Blackrock 24

"Happy" just means it looks right to me, my happy and yours are different, which is a good thing.....

The second rig looks better, but (to me) the mizzen is still too far aft and the keel too far forward. I want the fin's leading edge root to start at about station 3 in a 10 station waterline.

The Enavigo 33 is below, look at her keel profile....that's just wrong and any designer would tell you so with one glance. They built it and found "a large amount of weather helm".



Here's another one, the Stevens 47, designed by Sparkman and Stephens, again the keel is too far forward and she has weather helm. This case is a bit more subtle than the Gondola design, but looking at the profile causes you to say....Hummm?

http://sparkmanstephens.blogspot.ca/2011/09/design-2390-stevens-47.html

 

I stand corrected on the garage size. The two I've had were both about 22'. And my 16' skiff had a cuddy stepped mast so it could be short enough to fit in the garage.

Your SAD ratio isn't low for that type boat. You basically need to raise the water line a couple inches with added weight and extend the keel a bit to bring draft up to about 25" or there abouts. At least another 500 pounds of lead is needed. 750 would be better.

And I'd shorten the waterline by a foot as well. DLR's in the 350 range make good boats in this size if the crew is not hiking out. With a low DLR and the high crew position, the SAD would probably need to be around 1.8, and I'd count all the mizzen in that number (do what you like for calculating the coe lead). When looking at the various scattergram charts of design ratios, you need to realize that the trends are length dependant. A thirty foot average boat will have very different average values than an average twenty foot boat. That info is often lost in scatter charts. Some of the reasons for this are related to physics and geometric similitude. Some are artifacts of the 8'6 road limit and other nonphysics trends. I think your design ratios have led you a bit astray because of your choice of datasets, which I think were for larger craft. The old Plyboats Manual had a very good set of scattergrams specifically sampling small boats. And design ratios in and of themselves aren't enough. You still need to provide the stability.

Have you done a weight analysis yet? Where is the vertical CG location? One thing that I worry about when I see tucked-under quarter berths in this size is that the crew is seated so high that when the boat heels, there is nearly no righting moment added by the crew. The crew really needs to be low if staying inboard. Measure the angle from the boat's (empty) CGv to the crew's belly button. Now imagine the boat heeling 20 degrees- How does the crew respond and what happens to the added RM of crew weight. Small boats like this tend to sail at fairly high angles of heel.

Run the numbers for the Seaward I pointed to earlier. I figure it has about 40% more stability than your design suggests because of beam,weight,lower cg, lower crew, and better shape. It has a SAD of about 1.9 best I can figure (but has a genoa available). You'd need to reduce your SAD to account for the stability difference once you matched the DLR of the Seaward.

Please don't get too upset with the remarks, its just that everything in your design that departs from the norm has a seriously negative effect on stability. If the crew is young and agile and can trapeze or kickstrap well out- butts outboard- then you can accept a lower hull stability, but I didn't get that impression from the rig or your comments.

-data from the Plyboats tables for boat's under 30'. These are older craft.

average displacement for 22' Lwl = 5500#
average lwl for a 2700# boat = 18'
average Sail Area for a 22' lwl = 320sqft
average SA for an 18' boat = 220sqft
average beam for a 22lwl = 9'
average lwl for a 7'10" beam = 18'

sample was 622 boats under 30'

 

Tom,

Looked at those quite extensively and considered many of their features. Thx.

Michael,

Life is a compromise....and so is my keel. I originally wanted beachable, but finally decided to go with lead as low as my design parameters would allow for CG reasons. Thx.

Tad,
I have to consider your input seriously. I believe I see what you are saying. I've got my centers based on calculations and I can't disregard them entirely. I essentially have a cat rig with a larger than standard mizzen. That puts a lot of sail area very far forward and (with my limited experience) that suggests keel area forward. Granted, with a deeper keel, the keel would be drawn in, in both directions. If I were to count 100% of my current sailplan for balance purposes, I would still have almost 7% lead. Using the standard 50% i'm pushing past 17% lead. So, maybe you can see why I'm hessitant to cut the keel back on the leading edge. Though maybe not the best way to design, I can still pull the rake out of the masts and I can pull the mizzen almost 4' forward if need be. I don't plan to lock in mizzen position until after doing sea trials. My gut (and my numbers) tells me to let the rig and lateral centers stay.

