Cheap, Capable Cruisers II

St. Pierre Cruising Dory

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• 28' LOA
• 23' LWL
• 8' 4" beam
• Weight 4000#; Displacement 6000#; leaving one ton for goodies. (This may be too light; Jay Benford lists his 26' dory, a foot wider on the same LWL, at 8000# Displacement and D/L 290).
• Enclosed volume: ~ 650 cu.ft. This is a rough calculation, but is probably accurate to within 10%. So we're being pretty crude, though at this stage it isn't a problem.
• Total hull surface 477 sq.ft. including the deck.

This type boat is deep-water capable, and is certainly buildable in plywood with the same skill level and techniques as a Wharram cat.

Catamaran

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Second example: 28' Wharram-inspired catamaran (but not a Wharram design), with mildly curved hull lines, tombstone transom and 2' draft, with a vortex keel. As drawn we get:

• LOA 28', LWL 24'
• Beam hull 60", BOA ~15'
• Dry Weight ~1700#, figured at 2# per square foot surface area, a reasonable figure for this sort of boat
• Disp 3950#, giving us our one-ton carrying capacity well in hand.
• L/B 10.4/1
• Wetted surface 145 sq.ft.
• 836 sq.ft. skin area incl. deck (includes both hulls).
• 738 cu.ft. enclosed volume.

As to the proas, I've illustrated three different approaches, trying not to exaggerate in any particular direction.

Cruising Proa I

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You can see that this is essentially an expansion of the 31'er we were discussing earlier:

Probably the quickest to build of the three, and could be made even simpler with a plain deck line and sitting headroom. I've not shown, but would add, a lee platform with enough flotation or enclosed volume to ensure the boat would come up from a knockdown on its own.

On this sketch I've illustrated a different way of handling the reserve buoyancy of the log. It is now well understood that by reducing the reserve buoyancy- in other words, by making the log as near to neutral buoyancy as possible- that the ride of the boat is considerably improved, stress is reduced, beam seas & motorboat wakes do not shake the wind out of the sails, etc.  But at the same time you want to have enough reserve buoyancy so you won't have to worry about a windward capsize in a backwind situation. Solution: move the buoyancy to a separate float located higher on the outriggers. This has the added benefit of making it extremely difficult to capsize the boat to windward, as now the center of buoyancy will be higher than the CG, allowing one to roll past 90 degrees and still retain positive righting. I've drawn it with round sections though it could be any reasonable shape.

We can also make good use of the buoyancy structure for storage. In this case we are using it as a housing for a huge deck tent, so we'll call it an "omar" (after "Omar the Tentmaker"). When going offshore, the omar is sealed with waterproof tape. The example here has  2770#  buoyancy.

• LOA: 38' 7", LWL: 35' 5"
• Beam hull: 5' 7"
• Draft: 2' as with the catamaran above.
• LWL/BWL hull: 15/1.
• Disp main hull: 3250#, log: 1000#; total = 4250# with the ton capacity well in hand.
• Wetted surface;  hull: 113 sq.ft.; log:  52,  total = 165 sq.ft.
• Enclosed volume:  563 cu.ft. main hull, 22 cu.ft. log, 43 cu.ft. omar, total = 628 cubic feet.
• Surface area, main hull   621  + 70 log + 99 omar = 790 sq.ft., giving a dry weight of about 1600#.
• Log total submerged disp = 1400#

Cruising Proa II

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Version 2  has a John Shuttleworth style above-waterline step to increase interior volume on the same length of hull, so we'll call it a "step proa:"

Essentially, here we are adding interior volume to the first version by making the hull fatter. The step adds some 121 cubic feet to the enclosed volume to the hull, an increase of 21%, at the expense of a gain in surface area of 57 sq.ft., or 9%. That's not too bad.

It wouldn't be very hard to add a Tiki 26 type folding deck shelter between the beams. It could serve as a pilothouse under way, or room for extra bunks when stopped.

