Scaled Down Cruise Ship

[Tony Stark]

Hello there, this is my first post, go easy on your boy. The power/displacement ratio of cruise ships has grasped my attention. They require very little power for their weight. I want to build a 65' boat, and I love the idea of needing very little power to move. (I mean less than 50hp) So far I've found very little useful data about this specific idea. I know that the speed will be limited to 10 knots, and that's fine by me.

I feel particularly drawn to this idea because it opens up the realm of practical solar powered electric boats.

Thoughts?

 

[HJS]

It is not a question of size but of speed in relation to length and total weight.
Seven knots would have been more appropriate.
JS

 

[bajansailor]

Welcome to the forum Tony.

So we have 65', 50 hp and 10 knots - I think this is one of those classic cases where you can have any two but you cannot have all three.
Unless perhaps you have a long, skinny and light canoe shaped hull.

You mention that you want to build a 65' boat - this is an enormous project, and not for the faint hearted.
Re how you like small cruise ships, do you want this to be like an even smaller version? There is a limit really as to how far you can scale down, as people stay the same size.
Do you have any sketches that you can post showing your thoughts so far?

 

[jehardiman]

Do some google searches for "narrow boats" and "canal boats". These vessels are similar to the aspect ratios of modern cruise liners. Seaworthiness for them is another matter, being almost non-existent. When starting a design a full Statement of Requirements (SoR) is needed. You have to decide where and how you want to operate before selecting a particular hull form.

 

[Tony Stark]

I should add more details: I want it to be seaworthy and cross oceans.2 decks, cabins on the lower deck and galley/driver seat/ living area/ etc on upper deck. Length: 20m Beam: 5m, my bad I should have mentioned the beam. I know that speed will be 10kt. I am thinking that the width will add stability. Lithium ion batttery packs on the floor in the hull would aid centre of gravity. Am also considering active fins for stability. Here are a couple of sketches I made!

 

[bajansailor]

I want it to be seaworthy and cross oceans.

I know that speed will be 10kt.

If you want to be able to cross oceans on a 20 metre long vessel, you will probably have to do this at a speed that is less than 10 knots.
Your best bet is probably something like a classic trawler yacht - here is a nice one for sale.
1975 Southern Marine Trawler Displacement Flybridge for sale - YachtWorld https://www.yachtworld.com/yacht/1975-southern-marine-trawler-displacement-7627277/

And if money is no object (and this will have to be a requirement really, if you REALLY want to build a new 20 metre long vessel), then the obvious boat for what you want is one of Steve Dashew's FPB 64's -
FPB 64 TOCCATA is for sale - Berthon International https://www.berthoninternational.com/berthon-blog/fpb-64-toccata-is-for-sale/

This FPB 64 was sold 2 years ago - she had an asking price of US$ 2.2 million.
Circa Marine FPB 64 – Sail Northwest https://sailnorthwest.com/boat/circa-marine-fpb-64/

 

[bajansailor]

Another vessel that should come close to your design brief is the Arksen 65 - she is 20m x 5m.
https://www.arksen.com/arksen65/

 

[wet feet]

The main advantage that cruise ships have is that their principal cargo is fresh air-at least by volume.Which is why their underwater volume is so small compared with real freight transporting vessels.Which is a long winded way of alluding to the fact that you need to focus on displacing as little as can practically can be achieved as it takes power to move that water aside.The two outstanding examples that come to mind can probably be found on this forum with a bit of searching.one would be Tlingit by William Garden and another is Whio-hailing from New Zealand. How does Whio do it? https://www.boatdesign.net/threads/how-does-whio-do-it.65157/

 

[kerosene]

This FPB 64 was sold 2 years ago - she had an asking price of US$ 2.2 million.
Circa Marine FPB 64 – Sail Northwest https://sailnorthwest.com/boat/circa-marine-fpb-64/

I don't think you get it. $2 million is insane amount of money, even 900k is.

So he wants something that (nobody thought of), has 2 stories, goes 10 knots on 50hp and costs maybe, I dunno, $150k - or maybe he "has $50k now but will be able to save some as we go".

This would fit the typical 50+ ft 1st time boat dreamer we get here.

reality in short:
20m x 5m, fully equipped boat will not go 10 knots on 50hp.
There really isn't anything too magical about cruise ships. With shops scale gives massive efficiency benefits that is why we get 20,000 container carrying cargo ships.

You cannot scale boat design as different things scale at different rates (moment arms, displacements, areas etc.).

Ocean crossing 65ft boat is massively expensive. I hope you have owned a 50ft one before.

Building your own is never the cheap route.

 

[kerosene]

there is an Arksen 65 for sale too. 1.8 million GBP

and FPB which is pretty sleek has 236hp motor with 11 knot top speed.

 

[DogCavalry]

TS, there's an attribute called hull speed, which is the relationship between length of a vessel, and It's speed. The concept is somewhat obsolete these days, but still informative. The relationship is square root of length of the vessel in water, times some constant depending on units, times another for hull shape. Those liners go so well because, for their length, they are going very slowly. A 65' loa boat has a short enough waterline that hull speed is very limited compared to the cruise ship. Naturally this is a great oversimplification, but it serves to illustrate the challenges.

 

[Tony Stark]

Appreciate all the replies!

 

[DCockey]

Consider two hulls with similar shape but at different scales. For similar wave making the speed needs to be proportional to the square root of the length. So for a 65 foot boat to have a similar wave pattern to a 650 foot ship with a similar hull shape traveling at 15 knots the 65 foot vessel needs to be going at 0.316 x 15 knots or 4.7 knots. A 65 foot boat with a speed of 10 knots would be making waves similar to a 650 foot boat with a similar hull shape traveling at 31.6 knots.

 

[jehardiman]

I should add more details: I want it to be seaworthy and cross oceans.2 decks, cabins on the lower deck and galley/driver seat/ living area/ etc on upper deck. Length: 20m Beam: 5m, my bad I should have mentioned the beam. I know that speed will be 10kt. I am thinking that the width will add stability. Lithium ion batttery packs on the floor in the hull would aid centre of gravity. Am also considering active fins for stability. Here are a couple of sketches I made!

That sketch is way too shallow. Wet Feet is correct when he describes cruise liners as mostly air. Their SoR is also reflected in their draft. They don't need to be very efficient but they need to get into shallow harbors; hence the high block shallow draft hull form. If you want an efficient hull, you are going to need significant draft and deadrise.

 

[Andrew Kirk]

Here's a Youtube video about solar powered narrow boats on English canals. The gist of it is that you can move with solar power but you will also drain the battery bank. You then need lots os time, like a day, to recharge with solar. If you have somewhere that you can plug into the mains whilst docked that's a big help.