A question about hull shape on a sail boat

Discussion in 'Boat Design' started by SeanL, Nov 24, 2022.

  1. SeanL
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    SeanL New Member

    I have noticed in the last few years that many sailboats are now being built without what I would call a taper, from the widest part (max beam) to the stern. These newer designs, when looking from above, keep the beam of the boat nearly the same as it's widest point midships to the stern. Obviously, one advantage would be to increase the cubic ft inside the hull so that given a set length, say 40 FT there would be more room inside towards the stern. Can someone explain to me any detriments towards this design? I'm curious as to specific drawbacks to designing a hull in this shape. I will use this pic as an example as it seems that the transom is quite wide given the length of the hull

    Thank you in advance.


    Attached Files:

  2. bajansailor
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    bajansailor Marine Surveyor

    Welcome to the Forum Sean.

    I think that the yacht in your photo is an RM 13.50?
    Here is one for sale.
    2009 RM Yachts RM 1350 Cruiser for sale - YachtWorld https://www.yachtworld.com/yacht/2009-rm-yachts-rm-1350-8233352/

    These are my very simplified thoughts re your question.
    One possible 'disadvantage' (if one can call it that) that I have found with hull shapes like this, where they are very wide aft at the waterline, is that they are not as well behaved when heeled significantly, compared to a more 'conventional' older type of hull form.
    A more 'conventional' yacht with a narrower stern will generally have more 'balanced' waterlines, re their shape forward and aft of amidships. And when this yacht heels, the waterlines remain fairly well balanced (perhaps with a bit of weather helm).
    A classic example would be any typical older S & S design , such as Stormy Weather. Copied below is a drawing showing her waterlines - they are very well balanced relatively fore and aft. The lines below the centreline are the diagonals.
    This drawing is from Drawings http://stormy.ca/marine/drawings.html

    Stormy Weather - waterlines.gif

    I have found with yachts like (some of) the American Hunters that they generally do not like being sailed well heeled - their waterlines are very 'unbalanced' then, and the ones I have sailed will often tend to round up / 'spin out' when a gust hits them.
    But I think that they were designed ideally for relatively calm (up to 15 knots apparent wind) wind conditions while having excellent accommodation down below - and they are very good at doing this, hence why they are so popular.
    If you take this hull form to the extreme, with the downhill sleds designed for fast sailing down wind across oceans, they can sail very fast indeed - but they are probably not very comfortable when beating to windward.
  3. jehardiman
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    jehardiman Senior Member

    Ok, there are some things to consider..

    1st: the hull you showed is a very specialized shallow draft hull which does not represent best modern practice for general hull forms or appendages. There is waayy too much to unpack in that shape. The designer had obvious reasons, but nothing to do with your question.

    2nd: It has to do with how light a modern vacuum bagged composite hull can be built. This means that the actual amount of submerged volume needed is considerably less that hull even 20 years ago. Now you can use the actual hull structure similar to a sharpie or bugeye "pry board" of 100 years ago. Especially with twin canting keels....which make the hull effectively two separate hulls on different tacks. For example Hugo Boss.


    3rd: Note that these are not "sea kindly" hull forms like Stormy Weather show above by banjansailor. They are for a specific purpose and specific conditions. There are many different hull forms to choose from, and sometimes present "fashion" dictates what most manufactures build. This is because that is what sells, not what the buyer needs.
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  4. Dolfiman
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    Dolfiman Senior Member

  5. mc_rash
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    mc_rash Senior Member

    Increased initial stability is achieved by a wider beam/ stern which might be necessary due to less displacement resulting from new production methods.
  6. wet feet
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    wet feet Senior Member

    While I agree with the points mentioned,there are a couple of others which might be considered.The greatly increased amount of internal space allows for more accommodation within the hulls and that can be a factor for some potential purchasers.In addition,having a boat with a resemblance to the current crop of race boats adds to the image of a sporty boat,whether the boat actually has sparkling performance or not.I suppose it is a fringe benefit that the more common use of twin wheels and twin rudders places the helmsman in a better position for seeing the surroundings and actually having an effective rudder to steer with.We shouldn't forget that there are commercial reasons as well as dynamic reasons for the evolution.
  7. DCockey
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    DCockey Senior Member

    Probably the primary reason for many production boats.

