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PostPosted: Tue May 17, 2011 11:36 pm 
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First the legal disclaimer for anyone silly enough to read the following. The author of this text and photos and the internet site publishing the data informs readers that the advice and data given is completely useless an if followed is likely to cause disease, outboard motor failure mass panic and harm and damage to anyone within a 100km radius of users of this information. Therefore anyone reading or following these instructions is completely responsible for the consequences of their actions and the author and others associated with this data are not and will never be responsible for the consequences of following these directions.

If anyone has trouble following these directions they are welcome to ask for advice which may be provided subject to the legal disclaimer above.

Now for the fun stuff.

Anyone using a Tohatsu/Mariner/Mercury 4hp, 5hp or 6hp with a high thrust propeller should be aware of the excellent performance in forward gear and the hopeless performance in reverse gear where the exhaust gasses cause the propeller blades to cavitate before any useable reverse thrust is developed. This long post explains how to modify the propeller to make it perform in reverse in a useable manner.

I did the modification on a new propeller and trialed it in INMA over several days cruising. My observations were the forward thrust was not noticeably changed but the exhaust noise may have been less audible, my guess is the hub exhaust was quieter with the modified propeller. The reverse thrust was tested but I found myself limited to about half throttle because the progress backward started to load the rudder up to an uncomfortable level. In 4 days cruising with lots of docking there was no cavitation of the propeller in either forward or reverse.

I did motor through seaweed which highlighted that the modified propeller buried itself in a ball of weed and soon stopped in both forward and reverse. The venturis in the modification made removal of the mass of weed doubly difficult. In fairness our 3.5hp dingy motor with a weedless propeller made a similar mess of the weeds before it died in a green ball of weeds. The good news is the modified propeller survived several meals of seaweed and it also digested a rope surviving more abuse in 30 minutes than I have given the outboard in six years. The rubber hub in the propeller worked wonders. Lesson learnt, high thrust just sucks more weed in faster to muck up your progress and the modified propeller survived.

The Modification.
Start with a standard Mercury 48-812951A02 propeller. If you do not have a high thrust propeller I suggest buying one from http://www.crowleymarine.com/mercury-outboard/681.cfm . Don't think modifying other propellers will give a result, the high speed propellers are designed to slip and probably will lose performance in both directions. You will also need some rubber hose 12mm outside diameter and some epoxy glue. I used automotive fuel injector hose because I had it on the bench.
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You will need the following tools:

Nylon ties to help mark out the hub.
A ruler to measure the hub.
White paint or whiteout to help mark out the hub.
A punch to mark the position of the hole.
A hammer to hit the punch.
A small drill to drill a pilot hole a medium drill to open out the hole and a 12 mm drill to finish the hole.
A drill press or a hand held drill and a vice or workbench to hold the propeller.
A fine saw to cut the hose.
an applicator to apply the epoxy
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Place the nylon ties around the propeller to help get a circle to mark the center of the hole. I painted the hub white to make it easier to see where I was working. The dimension from the edge of the hub will be 17mm to the center of the hole.
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I marked a flat strip with two marks 42mm apart then laid the strip on the hub measuring from the rear edge of the blade to the desired position of the hole. I had assistance, if by yourself use masking tape to measure the dimension on the curve. Use the center punch and hammer to mark the hole position.
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Next drill the hole starting with the small drill, then medium then 12mm drill. I had some trouble drilling the 12mm hole and getting a good finish in the soft aluminum.
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Cutting the rubber hose to make the venturi is best done with a fine saw cutting straight then 45 degree cuts to produce the desired venturi. I did not bother to measure the cuts, preferring to cut several about the desired size then pick the best 3 for use. I did try precision cutting and it was too difficult (easier to throw a few duds out or keep the long ones as spares).
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Put the 3 best venturi in the hub and position the venturi angled inline between the blades. The water runs between the blades and the venturi needs to be positioned across the flow of the water which is generally aligned with the blades. You will get to realign them later but I found it best to play with the angles sighting between the blades until I was satisfied with the angle.
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Make sure there is 1mm to 2mm rubber inside the hub to permit working the epoxy into the hole.
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Next mix a small amount of epoxy (I used a nylon tie) then place a dob of epoxy on the inside of the hub on the exposed rubber, work the epoxy around the rubber then rotate the rubber several times to work the epoxy around and into the hole. Position the venturi with the blade checking its position with the other venturis. Repeat the epoxy procedure with the remaining venturi.
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Leave the propeller to let the epoxy set.
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Put the propeller on the outboard per the manufacturer's instruction.
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This design is not optimised, I looked at Moody Blue's modifications on an 8hp outboard and copied them using common sense to position the venturi. It worked fine on my yacht and should work on other craft but that has to be a risk and responsibility taken by who ever uses this data. I don't believe its worth trying to optimise this modification, it works.