• Disp hull 3250#, disp. log 1000#, total 4250# with at least a ton carrying capacity.
• DLR, hull = 32, log = 18.
• Hull LWL/BWL 14.8
• Hull beam 6' 7"
• Enclosed volume, cu.ft:  hull 684 + 68 log = 752 cu.ft. total.
• Wetted surface;  hull 116 sq.ft., log 58, total = 174 sq.ft.
• Surface area, main hull   678  +  163 log  = 841 sq.ft., giving a dry weight of about 1700#.
• Log total submerged disp = 4400#. This time we've shown a full 100% buoyancy log.

I've given this boat a Gibbons rig of 450 sq.ft., for a B# of about 1.3. With the displacement and distribution as shown, this means the log will lift out in winds in the low 20 knot range, which is quite low for a cruiser.

But, with such low DLR #'s and such a large log, there's the possibility of adding water ballast. And this brings up the old question of how heavy a log you really need, if the boat can come up from a knockdown and you can add extra ballast when it is called for. There's a long list of advantages to keeping the log light. The overall displacement is reduced, B# improved, you can fly the log in quite light conditions and save a lot of wetted surface. Reaching and running performance is improved. And when you are on a beat and the wind comes up you simply pump in extra ballast as needed; why carry it around all the time?.

Cruising Proa III

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Version 3 shows that another approach to getting interior volume on a long, narrow hull is to move the volume out of the hull and into a deck house (or houses). That is what the Micronesians did, and is illustrated here.

So we get a very narrow and structurally simple hull; with more careful design this could allow for speedy construction. The idea here is to strip-plank the hull, finish it, then turn it over and stitch-and-tape the already built house to it.

The displacement of the log has dropped to 800#, and to compensate we've increased the CL / CL beam from 18' to 21'. I think there's capacity for a bit more beam if you want it. An 800# log on a 4300# boat is about  a 19% ballast ratio. That's cutting it pretty fine, and I wouldn't want to push it that hard without some good calculations backed up with model tests. But on this sort of boat, weight can be easily moved to the windward end of the deck house, and since the log is big enough to carry quite a bit of extra ballast if needed, this could be a good place to use a minimal-weight log.

Probably the log won't need to be artificially heavy, since the house itself contributes righting moment by its position. Additionally, if the windward end is sealed, the house serves the same self-righting function as the omar.

To compensate for the wider beam we've taken advantage of the deck house structure to add on a simple swing beam setup, which should cut the beam to about 16', allowing the boat to be berthed in a normal width 40' slip. There is also a small cockpit at the mast base, so the crew will be less exposed to bad weather.

• Disp hull 3300#, disp. log 800#, total 4300# with at least a ton carrying capacity.
• Hull LWL/BWL 17.6
• Hull beam 39"
• Enclosed volume, cu.ft: Hull + deckhouse = 669 cu.ft. + 35 log = 704 cu.ft. total.
• Wetted surface hull 111 sq.ft., log 63, total = 174 sq.ft.
• Surface area, main hull   778  + 105 log  = 883 sq.ft., giving a dry weight of about 1800#.
• Log total submerged disp = 2230#.

I put a ~ 450 sq.ft. settee rig on this boat to contrast with the Gibbons on the step proa. The settee has the advantage of a lower center of pressure, in this case 18' versus 20', for a 10% reduction in moment arm. That's a problem for the Gibbons- it has a fairly high CE, though the flip-side of that is the sail has more area higher in the air to take advantage of the velocity gradient.

In comparing the deckhouse proa to the shuttleproa, we note that the former needs more material to build, and that if you added a small pilothouse to the shuttleproa it would have substantially more space for the same weight. But with the deckhouse concept you are not as confined to the narrow hull, and the accommodation can benefit.

Michael Schacht: The shuttlestep is interesting, adding a good deal of room, in the most needed dimension, for little material increase. But I would guess that it signals at least an extra 100+ hours construction time. I wonder which I would prefer: the extra hull beam, or a deck cabin? I think I would like the deck cabin, but it does add considerably to windage.

J: Yes it does, and high up, though historically that arrangement can be found on a lot of proas, so it not likely to  be as problematic as first appears.

