Fine tuning your rib?

[jonp]

Over the years I've owned so many different boats, you could say I'm a boat slut My first was at 12years old and I'm now 50. Over this time, I've become quite picky at trying to get the most in performance from each boat, with constant research into prop styles and power to weight, etc.

So now that I've finally found just how much fun a small rib can be - with the latest model Zodiac 550 tour - I'm once again tweaking to get every bit of economy and performance from such a relatively small engine (70 Yam 4 stroke).

Most manufacturers sit the engines so that the cav plate runs parallel to the keel, and put on an alloy prop that gets the outfit within 400rpm of the max rpm range. This is how almost every boat I've ever bought has been - it seems that most of us are happy to go along with those settings and think no more about it.

What I've found, is that by raising the outboard (as recommended by many sites, so the cav plate is either just skimming or slightly above the surface) you can free up a few hundred rpm and increase the speed, etc.
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The next thing I look at is normally prop experimenting - which is where the difficulty starts to step in. With so many props on the market with claims of high performance (especially on the stainless side of things), it can be very hard to find and test. Especially adding in the extra cost of stainless.

Over here, the main dealer is Solas props - or you can stay with the manufacturer's own brand. In the past couple of years, I've owned 2 Yam 70 4 strokes (as well as a Honda 90, a Suzuki 90... that's a lot of tinkering!) and found that the Yam stainless prop gave no advantage in speed or rpm over the much cheaper alloy prop (a free exchange of props made that easy to test). So then to decide whether to test out the extremely expensive Solas or go for one of the US props with their high performance claims.

As most of us realise, a little more speed reduces prop slip, giving better performance. But at the price of running an engine in the higher rpm range. I've found at 5000rpm my outfit seems to dance along at 45kmh, which seems to suit this boat just right. Now my intention is to bring down that rpm, but still end up with the same speed and top rpm.

There are some manufactures claiming that midrange improvement, and I wondered if anyone has found the "magic" prop which does as advertised. Anyone using Ballistic High Performance?

Jon

[Roflhat]

Have you tried surface piercing props? They're used for racing but if you're after good top speed and economy might be worth trying. They only really work on light boats, not ideal if you want to carry a lot of equipment or people.
On a tohatsu 50 a standard prop would be 15 pitch, whereas you can run an 18 pitch cleaver/chopper without difficulty at the same RPM, which on a thundercat usually brings it from 41 to ~49 knots.

Not sure if anyone has looked into them with regards to maximising fuel economy but I guess they would - more speed at the same RPM. You need to lift the engine a fair bit to make them work.

[jonp]

Originally Posted by Roflhat

Have you tried surface piercing props? They're used for racing but if you're after good top speed and economy might be worth trying. They only really work on light boats, not ideal if you want to carry a lot of equipment or people.
On a tohatsu 50 a standard prop would be 15 pitch, whereas you can run an 18 pitch cleaver/chopper without difficulty at the same RPM, which on a thundercat usually brings it from 41 to ~49 knots.

Not sure if anyone has looked into them with regards to maximising fuel economy but I guess they would - more speed at the same RPM. You need to lift the engine a fair bit to make them work.

That sounds very interesting, will have to to look into those. Today I tested with a stainless solas prop the same size and pitch as the aloy prop Im currently running, Didn't notice much difference except the stainless was 400rpm slower than the aloy.
JonP