Need some riverboat hull design help ??

[Hunter1]

I am thinking about building a long narrow36"-40" X 26'-28' low resistance riverboat that will have a shallow draft with a 40-90 hp Johnson E-Tec jet outboard for power. I would like some idea's on hull design if anybody would like to make some suggestions. I will be using this boat in waters that the majority of people are running jet boats, the average river flow speed is about 6-12 mph. I'm thinking about having a 2'' rise in the bottom of the hull over the last 4' to add a liitle clearance for the outboard jet, maybe an 8 degree v over the first 4' of the bow as well as some rise to split off some air and allow a little clearance for landing. What about turning? I would imagine adding a curve to the bow and stern area of the hull would increase the manouverability of the boat on turns but how much curve. Any thoughts greatly appreciated!

[cyclops]

26'-28' wide? on 90 hp? you will need all of that just to slide downriver at 2 mph. Forget any upriver.

[Hunter1]

26'-28' in length and around 40'' wide.

[cyclops]

That 90, MIGHT, plane with 1 or 2 small people in it. If you balanced the weights perfectly. Outboards that are converted to jets at the bottom are known for poor thrust compared to the normal prop setup. Do you think a 40-45 hp prop job would plane that much aluminum or wood? Forget steel or Glass.

[Gilbert]

Hi Hunter,
If you could upload a sketch or drawing of what you have in mind you might be more likely to get helpful feedback.
Gilbert

[Hunter1]

Cyclops, This style riverboat was used for many many years by guide outfitters to access some very remote area's in fact my relative a retired guide outfitter has a 36' long by say 38" wide riverboat powered by a 115 Yamaha jet. My uncle found the 36' boat to long in some narrow rivers so he is now having a 32' boat built. This design usualy has a very shallow draft, they never plane in fact I have talked to 1 fellow that pushed a 24' with a 10hp prop outboard in a moderate river. It does not take alot of hp to push this style boat up river but naturaly the heavier the load the slower you travel. The advantage of this style riverboat is that it is cheap to build out of plywood, epoxy and fiberglass, it really does not require a lot of power to push up river(I saw a factory built fiberglass boat like this weighing 600lbs go up some pretty skinny fast waters with a 40 Merc jet this year), you always have a shallow draft even loaded. My problem is I'm not sure what hull geometry to use, what is really important or minor. I feel the stern area needs to be rounded to some degree so the boat will turn but I'm not sure how critical this is. Does anybody have any idea what the draft would be on a flat bottom boat 28' X 38" weighing 1500lbs.

[cyclops]

If turning is a real concern, add a turning fin or keel only at the very front of the hull that is always submerged. Any combination of keel area at the bow that equals about 2 sq. ft. should give very positive direction in ANY wind or current combinations. Try to make the keel at least 3" deep and is about 8' long. You could do the 2 keels @ 4' each. Moving at speed they will work. As you stop, any sideways movement will cause one keel to block out the other one . 3" X 8' should hold very well. Make it a sealed piece of wood that is removeable and the bolt holes well sealed. What you are building is known as a Garvy in New Jersey. Great for the shallows.

[Hunter1]

I found something similar looks a bit narrow but close http://www.dngoodchild.com/5333.htm

[cyclops]

At those widths I believe the horsepower and speeds.

[PAR]

Hunter1, I as well as many have worked with low power applications and your idea is a sound one. It's not a new concept, but rather an old, well used one. Reciprocating power was expensive and very heavy per HP a century ago. This let steam and electric ride the powerboat wave for some time, before gas and "heavy oil" engines could be built with enough power to drag their bulk along with them.

The hull designs for these craft required very easily motivated shapes, for the limited propulsion available, resulting in long, narrow shapes. These shapes are much more difficult to design then the shoe boxes we see today. Power available on tap, isn't an issue any more so we can drive just about any shape to reasonable speeds. Displacement speeds will require very little power to get a well shaped narrow boat of this length to hull speed. Going faster then this will require some good thought and engineering.

Clearly you're in need of a design, or at least a lot of study on design is in order on your part. Narrow boats need special attention to some issues that don't generally come up in boat's of considerably wider beam.

What type of speed do you want? Do you have a target displacement? What is this boat intended to do? Accommodations? Stowage? Certification? The list of questions you need to answer is long and detailed.

If you'd like to drop me an email, we could go over some options. I can think of a few designs that may be available, that will suit your needs. You can try Phil Bolger. His Tennessee is 30' long, but near twice as beamy as you desire, though still quite narrow, requiring only a 10 HP outboard to bring up to hull speed.

[JEM]

Interesting concept.

To get the draft numbers, you'd have to provide more info on hull shape.

If you're looking for max draft and easy to build, I'd say a garvey style with maybe a slight V to add stiffness but not take away from turning ability.

[cyclops]

As to the horsepower to speed efficiency logic. I am cutting up the parts for a 1930's-- 21' X 4' 8"-- 4 seat runabout built out of solid woods. It was designed to be powered by a 15 hp or a 25 hp engine which would give a speed of 25 mph. It weighed a lot. But the fact remains. Boats that have 5 to 1-- (L to W)-- ratios or greater, are unreal in their efficiency. You will have a ball taking her out.--------------------------------------------- Hull rigidty becomes a real concern in unexpected rough water or if the hull starts filling with water when it is out of the water. It can and will bend or crack very easy compared to normal boats.