centre prop

[copernicus]

<span style="color:blue"> </span> I am at present restoring a Nobby, she is fitted with a wing type prop shaft which sticks out at an awkward angle from the hull, my problem is that the sternpost is questionable, some damage at the lower end could be repaired but at the junction inside where the section decreases there is a large crack, I could sister it but I would always be wondering, IF I go ahead and change it I am wondering about increasing its thickness from the present 3", to 4" which would give me room to bore out for a prop shaft , removing the horrible installation which she presently has and sweetening her lines no end. obviously this would mean re jigging the rudder, which would need an aperture in it and also resizing to retain a similar effective area, I have been told that there was an article in Classic Boat a couple of years ago which set out the reasons why this should be avoided, I have spent some time on the phone today to the lovely people at Classic Boat who have patiently searched their files and their memories for this article, does anyone out there either know of this article, know the reasoning or have any input into this? I appreciate the originality angle, I used to restore classic cars for a living, but this boat didn't have an engine at all when she was built in 1919 so anything she has now is non original, the sternpost and knee look like having to go anyway so if thats the case why not fit the centre prop? please help

 

[fluffc]

Adding a prop in the sternpost is going to involve redesigning the aft end of the underwater profile, to make allowance for this flailing metal object.

This could unbalance the carefully designed underwater lines, and is a bit of an unkown quantity.

Keeping the propellor offset I believe offers the least effect on the hydrodynamics of the hull, even if it does make manouvering an artform.

 

[Seanick]

This is an interesting dilemma, one which many have pondered over when rebuilding a traditional vessel. There are three solutions.
1st, stay with your wing prop, 2nd, put the shaft on the center line, and 3rd, twin screw via hydraulic drive.

The problem with putting an apperture in your rudder is that it really effects the efficiency of the rudder a disproportional amount to the quantity of rudder removed. Presuming you are going to put a healthy gaff rig on your Nobby, your low aspect ratio rudder will need all the help it can get.
I know of several gaffers who have moved to the center line and all report the same. Improved handling in marinas under power, BUT, massive increase in weather helm when sailing. For example, a helm that used two fingers on to steer before the apperture now requires two hands. This may also be due to the 3 bladed prop which is disturbing the flow past a large chunk of the rudder. This could be improved with a feathering prop.
In two cases additional area was added to the rudder.

Other probs to circumnavigate are your new tube will go through the centerline bolts of your sternpost /knee joint, so that will need a bit of thinking about.

Consider sticking with your wing prop. It does not compromise the boats structure, its simple, and you have the bits. The nobby is not a flighty plastic tub, and all manoevering will be of slow speed and well planned. She will turn fantastically one way, you shouls be able to spin her on the spot, but try to spin the other way and you will be dissapointed. I regulary move a 42' Fife with a wing prop in a marina, and have only hit the jetty once!
Other advantages are no vibration or bias on the tiller when motoring, ability to clear a fouled prop, being able to use a folding prop(cheaper than feathering).





The final option is twin hydraulic drive. This is all the rage on East Coast Smacks. The lovely Transcur, Victorious, and the Pilot Cutter Mascotte have fitted this system. Put the engine anywhere, coupled to a pump, piped to two motors. Twin folding props give control under power, full rudder gives control under sail. Its a great system, but of course a little more expensive.

So theres a few ideas. What did I do n my own boat, in the same situation? Well I fitted the new stern post and apron, thought about it for a number of years whilst rebuilding the rest of the boat. During that time a prop came my way, then an engine with g/box, so I stuck it back in the middle! If I had unlimited funds and time I would have gone hydraulic. How does she sail? Ask me next year!

 

[Victorious]

Gotta aggree with Nick.
Problem of centre prop is not so much the size of the post as the keel/sternpost bolts which will be in the way of a shaft tube running down the centerline from the gearbox to the prop.
And indeed the rudder apeture can have dire effect on sailing.

Victorious hydraulics were a tad expensive but money very well spent IMHO

 

[copernicus]

Many thanks to you all, just before I checked this post this morning I was sat on a box pondering the sternpost, sad I know but there ye go! we took the rudder off the sternpost yesterday which exposed the bolt going through it, looking inside this morning I realised how many bolts there are in the way, all things considered I think that even though the post has to go I'll leave that side alone, hydraulic drive is something I'd seen in an article on hire boats in Singapore or somewher out there and thought it would be a good way of getting the engine out of the middle of the cabin, my main objection to the wing installation was that I thought it would be easy to foul and I thought it must be a drag, spoiling the flow over the hull. Must admit that the twin screw idea had occurred in passing but I thought I was being fanciful on a boat this size, I would certainly be interested in hearing about anyones experience with this, if you would like to you can e mail from the contact page on our site www.speedwelldiary.co.uk . thanks again for all your help.

 

[Mirelle]

If you go for a centreline prop (which I don't advise) do be sure to avoid having any cut out in the rudder.

I had a two and half tonner which had an evil weather helm; she had been engineless when built but retrofitted with a centreline Stuart 4, so all the cut out was in the rudder. On inspiration, I took out the engine and filled in the hole in the rudder - the boat was completely transformed!

Albert Strange knew about this - those of his boats designed to have engines have all the cutout in the deadwood, well ahead of the rudder.

I'm saving up for hydraulics...

 

[Guest]

Hi-Bollix??? I sent you an email, did you get it?

 

[Mirelle]

Thanks; just found it and replied.
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