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PostPosted: Wed May 18, 2011 11:55 am 
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Greg,
You missed one important part, and that is to drill the final holes at an angle approaching that of the internal web on the propellor. This increased angle as opposed to your drill at right angles to the centre line, increases the venturi effect by a further 50%. Using your flat rear view of the prop you can see the angles used for the internal webbing.

If you look carefully at my original pictures you can see evidence of the angle required, and I stress again, drilling straight down at right angles is less effective than getting the hose to sit at an angle.

Well done though for taking the time to detail the proceedure.

Jeff (Moody Blue)
Inventor of this system.

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PostPosted: Wed May 18, 2011 1:10 pm 
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Jeff, I had enough trouble drilling the hole aligned with the hub without adding other angles. My preference for hand drilling on the smaller propeller resulted in me taking the simple method of no angles. Plus I did not have any measurement for the angle.

I was also keen to stay clear of the web and drill the hole a reasonable distance from the blades etc.

I had less interference than your setup and opted to add epoxy to secure the rubbers to make assembly simpler.

As I noted the propeller as modified did the job and gave adequate reverse thrust without cavitation, I was not brave enough to try full power in reverse to see whether I could get it to cavitate, the speed at half power was enough for most needs.

In case nobody noticed, the propeller modified in the photos is my old used propeller which is the second one I have modified. The first propeller I modified was a new one shown in the photo of the propeller on the outboard. The newer propeller on the outboard is sold to a TSP member.

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PostPosted: Wed May 18, 2011 9:56 pm 
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Tips
Use a vice to hold the propellor at the desired angle.

The first smallers holes can be at right angles to the hub centre.

The last hole can be done by using the highest drill press speed, patience, and slow advancing of the drill bit.

Doing this allows the bit to act more like a router than a drill bit, and the slow advance stops the drill bit catching and gouging out excess metal.

Cheers
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PostPosted: Thu May 19, 2011 11:02 am 
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Greg and Jeff, that is brilliant. I can see how it works, I had to check to see what the date was tho! :lol: A couple of Q's

* The prop used is not the sliding one is it, it's the lower pitch/larger diameter one? The sliding one is supposed to re-direct the exhaust in a similar fashion, but apparently isn't that great.

* Any feedback on the longevity of rubber and epoxy? My initial reaction was why didn't you use ally tube and weld it? I guess rubber is cheap, easy, and simple to replace.

* I'd use a 12mm hand reamer, this would give you an excellent de-burred hole and you'd be able to get a good controllable angle, and I was wondering wether just the relief hole at an angle would be enough, might try that first before adding the tubes. Better still we'll get Zeb to experiment, he loves that stuff. :lol:

POTY gentlemen, well done.

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PostPosted: Thu May 19, 2011 12:06 pm 
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Paul,
I did try just angled holes first, but the effect, if any, was not noticeable.

I then tried rubber as it was on the bench to find out what is the best angle in two planes ie in relation to the centre line, and as Greg has done, added a little more twist.

The intent was always to refine it then fabricate and weld aluminium pieces in place. Issue there is keeping the heat away from the rubber shock hub while welding.

