Opinions on bass boat design

[Shrimpkin]

Just messing around in sketchup designing hulls, wondering what you guys think about it and whether or not it would actually handle well. I am still trying to learning about stability and hydrodynamics.

 

[messabout]

Shrimpkin: The respondents would need far more information about the boat to answer your question adequately. You have not detailed the forward stations, have not given displacement figures, expected operating speeds, power of the propulsion unit, what kind of water it would be operating in, and more.

Designing boats that faithfully satisfy the intended use is not a casual exercise.

 

[Shrimpkin]

Shrimpkin: The respondents would need far more information about the boat to answer your question adequately. You have not detailed the forward stations, have not given displacement figures, expected operating speeds, power of the propulsion unit, what kind of water it would be operating in, and more.

Designing boats that faithfully satisfy the intended use is not a casual exercise.

Point taken, I am still in the process of learning Freeship. I have built the above model and run hydrostatics on it. The above should draft at 0.85ft at approximately 2200lbs. I am still trying to learn about the hydrodynamic aspect of Freeship.

 

[PAR]

FreeShip will run the hydrostatics on this hull form, but it will not tell you if it's well suited to your needs. This is a garvey hull and there are lots of these designs available, for 'glass, plywood and metal construction. You'd be well advised to get a set of plans, so there's no guessing about hydrodynamic suitability for your needs.

This is the Glen-L "Bass Boat " and a very modest dead rise garvey design, in plywood. It can be built at 17', which is a stretch of their 15' 6" version, but a stretch to 18' is certainly possible too. Plans are 135 bucks, so . . .

 

[Shrimpkin]

FreeShip will run the hydrostatics on this hull form, but it will not tell you if it's well suited to your needs. This is a garvey hull and there are lots of these designs available, for 'glass, plywood and metal construction. You'd be well advised to get a set of plans, so there's no guessing about hydrodynamic suitability for your needs.

This is the Glen-L "Bass Boat " and a very modest dead rise garvey design, in plywood. It can be built at 17', which is a stretch of their 15' 6" version, but a stretch to 18' is certainly possible too. Plans are 135 bucks, so . . .

As far as I can tell all of the large manufacturers of aluminum bass boats with engine ranging in the 60-120hp, 17'-19' range use a very simple shallow v hull with strakes along the bottom. The chines travel straight forward and the sides of the boat flare out based on how much beam you put at the transom. I'm curious how much deviation it would take to alter the handling of a boat like that, I wouldn't think it would be hard to replicate the design given a few points of information.

For instance, I know the beam on a alumacraft pro185 is 90" and the length is 18', I also know the transom height is 21". Doing a little math based on pictures I can estimate where the keel begins to rise and I would just use a tangent radius to meet it at the stem point of 18' length and 21" height, make the chines the same radius and interect them at plane across the boat at 21" height and draw lines from the stem to the chines.

When I modeled what I thought was close to their design it gave me basically the same draft and when putting an estimated engine weight at the back of the boat the LCB was in the middle (meaning it would float with the deck plane with the water line or thereabouts). I also know from basic design principals of boats that a small section chopped out of the hull to allow the water to rise and meet the engine will reduce drag because I could raise the engine.

I am very reluctant to buy a set of plans as I honestly can't find what I want. I have found one or two boats that I would build but neither of them are built from aluminum, instead using plywood and fiberglass.

 

[Mr Efficiency]

Unless you are competent at welding alloy, which is tricky, then it may not be such a good idea, and with the added complication of a design error(s) you may make, could be a more costly mistake. The fact that your drawing has a vertical transom suggests you aren't across all the requirements.

 

[Shrimpkin]

Unless you are competent at welding alloy, which is tricky, then it may not be such a good idea, and with the added complication of a design error(s) you may make, could be a more costly mistake. The fact that your drawing has a vertical transom suggests you aren't across all the requirements.

The model above is just something rough thrown together in sketchup because I didn't have access to the working design. My working design has a 12 degree transom and a 7 degree deadrise and a total of 5 strakes running the length of the boat and chine step about 1/3 of the way up the side.

Also welding aluminum is something I can do very well. I run a fabrication shop and will build just about anything. I have a 350P Miller Aluma Pro MIG machine and I constantly deal with welding 12ga (.105") 5052, which as far as I can tell is what they use for boats (maybe they source 6061 plate but I'd be surprised since I have a hard time finding any thin 6061 plate from ANY of my alloy vendors). I get 5'x10' sheets for around $180 ea due to the volume we order and by my calculations I will use around 6-7 sheets. It seems to make financial sense to build out of aluminum given it's longevity for me as well as the resources I have available.

You are right, there are still many things I need to work out like what to build the ribs (thwarts?) out of as far as thickness, how to build the transom (thick plate or boxed frame out of 12ga with ribbing support). Right now I am trying to determine just how much it will affect the boat if everything isn't precision engineered. Just how precise does a shallow V 18' boat with a 4-stroke 60 on it need to be to go fishing in calm waters? I'm not building something that's gonna go 60+ mph, maybe 40 mph max.

