feasibility of motorized skin on frame cs 17?

[DanSkorupka]

Hi I'm new here.

Sorry in advance for such a long post.

I have been looking for an efficient, multi-purpose semidisplacement boat for some time. A couple of years ago I stumbled upon the Happy Clam by Atkin. It is seaworthy, has amazing fuel economy, and is aesthetically pleasing in my opinion. HOWEVER it is a very complicated hull by any standard. study plans call for 428 hours build time for a commercial shop.

I found out about the Core Sound 17 and I think this will fit my bill. I have long been frustrated with the lack of any motorboat that will travel in the low teens with a small motor, carry a substantial load, and be build able by an inexperienced woodworker.

My idea is to build a core sound 17 hull shape as a dedicated motorboat i.e. mast absent. I have also been intrigued by large skin on frame boats, and the possibility of propelling them by internal combustion. it may sound crazy but look at the umiaks still used by Alaskan and Canadian first nations.

I have heard that in a force 5 wind 25 to 35 square feet of sail equal one horsepower, and in a force 4 wind the equivalent is between 50 and 70 feet squared per horsepower. given about 120 ft^2 of sail: 120/30 = 4 and 120/50 = 2.4 . I have heard the largest safe motor for the cs 17 is 10 hp.(is this true or just someone trying to sell bigger motors?)

if there is any question if a CS17 can plane with a motor there are two facts I would like to point out

1: people have planed under sail without removing the motor

and

2: this

Having had a chance to check out a CS-17 with an outboard I can tell you a 6 HP isn't way too big, if you like to go fast. I had a mildly hot rodded 5 HP Mercury on one a couple of summers ago and it was cooking in the high teens, low 20 MPH range. Yes, it was bow up pretty good, but it could be trimmed down with weight or crew location and she was experiencing a bit of a yaw tenancy, suggesting she was "on the verge", but I'm pretty good at keeping them upright. With the rig stowed, rather then up, she'd probably be a much more reassuring ride at these speeds.

Though for practical purposes, a 2 HP is more then enough engine, the 5 HP got me and a 170 pound friend up to 18 MPH in calm water.

so finally what I propose is a skin on frame version of the core sound 17 or something very similar, with a heavily overbuilt transom.

thank you,

Daniel

[PAR]

A skin on frame version of a CS-17 wouldn't be practical, nor advisable. The broad, relatively flat panels wouldn't lend well to this technique, not to mention bottom loading on a powerboat version. Skin on frame works in small, lightly loaded craft, such as canoes and kayaks, possably small shapely skiffs, but not a craft like a CS-17.

I'd also strongly recommend you look at another design, certainly one of Graham's first, as to a powerboat hull form that can motor with modest outboard size. A sailboat on plane is quite different then a powerboat in the same mode. The hulls are shaped differently and you'll find a CS-17 just pointing it's bow to the heavens, when throttled up and an uncomfortable sensation underway. 10 HP on a CS-17 is way over kill, a 2 HP outboard will easily push her along while a 5 HP could be all you need for contrary currents, windage and chop.

With the huge number of designs is this general size range, it would be best to suggest you need a design more specific to your needs. In other words, attempting make a silk purse from a sow's ear usually just nets a leather handbag.

[Ray Frechette Jr]

A skin on frame version of a CS-17 wouldn't be practical, nor advisable. The broad, relatively flat panels wouldn't lend well to this technique, not to mention bottom loading on a powerboat version. Skin on frame works in small, lightly loaded craft, such as canoes and kayaks, possably small shapely skiffs, but not a craft like a CS-17.

Aw come on PAR. It sure would be exciting to watch the Maiden Launch!

[hokeyhydro]

In other words, attempting make a silk purse from a sow's ear usually just nets a leather handbag.

Yes indeed, a SOF powerboat is a no-go. Good advice, PAR, but I laughed over silk purse...sows ear...leather handbag deal. Last leather handbag (Coach) I bought Milady cost as much as 6 sheets of 6mm BS 1088 or a 45lb jug of 105 WEST.

[MisterMoon]

FWIW, I think the report of getting a CS17 in the teens with a mildly hot rodded 5 HP might be a little hopeful. Wide open with a brand new 6 HP motor doesn't get me quite to 7.5 knots (8.6 mph) on mine. She's a lot happier at 5.5 knots (6.3 mph)... I had a flattie powerboat of about the same size and weight (AF-4) and it would only go about 20 mph with 15 HP.

I'm guessing "mildly hot-rodded" was an understatement!

[DanSkorupka]

Thank you for your help, I was worried about the large flat areas. I will not likely be building my boat for quite some time, and have made no commitments at all. Regarding gas motors on SOF boats, the larger Umiaks have been powered quite successfully with internal combustion outboards, although not at planing speeds and they were rounded shapes.

I am confused as to why a sailboat will plane easily under sail but will bow up and porpoise under power at the same speed.

The Rescue Minor by Atkin, and the Paiute by Weston Farmer are very sailboat like hulls, and look to me like they have a fair amount in common with the core sound series. They both are vee hulls with a large amount of rocker, and both are very close in size (24-25 foot long, 6 foot beam). Both plane with a small (20 to 30 hp) motor and if properly trimmed will not nose up. I have read that rockered semi-planing hulls, when properly trimmed, under motor, do not squat and nose up but instead it is difficult to tell low range planing from non planing without looking at the wake. The source for this information was an article about the Atkin Scandal skiff. The rider said it simply seemed to be levitating.

After reading your comments I have decided against skin on frame, and if I ever build will use plywood.

The motor I intend to use is a 5 horsepower briggs and stratton. On other forums there has been much confusion about these, which I would like to clear up. It uses a marinized version of 189 cc commercial Intek block as its powerhead. this has been given a heavier flywheel, and a carburetor more suited to low speed operation. It is very like the premium lawnmower engine off of which it is based. It is actually conservatively rated; the lawnmower version produces over 6 hp at 2800 rpm, and over 7 hp at 3600 rpm. The governor limits the outboard to 4000 rpm. Yes it is loud, yes it vibrates heavily. I own one and was once employed in small engine repair.

Most use would be on slow moving rivers in mild weather, except on Nauset Marsh (cape cod)in late summer. I might also use it in efforts to clean up said slow moving rivers. I would use thicker plywood than the plan calls for, I think 1/4 inch is kind of flimsy for my purposes. Thinking 1/2 inch hardwood ply. Weight is of little concern, I will be using a midsize suv to tow it, so I can deal with a hefty hull.

[Charlie Jones]

The subject of planing sailboat hulls, vs planing power hulls has a good bit to do with centers of effort. I know our boat, a fairly heavy non planing hull, doesn't like to get "pushed by the rear" and will squat if you apply too much power. That same hull will happily exceed hull speed under sail.

I'd really suggest you correspond with Graham on this. He's readily available to folks, designs both sail and power ( some LARGE power) and would, I'm sure be happy to assist you. The number and email address are both on the B and B website.

[Ray Frechette Jr]

Graham's bay River Runner is a boat similar to the CS series optimized for power use and might be a better fit. No longer advertised on website but still available by all counts.

[Bruce46]

Although not a design by B&B you might want to look at http://www.dropsheet.com/diablo/html/

[Tom Lathrop]

The main reason speed and trim angle of the same boat under sail and power is so different is the location of the center of thrust as mentioned earlier.

A sailing rig applies thrust at a point well higher than the highest point of the hull. Therefore the thrust applied at this high point forces the bow down and allows the hull to generate lift from the wider area of the bottom well forward of the stern. An outboard applies thrust from aft and well under the hull. This forces the bow up and lift must all be generated from the smaller area of the bottom near the transom. In addition, the bow high attitude makes more waves that those generated by sails and that robs a lot of energy also.

Just the opposite happens when we try to make a sailboat out of a planing powerboat. The big wide and flat transom that works great under power generates a lot of drag in a sailboat. You are neither the first nor the last to want to change one into the other. It can be done, as Graham did in the BRR and BRS, but you need to know what modifications will result in a decent boat of another color.

As usual, there is more to the story, but this is the basics.

[DanSkorupka]

Earlier I mentioned Atkins Rescue Minor, this was a mistake, I was thinking of Able Mable and forgot the name, Also by William Atkin. Not a B&B design, or even a plywood design, but very like what I am thinking of, except it is too big for me and my finances. It is a rather sailboat like hull and will plane with surprisingly little power. This is probably due to his use of a slow turning 186 cubic inch engine (no longer made) and the placement of its great heft a long way forward, and based on what I have been told it would nose up and porpoise harshly if an outboard were used without a heavy forward load (Is this true?).

Here is a link Atkin's Able Mable

I have not seen the body or sheer plan of the core sound boats, as they are not free on the web. From photographs of the cs 17 I think they look very similar under the waterline, except for a higher LOA/beam ratio on the Able Mable.

Are there lower priced study plans fore the CS series boats?

[PAR]

The subtle differences between the Able Mable and the CS-17 that you are not seeing, or do understand as significant, are the the very reasons that these two hulls are first, dramatically different in terms of application and second not even remotely similar. I'm not trying to be disrespectful Wanderer, but the two aren't even close enough to warrant a comparison.

The easy way for the novice to identify the difference, is to look at the chine profile. You'll note the chine is buried, from the first 1/3 of the LWL and continues it's decent all the way aft.

In fact, this particular chine (Able Mable) flattens out in the last 1/3 of the LWL, but never shows a hint of return, typical of a (pre 63 series) early, warped bottom hull form. After the early 1960's these hulls changed shape to a large degree (to the trained eye), but this older design is a classic form. A sailboat of similar size would have a single chine that just kisses or is slightly immersed around the midship or slightly aft of this, then the curve would sweep back up above the LWL to present an "easy exit" for water flow at the transom. This is true of the CS series and the chine just kisses the LWL (static, light load)

This is a Okoume built, CS-17 I did a couple of summers ago, note the chine line. This is her maiden trip, so details are missing, like topping lifts and a cove stripe, but the chine is just kissing the LWL, just aft of midship. This is typical of a sailing hull, even one designed to plane off.

There are other variations to this chine line location thing, but these two are simply typical. You'll need considerable more study before redesigning basic hull form dynamics, such as mentioned here. Again, this isn't meant as a personal dig at you, but an observation of your skill set in this regard. To a trained eye, the two hull forms mentioned are worlds apart and you have yet to get to this level of understanding. Keep reading, studying and paying attention. There's lots of good information out there, though admittedly, the net isn't the fastest, nor best source to expand your hydrodynamic knowledge.

[DanSkorupka]

I never claimed to be an expert in hull design, I am not a boat designer and as such am not proposing to alter the shape of any craft. I am not that good at analyzing photos, and already mentioned that I don't have any diagram of the cs 17. I have not been trained in boat design or hydrodynamics, and have had no source other than the internet to educate me up to this point. I was unaware of just how important the chine profile is.

My logic was to provide an example of a rockered bottom which could plane under motor IF there were sufficient weight forward to keep the bow down and the speeds were kept within reason. I thought this could be applied to a wide variety of hull designs. I have personally seen a moderately rockered boat, 16 or 18 feet in length plane with a 6 hp mercury on the Sudbury (or maybe Assabett) river . The boat was a soft chine rounded bottom type, and had 5 people in the forward third of the boat, and only 2 further back if i recall correctly. Its speed was in the low teens.

When I said that the Able Mable was very like what I was thinking of I did not intend to have it taken as it was. I DID NOT intend to pass it off as a oversized core sound. Yes, I admit, there are different boats meant for different purposes with different propulsion, with important differences in hull shape.

The purpose of my prior post was to correct an error in an earlier post, and to respond to Tom Lathrop's statement on the high point of thrust of sail. I know this makes a big difference, and am aware that in high winds the bow of a sailboat is pushed too far down. My message is that motor trim and weight distribution can, at lower speeds make a crude, incomplete, but somewhat effective substitute for the nose down force of a sail, enabling it to use more forward of the stern. I know that the top planing speed will still be substantially slower than a sail on the same vessel. I know that the effect of doing this always will be less than the force applied by a sail.

I acknowledge that the Able Mable's chine will give it a lot more stern planing surface than a normal sailboat. I should have better worded my prior post to illustrate that the Able Mable is very unlike any modern small production powerboat I have ever heard of, in the sense that modern powerboats tend to have great transom width and absolutely no rocker, a point I wanted to emphasize more than any similarity (or lack thereof) to the B&B sailboats.

I retract my claim of the Able Mable being a "rather sailboat like design" in light of further thought and your valuable information.

Also regarding chine profile is another poster put a 15 horsepower engine on the admittedly different Bay River Skiff, and planed successfully, albeit limited in speed and with undesirable levels of nose up. It also has a chine that is not buried for much of its aft run, and rises back up at the stern, but is not as much out of the water as the core sound is.

I know I am in deeper than I should be, and while I do not know much now I am able to learn as long as I can obtain the information, and learn I will. I came to this forum to learn better than I could by just reading, not just to spew ideas.

All I intend to ever do is build a core sound 17 (out of plywood, I was having doubts about SOF even before starting this thread, and now know better), put a 5 or 6 hp engine on it, put doel fins or other foils on the motor, have some friends in the forward half of the boat and go cruising around local rivers. My favorite nearby river has a speed limit of 10 knots, and I don't like high speeds, so I'm not looking for something that will fly around at some insane pace, but just to go 8 to 10 knots without spending the entire time at full throttle. I have neither need nor desire to be able to go over about 12 or so knots. I am not planing on any radical redesign, or even altering the hull. My only change would be to remove the masts. If I want to sail in future I can still add them.

[John Burritt]

If you're really looking for a power boat, go to Graham's web site and look at the Marisa. Its roughly the same size as a CS 17, but designed for very efficient motoring.

[Tom Lathrop]

Wanderer,

A 16 or 18 foot boat that will plane with 7 people aboard powered by a 6hp Mercury. I am in awe and would love to know its design.

[DanSkorupka]

I wish I knew what it was, I think it was strip planked and bore a vague resemblance to the American Sail American 18 daysailer (but it isnt) and is somewhat narrower. I am not entirely sure if it was 7 people or five, I think more than one were children, i only ever saw it once, when it passed by me. It was a very low range planing, distinguished primarily by the shape of the wake, and the fact that it was noticeably faster than hull speed. probably about 7 to 10 knots.

While I unfortunately do not have that plan, i have found something that performs a lot like it, but would take many years to build.

The plan I found was claimed to be a seabright skiff, put was almost entirely unrelated to the pulling boat of the same name, save the box keel.

The Happy Clam is a John (not William) Atkin design, 17 feet long 5 foot 6 inch beam, and built for small inboard power. It has a box keel the size of a canoe, draws over 11 inches, and planes with 5 mph at 14 knots, and the designer expects 20 mph with a 10 hp motor. It was boasted to have been clocked off the official mile (I assume nautical?) off loyds neck at 14.8 per hour with a 5 hp Palmer BH 6 inboard.

A detailed description that appeared in Mechanix Illustrated is available here. I would love this boat but I could never build it. A look at the build time on page 9 of the pdf will explain why I am looking for something simpler. That was a commercial shop back in the 1950s doing it. I don't even know how to guess how long it would take a homebuilder. I know it is far beyond what I am able to do personally.