Surface-drive Offshore Center-Console Fishing Boat?

[slowrunning]

I can't help but ask if a stepped hull might work.

A good article on them.

Stepped Hulls - Is A Stepped Hull Boat The Right Choice For You? | BDOutdoors https://www.bdoutdoors.com/stepped-hulls-hull-boat/

Those Freemans are pretty nice.

I have been researching bottom designs to create a higher riding bow. A large, single step, forward of the COG along with a steeper angle of attack forward of the step may be an option. I absolutely want to rely on bottom design vs ballasting or some other means.

 

[slowrunning]

The thrust line for a surface piercing propeller is different from ditto for an i/o drive, and the sp drive trim function is NOT for changing the boat running trim. The hull design process for a sp drive is not different per se from the process for any drive, but you have to consider the thrust line variations as a function of speed and load.

That said, erratic behaviour of sp propelled boats can often be traced back to bottom shape or weight distribution that is not suitable for spp, somewhat analog to what happens with waterjet installations. A hull (or center of mass) that is good for i/o drive is not necessarily good for a straight shaft, sp propeller or waterjet.

There are numbers of Arnesons (and Levi drives, Surfury drives aso) successfully operating in commercial environments; the point is: the hull must be adapted to the drive. Both Arneson and the other suppliers will assist in this process, provided they get involved early, but don't expect them to save your bacon if you install their equipment without due knowledge.

As always a reasonable SOR is the starting point; getting in love with a particular hull and creating a well-behaving vessel by "backwards engineering" is seldom a comfortable route. However, a driveline specification, adapted to the SOR may very well be a starting point for a successful design spiral.

Thanks, if this moves forward it will inevitably end up with an engineer and one with solid marine CAD capability for validation/tweaking and a set of plans to cut from. I'd like to be fairly educated and have a 90% solution prior to that though. Learning is most of the fun at this point.

Understood about the drives not providing much trim. As I see it so far: Best to assume a fixed trim angle on the drives, parallel with the hull bottom? We'd want a hull with an inherently high riding bow and then rely on trim tabs or something similar to control the boats trim?

 

[Mr Efficiency]

I dare say stepped hulls are popular with surface drives, if only because they are a device to get greater speed, much like the drives are, but not to compensate for the trim limitations of the surface drive boat. baeckmo might speak to the typical engine installations as regards distance from the stern, but it seems to me that taking them forward is only going to make it more difficult to trim the boat.

 

[baeckmo]

........... baeckmo might speak to the typical engine installations as regards distance from the stern, but it seems to me that taking them forward is only going to make it more difficult to trim the boat.

The statement you make here is only valid if you restrict yourself to the common hull designs used primarily for i/o drives, ie prismatic monohedrons with constant deadrise and transom beam ~equal to maximum midship beam. But the designer has more variables available to arrange the center of dynamic lift in order to balance varying thrust vectors. As long as you understand the variation of dynamic pressure along the bottom as function of bottom shape, speed and center of mass, there are a number of design tools to use.

One such tool, often overlooked, is the planform. Say that you have a longitudinal center of mass (LCM) that is more forward than optimum for a "classic" prismatic shape. You can then either shave off some lift aft by changing the planform (tapering chines aft for instance), or you can increase lift fwd by the use of dynamic lifting components (strakes, pads, wedges et c.) or even increase fwd beam. Look for instance at the hulls used for many foil vessels, like the Rodriguez (RHS) or many of the Russian foilers. There you can have a thrust line causing a nose-down attitude in some operational conditions, but this is partly balanced by the hull shape aft of midships.

If my memory serves me right (which it seldom does, according to my wife....), the series 65 hulls (25 degree deadrise version of the ser.62) was tested in two variants; one with the "standard" planform, and one with a more slender rear body. The latter was a modification in order to operate with a forward LCM/high thrust line. (The original report on the 65 series was made by Hadler et al, around 1973-1975 I think, it occurred in SNAME. Another on a similar shape was produced by Keunig and Gerritsma, Delft).

As we have seen in a number of threads here, erratic behaviour due to unsuitable weight distribution is not unique for SP propulsion or water jets. But trimming problems will turn up directly if the designer tries to use a "traditional i/o design", particularly if the boat is supposed to be safe and well behaved in offshore conditions (including low-speed loitering). This is the point I want to make here; when considering SP or waterjets, the hull shape should be checked carefully during the design spiral, do not copy an existing hull on its merits with other propulsion systems. A well balanced boat may well have a more centered C of M than you find on many planing boats today. The reason: there are fewer competent designers than there are copycats.

 

[slowrunning]

baeckmo - Thanks for the info, I'm not sure what your background is but I imagine it concerned boat design? I had to do a bit of research to interpret some of what you said but I'm fairly convinced that it shouldn't be a problem to design a hull that performs well with the engines placed a bit forward of the stern. I'm curious to know if you would think a stepped hull should be a primary feature of the bottom surface?

 

[Mr Efficiency]

I assume if you are buying new drives, consulting with the makers about the suitability of any design you come up with, would be wise. They, beyond all other people, should be the best judges.

 

[gonzo]

Another goal of the project would be integrated/welded everything: sea-chest, fuel tanks, piped and welded fuel/water/drainage plumbing instead of hoses/clamps/fittings and 5200

That will require a very careful engineering analysis to prevent fatigue failures. Hoses are the easiest way to relieve stress.

 

[gonzo]

Saddle fuel tanks built all the way aft

Fuel tanks should be installed with their CG as close to the CG of the boat as possible. Otherwise, the trim will change significantly as the fuel gets used.