Pocket cruisers

[SAQuestor]

Originally Posted by Guillermo

Leo,
Let's revisit WHIO (http://www.steamlaunch.co.nz/Whio.html), at the post which opened this thread:
{snip}
At 10 knots she will do 540 miles (allowing for a non-usable 10% in the tank).
What do you think about her?

Originally Posted by Willallison

Whio is an interesting and elegat little boat - but I don't think I'd like to spend more than an occaisional overnight on her....

Whio is an elegant boat. She has a wonderful ‘classic’ look while using modern methods and materials. The engineering thought and execution is, in my opinion, impeccable.

Frankly, I’d be proud to have built a boat that was half that nice.

If I lived in the same area that Whio cruises, she might be the perfect boat. But I don’t – and the areas I want to cruise a boat are far apart with extremely divergent conditions that may be encountered. So Whio’s minimalist approach, while right for the area and owner, are not right for me. And like Will, I would not want to spend more than a couple consecutive nights on her.

Best,

Leo

[SAQuestor]

After considering Guillermo’s question above, I’d like to take a bit of time to branch off and explore a similar but different path.

Disclaimer – I am not a math whiz. Never was. Never will be. But while that is true, it is also true that I can understand clues when whacked really hard with a cluex4. Hopefully someone will be gentle with the cluex4 about similitude that I’m asking for now.

Please see the attached drawing from Paul Gartside. I chose this particular one for an illustration because I like its lines, but there are many other examples available on the web that are in the same family.

19 ft Steam Pinnace, Design #132

Construction: Strip planked/cold molded. A fine, high transom stern as seen on this little launch is ideal for smaller power plants like steam engines. The extra waterline length will allow it to out-perform similar size boats of the fantail stern model. Add to this, simplicity of construction, and it is surprising that it is not seen more often.

This little boat was drawn especially for the home builder and the plans are very fully detailed. Construction is strip planking overlaid with two layers of cold molded planking for a stiff, light, frameless hull. Suitable for steam engines of 2 - 3 horespower or small electric plants.

Specification Table for 19 Ft Steam Pinnace

Length Overall 19 ft 6 in
Length Waterline 18 ft 11 in
Beam 5 ft - 7 in
Draft 1 ft 10 in
Displacement 1735 lbs

Looking at the lines drawing – it seems to my eye that this design has easy flow lines – water will easily flow around the boat and therefore requires little power – as Mr. Gartside says – 2-3 hp.

As you might remember from my earlier posts in this thread, I want a transportable boat that is suitable for various near-shore excursions of varying length – from a week to 3-4 months.

Let’s apply what I understand about the principle of similitude.

I’m going to work in millimeters, as that is so much simpler for my feeble math-challenged mind to comprehend.

I want a total beam of 2800mm (about 9’2”). So I do the simple math and it comes out that I need to multiply by 1.645 to achieve my desired beam.

Using the same 1.645 constant, that means the boat also lengthens to 9777mm (32’1”) and the draft goes to 919mm (~3’).

I seem to recall that displacement would change at a different rate - the square or cube or something… – so I’ll stay away from that for now.

So… I’ve applied the same constant to the design – in my mind that means that though the lines expand, the relationship between them remain the same. Correct?

Ass-u-ming that is so, then the horsepower requirement will go up based on increased skin friction and increased displacement. I doubt in a linear fashion, but I suspect not in a logarithmic manner either. Let’s get really extravagant and for arguments sake say, 8x or 24hp.

So for guesstimates sake, we spec out a diesel of about 32-35 horsepower for that extra bit of power for those times that unexpected adverse conditions force us to plow through wind and wave to get to a safe harbor.

N.B. – real numbers will be calculated by professionals that understand the math much more thoroughly that I do.

Regardless, we should now have a hull with sufficient volume to support the required tankage, assorted gear and amenities required for a couple to cruise for a few weeks to a few months while requiring very little power for ‘normal’ flat water conditions – rivers, canals and sheltered lakes.

By expanding all the dimensions by the same constant – 1.645 – the waterline length increases to 9484mm (31’1”) – leading to a hull speed of about 7.5 knots. But, from what little I understand about reading hull lines, it would be difficult to get too much more speed from this enlarged hull form because of the shape of the buttocks lines – the stern would drop into the ‘hole’ that the wave train generates at S/L of 1.34.

Reducing speed to a more reasonable S/L of 1.18 – ~6.5 knots, I would GUESS that the boat wouldn’t require much horsepower and would sip diesel at a very low rate.

Now… Here’s where the cluex4 comes into play. What the heck am I missing here? There has got to be some factor(s) that either I don’t understand or am missing completely, otherwise there would be more boats of similar lines – give or take a bit – being designed and built by either home builders or custom shops.

