small boats are full of compromise. I am also planning on having a forward hatch to reduce the need to go forward via the topside. The flair provides an overhang that will shed rain water while at anchor and help with additional ventilation during those times. True on catching water. As a coastal cruiser, I expect to have more options in choosing when I venture out. On the other hand, I've got a fair amount of forward sheer and a mast to deflect significant amounts of water.

    I think all of your concerns are valid and I believe I have considered them all at some point. The sail area of the boat is still being analyzed and might see a reduction from what is shown. I have also focused on keeping a low profile for the boat and a low center of effort on the sails. Another interesting feature of the balance lug is that the sails can be flown any where on the mast. They can be flown high in light winds and low in stronge winds. As it is drawn right now, I could fly the main 6" lower on the mast and the mizzen, a full 2' lower should the need arrise.
  4. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    Have you considered a pop-top? They can really change the whole feel of the boat. If you are in the Gulf, mosquitoes are a problem, so you can have an enclosed bug screen with standing headroom.
  5. michael pierzga
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    michael pierzga Senior Member

  6. Tad
    Joined: Mar 2002
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    Tad Boat Designer

    Neither the keel profile nor the rig look happy to me......

    The Romily rig provides a clue......

    Around 50 years ago L Francis did this, she's a nice boat that works well. I would consider the proportions of keel, rig, and house/side deck of Design #102 carefully....


  7. sean9c
    Joined: Jan 2011
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    sean9c Senior Member

    I'm going to agree with Tom, the reverse cabin side angle looks odd, as far as it adding room down below it just adds room up at the ceiling not where your body is. It'd also be a real ankle banger if you're using the side decks. As it effectively makes the side decks narrower why not just make it raised deck, have the house sides go to the rail.
    I'd be tempted to figure out how to make it out of plywood, a lot quicker, cheaper, easier. If you're going to strip plank it why bother with toothpicks, gaps, filler etc., just strip plank it gluing you go.
    Doesn't seem to me that that rig will be anywhere balanced with just one sail or without both sails reefed equally. Is that a concern? Also thinking you'll need more area in the keel
  8. Petros
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    Petros Senior Member

    I like the concept, but I would put in a simple dagger board and do away with the near useless keel. I know it will take up room inside for the case, but it is light and simple and will really improve its performance, and it will save a lot of weight. A Weighted dagger board or centerboard can be much lighter since it will be much deeper in the water.

    Also, the lug sail is not very efficient, I would consider using junk rigs. they are lighter and much simpler to operate and very efficient in all points of sail except for pointing into the wind (which that keel will not allow anyway). You can use your same mast arrangement, and the junk is much easier to reef, and will not flog, very good light wind performance too.
  9. philSweet
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    philSweet Senior Member

    A few numbers indeed. Who has a 26' garage? What is the plan in terms of crew mobility and effort? Hiking/trapezing? When I think of pocket cruisers of that weight, I think 18', not 24'. I think you need to be more efficient and shorten the boat 6'. A Marshall 18 weighs 2200. A Precision 23 would seem to come come close to what you're after. It's beamier, has more lead, less sail, and has a very refined design in terms of light weight top hamper. I suggest you use the ShortyPen pocket cruiser lists to compare the specs of boats you consider to be comparables, and take the time to figure out what makes each one tick. I don't think light displacement, narrow beam, and a lot of sail go well together.

    Here's another that came to mind-

    Seaward's are a bit closer to your idea and have a pretty good rep for feeling solid and safe.

    <edit. Thought I was giving the post a bump. For some reason, I didn't see any of the above responses until I posted mine.>
  10. alan white
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    alan white Senior Member

  11. LP
    Joined: Jul 2005
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    LP Flying Boatman

    Oh...Where to begin?

    At the top I guess.

    Gonzo, briefly.

    Michael, good links. I was curious about the amount of peak a balance lug could carry and this design shows it can be carried quite high. I may play with the amount of peak in my rig. The raked mast with the balance lug is interesting.

