Props - Again

Nasher · 14 March 2005, 14:54

Hi all

My 6 year old Ocean 6.25 is fitted with a 1989 Johnson 150VRO that will be changed hopefully early next season when funds allow. In the mean time I’ve been trying to get the best out of it by changing props.

It arrived wearing a 15 x 17 standard Ali OMC prop that was giving a reasonable pick up, and was topping out at about 34 Knots with a few bodies on board. The best I saw was 38 knots. Worryingly this turned out to be at 6400RPM when I fitted a Revcounter, Tops should be about 5500 RPM.

So I downloaded 3 different prop calculators, and spoke to all the prop-shops to get advice. Everything/Everyone pointed to a 14.75 x 21 prop being ideal for the RIB/Engine combination. I purchased a Hustler Ali prop with removable hub at a good price.

First time out on Sunday with this prop was very disappointing. I had another adult and three kids on board.
With a cold engine, going into gear stalled it, and I ended up raising the tickover to get away. At first I blamed myself for badly adjusting the new surface mounted throttle I’d installed, but later when warm I had to drop the tickover back down.
Once warmed up, the acceleration was terrible, and it actually struggled up on the plane. Once up on the plane the engine struggled to 4750RPM and could only reach 32 knots. I checked I was getting full throttle opening, so have concluded the engine just can’t handle the 21 inch prop, which is very strange considering Everything pointed at it being ideal. The engine sounded great, and I checked the plugs later which were all a nice colour so I didn’t have a dodgy cylinder or anything once warm.
Suppose I’ll just have to commit to another prop at about 15 x 19 and hope it works.

In retrospect I should have changed props back to the 15 x 17 to see if I could replicate the earlier performance and completely rule out a dodgy engine.

Nasher.

Kernow Buoy · 14 March 2005, 19:23

Just a thought for you Nasher, Castle Marine (http://www.castlemarine.co.uk/ ) re-pitched an alloy prop for me, they can increase alloy props by 2" and charged me £40, if you sent them your 17" you'd get it back as a 19", when mine came back it looked like new. I believe Steel Developments offer a similar service.
regards
Kernow

jwalker · 14 March 2005, 20:00

Quote:

Originally Posted by Nasher

The engine sounded great, and I checked the plugs later which were all a nice colour so I didn’t have a dodgy cylinder or anything once warm....

In retrospect I should have changed props back to the 15 x 17 to see if I could replicate the earlier performance and completely rule out a dodgy engine...

Nasher, this sounds very familiar. I had a similar experience with my VRO too. Over a couple of days it just got worse until it wouldn't pass 20mph. Plugs looked good etc. etc.

It turned out to be corrosion of the wire inside the plug caps which terminates the copper core into a clip. They didn't appear to fully prevent the spark, it just got weaker and weaker. The wire is just a wee piece of piano wire pushed right through the HT cable. Made replacements from brass wire and soldered them to the copper core and that was that....me prop was fine all along.

I hope your solution is as simple.

Nasher · 15 March 2005, 07:16

Kernow

Thanks for the suggestion, however the 17 inch is the only prop I've got that sort of works at the moment and wouldn't want to ruin it.

JW
Had that problem last year thanks, and replaced all the leads with Silicone Hot Wires. Made hell of a differance to how it runs.

Nasher

Wile E Coyote · 15 March 2005, 13:59

All I can say is that I have been going in prop circles for a while now. It would seem that the 1" per x amount of RPM is all nice and dandy on paper. When it comes to putting it in practice with an outboard and a boat and some water, not the same. Only advice I can give is to go up in small increments with the pitch. These rule of thumb notions that in your case a 21" pitch should do fine does'nt work. The other draw-back is how the hell does one get hold of all these props for trial? I have problems with a 14" pitch with over-revving. With a 16" pitch, I need oars to assist getting out the hole. So much for prop advice. According to size etc. of my boat it is said I should be running a 17" pitch. Never. Like I say.........it's a nightmare. And it's not always just the prop that's the factor. Engine height etc. Good luck.

Wile E Coyote · 15 March 2005, 14:03

See if you can maybe borrow a prop somwhere. There are some good folks out there. It's a lot cheaper than buying one. What make of motor do you have. Don't recall seeing make, just size.

Wile E Coyote · 15 March 2005, 14:05

I see it's a Johnson. Maybe I can help. I have an adjustable prop for Evinrude/Johnson. Not the best prop in the world, but one can play with the pitch and have a fair idea of where it needs to be.

Nasher · 15 March 2005, 14:57

Wile e

Thanks for the sympathy, it’s so frustrating, especially as I’m trying not to spend too much, I’m hoping to swap it in for something like a 200E-Tec early next year when funds are in place.
Anyone fancy a new business venture lending out props for a small fee.

Thanks also for the offer of the adjustable prop, does it fit your 70HP? If so It won’t unfortunately fit my output shaft.

I’ve found to my cost that the V6 150 Johnsons have a larger shaft than even the V4’s of slightly smaller output. I now have a collection of Props in the garage all advertised as being for V6’s that won’t fit my shaft. Why can’t other people use a vernier gauge properly? They are all £30 - £40 jobs of e-bay, I suppose I should put a list on the for sale page or offer them for loan or swap.

Anyone know of a collection of suitable props available for loan, or even a used 175HP or 200HP E-Tec(Some hope), Ficht, Optimax or HPDI at a sensible price. Don’t mind a load of hours on it, or even some damage, I can mend most stuff, and being a bit scruffy would blend in with the rest of the boat.
Just need something more efficient.

Nasher.

Wile E Coyote · 15 March 2005, 15:23

I was going to look when I got home, but now that you mention it, I seem to recall it would do something like 140HP V4. Doubt whether it will do V6. I will look on the ProPulse site, but I doubt the splines will be the same.

I know how you feel with this one. Pain in the arse......

Steel Dev · 21 March 2005, 11:12

Hope this helps. For prop selection you need to take into consideration not only the pitch size but also the blade shape, blade area and the amount of cupping on the blades. A V4 propeller will not normally fit a V6 engine as the gearcase size is different. For your OMC the V4 has 13 splines and the V6 has 15 splines so will not fit . We have a new site at www.steeldevelopments.co.uk which hopefully will answer a lot of common prop questions but if you have any queries I'm always willing to help

Julie

Nasher · 21 March 2005, 19:41

Julie

Many thanks, I'll have a look at the new site.

I'm going to try making up a copy of the shaft and splines so I can use it to measure props at jumbles etc.

Nasher