Pocket cruisers

[Pericles]

Here is a Down Easter that will not break the bank.
http://boatplans-online.com/studypla...abin_study.htm

It would probably benefit from having flat chines added to hull, but that would increase the beam somewhat.

Pericles

[fcfc]

For SAquestor : I think Will is not fully wrong.

Running some numbers, you will find for scaling up a bluejacket 24 (empty 1850 lbs, full 2850 lbs, len 24 ft) to a 6100 lbs model,
you will need to scale it by 1.28:1. (6100/2850)^(1/3)

So that makes now a 31 ft boat, about 10 ft wide. It will weigth 6100 lbs full.
But when bluejacket goes to 15 kts, you will need to go to 17 kts to have the same hydrodynamic effect. And you will need 120 hp when bj 24 needed only 50. speed is (scale factor)^0.5 , but power is (scale factor) ^ 3.5.

An then you will have to build a 31 ft hull, 10 ft beam , with engine, batteries and cruising layout (galley, head etc ...) of about 2900 lbs.

A scaled up empty BJ 24 would weigth 3900 lbs. 1000 more.
Ligthening 25% a BJ hull is not realistic unless you go extreme building technology far beyond the intended price.

BTW, 2900 lbs is about the weigth of Whio. Seriously narrower and about empty inside. And certainly without the scantling necessary to go planning loaded at 6100 lbs without breaking at the first slam.

Without forgetting that the 120hp outboard will eat alone nearly 20 % of
your intended cost. And you will be surprised by the fuel burn.

[tom28571]

Just got back to this thread today,

There is so much in the last few posts that it is hard to know where to start or what to address. As always, everyone will read and draw their own conclusions. I also want to stress that I am not the guru here, just another amateur designer seeking the good stuff.

I think there is one area that may need some clarification. The aspect ratio relates only to the waterplane, the vertical projection of the boats bottom from its waterline, not the actual area or have anything to do with the overall beam or length of the boat. My initial projected aspect ratio was approximately 0.35. For initial calculation, I did use 65% of this area to get an estimate of the planing waterplane. Like most of the figures related to planing, this does not even remotely remain fixed. For example, look at the series of photos in: http://www.bluejacketboats.com/Bluej...4%20photos.htm

I am sorry that I have to use one of my own boats for all of this but that is what I have and know most about.

Notice that the area contacting the water is progressively less for each increase in speed. Clearly, using one number for area in calculating bottom loading or aspect ratio is misleading. As speed increases, the area contacting the water decreases, the bottom loading increases, the aspect ratio increases and the trim angle increases.

From close inspection of these photos and knowing the weight of the boat, we can estimate all these numbers. We could perhaps also derive a coefficient of dynamic lift related to the trim angle. The weight of the boat in this condition is about 2400lbs, the LOA is 24', the max WL beam is 80" including flats and splash rails, the transom WL beam is 72" including flats and splashrail. Using the photos and these numbers, we can each come up with the useful characteristics that will be far more accurate than fixed estimates.

One thing of particular interest is the trim angle. (Not considering the percentage of boat weight supported by buoyancy) The trim angle will ALWAYS adjust itself to provide the dynamic lift to support the weight of the boat at the speed it is running. Dynamis lift is proportional to the sine of the trim angle. Speed, trim angle and weight are interdependent and can not be considered alone. Confusion can come from these photos in noting that the trim angle is slightly greater at the highest speed which may be counter intuitive. This undoubtedly results from the bottom loading going up faster than the speed so that more trim angle is required to support the weight.

This can quickly become a chicken and egg discussion and there are other considerations like the CG that play a part.

Ignoring luck, all good designs start with an exaustive list of goals that have been refined to weed out incompatibilities and impossibilities. Then these goals or requirements are weighted in order of their importance to the individual design. That the resulting boats are highly varied should be no surprise and our discussions often become tangled because we may be reading from a different list. The design goals of the boat in these photos were ordered to favor low speed planing and level trim over top speed and inshore performance over offshore capability. Another thing of note in the photos is that the transom never sinks below its at rest level. Well, maybe just a little.

Other items on the list were not ignored and impacted every other item. It is called the "design spiral" for a good reason. It usually feels like there is no ideal exit point from the spiral. That is because there is no "ideal" exit, only a result that is a compromised acceptable one.

Looking at the DE25, it might at first appear that the list of design requirements were the same as mine for the BJ24. They are actually quite far apart in many details that would be very important to individual owners. Different strokes.

