Aja
Well-Known Member
Lots of wind?
Donald
Donald
What makes a Yacht go faster
- Thread starter TonyS
- Start date
KREW2
Well-Known Member
Hi
This winter I took it back to the gel coat, hard work but I can now get an extra knot at 2200 revs on my volvo 2002. Having said that it was well overdue for a good scraping.
Keith
This winter I took it back to the gel coat, hard work but I can now get an extra knot at 2200 revs on my volvo 2002. Having said that it was well overdue for a good scraping.
Keith
billmacfarlane
Well-Known Member
You've missed out one very important factor - the nut behind the wheel.
G
Guest
Guest
Is it better that it be screwed on tight, or left a little loose?
valesailor
New Member
a) re jverges, if you can't take the weight off, at least put it in the middle. b) definitely trimming sails to conditions. I find that though I have negative roach on my in-mast reefer, the fact that I can easily reef it repeatedly by small amounts allows me to get the right amount of weather-helm, ie none (but clip on if you are alone like this otherwise the boat disappears over the horizon without you!!)
Ships_Cat
Well-Known Member
allows me to get the right amount of weather-helm, ie none
This belief that nil weather helm is the least resistance case has come up a couple of times in this thread. I was always under the impression that the least resistance case ie the fastest, was actually with some weather helm.
I have done a couple of quick reference checks and indeed, they confirm that the least resistance case is with some weather helm, the exact amount dependant on the hull and foils. For race boats this may be determined from tank tests but for general use the suggestion seems to be that around 5 degrees of weather helm gives the least resistance, so the fastest, case.
There are, of course, important safety reasons why one should have weather helm too.
John
This belief that nil weather helm is the least resistance case has come up a couple of times in this thread. I was always under the impression that the least resistance case ie the fastest, was actually with some weather helm.
I have done a couple of quick reference checks and indeed, they confirm that the least resistance case is with some weather helm, the exact amount dependant on the hull and foils. For race boats this may be determined from tank tests but for general use the suggestion seems to be that around 5 degrees of weather helm gives the least resistance, so the fastest, case.
There are, of course, important safety reasons why one should have weather helm too.
John
jerryat
Well-Known Member
Hi SHips Cat,
Yep, I agree with that. I've marked my rudder head with a 5 and 10 degree mark either side of dead ahead on each side, so that trimming can be carried out more accurately/easily by aligning the marks. We usually aim for 5 degrees of weatherhelm and that, for our boat, does seem to produce the best speed.
Getting it this accurate depends on sea conditions of course, but we do try!!!
Cheers Jerry
Yep, I agree with that. I've marked my rudder head with a 5 and 10 degree mark either side of dead ahead on each side, so that trimming can be carried out more accurately/easily by aligning the marks. We usually aim for 5 degrees of weatherhelm and that, for our boat, does seem to produce the best speed.
Getting it this accurate depends on sea conditions of course, but we do try!!!
Cheers Jerry
Ships_Cat
Well-Known Member
Whew, thanks Jerryat, was thinking I might be the only one.
We have a perspex window in the floor of the cockpit directly over the top of the rudder stock with a pointer on the top of the stock immediately under it. Is right between the helmsman's feet when at the wheel.
In case others wish to set up something similar an obvious alternative, which is often seen, is to tape the wheel at the rudder +- 5 and 10 degree positions.
Regards
John
We have a perspex window in the floor of the cockpit directly over the top of the rudder stock with a pointer on the top of the stock immediately under it. Is right between the helmsman's feet when at the wheel.
In case others wish to set up something similar an obvious alternative, which is often seen, is to tape the wheel at the rudder +- 5 and 10 degree positions.
Regards
John
billmacfarlane
Well-Known Member
When they're on my boat the tighter the better.
fireball
Well-Known Member
5 degrees of weatherhelm
Can you explain why 5 degrees of weatherhelm (or near enough) is faster than no weatherhelm?
On the yacht we have weatherhelm and eliminate much of it by putting in a suitable reef and flattening the boat a bit. On the racing dinghy I don't have weatherhelm - but then it is flat.
Can you explain why 5 degrees of weatherhelm (or near enough) is faster than no weatherhelm?
On the yacht we have weatherhelm and eliminate much of it by putting in a suitable reef and flattening the boat a bit. On the racing dinghy I don't have weatherhelm - but then it is flat.
Skysail
Well-Known Member
Re: 5 degrees of weatherhelm
According to Marchaj it is due to the rudder lift contributing to sideways resistance, ie it adds to the effect of the centreboard on a dinghy. Neutral rudder has no such effect. A few degrees of weather helm also aids 'feel' in going to windward.
According to Marchaj it is due to the rudder lift contributing to sideways resistance, ie it adds to the effect of the centreboard on a dinghy. Neutral rudder has no such effect. A few degrees of weather helm also aids 'feel' in going to windward.
