New Mast/Bowsprit

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machone

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I am thinking about adding a bowsprit and increasing the mast height of my Domp DK860. Where can I go to get advice on this? I've been to the original hull manufacturer and although bowsprits were a factory modification they do not have any drawings. The owners club closed 3 months ago and I am having difficulty finding a DK860 with a bowsprit for sale to take measurements. I know they are out there! There is no information readily available about mast height increase either but I was told by Abma that I can 'work this out myself'.... pythagoras?
 
Itcan be suprising how little a naval archietect might charge for a new sail plan,certainly worth enquiring as the investment in a new rig will be expensive.If not going to see a NA GET A BOOK ON YACHT DESIGN nd go on from there.The third possibility is talk to a sail loft-It will help if you have a some idea where the centre of effort is.
 
Following on from Mogy. If you add the sprit and a jib, your centre of effort will move forward. This is OK if you suffer from weather helm, and is used on some older boats to correct it. If, however, your boat is ballanced on the helm, then it will give you lee helm. Not a good idea.
Who designed the boat? If the builders offered another rig, the designer might have the drawings. Raising the height of the mast (& rig) also creats the possibility of ballast problems. If a foot or two, probably not. More, and you need advice, or be prepared to experiment.
DW
 
mogy said:
Itcan be suprising how little a naval archietect might charge for a new sail plan,certainly worth enquiring as the investment in a new rig will be expensive.If not going to see a NA GET A BOOK ON YACHT DESIGN nd go on from there.The third possibility is talk to a sail loft-It will help if you have a some idea where the centre of effort is.
Click to expand...

Sorry, centre of lateral resistance would be handy as well
 
DownWest said:
Mogy, there are pix of the actual boat on the web. Longish keel hard chine steel. Nice looking cruiser.
DW
Click to expand...

I think I was quiyte interested in buying one once.I am given to belive though that the samll dutch boats are mqde with thinnish plate.Stll as a steel boat farting about moving mast postions and adding bowsprits will be quick and strong!
 
There are pics of the boat here:
http://www.western-horizon.co.uk/archive_boat.php?boat_id=71

Here's one with a bowsprit but with an aft cabin - not sure if that makes a big difference:
http://www.sy-vinkona.de/Segelyacht/Vinkona.htm

Abma built it but he's either dead or retired. The yard are not interested as they build new steel canal motorboats and have lost/do not have the plans.

The steel plate has thinned in a few places and will be replaced but it was 4mm when new and still is in some places, I think that is fairly standard for pleasure boats.

If I have to move the mast foot I won't bother but I am interested in the mod because a Dutch club member said it stopped the boat heading into wind in a sea(weather helm I just read on wiki), not that this was a problem when my father owned her but if I'm getting a new mast anyway....

I'll contact a NA to ask about it. Know any good ones who do this sort of thing?
 
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Naval Architect

Following on from the above, can anybody recommend a small yacht 'new sail plan' experienced naval architect?
 
There are basically 3 ways you can reduce weather helm on that type of boat. It usually comes from a combination of unbalanced sail plan (centre of effort too far aft) and lack of power from the rudder. So the first thing to consider is moving the centre of effort forward by reducing the size of the mainsail - usually shortening the foot. You can experiment with this by observing the behaviour if you reef the mainsail. You can also have the mainsail cut flatter and/or flattening reef put in. The downside of this is approach is loss of sail area and drive because usually the main contributes more than the jib.

The more common way is to do exactly as you are proposing - move the CE forward by extending the base of the sailplan. This is very common on boats of this style - you will, for exmple see many Griffiths design boats with bowsprits or platforms. This approach has many advantages. It allows more flexible sail forward so you can balance the boat. On some boats it provides space to add a second forestay for a 2 headsail rig and finally it gives a good platform for stowing the anchor. There are few downsides, mainly the increased length can mean higher berthing fees and it is not always easy to build it strong enough to take the rigging loads. However, the type in your second picture is usually strong enough if it is triangulated to the stem - relatively easy to do on a steel boat.

The final thing you can do is to improve the efficiency of the rudder by increasing its area and most importantly adding some balance forward of the pivot. Again this is often done on Griffiths boats with considerable success. In some ways it is a "fix" because it does not deal with the lack of balance of the sailplan but does make overall balance better.

Don't get too hung up with getting accurate calculations. Moving the forestay forward a metre or so is not going to create any major structural issues except ensuring the new mounting is stayed to the hull. With a roller headsail you can experiment with area using the existing sail and then perhaps have a new sail made when you are happy with the result. When I did a similar thing to my Griffiths boat many years ago I discussed it with my sailmaker, and you might find that more useful than trying to get a NA to do calculation.

Hope this helps
 
bowsprit

Thanks Tranona. Will give it a go. If the windward stability changes it sounds like there are enough other adjustments that can be made to offset any negative effects.
 
Weather Helm

It sounds like weather helm is a concern on your boat. Before you do anything dramatic try reducing sail area when hard pressed. This will reduce heel so reduce weather helm. (this depends a bit on hull shape)
So reducing jib size can improve weather helm if it is simply an over powered problem.
If you are going for a new mast (aluminium I presume) then if you feel light weather performance is poor you can add another meter or so without too much concern. Of course the mast size and rigging must be suitable for the length etc. ie number of spreaders etc.
If your concern is light wind performance I would advocate a light weather jib on a bowsprit. The bow sprit needs to be at least 1 metre long to give a gap between jib and light weight jib out front.ie sail it as a cutter.
It might be convenient to move the main forestay aft a little to give gap between the jibs. Of course balance will be better with a bigger mainsail bit this concept of sail ballance is overated in importance to my mind.
good luck olewill
 
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