I can understand that this might not be a commercially viable idea – but why not for a transportable owner/builder one-off?

Please do not respond with simple comments like, “No, that’s a stupid idea.” Rather I’d like to see, “No, that’s a stupid idea because…”

Thanks in advance for helping with my education. Hopefully there are dozens (hundreds?) of other folks that will also gain some knowledge from those habitués that have the correct clues to dispense.

Best,

Leo

[Guillermo]

Leo,
Your idea is not stupid at all.
Those lines are lovely, derived from rowing boat's ones, so pretty efficient to motor at low speed/length ratios.

Your Lwl would go up to 9,45 m (based on Gartside's 18' 11 "). Asuming a waterline beam of 2,52 m, and a body draught (amidships) of around 0,4 m (As from Gartside's drawing, roughly), and keeping the Block Coefficient the same as for the Steam Pinnace (0,36) we come to a displacement of around 3500 kg (7772 lbs).

We need around 700 lbs/SHP for an speed/length ratio of 1.18 (6,58 kn), this calling for an SHP power of around 11 HP. To drive the hull at 7,48 kn (hull speed) we should use a figure of around 500 lbs per HP, so we'll need around 15,4 HP for that. Allowing for gear loses (3%) we'll need a continuous BHP of 16,6 HP, which brings us to an engine rated around 18.5 HP (assuming continuous as a 90% of rated HP). Let's make it 20 HP for that extra muscle sometimes needed.

I'd appreciate if you and Will could tell me what would you change in WHIO's design to make her acceptable for you to sail her for extended periods of time. An slender hull like that is pretty efficient in semidisplacement mode, so allowing for higher speeds with modest power, and also with a better seakeeping abilities than a wider hull. The derivation proposed by Leo on the Steam Pinnace, although being a lovely hull, will never be able to reasonably motor over her hull speed.

Cheers.

(P.S.: Doubling dimensions, you go up to 4 times the surface area and 8 times the volume).

[Tad]

So we have Whio at about 3000 pounds and the scaled Steamer at 8000 pounds, there's a great deal of territory in between. Whio is an elegant vessel but not too suitable as the family cruiser, more like a weekender at best.

As a half-way point I would suggest something like the sketch below. She's 28' LOA, 26'6" LWL, beam is 8'6", max draft is 2'0", max displacement is 5000 pounds. Engine is a Yanmar JH4 at 55 HP, maximum cruising speed is 13.5 knots at 2.7 gph. 9.5 knots is just over one gph. Four berths inside, a real galley, a separate head, she could in fact handle a small family cruise of 10 days or so.

Tad

[Guillermo]

Tad,
I liked the Down East 26 at your web pages. I've previously posted it at http://www.boatdesign.net/forums/sho...&postcount=148

In this new nice design of yours, what's the stern like? Is it veed with a tunneled propeller, or (what it seems to me) is it a flatted tug-like one?

Berth at companionway seems short. Does it open sideways?

Could the design be streched to 30' to get bigger room and improve performance?

Going diesel-electric would allow to take the engine from the middle of accomodation, thus allowing for a different arrangement. What do you think?

Cheers,

[Tad]

Guillermo,

The hull form is shown below, yes, the transom is veed and the propeller is in a pocket with full skeg to protect it.

The aft berths are two singles fore and aft, the feet extend under the cockpit seats. This berth can also be made up (with back cushions) as a huge double athwartships. Thus mom and dad have options to sleep together or alone, forward or aft. The boat would also be usable by two couples.

Can it be stretched? Sure, but this was an attempt at the minimum which could do what was originally requested.

Diesel-electric would certainly be possible, but it will add weight, complexity, and cost. A decent sound deadening box, flex mounts, soft coupling and thrust bearing are off the shelf items for this size. The ultimate would be a big Honda outboard way aft in a well.

Tad

[tom28571]

Very interesting interior Tad, don't think I've ever seen anything like it. Such quarterberths are common in sailboats but never thought of them like that in a powerboat. The helm arrangement looks like it might have been dictated by the engine placement. Helm vision aft looks pretty restricted.

A very nice looking boat although I have some personal issues with the unusual interior. Hey, you gotta give free rein to your pencil to ever get away from the usual. I think most of my qualms would be solved with an outboard which you probably have also considered.

[SAQuestor]

Originally Posted by Guillermo

Your idea is not stupid at all.
Those lines are lovely, derived from rowing boat's ones, so pretty efficient to motor at low speed/length ratios.

{snip}

although being a lovely hull, will never be able to reasonably motor over her hull speed.

Thanks for that Guillermo - but do you suppose that this speed limitation is the only reason that boats with this hull form haven't been designed and built of late?

Your calculations of the low HP required would seem to bring it in to the realm of practical as fuel prices rise - and rise some more.