    In the 1960's boats designed for racing started having reverse slope transom. My understanding is the primary driver was to gain an advantage under the measurement rules of the time which used the deck length as an input. Shorter deck length resulted in either a lower rating or a faster boat for the same rating. Soon production boats were following the trend. Similar reason why wings have appeared on some production cars.
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  8. jehardiman
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    jehardiman Senior Member

    Not really. As the reference Dolfiman gave shows, the actual waterplane is significantly finer than the hull, especially when sailing. This hull form is all about moving the CB well outboard of the CG on a set of sailing lines that are very fine. In a form without significant wall-sidedness initial static BM (i.e. Iwp/volume) is pretty meaningless.
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  9. ChrisVJ
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    ChrisVJ Junior Member

    Looking at some of them there is not enough depth in the aft hull for accommodation. There are several reasons for carrying the beam all the way aft. Some of the drawbacks, re balance etc, have already been mentioned.
    The broad beam aft allows easy planing off the wind which became possible as boats got lighter. (Somewhat following dinghy design.) To get past the lack of balance close hauled designers started making each bilge so that they became like individual hulls when heeled. There is a big benefit in offsetting the centre of buoyancy when heeled in such a hull compared to the traditional hulls.
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  10. philSweet
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    philSweet Senior Member

    One point that hasn't been mentioned yet is that we now have a hard stability criterion to meet, whereas in the past it was a matter of NA's sense of what was appropriate. That criterion does play a roll - the main proponents of the particular formula that became regulation were French boatbuilders with expertise in fat sailboats, and they made sure the rule favored their own sensibilities and production expertise. So when asking what are the drawbacks, it has to be done with the intent to score equally well on today's stability metrics, and those do favor fat sterns and large volumes. But as is almost universally the case, when a useful metric becomes a design standard, it fails to do what it did when used to evaluate boats that were not designed with it in mind.

    A second point is that pretty much every sailboat built nowadays is a motor boat with auxiliary sail. Motors have just gotten so good that you expect to be able to motor for days at a time at hull speed, and nobody is going to run 3 knots under sail for days at a time. So hulls that pay a lot of attention to light air and low speed performance basically don't exist anymore. Hulls of all sizes are optimized for higher speeds. This makes for finer entries and wider sterns. The top few inches of water doesn't act like linear wave theory says. It works like a shock wave. The bow wave produced by the top lamina has high entropy losses and does not admit much pressure recovery at the stern. So as you go faster, these losses increase as a percentage of total drag, and you need to put more effort into reducing these shock losses. The main disadvantage of these hull forms is that they are crap in light airs under sail. They don't have enough lateral area, they can't spread enough sail area, and they don't track worth a damn - but nobody cares because they don't sail like that - they turn the motor on.
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  11. SeanL
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    SeanL New Member