In the end the burrs work well in retaining the fuel hose with-out the need for adhesives.
Also they are easy to replace if one ever comes loose, as it's about 20 cents worth of tube carried in the tool box as spares.

The end result on the Mariner 8hp on the back of a Boomaroo 22, using the fuel hose in my original pics, was that it now reverses so hard that the tiller loads up and pulls out of your hand :oops:

The other issue is as per Gregs disclaimer. It takes a fair leap of faith to put drill holes in a perfectly good propellor :roll:

Cheers
Jeff

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PostPosted: Thu May 19, 2011 12:28 pm 
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Paul, the rubber works well and suits the handiman.

The sliding prop is used on the 10hp outboards and AZ100 has a good modification for their exhausts. Weed is a big issue with the sliding prop set up. I suspect the 8hp propeller Jeff modified is the same size as the sliding prop leg.

Moody Blue's 8hp props had more interference between the rubber and the hole, I ended up with less interferance so I epoxied the rubber in position.

The 4, 5, and 6 horsepower outboards have a smaller propeller than the 8 and 10 horsepower legs so I felt there was less room to play with hole positions and angles, I was very conservative keeping it simple.

During my maiden cruise I made so many errors trying to get into an isolated anchorage over weed that I wrapped the leg in weed a few times and eventually lost the tender tow rope into the propeller without any notable harm to the venturis so I believe they are more than tough enough. Besides replacing a lost venturi is simple enough on the water.

My idea was not to perfect it but to make a simple modification that does the job. You could spend enormous time developing something but in the end the simple one shown does the job. The flows inside something like the propeller hub can be complex so trying to develop a better system could consume several propellers before you find a truly better design, which may only do a better job flat out in reverse which is something most of us would never use.

At half throttle in reverse, my rudder got significantly heavy with the water flow in the reverse direction, any more throttle risked damage on INMA. Remember the engine in the well pulled the water over the rudder on the RL24 significantly increasing the water velocity on the foil.

The idea of an alloy venturi was considered but I figured the rubber did the job and alloy could be more easily damaged and more difficult to repair.

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PostPosted: Thu May 19, 2011 12:35 pm 
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Quote Jeff "The other issue is as per Gregs disclaimer. It takes a fair leap of faith to put drill holes in a perfectly good propellor" My first one was a new out of the box propeller, my faith in Jeff's work was justified.

There was a lot less stress drilling the old propeller in the photos.

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PostPosted: Tue Jul 05, 2011 8:59 pm 
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Hi Guys,

I just caught on to this discussion. I am sure that the example given here by Greg will work on the Mercury OB that does not have the sliding prop shaft. My prop was modified by Jeff and as he says in the comments above the way he installed the venturi tubes is slightly angled. The angle in designed so that when the outboard in put is reverse the gases are actually captured by the tubes as their angle is slewed!! (not sure of this term) to suck the gases out. The opposite is true when the motor is in forward gear, ie. the water is sucked in and forces the gases away from the shaft quicker.

Most certainly the modification by Jeff on the prop did work very well. I am guessing the the mods by Greg will also work because with the modifications at least there are escape routes for the gases when the prop is in reverse and the water pressure is increased from behind. I am guessing that this effect is the same as that experienced if the 8 HP Mercury had a sliding propellor.

Like Greg I bought a 4 bladed Solas propellor and it works. However I feel that although I have excellent reversing capacity now, I have lost a bit of power when in forward. Greg and I are going to be discussing this in a few days, I hope.

In conclusion if you have a reversing problem this modification of Jeff's and Greg's really does work.

Below is a picture of Jeff's modifictions to my propeller

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Below is a video of my modified propellor working (please ignore my confused Forward/Reverse commentary, I get like that way when I get excited :? :shock: )


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PostPosted: Sun Jul 10, 2011 2:33 pm 
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Hello all,
Very interesting, Is the original document by Moody Blue still available?
Peter

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PostPosted: Sun Jul 10, 2011 9:14 pm 
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Peter,
I never had time to document this procedure. I did post some pics of the first one I did on a Mariner 8hp.
Greg (Inma) put the time and patience in to document the procedure.

