A modern classic..?

Hi guys,

Long time reader, first time poster

Attached below is an idea of mine for a ‘tweak’ of a design. I really like the concept of Dudley Dix’s sport boat style with a ‘retro’ twist for his Retro 29. It is taken off his Didi 26 design and has a flat hull shape and a deep lifting bulb keel. I’m not completely taken however, and I would love to give it a bit more of a classic touch by lengthening the stern and bow a touch.


The bow I would leave as per design to the waterline and then re-loft the forward 1/3 stations and possibly add an additional bulkhead.

The stern I would lengthen slightly following the curve of the hull and add an additional bulkhead.


Essentially what I would like to know is how those changes would affect the boat. My thoughts are as follows but if you could tell me I’m missing something I’d very much appreciate it:

- Fuller bow above waterline would provide more lift and extended water line when heeled - benefit

- Extended stern adds weight – con

- Extended stern adds waterline length – benefit

- Extended bow gives more support for bowsprit – benefit

- Larger rig may be possible due to increased length and righting moment

- Balance of boat will change, sail maker to adjust sail size

- Extended stern – will it change planning abilities?

Cheers!!! - TJ


*Picture Shows from above original Didi 26, My altered Didi 29 retro design, and Didi 29 retro

 

You will need a lot more than a sailmaker to adjust the balance and behavior. Also, sailmakers will make sails to your design, they don't do naval architecture. That means that you have to redesign the new sailplan. In addition, the mast will probably have to be repositioned, with all the accompanying change in internal structure and keel position.

 

Cheers for the response Gonzo, Looking at the 26 and retro 29 Dudley has kept the mast and keel in the same position, would there already potential balance issues and would my extended stern be just too much? He did change the rig and it looks to be more main driven with a larger square top or gaff sail.

 

The drift surface, that is, the area projected on the longitudinal plane of the submerged hull, changes practically nothing when lengthening the stern or changing the shape of the stem. Therefore, this cannot be a reason to change the position of the mast.

 

To start with, the waterline will change with the hull modification. You need to calculate the position of the new waterline and for balance, stability, etc.

 

To continue with, sure the waterline changes, of course, but that's like saying nothing. Even an air breeze will change the waterline but from there to suggest that the position of the mast will have to be changed, there is a long difference. The change due to the modifications that the OP has indicated to us, will probably be so small that it will not mean changing the position of the mast.

 

Cheers TANSL and Gonzo, moving away from the rig, would there be any other consequences I haven't thought of? Perhaps the long stern may not cope well in a steep following sea? Would you say the righting moment will be increased substantially or negligibly?

 

Not being a designer, myself. The most significant change I see to the possible balance of the rig, and the balance of the boat, is bringing the rudder inboard. It looks like it is a little more forward, relative to the mast and adds aft weight compared to the transom mounted version.
Overall, I don't see there being any major worries, but do the calculations. I'm sure it will teach you something.

-Will (Dragonfly)

 

Cheers Will, the designer Dudley dix already made the changes between the 26 and the 29 retro but my adaption of the retro would leave it in place. Obviously the stern extension itself would add weight and potentially adversely affect balance, though the smaller transom would also save some.

 

We are mixing things that are not homogeneous. One thing is the position of the geometric center of the sails in relation to the center of drift of the hull and another thing is the righting arm of the boat at each angle of heel.
But all we can do is make assumptions and you should never listen to anyone who says "I guess ...". Without making some numbers it is not possible to give you a correct and concrete answer. So, if that is what you are looking for, please provide your boat details and ask your specific question.

 

Note the rig differences and listen carefully to the comparisons given. This all makes good sense to me. Note the hull differences are many but subtle, and the Retro is not really intended to optimize planing performance, but it is probably a better all round boat. It seems a bit light to me. I'd rather see a D/L in the 120 range <shrug>. It lacks what is considered normal sitting head room, so a revised sheer and a bit more displacement to get 4'6" headroom would seem a worthwhile thing to look at. Yes, the planing behavior of the 29 will be noticeably different from the 26, but I'm more accustomed to keel boats that weigh 3 times as much as either of those. The designer has a whole passel of rigs to choose from. On the long cockpit version, the quarter births are useless for anyone over the age of 10. This could be fixed with a very careful redesign of the cockpit, quarter births, bridgedeck, and rudder system. I'd want a big lazerette on one side. Otherwise, that aft extension is going to get filled with crap.