Props explained please.. - Page 3

JSP · 12 September 2009, 12:10

Quote:

Originally Posted by chewy

Thats a good incentive for him to get it finished sharpish!

It's ok for you lads with no binlids.

Hightower · 12 September 2009, 14:36

Quote:

Originally Posted by JSP

I am mate, but not with my boat. Wont be finished by then

So, what's to finish? Let's have a "put your RIB together" party!

JSP · 12 September 2009, 14:41

Quote:

Originally Posted by Hightower

So, what's to finish? Let's have a "put your RIB together" party!

Got new console glassed in, need to flocoat over the bare glass. Fit fuel tank, make a pannel for the back of the console, paint the whole thing and then fit steering, electrics etc. Doesn't sound much but we've got 3 of the kids birthdays in one week. Dame that christmass port!

Nos4r2 · 12 September 2009, 16:02

Quote:

Originally Posted by Hightower

Not in my experience of comparing two props on the vipermax.

It originally came with a 21" which was running towards the bottom end of the WOT range.

I bought a 19" off of ebay and tried this. Whilst giving me any more Top speed at WOT is actually gave me less speed at 3000rpm. If memory serves me it was about 3 knotts different. They where back to back trials in the same sea conditions.

No serious difference in acceleration either.

So with 2knotts faster speed at 3000rpm and 250RPM lower WOT I'd have to deduce that I'd be getting more miles per gallon at my usual cruising speeds with the 21"er.

Stainless Vs Ally by any chance?

IBWET · 12 September 2009, 18:24

Quote:

Originally Posted by Nos4r2

That works, but then you're going to have the most economical cruising speed/RPM with the prop that gives you the highest top speed-unless you can find a prop that slips significantly less at cruising RPM.

That's why I ultimately went to 4 blades;30% more surface area ment less slip,that borne out by the 3,500 speed test producing a better figure for a 23" prop over a 25" prop.

jwalker · 13 September 2009, 02:14

Yeh, but if you're taking your motor to the same rpm with a prop which loads it more, the throttle will be wider open. So you're cruisin at the a same noise level but using more fuel to keep it there. You are, of course, goin faster so I guess you need to do the economy figures if that matters to you. A better comparison might be running at the same boat speed and lower rpm and doing the economy figures.

But, I've found that the most economical cruise is not when the engine reaches max WOT speed on a given prop, it's a pitch or so below that. I also feel there is a sweet speed for a given hull, prop and sea conditions. For my old (O/B motored) boat, it was around 42 ish knots and when the water had just enough wave to make the prop break the surface occasionally. For my current boat it's similar conditions and speed but that happens to be flat out so it's rarely there - unfortunately.

Hightower · 13 September 2009, 11:07

Quote:

Originally Posted by Nos4r2

Stainless Vs Ally by any chance?

Both where stainless and OMC/BRP props

The 19" was an older SST and the newer 21" was a Viper. To look at both you couldn't visibly tell any difference in design/shape other than the pitch difference.

Hightower · 13 September 2009, 11:22

Quote:

Originally Posted by jwalker

Yeh, but if you're taking your motor to the same rpm with a prop which loads it more, the throttle will be wider open. So you're cruisin at the a same noise level but using more fuel to keep it there. You are, of course, goin faster so I guess you need to do the economy figures if that matters to you. A better comparison might be running at the same boat speed and lower rpm and doing the economy figures.

But, I've found that the most economical cruise is not when the engine reaches max WOT speed on a given prop, it's a pitch or so below that. I also feel there is a sweet speed for a given hull, prop and sea conditions. For my old (O/B motored) boat, it was around 42 ish knots and when the water had just enough wave to make the prop break the surface occasionally. For my current boat it's similar conditions and speed but that happens to be flat out so it's rarely there - unfortunately.

There might well be slight differences in fuel used, but my fuel meter didn't pick em up 32 litres/hour on both runs.

I think we have deduced that there are so many variables in prop selection Hull, sea conditions, trim, height of engine on the transom, outright performance, economy, driving style, application that the boat's used for, etc etc etc... That it's almost impossibe to select a prop off a shelf without trialling it.

Only advice I can give really is to borrow a few different sorts and see which one you like the best. That's why it's a missed oppotunity that JSP can't get his SeaRider ready for the cruise. There'd have been loads of backup props he could have borrowed and lots of help too.