When l purchased my LM30 last August l was surprised how little prop walk there was compared to my previous boat a Moody 33 so l would think it may well be a prop sizing issue. l would therefore have a chat with a local prop specialist to determine what size prop you should be running in relation to the engine/gearbox setup before splashing out on a replacement prop.
Thanks for your reply Nick. Mine is an LM 30 fin with a 36 hp bukh sail drive with a 15'" 3 bladed fixed prop, pitch unknown is yours a similar set up? Do you know your prop size? I've already spoken to Bukh and prop a manufacturer and they've recommended a 16 by 11 prop
All l know at the moment is that the prop is probably a 15" 3 blade fixed but l have no idea of the pitch. This prop does work well with the Bukh 20hp so a 16" dia, prop would seem to make sense in view of the difference in engine size.
l would tend to be guided by the experts as trial and error in trying different props is a very expensive exercise ! If l were doing this l would locate a secondhand prop and try that before committing to a new one.
On the other hand it might be worth asking the question ( on this board ) does anybody know the correct size prop for your engine set up, hopefully other LM30 owners may have that info.
Thanks very much for your help in this matter Alll the prop manufacturers I've been in touch with have recommended a 16 by 11 prop and all state that it's unlikely to change prop walk. They thought it might be an inherent fault with the vessel I e prop too far away from rudder. Hope someone with a similar engine size , 36 hp will let me know if they've found this to be so Thanks again for your input on this
l just wonder if the prop manufacturers may be getting mixed up with the LM32. This has the prop a lot further forward then the '30' and as noted in a used boat test ( see post in 'Ideas for sharing ' on page 2 ) going astern may not be straight forward. l read all the test reports l could find on the LM30 before purchasing and as far as l can recall nothing detrimental was said about the LM30 going astern. To me it's got to be a prop issue, although bear in mind that every single prop boat has some prop walk ! Just a thought the prop is'nt damaged eg bent blade or even a blade missing - has been known although you would probably have felt a severe vibration at all speeds ahead and astern.
l hope somebody else with a '30' comes along with comments on this matter.
Before spending vast amounts on a bow thruster, master what you have.
Once you have about 1 - 2 knots sternway, very little rudder is needed to steer a good course. The trick is to remember NEUTRAL IS A GEAR and use it. Once backing, know that the boat will pull to one side if in reverse, usually to starboard. Put it in reverse while still go slowly ahead, let the prop walk get your stern on the right course, then gun the engine in reverse for 2 -3 seconds, then shift to neutral and glide. You may have to do this several times to get steerable sternway. Once you have 1 -2 knots astern go to neutral. Use short 1 -2 seconds in reverse idle rpm to keep up speed. The rudder becomes a highly effective canard in reverse - do not oversteer. Never let go of the wheel or the rudder will slam to one side. I usually go hard astern while still making slight headway, and let the prop walk both stop the boat and swing it to the correct course (best to plan the maneuver so the prop will pull the stern to starboard). Go to neutral as soon as there is steerage astern. If you leave it in reverse, you'll likely loose control. Leave it in neutral to make a sharp turn either way into your slip or the Tiki Bar dock. Watch the tiller socket, it is your rudder position indicator.
The shallower the water, the more 'walk' effect the prop has - consider it a sidewise paddle wheel. A faster turning prop going astern seems to have less walk affect, I believe because it is pulling so much water over the rudder that the paddle wheel effect is literally washed away.
My boat Valiant is an LM-32 with the Volvo 36 hp, and I can make her dance - except in a cross wind. For docking/undocking, short strong bursts of forward power with the rudder hard over will act like a stern thruster - but warn your crew as it will really rock the boat.
Practice in calm conditions and you'll have it down in no time. The marina crowd will gather to watch you cruise stern-first down the fairway. Best wishes - and don't forget 3 short horn blast for going astern!
Capt. Heart of Palm, S/V Valiant
Last Edit: Jun 30, 2016 12:58:29 GMT by chrismunson
Before spending vast amounts on a bow thruster, master what you have.
Once you have about 1 - 2 knots sternway, very little rudder is needed to steer a good course. The trick is to remember NEUTRAL IS A GEAR and use it. Once backing, know that the boat will pull to one side if in reverse, usually to starboard. Put it in reverse while still go slowly ahead, let the prop walk get your stern on the right course, then gun the engine in reverse for 2 -3 seconds, then shift to neutral and glide. You may have to do this several times to get steerable sternway. Once you have 1 -2 knots astern go to neutral. Use short 1 -2 seconds in reverse idle rpm to keep up speed. The rudder becomes a highly effective canard in reverse - do not oversteer. Never let go of the wheel or the rudder will slam to one side. I usually go hard astern while still making slight headway, and let the prop walk both stop the boat and swing it to the correct course (best to plan the maneuver so the prop will pull the stern to starboard). Go to neutral as soon as there is steerage astern. If you leave it in reverse, you'll likely loose control. Leave it in neutral to make a sharp turn either way into your slip or the Tiki Bar dock. Watch the tiller socket, it is your rudder position indicator.
The shallower the water, the more 'walk' effect the prop has - consider it a sidewise paddle wheel. A faster turning prop going astern seems to have less walk affect, I believe because it is pulling so much water over the rudder that the paddle wheel effect is literally washed away.
My boat Valiant is an LM-32 with the Volvo 36 hp, and I can make her dance - except in a cross wind. For docking/undocking, short strong bursts of forward power with the rudder hard over will act like a stern thruster - but warn your crew as it will really rock the boat.
Practice in calm conditions and you'll have it down in no time. The marina crowd will gather to watch you cruise stern-first down the fairway. Best wishes - and don't forget 3 short horn blast for going astern!
This all sounds like excellent advice. Now would someone please do the same for the LM 24/27 which has a long, very shallow keel. My 8 years of manoeuvering Marguerite leads me to conclude that you cannot reverse such a boat into a confined space like a berth without the aid of numerous, strong crew to push, pull and fend off. I would love to be told how to manage things better. Charles
Like Chrismunson wrote, a few (fractions of) knots sternway, might exceed propwalk effects. when rudder and tiller are not precicely aligned the poor helmsman might blame the prop, but one degree off center of rudder position might have a more significant effect. flowing water of either direction will induce a momentum at the rudder. Rudder design, alignment and even barnacles etc. might have significant effents (in my opinion). Also bearing clearance might have an effect. If tiller and rudder-blade are not 100% close coupled, going astern might result in 'leaning' the rudder blade to one side, without movement of the tiller. As soon as the boat speed is sufficient to induce a momentum at the rudder, this effect will interfere with prop walk. Such interference is, what makes prop walk sometimes a mystery (in my opinion).
In my LM27, which is equipped with a becker rudder (large cross-section with flap at end)this effect is quite pronounced. Propwalk in my LM is only noticable, as long as the boat is hardly moving and there is no flow and no momentum at the rudder. Going astern the "nervous" flap will outperform the propwalk very soon even at slowest speeed but also allows using the rudder for highly sensitive "canard-like steering" which requires extreme attention (for good reasons, canard steering in aircraft is always done by computer control. It's so sensitive and needs permanent attention, that human pilots would crash within short time). But as Chrismunson wrote all such consideration might become obsolete in crosswinds - the superstructure of my LM27 wants to go sailing on her own, like a mongolian wild horse (not to say a stubborn donkey). In cross winds I have to eliminate all degress of freedom for the LM (by rope and careful maneuvering), otherwise wind induced drift will exceed all maneuverability that can be achieved at slow speeds tolerable in a crowded marina. (i'm not complaining - it just never gets boring ;-) ).
