115hp EFi only 36 knots?

[SARK]

Originally Posted by mikehhogg

does the word economy come in to any of these equations, how many knots are enough,how fast is fast enough.

As said before, I think my outboard might be under performing, or set up hindering it.

Lower revs doesn't always mean better fuel economy, its like putting you foot down in 6th gear at 45mph, just straining the engine. That's why there is a WOT rev range

5000 revs is ok if I'm loaded, but not if I'm on my own with little fuel. Just thought it wouldn't struggle on a 19" prop

[johnrambo]

Stainless does not “gain” you any pitch, that is impossible, pitch is pitch, it’s a myth. What’s actually happening is just “loosing” less pitch due to the blade twisting under load. Stainless is three times stiffer then aluminum for the same section geometry. Stainless blades can also be a little more efficient because you can go with a thinner blade section due to the extra stiffness.

I also have a 2001 merc, same engine but it’s stamped 75hp (75,90 and 115 are pretty much identical engines, just a software difference providing a little more power at the top end) Running on a 5.5m zodiac hurricane with a 17 pitch merc 3 blade. I get max 32-33Kts at 5200 rpm loaded(3 adults, 2 kids and 20 gallons of gas).

[Limecc]

Originally Posted by SARK

With a couple of extra people on board & more fuel, think it will bring me down a few hundred revs? So 17" better?

Any experience with Stainless getting more revs?

Thanks

I understand they give extra revs because the blades don't deform like alloy and are thinner, hence better performing while being more efficient. For every pitch they allow the available engine power do more and spin the prop faster. The prop spins faster, the boat goes faster. Props have power curves like engines have power curves.

Some say the gearbox is more at risk but I'm not buying into that when the rubber bushing ought to provide the needed protection. I stand to be corrected.

My memory is not very reliable but think modifying your 16p prop to a 17p (bending the entire blade) will cost about £40. Cupping or slight cupping (bending the trailing edge of the blade) increases efficiency and according to the amount added can reduce revs significantly. Apparently it can make a 17p behave like a 19p. Get your 16p cupped at the same time they alter the pitch?

Extra weight in the boat or towing will reduce your performance significantly. Better for you to be nearer 17" than 19" imho. Get the pitch right for the use you will most often be giving it.

[Limecc]

Originally Posted by Limecc

Props have power curves like engines have power curves.

Informative read.
https://continuouswave.com/whaler/re...owerCurve.html

[Voodoo]

Originally Posted by lakelandterrier

TBH I'd be more concerned abut a comfortable and economic cruising speed/ set up then maxing out on speed alone.

How often will you be travelling WOT, and for how long?

Personally I'd want a set up that gives me a comfortable ride over a day out than a technical what is is possible to max absolute speed.

You and I may have different wants from our boats, so this is not a criticism, just a view.

I hope you manage to get the set up that delivers foe you.

LT

Absolutely right...in our case we have a mercruiser 350 magnum in the rib, 300hp on a bravo drive, so I took off the 3 blade prop and fitted a 4 blade Mercury Evolution 4, dropped the pitch a bit and it works really well...better out of the hole, easy cruising at 25 knots with plenty in reserve if I need it and no strain on the machinery. (and you can still have a conversation)
Only time I was interested in top speed was when racing a few years ago, and even then we deliberately dialled 2 or 3 mph out of the prop to make the boat more driveable in most conditions. Steel Developments are are really good at the small changes that make a difference.
It really is horses for courses so I hope you get the setup that you want for your boat

[SARK]

Originally Posted by johnrambo

Stainless does not “gain” you any pitch, that is impossible, pitch is pitch, it’s a myth. What’s actually happening is just “loosing” less pitch due to the blade twisting under load. Stainless is three times stiffer then aluminum for the same section geometry. Stainless blades can also be a little more efficient because you can go with a thinner blade section due to the extra stiffness.

