Seeking guidance on design of 18' cat-ketch double ender

Hi, I just joined to seek assistance with a boat I'm designing in Freeship for glued plywood lapstrake construction. This is an 18' double ender based on the Noman's Land boats . This isn't any particular set of plans. My references are drawings in "American Small Sailing Craft" and John Leather's "Spritsails and Lugsails". Rig would be cat-ketch spritsail. Intended use would be for day sailing and dinghy camp cruising on either Puget Sound or coastal Bretagne, France. I would like the boat to be trailerable with water ballast and sealed buoyancy compartments. My understanding is that I should have buoyancy up high in the ends and down low amidships, so I plan to seal up the bow and stern and have sealed compartments below the side benches. I welcome any feedback on the design characteristics of this hull.

Freeship Design Hydrostatics
Design length : 17.860 [ft]
Length over all : 17.930 [ft]
Design beam : 5.917 [ft]
Beam over all : 6.484 [ft]
Design draft : 1.300 [ft]
Midship location : 8.420 [ft]
Water density : 63.989 [lbs/ft3]
Appendage coefficient : 1.0000
Volume properties:
Displaced volume : 30.121 [ft3]
Displacement : 0.860 [tons]
Total length of submerged body : 16.178 [ft]
Total beam of submerged body : 5.851 [ft]
Block coefficient : 0.2448
Prismatic coefficient : 0.5104
Vert. prismatic coefficient : 0.4261
Wetted surface area : 70.799 [ft2]
Longitudinal center of buoyancy : 8.887 [ft]
Longitudinal center of buoyancy : 2.887 [%]
Vertical center of buoyancy : 0.943 [ft]
Midship properties:
Midship section area : 3.648 [ft2]
Midship coefficient : 0.4796
Waterplane properties:
Length on waterline : 16.178 [ft]
Beam on waterline : 5.851 [ft]
Waterplane area : 54.383 [ft2]
Waterplane coefficient : 0.5746
Waterplane center of floatation : 9.057 [ft]
Entrance angle : -89.984 [degr.]
Transverse moment of inertia : 103.48 [ft4]
Longitudinal moment of inertia : 571.24 [ft4]
Initial stability:
Transverse metacentric height : 4.378 [ft]
Longitudinal metacentric height : 19.908 [ft]
Lateral plane:
Lateral area : 16.203 [ft2]
Longitudinal center of effort : 8.785 [ft]
Vertical center of effort : 0.776 [ft]




The following layer properties are calculated for both sides of the ship:
| Layer | Area | Thickness | Weight | COG X | COG Y | COG Z |
| | [ft2] | | [tons] | [ft] | [ft] | [ft] |
|-------------------------|--------|-----------|----------|---------|---------|---------|
| Layer 0 | 18.270 | 0.000 | 0.000 | 9.154 | 0.000 | 0.958 |
| Sheer Strake | 14.919 | 0.236 | 0.000 | 8.955 | 0.000 | 2.653 |
| 2nd Strake | 16.358 | 0.236 | 0.000 | 9.460 | 0.000 | 0.969 |
| Garboard | 23.629 | 0.236 | 0.000 | 9.644 | 0.000 | 0.656 |
| 6th Strake | 15.604 | 0.236 | 0.000 | 8.758 | 0.000 | 2.235 |
| 4th Strake | 16.425 | 0.236 | 0.000 | 9.149 | 0.000 | 1.467 |
| 3rd Strake | 16.867 | 0.236 | 0.000 | 9.432 | 0.000 | 1.201 |
| Deck | 26.004 | 0.000 | 0.000 | 8.824 | 0.000 | 3.072 |
| Rub Rail | 5.014 | 0.000 | 0.000 | 8.886 | 0.000 | 2.920 |
| Coming | 5.116 | 0.000 | 0.000 | 8.742 | 0.000 | 3.045 |
| 5th Strake | 15.136 | 0.000 | 0.000 | 9.254 | 0.000 | 1.860 |
|-------------------------|--------|-----------|----------|---------|---------|---------|
Total 173.34 0.000 9.270 0.000 1.447


Sectional areas:

| Location | Area |
| [ft] | [ft2] |
|-----------+----------|
| 1.000 | 0.000 |
| 2.000 | 0.293 |
| 3.000 | 0.753 |
| 4.000 | 1.277 |
| 5.000 | 2.011 |
| 6.000 | 2.795 |
| 7.000 | 3.353 |
| 8.000 | 3.618 |
| 9.000 | 3.609 |
| 10.000 | 3.326 |
| 11.000 | 2.824 |
| 12.000 | 2.229 |
| 13.000 | 1.638 |
| 14.000 | 1.124 |
| 15.000 | 0.720 |
| 16.000 | 0.421 |
| 17.000 | 0.148 |
|-----------+----------|
NOTE 1: Draft (and all other vertical heights) is measured above the lowest point of the hull! (Z= 0.072)
NOTE 2: All calculated coefficients based on actual dimensions of submerged body.

 

Seems like an interesting project,but a screenshot,if you have image posting permissions,would have been more useful than a cascade of figures.I might have misunderstood the description of buoyancy arrangements in that it isn't clear whether the sealed compartments below the side benches are for buoyancy or water ballast.It would also be useful to know more about the methods under consideration for filling and emptying the water ballast tanks.

