Displacement speed HP requirements for 20' flat bottom skiff

I have put together a design for an extremely simple, 20' x 6' flat bottom skiff with one purpose in mind-getting my whole family out on the lake safely and comfortably. Here is the basic design:



Construction will be screwed and glued plywood, glassed on the bottom and chines. Flat bottom, plumb sides, high sheer height, and a load carrying ability of about 524lbs per inch by my calculation. The weight of the completed hull should be about 800lbs, and the weight of boat, motor, crew (5-two adults and three children), and gear should be 1800lbs.

On the lake we don't need to do more than 7 knots, unless maybe a storm blows in and we want to get off the water fast. I plugged the appropriate numbers into Wyman's Speed Formula and found that I'd need about 7 HP to do 7 knots. Assuming that is correct, I figure I can run a 15 HP four stroke at half throttle to get around normally. If a storm were to blow in or we just wanted to go a little faster for a short time, the same formula tells me that full throttle (15 HP) should give me 9 knots. Definitely not planing, but fast enough.

Looks great on paper, but can someone give me a real world check on my math and design? Would I be well advised to go with a high thrust (deeper reduction) 15 HP vs. a regular 15 HP?

 

That's a full plane mode hull form, so displacement speeds will be very inefficient, dragging the transom and all. At 1,800 pounds a 15 HP will push that boat in the high teens at WOT. Lugging along at 1/2 throttle will not be good for the engine, unless you elect to go high thrust and even at this, it's not going to like chugging along at 1/2 throttle all the time.

You should design the boat for what you need and not try to make it do more than it can. Simply put, if you're primary need is displacement speed, then a displacement hull is the logical choice, knowing it'll never get up on plane.

 

That's the dilemma, I suppose. Displacement hull designs (motor driven anyway) seem to be hard to come by. I considered Michalak's "Dani Jay" for a while, but I think it would be a little cramped for five people even though it would handle the weight requirement with no problem:

http://duckworksbbs.com/plans/jim/dani_jay/index.htm

Another 2' of cockpit room would be desirable, but I doubt a boat like this is easily lengthened. Perhaps I need to investigate it more.

 

That's a more difficult build than a slab sided flat bottom. You design could be fixed, it needs some rocker aft and would handle and steer better if the stern quarters where pinched in a bit. Of course where and how much to place the rocker, how much to tuck in the aft sections, etc. require some understanding.

 

You also don't need a 20' boat to do what you are asking. My old skiff at 15'9",with a small pram bow, shallow vee bottom, made of 7mm Okoume, would easily carry 5 adults totaling 1000 pounds. Hull bottom and sides were gotten out of two sheets of 16' ply book matched and cut together. That is a pretty easy build done ply on frame. I mostly ran a 5hp on it, but later converted to a Yamaha 25. It was designed as a sailboat and couldn't quite use all the 25 efficiently, but it would carry a hell of a load of traps at 12 knots. You are showing a planing bottom and a topsides that are not really suitable or safe for a planing boat. You would be doing yourself a huge favor if you made your boat look like other boats designed for your purpose. You would benefit from some vee, and you need some flare to the topsides.


Picture shows 4+photographer heading out for a day on the Florida Bay with the 25 on center and a 3 for backup and trolling. Boat is a 16' version of the old Mirror Dinghy type. I built this boat as a teenager 30+ years ago. (I'd like to think I could do a bit better now). It's still around, but I don't have it anymore. It weighs about 375 pounds.

 

Agreed, I've been nice about the realities of the design posted. It has a lot of issues associated with the shape choices made. The first thing Yota should do is decide what this boat is going to be, then develop a set of shaped well suited for this goal(s). If it's a displacement putt putt, then he should be looking maybe toward a driftboat type of deal, possibly a dory or a rowing skiff. John Gardner has several of these types in his "Building Small Craft" book. Most can be stretched to some degree if necessary. Glen-L and Bateau are also places to look, if only for inspiration for the 2.0 version of this design.

 

PAR is right.
Your design is not adapt to the performance you want.
Here an example, my design, what is exatly you nedd.
Is not 20 ft, and is not designed for to be built in plywood, but this is the geometry.
Same disign, but in plywood and 20 ft long will have 7 knots with an engine 7 hp, and 12-15 knots with two engins 7 hp.

 

obviously the 20 ft in plywood will have several reports not only in length, but also in draft

 

WindRaf, the boat is nice but seems likely to acquire a huge boat trim as captain at the stern and the boat starts to navigate. Have you tested with an already built model ?. It would be interesting to know the results.
On the other hand, it seems that the most logical way, perhaps the only, to build it is using GRP and a mold, which is not cheap, am I wrong?.

 

That's an interesting point you touch on. About two years ago when I first began investigating the possibility of building my own boat and began trying to learn all I could about boat building, I started by consulting forums on fishing, pleasure boating, etc... I had my eye on some plans for a 16' garvey type boat from Bateau:

http://bateau.com/studyplans/GF16_study.htm?prod=GF16

I was (rather rudely) told that I would kill myself and my entire family if I attempted to take them out on the water in it by some folks over at a pleasure boating forum which shall remain unnamed. That was what began to steer me toward looking at larger and larger boats.

Well I appreciate it. I'm not a pro, just a guy who wants to build a decent boat, and I'm learning as I go and I'm looking for constructive criticism from folks who really know what they're doing. What I'm finding is that I sought information from the wrong places in the beginning (I didn't find the Boat Design forums until fairly recently) and am having to rethink some assumptions I had made.

 

TANSL,
- A trained eye immediately understands that the boat has a center of buoyancy very backward
- That the stern is inclined to give negative trim engines
- That to build it in plywood just use the Last 4 inch wooden full and then round it

 

A trained eye, in what?
I gather from what you say that you have not yet navigated the boat. Perhaps you bring any surprises.
Tell me, please, your experiences with this boat in the water.

 

Both these creations, the OP's and windraf, look like they would get booked for impersonating a boat, you must be better off with an established design tailored to your requirements.

 

TRANSL,
why you go looking for a bad answer?
I have already posted several times drawings, photos and even video on boats made of that geometry

 

On page 135 of the attached pdf is a suggestion to use a well for an outboard on a 32 ft dory. The boat has long overhangs and really is more comparable to much shorter versions of other boats. Long overhangs are nice for keeping your feet dry at the shore too...

On page 152 is the 26ft St Peirre fishing dory showing a well for a small outboard motor. I would put a bulkhead all the way across where the motor mounts. You don't want to spend much time behind that stinky motor anyway.

The canoe-like shapes of these boats would make them easy to drive and they are designed for use in rough water.

http://www.pssurvival.com/PS/Boats/The_Dory-Boat_Book_1978.pdf