3-part dory design

I am presently designing a plywood 24ft dory (with about 5ft beam) for a rowing club.
It will accommodate 6 rowers and will break down into three 8ft sections which can be transported on car roof racks. The dory will dismantle at upstanding bulkheads.
I have seen small yacht tenders which break into two for stowage but not anything on this larger scale.
All advice and comment would be much appreciated.

 

Yes it can be done. Just give close attention to the structural parts and fasteners at the joints. Phil Bolger has designed a really big boat , I dont remember the exact length, but it was upwards of 40 feet. It too came apart such that there were three elements.

The Banks type dory design is certainly a simple one for construction purposes. It may not be the best choice for the rowing club if speed is an important factor. The length, that you mention, suggests that the construction will be of plywood. A Swampscott type dory is a more intricate build but it would probably be somewhat faster than the Banks type. The Swampscott, built with glued ply lapstrake construction, will surely be less apt to twist in a lumpy sea.

Tell us more about your project. What kind of weather and water conditions will the boat likely encounter? Is economy and simplicity a deciding factor? Will you be tempted to put a sail on the boat? etc.....

 

To explain in more detail. My home port is a tidal estuary where there are about ten sailing clubs.Because access for sailing is limited by tide times there is also an interest in rowing.
The idea behind this dory is that it would be a one design which would be easy and inexpensive to build and each club could build one (or two) for rowing competitions which would take place at each club with about ten participants.
Actual efficiency in the design is not too important as all participants would have the same design
The nice thing about a six- man (plus cox) three- part dory is that the building effort can be split three ways with each pair building a section, the transport can be split three ways and the rowing effort is split six ways!!
I have made a card model with a fine entry, flat floor and pinched stern which looks the part. My main concern is the strength at the joining bulkheads. The boat is intended for calm sea conditions but you know what can happen. I would hate the boat to break into two or three sections if wash or waves came along.
Thanks for your interest.

 

A bit further developed (or simlified) model could have separate bow and stern sections, and unspecified number of "midship" modules, which could be used so many as it's needed at an event (number of members in each team could vary).

 

Your usage descripton implies that the simple Banks style dory would be the preferred planform. One other question: Do you intend to use single banked or double banked rowing stations? (that is; will you use 6 oars or 12?) If you use the Banks style design, there are long overhangs on both ends. That will naturally decrease waterline length. The long overhangs are a most effective scheme for handling rough water. One advantage is exchanged for another as in most boat designs.

I would guess that the crew, with cox, could constitute a live weight of 500kg more or less. The assembled boat might weigh somewhere between 130 and 160 kg. I can envision a boat with 12mm bottom ply and perhaps 9mm sides with sufficient longitudinal stringers to keep it reasonably stiff. This would be a boat with relatively sparse scantlings for its' length. One of the compromises is that you would want the individual sections to be as light in weight as possible while maintaining some structural integrity. The reason for lightness is obvious in that you will carry the sections on cartops. Another reason for light skin is cost containment.

The connecting bulkheads will need to be robust and reinforced, near the fastenings, with stringers. This is the place where lightweight is not the important factor. Considering the total displacement and the length of the boat I'd say that the long bow and stern overhangs should be kept. Think of plowing into a wave of say, 1 meter in height. The instantaneous, but moving, load on the bow section can be considerable. The longer the lever arm, the more serious the consequence. The overhangs will shorten the lever arm in a useful way.

It might be worthwhile to Google the Mystic Seaport Museum In Mystic Connecticut, USA. They have a library of plans and drawings of many traditional boat types including dories of various kinds. Saint Pierre dories were used in the Maritime provinces of Canada for a hundreds years. They are of the general Banks type and many of them were as long or longer than the one you propose. You may be able to dredge up some information about the old time boat building port of Lunenburg Nova Scotia. The Lunenburgers have been building dories for ages.

If your sections of the same boats are to be built in different locations, then the perfect fit of the sections will become a problem to be dealt with. I think that you could build rigid templates for the outside of the joining area that would simplify the problem. In addition, the use of a standard template could assure that all boats are essentially the same. It seems to me that a symetrical boat could be designed such that the bow and stern sections are identical. That too, would simplify construction if done at different locations. More than that, a damaged bow or stern section could be replaced by one from another boat

You have a most laudable plan in the works. I wish you well in your efforts. Please keep us posted about your progress. You may be interested in another thread on this forum that involves rowing. It has been going on for a very long time and it has attracted hundreds of respondents. Use the search function to find the screen name; Nordvincrew.

 

It is also important to realise that one oar per person is considerably more efficient and easier to row (as the oars are a bit further apart). It also allows for the weight to be more concentrated in the middle so the boat will behave better in a seaway, as well as reducing the hogging loads and the loads on the hull due to overcoming rotational moment of inertia about the transversal axis.

I have long been a proponent of take apart boats, for all their obvious advantages. One of the greatest design challenges though, in this particular case, is how to carefully place the cutting bulkheads in such a way as not to interfere with the thwarts. The bulkheads can off course be quite cut away to a very low freeboard, as the the water in between the bulkheads is not much more than a static film but typically in a nice shell the best position for the thwarts is only few inches above the waterline too.

The question of overhangs; i think is more to do with the automatic flare that the overhangs produce than anything else. So for a given overall length ( 24 feet = 3 sheets ply) one can create a shape with rapidly augmenting buoyancy (flare) that also does not have much overhang to maximize waterline length. This is the direction i would tend to go in but one has to carefully consider the constructibilty of it as it gets to the point where the design calls for so much twist in the end sections that it is no longer easy or feasible to build. With overhangs the flare comes naturally without having to torture anything.

 

Thanks for the pointers sofar. Very helpful.
The intention is to have six oars. The freeboard will be fairly (40 to 60mcm) low to allow moderate oar lengths.
For "distributed building" I would make up standard bulkhead templates and apart from (1) enclosed sections at the bow and stern for bouyancy and storage, (2) specifying the plywood thickness (6mm hull,12mm floor) and (3) specifying the stringers, the clubs would have a fairly free hand in the construction. It would also be sporting to have a maximum crew weight
I am thinking of making the bow by clenching together the hull material onto a section of 12mm ply to give a fine entry. In my model this gives a fine entry and nice flare with a complex twist to the first main bulkhead.Too little buoyancy? Maybe.
I should really now get the drawings together and post them on the Forum for some expert comment!
Thanks again

 

My experience with ply hulls comes from building lightweight sailing dinghies (specifically, the NZ Cherub 12ft restricted class). My recommendation is that, rather than applying the ply as a flat skin, you force it into a slight curve in both dimensions (e.g. make an arc bottom like a Star rather than a flat one like a Banks dory). The resulting stressed skin adds rigidity, and allows you to produce surprisingly sweet bow sections if you choose your ply carefully.

 

hERE IS A FOLDING BOAT DESIGNED TO FIT INTO A CONTAINER. May give you some ideas. http://www.seatrotter.com/

 

Thanks so far.All contributions have been excellent.
I have now moved on from FREEship to DELFTship 3.2 which has the same software but much more support including a User Manual. I have also ordered some of the recommended books.
So... thanks guys but you will not be hearing much from me as I try to absorb all this technical stuff.