 

Apologies for the formatting, wet feet. I uploaded the images to a gallery here and thought this would be the way to link them. Regarding the side benches, those would be sealed buoyancy in addition to compartments in the bow and stern. Water ballast would be in the floors, filled by opening an inspection port and pulling a plug. Emptying could be done with a manual pump plumbed to the side or CB case. I am a complete novice with no training in naval architecture, so feedback on any point is welcome. Is a displacement of 1700 lbs ridiculous for a boat this size? Should I slim down the waterlines? I imagine I'll need to eventually contract a professional to work out the details before I can draft a set of plans.

 

Historically,people would compile a list of the characteristics of similar boats so that it would indicate the "norms" for the type of boat and only a bold individual would deviate too far from well known precedents.In this instance I wouldn't know what designs might be directly comparable and consequently can't comment.I would suggest browsing American Small Sailing Craft by Howard Chappelle for a solid start regarding shapes and proportions.
Are the seasonal wind patterns consistent and predictable?How much experience of similar sized open boats does the prospective sailor have?It might seem like a lot of questions but only by answering them will some guidance begin to emerge.

 

The prismatic is very low, which is fine for light air sailing but slow when it blows. She would be a better boat if that aft hollow in the curve of areas was straightened out. With such a fine stern the helmsperson (and all loading) will be pushed forward for proper trim. Loading makes a big difference in small boats, so you need to look at hydrostatics in a light condition and in your heaviest imaginable condition.

 

... So basically, you want to drop the planks a little in the stern and lower the inflection point in the aft displacement curve a lot (and move the inflection point in the aft waterlines back to at least 95% of the WL). This will bump the Prismatic up to a more reasonable value. You want to check a range of loadings covering about 500 pounds at 100 pound increments. Prismatic should be fairly consistent. Most of the RM comes from the crew, so the ability to carry sail power doesn't change much with load. Don't be afraid to use a trapezoid section on the horn post.

Have you run a developability check to see how much swoop the planks have and how they nest? That is a major constraint on ply-plank designs. Please post the image of the plank nests so we can better understand that constraint.

Mostly for aesthetic reasons - but also practical ones - I try to have the sheer strake where it can be fitted with an actual plank. So shaped within the limits of taper and edge-set for a conventional cedar plank.

Chuck Paine's Francis 26 lines from Paine's website -
She was heavier and more stable, so a bit rounder, but you see the difference.
26’ FRANCES. A small double-ended cruiser. – Chuck Paine Yacht Design LLC https://www.chuckpaine.com/boats/26-frances-double-enders/

If you really want a stern that fine, you should probably rig it as a yawl. That will put things where they need to be. See boats by Albert Strange, who was partial to these.
CHERUB 2 1/2 - a modern version of albert strange’s favourite boat https://www.jamieclayboatbuilding.co.uk/Website/FUTURE_PROJECTS_CHERUB_II.html

Cherub II -

 

Phil, thanks for the comments. I was not receiving notifications, so sorry for the late reply. Less hollow aft was almost unanimous advice from everyone who saw this design. I have reworked the lines, but probably not to the extent you suggest. There are two sets of waterlines attached - one at 2° trim and one parallel to the keel. Developability checks out. I've modeled my revised design in cardboard. I'll post up the developed plates when I get home. I'm not familiar with the term horn post. Could you please explain?

 

Tad, thanks for the comments. I didn't see your post until today. I have revised my design to reduce the hollow aft and got the Cp up to 0.545. What would be a good range of Cp to shoot for?

 

Here's a cardboard model of version 2 made from the lines plan and developed plates. Scale is whatever FreeShip spit out. The rig is just there for fun, it is not proportioned.

 

Tad spoke of trim and loading. I have a 15' sail-n-oar vessel that has a push-pull tiller to move crew weight forward. It's OK but I'm not a fan.
How do you plan to propel her without sail? My 17' sail-n-oar weighs ~550 pounds ready for the crew and is about as much as I care to row.
The same 17' boat, a Salish Voyager, was initially offered with water ballast but that is no longer an option as it didn't deliver. The space in the bilge is much better served with room for ground tackle, wet boots and metal or rock ballast. Armstrong 10"x20" hatches are great for that application.

 

I'd like to have two rowing stations, one at the forward end of the cb and one at the mizzen thwart. Otherwise, I'll have an outboard either on a side bracket or an off-center well with a flush plug.

 

A little is a lot when it comes to Cp, so somewhere between .52 and .54 would be my goal. .52 would favour rowing and light air sailing, whereas .54 would be better for higher speeds in more wind, and draggier at low speed.

 

Minor note of FreeShip. You will want to turn off hydrostatics in the layer for the rudder. Check that the reported vessel waterline does not include the rudder's waterline. It's normal for boat computations to use the canoe body - no rudder and no board, but to include the keel in long-keel boats. Some people only included down to the rabbet lines though, so you have to be a little careful when comparing prismatic coefficient of these older designs.

It looks like you may have done that already, but I can't tell for sure

 

Thanks. Yes, I caught that when I first added the rudder to the model. What do you think of the revisions, Phil? Maybe I should consider adding a transom and model it after the Kingston lobster boats instead. Also, I just realized I didn't post the developed plates to show the shapes of the strakes. I'll try to do that today.

 

Having experience with transom and double ended sail & oar boats I'll say a double ender loses a couple feet of useable room* and greatly complicates mounting a motor.
* Or, if you have space, you add a couple feet to maintain the useable capacity.