Resistance calculations for a 1938 monohull

I'm restoring a 1938 ex-lifeboat for future use on rivers & canals. Therefore I'm exploring electric propulsion.

I thought it sensible to properly calculate the power needed at various speed points in order to work out the endurance/range, which would dictate the size of battery bank/charger etc etc.

So I assumed they'd be formulas out there so did my research (inc. these forums) I now have a great set of partial figures before coming to a crash and realising they can only take you so far. Only a tank model or CAD will actually deliver a full set of resistance numbers.

So I started looking at FreeShip/DelftShip and after several hours in those realise I'm out of my depth (chortle, pun intended). I think I probably could get closer if I spent enough time messing around in it but I have to be pragmatic that when it comes to the ballast keel and enclosed prop tunnels I would end up with a Picasso (not in a good way) rather than a Rembrandt.

If I were designing/building a boat(s) rather than restoring something already existing then I would of course persevere and 'master' the software. I will just be using the 'calculate resistances' button for a one off purpose.

Is there any kind soul here who can donate an hour of their time? I do have the Lines drawings.

 

If you have a reliable body lines drawing in dxf or dwg format I can do with my application ARQN, a power estimation using the Holtrop-Mennen method. That is, for the displacement you indicate, trace the power-speed curve.
Make it clear from the begining that the method Holtrop-Mennen is not far, a tests in a test tank.

 

Welcome to the forum Jim.

I have (as well as others here) some experience with lifeboat conversions. Which do you have and can you post "reconciled" lines? By reconciled I mean just what TANSL has hinted at, a reliable set of lines that truly convey the shape of the boat. Weights and centers would be nice too, though these could be worked out both mechanically and digitally. Most modern lifeboats are ungodly heavily built and very tender (intentionally). Their shapes usually don't need much power, but because of their typically massive builds, considerable drag making accouterments and the full up load their designed to carry, considerably more power is necessary than a displacement cruiser of similar size and shape..



I'll assume it's one of the, in tunnel twin screws of the first real powered RNLI boats. These have a rich history and lots of information and help is available on your side of the puddle. Power for these ranged on size and operational envelop, but as little as twin 12 - 15 HP diesels to twin 40's where common.

On the other hand, it certainly could be a ships lifeboat, which is a different beast altogether.

I don't think you need to do as much "resistance" calculation, as you might think, as repowering is fairly straight forward, even with electric. Of course, to get the last few percent of efficiency from her, you will want some resistance calculations performed, but generally these boats have enough drag, that you'll be kidding yourself with "upgrades" to improve efficiency.

 

@TANSL

Alas I only have the lines as drawings and bitmaps (see below)

@PAR

Fantastic photo! Took me a few minutes to find that boat. RNLB Jose Neville. 28th and last of the 35'6" (10'3" BWL, ~7t) Single Screw Liverpool Class, built 1941. I've been on one of those, smashing little boats. Ours shares much heritage with the Liverpools so the looks are 'related'.

Ours is also a real/full/proper lifeboat (I forget that it's such a wide category, including ships boats etc). A twin screw Watson 41 (41' LOA, 11'8" BWL, ~14t), originally twin 35HP Petrols then the class was retrofitted with twin 47HP Diesels in the early 60's (see attached, taken in 1939).



Re: Reconciled Lines. I think so, but I can't be the judge of how complete/useful they are in their current format.

Alas I don't own the Lines drawings although I have bitmaps of them courtesy of RNLI Archives. I gave a written undertaking not to disclose them or use them apart from for private/research/restoration purposes - therefore I can't in good faith upload them to public forums. I'm happy to e-mail them privately of course for the purposes of judging their worthiness and/or CAD.

Interesting you've been involved in Lifeboat restoration/conversions previously.

 

Well, if you want, I can examine the bitmaps to see if I can do something right enough with them . However, the method of forecasting power I have is only reliable for vessels whose dimensions and Froude number fall within certain limits. The first thing I should check is that your boat is within the range of application of this method.
my e.mail : 657677483@orange.es

 

Thanks TANSL, large e-mail on its way to you as we speak!

Edit: Oops, bounced...
"A message that you sent could not be delivered to one or more of its recipients. This is a permanent error.".

I'll try without an attachement to see if it's a size related issue and a poorly described error message.

 

You may need to have a minimum number of posts, before you can send and email. Send it through your browser. I'd also be interested in seeing the lines, though I respect them trying to protect the lines, they can't possibly exspect any level of domain rights after 3/4's of a century other then the gentleman's agreement you've made with them.

 

TANSL has had a quick look at the Lines info (low-res versions) I have and think there *might* be enough to get the job done so is going to look further (at hi-res copies)

@PAR, I've sent you a PM on sending you a copy (the e-mail stuff to/from TANSL was real ones not forum PM's).

Cheers guys, cross fingers but it's easy to count chickens.

 

Sorry Jim but I have not been able to get anything useful. I have sent an e.mail explaining a little more the existing problem.
It would be interesting to know if anyone sees possibilities that I have not seen.
Cheers

 

Thanks TANSL, great that you gave it a look - I've sent you an e-mail in response.

 

I took a quick look at the lines and this boat's design is very propose driven. She's design to load down evenly and motor. She could be rowed, but you wouldn't want to, unless you had a sizable crew of 18 year olds, with nothing better to do. Sailing would be difficult, as there's a fair bit of drag associated with the shape choices that make her a powerboat. As an electric conversion, she'll require substantially more KW than a different set of shapes, though some things could be done to make modest improvements in her low speed handling and drag.

You're looking at two 20 KW units (my minimum recommendation, assuming you'll want some reserve capacity), with their related gear and energy storage. This is a significant set of batteries, controllers and the like. I guess the real question is, "how married to electric is this project" and how much more, over more conventional repowering are you willing to spend, to have this conversion?

 

@PAR - Thanks, I've sent you a detailed response via e-mail.

@TANSL - Many thanks! Again, several e-mails back and forth.

As a general update, TANSL has come up with a set of approximate figures. He has reservations over their accuracy, due to the limited set of data I was able to provide. However as I was only after approximations I'm happy to have a set at all and accept them for what they are.

Cheers guys!

 

TANSL's approximations can be accepted as much more than that.

 

Absolutely, his accuracy is only limited by the poor information I've been able to provide him.

Plus of course he's not doing a full exercise but a 'quick favour' to generate some 'rough' numbers... he's gone well beyond and above in this.