Pocket cruisers

If you want an economical boat, then what you want is not a planing hull, but a semi-planing hull, but a boat like the Andreyale 10m, designed by Joubert-Nivelt. This boat weighs only 2500kg and can make 15 Knots with a 60 hp engine, but the hull does not plane. Kind of cuts through the water. They should use a more powerful engine, because that speed is at 3000rpm and that is the max rpm of the Nanni engine they use, but not much, I guess that the optional 75hp would be fine.

But this boat, even bigger than the one you are talking about, does not gain in having a bigger engine than that. It is not a planing hull and the power would be wasted.

I Guess that with a 8.5M boat you can get around 13 knots (cruising speed) with a 60hp engine, wasting only about 7L/hour.

Nigel Irens, that has designed several boats (the Range series) with a hull that works the same way, explains what I am saying:

“As the performance of these semi-displacement hulls is very much linked to their waterline length, it follows that the bigger boats are more efficient for a given speed. That said, it is important to understand that this hull form should not be used if cruising speeds in excess of a speed/length ratio of about 2.5. are required - in this case 18.4 knots.(i.e.2.5 x the square root of the waterline length in feet). It would not be practical to install more power than is necessary to achieve a speed/length ratio of about 3.5.”

This means, that for being efficient, regarding fuel consumption, an 8.5 M boat designed with the kind of hull you are talking about, should not have a max cruising speed superior to 13k and it would be useless to try to go faster than 17.5K (even with a lot more powerful engine), at least according to Nigel Irens, and it seems to me that he knows what he is talking about

I would go for it. A speed of 13K, wasting only 0.6L/mile seems a good deal to me

 

What kind of hull am I talking about?

 

Humm, have you forgotten?

Will has answered:

We are talking about the boat you have suggested, a “A plumbed bow boat, with 8.5 m (28') Lwl, 2.5 m beam, 2500 kg loaded displacement …for an 18 kn ride.” A boat with“Relatively high speeds and a low displacement combination make possible to reach pretty long distances”, obviously a relatively fast boat with a low consumption.

Willallison has suggested a smaller deadrise. It looks that to be really effective it should have even a smaller one, at least if we are considering the successful experiences of Nigel Irens and Joubert-Nivelt in creating relatively fast boats with very low consumption (they are the ones that have managed to better comply with that objective, if we exclude cats, at least to my knowledge).

Now it is my turn to be confused…. What do you mean, isn’t your objective speed with very low consumption?

 

Desired speed with lowest possible consumption, yes. I've never said I wanted to go into the semiplanning mode. That's your assumption, not mine.

 

You were talking about a boat with 8.5m with a beam of only 2.5m. I thought you were talking about a slender and light boat. I thought you were considering what Nigel Irens had said about it, regarding speed and fuel consumption in a slender and light hull.

The link to this paper has been posted by fcfc long ago and it is very interesting:
http://www.nigelirens.demon.co.uk/images/NigelIrens.pdf


‘THE APPLICATION OF SLENDER HULL TECHNOLOGY IN POWERED YACHTS AND SMALL COMMERCIAL CRAFT’.

Nigel Irens Design

This paper sets out to examine the role of slender-hull forms in the current powerboat market (both leisure and commercial), and suggests how that role might change in the future.

Although fuel prices are currently on the increase fuel is still a relatively cheap and available commodity but that the availability of suitable berthing facilities (mainly marina berths) is not. This has encouraged the development of short planing-hulled vessels.

Whilst these are usually fuelefficient at high speed in reality they may often be operating at low to medium speed - often because of adverse sea conditions. At these lower speeds their performance is often unsatisfactory
and inefficient.

SLENDER HULL’ DEFINITION:

In the context of single-hulled vessels the term ‘slender-hulled’ is used here in the absence of a more accurate generic term to describe any vessel that reaches its design speed with little or no dependence on dynamic lift. The following parameters describe the nature of these hulls:

Design speed would normally be in the region of up to 2.0 x _dwl(ft) although 2.5 x _dwl(ft) would still be realistic as a maximum cruise speed – at which point there would a degree of dynamic lift would be present.

