Thread: Sails for a putt put launch

Howdy,

Have a word to David about what he suggests.

Basically there are a couple of ways to go.

First ... with the prop dragging and no proper leeboard/s she will never sail really well. So it makes a lot of sense in that case to just use the simplest possible rig. A freestanding mast could be dropped in place with a modest rig. But you won't get a lot of performance. Forget the jib - it adds a lot to the inconvenience of the rig and also look a little silly because it will show that it was drawn up for a boat 10ft 6" long rather than 14 or so feet. But the mainsail alone won't give that hint.

The advantage of the freestanding rig is that you can rig and derig in about three minutes flat.

A modern stayed rig like the mirror will take between 15 and 25 minutes to set up and all the stays and bits and pieces will all get in the way.

The mirror rig cannot be set up to be freestanding bacause it will break. But you could use the sprit, sail and boom on this boat.

Fitting a leeboard ... more complication for the poor sailing performance brought around by the prop.

Actually .... I may well be overstating the case ... but I would strongly recommend giving David a call or an email because he has designed several boats with both an inboard and sails ... so he will know better than I will.

BTW ... with some inboards you can let the prop freewheel to reduce drag ... but be careful ... It does mean the gearbox is going round and some gearboxes need the engine running to be lubricated.

Best wishes
Michael

Hi Percy,

If you can make it down to Golden Beach this weekend you should get some inspiration,

Have a talk to Ian Primrose who has a nice steadying sail fitted to 'Lattitude 27s', a beautiful David Payne double ender,

Its an opportunity to see a number of Putt Putts and talk with their builders

The Regatta Site is here:

http://www.caloundra.org.au/cg%20web/index.html

Originally Posted by Boatmik

Howdy,
BTW ... with some inboards you can let the prop freewheel to reduce drag ... but be careful ... It does mean the gearbox is going round and some gearboxes need the engine running to be lubricated.
Best wishes
Michael

Que ??!!
I had understood that a free-wheeling prop actually increases its drag, in the same way that helicopters use auto-rotate to soften the bump when the engine stops. Or a centreboard generates more lift when moving than stationary.
Did I get this wrong too ?
cheers
AJ

wow AJ ... never thought of that!!!!

Actually the old trick for old style racing yachts was to use a two blader and have a mark on the shaft so the blade leading edges would line up with the keel.

But under power a two blader does reduce efficiency and cause vibrations too as both blades will lose a bit of thrust when they go behind the skeg when the boat is being powered leading to a pulsing in the power. A three blader is usually chosen for that reason!!!

I did do one long trip from Adelaide to Sydney where we let the prop free run at times. It seemed to increase the speed.

So .. now I feel very uncertain whether it was a perception or a reality.

Does anyone know for sure?

MIK

This isn't as easy a question to answer as it appears. If a fixed pitch, two blade wheel is tucked in behind a fairly substantial, slab sided deadwood, then, your best locking her down vertically.

If the same prop is behind a foil shape then let it spin for decreased resistance.

Of course feathering and folding props are better choices, if you can afford them.

A locked blade behind a fin will produce huge eddy drag. If left to spin, it will produce some drag, especially tip vortices, but to a lesser degree then if locked. Behind a square sided deadwood, there is enough drag associated with the appendage as to not have a marked effect with a prop locked in column behind it.

The drag of a free wheeling prop is usually sufficient enough to charge batteries, if the shaft has a pulley with a V belt to an alternator. On a moderate to heavy cruiser, driving an alternator will cut a half a knot off her top speed. In a lighter weight cruiser/racer, then maybe a full knot. It takes around 3 to 4 HP to overcome a cutlass, stuffing box and spin the fully loaded alternator. That's a lot of prop drag.

If there is a fair amount of separation behind the deadwood or fin (prop located well aft of appendage), letting it spin will reduce drag, because it will be in disturbed, but free flow when there is little leeway and freer flow when substantial leeway exists.

This is the watered down version, as it could easily be a full blown reference volume.

G'day Paul
Is there any on-line references you can point me/us to ?
I recall reading somewhere several years ago, of a large vessel of some sort which lost an engine, was unable to feather the screw, & went to some trouble to lock the shaft to reduce its drag.

This made sense in the sense of water being a thicker fluid than air, & matched experience - being easily able to feel the resistance of a stationary airscrew/fan being far less than a free-wheeling one.

If it comes to that, when fishing, a stalled lure has far less drag than a spinning/wobbling one.

cheers
Alan J.

Nice to see you back here Paul! I was starting to miss you!!!

I'd be guessing the more energy you pull out of such a recharge system the more drag there would be just from the conservation of energy.

So if using the prop to recharge then it is not really the freewheeling but it is turning agains the resistance of the generator and belt/s etc.

This put me in mind of a first year Engineering experiment which was to measure the power output of a "Pelton Turbine". A simple waterwheel with cupped blades so that they would turn the water 180 degrees. It was attached to a brake and we measured the output to find when the turbine developed the most power.

When it was static the power output was low and when it was freewheeling the power output was low. The highest power was available when the wheel was spinning at half the velocity of the water stream because when the water rebounded off the blade it was at a complete stop relative to the observer.

That is me being open minded!!!

So maybe there are two minimum drag points for a propeller too?

There is an article for autorotation for a helicopter on Wikipedia.
http://en.wikipedia.org/wiki/Autorotation_(helicopter)
It shows that pitch adjustment is critical to make autorotation work because some of the blade is being used to spin the blade and some is being used to create downforce.

However ... what is the pitch is fixed to what is probably a high value like with a prop!

Still leaves us with the question. You could ask at the "boat design forum" ... there will be some guru who will know.

Or you could ask at "Sailing Anarchy" ... you would probably not get an answer, but you would have the members flaming each other for months to come!!!

MIK

I think that the matter of prop drag will not be able to be generalised.

there are a great many prop geometries and shaft friction factors.

consider a prop that is slow reving and has big wide blades....as oposed to a higher reving prop with long thin blades.... the drag situation has to be different.............then you consider pitch.

to be sure of the facts you would have to test a particular prop fixed and spinning then you would need to seperate the free spinning prop figures then add the shaft friction effects.

it occures to me that a prop of small cros section with sufficient pitch to spin its self freely running on a low friction shaft will present much less drag than others.... then you look at the size of the prop in comparison to the hull.

very complex matter and imposible to generalise.

cheers