Let's see how these boats are "similar," keeping in mind that accuracy better than 90% is not expected, and that none of these sketches are real boats. As to enclosed volume:

• 28' Dory, enclosed volume = 650 cu.ft.
• 28' cat, encl vol. = 738 cu.ft.
• 38' Proa #1, encl vol. = 628 cu.ft. (hull & omar = 606)
• 38' Shuttleproa, encl vol. = 752 cu.ft. (hull only = 684)
• 38' Deckhouse proa, encl vol. = 704 cu.ft. (hull & house = 669)

First off, we see an amazing similarity of enclosed volume, with the cat being a bit higher and conversely with probably more unusable volume (in the ends) than the dory. The proas- if we allow that 20% of the log volume is usable (say for anchor/line storage, etc.), come in a bit smaller than the cat, and proa #1, despite the extra 43 cu.ft. in the omar, is even smaller than the dory. The cat clearly "wins" on absolute volume, though the dory's volume envelope is more flexible and therefore more useful.

M: OK, I buy that proas have the worst interior. In fact, without some sort of bridgedeck structure or enclosure, I doubt if a cruising proa could be made a viable option in the West. Even the Micronesians added them on.

J: I've been thinking along the same lines, wondering what is the best way to do it. Do you need the deck house at all times, or not? That is a major point. If you want it always, then it has certain fixed dimensions required by the human body. If you want it only when parked, then it could be collapsible like Wharram designed on the Tiki 26, a pop-top tent-like affair, or perhaps a hinged arrangement like you originally had planned on Roz.

You could even consider some sort of elaborate insulated or double-wall deck tent, possibly with rooms, equipped with good folding furniture, for use in harbour, making do with tight accomodations at sea. Just make sure it is a snap to set up. The house doesn't have to be on the windward side, either, if you don't need it full-time (maybe even if you do, but I don't think that the Caroline Islands style of the larger house on the counterpoise is going to work if you need to go to windward more than slowly and occasionally). But you could center it over the hull.

M:The old Corsair F-27 had a pop-top. It worked OK, and gave standing headroom in the cabin. You could even sail with it up, if you wanted to. I think the pop top is fine for a smaller proa (Under 40' is small in a proa, I'm learning!). I DO like the Wharram deck pod concept. Not a full fledged saloon, but more a cozy helming station with a berth or two, to keep out of the elements during passages.

I am also a fan of a recent concept in powerboat design: convertible cockpits. People are realizing that the cockpit is where you want to be in a boat. It has the best view, it's where the action is, you have good air and sunshine, etc. When I go powerboat cruising with friends, we always hang out in the cockpit, even though there is a perfectly good saloon below. In the evening, we put up the canvas top, and put on the side curtains, and it makes a nice little "room" with a view. The saloon below is pretty much wasted space. Now they are making powerboats that have an electric hardtop, and slide down side windows, so the cockpit can be either indoor or outdoor at the touch of a button. While of course the electrics are inappropriate for a Wharram-like proa, the concept is not. If a proa is designed with a nice social cockpit, then it can also double as the saloon, if it can be enclosed in inclement weather and at night. Then we can devote space below to berths, the head, and the galley.

J: It would be really interesting to see a "proanized" work-up of the convertible cockpit. That might be a big reason to go with a deckhouse proa concept. To a large extent fixed furniture in a boat causes problems if some use for the space arises other than that assumed by the designer. Convertibility could be a big plus, and as the proa boom does not sweep the deck, there could be possibilities here that don't exist for catamarans.

Now, on to surface, or skin, area. This is a good guide to building time, all else being equal.

• 28' Dory, surface area: 477 sq.ft.
• 28' cat: 836 sq.ft.
• 38' Proa 1: 784 sq.ft.
• 38' Shuttleproa: 841 sq.ft.
• 38' Deckhouse proa: 883 sq.ft.

The catamaran and the proas have nearly identical skin area, and since we know as a matter of experience that they will take about the same thickness of skin, we'll say that from a structural point of view that the 28' cat and the 38' proas are equivalent.

M: I am a little disappointed by this, since I would have assumed the fact that one larger hull would easily have less surface area than two smaller ones, given congruent enclosed volumes.

