A question re directional stability as a result of keel modifications

Bajansailor

Bajansailor

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I asked a question on the PBO Forum about the possible effects of deepening the keel (skeg) at the forward end of a displacement motor boat, and posted a few photos of the work that has been carried out so far.
Here is a link - http://www.ybw.com/forums/showthread.php?t=333082

No replies yet, hence my reason for now posting 'over here' as well.
I would be very interested in any opinions (either for or against) re these keel modifications.
 
The problem basically is that they want the boat to lie beam on to the seas while they are fishing. But because they have a fairly steep rake on the keel, this allows the bow to be 'blown off' when they are beam on (we dont really have significant tides as such here, everything is pretty much wind dependent).

Hence they end up at perhaps 135 degrees to the seas, rather than 90 degrees, and they dont like that - I think mainly because of how the fairly long gill-net for flying fish is deployed.

The philosophy of the gentleman doing the re-fit work on the boat is that if he increases the depth of the keel forward, this will give more sideways resistance to the bow being blown off, and they should then be able to lie beam on to the seas quite happily.

This may be so, but my concern is that with such a relatively deep keel up forward, the front end of the boat is going to try and take control - eg, if say the boat is motoring down wind at say 8 knots, and she does a wee surf on the back of a long ocean wave, then I think that there is a good possibility that the rudder is not going to have much effect if a course change is desired, because it has to turn the bow sideways - and this is being resisted by that deep keel.
(Significant wave height here is typically around 3 m when the tradewinds are blowing, but with a long wavelength in deep water)
 
Surely the centre of gravity, and the position of the cabin and top sides will have a big influence on the response to cross winds? In terms of steering, I could not see if there are 1 or 2 engines? Again, this will be a bigger influence?
 
The windage is mostly forward, with a nice sheerline and a high bow, combined with a forward wheelhouse.
The centre of gravity will be aft of amidships, due to the weight of the ice box and perhaps 1.5 tonnes of ice within, hence she will be trimming down by the stern a bit, which does help directional stability (all else being equal).

She did originally have twin shafts on 'A' brackets and twin spade rudders, but these were taken out and a shaft log and skeg added to the keel for a single propeller and rudder.
The rudder seems to be relatively small (it was resting on the top of the ice box in the cockpit) and I suggested that they might like to consider adding some more blade area to it.

One reason why I am concerned is that somebody did something similar to a fibreglass 'pirogue' here recently, where he added extra depth to the keel, and then they later had to cut away the forward half, because she was directionally unstable while under way.
 
Given the high windage forward I would suggest trying a mizzen steadying sail. The keel will have beggar all effect when anchored as the boat is not really moving relative the water, otherwise the deep aft skeg would hold her head to current. The force is all generated by the wind on the superstructure.

They could also try anchoring by the stern, that would probably work.

Like you, I suspect that a deep fore end to the keel could really screw up handling in other sitations, but they will understand that soon enough . . .
 
Bajansailor said:
Hence they end up at perhaps 135 degrees to the seas, rather than 90 degrees, and they dont like that - I think mainly because of how the fairly long gill-net for flying fish is deployed.
Click to expand...

I thought a boat always drifted to 90' to the local wind (more or less) which may not always be 90' to the swell.

the answer could be in the gil net, just have a row of cleats down your side and fasten a rope 30m from the (stern) net rope, move the gil net 'training ' rope until it brings you to 90' :)
 
DAKA said:
I thought a boat always drifted to 90' to the local wind (more or less) which may not always be 90' to the swell.
Click to expand...

I thought it was the other way round, boats will sit at 90 degs to the swell, but not to the wind.

Imagine the side of the boat above the W/L as a 2D area, and locate it's geometric centre, then do the same for the hull and stern gear under the W/L. If the centre point above the W/L is not vertically above the centre point below, then all other things being equal (ie. no tide or swell), a cross wind will create a turning moment?
 
Nick_H said:
I thought it was the other way round, boats will sit at 90 degs to the swell, but not to the wind.

Imagine the side of the boat above the W/L as a 2D area, and locate it's geometric centre, then do the same for the hull and stern gear under the W/L. If the centre point above the W/L is not vertically above the centre point below, then all other things being equal (ie. no tide or swell), a cross wind will create a turning moment?
Click to expand...
Mobos tend to always lie beam on to the wind when stationary. Can't explain why because I understand your logic but they do. When you apply power and go astern, the boat will then tend to weathercock ie point it's stern into the wind. I guess the latter is because when the props rotate, they exert a grip on the water towards the stern but the bow gets blown off
 
Monique said:
Fishing beam onto the sea?

BBBBAAARRFFF!!!
Click to expand...

I agree absolutely, and the fishing boats here are no exception when it comes to rolling - defo need one hand to hang on, and one to fish with (well, I did, when I went out on them).

But fishermen are a die hard conservative bunch, very resistant to change - little one cylinder Petter diesels started to be introduced to the fishing fleet in the late 50's (they were sail power until then) but many fishermen were very sceptical of these new fangled devices, and still carried their sailing rigs (with the masts stowed on deck) up until the early 70s.
Very sensible really, as nobody had VHF radios in those days, so it was good to have an alternative method of propulsion to get home if the engine decided to break down.

I have added a few more photos on the PBO thread of this fishing boat with another section of keel added.
http://www.ybw.com/forums/showthread.php?t=333082
 
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