Phil,

No worries. Your sincerity is obvious and no one can fault that. I can't say that I'm big into number sets and the ones that I've been using are out of L & E's Yacht Design text. You are most likely correct that they are skewed towards larger craft. I can't say that I'm overly eager to dig into to my hull again. To throw some numbers at you though. My VCG is 1' above the waterline. Angle of vanishing stability is around 108 deg. Righting moment at 20 deg. heel is 4000 ft.lbs. and continues to rise to 4800 ft. lbs. at 48 deg. heel. My displacement is actually 1.4 tonnes and my garage is really 40' long (but don't tell any one ) Bellybutton factor is 2'. Cockpit coaming at it's widest part puts your backside at 3' from centerline.

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I think that the areas I could consider for change right now are cabin height. rig height and keel depth. I would not be eager to lose the interior volume, but I could feasible lose 6-8" off of the cabin, lower the main by that much and throw the same onto the depth of the keel.

 

Thanks, LP

That makes me feel a bit better. I had initially gotten off on the wrong impression I think. I was a concerned that you were basically pursuing the design ratios as goals in-and-of themselves, rather than as a means of quantifying the experience you were after. And your initial post didn't address stability at all. Your last post cleared that up.

My advise at this point is to really work out the cockpit details carefully because I think there will be a sense of being perched on top of the boat while sailing. I'd want the option to be able to hike out. A 4" flat support under the upper legs is wide enough for occasional use. And you won't want to be pulling down to sheet the main. Up from the center would be good.

 

Why having side deck? they are not usefull in this design. Go directly to the side with the cabin trunk, it will be simpler to built and stronguer too.

 

I also prefer a flush foredeck on smaller boats. It offers full headroom when seated on the berths instead of banging your head against the cabintop. As said, its stronger, simpler to build and far less tendency to leak or promote rot and also adds to righting moment. When done right a flush foredeck offers the looks of handsome classic designs.

 

Some thoughts

-about your construction plan -is this just for you, or are you looking to sell plans? If customers don't know and trust the technique it could be a big negative.

About sail plan -it's good that it is simple and keeps the force close to the water, but lugs are hard to manage when the wind kicks up. It takes a monstrous force on the halyard to keep them flat in a blow.

About the keel, did you consider twin keels? You may be satisfied that your stub is enough area, but heal the boat 15 degrees and it's useless -but twin keels would just be reaching max performance (good for an over-canvased boat). Twin keels would be nice for beaching, which may happen more than you intend given poor windward ability. Twin keels also protect hull better than twice as well.

This may be more advanced than you want, but when I see a small shallow draft keel boat I always think I would prefer to save on keel weight and spend big on heavy batteries for electric propulsion.

 

Phil,

Does mean you'll go out on it with me if I were to stop by while pulling it down to Florida?

Cockpit height is 2' above waterline. The cockpit coamings rise another 12" above that. If you look at the midship section, the far side of the cabin wall is shown in light grey. The flaired cabin sides transition to becoming a flaired coaming that will be comforable to lean against. I'm hoping this also reduces the feeling that you are be pushed to the lee side when heeled. The cambered seating area also slopes outboard at about 10 degrees where your butt goes. This way you don't have to brace yourself until past 10 degrees of heel. I don't think I'll need to be hiking out on this boat. My current sailboat is 16' with 140 sf of sail on a 6' beam with no ballast. http://www.cmdboats.com/mbirdk.htm There are times on this little boat that a little more leverage would have been nice and I would sit on the coaming with a cushion under my seat. I've been inclined to build a moveable hiking chair that would give a good solid place to but the arse. Most of the time, I'll let the main luff a little or pull in a reef and relax.

Daniel,

Good to have you drop by. I wage the raised deck battle in my head on a regular basis. For basic utility, I think that it would be the was to go. There are many points in it's favor. I have a hard time with the raised deck appearance. I am working on a the raised deck version, but if I can't get the looks right, I won't build it. All of that deck area forward would be such a blessing. Time will tell and I still have a few months before any pieces get put together. I'm open to examples if anybody has a good one.

Tom,

See above.

Skyak,

No plans to sell the plans. The method is not undocumented. I have complete trust in the method as I have it laid out. The key to a good set on a balance lug is not the halyard, but the downhaul. The sail is hoisted then the downhaul is set. Most likely, I'll have a 6 to 1 purchase on the downhaul. The keel is an ongoing study. It may be two years before I really have to get it fully configured. I have a couple of options for centerboards if I decide to go that route.. I can house one in the keel or I can place one off the centerline and hide it in the face of a settee. No electrics for me. Give me gas.

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The raised-deck argument is a valid argument and can't argue with any of the points presented. The design as it stands is a project that I've worked on over several years and I have grown fairly attached to it and it's crazy flaired side-walls. A little bit of form follows function IMHO. I guess the question is always about which function to set your form around. If I can work the general appearance into a raised-deck, I'll be onboard and ready to run with it.