There is, therefore, no pretence that slender single-hulled vessels
can be as suitable for high speed operation as dynamically lifted hulls.

Favorable displacement/length ratios are at the heart of a successful non-dynamically lifted vessel. Typically they will need be less than 100. [where DL = Displ. (long tons) /(0.01 x dwl )³ ].

.....
…it is only during the 20th century that the search for higher speeds has lead to the inevitable use of some kind of dynamic lifting technology to break free from the clutches of excess wave-making drag and, ultimately, skin friction – the Achilles heal of the slender hull.

As operating speed/length ratio increases it is well established that there comes a point where at least some degree of dynamic lift is necessary to avoid the need to apply exponential amounts of power.

The point at which this dynamic lift becomes essential depends very much on the configuration of the vessel in question; if it has a favourable displacement/length ratio, for example, then a nonplaning or semi-planing hull form can be driven faster without incurring excessive drag penalties but with only modest amounts of dynamic lift being generated.

To some extent favourable (low) displacement/length ratios may be achieved simply by using lightweight building technology but it is more likely to be the result of a fundamental design decision to cut down on both the living volume and complexity of its on-board systems ..

There are several areas in which slender- hulled vessels excel:

a) The dilemma with any planing-hulled vessel is to choose a dead-rise angle that is low enough to allow planing to occur at relatively low speeds (and reasonable power) and high enough to limit the hull’ s propensity to slam when powering to windward.

The sections of a slender-hulled vessel can be as deep as necessary to promote a more gentle upwind ride as the hull is not being asked to provide dynamic lift.

The most sea-kindly planing hulls usually have high dead-rise angles but, in consequence, high installed power requirements.

If they are designed to be able to make way fast upwind in almost any conditions (e.g.lifeboats) then they will also be heavy – which will again call for more power.

b) A relatively slender-hulled vessel can provide a good quality of ride upwind as a result of being long enough to span wave crests, ...

c) Planing-hulled vessels can be difficult to manage in following seas; by their nature the half-angle of entry of their waterlines is often very high – especially above the chine. If powering downwind in strong conditions there is a danger that such a vessel will plane into the back of a wave where it will decelerate very rapidly. As the mass of the engines is often
well aft on a planing-hulled vessel there can be a tendency for the stern to try to overtakethe bow. This is aggravated by the fact that the rudders - which are usually designed tooperate in the back-wash of the propellers – will lack area when the power is suddenlytaken off .

The result is that directional stability is difficult to achieve and progress can beslow and laborious at best - and dangerous at worst.

Slender hull drawbacks:

.. many marinas charging an increased tariff per metre for vessels over certain threshold lengths.

There are also other financial burdens to be born by the owner of the longer vessel: In the following comparison between a standard planing-hulled and a slender-hulled a common nominal displacement of 5.5 tonnes results in the planing-hulled vessel having a length of 10.20 metres against 12.00 metres for slender-hulled model – an increase of 18%.
…
In general terms the planing powerboat has a shape that is ideal from an accommodation point of view, the .. compared to the slender-hulled equivalent model ..

……………………………………………………………………………………………….
CONCLUSIONS

The first and most obvious conclusion that can be drawn ...is that high speed is an expensive option – no matter what hull-form is chosen.

Non-planing or semi-planing RANGEBOAT-type hulls are very dependent on waterline length if they are to produce really convincing performance figures. ..

The reality of the experience with operating the RANGEBOAT is that the vessel settles to a comfortable and sustainable speed of between 13 and 15 knots. The chart shows that fuel economy at those speeds is satisfactory. ..

