Design Thoughts

Folks,

I built a boat a while back, and am contemplating building another. Having been unable to find a set of plans that meets my "needs" I'm toying with a design of my own. Would love a little input from those more experienced than I.

At this point I'm mostly interested in thoughts on the hull shape. I haven't done enough work on the innards to figure out weight and distribution to know what I expect the waterline to be which makes it tricky, but I gotta start somewhere.

It's a 25' constant deadrise 20deg hull with a chine flat that carries all the way forward for initial stability and spray deflection. The lines are designed for plywood sheet construction. I'd like to power her with an inboard. Preferably diesel but given costs it may end up being gasoline. I'm guessing something in the 200hp range would be good for around 20-25kts.

The intent for this boat is fishing and diving off the California coast where things can get pretty hairy. I want a hull that can make it home in comfort when the weather comes up, and that has a place a couple of people can overnight when desired.

Lines and early renderings attached. Would LOVE to get your thoughts.

Thanks!
-Ben

 

It might be interesting to place a keel/skeg that, besides protecting the propeller and rudder, increase the drifting area of the hull.

 

I've thought about that. Would that add considerably to the drag (ie: make the whole thing less efficient)? I was kind of thinking that with the constant 20deg deadrise I'd be okay from a tracking perspective without the keel, but protecting the prop is always a good thing.

 

You should protect the propeller and rudder.
This hull, like planning boats, at low speed will be difficult to govern.
The increased draft will not be very important but the improvement in stability of course will be very large.

 

For a dive platform, that hull will be a rolly thing, though comfortable in a slosh at speed. You should consider a different approuch than the constant deadrise hull form. I'd make the entry finer, so you can get through the harsh stuff, but decrease her deadrise, so she'd enjoy more initial stability, which the divers would appreciate. This is a high speed hull aft and a wave basher forward. Unless you intend to target speeds over 30 knots, a different set of shapes would be more desirable, given your needs.

 

Both diving and fishing require a boat with high initial stability for comfort and safety. For diving, you also want a boat that doesn't pitch too much.

 

If I were the OP would ask the following questions: What is and how much is "high initial stability?". How pitch is prevented or reduced?.

 

Pretty sure I understand the "high initial stability" concept. The chine flats were intended to create a degree of that. I guess what I'm hearing is that it's not likely to make enough difference?

PAR, I'm targeting speeds in the 20-25kt range for this boat, which I'm hoping I can achieve with about 200HP. I'm more concerned with getting to and fro comfortably than with how much she rolls on an anchor since the diving application is experienced watermen going hunting, not tourist diving for novices. That said, if I can make it more stable at rest and still have a the ride characteristics I'm after that'd be excellent. I based the hull lines at least a little bit on the Farallon boats that are common here on the West Coast among commercial fishermen. They've got a constant 20 degree hull and chine flats for initial stability, though I do believe they come to a somewhat finer entry. I flattened it out slightly to optimize space in the cabin figuring that most of the time when you're running the bow is out of the water. I gather that'ss probably the wrong approach, but hey... I'm just getting started with this.

Am I correct in inferring that it'd be wise to flatten the deadrise at the transom and then increase it moving forward? If so, would it be wise to target about 20deg under the CG of the hull? What would be a good starting value at the transom?

 

So here's a somewhat revised version of the lines. Less deadrise aft transitioning to a sharper entry. Doesn't show a skeg, but I'd plan to include one based on feedback here, though I'm also starting to wonder if it makes more sense to hang an outboard on a bracket off the back. I like inboard boats much better, but I'm realizing that at 25 I won't be able to bury the engine completely in the bilge and a motor cover in the middle of the cockpit is just as much in the way as a motor on the stern.

This seem like it'd handle big water, roll less, and run around 25kts?

I didn't dimension the drawing this time but that hasn't changed much with these revisions...

 

If you run 20 degrees mono, you will have lots of stability. Certainly a finer entry in the front COUPLED with large trim tabs will enable you to push the bow down a bit running in rougher water.