I'm glad the forum is here and my little project is interesting enough to evoke some commentary.

 

Sure- I'll go out on anything. Everyone knows that. Just don't frown at the #80 dufflebag I bring with me.

I used to get a lot of such invites. Often, they had something to do with the wife or girlfriend refusing to go on board unless I went along- usually following work on the engine/fuel system/electrical system by the husband. I called these "bring your own fire extinguisher" invites. I did a lot of Florida boat parades under these circumstances. I was the guy refuelling the generators hung over the side like so many fenders.

 

Blackrock again.

This was nearly a complete redesign. I drop the keel entirely and went with a good sized centerboard that should work well with the sail rig that I have. I am excited to have the centerboard as it lets my work in an original design criteria that I had dropped in preference for the simplicity of a fixed keel. I originally wanted something that I could beach or could take to the ground. The ballast is upped to 800-1000 lbs and rather than simply throwing on more ballast and moving the waterline, I reshaped the entire hull to use the added displacement to my advantage. Removing the keel(adding the centerboard) also lets me live within my overall height requirement and increases shallow water launchability. The centerboard is offset and blends into furniture for the most part. Cabin intrusion is minimal. I tried to work the raised-deck aspect, but could never get the ugly wiped off. I've never been a fan of the appearance of raised-deck cabins, but the advantages of that type design warranted attempts at making one work.



The drawing is a work in progress and I'm working an idea that shortens the centerboard, but increases it's extension. I figured I'd put it "out there" for some feedback before cleaning things up.

 

A couple of updated drawings.

Attached is the latest profile with the centerboard that I've settled on. I'm deciding whether to have it reside fully inside the hull or to have a slight amount always exposed for lateral resistance in the retracted position. The bottom 3" are going to be a lead shoe. It would be accompanied by matching fixed shoe on the opposite side for balance, additional ballast and grounding/beaching.



The midship section is also attached showing the offset board that allows for a clear central cabin.

 

Having the centreboard slightly protruding from the bottom of the hull may help avoid the board getting jammed wth small stones. At least, the centreboard on my boat is like that and it does not seem to get jammed.

If you do have the centreboard protruding when fully raised you will need some robust arangement so that it can take the whole weight of the boat when grounding. Also, with your offset centreboard the boat will dry out at an angle if the centreboard protrudes. You could have a shallow bilge keel on the opposite side to counteract but i am not sure your centreboard is far enough offset for that to be laterally stable when dried out.

 

John,

I figure having two lateral points of contact is a starting point. I don't anticipate too many occasions for the boat drying out, but it will at least start out horizontal.

 

Constrution design question.

I'm trying to look pretty far down the road as I draw up my plans for this vessel.

When it comes time to lay in the settee and V-berth panels, I'm thinking that I'd rather not be filleting and taping while standing on my head. Right now, I have it drawn up with SPF cleats attached to vertical surfaces.
1"x1/2" for single side attachment. I plan to up it to 5/8" if I continue along this route and for each side at the top of a vertical support (frame top), 1/2'x1/2". This is a lot of little pieces, but the work is done either outside
of the boat or with easy access inside the boat. I will likely fillet edge to hull locations for obvious reasons.

Are there recommendations for an alternative method that I have over looked?

 

Cockpit details.

I'm trying to tie up some loose details with my cockpit. I've attached a drawing of the centerline cut in the cockpit area of Blackrock. I've drawn an outboard profile from dimensions taken from the internet.
It has a leg dimension of 17 1/2" from transom top to cav plate. I admit to having little knowledge on the small outboards. A short legged o/b should have a transom height of 15" and a logged/shafted should be 20".
I'm trying to decide if I've got a bad dimension and what I should with the transom height. Tied to this too is where/how deep the set the foot well in the cockpit for comfort. The orange line in the profile are at 16".
Common sense IMHO puts the well no deeper than a persons knees. The sloped sole ranges from 20"-21" fore to aft. My concern is that this is deeper than is comfortable.

I am looking at designing to accept a short shaft (15") motor. I like the lower placement of the motor for CG reasons and less intrusion into the cockpit. I think it will also be easier to design a sound deadening
enclosure for the motor. I believe that I need to lower my internal transom to accommodate a short shaft motor, but I'm not absolutely sure. This will also affect my footwell dimensioning.

I am thinking I need a 15" dimension from hull bottom at motor well to internal transom top. Is my thinking correct here? Also, I am looking for input regarding any oversights anyone might see with regard to my
engine placement or other cockpit features.

I'm also attaching aft and mid-cockpit station cuts (not to scale) to help visualize what the design more fully.