How will she plane???

I am currently infatuated with the old design, Caballero. It was designed by naval architect Charles Ungerbuehler, probably in the late '40s or early '50s and the offsets and a few diagrams are available free. I have no expertise on boat design other than I have looked at many designs on the internet over the years.
My concern is this:
On most of the modern planing powerboat designs I see, the bottom is completely flat. However on Caballero, the bottom rises from amidship to the transom of the boat. Who am I to question Naval Architect Charles Ungerbuehler, but will this design plane easily, or will the rise in the bottom cause bad behavior in getting up on a plane. I know outboards are much more powerful now. In his design notes, Ungerbuehler mentions 25 and 33 HP motors, which I assume were the most powerful back then. My question to some of you who are much more knowledgeable than I is if you were building this boat, would you go with the designers plan for the bottom, or would you be inclined to alter the plan to straighten the bottom. My hope is that the boat if powered by a 50 to 70 HP motor would plane easily and cruise at around 17 or 18 MPH.
Please excuse my lack of knowledge and poor use of terminology, but your comments would be very much appreciated. Thanks, Lynn

 

Unfortunately it is not possibleto zoom in and see details, but provided you stick to the original plans and maintain strict discipline when it comes to weight, that bottom is an "easy glider". It has some warp, ie the bottom rise is flattening going aft. This makes her a moderate speed vessel; nice in not too rough seas. 70 hp might bee overpowering her, pushing her nose down. A modern 40 (maybe 50) hp:er would certainly do fine.

Note that this bottom is developable if there is a certain curvature in the frames; it is a common fault to believe that straight frames result in a developable surface when there is a warp. The sides may be a different story though. Can't see those details either, sorry...

 

Baeckmo, thanks for your comments. I am attaching a link to allow readers to see the plans and diagrams close up. I am OK with just a 40 or 50 hp. Smooth and slow works for me. I was also wondering about increasing the beam about 6 inches. Other than that I would prefer to stick with the original plans for the bottom if I can be confident the boat will plane and cruise at a fairly slow speed. Thanks. Here is the link:

www.svensons.com/boat/?p=MechanixIllustrated/Caballero

 

20 ft long weight 1 ton......If you are buying an outboard remember that the power to get it on the plane and he power to keep it on the plane are two different things.. If you have plenty of power then once its on the plane you can cut back the rpm and it gets quieter...if the motor is a bit minimal you will need to keep the rpm high and thats noisy ...I bet it will drop off he plane at about 15 knots so go for 20 as a cruise this will make it easier to handle.....I had a 20 ft stabicraft about thesame size and weight with a 70 HP had to run it hard all the time ...115 was quieter once up on the plane

 

Most all of the classic type boats have this type of hull so you should have no problem. Here is an example. Make sure you know how to strengthen the transom. https://www.boatdesigns.com/products.asp?dept=339

 

She will plane fine with the motors listed. 50hp is plenty to give spirited performance if you don't over load it. The bottom is best suited for inland waters because of the fairly low deadrise (V) forward which will make it pound in rough stuff.

 

Lynn, in order to create lift, a planing surface must present itself at an angle to the water which is acting upon it. In order to do this, it will trim up by the bow. As the buttock lines of Caballero slope up towards the transom when the boat is at rest, it must trim higher by the bow in order to this than would be the case if the buttock lines were parallel to the waterline. However, quite apart from any other side-effects that may ensue, if you were to change the bottom shape to make the buttock lines level, then you would move the centre of buoyancy further aft. This will (generally) increase the speed at which the boat will climb onto the plane and increase the amount of trim that will occur in the process.
Caballero has quite flat sections, so one would expect her to plane quite easily and with relatively little power.
Or, to put all that a bit more simply.... what Tom said...!

 

Will, Tom- as usual, hard to argue with either of you.

