ibasquarepeg, lots of reading ahead of you. Your post has some large scale misconceptions and those could only be converted to facts when you did more reading. I'd like to recommend some books, and then take a short stab at pointing out some phrases that seem to indicate the areas where study will help you become more informed.
First, is the concept of boat speed, and hull form, there are numerous discussions printed and some online but I'd point you t Dave Gerr's (pronounced like bear with a G) books. The Nature of Boats is my first recommendation
http://www.alibris.com/The-Nature-of-Bo ... ok/4595611 and I point you to Alibris to buy a used copy for pennies instead of the new or even hardback new editions. (I used to give this book to friends for seasonal gifts in decades past).
What is not firm in your understanding is the three speed classifications; displacement, semi-planing and planing and what separates these as classes of boats. Your post(s) show you're relying on vague concepts not clear and defined facts of the boats, shapes and power required. That is the first part of your project that need attention because all the other decisions will come as a result of this learning and the following class selection.
I can also recommend all the Gerr books as the best in their categories, and that's because there is no one writing who is actually in Dave Gerr's league providing references and simple math to help you learn the topics.
Next is the SOR or Statement of Requirements and this is a design document which is prepared, drawn up, written down into an unchangeable concrete standard for ALL your decisions. There are no "well I kind a sort-a.." There are no "I wouldn't mind if...." There are no " about this long or about his fast or sleep about this many...." An SOR is the basis of any new boat design and build. But there is on detail I'll share with you as the designed and builder of a few welded aluminum boats.
99.95% of all new owners can't write this document and stick to it;
AND 150% of all boat building problems which are due to change orders and 'rethinking' after the boat is designed; could be avoided if the SOR were adhered to;
AND 1,000% of all cost overruns from changes are due to the previous two facts.
So the first step for you is to make/write/find/create/"OWN" a SOR that meets your needs- after you read Gerr's book(s). AT this time you're in the casting about stage, still learning the terminology and concepts that express your ideas. Soon you'll have to prioritize those ideas and desires, based on facts, into an SOR document. (of course you can pay a design office to do this...?)
ibasquarepeg wrote:a) I would like a higher cruising speed than a displacement hull(10-12 knots) but less than a full out planing hull(30-35knots)
This shows that you're misinformed about the speed ranges, below 10 is displacement 10 to 15-6 is semi-displacement and >16 is planing and 30-35 is only a few small percentage of all planing boats. Probably 85 % or more cannot achieve 35mph. A displacement boat will cruise at 6-8 and run up to 9.5-maybe 10? but the cost in fuel likely
triples to go the extra knot or two.
If you want to cruise above the displacement hull range but don't want to plane(???) then you'll need to contemplate the semi-displacement range or the planing range operated slowly. The design factor for planing hulls is 5 the design factor for displacement hull is 7 and the design factor for semi-displacement hulls is 43. (there are no typo's in that line; 5, 7, & 43 )
Your above quote reaches so far that about 75% of recreational craft could fit somewhere in that statement so you'll see it not to helpful really getting you closer to your goal, its to wide/sweeping/vague/far encompassing to be realistic.
ibasquarepeg wrote:b) I see these commercial grade motors on the sites of John Deere, Cummins etc with their inherent reliability and low RPM horsepower (high torque) and wonder why they're not used? Perhaps they're too heavy?
Obviously those engines (electricity drives motors, marine power is from internal combustion engines so I use that term) are widely used. What is apparently outside your experience at this time their application? They are used in countless commercial boats that travel far, stay out long, and do not (90%) plane.
There is a small part of each engine manufacturer's product line that features higher RPM diesels of lighter relative wt and these are most used in the recreational market where speed (see my first post) is priority. However as Chaps, and Ctchucker have mentioned quite a few times; they cost more because they're longer term investments (Ctchucker is being pessimistic by implying a 20yr. diesel life is unexpected- I've seen 40 on lots of engines, depends on use and regular maintenance.) Deisels are heavier for each HP delivered due to the increased compression and crank mass, and we can look at another comparison in the wood shop for an example.
An 117VAC router that is used a few hours a year costs affordable money and will 'last a life time'. The same job can be done with spindle shaper (table) but the two tools work with similar action, rotary cutting bits even if they do so in a different manner. The hand held router turn 20,000 rpm and develops its cutting power due to speed of the bit and it cuts fine but is not really reliable for a 2,000 hr per year cabinet shop.
The spindle shaper is belt reduced to about 3-5,000 rpm most, and has much more torque on the cutter head, but can be run for 3,000 per year and just needs new bits to cut as good as it's first day.
NOW; we've had posters being realistic about outboard engine's performance (routers) they're very cost effective considering all the 'stuff' the have built in... and can be essentially considered disposable, like a wood shop's 3hp Plunge router purchase.
On the other hand if you're running a day shift and thinking about going to night shift to keep your cabinet shop door panel forming shaper's running enough to do the work you have?? well now the cabinet mounted spindle shaper (diesel engine) is the better investment. But the cutters for the spindle shaper do cost more than the most expensive hand held router by three to four times!!! That's not buying the shaper, just the cutters.
So, your remark b) can be answered by the combination of the posts that have been put up. #1 cost differences ,#2 expected use or number of hours per year & #3 installation and boat space is much more involved. Those engines you describe ARE used in boats, but with the above conditions in the SOR for those boats.
ibasquarepeg wrote: c) why not take a lightweight production planing hull(aluminum), drop the cruising speed to just above planing under the worst conditions, say my fifteen knots, and you could reduce the horsepower needed to move the craft.
Planing hulls will not perform as you've assigned them. They will do OK 'walking', in displacement mode, and they do fine 'on step' but they wallow like hogs in between. They won't steer, they're wandering due to the relationship of the bottom's buttock lines to the very small 'keel' (outboard leg) and since the Center of Gravity is so far aft on planing hulls they balance once they're 'up'. But if that same boat is run at the 'transition speed' is is not well balanced so there are no planing production hulls that will perform in the speed range you're describing. (see previous hull range design remarks) Fuel use of a planing boat in transition is higher than either on plane or in displacement mode.
It is critical for you to understand that boat movement resistance is not like a wheeled vehicle. There is no rolling friction reduction once a boat is moving. Water resists uniformly at all speeds based on some ratios that D.Gerr's books will explain.
Next 'thing' or issue you've miss-assigned is engine rpm vs torque curve vs power delivered to the prop vs propulsion vs hull behavior. This is not a simple concept to understand so I'll defer to Dave Gerr. In short, there is a kind of Catch22 involved but your concept doesn't acknowledge those physical facts.
This area of 'tuning' an engine to the hull to the load to the speed you're describing is harder than it appears in the phrases you're using.
ibasquarepeg wrote:Interestingly, Ranger Tugs makes a small semi displacement cruiser that cruises at fifteen knots.
The planing formula is 1.34 x the square root of the waterline; so a 30 boat (square root about 5.5' X 1.35) going over 7.5 knots -let's say 8 knots is planing. So the company you're discussing is sort of fiddling with the terminology- it helps to read more on this. I think the Ranger is actually planing if it 'cruises' at 15knt.s ??
ibasquarepeg are you familiar with making "value tables"? just a spread sheet to help quantify decisions using assigned values - they could help you as you move ahead; but first you'd be best served to get more of the terms correct, and to grow your overall class design awareness. I'd say it was a good move to get and read D.Gerr's books?
Cheers,
Kevin Morin
Kenai, AK