Ships_Cat
Well-Known Member
Re: 5 degrees of weatherhelm
Is along the lines as Skyva2 says. I am no hydrodynamics expert but is basically like the following (an expert may like to round it out or make it clearer).
The keel, as we know produces lift to counteract the sideways force from the sails and to do this depends on leeway angle. But the process of producing lift produces vortices which induce a resistance to forward motion. This induced resistance is proportional to the square of the lift produced.
We also all know that the rudder produces lift too, as long as it is angled to the water flow ie has weather helm (and leeway angle is there too), and that can be used to unload the keel to some extent ie the lift required from the keel to counteract the side forces from the sails becomes less.
So, because the induced resistance from a foil does not increase linearly with lift (it increases as the square) if the lift can be shared with another foil ie the rudder, then the combined induced resistance from both foils (ie the keel and the rudder) is less than if all the lift was from a single foil (ie just the keel). This sharing of load between foils is part of the reason for experiments with two rudders on America's Cup's boats, for example, and why bi-planes used to be popular on slow speed aircraft.
Pretty obviously there is a limit to this rudder effect in that at high angles foils stall and become drag devices rather than lift ones. The suggested optimum as a rule of thumb seems to be that 5 degree or thereabouts weather helm gives an effective result for a boat where accurate testing has not been done.
As I say, an expert hydrodynamics person may be able to tidy that explanation up a bit, but basically that is how I understand it works.
I think there are other advantages less easily quantified, but, for example, I find a boat with neutral helm to be very lacking in feeling and responsiveness - that may just be a personal issue though (like peeps views on "road feel" in car steering). I think Skyva says similar.
John
Is along the lines as Skyva2 says. I am no hydrodynamics expert but is basically like the following (an expert may like to round it out or make it clearer).
The keel, as we know produces lift to counteract the sideways force from the sails and to do this depends on leeway angle. But the process of producing lift produces vortices which induce a resistance to forward motion. This induced resistance is proportional to the square of the lift produced.
We also all know that the rudder produces lift too, as long as it is angled to the water flow ie has weather helm (and leeway angle is there too), and that can be used to unload the keel to some extent ie the lift required from the keel to counteract the side forces from the sails becomes less.
So, because the induced resistance from a foil does not increase linearly with lift (it increases as the square) if the lift can be shared with another foil ie the rudder, then the combined induced resistance from both foils (ie the keel and the rudder) is less than if all the lift was from a single foil (ie just the keel). This sharing of load between foils is part of the reason for experiments with two rudders on America's Cup's boats, for example, and why bi-planes used to be popular on slow speed aircraft.
Pretty obviously there is a limit to this rudder effect in that at high angles foils stall and become drag devices rather than lift ones. The suggested optimum as a rule of thumb seems to be that 5 degree or thereabouts weather helm gives an effective result for a boat where accurate testing has not been done.
As I say, an expert hydrodynamics person may be able to tidy that explanation up a bit, but basically that is how I understand it works.
I think there are other advantages less easily quantified, but, for example, I find a boat with neutral helm to be very lacking in feeling and responsiveness - that may just be a personal issue though (like peeps views on "road feel" in car steering). I think Skyva says similar.
John
BrendanS
Well-Known Member
Re: 5 degrees of weatherhelm
Cor! you learn something new every day in this place. Never knew that - being more a motor boaty type, I've never looked into these things
Cor! you learn something new every day in this place. Never knew that - being more a motor boaty type, I've never looked into these things
bbg
Well-Known Member
Re: 5 degrees of weatherhelm
Thanks for the explanation. Without disagreeing with anything you say, I had always believed that the 5 degrees of weather helm improved pointing / reduced leeway rather than increased boatspeed. Either way (reduced leeway or increased boatspeed) there will be an improvement of VMG - but in one case it wouldn't show up on the boatspeed display.
Any insight to this?
Thanks for the explanation. Without disagreeing with anything you say, I had always believed that the 5 degrees of weather helm improved pointing / reduced leeway rather than increased boatspeed. Either way (reduced leeway or increased boatspeed) there will be an improvement of VMG - but in one case it wouldn't show up on the boatspeed display.
Any insight to this?
fireball
Well-Known Member
Re: 5 degrees of weatherhelm
Thankyou - very helpful! /forums/images/graemlins/smile.gif
Thankyou - very helpful! /forums/images/graemlins/smile.gif
Ships_Cat
Well-Known Member
Re: 5 degrees of weatherhelm
Getting out of my own depth here for the asked for comparison between windward gain and speed gain, because as I have said I am not any sort of hydrodynamics expert, but think it would go something like this:
Just looking at the variables as far as the boat itself is concerned, lift is proportional to velocity squared, angle of attack, foil area.
