Barrelback replica - increasing deadrise

Hi everyone, have a question to put to the experts here...

I am a boatbuilder from Sydney and will soon be building a 17-19ft Chris Craft style barrelback. I use the word replica loosely because I am thinking of making a couple of changes...

I would like to increase the deadrise angle so to make the boat a bit more friendly to the harbour here which can be quite choppy sometimes with ferry wash etc. I think this would also help to eradicate the tendency of boats with flat-bottom sterns to "slide" around. I also would like to add a keel to help it to track better and be less susceptible to the influence of wind. Nothing big, just tapering from say 6 inches aft where the shaft comes out to nothing at the base of the stem.

Now with these modifications what would be the chances of moving the engine aft a couple of feet and using a v-drive to increase cockpit space? I have no problems with having an under-floor fuel tank further forward to try to balance the trim out a bit and I was hoping that the extra volume aft from the increased deadrise would make this more feasible also.

Any constructive criticism/suggestions on these topics would be much appreciated. I specialise in restorations and repair mostly so my knowledge on the naval architecture side of things is quite limited.

Thanks
Carl
Carl Rickards Classic Boats/Sydney Harbour Boatbuilders

 

Hey Carl,

I don't really have any suggestions, but you might want to look at the Van Dam Wood Craft web site. They have built quite a few variations on the classic runabout and most of them have more dead rise than the originals.

http://www.vandamwoodcraft.com/

Check it out, you might get some ideas from the hundreds construction pictures.

 

The changes you've suggested could be done, but you're fooling with the balance of the boat, quite an amount, which should only be attempted by experienced and knowledgeable persons. Craft of that size are very trim sensitive, so you'll want a pro to look over your modifications on paper before you commit to making saw dust.

 

Thanks guys, I know I am fooling with it quite a bit. I have done some preliminary sketches which I will try to expand on and post up to show you tonight. Trim tabs might be on the cards too, just to give the driver more control of that aspect. I want to get most of the way through the design process before I go to a real expert, though I am related to and know well some of Australia's most recognised boat designers (Halvorsens, Warwick Hood).

 

Most designers would rather work from unaltered existing designs and modify them to suit your needs. This is much preferred then to sort though the speculations of an unknown quantity of "refinements" of equally unknown quality. Not trying to offend, just an observation for someone who's received this type of request and had to speculate on the "modifier's" qualifications and justifications for extensive alterations to well founded designs. In short, it's much cheaper and easier on the designer, to work on the changes early in the design process, rather then later.

Frankly, it sounds like you need an "In the Spirit of" custom or semi-custom design, based on the barrelback.

 

Hi Carl,

If you look at my website, you will see my Cherubini Classic 20 (CC20) which is a retro style speedboat that I designed with a 20-deg. deadrise bottom.

http://www.sponbergyachtdesign.com/CC20.htm

http://www.cherubiniyachts.com/20.html

This boat performs like a champ with an exceedingly smooth right and is able to make tight turns at high speed. It has topped out at nearly 60 mph on about 240 HP. It does not have a keel. The deeper you go with the deadrise, the less you need a keel.

Since I designed this boat, I also re-designed a Chris Craft Cobra 21 with essentially the same bottom as the Cherubini Classic 20. The original Cobra, designed in 1954 and produced in 1955, came with the traditional concave sections forward and flat bottom aft, and this particular owner for whom I did the design wanted to build a reproduction Cobra that would not have the handling problems and hard ride of that type of design. The boat should be nearing completion soon and we are hoping for a favorable run. I gave him new lines and complete framing patterns to build the boat.

The original CC20 was designed for a jet drive using Mercury jet pumps. This necessarily put the engine and drive in the aftermost part of the boat, and so the deck and cockpits were designed and tooled forward of the engine space. The latest version is a diesel I/O drive which seems to be more popular than the gasoline jet. We have definitely considered using a small block V-6, 4.3 liter gasoline engine and a V-drive to an underslung propeller. It has to be a V-drive set-up because it costs too much to retool the deck and cockpits. Cherubini has not built a prototype one like that yet because they don't have the budget for it at this time. They are a very small but growing builder, and hopefully they can devote one of their hulls to this set up in the not-to-distant future. I think it will be really nice.

Eric

 

Brilliant stuff Eric, looking at your successful design makes me so much more confident in undertaking this project because you have shown me that it can be done! I think your design is almost exactly what I want bar a couple of things - I prefer more of a plumb stem, and a more gradual change in the deadrise angle running forward. Obviously having a sharper v forward reduces pounding, but I am a little bit puzzled as to why you added such a dramatic change.

The two reasons why the sudden angle change doesn't appeal to me are that I want the boat to be slipway-friendly (your design would be a bit of a nightmare on a traditional slipway), and that I want to be able to build with sheet ply below the chine. Just thought I would clarify that for you too.