Scaling up a boat is not so simple. Scaling up a powerboat, particularly a planing powerboat, is not like scaling up a displacement sailboat. I don't think the usual ratios apply very well. I would prefer, for other than small increases, to start with a clean sheet of paper. I'm a little uncertain about this though.

[SAQuestor]

Will, fcfc and others, I would suggest that none of the statements you have made are incorrect. In fact, I agree with much of the content.

Perhaps I am not the “great communicator” that I fancy myself. Perhaps it is the limitations of this medium where nuances are difficult to convey. Regardless of the exact reasons, it appears that we are not on the same page when discussing a 9 meter-ish long by 2.8 meter-ish wide planing hull. If we could verbalize here, I think it would be like one person saying to-ma-to and the saying tom-at-o. Same vegetable – different pronunciation.

A couple of points. The BJ28 design looks similar to the BJ24. The construction techniques are similar. But it is a different boat, not a scaled up version of the BJ24. I hope we aren’t discussing and comparing the BJ24 design to a much larger boat. The BJ24 is a wonderful boat and attempting to increase its size by 25% would appear to be a foolish endeavor. That said, scaling an existing design up or down by ~10% seems to be within the realm of “standard practice” – at least as best as I can determine and as Pericles pointed out the Alaska Skiff of Renn Tolman has had that done to it by numerous people. And as Tom just pointed out, a fresh sheet of paper may be the best solution overall.

Next, it appears that my Excel file has been misinterpreted. I originally created the spreadsheet to attempt to determine what the maximum length of a hull could be while staying under the legal width for towing here in the US and also maintaining something close to the .35 aspect ratio.

The numbers in the file are not set in stone. They desired length or beam can be manipulated to produce an aspect ratio in the area of .35. The file is supposed to merely illustrate that one can change the beam and length of a boat and maintain something close to the .35 ratio.

The bottom area and bottom loading sections are new as of 11/17/06 and as I said in an earlier post, they are subject to change as better formulas are determined and applied. Anyone can change the formulas in the appropriate cells to reflect the .65 reduction factor that Tom used in his calculations. That should yield a closer approximation of the gross boat weight when in a planing condition. And having done that, it may not be practical to use a planing hull for those fortnights away. Willard 30 here we come!

Best,

Leo

[fcfc]

Or dreams...

My conditions (money and living place) only allow me a trailerable boat. I live nearly 1000 km away from sunny sea and I am not rich enough to remotely own a boat for a limited use during summer holidays only. The nearby sea may be OK for some week ends, but I will never spend summer holidays there.

Boats I could afford (homebuild or used, barely brand new if mass produced and certainly not one off built) would be like a scaled down andreyale 33. http://www.tofinou.com/en/andreyale10.php , shortened and lightened a bit to fit on a trailer, (around 31 ft, 4600 lbs). or like admiral tender http://www.admiralstender.nl/ , lengthened and weighted (and of course a cabin).

Now the problem:
Meanwhile dreams, I do sail. The only way for me is to rent sailboats on the sunny location . They are egg bathtubs like Bavaria 30, or so. They are among the cheapest holiday liveable thing on water to rent.

And I would like in my trailerable powerboat 8.6 ft beam, 4800 lbs displacement the same layout of a sailboat 11 ft beam and 8800 lbs displacement.
I wont spend any buck on a powerboat with only half of this accomodation. I would prefer rent a sailboat.

And for now, I can turn the problem any side, I have no found a way to cram such layout in a trailerable boat. (And owning a such a sailboat is no go since not easily trailerable)

So for me, its is still chimera for a long time.

[Guillermo]

Originally Posted by Pericles

Guillermo, your Jet Jane 25 designs are very exciting and thought provoking. The evolution from traditional pilothouse to what looks to be a streamlined sport fisher with Carolina bow flare, (also flair and great looks) is intriguing. Which will you build?

I'm working on the more agressive looking version. I hope to be able to post something else soon, but this will take its time as it is not an earning money project (for the time being) and I have only spare time for it.
Cheers.

[Willallison]

Originally Posted by tom28571

I think there is one area that may need some clarification. The aspect ratio relates only to the waterplane, the vertical projection of the boats bottom from its waterline, not the actual area or have anything to do with the overall beam or length of the boat. My initial projected aspect ratio was approximately 0.35. For initial calculation, I did use 65% of this area to get an estimate of the planing waterplane. Like most of the figures related to planing, this does not even remotely remain fixed.

