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-   -   Why the Bristol 32 backs so poorly (http://www.sailnet.com/forums/seamanship-navigation/102668-why-bristol-32-backs-so-poorly.html)

davidpm 08-17-2013 09:32 PM

Why the Bristol 32 backs so poorly
 
I've been playing with the Bristol 32 and comparing its performace to other boats I've sailed like the Catalina 27 and 30.

I think I have a theory I would like to explore with those of you knowledgeable about these things.

First the behavior I'm trying to understand.
With the Catalina's putting the boat in reverse caused the stern to prop walk to the port. Once it has way on it could be steered in reverse pretty well.

With the Bristol the behavior is reverse seems to be random, it obviously is not which I why I'm doing this exercise. Sometimes the stern will kick port sometimes starboard. Often when is reverse the boat will not steer based on the helm but will choose its own path.

I think I may have an answer.
If you look at the underwater profile of the boat most of the keel is under the wheel. The forefoot is cut away. I'm thinking that while a fin keel boat pivots someplace near the mast in either forward or reverse the Bristol pivots nearer the wheel in reverse.

I think that what is happening is that in reverse the bow is very easily blown downwind or down current turning the boat in a much more effective way than the prop walk or rudder.

In order to predict what the boat will do in reverse I have to think in terms of how the current and wind will move the bow much more than in a fin keel boat.

What do you think?

chris_gee 08-17-2013 10:55 PM

Re: Why the Bristol 32 backs so poorly
 
It is pretty similar to the Contessa 32. The pivot point is well back from the mast. Thinking of the centre of lateral resistance. I would guess about four feet back. And you have a fair amount of lateral resistance with a cutaway full keel.
Also the centre of windage is forward giving a greater moment to the force of wind, than say the effect of prop walk at a lesser distance.
At very slow speeds the rudder is less effective.
You can get more prop walk by high revs briefly before the prop really grips, but you may not have much room for that if you have to get her moving first to clear a berth.
Where it gets interesting is when the wind is acting opposite to the prop walk. In which case you essentially have to go with the flow and work it so you can ultimately go ahead.
I am not sure that current would make much difference in the absence of wind because I would have thought it would act at the centre of lateral resistance i.e. say pushing the stern one way and the bow the other.
I agree though sometimes it can make life interesting.

davidpm 08-17-2013 11:30 PM

Re: Why the Bristol 32 backs so poorly
 
Quote:

Originally Posted by chris_gee (Post 1075440)
Also the centre of windage is forward giving a greater moment to the force of wind, than say the effect of prop walk at a lesser distance.

That is as good way to explain it.
If the pivot point is closer t the rear of the boat then the lever action of the prop walk and even rudder control is much smaller than the lever action of the wind on the bow which has a longer lever.

There is an amplification effect also when going in reverse.
It feels like what happens when you are trying to backup a trailer with a long extension. If you get off just a little it rapidly causes your heading to go off.

It would be very useful to be able to mentally calculate all this and be able to successfully back down a fairway even if it was at a bit of an angle.

CalebD 08-17-2013 11:36 PM

Re: Why the Bristol 32 backs so poorly
 
david,
I believe your suspicions about the underwater profile of say, your Bristol 32' are likely true.
This is why I suggested that if, given light conditions, you start by pushing the boat in the course you want it to take in reverse, and only after the boat is moving on the line you want kick in reverse gear. This only works on a finger pier so if you are on a mooring all bets are off which way it will go in reverse, no matter where the rudder is pointed.

davidpm 08-18-2013 11:05 AM

Re: Why the Bristol 32 backs so poorly
 
Quote:

Originally Posted by CalebD (Post 1075456)
david,
I believe your suspicions about the underwater profile of say, your Bristol 32' are likely true.
This is why I suggested that if, given light conditions, you start by pushing the boat in the course you want it to take in reverse, and only after the boat is moving on the line you want kick in reverse gear. This only works on a finger pier so if you are on a mooring all bets are off which way it will go in reverse, no matter where the rudder is pointed.

That is exactly what we do and it works most of the time.
Once however when the current was running towards the dock the exaggerated steering in reverse took over and the boat turned sidewise to the dock and we got pinned.

I'm thinking now that more speed and less helm may have worked.

Seaduction 08-18-2013 04:17 PM

Re: Why the Bristol 32 backs so poorly
 
Quote:

Originally Posted by davidpm (Post 1075453)
That is as good way to explain it.
If the pivot point is closer t the rear of the boat then the lever action of the prop walk and even rudder control is much smaller than the lever action of the wind on the bow which has a longer lever.

There is an amplification effect also when going in reverse.
It feels like what happens when you are trying to backup a trailer with a long extension. If you get off just a little it rapidly causes your heading to go off.

It would be very useful to be able to mentally calculate all this and be able to successfully back down a fairway even if it was at a bit of an angle.

Strange phenomenon, indeed! I don't think anyone here has trouble backing up their sailboat.:rolleyes:Later edit: Just joking

davidpm 08-19-2013 10:07 PM

Re: Why the Bristol 32 backs so poorly
 
Quote:

Originally Posted by Seaduction (Post 1075700)
Strange phenomenon, indeed! I don't think anyone here has trouble backing up their sailboat.:rolleyes:

I hear what you are saying Thanks buddy.:)

CalebD 08-19-2013 10:22 PM

Re: Why the Bristol 32 backs so poorly
 
Quote:

Originally Posted by davidpm (Post 1075586)

I'm thinking now that more speed and less helm may have worked.

I'm no expert but I'd bet that you are absolutely right.

Barquito 08-20-2013 10:32 AM

Re: Why the Bristol 32 backs so poorly
 
It's really not random. This simple equation describes the dynamics:

http://i45.photobucket.com/albums/f9...93516eqn26.gif

MarkSF 08-20-2013 02:01 PM

Re: Why the Bristol 32 backs so poorly
 
My Bristol 31.1 can be a bit "interesting" in reverse too. I know the keel design is different, but I think the reversing problem is for the same reason: the tall bow section acts as a sail, while the overhangs reduce the resistance to rotation. You probably have a centre of resistance more aft than me, too.

Reversing out of the slip is nothing compared to how interesting anchoring can be.

However, once you have her moving at 2-plus knots, she reverses just fine.

My technique for reversing out of the slip is to rotate the boat so as to anticipate the prop walk, then go to reverse at idle. Maintain idle until moving, then go to neutral. Restrict rudder movements to only a third of a turn of the wheel, or the rudder will stall. Even then, if a gust of wind hits the bow you are off who knows where.

You've got to laugh, haven't you?

If the bow spins round and you are heading the wrong way down the fairway, just go with it - make it look like that's what you intended all along. I've got very good at back and fill......

These days I tend to assume that the boat has no steering in reverse, and plan accordingly.


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