Every effort has been made to reduce noise levels and the objective is to make it possible to use the vessel in much the same way as a sailing yacht is used, so that passages of 100-200 miles could be realistically undertaken .
.....
Future trends in fuel cost are unpredictable but likely to be more volatile than they have been in the past.
…….
The real question ..is : what proportion of the time spent underway in the average planing-hulled leisure powerboat is spent at the high speeds for which the hull was really designed ?

Figures may not exist to answer that question, but realistically it appears that high speed operation is very often limited by sea state.

The strongest argument to support the use of slender-hulled power vessels must surely be that they offer a pragmatic solution to this problem. Like their planing counterparts reality will dictate that even their lower cruising speed will often be further reduced as a result of adverse sea conditions, but they are fundamentally well adapted to performing in a seaworthy and safe way at any chosen speed.

 

It's all pretty simple really. If you want to do 18 knots with a 28' LWL boat then you are better off with a planing hull - though having said that, I've been aboard boats that are most certainly of semi-planing hullform that are still quite economical at up to about 20 knots.
The secret to sensible planing characterisitics - and by that I mean the ability to remain comfortably on the plane at lower teen speeds, is to keep the bottom loading as low as possible (Tom where are you!?! ). Off the top of my head bottom loading should be kept to about 250 kg/m^2 (of the waterplane area) and the power to weight ratio is ideally no more than about 25 kg/hp

 

Of course. What I am saying is that in a semi-displacement 28ft hull, the kind Nigel Irens is talking about, you can go at 13 knots wasting half the fuel that you will need to go at 18k in a same size planing hull. And has Nigel has pointed out, you can only do 18knots in a planing 28ft hull with almost flat sea. If you have waves, probably the semi-planing hull is going to manage to go faster than the planning hull.

 

Vega - your post appeared whilst I was in the midst of typing mine, but it furthers the point:

With a LWL of about 28ft, Irens semi-planing form (and that's really all it is) is good to about 13 knots. If Guillermo wants 18, then he's better off with a planing hull. But of course there are the compromises that go along with this...

 

Ha - we are cross posts again!
You are quite correct Vega - and that was the compromise I was talking about. In general, planing hulls become less useful once you enter the realm of semi-planing and particularly faster displacement speeds.
But by keep weight low, trim correspondingly low (as for instance Tom has done with the Blue Jacket) a planing hull can be almost as good at lower speed as a semi-planing form, with the advantage of being able to travel faster when conditions permit

 

For Guillermo:

The power prediction I used is simply formula taken from a book. (Crouch I think).
For 18 kts,as the others said, the hull would probably be planning. So the requirement of the maximum possible waterline is not strong. So you will have a bunch of small planning boats, like the seaward, the sciallino, and even small antares like the 760. Note the antares 760 is category B4 too. But one problem is all theses boats need some planning surface, and all are wide, above trailerable limit.And weigth are also a bit above euro trailerable. Not by much, but above.



So that why I though of a non planning boat. Say lwl= 9m, Bmax 2.5m, cruise weigth 2.8 tonnes, empty weigth 2 - 2.2 t , cruise around 12 kts.

But I hit the following problems:

I computed Fn= 0,65 and Fnv=1,66 Clearly characteristics where you do not want to operate a hull. You need to be very narrow and ligth to go there. If I want to follow N Irens indications, I must have D/L better than 100 and lwl/bwl better than 4.5.

So reworked to lwl = 9.3m, bwl=2.06m, D=2.8t, L/B = 4.5, D/L = 100.

Now, the funny things start:

Cp should be around 0.65 - 0.7. Cm can be estimed to 0.6 - 0.65 (round chine) . This give a maximum canoe draft of 35 cm. This in cruise conditions. Waterplane area around 13 m². In light condition, should be 5 cm higher (wild guess) . So light canoe draft 30 cm.

Bare minimum engine heigth = 60 cm.
Bottom hull thickness + some space + engine + some space + soundproofing thickness + floor heigth = 80 cm at very minimum.