Lynn- as has been mentioned, this hull ought to do fairly well at low to moderate planing speeds with something around 50 hp. More might be OK if you're going to be carrying heavy loads and select a prop accordingly. Don't overpower it just to get more speed; 20 knots or so would be a nice fast cruise with something like this. Expect a comfortable ride in calm conditions, with some pounding and slamming in heavy chop.

 

Thanks for your replies, gentlemen. You said what I hoped you would say, and alleviated my fears regarding the planing question. I like the look of Caballero, and I would be satisfied with the way you describe the smooth ride. I wouldn't care to set any speed records in a chop. The boat would only be used in inland waters, like our local lakes in Mississippi, or an occassional cruise on the Rivers. Again, thanks for your help. Lynn

 

Lynn,

If you like the look of the boat, and are happy with the assessments you've seen of her performance potential, it's time to think about construction considerations.

Most notably, there are a lot of structural details that are not shown on the drawings you posted. Unless there are a few more sheets of drawings hiding somewhere, you'd have to figure out a new construction method and recalculate all the scantlings. (Actually, for a boat designed in the '40s and being built in 2009 from modern materials, this would be a good idea even if full plans are available.) She was likely designed as plank-on-frame construction; today, I'd be tempted to suggest wood/epoxy strip plank or plywood/epoxy composite as a stronger, lighter, and easier to maintain alternative. Unless, of course, you really love camping out at the pier for a day while the planks swell up at each launching...

There are books that can help you with this (Dave Gerr's "Elements of Boat Strength" comes to mind). But outdated, incomplete plans don't make for easy work. I'm not trying to turn you off this project- if you build it, it'll be quite an eye-catcher for sure. Just making sure you're aware that there are going to be a lot of challenges to overcome if you go ahead with what you have.....

 

Thanks Matt. Yes I am well-aware that building her with the limited information available would be quite a challenge. I agree with your suggestion of plywood/epoxy composite construction. Thanks for the suggestion of Dave Gerr's book. I am planning to build a small model first, and I will be researching to get some ideas on joinery and construction techniques that would work well for this boat. If I decide to go ahead with Caballero, I'm sure I'll be back here seeking or begging for technical and moral support. Thanks again. Lynn

 

I would ditto what Matt said about updating the construction being a likely good route. It may be possible to duplicate the bottom in plywood but the forward topsides have flare that cannot me done in sheet material. Everything can certainly be gotten with cold molded ply though. This is a superior way to build a boat anyway and would be my choice for any compound surfaces.

A great first task would be to build a model from the offsets. This will tell you which panels are developable and which must be build up. For those surfaces that are developable from sheep plywood, it will also give you the panel dimensions in case you would like to try stitch and glue construction. You might get Sam Devlin's book on the subject which is a very good introduction.

Looks like a great project. Take your time and it will take some serious time to turn out such a nice boat. Don't worry about the engine for a while. Good luck.

Lynn, I see that you responded while my slow fingers were working.

 

If you take a look at the link in Lynns 2nd post, there are some other drawings with the construction sections and the like. It is indeed plank-on-frame as Matt surmised.
There are many similar looking hulls around that are designed for stitch and glue....have you considered mating the styling and layout of Caballero to a more simply constructed hull? As Tom points out, you would lose the ability to incorporate flair into the topsides, but that's about the only obvious downside....

 

Tom and Will, thanks for your suggestions and encouragement. The designer does comment in the article that large sheets of plywood cannot be used on either the bottom or sides. I really can't see why not on the bottom, but there is flare in the topsides. The Ashcroft Method could be utilized to achieve that, and I do like the look of the flare. Will, I would be open to any suggestions about a more modern hull, but I love the "Retro" style of Caballero.

 

Typically, when you bend a sheet of ply to form the bottom panel of a planing hull, the resulting sections will be convex in nature. Caballero's sections are straight, so I would doubt that they are developable.
I'm not suggesting that you change the styling to something more modern - just the building technique. As for suggestions, I'd look no further than my esteemed colleague Mr Lathrop.