Boatmik
30th December 2007, 08:18 AM
Howdy All,
Robhosailer made some statements that open up an interesting discussion.
Michael,
"small dimensioned longitudinal pieces of timber on the outside of the hull over the length of the cockpit" are great idea! They are excellent as stiffeners and they are great for protection of bottom ...and they are taking part similar to centreboard, keel, leeboards and chine runners (bilge runners).
Efficiency in sailing with half rised daggerboard (centre board) will be better thankful of those "small dimensioned..." I think so ...:roll:
Sounds well...???
There are a lot of comments on the net about adding a shallow piece of timber to the bottom of a boat to assist in tracking (in canoes) or upwind performance with centreboard part retracted.
Maybe that shallow keel would be an inch (25mm) or 3/4" (19mm) deep.
Canoes and Paddling
With a canoe, the general thrust of the literature is if you have to consider adding a shallow keel to get more directional stability then the boat has not been designed properly.
It should achieve the required directional stability by shaping the bottoms of the bows with enough fineness (narrowness) to give the boat additional directional stability -they act as a fin at the front and a fin at the back to keep the boat tracking.
My balsa canoe was fairly extreme in this regard but demonstrates the point nicely. It is the ends of a canoe that give it directional stability.
http://www.storerboatplans.com/Balsacanoe/balsa%20stem%20detail%20a.jpg
With a very simplified plywood boat it might be necessary to add an external keel as it might be difficult getting the right shape in the ends.
With my plywood Eureka canoes I worked very hard to get approximate the correct shape in the ends - so they don't need an external batten - but with really simple boat it might be necessary to have a keel batten and/or a skeg at the back.
By the way - if you have fibreglass canoe and it wanders all over the place - it is because the fineness in the ends is the first thing most manufacturers like to get rid of because it is harder to build. There are some good 'glass canoes around (even though most are a lot heavier than properly designed plywood ones) but you can see which ones are good in the showroom by llooking at which have hollowed out ends like the balsa canoe above.
Sea Kayaks are often much more radical in the ends than the balsa canoe.
A good strategy for the depth of the keel required is to making it bigger and deeper than required and plane it down if the directional stability is excessive. The depth of the keel AT THE ENDS OF THE BOAT is where it makes the difference - in the middle it doesn't help much.
Sailboats
Sailboats are different - a deep hollow forefoot on a light boat is asking for trouble. When going down waves the bow tends to bury a bit and the rudder comes out of the water a bit. If the bow is hollow like the balsa canoe it might be able to generate significant steering forces in that situation and the boat will spear off in one direction or another no matter what you do with the rudder.
But back to the point of the discussion.
A shallow batten as we are talking about will have some effect, but because of the higher forces involved in controlling a sailboat (think about steering a canoe at 3 knots or a sailboat at 12 knots) the effect of the batten is so much less than that of the rudder or hullshape it is diminishingly small.
It won't provide any useful effect in either directional stability or help the boat go up to windward with the centreboard retracted.
As it gets deeper it is much more effective.
But the problem is the opposite of the canoe. If the keel is the length of the boat and has some depth - eg a foot of depth on a 16ft boat - it will add so much directional stability the boat might become a poor at changing direction - sluggish on the helm and perhaps start failing to tack correctly without losing all speed during the turn. So the tendency is for keels to get shorter and deeper for sailboats to retain adequate manoeuvrability - but it was also found that this improves performance substantially
Some directional stability is useful - and a medium length and deeper keel can be useful for that - but it is possible to control directional stability using hullshape and volume distribution and use very narrow deep centreboards or keels and still get reasonable or good directional stability without a long keel.
Now with the example of Roberts - we are looking at putting some battens on the outside of the bottom of the boat to reduce flex of the bottom. I don't expect them to have much effect in upwind performance at all.
At one stage I sailed a Heron class dinghy that had bottom skid of the type we are talking about - it didn't seem to make much difference.
One of the biggest determinants of good upwind performance is depth of the centreboard or keel. Many traditional looking boats have very poor upwind performance for just this reason - inadequate lateral plane or depth.
Beth - the boat we are talking about here was very poor with the original centreboard - which had an in-water depth of around 2ft 4" - upwind performance was very poor.
I built a slightly longer board - just under 3ft below the bottom of the boat - and performance jumped dramatically - so it went from just feeling slushy and vague and not having performance comparable to even the poorest performing conventional racing classes to suddenly having enough pace to match Lasers around a racecourse.
This sort of speed can be fobbed off by some as being unimportant - but it means the boat can cover real distance in adverse conditions with little effort. Important for safety and an increased cruising range.
BUT - if you sail habitually in REALLY shallow water and having even a foot deep centreboard or keel means you can't go sailing - then it is well worth exploring the options of very shallow long keels.