That means if you want to have the engine under a floor, that floor must be 45 cm above WL. ANd the cabin roof 2,25m above WL if you accept 1,75m headroom + 5 cm roof thickness , all this for a 2,06 m Beam at water line.

I do not think it is possible to have the engine under a floor like the planning hulls seaward, sciallino or antares. Theses hulls can do this because they are a bit heavier (I guess 3.2t, 3.5 t in cruise condition) , much shorter at LWL (guess around 6.5m) and lower Cm(V bottom with chines just under WL Cm +- 0,55). This give them a 25 cm deeper hull although wider at wl. (guess 2.5m)


The same for the head position. If the max draft is 35 cm, you will have hard time to put a floor more than 20 cm uder the WL.You will need to have a roof top 160 cm above WL. If you have 1m freeboard at that position, you would need a 60 cm hight roof. No. But you cannot add too much freeboard, since at one third of the length, the beam at waterline will be around 1.2, 1.5 m.

So you will have to go to layouts like the commuter27, but with an engine box somewhere, the engine box size is likely to be 80 cm len, 65 cm width, and height above floor 65 cm (Extra minimum dimensions), if floor 20 cm under wl. Or like the motorlaunch 40, without the aft cabin, with a two level roof (but, no standing room in the front roof) , and narrower. It is likely to have no more than a 30-40 cm passage each side of the engine box. 40 + 40 +65 is 155 cm wide. I remind you are 20 cm under WL, And Bwl is 206 cm. And then on the side of this passage, you can only have a shelves, seats, or narrow galley. No berths or head, unless you block one side passage. Remainder width to max beam is 45 cm each side.

 

Excellent post, fcfc (By the way; May I have your name, please?)
Too tired today to work on the boat, I'll be back to it on weekend.

Just a moment more to tell all of you about a most interesting 100 knots concept yacht that is being developed by Donald L. Blunt and Associates (http://www.dlba-inc.com/) (Somewhat bigger and faster than my humble 28 odd boat!). Her main data (lightship condition) and parameters are:

LOA: 46.00 m
Lp: 39.10 m (Chine length)
BOA: 8.10 m
Bpx: 7.10 m (Maximum chine beam)
Bpt: 6.04 m (at transom)
CAP: 19.71 m
Ap: 242.63 m2 (Planning area enclosed by the chines)
Beta Transom: 8.00º
Beta Amidships: 20.38º
Design Speed: 100 kn
LCG: 16.51 m
Displ: 200 MT

CAP/Lp : 50.4%
(CAP-LCG)/Lp : 8.2%
LCG/LOA: 35.9%
LCG/Lp : 42.2%
Bpt/Bpx : 0.85
Lp/Bpx : 5.507
Ap/VOL^2/3 : 7.212
Lp/VOL^1/3: 6.741
Fnv : 6.825

Propulsion power: 36,077 kW (!)

Good night, everybody!

 

Far be it from me to level criticism at one of the current greats of marine design - but if I'm going to do 100 knots, I'd like windows big enough to see where I'm going!

 

I think the post of Nigel Irens is full of good reasoning.

I have not followed this thread for a few days and you guys have gone far and wide. Frankly, I am not convinced that a reasonable cruising boat of 28' LOA can be made to get 18kts with good fuel mileage and not be planing. That is a monohull, of course. To get that speed, a non planing hull will need a high L/B ratio. A sufficiently high L/B ratio to achieve that speed in non planing mode will likely be so narrow in beam that stability will be serious concern.

I believe Weston Farmer achieved this speed in his Coyote, but this was a lightly built open launch which he admitted was pretty tender. He had modified an even narrower hull to a bit more beam in which he said that the operators should have their hair parted in the middle for proper ballance.

Will, it was after studying this option that I decided to try the light weight planing route. Since I am willing to forgo much of the stuff aboard that many might consider necessities, I was able to get a boat with roomy accomodations in 24' LOA and still get a bit less than 2 gal/hr on cruises running at various speeds from 6 mph to 23mph and probably mostly at about 13mph. The aspect ratio is about 4.5 although less than that at higher speeds when the forefoot is out of the water.

