Surface-drive Offshore Center-Console Fishing Boat?

[slowrunning]

Without getting too deep in the weeds, I'd love to hear the crowd's thoughts on why this would or wouldn't be a good idea. And, if you WERE going to build something similar, how you'd do it.

Rough specs:
Aluminum build, center console, tournament style, fishing boat.
35' LOA, closed transom, 2-3' dive platform covering the props.
11' Beam
24.5 deadrise
A pair of 480hp Cummins QSB's or Mercury 6.7's. Side-by-side, mounted low under a seating console.
Arneson ASD 8's
Bow thruster

Intent would be long offshore runs with significant time at cruise, think day trips from NE Florida to the gulf stream (70 miles). Lots of trolling/drifting. 5-700 hrs per year. Capability for a 50mph cruise would be the bare minimum.

 

[Mr Efficiency]

I love the 24.5 degree stipulation, that is being very exact ! I think what you must nail down, with that type of drive, is not to find yourself wanting the bow running a little higher, because it won't be readily achievable.

 

[bajansailor]

Welcome to the Forum Slowrunning - the name sounds at odds with your desire for speed most of the time!

Surface drives are complicated beasts relatively, especially so when compared against a 'conventional' propeller set-up.
It sounds like you are considering either Cummins inboard diesels, or Mercury (Mercruiser?) inboard petrol engines?

Have you considered using water-jets instead?

 

[slowrunning]

Thanks for the replies. Understood about the bow rise topic, seems the surface drive boats run pretty flat. Any way to design the hull to naturally run a bit higher in the bow, maybe rely on trim tabs to bring it down when wanted? Or are the drives going to force the bow down no matter what?

Bajan - Mercury offers a 6.7 diesel that from what I've gathered is essentially a repackaged 6.7QSB Cummins. Not real sure on the differences but it seems a bit cheaper than the cummins.

I've been on several RHIB's with Hamilton waterjets. They're an option but I'm really stuck on the simplicity of surface drives, I also don't know that the waterjets would get the speed.

 

[Mr Efficiency]

Thanks for the replies. Understood about the bow rise topic, seems the surface drive boats run pretty flat. Any way to design the hull to naturally run a bit higher in the bow, maybe rely on trim tabs to bring it down when wanted? Or are the drives going to force the bow down no matter what?

Having a boat that needs trimming down, rather than up, appears to be the answer, using tabs or interceptors, and buying an existing make that is proven with the surfacing drives, seems highly advisable. Otherwise you become involved in what could turn into a very expensive experiment that doesn't meet the performance requirements. You could say similar about jet drives, they work best on hulls that are tailored to suit them. How far would you expect to travel, on a day trip, up and running ? Presumably you would spend a lot longer just trolling and fishing, unless the grounds are very distant, you are going to be paying a huge premium for say an hour less of travel time, and you could save a bomb in fuel costs and other outlays just by leaving the dock a little earlier, and getting back in a little later. I think the idea only makes sense if (A) the grounds are very distant, and (B) the fishing there is top notch.

 

[slowrunning]

Having a boat that needs trimming down, rather than up, appears to be the answer, using tabs or interceptors, and buying an existing make that is proven with the surfacing drives, seems highly advisable. Otherwise you become involved in what could turn into a very expensive experiment that doesn't meet the performance requirements. You could say similar about jet drives, they work best on hulls that are tailored to suit them. How far would you expect to travel, on a day trip, up and running ? Presumably you would spend a lot longer just trolling and fishing, unless the grounds are very distant, you are going to be paying a huge premium for say an hour less of travel time, and you could save a bomb in fuel costs and other outlays just by leaving the dock a little earlier, and getting back in a little later. I think the idea only makes sense if (A) the grounds are very distant, and (B) the fishing there is top notch.

I think the draw is that there are almost no center-console fishing boats in this size range with inboard/surface drives. I've always assumed there's good reason the big builders aren't doing it but can't pinpoint why. Cost, space to fit inboards, easier to design a hull - VS - a hull/integrated propulsion system? 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, fully welded T-top, rod holders, you name it. Very sleek, overbuilt boat with nothing to come loose, rattle, rust, or break. Looonng-term project but quite a bit of effort to put into a boat unless there's going to be something special about it. I think the uniqueness of surface drives on this style boat could be that. If outboards, I think it better to just buy 1 of the hundreds of proven hulls out there.

As far as cost, with all things considered, fuel is a fairly small factor. I think the cost savings would be realized 10 years and 6k hours later when you've saved the cost of repowering 2-3 sets of twin/triple outboards but had the same or better performance throughout.

But if things don't add up or the boat wouldn't achieve the same or better ride/handling than a comparable outboard boat then I'll stop wasting brain power on it. If it's feasible, I really want to start working on a running surface, or whatever comes first when starting something like this. I think the fabrication/construction part is covered, the hydrodynamic/engineering side is where I need help. I've studied a lot of different hulls with surface drives and they seem to work on a bunch of different boats, so I'm hopeful.

 

[Mr Efficiency]

Low speed handling, in confined spaces, may be an issue with this type of drive. Also, in alloy, it can be difficult to imitate the bottom shapes of the best glass boats. A cat probably is less restricted in that shaping respect.

 

[slowrunning]

Low speed handling, in confined spaces, may be an issue with this type of drive. Also, in alloy, it can be difficult to imitate the bottom shapes of the best glass boats. A cat probably is less restricted in that shaping respect.

My partner thinks a cat is the way to go. I think fitting the diesels below the deck of a catamaran will be nearly impossible without building a really large boat. A flat deck from bow to stern is necessary and the diesel engines are fairly tall.

 

[Mr Efficiency]

I take it from your opening post, that the engines located under a seating consoles, involves jack-shafts ? Or else the engines are at the stern. If further forward, then that alone would create the likelihood of a flatter running attitude, something that could become a problem with surface drives, and their characteristics as regards boat trim.

 

[slowrunning]

I take it from your opening post, that the engines located under a seating consoles, involves jack-shafts ? Or else the engines are at the stern. If further forward, then that alone would create the likelihood of a flatter running attitude, something that could become a problem with surface drives, and their characteristics as regards boat trim.

Correct, side by side under the seating console (hence the need for a wide boat). Jackshafts to arnesons was the thought. Engine centers would be roughly 8 feet forward of the transom. Saddle fuel tanks built all the way aft, 150'ish gallons each, and about 60'ish gallons of livewells on the transom. Lots of fuel/water weight torward the stern. Could probably put batteries in the transom.

Unfortunately, there's not a lot of engine placement options on this style boat. Center console or seating console seem to be it.

I'm hoping that the running surface can be designed to provide bow lift. This is where I'm clueless.

 

[Mr Efficiency]

Ballasting is always a possibility, but weighing down planing boats with ballast to get them trimming as you want, is a blunt instrument, that you will pay for in fuel use.

 

[fallguy]

I am clueless, but curious. If the sd pushes the bow down by nature; how do you get the bow up? Obviously, not by running a trim tab down!

Tried to read the Arneson webpage, it wasn't too mobile device friendly and am too tired.

 

[Mr Efficiency]

Realistically, surface drives were designed for high speed, putting the prop away from water dragged along by the hull, and gaining a few knots out of that, plus less propellor immersion getting some extra revs, I don't know that it is really a general purpose drive. It certainly has limitations for trimming the boat, by use of the drive, but I guess you could say similar about conventional shaft drives, where there there is none.

 

[fallguy]

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.

 

[baeckmo]

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.