Subject: Re: [harryproa] Re: Ratio of ww to lw hull lengths
From: Mike Crawford
Date: 2/16/2009, 1:10 PM
To: harryproa@yahoogroups.com.au
Reply-to:
harryproa@yahoogroups.com.au

Gardner,

  I'm sure you'll find a nice compromise.  The contrarry ended up being pretty neat, and the first draft of the new boat has a lot of potential.  The process is just a very long series of trade-offs between conflicting design goals.

  Personally, I like the proportions of the harryrproa design, and since what I'm looking for is a glorified daysailer and/or coastal cruiser, the format works for my needs.  a 26' to 30' ww hull will be plenty.

  In the end, the argument for the shorter ww hull is one of optimization: how fast can you make a boat for a given cost and weight?  If speed isn't the primary criterion, though, it's not.  If a 50' hull is your limit, then the ww hull will have to end up being whatever it ends up being.  Even if you end up with a 40' ww hull, It's not as if you're going to be bludgeoning your way through the water like a 20,000 pound Lagoon 440.

  Of course, there's nothing wrong with a catamaran.  You could end up building a harrycat, with a schooner rig in one hull and most of the accommodations in the other hull, and still have a great boat. 

  You'd have all the advantages of the unstayed rigs without having to worry about how to fit them through the saloon of a normal cruising cat (and then support that saloon).  It's nice to have six feet of reinforced structure in which to bury those masts.  You would also have the option of shunting instead of coming about or gybing, and in extreme wind scenarios (either two knots or thirty+ knots) this can be a big advantage.

  After that, the more you can keep your weight down, and accordingly the length of the ww hull, the more you'll experience the benefits of the proa.

       - Mike

 

Gardner Pomper wrote:

Mike, 


Thanks for the excellent reply. Both yours and Roberts have definitely helped  me understand better the advantages of a shorter ww hull.

In response to how much you can pack in a proa, I thought I understood the tradeoffs in space and accomodation, but perhaps I am not being realistic. I am trying to get the accomodations of my 30' cat into a 50' proa, with some slight improvements. So maybe that is equivalent to a 33' cat. Basically 2 double bunks, 2 single bunks, 2 heads, a galley and a bridgedeck table to seat 6, plus a little walking around room. I have a layout that I like, but I ended up with a 40' ww hull to make a fine entry. I am currently using 26' of the length for 2 7' bunks and a 12' bridgedeck. I can probably rework that to a 33-35' ww hull.

I think that the length of the lw hull does start to become something of an issue when it starts exceeding 50'. Not just for dockage, but just manevering around a crowded anchorage. My gut feel is that you should be able to match the accomodations of a cat in a proa that is 50% longer. If you actually need one twice the length, then I think proas will have an acceptance problem beyond just education.

Thanks again for your thoughts. My layouts are usually posted in the Files section of the forum, under "Gardner's layouts" specifically to draw such comments.

I will go back and try to rework both hulls.. ww for the length issue and lw for the torque issue due to the center cutout.

- Gardner Pomper
York, PA


On Mon, Feb 16, 2009 at 8:12 AM, Mike Crawford <jmichael@gwi.net> wrote:


  If you're going to be tacking, it's nice to have both hulls as long as they can be; the leeward hull is going to be taking much of the fore/aft stress when sailing hard, and on a tacking boat, the hulls switch off as to which is leeward.

  This is one reason why some designers create trimarans with amas that are longer than the vaka, or main hull, or alternately, build picklefork tri's that have amas which extend forward of the bow of the aka.  Both are an attempt to get the leeward bow out as far as possible to resist sailing loads, avoid pitchpoling, and keep from stuffing the bow under a wave (and therefore pitchpoling).

  So, you can think of a harryproa as two-thirds of a fast trimaran with long amas -- just remove the windward ama and you have roughly the same geometry.  Assuming you're not tacking the proa at high speeds or in high winds. the ww hull doesn't need to resist significant sailing loads while tacking, and can stick with the shorter length.  The longer the lw hull gets, the faster and more sea-kindly the design will be.

  If you look at the solitarry and texel harry at http://www.harryproa.com/designs.htm#other , you'll notice that there's even a bigger disparity between the hull lengths than in a normal harry.  Since accommodations aren't the goal, speed and seakindliness rule here.

  Other thoughts/points:

  - If you're heading to windward at 45 degrees, and the line from the ww bow to the lw bow is also roughly 45 degrees, you'll be meeting the waves head-on with both bows.  This will create a much kinder motion for both the boat and the crew.

  - If you want to limit yourself to X pounds of weight, and you know that one hull will be to leeward most or all of the time, it makes sense to borrow length from the ww hull and add it do the lw hull.  That's the hull that's going to add speed and resist pitchpoling.

  - A shorter ww hull will place fewer sailing loads on that side of the boat, requiring less structure and weight.

  - A shorter ww hull will make the boat easier to turn.

  - The smaller ratio of ww hull to lw hull just looks darn cool.  imho.  I actually think I'd prefer a 3:5 ratio instead of 2:3.

---

  That said, none of this solves your issue of dealing with the weight of accommodation. 

  This is a classic multihull problem, made worse by the proa design.  If you can keep a mulihull simple and light, it will fly.  If you can do the same to a harryproa, it will likely fly even faster.  But as you add weight to a multihull, it slows considerably. 

  Proas are more vulnerable here in the sense that many people think they already offer too little accommodation for their length.  A 70' proa might only carry the same accommodations as a 45' catamaran.  If you start trying to add in all the accommodations a 40' or 50' boat "should" be able to handle, you'll have a slow proa. 

  This is not only because of the design involving the longer lw hull, but also because the hull is pointy on both ends.  There's no wide sugar-scoop transom that can handle a genset, island bed, corian counters, and storage tanks in each hull.  If you want to carry the load in a proa, the hull immediately gets longer, again causing people to comment on how little accommodation there is for the length.

  But that's only if you think in terms of length.  If you think in terms of cost and speed, the equation changes. 

  The flip side of the standard "too little accommodations" comment could be applied to the average cruising cat: it's got too little waterline for its weight, and too much cost for its speed.  Given the same expenditure, the proa will be faster and more sea-kindly.  A 70' proa is definitely going to outsail that 45' catamaran, will weigh less if done right, and will also cost less as well (smaller sailing loads, no compression, less structure, no standing rigging, and so forth). 

---

  It sounds to me like you're bumping up against the limits of the proa design.  The simple fact is, in order to carry the same weight as a catamaran, you'll need a much longer lw hull.  If a 10m ww hull and 15m lw hull don't provide enough accommodations, you have two choices:

  a) Slow the boat down and increase sailing loads with a longer ww hull. 

  b) Lengthen both hulls to keep the 2/3 ratio.

  Since the proa will still cost less than a cat, even at the longer length, and should still be faster, it's hard to imagine cost or speed being the issue.  My guess is that, when compared to a catamaran, the limit you'll run into with a proa is how long you're willing to go.

       - Mike



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