Is there some other major (or minor) drawback that I'm unable to see?

Thanks,

Leo

[Guillermo]

Tad,
I love the lines.
2.7 gall/hour at 13.5 kn is pretty attractive. My good old 'Marie' is only able of 7.6 kn at that consumption at its best (But she's moving 13 tonnes, with pure full displacement lines, of course!)
What about her initial stability? Wouldn't she be somewhat tender?
I coincide with Tom's concern about rear visibility from the helm position, precisely on the starboard side. COLREGS state you must give way to a vessel coming as abaft the beam as 22.5º on that side.
How much cost would a diesel electric system (such as the Ossa powerlite) compare in price against the usual stuff? (only order of magnitude)

Leo,
I do not know why that kind of hulls are not more usual among boaters, but I imagine it's mainly because till now fuel has been very cheap and time to enjoy boats very short.

About drawbacks of the design for your intended use, I see a couple, at a very quick sight. One of them is some lack of rear space, due to the closing sidelines, and the other one is that probably her prismatic coefficient is around 0.5-0.53, which is low if efficiency is wanted at near hull speed. It is better to keep this prismatic coefficient high than low, as the loses due to it being high when going at lower speeds are lower than the contrary. Maybe lines should be altered somewhat to better cope with cruising needs. Probably some alterations to the freeboard should be also made to arrange for internal volume.

If those lines are suited to what you want, depends on your precise needings and would need a detailed study, I'm afraid.

[Tad]

Originally Posted by tom28571

Helm vision aft looks pretty restricted.

Well....that's nothing a couple of big truck mirrors (P&S) wouldn't solve

Actually I was thinking to use a trick I recently noted on another boat. They had the enclosed head in a deckhouse like this, aft of the helm, but installed windows in the bulkheads so normally you could look right through. Just pull the shade when the head is in use. I my proposed boat the forward window could be very small as it's quite close to the helmsman's face. This actually works quite well.

Other visibility options would include a sliding hatch over the helm so I could sit on the roof and steer with my toes. I would also have full controls in the aft cockpit for fishing and for docking.

Guillermo,

She is a small boat and will move underfoot, she will be more tender than your average hard-chine box-boat, that is guaranteed! But there is always a tradeoff, a slim fine-lined form was the primary goal, and useful interior volume the secondary goal in this case. The reason for the deep vee hull is to move the heavy weights below waterline. I would pay a great deal of attention to reducing weight up high, foam or honeycomb cored cabin sides and top, engine, tanks, and batteries are down low, overall vertical height is minimized.

I would use this hull as a test bed for various (unending actually) stabilization systems. The first one I would like to try would be fixed foil-shaped bilge fins fairly far aft, angled outboard but not extending beyond the beam or draft of the hull, with very short cord length (perhaps 2-3' at the root and tapered).

I would also like to test a flume tank on the roof, a vertical dagger board on centerline in the cockpit, pivoting bilge boards above the waterline aft and extending horizontally into the quarter wave, and maybe even some bat wings. If I was cruising the West Coast in this boat I would fit a short pair of stabilizer poles and tiny paravanes.

Just guessing I would think that a straight diesel-electric system, engine, generator, motor, controller, and installation, will easily run double the cost of an engine and shaft.

All the best, Tad

[tom28571]

Originally Posted by Tad

Well....that's nothing a couple of big truck mirrors (P&S) wouldn't solve

I would use this hull as a test bed for various (unending actually) stabilization systems. The first one I would like to try would be fixed foil-shaped bilge fins fairly far aft, angled outboard but not extending beyond the beam or draft of the hull, with very short cord length (perhaps 2-3' at the root and tapered).

I would also like to test a flume tank on the roof, a vertical dagger board on centerline in the cockpit, pivoting bilge boards above the waterline aft and extending horizontally into the quarter wave, and maybe even some bat wings. If I was cruising the West Coast in this boat I would fit a short pair of stabilizer poles and tiny paravanes.

All the best, Tad

Good luck with the bilge fins. I know of one boat they were tried on and there was very little noticeable improvement. This was an old Navy whaleboat, a noted roller.

Why not a big gyro wheel

[Guillermo]

I've tested long bilge fins in my boat (around 2 m length 0,20 m wide), but dampening is very small. And they produce annoying 'bangs' when short beam or quarter waves hit the hull when heeled.

[Tad]

Tom,

If the stabilizing gyro was also the generator in the diesel-electric propulsion system.....we might have something!

Tad

[Tad]

Bilge keels would look something like this....

[Vega]

This one has some kind of stabilizing keels. I don't know if they work, but it looks to be a nice Design. It's an Aluminium boat by Dutch Architect Firm: Ankersmit Marine .

http://www.ankersmitmarine.com/engels.html