Cheers

Jeff
(Moody Blue) Sonata 26

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PostPosted: Mon Jul 11, 2011 5:34 am 
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The webs supporting the outer hub are in a different position on the bigger propeller compared with the smaller 4-6hp propeller. This results in the position of the angled hole in Jeff's design not being practical on the smaller propeller.

Anyone copying the modifications on different propellers would need to make sure that the area being drilled is suitable for insertion of the rubber eductor.

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PostPosted: Tue Jul 12, 2011 12:29 pm 
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Thanks for documenting the process Greg, beautifully done and very helpful. Not sure if it has been raised, but have you considered doing a Cruising Helmsman article about it?

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PostPosted: Tue Jul 12, 2011 4:52 pm 
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Mojo, I doubt there would be a need for the modification for most users of the smaller outboards.

It really is only trailerable yachts that seem to need the lighter outboards and a good reverse.

And I have not done anything to optomise the modification, it just works.

I do have to observe that Mercury should have put more effort into providing a better reverse thrust to go with the high thrust propeller.

Its interesting that the larger new 8 and 10hp mercury products have issues with reverse thrust.

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PostPosted: Thu Sep 15, 2011 1:25 am 
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I bought the hithrust prop from Greg and he kindly completed the exhaust modification for me. Prior to this I was using a low pitch prop on a new 5hp 2stroke. The boat is 8m long and all loaded up was 1600-1700kg. Wow, what a massive difference. The old prop in reverse was near useless. It would instantly suck it's own gas when put in reverse. The prop would get no traction at all as it was spinning in gas, not water. The modification changed all this, she now reverses nicely and has plenty of bite.

Performs well in forward, but I suspect this is more due to changing to hithrust blades rather then the modification. Can sit on 6.5-7kts boat speed, the old prop maxed out at 5kts.

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PostPosted: Mon Apr 16, 2012 3:33 pm 
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Since this thread stopped, I have modified a couple of propellers for others and I've been told by others that they have copied the modifications.

Could people who have tried these modified propellers place their observations here so others can gauge the value and issues modifying small outboards.

I have also had an American outboard dealer tell me the mods won't work and the loss of forward thrust will be a big problem. That's not my experience but he was an expert who quickly made a judgement without actually testing anything, I am interested in other's experience.

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PostPosted: Mon Apr 16, 2012 4:45 pm 
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As the inventor of this tube method, I think I'm WAY MORE clever than any "Yankee" Expert :oops: :oops:

Jeff


Maybe feedback from Nick on "Hot Chocolate" as that is the oldest mod that has now been running for over four years now, and to my knowledge has never lost or had to replace the pressed in hose parts.

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PostPosted: Fri Apr 20, 2012 11:09 pm 
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Hi Inma,
In reply to your request for other peoples outcomes on the prop mods.Last year after reading your post on how to do it ,I used my old prop to do the mods with .Engine is a Yamaha 8 hp with a 9 by 7 high thrust prop.Prior to the mods ,in reverse it was absolutely useless tried everything slow ahead,low revs ,high revs,it just cavitated as you would know very awkward when trying to dock.
Anyhow the good news is IT WORKS ,in fact its that good I,ve left the old prop on and will do the same to the newer prop this winter.
The holes were drilled at right angles to the hub,and the fuel hose epoxied in at an angle also the end angle cut.
Cheers David

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PostPosted: Mon Jun 04, 2012 5:04 pm 
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I was just looking up the american site for parts and notice the price of the high thrust propeller has virtually doubled to $143 which is disapointing. With modern manufacturing parts like propellers are meant to get cheaper as the quantities increase which is how the accountants justify Tohatsu making Mercury, Mariner and Evinrude, pitty the price did not drop.

To add insult, someone recently advised me that Mercury dealers are no longer allowed to change the propellers free on new 5hp outboards. This may need to be tested when someone gets a quote for a Mercury outboard.