I also have a 2001 merc, same engine but it’s stamped 75hp (75,90 and 115 are pretty much identical engines, just a software difference providing a little more power at the top end) Running on a 5.5m zodiac hurricane with a 17 pitch merc 3 blade. I get max 32-33Kts at 5200 rpm loaded(3 adults, 2 kids and 20 gallons of gas).

I said gained revs, not pitch.
Yeah understand aluminium warping.

Maybe mine is a 100hp with a 115hp cowling, ha. No I'm sure I've checked the serial number when I bought it.

33 knots is good with a 75hp & loaded, so you see why I'm questioning mine with an extra 40hp, & unloaded, getting just 3 more knots.

[SARK]

Originally Posted by Voodoo

Absolutely right...in our case we have a mercruiser 350 magnum in the rib, 300hp on a bravo drive, so I took off the 3 blade prop and fitted a 4 blade Mercury Evolution 4, dropped the pitch a bit and it works really well...better out of the hole, easy cruising at 25 knots with plenty in reserve if I need it and no strain on the machinery. (and you can still have a conversation)
Only time I was interested in top speed was when racing a few years ago, and even then we deliberately dialled 2 or 3 mph out of the prop to make the boat more driveable in most conditions. Steel Developments are are really good at the small changes that make a difference.
It really is horses for courses so I hope you get the setup that you want for your boat

This isn't a question of how do I go the fastest, but a question of should my RIB be going faster, or is there something wrong.

Mostly I'm doing 25-28 knots, but I do like to go flat out once in a while when it's calm.

Yeah everyone seems to highly recommend Steel Developments.

Thanks

[SARK]

Originally Posted by Limecc

Informative read.
https://continuouswave.com/whaler/re...owerCurve.html

Was interesting about the reserve power.

Think stainless is a problem if you hit something, whereas aluminium will dent or chip helping to absorb some impact.

I assume the lower the pitch, the greater chance of cavitation, to which I need to lower my engine height, potentially causing more drag, & inturn lower revs again?

Is it awkward to adjust height? Might YouTube it

[Limecc]

Originally Posted by SARK

Was interesting about the reserve power.

Think stainless is a problem if you hit something, whereas aluminium will dent or chip helping to absorb some impact.

I assume the lower the pitch, the greater chance of cavitation, to which I need to lower my engine height, potentially causing more drag, & inturn lower revs again?

Is it awkward to adjust height? Might YouTube it

I think you are getting the terms ventilation and cavitation mixed. If your prop is breaking the surface and drawing air then that is the former. Cavitation will happen if conditions are right no matter how deep the prop is submerged. You don't have cavitation. If your anti-ventilation plate (not anti-cavitation plate) is 2" above the bottom of the hull it is likely ventilation.
https://www.iims.org.uk/introduction...er-cavitation/

I was easily able to raise my engine in under an hour using the trailer/jockey wheel method but I couldn't re-find a link so I'll describe it:

1. Put a wood block under the skeg and lower the jockey wheel and you have a see-saw. Lift the trailer up at the front to the point that the skeg takes the engine weight at the back.
2. Remove the upper fixed bolts and loosen the lower bolts.
3. An assistant can raise (or lower) the jockey wheel. Line up the new engine holes with the top holes of the transom, the lower bolts will slide in the slots on the engine plate and keep the engine from tipping.
4. Re-fit the top bolts with sealant and nip them up.
5. Remove, clean and re-fit the bottom bolts with new sealant.
6. Tighten all four bolts to specified torque.

[SARK]

Thanks Limecc

And yes I do have them mixed up, ha
I plan to go out tomorrow, so I'll do the trim test.

[willk]

OP - I've just skimmed this thread and there's a lot of detail - but props and trim aside - do you actually know that the engine is delivering all the horses to the prop shaft? I may have missed a comment in there somewhere, but I sense that you have a concern about that? I'd be doing a compression test pronto. If that checks out then OK, you may still have lost a few horses over the years. Ultimately a dyno would tell the truth but money for what, closure?
You need to get the rev range right - upper range for light ship. After that you can tweek away but no point flogging stray horses.