Best wishes
Michael Storer
Robhosailer made some statements that open up an interesting discussion.
Michael,
"small dimensioned longitudinal pieces of timber on the outside of the hull over the length of the cockpit" are great idea! They are excellent as stiffeners and they are great for protection of bottom ...and they are taking part similar to centreboard, keel, leeboards and chine runners (bilge runners).
Efficiency in sailing with half rised daggerboard (centre board) will be better thankful of those "small dimensioned..." I think so ...:roll:
Sounds well...???
There are a lot of comments on the net about adding a shallow piece of timber to the bottom of a boat to assist in tracking (in canoes) or upwind performance with centreboard part retracted.
Maybe that shallow keel would be an inch (25mm) or 3/4" (19mm) deep.
Canoes and Paddling
With a canoe, the general thrust of the literature is if you have to consider adding a shallow keel to get more directional stability then the boat has not been designed properly.
It should achieve the required directional stability by shaping the bottoms of the bows with enough fineness (narrowness) to give the boat additional directional stability -they act as a fin at the front and a fin at the back to keep the boat tracking.
My balsa canoe was fairly extreme in this regard but demonstrates the point nicely. It is the ends of a canoe that give it directional stability.
http://www.storerboatplans.com/Balsacanoe/balsa%20stem%20detail%20a.jpg
With a very simplified plywood boat it might be necessary to add an external keel as it might be difficult getting the right shape in the ends.
With my plywood Eureka canoes I worked very hard to get approximate the correct shape in the ends - so they don't need an external batten - but with really simple boat it might be necessary to have a keel batten and/or a skeg at the back.
By the way - if you have fibreglass canoe and it wanders all over the place - it is because the fineness in the ends is the first thing most manufacturers like to get rid of because it is harder to build. There are some good 'glass canoes around (even though most are a lot heavier than properly designed plywood ones) but you can see which ones are good in the showroom by llooking at which have hollowed out ends like the balsa canoe above.
Sea Kayaks are often much more radical in the ends than the balsa canoe.
A good strategy for the depth of the keel required is to making it bigger and deeper than required and plane it down if the directional stability is excessive. The depth of the keel AT THE ENDS OF THE BOAT is where it makes the difference - in the middle it doesn't help much.
Sailboats
Sailboats are different - a deep hollow forefoot on a light boat is asking for trouble. When going down waves the bow tends to bury a bit and the rudder comes out of the water a bit. If the bow is hollow like the balsa canoe it might be able to generate significant steering forces in that situation and the boat will spear off in one direction or another no matter what you do with the rudder.
But back to the point of the discussion.
A shallow batten as we are talking about will have some effect, but because of the higher forces involved in controlling a sailboat (think about steering a canoe at 3 knots or a sailboat at 12 knots) the effect of the batten is so much less than that of the rudder or hullshape it is diminishingly small.
It won't provide any useful effect in either directional stability or help the boat go up to windward with the centreboard retracted.
As it gets deeper it is much more effective.
But the problem is the opposite of the canoe. If the keel is the length of the boat and has some depth - eg a foot of depth on a 16ft boat - it will add so much directional stability the boat might become a poor at changing direction - sluggish on the helm and perhaps start failing to tack correctly without losing all speed during the turn. So the tendency is for keels to get shorter and deeper for sailboats to retain adequate manoeuvrability - but it was also found that this improves performance substantially
Some directional stability is useful - and a medium length and deeper keel can be useful for that - but it is possible to control directional stability using hullshape and volume distribution and use very narrow deep centreboards or keels and still get reasonable or good directional stability without a long keel.
Now with the example of Roberts - we are looking at putting some battens on the outside of the bottom of the boat to reduce flex of the bottom. I don't expect them to have much effect in upwind performance at all.
At one stage I sailed a Heron class dinghy that had bottom skid of the type we are talking about - it didn't seem to make much difference.
One of the biggest determinants of good upwind performance is depth of the centreboard or keel. Many traditional looking boats have very poor upwind performance for just this reason - inadequate lateral plane or depth.
Beth - the boat we are talking about here was very poor with the original centreboard - which had an in-water depth of around 2ft 4" - upwind performance was very poor.
I built a slightly longer board - just under 3ft below the bottom of the boat - and performance jumped dramatically - so it went from just feeling slushy and vague and not having performance comparable to even the poorest performing conventional racing classes to suddenly having enough pace to match Lasers around a racecourse.
This sort of speed can be fobbed off by some as being unimportant - but it means the boat can cover real distance in adverse conditions with little effort. Important for safety and an increased cruising range.
BUT - if you sail habitually in REALLY shallow water and having even a foot deep centreboard or keel means you can't go sailing - then it is well worth exploring the options of very shallow long keels.
Best wishes
Michael Storer