Coastal Cruiser

Bert
That is a worrying tale... nobody likes to get ripped off....
You haven't given us much to work on in terms of the actual design, but as Rick has said, in general it would need to be designed specifically for the task at hand if you want to optomise the performance / efficiency. Be carefuul about altering the plans - in most cases you can't simply build the boat lighter. It will tend to adversely effect stability and often compromise the structural integrity of the design. ALWAYS xheck with the designer and get his/her approval (in writing) before you make any alterations.

Rick
Please... give me some credit. I am perfectly aware of the differences in the loads that a planing and displacement hull are subjected to. I'm also perfectly aware of the momentum that a 1000kg boat can have when it's out of control. I can assure you... they are certainly great enough to puncture a thin skinned composite hull.
So, as far as the 1000kg target goes, lets just agree to disagree, shall we?
As I've said, I consider it to be an interesting project... I wish you well with it.

Dave,
Often there are constraints placed on a design by 'external forces' - like it must fit into a marina berth that is say 10m long for instance. When a designer refers to the length not being constrained, he/she doesn't (or shouldn't) mean it literally, they mean that the design can be optomised to suit the design objectives - efficiency for example - rather than to suit those constraints.

 

Dave
The calm water drag on a displacement hull has two main components namely viscous and wave. For given design speed and set displacement there will be a single hull that has the lowest drag.

The lowest drag hull is usually long and slender. If you have a very slow design speed the lowest drag will have more familiar proportions.

Rowing shells are good indicators of the shape of optimised hulls for their respective weight and speed. A single displacing 100kg is about 8m long and optimised around 15kph or a bit more.

Taking the 1000kg boat designed for 10kts the length for lowest drag is 16.3m and power to the hull to get 10kts is 1535W. However there is not much cost in "constraining" to 13m particularly if the hull has a hard chine. The power increases by less than 10%. Also conditions are rarely dead calm and the fine ends of the lowest drag hull do not offer much benefit in more than a ripple.

Increasing the weight to 2500kg pushes the lowest drag length to 21.2m and power goes up to 2589W for the lowest drag hull.

The lowest drag catamaran displacing 1000kg, designed for 10kts, will be 9.7m long and require 2352W on the hulls.

So a catamaran of unconstrained length will require 53% more power than a monohull of unconstrained length.

As the maximum allowable length is reduced (constrained) there will be a point where the catamaran configuration offers less drag than the monohull.

With the original idea on this thread I was trying to get some form stability into the main hull but the extra beam to achieve this adds drag. The stabilised monohull has stabilisers independent of the main hull that skim the surface. I have quite a lot of experience with this configuration and it is superior in terms of drag to a catamaran or monohull with form stability providing the length is commensurate with the design speed and displacement.

The ends do not need much volume so extra length can be gained with very little increase in weight.

It is no surprise to see the concept emerging for performance vessels like the LCS Independence.

As a hull configuration it gives solar/wind turbine power a realistic ability to compete with sail or diesel for passage making.

The power figures may seem extraordinarily low by comparison with what you have seen before but there are not many hulls designed to achieve lowest drag. It has not been a priority in pleasure boat design for maybe 80 years. To put it in perspective my V14 hull displaces 92kg with me on board and I can hold 6kts on that all day with just 130W, which includes all the losses. Actual power to the hull is 90W. It is 6m long and optimised for 6kts.

So after all that it should be clear that by "unconstrained" I mean permitting the length that results in the lowest drag hull as an acceptable design length.

Rick W

 

Like I said in a previous thread, I had good service from them, thus maybe I should give them the credit of the doubt. Deep in my heart I hope he is working day in day out to design a nice electric boathull, as he realises that life goes on and electric boats are no longer a stupid dream.

I am an optimist, always see solutions for problems.

Thanks for the advice Will. I was not planning to modify the hull, maybe the deck or small internal configurations. Like a "drop down" roof, simular as with caravans. Maybe a little stronger construction to reduce wind resistance. The electric motors I like to have underwater in a bullet type of stainless steel casing, instead of in the boat. But I can only start planning as soon I have the plans.
I ordered the Flareline 18, with the posibility to extend it by 2 feet. (According to the designer)
Can't wait to start.
Bert

 

Hi Rick,

The Flareline 18 is 509 kg's , I probably will extend it to 20 feet and is thus slightly heavier, are you willing to give me an indication whether I am out with my 20% extra power needed to push this boat, at similar speeds as you have calculated for your boat?