Foil area is fixed for a given boat. If the lift is spread between two foils through weather helm on the rudder the induced resistance from the foils decreases so boat speed increases and the lift increases as the square of that. So the increased boat speed allows the boat to sail closer to the wind ie less angle of attack is required to get the same lift.
However, there are obviously alot of other things going on as the boat is normally heeled and not in smooth water, etc. But one easily recognisable thing is that as the boat sails closer to the wind due to decreased leeway then the rudder becomes less effective as a lift producer because it comes more and more into the trailing vortex from the keel. So limiting effects are going on.
How all that pans out into actual quantities as far as the split between increased speed and decreased leeway I do not know and obviously has alot to do with the boats design (a simple matter being the distance between the keel and the rudder, greater distance keeping the rudder out of the keel's trailing vortex - and rudder is hopefully an efficient spade /forums/images/graemlins/smile.gif). But my understanding is that the induced resistance from the foils is roughly around 10% of the total resistance (including wave resistance, etc) for a moderate performing yacht, so for a boat in a race there would seem to be plenty of room for performance gains by reducing it.
For example, for a 34 foot OA yacht (say):
A gain of 0.5 degree to windward gives an advantage of more than 1.5 boat lengths per nautical mile sailed.
And considering that resistance is proportional to the square of boat speed, if the induced resistance from the foils is reduced by 2.5% (that is 2.5% of the assumed above 10% total resistance contribution from the foils, so total resistance reduces by 0.25%) then boat speed will increase by Square root 0.25 ie around 0.5% (assuming the boat has not reached hull speed). Then that gives a gain of around 1 boat length per nm sailed.
So putting those together gives around 2.5 boat lengths gained per mile sailed which would be very important in a race. For a 12 nm race would mean an advantage of 30 boat lengths (but, again, the advantage from speed will be reduced if the boat is inhibited by reaching hull speed during the race).
Again, I am not saying that 0.5 degrees to windward and a 2% reduction in induced resistance from the foils is what one would achieve. Just using those to show that important gains are achievable even making what would seem to be conservative assumptions. Also, have to remember that alot of other related influencing things will be going on as well, some of which may be of disadvantage.
Would welcome any actual data (and corrections /forums/images/graemlins/frown.gif, if any), from someone better versed in hydrodynamics.
John
Getting out of my own depth here for the asked for comparison between windward gain and speed gain, because as I have said I am not any sort of hydrodynamics expert, but think it would go something like this:
Just looking at the variables as far as the boat itself is concerned, lift is proportional to velocity squared, angle of attack, foil area.
Foil area is fixed for a given boat. If the lift is spread between two foils through weather helm on the rudder the induced resistance from the foils decreases so boat speed increases and the lift increases as the square of that. So the increased boat speed allows the boat to sail closer to the wind ie less angle of attack is required to get the same lift.
However, there are obviously alot of other things going on as the boat is normally heeled and not in smooth water, etc. But one easily recognisable thing is that as the boat sails closer to the wind due to decreased leeway then the rudder becomes less effective as a lift producer because it comes more and more into the trailing vortex from the keel. So limiting effects are going on.
How all that pans out into actual quantities as far as the split between increased speed and decreased leeway I do not know and obviously has alot to do with the boats design (a simple matter being the distance between the keel and the rudder, greater distance keeping the rudder out of the keel's trailing vortex - and rudder is hopefully an efficient spade /forums/images/graemlins/smile.gif). But my understanding is that the induced resistance from the foils is roughly around 10% of the total resistance (including wave resistance, etc) for a moderate performing yacht, so for a boat in a race there would seem to be plenty of room for performance gains by reducing it.
For example, for a 34 foot OA yacht (say):
A gain of 0.5 degree to windward gives an advantage of more than 1.5 boat lengths per nautical mile sailed.
And considering that resistance is proportional to the square of boat speed, if the induced resistance from the foils is reduced by 2.5% (that is 2.5% of the assumed above 10% total resistance contribution from the foils, so total resistance reduces by 0.25%) then boat speed will increase by Square root 0.25 ie around 0.5% (assuming the boat has not reached hull speed). Then that gives a gain of around 1 boat length per nm sailed.
So putting those together gives around 2.5 boat lengths gained per mile sailed which would be very important in a race. For a 12 nm race would mean an advantage of 30 boat lengths (but, again, the advantage from speed will be reduced if the boat is inhibited by reaching hull speed during the race).
Again, I am not saying that 0.5 degrees to windward and a 2% reduction in induced resistance from the foils is what one would achieve. Just using those to show that important gains are achievable even making what would seem to be conservative assumptions. Also, have to remember that alot of other related influencing things will be going on as well, some of which may be of disadvantage.
Would welcome any actual data (and corrections /forums/images/graemlins/frown.gif, if any), from someone better versed in hydrodynamics.
John