Locking the prop

Re: Locking the prop - whoopee, here are some references

Not going to argue on this but you may be interested in knowing that even the "experts" have different opinions on this. In the well respected book "Principles of Yacht Design", Larsson & Eliasson, they claim that the drag from a fixed propeller is not 2x, as in the US Navy Towing Manual, but is 4x the drag of a freewheeling propeller. The few actual designers/builders of sailboat propellers (ie those with a high aspect ratio to minimise drag) whose opinion I have heard, claim that their propellers produce less drag when locked.

So, which is correct or are they all wrong?

For Wally_Vela with a propeller which has "a cover factor of 85%" (assuming that you are referring to BAR), that propeller should not be on a sailboat if the figure quoted is true. It may or may not produce more drag one way or the other of locked or unlocked, but whichever it is academic, it will produce inappropriate high drag whatever you do.

John
 
Re: Wayward Son

The stern walks in when the prop is in forward as well!! The paddle wheel effect acts whenever the prop is turning. What causes paddle whell effect? Go back and read again. The paddle wheel effect is caused by pressure differences, between the blades at the top of the revolution and those deeper in the water. A boat (single screw) with a right hand prop, will turn far quicker to port than to starboard, due to paddle wheel effect, whilst travelling forward. Why does a twin screw boat with outward turning props turn on a sixpence? Because the paddle wheel effect is doubled with one prop going forward and one going astern. As for the priop being designed to go forward, correct,but this is causing the paddlewheel effect, rubbish!
 
Re: Locking the prop - whoopee, here are some references

to be quite honest - I would be suprised if the experts agree on much at all, but in this case its not even grey is it - quite the opposite in fact . ....... but there again I am a bit cynical where scientists are concerned

/forums/images/graemlins/cool.gif
 
Re: Locking the prop - whoopee, here are some references

QUOTE:

For Wally_Vela with a propeller which has "a cover factor of 85%" (assuming that you are referring to BAR), that propeller should not be on a sailboat if the figure quoted is true. It may or may not produce more drag one way or the other of locked or unlocked, but whichever it is academic, it will produce inappropriate high drag whatever you do.

John

UNQUOTE

/forums/images/graemlins/blush.gif OOOPS /forums/images/graemlins/blush.gif
Should have been 65 and not 85.

May be so, John, but what would you have put on a Centaur that is fitted with a 25 SHP continuous?

The power is necessary because of the steep seas that are very common down here whenever it has blown NW or NE (most times) or SW (not so often) for a couple of days because of the long fetches and the sharply shelving seabed. Difficult to fit a larger diameter because of (lack of) clearance.

Fair winds! /forums/images/graemlins/blush.gif
 
Re: Locking the prop - whoopee, here are some references

Pardon me for jumping in, with not very much to add....I see that you have a similar engine to mine; I have the Yanmar 4LH-TE and was advised by the previous owner and the manual to NEVER engage gear to lock the prop. As I had always done just that (for the same reasons that have been given here ad nauseum and also with the desire to reduce wear) I had a terrible job to get it back OUT of gear when I wanted to start the engine. The only way to do this was to start the engine in gear, and that required me to wire out the interlock. I really don't know what to do but for the time being I am leaving it freewheeling.
 
My previous boat was fitted with a Volvo Penta 2030 (27HP) OK so it's really a Perkins! and from memory suggested that you should run the engine if left rotating in neutral for more than 10 hours to circulate gearbox oil (never understood this because I thought that the rotation would achieve this anyway). I too found that if I put it in reverse when sailing then I could not get it out above about 6 knots and had to start up in reverse.

My new boat (3 months) has a Yanmar 4JH4E (54HP) but as I opted for a performance pack it can as standard with a Volvo 3 blade folding prop. Oddly enough, simply sailing in neutral the shaft still rotates, putting it into reverse to lock the shaft obviously stops this and then neutral can be re-engaged easily and the shaft does not start to rotate.

Both boats were purchased new and on both the shaft could be heard rotating (rumbling) which stopped by engaging reverse, it is therefore extremely unlikely that the noise is due to worn bearings.

As for the folding prop. Don't know if it sails faster because it's always had it. Performance forward seems OK but seems to need more revs than I would expect at lower speeds but fine once revs get up. As for reverse, well that's another story, there seems to be virtually no reverse thrust, probably about 10% of forward. Trying to slow the boat down (never mind stopping it) would probably be just as good by chucking a bucket over the stern as a drouge. What you do get is tons of prop walk. Trying to reverse the boat from a standstill causes the stern to turn through about 90 deg to starbd before the revs can be reduced and any steerage way can be achieved. I was seriously considering binning the folding prop for a real one.
 