 

[Mr Efficiency]

I'd say it does not have to be within precise dimensions, or something horrible will happen, an eighth inch out here or there will make no discernible difference in a relatively sedately moving boat. If you are intending to use unswaged light gauge material, you will need an elaborate framing set-up, or have "dishing" galore, so you don't really save weight, or cost, or time, by using it. In the absence of equipment to press longitudinal ribs into the sheeting, most cottage builds resort to heavier plate construction. I have never seen an alloy welded boat made with plain flat sheet of very light gauge material, it makes little sense to try.

 

[Shrimpkin]

I'd say it does not have to be within precise dimensions, or something horrible will happen, an eighth inch out here or there will make no discernible difference in a relatively sedately moving boat. If you are intending to use unswaged light gauge material, you will need an elaborate framing set-up, or have "dishing" galore, so you don't really save weight, or cost, or time, by using it. In the absence of equipment to press longitudinal ribs into the sheeting, most cottage builds resort to heavier plate construction. I have never seen an alloy welded boat made with plain flat sheet of very light gauge material, it makes little sense to try.

So the strakes longitudinally along a boat like an alumacraft pro 185 are pressed into the sheet, they aren't angle strips formed to the underside of the hull and welded in place? My current boat is a custom aluminum boat my grandfather had made in 1985 and it has pieces of angle welded like I described. I know as I can see inside the hull and it is not press formed. Granted, it is of heavier gauge material than I plan on using. My plan was to have a plywood "mold" to lay the sheeting across, tack the hull together, and then have form cut ribs on the inside to hold the structure rigid with angle welded along the bottom side to keep the plate from what I believe you described as dishing and what I would call malforming. I have a 6'x12' plasma cnc and a 5'x10' waterjet at my disposal as well to cut all of these things to within .005" accuracy. I agree that form pressed strakes would be not only lighter but easier to create given you have the machine.

 

[Mr Efficiency]

So the strakes longitudinally along a boat like an alumacraft pro 185 are pressed into the sheet, they aren't angle strips formed to the underside of the hull and welded in place? My current boat is a custom aluminum boat my grandfather had made in 1985 and it has pieces of angle welded like I described. I know as I can see inside the hull and it is not press formed. Granted, it is of heavier gauge material than I plan on using. My plan was to have a plywood "mold" to lay the sheeting across, tack the hull together, and then have form cut ribs on the inside to hold the structure rigid with angle welded along the bottom side to keep the plate from what I believe you described as dishing and what I would call malforming. I have a 6'x12' plasma cnc and a 5'x10' waterjet at my disposal as well to cut all of these things to within .005" accuracy. I agree that form pressed strakes would be not only lighter but easier to create given you have the machine.

I have no idea how that brand name boat is constructed, but in this country only plate (read 3mm or more) would have stiffeners or strakes welded on. Mass manufacturers would have pressings in 3mm (1/8") even. You are using much lighter material than that. By malforming I take it you mean distortion from welding heat, dishing of relatively unsupported flat panel is more to do with water pressure on the panel from wave impact, even the wakes of other boats. It looks sh*t-house, and if bad enough can effect the performance of the boat.

 

[Shrimpkin]

I have no idea how that brand name boat is constructed, but in this country only plate (read 3mm or more) would have stiffeners or strakes welded on. Mass manufacturers would have pressings in 3mm (1/8") even. You are using much lighter material than that. By malforming I take it you mean distortion from welding heat, dishing of relatively unsupported flat panel is more to do with water pressure on the panel from wave impact, even the wakes of other boats. It looks sh*t-house, and if bad enough can effect the performance of the boat.

No, I am talking about using plate close to 1/8". 12GA is .105" or 2.66mm. I could step up to .125" if it makes a large difference. Also, there would of course be an internal structure spanning the beam as well as aluminum angle longitudinally along the hull. Let me see if I can get my hands on the model from my other pc.

EDIT:

I could only get an earlier version in backup. Still it has stuff to illustrate the idea.

Imagine a piece of angle roughly the same thickness as the hull strapped along the length and welded. There is only one in place at the moment but the other 4 go along the length.

Also there would be ribs like this at key points in the boat, where the cockpit starts and stops for instance, and one further forward to support the casting deck, maybe two further forward for the casting deck.

 

[Mr Efficiency]

To the imperfect extent your drawing illustrates the shape of it, I can't see that your boat wouldn't handle OK, bearing in mind that in flat water, a bath-tub might seem to be doing OK. Having said that, there is very little boat in the water forward, and moving right forward, at rest, would have the bow dipping quite a lot.

 

[Mr Efficiency]

You don't want bulkheads contacting the sheeting, unless you have comprehensive longitudinal stiffenings in conjunction with it, or it will create hard points, and/ or will dish.

 

[Shrimpkin]

You don't want bulkheads contacting the sheeting, unless you have comprehensive longitudinal stiffenings in conjunction with it, or it will create hard points, and/ or will dish.

Hence the aluminum angle welded length wise on the bottom...

 

[Mr Efficiency]

You would need them pretty closely spaced, and they may interfere with the prop in turns, but it would be doing a similar job to internals, albeit with a lot more welding on the outside ones.