Based on the specs on your website Tom (LOA 24'3", BWL 80"), I estimated your LWL to be about 23'6", or 282" - thus giving an aspect ratio of 0.28
So when you say you aimed for an aspect ratio of about 0.35, I can only assume that you did that based on some sort of shortened WL in an attempt to represent the WP whilst on the plane...?
All the info I have suggests that aspect ratio is calculated on the at rest LWL / BWL, as these figures are too difficult to accurately determine whilst on the plane.

In regards to the 65% of AWP, it would be interesting to go back now, with a series of photo's and see just how close to the mark you were. Given the level running of the BJ24, I wouldn't be surprised if the figure turned out to be somewhat higher. Though for many boats, it probably wouldn't be high enough!

fcfc - if you are a long way from the water and thus unlikely to use a boat very often, then I think there's a lot to be said for chartering. Afterall, a boat is costing you money whether you use it or not.....

Originally Posted by fcfc

Or dreams...

My conditions (money and living place) only allow me a trailerable boat. I live nearly 1000 km away from sunny sea and I am not rich enough to remotely own a boat for a limited use during summer holidays only. The nearby sea may be OK for some week ends, but I will never spend summer holidays there.

Boats I could afford (homebuild or used, barely brand new if mass produced and certainly not one off built) would be like a scaled down andreyale 33. http://www.tofinou.com/en/andreyale10.php , shortened and lightened a bit to fit on a trailer, (around 31 ft, 4600 lbs). or like admiral tender http://www.admiralstender.nl/ , lengthened and weighted (and of course a cabin).

.

Not really expensive, now we will build the new 31 ft and 40 ft from Van De Stadt design. The 8,50 (so called Admirals Tender) and the 9,50 (or 31 ft) will have different cabin solutions soon.
See http://www.classicsolution.nl/
and
http://www.stadtdesign.com/products/vds608_6531.htm
or
http://www.stadtdesign.com/products/vds608_653.htm
or
http://www.superioryachts.biz/
http://www.admiralstender.nl/

Than see also our gallery with this boats during construction: http://www.boatdesign.net/gallery/sh...00&ppuser=7697

[Guillermo]

Nice boats, Luigi. Keep us informed on the 8,50 and 9,50 cabin solutions.

Pericles,
I borrow your post on the Tolman skiffs to highlight here their interesting hull lines derivation on Sea Bright skiffs' ones.

[SAQuestor]

Originally Posted by Guillermo

Pericles,I borrow your post on the Tolman skiffs to highlight here their interesting hull lines derivation on Sea Bright skiffs' ones.

Global warming hasn't reached Alaska yet! Look at how they are bundled up. In the second picture notice the fellow holding the coffee cup by the top. His hand is near cooked lobster color from the cold.

[Pericles]

Guillermo,

My pleasure. I am guessing here but it looks like Renn is modifying his first design, the Standard 18 footer skiff that does not have chine flats on its 7'6" beam. The Widebody has 3" chine flats and the Jumbo has 4" chine flats and are absent on the model. As Tolman Skiffs are working boats, powered by outboards, could the reversed V serve to protect the propellor?

Tom, you mention that the chine flats on "Liz" taper in width from zero at the bow to 12 inches at the transom. Does that mean 6 inches per side?

A 30 footer with a maximum beam of 8'6" would be pushing the envelope for trailering in the UK. However, the DownEaster 25 http://boatplans-online.com/studypla...abin_study.htm
has a maximum beam of 8'4", a hull weight of 1750lbs and a displacement of 2800lbs. The draft, inclusive of skeg, is 14" and PPI at DWL is 600lbs. Hull is 80 hours work, with 250 + hours to completion and the plans are $165. That would be fine for the UK. Being careful with bulkhead placement would enable the DE 25 become a DE 27 or DE 28 and that 10% in length would be invaluable to those who would welcome four real berths. A 90 hp outboard would deliver 22 mph with 3600lbs all up. Perhaps even more with chine flats!

I believe I have two boats in me and am house hunting for a somewhere with the space and access to build them. Here is one possibility, http://www.swalemarina.co.uk/ and here is another, http://www.ironwharf.co.uk/.