Re: Locking the prop - whoopee, here are some references

Lemain,

I am responsible for starting this post so blame me for the ad nauseum! I thought that I had asked a simple question - but it has turned in to a bit of a marathon issue.

Interesting that you've had the locking in gear problem with your N42 as I have had with my N39. It's obviously the engine/gearbox configuration in the Nauticat range that has this issue (amongst others).

Like you I don't know what to do which is why I asked the question in the first place. I am less concerned with the technical arguments on here about whether there is more or less drag when locked and more concerned about undue wear on gearbox/shaft seals/engine. I've decided that I'm gonna freewheel for the rest of this season and then invest in a folding prop this coming winter. It seems to be the only real solution.

Rob
 
Re: Locking the prop - whoopee, here are some references

Robin, I have been tempted to buy either a folding or variable (e.g. Maxprop) propellor but I have doubts. Folding props are ideal for pure sailing yachts - where excellent (competitive) performance under sail is required at the expense of complexity, lower strength and prop performance. The variable (Maxprop) propellors are excellent when uder power - probably a better choice for a N39 which is a motor-sailor than the N42 which was designed to be a sailing yacht with an auxilliary. Then again, like most cruising folk, we spend a high proportion of our time under power so a Maxprop looks interesting. However, I am concerned about the consequences of the prop being fouled; most areas of the world can make up a new fixed prop whereas the repair of a Maxprop is a specialised task.

Like you, with all this clarity of vision, I have decided to do nothing for the time being /forums/images/graemlins/laugh.gif
 
Re: Locking the prop - whoopee, here are some references

Hmmmm.........I hear what you say but I think the possible performance improvement from a folder makes it worthwhile. The N39 itsn't a motorsailer but a straightforward yacht - designed by Nauticat for fast sailing - and whilst we're not racers an extra kt would be very nice. But the no' 1 priority is to get the locking issue sorted and to get away from rumble and gearbox/shaft seals wear issues.

rob
 
Re: Locking the prop - whoopee, here are some references

Robin, Sorry, I had believed that the N39 was one of the motor-sailors - so we are in the same position and it seems that we think alike.

Is your belief that you would get an extra knot when under sail with a folding prop backed by any evidence on this type of yacht? It sounds like a huge %age increase. I had always thought that folding props were less efficient, when driven, than a properly selected fixed prop and given that when under power economy, range and endurace are all issues I would be loathe to sacrifice any material performance unless the sailing performance was dramatically improved.

I am with you 100% on the rumble and wear issue - there must be gears idling in the gearbox, as well as seal wear, plus the cutlass bearing. It feels wrong to let it idle for days on end.
 
Re: Locking the prop - whoopee, here are some references

I regret my proposition of a 1kt improvement in speed is not based on any scientific study - it's merely what I've picked up from hereabouts and the suchlike. Many seem to particularly rate the Bruntons Autoprop - but at about £2k it needs to be good. Agreed that a 1kt improvement under sail is a huge proportionate increase in speed. I'm also told that under engine one can expect a similar improvement. I've a friend with an Oyster 435 who has supported these claims and reckons it's a good investment. Also promised is better astern performance - it's seems to be widely talked up!


I don't know whether you've followed any of the travels of the N43 "Triumph" which has recently crossed the Atlantic from Florida - in the log they have mentioned that "nobody should ever cross the Atlantic without a folding prop" - they'd done the maths and reckoned that their folder had saved them three days at sea.

Good luck with your trip - I see you've moved from Chichester to Falmouth - you're obviously on your way - keep us up to date with progress; one day I hope to follow you!

rgds

Rob
 
Re: Locking the prop - whoopee, here are some references

Robert, You can take or leave my advice with whatever seasoning you wish to apply - especially as we know each other.

For what its worth, we have a three bladed prop and we will be changing it to a folding or feathering one as soon as funds allow.

Despite the arguments and alleged figure proving one thing or another, my experience of Yacht type props is that locking them makes us speed up. Not a great amount, but its certainly been measurable in the past. Reading through the posts I suspect that power boat props don't behave in the same way.

I also worry about the shaft spinning all the time and wearing our bearings etc. Another good reason to lock the shaft by putting the engine in reverse.

Pay your money and take your choice as they say.
 