 

I think in a few years electrics will kinda take over
motors will become way more efficient and batteries will advance by leaps and bounds
those new 3D solar cells are way more efficient
lots of things will be improved and given that the electronics field is the one advancing fastest its hard to say what exactly will be the next big leap but its coming
likely sooner than later

my two cents
B

 

I disagree.

Imagine two hulls, one with smaller wetted area but with a wider beam than the other. One hull will have lower viscous drag and higher wave drag than the other, but both hulls will have the same total drag.

There are infinitely many ways to repeat that process. The trick is to pick the combination that is best given all the other myriad design constraints and requirements. Apparently, carbon-based naval architects are quite good at this sort of thing

Cheers,
Leo.

 

Bert
It will take a lot more than 20% more power than 2.1kW the 1000kg
13m hull requires to get it up to 10kts.

The hull you have chosen is designed to plane. At displacement speed dragging the transom through the water will absorb a lot of power.

Rick W

 

Leo

I just picking the one hull that has the lowest drag displacing 1000kg when it is doing 10kts.

Rick

 

Wooops Rick, thanks
That was suggested by the NZ company. Now I understand why it takes so long to get the plans. He must have ralised that there maybe better designs from him and is working on some kind of modification.

There are not too many companies in the world who has ready available plans for me at an affordable price. Maybe I should have selected the Fisherman 18, but it will be just too small by 2 feet.

 

Bert
The boat shown on this link is more like what you should be looking for:
http://www.wolfeboats.com/
Denny Wolfe has been posting on the Efficient Electric Boat thread. He did some useful comparisons on small motors and props.

It will not get to 10kts without a lot of power but it will do 6 to 7kts with a 4.5kW motor like I have.

If you want to get to 10kts with 4.5kW then you need a longer boat.

Rick

 

That's a good strategy It's just that there is not necessarily one hull that has the same "lowest" drag for the same displacement and design speed.

Leo.

 

So that begs the question. How many hulls are there and how much do they vary?

Rick W

 

There could be infinitely many, varying in drag by less than 1%. The search space can often have regions where there are many candidate designs with almost identical performance at a single design speed.

That's why it is rarely a good idea to "optimise" for a single design speed. It is much better to search for hulls with the best performance for a range of speeds, even if that range is the target speed +/- a few percent.

Imposing practical constraints (e.g. on length, beam, draft, shape, initial stability, seakindliness, etc) can also sharpen the distinction between otherwise identical candidate designs.

Leo.

 

Leo
You have side stepped the question.

Within that infinite array of possible solutions that have less than 1% variation how much will they vary in length for example? (I was answering a question on what I mean by unconstrained length)

The above was prefaced by calm water resistance so the issues of sea-kindliness and stability do not come into it under the terms of reference. The big feature of the stabilised monohull is that stability is afforded by something other than the immersed hull.

To avoid absolutes, lets say I want a hull displacing 1000kg that will exhibit calm water drag within 1% of the lowest possible drag for 10kts. What will be the range in length of the myriad of possible hulls?

Rick

 

That depends on the allowable hull shapes. In Godzilla it depends on which hull series you use, e.g. Series 1, 7, ..., 32. The optimal length could easily vary by +/- 5%, and even more in some cases.

It is fairly easy to test what happens if you fix the hull shape (e.g. your chined hull) and vary only the length and draft while keeping displacement (and hence beam) constant.

For your specific 10kt case, the boat is travelling near the wave resistance hump, so length is the most important parameter, and it won't be quite as variable as for lower speeds where +/-20% is possible. At very high speeds, the optimum could vary by +/ 50%. Near the hump speed I wouldn't expect the optimum length to vary by more than a couple of percent. It is similar to the case shown in Figure 2(c) of
http://www.cyberiad.net/library/multihulls/multipep/results.htm

Remember too, that separation near the stern can play a role which complicates matters further. Now you have three drag components, and you can trade off increased skin-friction for less wave resistance and less form drag, and similarly for the other permutations. Of course, how to estimate form drag is still a very vexed, unsolved problem.

Incidentally, your experiences on the Murray and with Greg's record attempt have been very inspiring and given me a lot to think about.

As a hobby interest I have just started on an optimisation project myself. I wouldn't be surprised if I end up with a flat-bottomed hull similar to your V14. However, I am using a 32 parameter series, including squat, the moving leg masses, and I am also going to try to account for changes in displacement as on-board water and food are consumed, and the pedaller loses weight. Who knows, my "optimum" hull might be completely different to yours. It really is a fun problem!

Leo.