    Tad, Define Happy. :D Seriously though, I'll bite the bullet on being overly zealous sail area. Are you also looking at centers placement? I show about 15% lead on the Hereshoff cruiser and 14% on Romily at 50% mizzen. Less lead for a higher than 50% since the mizzen is not tucked up underneath the main. I have considerable lead if only 50% mizzen is used for sail balance, 19%. If a larger percentage is used, say 75%, then lead drops to 12%.

    Petros, thanks for the comments. The keel is a compromise at best. Wouldn't a fully battened balance lug be a junk. Hmmmm.;)

    Sean, to each his own. I like it. Study it and you'll see that the room is beneficial. Already did a glue as you go. Never again. This will be quicker and easier. If not, it will be cleaner. I am still asking myself the raised deck cruiser question. It'll take both sails to balance and most likely, if you reef one, you'll reef them both in the current proportions. Keel area is adequate.

    Phil, Ummm...I have a 26' garage and my 16' sailer with the mast down uses it all. Odd that you would suggest shortening the boat for it to be more efficient. I'll take a look at the Shortypen list. I don't believe that I have used the pocket cruiser term on this design. I'm not sure if the term fits, maybe.

    Dave, Madrigal is a unique design. Interesting blend of curves.

    Thanks to all. Your comments have been enlightening and they'll help me reaffirm or fix design details. The rig will get attention and maybe the keel, too.
  12. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    I have a 26' garage too.

    A "slice 'o pie" style of board could be completely enclosed in the low aspect appendage, without intruding into the cabin living spaces. You can double aspect on the fin and it'll help balance the boat in various positions/points of sail too.

    Initially, I didn't like to outward sloping cabin sides, but it's grown on me. I now think the combination of shapes will look pretty cool, considering the other shapes in the cabin. I'd have gone with a raised foredeck and broken sheer treatment, but this is an aesthetic that many can't handle and it's hard to draw up a broken sheer and make it look right. We can't all be Sam Crocker.
  13. LP
    Joined: Jul 2005
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    LP Flying Boatman


    Here is a quick and dirty sailplan update.


    The center aren't marked, using 50% of the mizzen brings the lead in about 2 points ahead of the Hereshoff and Romilly. I don't know that I can rely on the 50% mizzen rule with this design though. Using a higher percentage does bring the lead down. I have raked the masts. I was trying to avoid this as it causes the sails to try to align on the boat centerline when winds are light. It does give me options for adjusting lead by adjusting mast rake. Unfortunately, when straying from the norm, empirical design information gets sketchy. Proper lead for this design can only be guessed at and being able to adjust the rig once it is standing will be highly advantageous.

    I do believe that this is a "Happier" rig, Tad. Thanks for the couteous guidance. The DA is down to 9 and change.

    Hi, Paul. Yeah, I think some kind of swingy thing is in the future. Time will tell. Thanks for the comments on the cabin.
  14. tom28571
    Joined: Dec 2001
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    tom28571 Senior Member

    For a couple of other perspectives on how the ballast and CLR issues were handled, look at these boats with similar design goals.

    The first is a particularly innovative way to get extremely shallow draft, efficient CLR for windward performance, no intrusion into the cabin and ability to take the ground level.

    The second is a more common treatment of these features and has very little centerboard intrusion to the cabin.

    Not suggesting that you drop your design and build one of these but offered for information as to what works well.

  15. michael pierzga
    Joined: Dec 2008
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    michael pierzga Senior Member

    Id get rid of the stubby keel and use a ballast lead shoe. The object is shallow draft and a boat that is easily trailed.

    A normal transom hung kickup rudder, with no skeg, will make the boat handle better and also trailer and launch better. .
    If windward sailing ability is valued then Two centerboards... port and starboard, off centerline...will generate enough lateral resistance and take up the minimum interior space.
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