Re: Locking the prop - whoopee, here are some references

I go along with what you say too John.

Also, I suspect that those who claim great increases in sailing performance from going to a folding/feathering one did not have good narrow bladed sailboat props in the first place. From what I read here and see on other boats, wide bladed style props are frequently what seem to be fitted, but as Wally_Vela has pointed out some boat designs do not allow the fitting of the greater diameter that a sailboat prop requires.

Am not disputing folder/feathering has lower drag, but must be seen in the light of compared to a good fixed sailboat prop the difference will not be as great as some claim and there will be a drop off in motoring performance, unless a very good variable pitch prop is fitted (very expensive), and higher maintenance which may not suit all.

John
 
Re: Locking the prop - whoopee, here are some references

John,

I'm leaning toward the idea of locking - the practical responses on here seem to weigh in favour of this as regards speed. But as you know I'm no racer and I'm more concerned about mechanical wear and tear. Someone on here suggested locking the prop and then when coming to unlock just tweaking the starter sufficient to release the "jam" from the gearbox and then start the engine proper. This idea seems to get around both the rumble and the need to start the engine whilst in gear. I'm gonna give it a go at the weekend.

rgds

Robert
 
Re: Locking the prop - whoopee, here are some references

John

Our current boat originally had a folding prop and indeed we still keep this as a spare, but the previous owners changed it for a Brunton which self pitches and feathers it's 3 blades. I had never had a Brunton before but know from talking with other owners of the same boat as us that it performs better under power than their folding props which is not surprising but also much better than fixed 3 bladers which is surprising. The blades reverse themselves in astern and Brunton claim a crash stop in reverse in a boat length from 6kts, I haven't tried it but we sure have good brakes going astern! With just a whisper of wind assist the Brunton increases it's pitch like switching in overdrive and for the same revs the speed goes up 1/4 kt or more, if the wind and waves are stopping us the converse is true, it lowers the pitch like dropping to 3rd gear for more power and you can regain speed with a touch more revs if needed. Brunton claim that you can always extract max HP from the engine because the self pitching allows max revs to be reached even if punching a headwind/sea or if towing. That said we rarely use this outside of harbour because we can sail as fast upwind in those conditions more comfortably, quieter and for free!

Under sail our boat is a light wind beauty, we can get going in the lightest of breezes thanks mostly to the design expertise of Doug Petersen and a decent sail area but the Brunton for sure is playing a big part in these conditions as well. We sailed home last weekend with a light tailwind that went from F1/F2 with a couple of gusts nudging F3. Because the wind started right on the tail I didn't even bother to hoist the main for the short trip of around 17mls and we sailed the whole way with just the genoa even when the apparent wind went to about 100 degs. Under these conditions a fixed 3 blader on an open shaft like ours would be like towing a big bucket behind and I even worry about the extra drag of a fishing line! /forums/images/graemlins/laugh.gif

The drawback as you say is the cost and some extra maintenance. The cost in our case was not paid by us as it luckily came with the boat. The first haulout I did adjust the blade bearings because on our earlier type the blades 'rattle' at tickover ahead (50 rpm more stops it) and I actually bought a bearing replacement kit but kept this as a spare as adjustment was still within tolerance. Now 3 years on the only maintenance needed has been to clean and bright polish it before relaunching.

Robin

PS Congrats to NZ's Michael Campbell for his USA Open Golf win, absolutely stunning golf last Sunday and to beat one charging T. Woods esq down the stretch too, wonderful.
 
Re: Locking the prop - whoopee, here are some references

Sounds nice! How much do they cost?
 
Re: Locking the prop - whoopee, here are some references

Yes they have a good reputation Robin, but I have never been on a boat with one. A friend has a boat of very close design to ours and has a true variable pitch prop and regards it as well worthwhile performance wise. Have known of a number of owners on cruising boats who have dumped folders because of performance issues

Brunton must be good as they sell a shaft lock for fixed props which among their claims for it are that it reduces drag /forums/images/graemlins/smirk.gif.

Yes Michael Campbell did ok. Just hope he keeps it all mentally together this time round and does not go to pot again.

Regards

John
 
Re: Locking the prop - whoopee, here are some references

Simon

See http://www.autoprop.com/

The bad news I suspect is they wouldn't fit in the small aperture of a Co32, they need a little space fore/aft to swivel the blades. Prices I don't know but I suspect £1500 +/-, We didn't have to buy ours fortunately!

Robin
 
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