How do they match engine size to a boat on paper?

That's part of a naval architects job is to marry HP with hull size/weight/and design. However, sometimes the builder will build the boat heavier than it's design specs from the naval architect and a once great riding hull, is a pig. Sometimes the manufacturer will take a great proven hull design and get the center of gravity wrong, and make it as wet as a surfacing submarine......etc.......many variables to a good boat.

Much also has to do with who manufacturers have a relationship with. Most smaller production boats will give you a choice of a few motors and leave the decision to you (unless you're buying one in stock). One may only give you 2 or 3 extra knots, but cramp your engine room and guzzle fuel, but you choose. For in stock the dealer chooses which package he thinks he can most easily sell or the manufacturer decides based on his commitments.

Engine size/weight/power is just a small part of the things a naval architect has to balance. In vessels where a single designer handles the tradeoffs, odds are good the end product will be fine. But with multiple designers the opportunity for screw-ups goes up... I've seen weight and balance issues totally deep six a vessels performance due to "the left hand not talking to the right hand" during design. But back to the original question... engine size vs. efficiency. When boats are repowered its interesting to see how many older "planing" boats get repowered to only do 8-12 knots... hull speed cruise with radically better fuel efficiency and reliability. The question you pose is one that inspires kids to try and find the answer... and end up as naval architects. No easy answers... indeed one can make a career out of looking for the answers...

The basic thing to look out for is weight. Then you can run your power to weight ratios against other boats/brands. All boats advertise specs on weights (usually half loads).

For example boat "A" and "B" are the same boat or similar, have the same HP, but boat "B" weighs more - in theory this will cause boat "B" to use more fuel.

Sometimes going to a bigger HP motor may not have a good effect because they usually weigh more than a less HP motor. This is why the IPS/Zeus are such good systems, usually you can go down a few size in HP compared the usual shaft driven boats, but the boat will run with similar running speeds, BUT better on fuel.

Hull form is next on the list... but that's another can of worms. and sometimes it contradics the above statement.

Far

It's all about balance

Engine selection is one big chunk of the design spiral and also one that usually changes several times during a build cycle as owners/clients change their mind when new engines hit the market.

Most designers/builders work with a target speed and try and design structure to match the size engine that will provide that speed and build the boat accordingly. Big changes in engine power dictate either increasing structure to handle the larger loads (tough to do on an already built hull) or living with more structure than needed and having a heavier, less efficient boat.

For an NA or builder, nailing a target speed is usually not too difficult but last minute changes/additions do often cause loss of hair and sleep!

Most definitely. I've long been a proponent of long sleek wave piercing monohulls (look at the Hobie 33), with the new Gyro stabilizer systems you can make them roll comfortable now as well without the drag penalty. Their real advantage though is pitch comfort riding into a sea as you slice trough it rather than pounding into it and surging over it over and over and over again... These long sleek hulls take less power and achieve higher speeds. Add in a combined cycle steam plant for regenerative heat capture from the prime mover, and suddenly you start getting pretty ****ed efficient. I was also considering solar, wind and tide for electrolysis energy to collect hydrogen for a small fuel cell for water and emergency electric.

Another nice thing about the design is you get a lot of length and performance in a low volume hull, so costs to build and outfit are 1/4-1/3 of the normal "$1 million a meter" heard quoted for another top name yacht builder for boats over 160'.

Trawler is an inefficient design for hauling passengers. It is meant to haul a large volume of fish, not a few people. Look at the old Sardine Boats rather than Trawlers. Sardine boats ride great in nearly anything and are much faster and more efficient than trawlers.

Pelagic Dreams, now we're getting somewhere. I would have to say the motor selection is not as important now, and hydrodynamics is. I was thinking you were getting a planing hull.

If the boat is still in the concept stage (still on the drawing board), I would suggest to maybe pump the length to say 70', and bring the beam of the hull in... or play with the beam's dimensions

The idea is to get a better "beam to waterline length ratio", and thus improove drag. Is it possible to go to a alloy hull construction...?

Far

Yes, with a full displacement hull, 1 mpg should definately be possible. The 76' Northern Marine I used to run here and there would get 1mpg at somewhere in the 8-10 knot range, you could cruise it at 12, but it burned more fuel.

I could get 1.5 GPM at 10 knots on a 75' Hatteras MY, and I also worked on a 97' steel displacement hulled canoe stern MY that also got 1.5 gpm, but this boat was built 25 years ago.

My hull is 103' X 18.5 with new twin 385hp cats @1700 and I get about 1.3 mpg @ 9-12 now if I open her up to 2200 then I will be doing about 15-16
I am on inland lakes and river and this setup works great. I see most boats my size are about 700hp and some like the hatteras have there boats up to 900hp per engine. I don't think I there going to get 1.3mpg. I would think these big hp boats would do great on the open water but on inland cruising they could find trouble for there wake. I guess this is a bit of the repower trend.

Thats not entirely true.......I ran a 75' Hatteras MY with 12v71 TI's that were around 850hp each. I did the entire Great Loop in the boat, and 90% of the time at 10 knots or less. That boat at 10 knots, burned 13gph so 1.3 GPM, at 8 knots it burned 8gph or less, so 1 mpg. These fuel figures included 1-2 hours at cruise speed of 17 knots/80gph (1/2hr bursts here and there if there was space). Not as efficient as say a full displacement, but pretty darn respectable none-the-less.