Semi displacement powercat

I’m sorry but there are some misnomer being posted and also not wholly correct definitions thrown into the mix too.

There is no such thing as an “efficient” boat. Define efficient?....if a boat satisfies its SOR, it is a successful design.

Only if part of the SOR requires some aspect of the design to be more favourable than others - i.e efficient - then one could consider it to be slightly better than another – everything else being equal. But to call it ‘efficient’ belies the whole understanding of what ‘design’ really is.

A successfully designed boat is one in which all the conflicting elements that exist are brought together as one whole. Identifying one aspect of the vessel is merely that, one aspect of the boat. But the contradiction is that a successfully designed boat is greater than the sum of its individual parts. In other words, to call a boat efficient, can only be efficient with respect to one aspect of the design. Since there are far too many elements that are required to be satisfied. It is impossible to make each one “efficient” and work in harmony with each other. It is simply not possible.

Thus the hull form, just the hull form alone, may be called efficient. But for it to be ‘efficient’ it must be pushed at the “exact” speed at that displacement /draft. For that to occur, the engine being used to push it must be at the exact power rating. One doesn’t buy motors of 68.7kW power, they are bought off-the-shelf. Therefore the motor would be 65kW or 70kW, thus the ‘efficiency’ is beginning to drop off already from being “efficient”.

Then if the displacement is not 100% exactly as predicted for that hull to run at most efficient the “efficiency” is being lost again, since higher displacement = more power required over and above the optimum for that “efficiency” label. And then if the flow into the prop is not 100% exactly as predicted, the prop is not working at its most efficient and so on and so on.

And that is just the drive train…then one can look at the living/accommodation space as another example. To say it is efficient is just another way of saying we haven’t got enough space for what we really want, so we’ll make do with what we have by making if more “efficient”. Simple language compromise, again.

The whole vessel is a compromise every part of the boat, from the hull to the structure to the outfitting to the layout etc etc. So, to call a boat efficient…what part of it is efficient…because all the other parts are a compromise to satisfy the SOR. Satisfying the SOR is all that matters unless one is designing a single objective SOR vessel, which is rarely the case, as it would be minimalistic too.

With regards displacement boats, it is not defined by its length. It is defined by the pressure distribution that surrounds the 3-dimensional shape, we call a hull. The shape of the hull defines whether the pressure shall be hydrostatic or hydrodynamic. It is conventional to describe this in terms of Froude number, which relates to the speed for a given length.

Displacement boats usually run in the Fn less than 0.40. So looking at the principal dims of Skoota 28, its Lwl is 8.30m This means to run at a speed less than 7knots to be FN = 0.40 or less. So if a hull goes beyond Fn = 0.40 is is no longer a displacement boat? It all depends upon the hull shape.

We have designed many ‘displacement hull’ vessels that run with Fn well over 1.0. They are not planning hulls in and way at all. What defines them as displacement hulls is the shape of the hull. The hull shape does not offer the opportunity for the redistribution of pressure around the hull to provide hydrodynamic lift nor does it prevent the pressure redistribution from making the hull squat excessively (traditional displacement hulls) to prevent it from going past the main prismatic resistance hump at Fn = 0.45-0.50.

Thrown into this mix which assist this is, as noted, the slenderness ratio. The higher the LD (length-displacement) ratio the less power is required to push the boat. However, it is not about being fat it is about being heavy, for its length. The LD ratio is simply = length/displacement^(1/3). So looking Skoota its LD ratio is 6.1 which is low end of a good LD ratio hull.

Very ‘efficient’ cat hulls have an LD ratio over 8.0. Which would mean Skoota to be no more than 1.10tonne, less than half the weight it is currently displacing.

 

The word 'efficient' is catching and one can often see boat capable to reach certain speed at very small power labeled as 'efficient'. In reality, such boats are empty drums without any payload, and often too optimistic weight estimates. This illusive efficiency is often a problem of amateur yacht designers and their niche clients.

Say, if one is designing a pleasure cat, to sum loaded craft mass the weights of all liquids, equipment and persons on board should be taken and there is min weight of personal items specified in ISO12217. Instead, we are sometimes asked to reduce those loads in documentation, to come up with more 'attractive' mass figures. But what is the point, if the boat can/will carry more? Say, having 'efficient' and 'light' 39' powercat with only 8 persons in the CE-certificate, though it is clear that they are taking over 20 for a day trip?

Give clear statements, fair displacement and speed figures to the client, this is what the designer should do.

 

Load of pedantic rubbish, I simply pointed out that there is an interference drag penalty at certain speeds of squeezing the two hulls in tight together. What else to growl about, Ad Hoc ? The Easter Bunny not showing up ?

 

So here is my SOR
1. Achievable speed at least 15 knots- local regulation requirement to qualify for self drive rental boats
2. Trailerability- I should be able to drop/pick up my customers at any coastal point on the island. Usual average distance 30-80 miles one way.
3. Separate enclosed toilet
4. Easy beaching
5. Comfortable sea cruising with less rolling for 4-8 pax- my usual customers are families looking for a day out boat ride.
6. To stay competitive economical initial investment, operation and maintenance- a pair of new 20hp outboards cost approx €5k with the maximum consumption 12 litres/hour and cruise at around 8-12 knots at about 5-7 l/h for both.
7. Low risk operation- hence small engines as opposed to risks when inexperienced boaters drive 200 HP. By our law, any person could rent a boat for a day ride having just a CAR driving licence.

