High Thrust Props on Outboards.

[AndCur]

Folks

I have a Mariner 6hp 4 stroke outboard which I use on my Achilles 24. The combination of this engine and the standard prop will easily push the boat at hull speed.Even when pushing into a sharp sea I have never experienced cavatation etc. So was wondering if there are any advantages in changing the prop to the Sail power (high thrust version). I had spoken to a couple of outboard dealers who all seem to agree that stopping power in reverse will be improved if I change props but don't seem to think forward thrust will be much better. Hopefully you will be able to shed some light on this.

As always thanks for the reply's


<hr width=100% size=1>

 

[Talbot]

I changed the normal prop on a 30 hp yamaha for a hi thrust version. performance in calm weather was actually a bit worse. However, performance in bad conditions was better, and astern was much better.

<hr width=100% size=1>

 

[oldharry]

generally speaking high thrust props give a better blade area ratio giving a larger surface to push against the water, and are pitched finer to allow the engine to develop higher revs and so more power under heavy loading conditions such as a strong wind and head seas. Both elements will improve the performance in reverse.

If the engine is already performing adequately in bad conditions then a prop change will only give marginal improvements, and may not be worth the cost.

<hr width=100% size=1>

 

[andyball]

Tried a high thrust one on a yam F8 on a displacement boat.....reverse was hugely better, although the light boat meant just needing lower rev's to stop smartly rather than it wouldn't stop with the stock prop. Forward speed the same but at higher rev's...prob. good for towing.

Often the high thrust one has different hub to divert exhaust when astern, hence better stopping, & usually smaller pitch, so less movement for same rev's....if stock one seems ok, think you'll find reverse better, but simply higher rev's for same speed in forward.

Think the high thrust ones better if larger diameter matched to "high thrust" gearbox with space for bigger dia. prop + slower turning for same eng. speed.



<hr width=100% size=1>

 

[gasax]

I've just fitted one to an 8hp Johnson which is on a Robber III (25ft).

Slightly better forward speed in flat water (maybe), better in a chop and much improved astern. So I can now actually reverse out from the pontoon in a stiff breeze!

If you don't have any problems like that then the £70 would probably be wasted.

<hr width=100% size=1>

 

[rex_seadog]

On the Mariner 8 (2 stroke) Sailmate the prop moves backwards and forwards on splines. In forward gear the exhaust gases pass through a cone in the centre of the prop but when reverse is selected the prop slides backwards (i.e. aft) and the cone diverts the exhaust so that it exits for'ard of the prop thus allowing the prop to operate in clean water, as well as reversing the thrust provided by the exhaust gas stream. When we first bought the engine 5/6 years ago for our Hunter Delta 25 the high reverse thrust feature appeared to work as claimed but after less than 20 hours use the prop had moved backwards on its rubber bushing about 5mm relative to the inner bronze bush so allowing exhaust gases to escape forward even when in forward gear. Mariner (E P Barrus) replaced the prop under warranty and so far the new one has been OK. We also tried the prop without the sliding feature by removing the cone ( I think you need a different spacer washer if you do this) but there was a definite fall off in stopping performance so after one season we reverted to the original set-up. The pitch of the Sailmate prop is designed to give hull speed at near max revs so the same prop is used with or without the reverse thrust sliding feature. An engine used on say a planing dinghy would of course need a prop with a coarser pitch. It could be that the pitch of your existing prop is already more or less correct for the Achilles and it may be possible to just add the sliding feature. Tim Bennet at E P Barrus is very helpful and will, I'm sure, provide all the answers.

On a previous smaller boat I had a 4 HP Mariner Sailmate but this did not come with the sliding prop.


<hr width=100% size=1>

 

[Micky]

I asked a similar question on this subject but never got to the bottom of knowing Werther if i change to a sail power prop would be to any advantage. My outboard is a Mercury 8hp 1998/99 model, and pushes my Vivacity 21 at 4.5 knots flat out and uses about 10 litres of petrol in about an hour.
The engineers at one of my local boat yards seem to think a replacement prop would make a lot of improvements to speed and fuel economy, prop would cost £82.00.

This is where i get the problem, some people say it is all to do with hull displacement and a new prop would NOT make any difference on the Mercury as the engine torque is for a plaining hull, all speed and no torque..... I am lost.

Now the strangest part. I have an old Seagull long shaft century outboard, i love the noise of this old thing, this will push my boat along at 6 knots and run up the river and back for an hour on less then a tank of fuel, i don't know what the tank holds, probably 2-3 litres.