It looks like Mercury is making profit where ever possible. Remember Tohatsu and Evinrude are the same outboards so some shopping around for better deals might be possible.

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PostPosted: Fri Jun 29, 2012 12:07 am 
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Tonight Inma (Greg) kindly made the same modification to the high thrust prop on my new Mariner 5hp 2 stroke. (Yes, fingers do get crossed while watching 3 big holes being drilled into a new prop. No need to worry, though: Greg carried out the job calmly and professionally.)

Given my hibernation in Melbourne's winter it will be a few weeks before I get out onto the water for a trial. Can't wait to back into a marina berth at 6 knots! :lol:

Thank you again, Greg, for doing the work and to Jeff for the original idea.

Peter

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PostPosted: Fri Jun 29, 2012 1:27 am 
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sailingpeter3 wrote:
(Yes, fingers do get crossed while watching 3 big holes being drilled into a new prop. No need to worry, though: Greg carried out the job calmly and professionally.)


Private message to Inma:
You've got them fooled!

:D

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PostPosted: Fri Jun 29, 2012 4:44 am 
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To anyone doing it themselves, I learnt a lesson on Peter's propeller. Even though we center punched the hole position before drilling the first hole, the drill drifted off as it went through the soft alloy and not down the curve but in the worst direction possible.

I'd like to know why but may never know, I dropped the first drill size to about a 2mm drill and the other two holes were fine. Drilling the soft alloy took care and going up to larger drills in smaller size increments made it easy. I cant explain why but this time the soft alloy did not machine as smoothly as past holes in propellers. The final holes were fine but I was a bit frustrated even with the drill press.

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PostPosted: Mon Nov 12, 2012 11:14 pm 
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sailingpeter3 wrote:
Given my hibernation in Melbourne's winter it will be a few weeks before I get out onto the water for a trial.


I've just come back from two weeks on Gippsland Lakes and put the outboard through its paces. In a nutshell, the modified prop works brilliantly in reverse: it bites immediately and speeds off as if in forward - something I've never experienced with any other outboard / prop combination. Forward performance remains as good as ever. It's a joy - and safety feature - knowing that when reverse is needed it will work quickly and very well.

Happy sailing.

Peter
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PostPosted: Tue Nov 13, 2012 3:43 pm 
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INMA wrote:
To anyone doing it themselves, I learnt a lesson on Peter's propeller. Even though we center punched the hole position before drilling the first hole, the drill drifted off as it went through the soft alloy and not down the curve but in the worst direction possible.

I'd like to know why but may never know, I dropped the first drill size to about a 2mm drill and the other two holes were fine. Drilling the soft alloy took care and going up to larger drills in smaller size increments made it easy. I cant explain why but this time the soft alloy did not machine as smoothly as past holes in propellers. The final holes were fine but I was a bit frustrated even with the drill press.


If the final drill size is 12-13 mm then the second last drill size should be 10 mm.

For the final 12-13 mm angled hole select the highest speed the press has. This will allow the drill bit to act more like a lathe on the soft metal.

I used the same method for the first Mercury propellor and for Wanton's propellor with out any issues.

PS: Thanks for the compliment on inventing the idea. In reality I'm happy if I've contibuted to another TSP member being safer and less stressed.

Jeff

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For this message the author MoodyBlue has received thanks: INMA (Tue Nov 13, 2012 4:21 pm)
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PostPosted: Wed Nov 14, 2012 8:00 am 
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Able Skipper
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Does anyone know the part numbers for the Mariner and Tohatsu equivalent to the 6 x 8 3/8 Merc High Thrust prop ? ?

Ive got a 5hp 2stroke mercury and I'm pretty sure its got the "standard" tinny prop which is made of plastic I think. I'd like something that works better in reverse too.

Come to think of it, Ive got two high thrust props to suit an 8hp motor, maybe suzuki ? ? I'll check them out and see if anyone wants them.

cheers,
A

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