Power boat design for economy.

So you need 40hp to do 10 knots. The graph above shows only 15hp to do 10 knots for a similar boat. As I say, not very likely

My own rough-and-ready-first-estimate computer program implies your boat would do 9.8Knots with 40hp. Which is in the right ball park. A more refined program will give more accurate results of course

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

Hi Richard,
3000 rpm is 14hp at the engine, less gearbox and other losses, likely less than 10hp at the propeller, and propeller efficiency is not brilliant, lets say around 50% ? - - - so the propeller likely delivers ? - - so your call..... It all depends on where you measure the hp

The attached image relates to the industrial version - consider marinisation losses and adjust downward...

 

It also largely depends on the flow into the prop too...i.e. shape of aft lines of the hull and proximity of prop to the hull.

 

Fairly flat & clean - - - but of late needs some barnacles removed

 

Ok..I'll assume that's correct without piccies for verification.

What RPM is at the prop?

 

Hi Ad Hoc,
The manual is on the boat, but from, (failing), memory is 2.56:1 so should be around 1170rpm at 3000... - Which may well be correct as I remember at max rpm of 3800 the propeller ran to about 1400rpm which agrees with the mathematics... - The new series 60 legs are a lot taller (2.38:1 or 2.15:1) for some unknown reason... I would rather the torque and slow spinning propeller...

 

masalai
What's the objective...a prop more suitable...?
Well, from the values you have given and assuming you have easy clean flow into the prop not disturbed nor sudden change of hull shape...then you should aim for around 16~16.5inch diameter with around 12.5 pitch

 

Masalai even thats old school guesswork compared to Frostys wants . What he needs is the efficiency of a inboard power/shaft drive/variable pitch controlled system. If one wanted to get fancy set it up computer controlled with input sensers linked to balance out RPM-Pitch-Speed-for optimum Fuel consumption. Extra add on computer info features could analize your destination,recommend slight changes in course headings to reduce beating to weather resistance and save in the overall fuel costs for that leg of the journey. Christ it could recall historical hull resistance and optimal drive system info and decide to stop dead in the water until you went over the side and cleaned the hull. (yes master HAL). If money is no objective possibly a 25% in operational costs could be achieved, however as i posted in #93, go beyond the affordiable law of diminished returns and costs are not important as affordiability of operation or ownership of the machine is a mute factor.

 

Hi Viking,
- Frosty could still do fairly well with something akin to "Foreign Affair" which has a demonstrated record in the Coral Sea region and can sit on her bottom on a friendly sandbar... - Definitely a valid choice for most tropical island waters . . . and

In my view above 15knots for an 'un-crewed' pleasure craft is not sustainable, comfort wise, in the longer passages other than occasional-calm-weather-windows-of-opportunity, (for example 1000+miles across the Coral Sea)... The less complexity and "automatic control" the better for a live-aboard cruising boat...

Beyond that sits the rationale for a "super-yacht" and paid crew of 'captive servants'....

Hi Ad Hoc,
- I was responding to richards points/questions here http://www.boatdesign.net/forums/boat-design/power-boat-design-economy-37597-7.html#post548995 and here http://www.boatdesign.net/forums/boat-design/power-boat-design-economy-37597-8.html#post549040 Always looking at possible improvements - budget permitting.... Now I cannot remember the size of my propellers - I think I have the maximum diameter recommended by the manufacturer, tweaking by pitch remains... and at near Au$1500 a pair I am not in a hurry, at present... Gear selection linkage, (forward & reverse), seems to be presenting difficulties at present ? not really sure what, yet but something seems amiss?

 

It's admirable to try and save fuel,a depreciating and finite resource but at some point in time/length/gph/hours used per year-unless you are mooring out for free or own your own dock space and/or are travelling many hundred hours a year,you will spend much,much more on maintenance and moorage fees and especially depreciation than you ever will on fuel.

On aside note,there's been a 94' Sunseeker docked here in Vancouver for over 10 years,and AFAIK it hasn't moved-I can see it from my building.
If they don't own the space....

 

Maybe it could be suggested that many of the unused boats may have been forgotten by the owners who, maybe would appreciate the theft of such unwanted property, as the insurance payout is likely more than the used resale value of the boat...

I used to wander around looking at boats in marinas trying to select one that I would like to have...

Sadly I could never find one... They were all "illicit sex places", for wont of a better description or use... There always seemed to be at least one key feature of the design or options added thereto, ensuring permanent dependence on a powered, secure, sheltered marina berth when not out on a "fair-weather-day-trip" . . . If the boat itself was not crippled by lack of attention to reasonable care & maintenance etc... I like fuel efficiency as it means I can go further and need not save up for so long to go on a decent voyage... The next one is fairly major and requires significant planning for me to finance and organise... maybe early 2013?

 

Heard the other day of a story of a guy that had a sailboat that for years paid someone to maintain boat, dockage. One day finally he feels better and wants to see the old girl. The guy he was paying took her for an Island trip and apparently was living on boat.
The boat guys enjoy the boats more than many of their owners.

 

I thought you were going to say,by the time he decided to take her out, he had grown so old he forgot in which marina the boat was docked--

 

I actually got my boat from a guy that died before he could finish his boat.

 

Oh man don't say that--it's my greatest fear it happens so often--I lay awake at nights making deals with the spirits --just give me time to complete and sail her once -- preferably into a tropical storm anything after that is small potatoes. Thats it, leaving Florida (Daytona) tomorrow -- got to get back on the project