Of course, I also cannot bash to windward in a high chop at high speed. I was just not willing to compromise all the other economic and trailerability attributes to get that ability. It has not been a serious problem.

My bottom loading is always less than your 250KG/M^2 though.

WHIO comes close to some of the performance discussed here but is a planing hull and has far less cruising comforts than even my boats.

 

Also very quickly in the morning, before going to work:

To keep interiors uncluttered there is the possibility of going outboard. To avoid using petrol, I was thinking about the JetPac thing. They have 150 HP and 200 HP diesel units, being both based on the same International Engines HS 2.8 lt, 4 cil. engine. The 150 HP version runs at 3600 rpm. (http://www.swordmarine.com/) I have to check out consumption.

8.5 m LOA (so plumb bows) plus 1.5 m JetPac length brings us to 10 m, which should not be exceeded, just to 'accomodate' the boat in a 10 m place at marinas. I think also 8.5 m and 2.5 MT is an absolute maximum for the JetPac.

JetPac weights a dry 428 kg 'on air' but it floats around 150+ kg out of it, depending on stern deadrise (Maximum 20º), so net weight would be in the 250-285 kg region with the proper deadrise.

What do you think about this possibility?

Precisely.

Well, I find the concept interesting from an NA point of view, but from the point of view of a boater I would ban those things out of the water! (I'm not even so sure about the 20 kn thing...! )

 

This is my own evaluation:

Inboard diesel, straight shaft: engine likely in the cabin.
Pros: weigth centering, rather ligth, electricity generation (no problem to have 100A), hot water generation, reliable.
Cons: space used if cannot be put under a floor, heat and noise in the cabin. prop draft if big prop for efficiency.

Inboard diesel, V shaft (ZF15 gearbox): engine in the cockpit, rigth after the cabin. (see rangeboat)
Pros: weigth centering, rather ligth, electricity generation, hot water generation, still reliable.
Cons: price, mechanical acces to shaft & stuffing box. Not too many angle available. Still some layout problem. Need a side access to the cabin. prop draft if big prop for efficiency.

Inboard diesel, Sail drive: (same as V drive)
Pros: weigth centering, rather ligth, electricity generation, hot water generation.
Cons: Propeller placement,efficiency unknow .Still some layout problem. Need a side access to the cabin.
I think it need more evaluation.

Inboard diesel, stern drive: (engine full aft)
Pros: electricity generation, hot water generation, still reliable. No layout problem.
Cons: heavy aft weight, efficiency unknow (small prop, loss in transmission), complexity, price (nothing specifically made for "small" power).

Outboard gasoline:
Pros: No layout problem, simplicity, price.
Cons: some aft weight, No electricity (or very few: hard to have above 20A), no hot water. Some problems with safety reliability and efficiency.


This is hard to objectively evaluate. My instinct would say straigth shaft, offset gearbox, kind of homocinetic coupling (aquadrive, python ..) for some angle. This would be the less trouble system, good efficiency from a mechanical point of view and decent price (although much more than an outboard) . But there is the space/layout problem.
The other option would be a single 60 hp outboard, 4 stroke, kind of high thrust/big foot for heavy boats. No space/layout problem and cheap, but I think the biggest problem for a cruise boat, before safety and seaworthiness, would be battery charging. At anchoring, you can run half an hour an inboard diesel at 1500 rpm with an additionnal alternator to give 100A. Running an outboard at 2000-3000 rpm some hours because you get only 10-15A would be less acceptable by the neighborhood.


For the jet pack, I fear it remove any trailerability. You have more than 400 kg at the rear end of your boat on you trailer (For comparison, a yamaha T60hp high thrust is 113 kg), so I guess it is not possible. So you have to remove your pack before trailering. The only way to remove it is with a crane. And where do you put it in your trailer or car ?.