Hence i am still looking into options because i would love to get my Mercury to perform the same way as the old Seagull, after all, i just paid £500 for it, and the Seagull only £40.00, surly the Merc should perform better and maybe, even more economical.

Any ideas, anyone.


<hr width=100% size=1>

 

[rex_seadog]

Definitely something wrong here, Micky. The basic engine of our Mariner 8HP Sailmate (also 1999) is identical to your Mercury. In reasonably calm conditions we can achieve the hull speed of about 6 knots in a our twin keel Delta 25 with a displacement of around 2000kg. (We once tried a 5HP Yamaha and that gave us over 5 knots). Even flat out our Mariner never consumes more than 5 litres per hour. If 10 litres per hour is the genuine fuel consumption for your Mercury this seems to suggest that you have some other problem with the engine. It could well be that the existing prop is not the optimum for the Vivacity but even so I would have thought that the standard prop would have performed better than you describe. Strongly suggest you get full details of your existing prop (should be engraved on it somewhere) and talk to Tim Bennett Of E P Barrus (01869 363636) who are the distributor for both Mariner and Mercury. By the way, what is the HP of the Seagull? I tend to think of the small (about 1.5 HP) egg whisks but I'm sure that yours must be bigger.

<hr width=100% size=1>

 

[oldharry]

4.5kts at 10 litres to the hour is quite appaling! I put a Yam 8 on the back of a heavy displacement 20 footer, and got 5.3. kts at around 5 litres/hour. Your Merc ought to be doing a lot better than that, particularly if the old Seagull (and what my I ask is so 'nice' about the ear splitting racket and trail of oil they leave????) is giving so much more speed. Seagulls were not noted for their speed.....

So your engineer is right, the Merc should do much better than a seagull. See if he will put his money where his mouth is, and sell it to you on a 'sale or return' basis if it doesn't make an improvement

<hr width=100% size=1>

 

[Micky]

Hi Oldharry, tried the sale or return bit but it never worked.

Will take the spec of the prop and make more inquiries.

Seagull century outboard is 100, don't know what that is in terms oh HP, has much bigger prop then the Merc.

Thanks for the help so far, now got a couple of things to look for/at......cheers.

<hr width=100% size=1>

 

[Micky]

Thanks Rex-seadog, will take a look at the prop size then check it out. 10 litres is all the tank hold, from brim full all gone in one hour. Just got hold of a 30 litre tank, not the anser i know, but at least we won't run out again.

<hr width=100% size=1>

 

[oldharry]

If the prop is overcoarse for the job, it will be moving the boat quite rapidly at low revs, but not increasing the speed much as you open up the throttle. I.e you maight be getting 2- 2.5 kts at fast tickover, but opening up doesnt make much difference, plus the engine is not developing full revs.

If this is what is happening then you definitely have too coarse a prop, and a change will give a significant increase in top speed, and much easier handling at low speed.

<hr width=100% size=1>

 

[JimC]

I bought a Mariner 6 hp fourstroke for my Hunter Horizon 23 to replace the 5 hp Yamaha 2 stroke that was Hunter's original fit. I specified the high reverse thrust prop as fitted to the sailmate engines. The new engine did everything I expected of it and was a big improvement on the Yamaha in terms of noise, economy and low speed handling.

I had to pay extra for the special prop and the engine suppliers gave me the original standard prop (which they had taken off). One day I put this on the engine and tried it out of curiosity. The max speed was unchanged at about 5.5 knots. Also unchanged was the fact that this was attained at about two thirds throttle, the last third of throttle just increased the revs, noise and wash and pulled the stern down. What was different was the low speed control - a short burst of power in ahead or astern was no longer so effective in checking the boat.

Needless to say I soon put the special prop back on.

<hr width=100% size=1>

 

[JimC]

I bought a Mariner 6hp fourstroke for my Hunter Horizon 23 and was very pleased with it. I paid extra for the special high reverse thrust prop as fitted to the sailmate engines, and the dealer also gave me the original standard prop which he had had to remove.

One day I tried the original prop out of curiosity. The max speed was the same at five and a half knots. Also unchanged was the fact that max speed was attained at about two thirds throttle - the last third just gave more noise and wash and pulled the stern down. What was different was the low speed handling: with the special prop the boat could be controlled with little bursts of power in ahead and astern, the standard prop was far less responsive - going astern to slow the boat as you approached a berth took yards to take effect - if it worked at all. Needless to say I soon put the sailmate prop back on.

Hope this helps.

<hr width=100% size=1>