Question for Jeff H, re: plumb bows - SailNet Community

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Old 12-28-2005
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Question for Jeff H, re: plumb bows

Hi, Hopefully Jeff''s re-appearence means he will cast his eye over this.

Basically I have had a "notion" of sorts and I am wondering if it is at all practical.

We have a 73 vintage Endeavour26 (australian boat, no relation to the US version). It has a pleasently though not extremely raked bow. We plan to use it for some serious cruising and my wife has a concern regarding submerged objects in the dead of night.
The thought that came to mind was to build a collision bumber box that effectively would give the boat a plumb bow for added waterline length, as well as providing a second skin at the bow and a cavity filled with shock absorbing foam.

So the question is, in what ways would the boat handling and seakeeping characteristics possibly change. At the moment the boat is very comfortable in all seas and sort of goes like a mountain goat over even nasty chop, no pitching and behaving like a whitewater kayak. I understand that plumb bows nock a lot more and get pounded by a side sea, but I may well be mistaken. I would really like to open the floor dor discussion about the whole plumb bows thing as it would be a fairly major mod, but one that is within my capabilities, not too expensive, and seems genuinly worthwhile from where my understanding is now. I think I can keep the total weight down to less then 25kilograms, too, maybe less.

Thanks.

Sasha
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Old 12-28-2005
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Question for Jeff H, re: plumb bows

Without knowing more about the boat in question it is hard to answer your question. The current generation of plumb stem boats were design as a whole. When done properly the finer entries on these boats mean a more gentle motion and a faster performance. But those gains come in concert with changes in the longitudinal center of bouyancy, keel shapes and positions, and rig design.

When you talk about adding a false bow that would extend your waterline length, it would also move your center of lateral plane forward as well and so would require a rig change to prevent dramatic weather helm. It would be hard to predict the impact on motion comfort. The false bow would increase the longitudinal moment of inertia, (slower motion through a wider arc) but on the other hand it also would provide greater dampening. In theory the boat might sit bow down when static or could float bow up depending on the balance between the increased buoyancy and the weight of the false bow.

To some extent this is the kind of thing that can only be resolved by a knowledgeable designer looking at the specifics of the boat in question. I am not sure that I can give you meaningful information.

Respectfully,
Jeff
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Old 12-28-2005
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Question for Jeff H, re: plumb bows

I guess I was mildly bouyed by the thought of the Transpac racers that cut the bows off their production boats and installed plumb bows to meet measurment regs.

then again..I do not think those guys were all that concerned with motion comfort.

The Endeavour is a very strongly built medium fin keel with balanced spade rudder, built to the JOG standard.
It is a masthead rig with baby-stay.

Thanks for the initial thoughts on the subject. It occurs to me that with the bigger then normal engine and the tankage aff, she was not riding on her lines at the bow anyway, and was definately taking a nose-up view of the world. Restoring some balance at the front may be ideal.

Sasha
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Old 12-30-2005
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Question for Jeff H, re: plumb bows

Sasha-
I''m probably confusing people but IIRC maybe it was Sir Francis Chichester? who added a "falsie" to his boat to extend her length in order to enter an open water race. It CAN be done. Basic carpentry..although you want to make Real Damn Sure it will be strong enough to stay attached in bad wx.
But as to boat handling...it is a real black art to designing a hull that hull be totally balanced both while level and while heeled over (i.e. under any real speed) so you can expect the balance of the boat to change unless you''ve got a real good nautical architect designing the falsie.
I''m not a great fan of plumb bows, they can pound and slam and kill your motion in light wind with confused seas. You''d want to read up on some of the classics of boat and hull design to get a deeper feeling for the issue and how delicate [read: difficult!] good design can be.
And then too, by adding bow you''ll effectively be moving the mast aft, changing the entire balance of the sailplan. More complications.

For collision protection I''d sooner leave the exterior alone, and add reinforcing inside the bow, i.e. a Kevlar composite sealed bow storage area, Kevlar and steel fiber/mesh reinforcement inside the hull in the bow, etc. so that you could be reasonably sure of adding reinforcement--without compromising handling.
If the boat isn''t long enough, or the handling isn''t sweet enough...welll...that''s one reason people sell and change boats. Much simpler to buy a design that you know balances nicely, than to try doing it from scratch. Probably cheaper in the long run, since the false bow may also totally ruin the resale value of your own boat.
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Question for Jeff H, re: plumb bows

One minor point, I occasionally see reference to comments like Red''s that "plumb bows...... can pound and slam and kill your motion in light wind with confused seas." While a poorly designed plumb bow can be shaped in a manner that they can pound and slam, a well designed plumb stem moved the waterline forward and results in a finer entry angle. That narrower entry angle usually results in reduced pitching and minimizes the impact with waves in a chop. In other words a much smoother motion and less lost speed in a light wind and confused seas.

Respectfully,
Jeff

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