J: The difference is the length/beam ratio of the hull: 5/28 for the cat, 5.6/38 for the proa (version #1). And also in the log, which also has a longer L/B than the cat hull it replaces. You've added more material longitudinally, but in effect it is in the ends where there is little enclosed volume. If you just expanded one 28' cat hull up to 38', you'd see a better ratio, but you'd be introducing other problems.

M: OK, I get it. And when you ad in the connectives, it will really be very similar to the cat in build difficulty. Only that when you are done, you will have a boat with far greater performance potential.

J: Yes, and its a good example of the proa advantage. There's no way to compare the sea keeping ability of the 28' cat to the 38' proa, even though the same material is absorbed in building either one.

The dory has about 40% less skin area- but due to its weight it needs proportionately thicker skin. Plus it has ballast, and must have structural strength in excess of the cat or proas to carry that. So we'll also estimate that the relative material cost of building the three types to be essentially equal as well. All else being equal though, the dory will go together more quickly than either the cat or proas; less skin area = less structure = fewer operations to complete.

M: Not to mention the connectives, etc. Which is why dorys are always appealing!

J: Now, earlier you mentioned that for a proa, carrying capacity will always be the problem. There's a challenge for us: develop a hull shape that can handle more weight in a shorter length, without hitting that displacement wave drag problem."

I think the problem is a bit more complex than that, and really stems from the dividing line between when a catamaran makes more sense, and when a proa does. There are two keys: righting moment and Displacement/Length Ratio (DLR).

As the cat gets heavier, righting moment goes up, but if we only increase the load on the main hull then this isn't true for the proa. If we allow 4000# displacement on the 28' cat above, and 10' Cl to Cl beam (on 15' overall), we have about 20,000# maximum static righting moment. On the proa #1A, also at 4000# with 25%- 1000# on the log, and 18' Cl to Cl, we have a maximum of 18,000#.

Figuring strictly statics here: since the proa can self-right from a knockdown it can safely carry more sail area for a given amount of righting arm. If the 28' cat is rigged to fly the windward hull at say 30 knots wind, the 38' proa with the same sail area and arrangements would fly the log at about 27 knots. That difference would be OK. But if you load both cat and proa to displace 5000#, on the cat you can increase the sail area to mostly compensate for the weight gain, since you will now have 25,000# righting moment. But not with the proa, since righting moment would not increase. So your effective B# must decrease as loading increases.

M: To alter that equation the proa must move more weight onto the log- and when do you stop? More log weight = loss of light air performance, and more skin area + more structural loads = higher cost.

J: At some point, you are better off moving to the catamaran, picking up interior volume to boot. Yes there are the Ndrua / Alia types, but are they necessarily better than a cat of the same average length, given modern materials?

M: The Ndrua form has been completely ignored in the West, yet it was the largest, most successful - and most imitated - South Pacific voyaging canoe (according to Haddon & Hornell). It would be interesting to know the details of why this was so, perhaps there is an appropriate Western analogue.

J: There is also the matter of DLR. We need to keep the proa hull DLR below about 70 or we begin to pick up gobs of wave drag. In order to keep speed up as wave drag becomes a big factor we would need to increase sail area, but as we've seen it will be hard to do that without moving, step by step, closer to a catamaran configuration. There's a practical limit on just how burdensome the hull can get, particularly in relation to the weight of the log. Unless weight is proportionately added to the log as the main hull is loaded, the more burdensome hull will lead to a proportionately greater loss of performance. But as the greater weight of a loaded log is transferred onto the main hull by heeling, DLR increases, leading to higher wave drag.

The cat / proa dividing line comes somewhere before here: If you take a 40 foot Wharram Narai, loaded to design specs, you've got DLR 44. Carry enough sail so 70% of the displacement is transferred onto one hull (I don't think cruising cats get pushed much harder than that very often), you get a DLR (heavy hull) of 62. But on a proa of the same length and displacement that has 30% of the weight on the log, you get DLR for the main hull at rest of 62. And when pressed, with the log nearly out of the water, the main hull DLR rises to 88- way into the wave-drag death zone- and that's with no leeway for water ballast. Clearly in this case the catamaran is the better option, and you'd have to run out the proa a lot longer & wider to match up again.