Summarizing my conclusions ultimately led me for a catamaran with less horsepower engines (which automatically exclude planning cats) and approx to 7-9 metres boat - comfortable handles at sea and at the same time remain trailerable.
The question regarding Semi displacement v's Full displacement arose when I started to study this subject regarding advantages of one over another.
Still a bit confused about the sea handling, but it seems that speed, horsepower and consumption are very much similar

 

Rule 1 confuses me, you need to to be able to reach 15 knots to meet local regulations ? What is that about ?

 

Yes, correct, 15 knots are a minimum requirement to qualify for tourists self drive rentals. Slower boats should be crewed. The reason behind that as I see the authority don't want inexperienced boaters caught by weather at sea enabling to quickly return ashore.

 

I doubt a pair of 20hp outboards will push any boat capable of safely carrying 8 passengers, to 15 knots.

 

))) you can check Richard's blog of his adventure. His own Skoota 28 currently in a Bahamas sailed from Florida by a pair of 20 HP outboards. My boat will be lighter. I'm just weighed all the parts before assembly, something around 500 kg, + 200kg glue and paint+200 engines and fuel. Looks below 1 t. 8 persons are a max. Usually 4-6. But even 8 will add just another 500 kg. So it's perfectly in line with the design. About the engines, the installation will take place after a sea trial. One local engine repair shop agreed to temporary lend me some engines for that purpose. So if not enough, will fit paur if 25,, or even 40,, will see..

 

You seem to be going about things methodically, if you can build light and still satisfy the survey requirements, maybe light engines will do the job, but if the weight it has to test at 15 knots is higher, you may need more powerful engines, which are heavier and maybe not so good on slender hulls.

 

Yes I'm practicing a minimalist capitalist approach. Every cent I spend on a boat should generate a profit or its considered unnecessary. Things like watermaker, generator, radar and so on will not be on a list. The area of operation is restricted by 2 NM off coast. So if someone is lost at even 5 NM it will take just 20 min to return to the shore. Regarding the survey. Regardless that Cyprus is part of the EU, authorities here are practicing some amendments for the local businesses. Full documentation prepared by the local naval officer for this size self builds boat cost around €2k. They already checked my site and materials storage and agreed about the project. What I have to do is provide them with photos of each stage of assembly. Regarding the weight, for example, one structural segment- 2 bulkheads (6mm plywood) connected by crossbeam, framed and glassed is 17kg. Weighed by myself. The boat is designed with 6 bulkheads plus transom.

 

Mani Kandasa
Here is a link to a study I did about optimization of semi-displacement

Thanks Mani! Will check your report, hope to find some clue

 

Just a bit off topic. I remember my childhood when we spend all summer seasons cruising on a boat in my cold Baltic country, good memories. My father was an engineer in a diesel engine producing factory. He bought a discarded 9 m lifeboat and converted it to a cabin pleasure craft.,( He had a hobby of building small boats from his young ages). Then years later I enjoyed a sailing catamaran at a racing club. After that was a gap of 20 years of a busy life on land. Now being in Cyprus, surrounded by the warm Mediterranean sea I have the privilege to reestablish those fillings with my young son, so enjoyable!
Unfortunately, my father passed away a few years ago.And my wish to have an advice from him I redirected to a boating community. Seems like a life circle)) So I say thanks to all who participated in this discussion, placed comments, shared ideas and views. It's very valuable to me! And I believe to everyone. As when we share we grow!

 

I try to write simple explanations because many people reading threads like this don't know as much as professionals do. So, for example, I wrote "semi displacement" to imply a higher speed. Had I written "Skoota has a displacement hull" many readers would then think it was a trawler yacht capable of 6-7 knots maximum.

I also know that writing simple things might imply that I don't know more than the basics. It would be embarassing if that were true, after all I was the "multihull expert" on the committee which wrote the ISO12217 standard.

The SOR for the Skoota 28 was for a comfortable, economic coastal cruiser for two people that could be easily built in wood by amateur builders. It was not designed as a fishing boat, nor as a sexy boat to impress in "Ego Alley". More on the lines of "Drinks for 6, meals for 4, sleeps 2", just like many other cruising boats are these days.

I suspect that, like me, you only scan-read long posts, like my previous one was. So you probably missed the fact that we have cruised our boat 4500 miles. Our current cruise started in west Texas 7 months ago and we are now in the Bahamas. We do not live in an "empty drum". We have a 5ft wide double bunk with 6in memory foam mattress, a table that seats 6 comfortably and a cockpit big enough for 12 on day trips. Standing headroom throughout. Big galley and heads/shower

Nor is the boat empty. I have enough power and hand tools on board to rebuild the boat, a rigid sailing dinghy, a big inverter that runs the sewing machine and vacuum cleaner. Three computers, a 24in monitor so I can draw. We carry at least two weeks worth of food (we have a big fridge/freezer) and water and a cruising range of around 250 miles. 350W of solar power and a big battery bank powers it all without using the engines.

Since Houston we have averaged 6.5mpg and normally average 10 knots loaded, 12 knots in "weekend cruise" trim. I have a youtube video showing us doing over 16 knots in "lightship" trim and we have done 12 knots with 12 on board. That is with 20hp outboards. I have just come back from shopping for two weeks (we head back to the US shortly) With full water tanks and half full fuel tanks the antifouling line is clearly visible all round. I painted it 40mm above the designed WL so it is still floating pretty much on its marks despite its liveaboard cruising trim.

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

Let me know if you need CFD resistance calculations for your hull at 15 knots to get an estimate for the engine HP requirement.