    I'm very surprised and thankful of these responses. I'm looking to cash out and retire in 4-5 years. My thoughts are to hop on a boat and sail off to where ever. I grew up on the shore in the North East (USA). My dad always had Westerly's with bilge keels. I've spent my life welding / painting / fabricating so for me a steel hull sailboat makes perfect sense. With the advancement in chemical coatings in the last 15 years, a corton steel hull would easily outlast my time here on this orb. I like the idea of welded on safety rails, cleats and other ancillary things. I believe the less holes in a boat, the better. I've been leaning towards making my own boat. I don't have a family and I will probably be sailing with my fiance'. I'm leaning towards something in the 40-45' range. I have the bilge keel thing stuck in my head because I remember my dad used to ground his Westerly on the local sand bar 3 times a season and basically brush off most of the small growth that had accumulated on the hull. I'm OK with losing a knot or two in performance. Having spent quite a few hours at 160+ miles an hour road racing motorcycles in the 1990's...a "fast" boat? Well, it's all perspective. I'm not going to be competing in any races on my boat. I'm thinking about something with at least a 10 FT beam, maybe 12' and 4 or 4.5' draft, bilge keel, twin skeg mounted rudders with a tiller for ease of the auto pilot thing. Single mast, bow sprit and gaff rigged seems appealing to me at this time. From what I've read the rigging would be less expensive and having more sail available lower on the boat can be advantageous. I'll probably do a carbon fiber mast as I've seen how rigid and strong CF can be from hanging in the motorcycle world now for 35 years, it's almost a no brainer.
    Thank you all very much for your input. If anyone wants to poke holes in my proposed boat described above, please chime in. My criteria is max living space, sturdy / sea-kindly and ease of handling single handed or with another person
  12. bajansailor
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    bajansailor Marine Surveyor

    Hello Sean - your plans sound very ambitious!

    If your main desire is shallow draft / drying out bilge keels, then your best bet would be buying a 2nd hand bilge keeler - even a second hand RM yacht would probably cost less than the cost of you building a steel yacht as per your description above.
    If your main desire is a steel yacht - again a second hand vessel will only cost a fraction of the price of building a new vessel.
    If you REALLY want to build your own boat (and design it as well?) - have a look at this link :
    How I designed and built my own 24ft sloop in steel - Practical Boat Owner https://www.pbo.co.uk/expert-advice/design-build-steel-60681

    There are various companies producing stock plans for steel construction - here are some :
    I do like the Van de Stadt designs -
    Van de Stadt Design - Yacht Designers and Naval Architects https://www.stadtdesign.com/

    I am less impressed by the Bruce Roberts designs, but they appear to be very popular, perhaps based on very good (?) sales techniques :)
    BOAT PLANS, BRUCE ROBERTS BOAT PLANS, BOAT KITS, FOUNDED 1966, OFFICIAL WEB SITE 400 boat plans SAILBOAT plans POWER BOATS; powerboats; steel BOAT PLANS; aluminum boat plans; fiberglass boat boat plans; wood epoxy boat plans, plywood boatbuilding plans, https://www.bruceroberts.com/

    Take note of what the author of the PBO article (linked above) says about building a boat that is too big - ideally you want to build the smallest boat that will meet your requirements, as the costs go up very quickly as size increases.
    He mentions Tom Colvin designs, but these plans do not appear to be still available, as Tom crossed the bar some years ago.
    Try googling his designs - there is a lot of info out there about them.
    Re how you like gaff rig, maybe a Chinese junk rig will also appeal?

    Have a look also at the Kasten marine designs for steel construction, and see if any of these appeal to you -
    Kasten Marine - Sailing Yacht Designs http://www.kastenmarine.com/sail.htm

    Be aware that while the author of the PBO article pretty much designed (lofted) his boat as he built it, it will be much easier in the long run to buy 'proper' plans, and the cost will be a tiny fraction of the cost of building the boat.
    Last edited: Nov 30, 2022
  13. SeanL
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    SeanL New Member

    Yes, I am very impressed with Kasten. There are a few there that fill my needs 90%. I will be contacting him regarding possibly altering one of his designs with a bilge keel. I keep hearing that building a boat is a huge undertaking. That is true. But for me, any steel (or aluminum) fabrication is fun. It is not even like work. Another possibility is that I buy a steel boat and add / alter it to my liking. I would of course run any potential mods past a knowledgeable Marine Designer.
  14. gonzo
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    gonzo Senior Member

    The hull and deck is probably 10% of the total job at the most. Fitting out is where time and money gets spent.

  15. TANSL
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    TANSL Senior Member

    If you have to make the new molds, those of the hull, the deck and others, they can complicate things a bit and vary that percentage a lot.
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