An outboard would be the way that I would go. Half the weight of a diesel, less hassle with rudders, shafts, cutlass bearings, packing, forward location of the engine, an engine box in the middle of the cockpit, sound suppression, and a loss of valuable storage space under the cockpit.

You will not have trouble at low speed or backing down because when you turn the wheel you are directing the thrust with the direction of the prop. Completely different than backing down a ruddered boat.

There are lots of plans out there that you could get for the hull shape that you want complete with framing details etc.

Technically, you are not designing your hull. As design requires calculations and engineering or naval architecture background. Basically you have sketched what you think might be a good idea and hope it works.

You are going to spend quite a bit of money to build your boat and a few hundred dollars to get a proven hull design from a design company complete with DETAILS of construction is the most efficient money that you will spend.

 

Thanks for the reply, Barry.

First off, I'm 100% clear that the hull shape alone is not a boat design. I've got the experience with boat building and framing to understand how the parts should go together, and the engineering and math background to figure out the calcs to build something structurally sound once the hull shape is determined. The hydrodynamic implications of a particular shape is the place I'm weak, which is why I'm here. Once I get a shape settled I can then start the hard work of engineering the framing and other structural elements. In short, while it's a giant PITA, I'm not scared by what's involved in designing everything behind the skin. What I'm scared of is spending hundreds of hours and tens of thousands of dollars on a hull that doesn't perform like I want it to.

As for the calcs associated with the shape of the hull, my sense is that the sea-keeping success of most new hull shapes is about 10% math, 70% experience (personal and collective) and 20% luck. Don't mean to step on anyone's toes with that assessment, but having spoken with a number of designers over the years it seems that the state of the art with respect to predicting sea keeping from a drawing just hasn't reached the point of being something simulation and math can handle, so most designs are iterations on things known to have worked. In many respects that's what my intent was with the original drawings I posted since that basic hull form is very successful in this area (possibly with a finer entry than I drew). Then I came here to draw on the collective experience of others.

In any event, I was happy to spend money on plans before I built my last boat (which was a very expensive, and very successful enterprise) so I'm not averse to that. Believe me, while I enjoy designing things, I'm acutely aware that even a $1000 set of plans is a bargain relative to the amount of time I'd spend mucking around in Solidworks getting the details sorted out and the cutfiles ready to take to my CNC.

My issue is that I've struggled to find plans I like. If I decide I'm willing to go the outboard route there are more options for proven designs, but inboard plans for the kind of boat I'd like to own are proving harder to find. I have a strong preference for inboards in general (based on lots of experience with both inboard and outboard boats) which is what has pushed me to contemplating a design of my own, but I'm starting to doubt the wisdom in this size range given the engine would be protruding above the deck. If I went with a hopped-up 6-cyl gasoline engine and a 25degree deadrise I might be able to get to around 200hp and still be able to shoehorn the thing below a reasonable deck height so there's no elevated cover, but the service life of that motor would be pretty short. A V8 or diesel that's big enough to deliver the power without straining is just not going to fit below the decks of a 25' boat of this type.

At 30' I'd absolutely want an inboard boat, but I'm wavering in the 25' range.

 

Good dive boats usually have little or no deadrise aft. For trolling fishing, it also helps to keep the rolling down.

 

I would have never imagined. Why is that so?.

 

The topsides seem excessively high to me. I agree that you are better to buy an existing design and avoid a lot of dramas and possibly a boat that falls short of expectations. In any event, most dive boats in this part of the world (Australia) are power cats, and are well proven in the role, not to mention the ride out and back is very hard to beat.

 

If you went with an inboard/outboard you would be able to position the boat low enough to have a smooth rear deck. There are lots of boats in service from many manufacturers with this configuration with a smooth aft deck.
The I/O will keep the swim/dive platform clear and eliminate the steering, rudder, slow speed control, packing, cutlass, etc etc problems with a straight shaft.

There has been a strong resurgence in outboards in boats up to and over 30 feet. Usually twins for redundancy. Kingfisher, Lifetimer, and others. These are aluminum though. ( but I did notice the
welder in your shop)