Weight vs. ssquare inche of hull???

[otterbfishin]

I asked a similiar question in the "Designs forum". How much weight will one square inch of hull displace?? I'm trying to find out how wide I have to make the transom to support the 275 lbs of the 65hp outboard jet motor on my 16 foot river skiff. I'm running 5 degrees of deadrise for half of the hull (12 degrees forward). I want to be able to float the craft dead in the water at 6 inches of draft. Can you help?
Thanks,
otter

[RonW]

It takes approximately 62lbs of weight to displace one cubic foot of fresh water,and 64 lbs. to displace one cubic foot of saltwater.
There is 144 square inches in a square foot, and 1728 cubic inches in a cubic foot.
You have to multiply the width of your boat by the length to get square feet.Then divide by 12 to get cubic foot displacement at 1-inch emersion ,then multiply the cubic ft. by 62lbs. to get weight required to sink boat one inch.
But the bottom is constantly changing shape and volume, which generally means the weight required to sink it another inch is constantly increasing, because the volume of the boat is constantly increasing the further down it sinks due to hull shape.Lots of math.

The question you are asking is impossible to answer, without the set of plans to calculate from.All boat shapes are different, so the volume rates are different. But all boat plans from architects have a hull draft listed at a specified dissplacement weight. Unless you are doing your own designing, if so, then you have to figure the volume of the hull for each inch of emersion. Good luck, hope this helps....

[Ian McColgin]

One supposes, now that area v. volume has been clarified, that a 16' boat could be designed that would float level with a 275# object hung on the stern - sort of a Fat Albert of boats, broad at the butt and narrow at the bows - but then it would float and ride poorly when loaded.

65 hp sounds a lot for a little homemade boat anyway. Wouldn't you be happier with a Boston Whaler?

[Paul Pless]

http://www.imagestation.com/picture/sraid116/p5f521616d9d4f06586075cf3eeb3c68b/f8b4117d.jpg

Here is a picture of me in my 16' aluminum 'jon boat' driven by a 50hp jet rated Yamaha. This is at the top end of a class III rapid on the Coosa River and there are rocks about 6 inches under the white water at the transom of the boat.

It'll plane in about 3.5" and off plane sits level in about 6.5" of water. When trimming the boat, we that run jets often compromise the trim when on plane for better low speed shallow water performance. All up, with 12 gallons of fuel, full shad tank, 2 - 12 volt dry cells for the trolling motor, and 1 - 12 volt wet cell starting battery, this boat weighs approx. 770 pounds. I have moved the batteries, fuel tank, and baitwell as far forward under the casting deck as possible. This allows the boat to sit very level in the water when off plane and ultimately allows you to navigate in shallower water than if the boat were trimmed as a normal boat.

Trimming the boat in this manner, in conjuction with a footless outdrive, and keelless boat also allows the boat to be driven, when on plane, much like a big jetski, the outside bow chine will dig in just a little, and you can rotate the entire stern around the bow while on plane. My boat will even plane backwards under no power for consideralble distances by doing this.

Incidentally, jetdrives don't particularly care for transom widths greater than 54". Greater widths increase cavitation at the jet intake. Also jetdrives do not like deadrise at the the last 6 feet of the boat for similar reasons.

While I disagree with Ian's statement about such a high horsepower in conjuction with a homebuilt boat, I do question the use of wood in a jet drive application, for the simple reason that you will at some point inadvertantly ground your boat. Aluminum takes such impacts much better and is easier to repair if need be than wood or fiberglass.

[otterbfishin]

RonW,
Thank you very much for your fine explanation of calculating displacement. This is exactly what I was looking for. With a little "fudging" for the increased surface area as the sides flare outward, I think I'll be able to come close enough to estimate I'm searching for. Yea, lots of math, but I have the time.
Ian,
Yes a 65hp is alot of power for a "little wooden boat" but it is what is required to run the wide shallow glacial rivers here in Alaska. I indeed would love to be able to build with aluminum for this craft, but alas I don't have the bucks fro such a venture. A Boston Whaler on these rivers would be shredded in no time; draw too much water. I built and ran a 20'X4'jon boat here until at about 10 years the 1/2" bottom started to rot. Economy style wooden boat building with standard AC plywood and fiberglass.
Paul,
Awsome pic of your flying jon boat!! There is no class III water here, just a wide steady current of about 8 mph, with lots of sweepers, sleepers, and sudden gravel bars to look out for. I will trim in a similiar fashion so that I can drift with oars on some downstream runs (local technique for hunting moose).
Thanks to all of you,
otter

[Tom Lathrop]

Otter,

It's true that you don't give enough information to even make a good guess at the answer.

Design of a boat is a sort of spiral where you generally assume some parameter(s) and derive the rest and then do that several times until you get the boat to meet your goals.

If you assume that the beam over the aft part of the boat to be 60", then the answer is about 0.16lbs per sq in of added waterplane for a draft of 6" and an aft deadrise of 5 degrees. If the beam is changed very much, then the answer also changes. This is just geomometry with a bit of trig thrown in.

This doesn't do you much good since we don't know the original displaced volume and its distribution along the length of the boat. In other words, it is not a real simple thing for an amateur to do without some study of the basics. You can take what I gave you and guess at the added width and maybe come out all right.

[ 12-28-2004, 11:22 AM: Message edited by: Tom Lathrop ]

[RonW]

Otter- If I remember correctly, you have the plans for devlin's cracker or honker, anyhow a duck hunting skiff that you wanted to take the rocker out of the transom and try to refine it into a whitewater river sled with a jet.Also this is a stitch and tape design, I think you are screwing around with the wrong plans.look at this.

http://www.boatdesigns.com/cgi-bin/store/web_store.cgi?page=roguerunner.html&&cart_id=1043511_8126

This is a heavily built white water sled designed for a jet outboard.It will take a whole lot more abuse then a stitch and tape boat, and you can build it from 16 to 20 ft. Perfect for your useage.Plus you don't have to try and modify it, hoping that the modifications work properly.

You can build it out of 1/2inch douglas fir marine ply, maybe even 5/8 or a 3/4 inch bottom. Heavy sawed frames, construction adhesive,stainless steel screws and a oil based porch and deck paint.Forget the fiberglass, just put heavy wood runners on the bottom and side chime for abuse. You can still make a good looking boat that is so rough and tuff, it will take a beating, and douglas fir marine ply will last 30 years and longer with out water sitting in it. Maybe not the best ply for varnishing, but the best for durability and abuse.You can probably build it for less then $1,000.. And use and abuse it for the next 30 years or more.
And that is my opinion, build it heavy work boat fashion.But start with the proper plans for your useage.

[otterbfishin]

RonW,
Thank you very much for your opinion. I've reviewed this design, and agree that it has merit. The main problem is that it is a heavy boat. I have a 65hp jet outboatd motor already, and would like to build a light weight sled that will plane easily with it. This probably means stitch and glue with marine fir plywood and epoxy. The design considerations are that it must be able to float 900lbs with as little draft as possible so as to be able to drift without bottoming out in shallow glacial streams here. Four inch draft would be ideal, but six inches is probably more realistic. I'm not trying to re-invent the wheel, just come up with a craft that would suit my needs for duck hunting and fishing near my home.
thanks,
otter

[Ian McColgin]

Now that I understand the use a bit better, I must say that Otter is likely to make a good and interesting garvey.

I'd think 65 hp is abundant and the boat weight won't matter that much when you load her up anyway, so I'd myself lean towards something a bit more resiliant than stitch&glue, but it's now plain to me that Otter is thinking clearly about his exact needs.

Bodes well. Sorry about the crusty crack about getting a whaler.

G'luck