Prop/rudder optimization for longe range single engine displacement vessel 30k lb displacement

I started this thread originally with a question regarding a Yanmar pod drive, but the more I investigated it, the more convinced I was that a pod drive was not the solution. Especially so since the pod only allows thrust vectoring at low speeds with the rudder locked straight ahead and control with a joystick. Had thrust vectoring been allowed at any speed, the pod could have eliminated the rudder and associated appendage drag at cruise speed. The leg of the pod would probably be a perfectly fine rudder at cruise speed of 7-8kt.

The potential bot for this application comes in 2 basic forms:
1) A converted sailboat in the 50-55 ft length category, which would have the needed internal space for the needed 1000+ gal of fuel. Best case scenario , one could obtain such a boat after a dismasting or one that needs a lot of work to the rig and sails, since that would lower the cost significantly.

2) Building a new George Buehler style multichine hull intended for this purpose.

Option 1 would require keel modifications to reduce ballast and most likely a re-power, unless it had a fairly current engine. It is likely to need a different transmission ratio for a bigger prop since that would be the primary means of propulsion and sailboats tend to care more about prop drag when sailing than having the most efficient prop for motoring.

Option 2 would have a drive train layout intended for the purpose but not neccersarily optimized for drag. There are multiple potential modifications for improved propulsion efficiency, and one has to decide which of any options to pursue. I will make a separate post listing the options based on those listed as options by major ship builders.

 

What is your objective with this boat? What do you want to do with it?

 

Deering, this is something along the lines of "voyaging under power". Fuel capacity to go from Panama to Hawaii with reserves. 50-60 ft water line length. Beam around 12ft. Inboard diesel of about 80hp. Cruise speed in eco mode would be 6.5kts with the ability to go a little faster against a tidal race.

I have gone round and round on the range issue and it seems that feedback on this forum is 4000 miles for a powerboat making long pacific crossings. Given a target fuel consumption of 3nm/gal that means 1333 gal of fuel which is about 9000lb in fuel weight, + the fuel tanks themselves.

Idlewild had a capacity for 1000 gal of fuel and apparently the consumption was closer to 4nm/gal but she only had a 55hp engine and she may have been run pretty slow to get those numbers. She was built to set a few records and thats what they did at a heck of a cost for the portage down the peace river nearly 2000 miles. I don't think I will be going with an aluminum hull, too expensive. I think she also set a record for the longest un-refueled non stop voyage too.

 

You should be able to do much better with a sailboat hull or similar. E.g. Swan 65 S&S (50' LWL) is about 32 tonnes and should do 6.5 knots in calm with 15 kW power. The consumption would be 5 l/h and thus about 0.8 l/nm or about 5 nm/gal. Your planned displacement is less than half of that.

Here's a smaller boat with the same as your target displacement, but only 40' LWL. https://muldermotoren.nl/media/wysiwyg/Muldermotoren/PDF/Folding_prop_test_Volvo_Penta.pdf
It's consumption was 3.5 l/h at 6 knots and 6 l/h at 7.1 knots with the best propeller. Would be less than 5 l/h at 6.5 knots.

 

Intuitively, I should think you'd want a large, geared down prop. Also, I think if you ended up building something, it might pay to look at some research on low drag hull shapes. I'm sure it's out there. I know that people have figured out ways to make axi-symmetric bodies much lower drag by using the prop to affect the boundary layer, but I don't know if there's research on doing this. OTOH, if you did the trip in a submarine it would probably REALLY save fuel.....

I seem to recall that at one time there was a power boat which had the shape of an Alden Ocean Shell tripled in size!

My guess is that a specialized hull shape could be smaller and cheaper because it needed less fuel.

Those generalized estimates, like that 4,000 miles, contain so many assumptions that they'd be useless unless your boat conformed to all the assumptions.

Of course, we really don't know much about the constraints on your "mission". So we probably can't make great guesses.

 

I can't help with your quest but a few hours ago I watched a tv show about metal scrappers. They had a 46ft. steel yacht from which the rig and interior had been removed. They gas-axed the skeg and bilge keels off so they could fit the boat on a low loader to take away. It struck me as a sad waste as it looked good enough for your kind of conversion. Still, I guess there are many more out there.

 

Plenty of times the difficulty is connecting buyers and sellers. And of course these have to be cash sales and how many people have the money in hand ?

For now, unless I make my own scouting trip after a new hurricane passes through somewhere, it looks like I will be maxed out around 40 foot, probably too small for the pacific although there are reports that the 37' Willard got 7nm/gal at 6kt so perhaps such a large fuel storage would not be needed.