[SARK]

Originally Posted by willk

OP - I've just skimmed this thread and there's a lot of detail - but props and trim aside - do you actually know that the engine is delivering all the horses to the prop shaft? I may have missed a comment in there somewhere, but I sense that you have a concern about that? I'd be doing a compression test pronto. If that checks out then OK, you may still have lost a few horses over the years. Ultimately a dyno would tell the truth but money for what, closure?
You need to get the rev range right - upper range for light ship. After that you can tweek away but no point flogging stray horses.

Hi Willk

Compression, spark, & plug in all good. I had to get injectors cleaned & apparently they should be as good as new. Only issue I have is that its slightly hesitant from tick over, but it's worse when on muffs to a point where it can stall.
I was just surprised that with just me it couldn't push the 19p to the WOT rev range.
Maybe it's just because it's the older model (only done 360 hours though)

Trimming didn't too to much today, maybe gained a knot, but think once I'm proped right, Im hoping there's a bit of a powerband around 5500 revs.

Thanks

[beerbelly]

i think your expecting too much. the tornado is a good deep v hull thats good in a heavy sea but unfortunately deep v hulls are a little power hungry . you have a 115 that i suspect is as big as you can go on that hull get it propped right and it will probably do 37 knots or there about but thats not what ribs are about top speed is not what there good for . you will not get a feeling of a powerband from it regardless of how its propped . if your getting 35 now and your not reaching 6000rpm at wot put a lower pitch prop on it try a 17p or even a 15p then when you have got the rpm at wot right and gained possibly another knot or two you will be as good as it gets .at the end of the day its a relatively small outfit with a small outboard its never going to set the world on fire speed wise

[Pikey Dave]

Originally Posted by beerbelly

i think your expecting too much. the tornado is a good deep v hull thats good in a heavy sea but unfortunately deep v hulls are a little power hungry . you have a 115 that i suspect is as big as you can go on that hull get it propped right and it will probably do 37 knots or there about but thats not what ribs are about top speed is not what there good for . you will not get a feeling of a powerband from it regardless of how its propped . if your getting 35 now and your not reaching 6000rpm at wot put a lower pitch prop on it try a 17p or even a 15p then when you have got the rpm at wot right and gained possibly another knot or two you will be as good as it gets .at the end of the day its a relatively small outfit with a small outboard its never going to set the world on fire speed wise

^^^^^ wot he said.
My RC545 with DF140 tops out at 6kRPM/37kn. It’s bang on where it should be.

[SARK]

Originally Posted by beerbelly

i think your expecting too much. the tornado is a good deep v hull thats good in a heavy sea but unfortunately deep v hulls are a little power hungry . you have a 115 that i suspect is as big as you can go on that hull get it propped right and it will probably do 37 knots or there about but thats not what ribs are about top speed is not what there good for . you will not get a feeling of a powerband from it regardless of how its propped . if your getting 35 now and your not reaching 6000rpm at wot put a lower pitch prop on it try a 17p or even a 15p then when you have got the rpm at wot right and gained possibly another knot or two you will be as good as it gets .at the end of the day its a relatively small outfit with a small outboard its never going to set the world on fire speed wise

I tried an SS 16p vengeance & over reved, 37 knots, but I can see why people use stainless.

& yeah it is a decent V & high bow, wide beam.

Cheers

[SARK]

Originally Posted by Pikey Dave

^^^^^ wot he said.
My RC545 with DF140 tops out at 6kRPM/37kn. It’s bang on where it should be.

That's good to know, so seems like I am on the money then.
37 knots is fast enough, only when it's flat it would be nice for more.

What are you getting cruising wise? 28-30 knots is about 4000 revs for me.

Thanks

[Pikey Dave]

Originally Posted by SARK

That's good to know, so seems like I am on the money then.

37 knots is fast enough, only when it's flat it would be nice for more.



What are you getting cruising wise? 28-30 knots is about 4000 revs for me.



Thanks

I cruise just over 4kRPM, 25kn or thereabouts.