The first boat will be along the lines of Guillermo's original proposal on 13th May, but 10% longer, as trailering is not an option. LOA 33' BOA 12', Bow deadrise 24 degrees, midships to transom deadrise 12 degrees, chine flats zero at bow running back to 9" per side at transom, with 5 degree negative angle. Hull bottom 1" and hull sides 3/4". Superstructure is Paraglass 22, sandwiched between 1/4" Meranti. http://www.univar.co.uk/downloads/Pa...for_yachts.pdf

Construction in Epoxy/Meranti BS1088 using Sam Devlin's stitch & glue/cold moulding techniques. "West" 5-1 epoxy minimises print through compared with 2-1 epoxies. Power looks to be Volvo D3 190 hp with Aquamatic Duoprop. Fittings from Vetus. Awlgrip finish. Built to comply with RCD. NMEA 2000 with Octoplex cabling.
Four proper sized berths with decent facilities and the crew can sling their hammocks in the engine room.
(Ahaa Jim m'lad, it's a great life in Nelson's navy!)

Although I have been planning since early summer, sketches are still just scribblings on sheets of A4. When they are decent and I figure how to post them here, I'll do just that.

The second boat? A Viking longship, so I can travel to Valhalla. Cast adrift on the open sea and set on fire by a shower of flaming arrows. Pretty spectacular.


Pericles

[tom28571]

Originally Posted by Willallison

Based on the specs on your website Tom (LOA 24'3", BWL 80"), I estimated your LWL to be about 23'6", or 282" - thus giving an aspect ratio of 0.28
So when you say you aimed for an aspect ratio of about 0.35, I can only assume that you did that based on some sort of shortened WL in an attempt to represent the WP whilst on the plane...?
All the info I have suggests that aspect ratio is calculated on the at rest LWL / BWL, as these figures are too difficult to accurately determine whilst on the plane.

In regards to the 65% of AWP, it would be interesting to go back now, with a series of photo's and see just how close to the mark you were. Given the level running of the BJ24, I wouldn't be surprised if the figure turned out to be somewhat higher. Though for many boats, it probably wouldn't be high enough!
....

Will,

The waterline length is actually about 22' 10" but that is not the main difference between your figures and mine. The main difference is in the method of measurement. Your number applies to "beam/length ratio" and mine is based on Lord's method of calculating "aspect ratio". These are different concepts and the aspect ratio takes the geometric shape of the waterplane into account. I used a method similar to the one Leo posted in post #253 on page 17 of this thread. I think it is a more informative look at the actual bottom shape since it takes long skinny bows and narrow transoms into account and B/L does not. This method gives an aspect ratio of 0.35 for BJ which is not surprising since I forced it to be that way.

Trying to measure the aspect ratio from the photographs is not at all easy since the spray hides the actual waterline and the viewing angle is not perfect in all the photos. You are certainly correct that the aspect ratio will increase as the speed and trim angle increases but I cannot say accurately just what it is. In very high speed small planing boats when the hull is almost out of the water, the aspect ratio can become quite high, even 4.0 or more.

[Guillermo]

Originally Posted by Pericles

As Tolman Skiffs are working boats, powered by outboards, could the reversed V serve to protect the propellor?

I'm not sure about their intention when adopting those lines, but probably they are looking for better efficiency when planning (The inverted V sections look like an extreme version of Peter van Diepen's 'Flowmocean' lines) and maybe increased grip in the turnings, but we should ask them. I would like to know how this hull performs whith waves.

Hope to see your sketches soon.

Cheers

[tom28571]

Originally Posted by Pericles

Guillermo,

Tom, you mention that the chine flats on "Liz" taper in width from zero at the bow to 12 inches at the transom. Does that mean 6 inches per side?

Pericles

I missed this one earlier. The aft sections are monohedron. The chine flats are designed to be at a positive 1 degree angle of incidence relative to the aft buttock lines. Therefore the width of these flats increase going aft and they are 11 1/2" in width at the transom on each side. The transverse angle of these flats is about 2 1/2 degrees cathedral (down). The objective was to create extra lift to the aft bottom where the lift coefficient decreases on most boats. My thought was to make low drag trim tabs in the effort to get the boat onto plane early. The fact that the stern never sinks (as the photo series shows) and the longitudinal attitude is very stable indicates that these flats are at least partly responsible for this desirable behavior.

This was my one original contribution to the design of this boat. All the rest was following the guidance of other designers I studied in books. Of course, the idea may not be completely new since so many innovations have been tried in the past and never made it to the mainstream for all kinds of reasons.

Originally Posted by Guillermo

Nice boats, Luigi. Keep us informed on the 8,50 and 9,50 cabin solutions.

.

Here the new Classic Solution 9,50 with sport cabin (under construction) and classic cabin (near future). In picture 850 Admiral Tender under assembly: