Efficiency of Baglietto Ischia 80 or similar Vee hulls at low speeds

[Ad Hoc]

Welcome to the forum,

...The concern is which boat to use for this conversion project. .... while I would naturally be aiming at cruising at lower speeds most of the time (7–10 kts) to take advantage of those ~240 kW of batteries ...

However, your going about this the wrong way.
If you are happy to potter along at "slow speeds", then find a boat that you like.
If, it turns out to be like those you posted in #1, whatever efficiency you may or may not get is irrelevant. Those hulls are designed to run at high Fn.
Can they potter around a slower pre-hump speeds, sure, anything can.
But all of these boats are, in general, heavy for their length - its all the glitz they add. So as you approach the hump speed the drag goes up massively. These boats have powerful enough engines to overcome the increase in Resistance, pre/post hump.
Adding heavy batteries etc..does not hep, in this regard.

So, if you're happy pottering about, forget whatever efficiency you think you may get, you'll just have to accept the speeds you get and the less than ideal efficiency of the propulsion system owing to the design of the boat.
It may therefore be better to select a boat that is designed for such slower, pre-hump, speeds. The efficiency wont be much better, but just a more suited boat, overall.

Otherwise, best get a boat designed for your actual needs....= $$$$

 

[wrobelda]

Due my poor language I misunderstood phrase "aft extension plug" . Those hidden torpedo launchers?

A ~1.5m faired (tapered) aft extension, a solid hull section added to reduce the transom depth and width and thus smooth out the hull shape and reduce the drag. In practical terms, it would be a swimming platform plus some extra storage.

 

[wrobelda]

an underpowered large vessel is an unreasonable risk because it is very weather dependent. The required 600kw/4hours refers to a 200hp/160kw output.

Isn't any heavy semi-displacement vessel maxing out at 10kts also underpowered and weather dependent in such scenario? Because 2 x 200hp is more less what is typically installed in those?

But in any case, I could absolutely opt into larger, as big as 475kW/650Nm engines (times two) if needed, basically at the same cost. The problem is I don't really see how this would be a *requirement* for safety at all?

 

[trip the light fandango]

I can't help you there I can only repeat that 200hp is half the average and your range is half what you wanted by the rough calculations using AI, regards.

 

[wrobelda]

I can't help you there I can only repeat that 200hp is half the average and your range is half what you wanted by the rough calculations using AI, regards.

Fair enough, I'll look more into it.

 

[Ithaka]

Regarding the hull form, I think you are better off finding a cheap trawler, they are not that difficult to find used. An alternative could be to add a keel to the planing hull - should not be that difficult or expensive and it will help partially.

Regarding required power and energy storage, I could share my experience with a similar hull (planing, modified V, 14 deg transom angle), 32,000 lb, it needs about 50 hp (40 kW) to move at 7.5-7.8 knots in calm weather with a dirty hull. If I were to switch to electric, I would consider 2x 40 kW electric motors (double the power for additional reserve) and 400 kWh battery, which will give me 10 hours x 7.5 knots or 75 nm range. I will upgrade the generator to 20 kW (ideally a DC generator matching the battery voltages) but most of the charging will be from shore power.

In terms of weight, the battery cells will be 4,500 lb, say another 1,000 lb for enclosures, cooling, etc., another 1,500 lb for the motors, so total 7,000 lb added. The total weight of the fuel, engines and gearboxes that will be taken out will be 6,500 lb, so the net impact will be insignificant. Add some fuel for the generator.

In terms of cost, you can get the batteries at $85/kWh, so $34,000 plus another $25,000 for everything else = $60k. Don't use automotive parts... procure the cells and the BMS yourself.

Is it possible to do? Absolutely, LFP batteries are dirt cheap. Is it worth it as a retrofit? I do not think so. Is it the future for coastal boating? Absolutely, the same as for cars, coastal cruisers will move to electric boats. It is perfect for the average weekend trip or marina hopping.

 

[BlueBell]

Probably the cheapest way to answer your question number one , is use of cheap outboards with variety of props , dismantling original props of course.

The cheapest, simplest and most accurate way would be to tow the boat at various speeds and record the force necessary using a large spring scale in the tow line.

 

[gonzo]

Isn't any heavy semi-displacement vessel maxing out at 10kts also underpowered and weather dependent in such scenario? Because 2 x 200hp is more less what is typically installed in those?

You need to be more specific. For example "heavy semi-displacemen"t needs to be defined by displacement to length ratio, actual length, draft to beam, prismatic coefficient, so it can be compared to any other. What you call heavy, may be moderate to somebody else. If you are referring to a 30 ft boat, 400HP would be adequate.

 

[montero]

The cheapest, simplest and most accurate way would be to tow the boat at various speeds and record the force necessary using a large spring scale in the tow line.

It was probably after wednesday rum ration.

 

[gonzo]

It was probably after wednesday rum ration.

Seems like many of your sarcastic comments are based on a lack of knowledge. Towing models with a boat is a tried and true method. One effective method is to tow two models attached to a bar pulled from the middle. The model with more resistance will be evident as it will tow behind the other. It gives instat comparative data.

 

[montero]

My mistake .I tried to quote myself , comments about use cheap outboards to test . Instead I paste BlueBell comment.

 

[CocoonCruisers]

A ~1.5m faired (tapered) aft extension, a solid hull section added to reduce the transom depth and width and thus smooth out the hull shape and reduce the drag. In practical terms, it would be a swimming platform plus some extra storage.

Guys, i'm noticing the white boat in the initial pic has enormous trim tabs,
their mounting hopefully beeing dimensioned for the massive forces of the claimed top-speed of 27 knots in some waves.

So if the aim is to reduce the boat to a third of the design speed (a small fraction of the forces), then perhaps instead of an extension, one could just make much longer trim tab plates and be done. Twisted ones if you get ambitious, to get closer to a cargo stern. Probably with some kind of side board so they don't get washed over by sidewise flow underway. Perhaps even boxes with some buoyancy and a double use as swim step or dinghy lift. Calculated to break or bend in case of gross overload, so you avoid the attachements punching holes in the hull. The energy recovery effect at slow speeds should be quite comparable to what you can get with your solid swim step extension.

You might even be able to convince the current owner to let you try them on and record the fuel burn & rpm's; most sellers of fancy-but-out-of-fashion boats are rather desparate after all, especially in winter.

More generally speaking, the main problem of that hull for displacement speeds is the deep immersed transom. Probably you could play with the battery placement (and ejection of the original heavy engines?) to raise the stern quite a bit. The stability implications should be manageable if you don't take it too far. You'd live on light slopes as a result, but hey, weren't most of our ancestors in their self-build huts ?

If all that works out, and seeing you are into custom controls on the electric side, perhaps you could even get the extended trim tabs to flap around and stabilise you at anchor. (Don't let them chew on swimmers though )

(Disclaimer: this is gut-feel, tinkering intuition, and a reflection on some CFD i'd done a while ago in a broadly comparable direction. I'm *not* at all qualified to validate the structural and certification implications. Luckily the crowd here are quite likely to spot any terrible flaws of the idea. Best wishes for your project !)

 

[CocoonCruisers]

32,000 lb, it needs about 50 hp (40 kW) to move at 7.5-7.8 knots in calm weather with a dirty hull.

Thanks Ithaka for the data point, that is just half the living weight of the Baglietto 80 though. And that V boat of yours is probably quite a bit shorter, right ?

 

[trip the light fandango]

I can't help you there I can only repeat that 200hp is half the average and your range is half what you wanted by the rough calculations using AI, regards.

My gut instincts tell me that without a lot of reserve power this shape of craft will find manoeuvering is dictated by the huge bow overhang so the helm is correcting windage or current pushing/ blowing the bow around,[so in theory bow thrusters would be quite large] long buoyant trim tabs may help as cocoon cruisers mentions. The other problem is that the waterline length missing because of the bow overhang would have provided stability and hull speed. I didn't really want to comment because I'm a novice but there it is, I can see the attraction though, it would be an attractive low pollution relatively cheap boat with unique motive power for that type of craft.

 

[Adrian Durand]

Hello Wrobelda,

I've been doing a similar design exercise myself designing a solar powered boat that may never launch (pun intended). The modern internet being what it is all of the tools you could want to use for designing such a vessel are behind paywall. Not to mention if you do get access to these tools understanding them in the context of your problem is a career in itself. So what are we enthusiastic amateurs to do? Go old school and get a book. The one you want is:

Propeller Handbook - The Complete Reference for Choosing, Installing and Understanding Boat Propellers, by Dave Gerr

There are a bunch of places online for you to source this book. What it contains are simple formulas, tables, and figures (charts) that you can use to come up with an estimate of the power requirements for your boat propulsion. You will, of course need to add in all of the house power consumables in your calculations to determine how you're going to be able to motor around.

The maximum solar power collection I was able to estimate was about 9kW for a boat that has an overall length of 50 feet and a beam of 14 feet. A 100kWh battery would need at least 12 hours of sunlight to fully charge the battery. This means you aren't motoring nor are you using any of the house implements as they would subtract from the charging rate.

Fine, we're not in a hurry, we motor slowly or every other day. Well, you're going to have to go SLOW. My theoretical boat would manage 5.5 kts at 6 kW (probably optimistic) would need 20 kW at 8.5 kts and 150 kW at 15.5 kts. This illustrates that you'll be just barely tooling along most of the time and a calm ocean is your ideal conditions. The river is my use case so chop and high winds aren't as big of a problem because I can just tie up.

I'm planning on using Nissan Leaf drive units for the propulsion motors. This will allow me to crank up the output when I need to get out of the way. I don't expect this to be a concern for most situations because there are lot of boats that live on the water going my design speed.

As far as your boat choice is concerned, don't worry about hull design. At the speeds you're going to be able to operate at it should behave like a semi-displacement hull and there are a bunch of design data in the Propeller Handbook to help you get a rough order of magnitude. Your boat is about 4x heavier than mine so the drag calculations will reflect that.

The professionals are using catamaran style hulls for their boat design because it is more efficient. However they're also not using motors that are anywhere near as big (twin 30kW) as what you've been told to specify. The numbers for the Soelcat 12 match up nicely with the estimates I've developed.

https://soelyachts.com/synew/soelcat-12/#1535707794888-12c03631-dbe5

I think assuming 1.5 to 2x the power consumption for your non-optimized design is probably fair.

For my design journey, I'm starting with a 16 foot open fishing boat where I'll have two trolling motors for propulsion and a solar canopy that can generate 1600 W of power. This is just a proof of concept that will allow me to verify a bunch of my assumptions and understand the use case a bit better. My expected solar boat build is $20,000 plus the cost of the boat. As I teach my engineering students regarding estimates, double it and add 10% and you'll be close.

There's a lot I still don't know but I'll be taking the plunge in the next couple of years. Good luck with your build.