One more question, did you have problems with trim in the boat? That is my main concern sofar, but it seems like in your design everything even the passengers is in the aft half of the boat. How did you stop it from digging a big hole for itself?

Thanks
Carl

 

The CC20, of course, is built out of fiberglass, so we can do just about whatever we want on hull and bottom shape. I specifically wanted a high deadrise and convex sections forward to allow the boat to slice through waves, minimize pounding and give it a soft ride. We hit a bulls eye with that design--the boat rides beautifully. The deadrise is nearly constant for the whole after half of the boat, 20 deg. The more prismatic the shape is in the after part of the hull, the lower the resistance and the more predictable the speed-power characteristics. If you draw out the twist too much, carrying it too far aft, the boat will be slower and won't handle as smartly.

Obviously, if you want to use only sheet plywood below the chine, that is going to affect your design. On the Chris Craft Cobra, the owner is using plywood in the after half and strip planking and veneers in the forward half to get the bow shape just as I designed it. This is going together well.

As for trim, you'll notice that the chines are nearly a constant width, which allows the bottom to be a nearly constant deadrise. Trim is a function mostly of waterplane area, and we have lots of area aft. This allows us to carry a lot of weight aft, namely the engine and the passengers. So no, I did not really have a problem with trim; I did a very detailed weight estimate to make sure the boat would trim properly.

I had more trouble with getting the boat down to her lines. Initially, the boat was so light that the chine aft was not down in the water when at rest--it was slightly above the water. So in the course of development, we looked for ways to make the boat heavier so that she would sit lower and submerge the chine when at rest. Mostly we beefed up the fiberglass, which has the beneficial effect of making the hull and deck stronger and stiffer. We were not worried about speed slowing down due to the extra weigth--the boat goes plenty fast enough for the average boat owner. With changes in engines and powering, however, the equipment weight went up also. So what was originally a boat of about 2,300lbs. is now closer to 3,000 lbs. She still runs like a champ.

In your studies of other classic boats, you probably saw that many of them have a maximum beam somewhat amidships or forward of amidships even, with fairly narrow sterns. Such was the design concept back then, along with flat bottoms. Because of the lost waterplane area aft, the boats are very sensitive to trim and fore-aft weight movement. And a narrow stern and wide beam forward is going to affect the shape of the bottom of the boat, the amount of deadrise and twist, and therefore the ride.

This is why in the CC20, and fortunately in the way the Chris Craft Cobra was designed, it was important to NOT start from too traditional a shape--i.e. narrow stern and plumb bow. I could not suffer with a narrow stern. I started the design from scratch with the planing surface first and then shaped the topsides and deck to suit the look. I wanted that bow to be slightly raked to mimmick the look of the Dodge Watercars. And the transition from the stem to the bottom has kind of a nice slant to it--not a deep forefoot, and this keeps the bow from digging into the waves prematurely.

I am very pleased with the way the boat came out--we hit a home run with this design.

Eric

 

Great, thanks again Eric. I started doing some proper lines drawings last night so I will try to put them up tonight for people to have a look at.

 

First lines done

I have finished the first draft of the lines for the boat, I am thinking maybe it needs to a be a little beamier but it would be good if I could get some feedback. Thanks

 

What you have to do now before changing anything else is to calculate the displacement of the boat. What is the volume and where are the centers so that if you want to make changes, you have an idea as to which direction the changes should go. I also suggest you draw a plan view of the chine line so as to judge its fairness and fullness.

To my eye in the body view, the midship topside stations have perhaps a little too much concavity, whereas the forward stations perhaps not enough. Just a thought.

Eric

 

You might not be able to see it very well in that picture, but the chine is drawn on the half breadth view. It is hidden a bit by the shading of the covering board. I did notice that it might be a bit too full in the bow, because I need the chine to "disappear" so it will flow into the stem nicely. Now that I am thinking about what you are saying with the concave sections midships and my thinking I need to increase the beam, I think what I actually need to do is just increase the beam at the chine aft, taking out most of that flare midships and run it in finer giving me more flare up forward, making it disappear into the stem, and giving me more waterplane area aft to support the engine. Sorted. Now to do some new drawings then start with some calculations... Thanks again for your help Eric!

 

modify existing

Hi Carl- I have been searching the internet for someone who has done this and came across your post. I am an amateur builder in the planning stages to build a semi-replica 20' barrel back. I was planning on using a Glen-L design for the hull unmodified and was hoping to change the cabin layout to make it larger and accomodate a wrap around bench seat similar to your drawings. I want to be able to increase the main cabin size by moving the engine aft flipping it around and V-driving it so that the transmission/v-drive is under the rear seat. Obviously this will change the weight distribution and I was hoping to move the tanks forward to compensate. I was not planning on changing the deadrise. Have you had luck with your new design. Do you think this is possible?

Thanks,
Ben