So the proa has to exploit the square-cube law effectively in order to be successful, and that means a light weight for long length and wide beam. There's not a whole lot we can do about it. And, come to think of it, this has clear implications for water ballast; a heavy proa may not be able to use it effectively.

M: You may have just hit upon the proa's "Achilles heel". This might mean a freer interpretation of Oceanic proas is appropriate for Western adaptation... If righting moment is important, and I think it is, then weight-to-windward proas such as Terho Halme's  EQL-7 or Rob Denney's Harry proas are going down the correct path. W2W proas multiply the advantage of the double canoe when it is heavily loaded, and in fact could compensate with comparatively more sail area, or a narrower beam.

J: Well, it multiplies it up to the point where the DLR rises to make wave drag matter a lot. Once you hit that point, you lose the benefit. If you are carrying a too-high DLR on the windward hull, you will be held to a slow speed by the windward hull's drag, except when you have enough side force from the sail to move a lot of that displacement over to the lee hull. This won't happen in light wind, and a lot of the time it won't happen off-wind. Off-wind with such a draggy windward hull you may not be able to generate a forwards-enough apparent wind to make that happen. And if you do manage to move it over, then you need to keep the lee hull's DLR low enough to stay out of wave drag territory or you have lost the benefit again. Yes, you can compensate with more sail area- at higher cost- but only up to DLR ~70; above that, wave drag increases faster than the boat's practical capability to carry the extra sail.

M: Dave Culp told me once that the classic Pacific proa makes sense in very small sizes, where shifting crew weight is the most important thing, and in very large sizes, where righting moment grows by the cube.

J: How much weight belongs on the log (as a fixed static situation)? We don't know. How effective can water ballast be, and what is the precise dividing line where it would have been better to design a cat instead? Can't say. How little righting arm can reasonably and safely support how much sail power? We don't know that one either. How hard can we push static righting arm and rely on dynamic and the lifting rig? Don't know. How large should the damn log be anyway? I built the log of my new (and as yet unfinished) 21' proa with water ballast tanks in order to try that out; what Robert Biegler wrote recently about wave stability is really focused on the last major design hang-up I have before feeling like I know enough to make some valid judgements. We've made a lot of progress in understanding proas over the past couple years. But until I have good answers for these questions, I'm not quite ready to build a 38' cruising proa.

M: I agree. The "cheap cruiser", which is the area we intuitively believe the proa can contribute, is a difficult one. Extra length means higher performance, but also higher docking fees. Widely varying payloads are inevitable, as is overloading. Flying the ama for extended periods is a nice fantasy, and one that the Islanders could achieve, but not really what I would want for a safe family cruiser. Keeping both hulls firmly in the water would be more desirable, from that standpoint. I think that we are still on the correct development track by concentrating on the small boats. Even though there is a lot of chat about larger ocean cruising proas on Proa File International, I think the real interest and the real potential is in the small boats. Why not cruise in a safe, self-righting, usefully fast, shallow draft, and cheap little beach cruiser?

Comments

• I havent many boats under my belt but i have an idea for a proa. See what you think...2 canoes like a cat.but the rig is what makes it a proa.What if someone had 2 free standing aft mast configurations with a jib on each.Depending on the tack would depend on what hull you operate the rig out of. It would however be an atlantic/pacific proa depending on how you decide to run that day. Pacific on 1 tack Atlantic on the next??????  Understand??? It is very simple. Just add daggerboard and rudder to the akas. It would be a 1 way boat (Bow and stern) It is a really simple design and i am sure SOMEONE has done it some where. Imagine both jibs flyin at the same time. WOW… To me The “ULTIMATE” Proa is as you all know and love....."CHEERS" looks like a ripping good time. I could imagine cheers with that setup with an inside helm. Lift and pull would help with Pitchpoling you think? Anyways see what you guys think.

  Posted by  on  03/10  at  07:57 PM