Help with a simple skiff design

Hi guys, I've surfed this forum for a long time without registering!

I'm wanting to build a boat of my own design to use in rivers and protected inshore flats. The boat will be a flat bottomed skiff as big as I can make it from 4 sheets of 1/4" and 1 sheet of 3/8" plywood... plus some boards. I am a Mechanical Engineer, but I have little Marine Engineering experience, so I thought I'd ask the form!

This is what I've come up with:
Length overall: 16'-10"
Max beam: 54.5"
Max beam @ waterline: ~4'-2"
Power: 5hp to 15hp (planning to use a 10hp).
Speed: ~25MPH
Target hull weight: sub 150 Lbs.
Target cost: Less than a 12' Alumacraft flat from Academy.

I'm asking for you guys to look at it and tell me if you see any glaring mistakes!

*As designed, it does in fact fit on the required sheets of plywood.
*This drawing is missing most of the dimensions intentionally.

 

3-D sketch

 

http://www.boatdesigns.com/11-15-Power-Row-Skiffs-flat-bottom-skiffs/products/857/

some data for you. check your specs to these.

 

According to the images in post # 2, I doubt that the lines you have shown us are correct. The lines of the frames on the side should be all parallel. The bottom line of the bow section should be straight. Otherwise, the bottom panel will have double curvature in that area, which is useless and will make difficult its construction.

 

TANSL I am not sure if I get what you are saying but having twist is common and not at all a problem. It will cause cross sections not staying parallel and can even cause concave shapes.

See Ross Lillistone's Flint lines and construction photos below.

As for the Ironclad's plan. I think its bit weird to go for bottom split into two parts and only use it for the frontal V while most is totally flat bottom. I would keep some deadrise all the way to the back.

while at it check Ross Lillistone's Fleet. I have bought the plans as I think its a beautiful small boat.
http://baysidewoodenboats.com.au/designs/fleet/

 

Tansl I'm not sure I'm understanding you correctly either.

Are you talking about the flairing out of the sides of the craft? It gently flares out further in the front than the rear, which would make a compound bend. It's hard to tell from the 1st picture, but the side panel is actually wider (taller?) to accommodate the extra flair and make it a smooth curve like you see in the 3-d sketch.

 

What I mean is that, in my opinion, all lines "1" should be parallel and the line "2" should be a straight line instead of an arc segment line. But if you have checked the body lines plan and are happy with it, I have nothing to say.
My opinion is that "having twist is not a problem" but why complicate your life, in your boat, with twist, if it is useless?

 

Thank you for clarifying. I'm going to remove the curve @ 2.

 

Allow me a new review and apologize that not have made all at once. If the stern is as seen in "1", the side panel may not be as "2"

 

Good catch!

I did that to make it easier to draw in traditional AutoCAD - I shortened the transom and let it be flat! I meant to mention that but forgot

 

Welcome to the forum.

This is incorrect TANSL, the flare at each station can move around a lot, while still being a developed surface. In fact, it can make for a much better boat if applied appropriately, without undue difficulty hanging planking. Additionally, most warped bottom designs are more efficient if you can work some convexity into the forward bottom sections. The planking will do this naturally as you bend the planking on the jig or tie the seams together, so work with this too, as the boat will be stronger and softer riding. The transom shouldn't be plumb. You'll find most outboard brackets have the most adjustment, if the transom is raked aft about 14 - 15 degrees. I'd also be inclined to use much more entry V, since you're using some anyway. The boat will be more comfortable in a chop and will pound much less at speed too. 25 MPH should be the end of her speed range, as by the time you near 30 MPH, she'll become pretty uncomfortable to drive. She'll do her best in the mid to high teens, with occasional blasts into the low 20's for the fun of it.

If you use a 10 HP outboard, 1/4" planking on the bottom is right on the edge of being too flimsy. If a 25 MPH target is desired, you should strongly consider 3/8", if not 1/2" instead. If your plans are to stay in the mid to upper teens, 1/4" is okay. Making designs like this mind themselves underway, is mostly about the transition from the V sections forward to the flat or modest V sections aft. Where to place this transition zone, how it blends into the after sections, etc. is more about experence than anything else. Your design as drawn has little advantage of a straight, flat bottom craft. With more sweep to the chine forward, refined forward sections and a good guess at the blend area, you'll be very close to what most of us end up developing, after several attempts to nail it down. This is a fairly narrow design comparatively, so she'll not have the rock solid initial stability many fishermen prefer, but it will not be intolerable either. This narrowness will help her efficiency greatly, requiring less HP to get her up and scooting.

 

I built something similar without a keel. It would run ok with 5hp, barely plane under right condition with one person. But would run a little sideways with one person sitting in back on plane. It was ok, you got used to driving it a few degrees off into the wind. Hull flex was apparent, but not a real problem. Mine was shorter about 12 feet. And the weight was a bit more than 150 lbs by the time I added fiberglass, epoxy and paint. I tried to be cheap on fiberglass and epoxy, later the boat became waterlogged. We ended up using it for years as a little work boat to do seawall repairs and haul concrete bags. Best $400 I ever spent... but it wasn't a great navigator.

 

PAR, I agree with most of what you say, based on my boating experience.

However, and I admit I've done no calculations, this boat's bottom at it's widest point is as wide as a Sneakeasy, and just as flat. (I was on one once and the initial stability was impressive.) Obviously it's not going to be as steady as the longer and heavier Sneakeasy, but my guess is that it will be pretty good. Maybe not QUITE as stable as a jon boat.

I suspect that making the 150 lb goal will be tough. You might have to do a bunch of framing on the bottom to make it stronger and stiffer, at least if you want to go 25mph. Especially if you're going to be hitting driftwood now and then. Or are you using the 3/8 for the bottom?

I also suspect that making 25 mph will be tough without more horsepower, though I could be wrong.

It seems to me that there are quite a few designs that are comparable, and from which you can possibly learn things. I can't claim special insight. The only homemade power boat I've been in underway has been a Sneakeasy. Oops, scratch that, I also had a ride on the Infinite Modular Sharpie, though I think it's slightly larger than you want. ;-)
http://www.duckworksmagazine.com/03/r/projects/tims/
Tims being assembled:
http://www.duckworksmagazine.com/03/r/gatherings/kingston/DSC00211.jpg\
at full length, or close:
http://www.duckworksmagazine.com/03/r/gatherings/kingston/DSC00206.jpg
Bolger designs:
-Clam Skiff heavy due to really thick bottom
-Work Skiff similar
More Bolger designs at http://hallman.org/bolger/isometrics.html or in books by Phil Bolger which include discussions of why he designed them the way he did. And what went wrong on the ones he screwed up. Maybe your library or library network would have them, also they are still found on eBay.

I think a light motorboat based on Bolger's Folding Schooner (in a book of the same name) would be amusing and impressive, though I guess a 32 foot boat might be awkward to maneuver. Each half wouldn't weigh much more than your target weight. I forget which book Naval Jelly was in, but it's not that much heavier than your goal. It's about the same length as the Folding Schooner. Shortened and given a transom, it would be very unusual but probably reasonably fast on the moderate power. I think. But this is drifting pretty far from your stated goals. And it doesn't fold!
Jim Michalak designs:
-AF4 would need to lose the superstructure and go on a diet, maybe be slightly smaller. There's a shorter version called AF4 Breve
-Johnsboat Very similar to the aluminum jon boats
-Brucesboat Nice looking to my eye, supposed to go 15mph on 10hp, slightly larger than your stated size. Not as stable as a jonboat of the same beam
http://www.jimsboats.com/webarchives/2012/1jul12.htm#Brucesboat

If you can find Thomas Firth Jones' books in the library, you might find his discussion of the Puxe and Puxe II interesting.

Again, most of these I only know from books, and I'm not a powerboat expert.

 

PAR, thanks for your invaluable explanation but I'm not talking about the possibility of torture plywood but about pictures the OP has shown us.

 

There's no torture in the planking above TANSL, as drawn. In fact very little edge stress, though some cupping should be expected in the forward sections, but not a lot. I've bent much, much worse with few difficulties.

Rnlock, do not confuse displacement boat scantlings with full plane mode scantlings. Once a powerboat reaches a certain speed range, the scantlings increase dramatically, because of the seemingly exponentially increasing loading.

At this point, you need to do a fairly accurate weight study, so you can consider what the HP potential might offer. For example, given a 500 pound package, a modest modifier and 10% slip, you should be able to see the low 20 MPH range with a 10 HP outboard. This is fast enough for most fishermen, but also a light load. A 750 pounds, which is more reasonable for a boat of this size, now the speed drops to about 17 MPH and in some condisions, you'll fight to keep her up on plane.

Do the weight study, then consider what you want, because 1/4" bottom planking on a full plane skiff, just isn't wise, unless speeds will be kept below say 15 - 17 MPH. Even at this speed, you're going to experience some flex, buckling and twist with 1/4".

I have a 17' skiff design much like this and I employ two bottom thicknesses, regular, which is 1" and heavy duty which is 1.5" thick. Admittedly, my skiff is intended to take a beating and come through with few issues, but this heavy bottom thing is employed by many (including Bolger) for a number of really good reasons, which don't become apparent until you've powered one of these into a heavy chop.