New Engines - How Fast??

[willk]

Originally Posted by Chris

Not really

Still Chris, even if neither of us understand the physics, twins are rigged with a slight overprop. A quick Google will confirm this.

A Singleton should be wary of offering advice to a Twin on this matter.

[willk]

Originally Posted by The Beard

Erm.....what are.....ahem.....ventilation plates?

I think he means Cavitation plates, but what would I know...

Originally Posted by The Beard

Also, er.....does 18", 19" or 20" when that measurement applies to a prop. mean the distance it will travel forward during one full revolution

Yes.

[Chris]

Originally Posted by willk

Still Chris, even if neither of us understand the physics, twins are rigged with a slight overprop. A quick Google will confirm this.

A Singleton should be wary of offering advice to a Twin on this matter.

More to the point I dont understand your physics I am just working off the information that BRP give and that I have used when installing and setting up my 150.

The engines are designed to work best within a certain RPM range and I can see nothing in the installation manual that I used when fitting both my 115 and 150 to suggest that when running twins you should over prop the engines.

[Chris]

Originally Posted by willk

I think he means Cavitation plates, but what would I know...

Anti Cavitation plates or Anti ventilation plates. Same thing

[willk]

Originally Posted by Chris

More to the point I dont understand your physics

This is good, I have none...

Originally Posted by Chris

The engines are designed to work best within a certain RPM range and I can see nothing in the installation manual that I used when fitting both my 115 and 150 to suggest that when running twins you should over prop the engines.

Mine were rigged by the RIBS manufacturer in conjunction with the original buyer, a RNLI Cox'n/Instructor. What DOES your manual say about Twin Installations? (I didn't get a manual and have to rely on the internet )

[Chris]

Originally Posted by willk

This is good, I have none...

Mine were rigged by the RIBS manufacturer in conjunction with the original buyer, a RNLI Cox'n/Instructor. What DOES your manual say about Twin Installations? (I didn't get a manual and have to rely on the internet )

Loads of dealers rig these engine to run at the lower limits of the RPM range as you can get a bit more speed out of them by doing this however BRP came out with the optimum range to ensure that dealers were setting up the engines to get the best performance and ensure the best economy and engine lifespan.

I could probably get a couple of extra knots by fitting a 21" prop and pulling 5000 RPM but have actually decided to follow BRPs lead on this RPM thing.

[The Beard]

Originally Posted by willk

I think he means Cavitation plates, but what would I know...



Yes.

I could at this point say: "Ah, yes of course." However; although I've heard of cavitation, even seen it mentioned in a magazine, but, I don't actually know what it is and how cavitation plates prevent it. Or do they help it?

[Poly]

Originally Posted by The Beard

I could at this point say: "Ah, yes of course." However; although I've heard of cavitation, even seen it mentioned in a magazine, but, I don't actually know what it is and how cavitation plates prevent it. Or do they help it?

they don't they prevent ventilation - but for some reason the misnomer anti-cavitation plate seems more popular!

[willk]

Originally Posted by Polwart

they don't they prevent ventilation - but for some reason the misnomer anti-cavitation plate seems more popular!

I hang my head in the closest thing I know to shame, apologies to Chris

This seems to explain the relationship and nomenclature nicely: http://www.boatlicence.au.com/cavitation.htm

... so Anti-ventilation plate it is then, Polwart

[9D280]

I suppose this would be a good time to chuck in a couple of definitions:

Cavitation is seen on the back edges of the blades, and is the result of the pressure dropping so much behind the blade that the water will boil at the ambient temp it is at. (higher pressure = higher boiling point - conversely the lower the pressure the lower the BP). The boiled water forms steam bubbles, which can't stay in that state for long (milliseconds), and so the steam bubbles collapse rapidly, forming hthousandds of mini explosions. Keep it up for long and you start to erode the blade surface. Normally seen a short distance from the tip on the back of the blade.

Ventilation is simply that the prop pushes more water out the way than can get in to replace what has been chucked out. Gravity assists the stuff above the prop do drop, and so the "hole" is preferentially filled form above. There isn't a lot of water up there, so as the resulting walls of water either side fall in to fill the hole, alots of air gets into the mix & you know what happens next.


Theory of the plate is that it helps to keep the water above the prop air free by not allowing anything in form the top, so water is pulled in from the sides to fill the gap (where it is still 100% water) rather than a water / air mix air form above. Not a spectacularly teccie description, but hopefully it clears things up a bit.

[Cookee]

Originally Posted by willk

I'm running 14 x 19 on 90s and getting about 4750 (ono ). Keep in mind that twins should be one notch overpropped compared to a single rig.

I'm sorry but I still don't understand your advice - just because a manufacturer and an RNLI Coxwain set it up like that certainly doesn't make it right. Any engine manufacturer will tell you not to over prop any motor whether it is single twin, or triple inboard or outboard, it will shorten the life of the motor - no doubt at all! Running below the recommended rev range means the motor will be labouring and I have never come across any good reason for this - ever!

[GordyP]

Originally Posted by 9D280

I suppose this would be a good time to chuck in a couple of definitions:

Cavitation is seen on the back edges of the blades, and is the result of the pressure dropping so much behind the blade that the water will boil at the ambient temp it is at. (higher pressure = higher boiling point - conversely the lower the pressure the lower the BP). The boiled water forms steam bubbles, which can't stay in that state for long (milliseconds), and so the steam bubbles collapse rapidly, forming hthousandds of mini explosions. Keep it up for long and you start to erode the blade surface. Normally seen a short distance from the tip on the back of the blade.

Ventilation is simply that the prop pushes more water out the way than can get in to replace what has been chucked out. Gravity assists the stuff above the prop do drop, and so the "hole" is preferentially filled form above. There isn't a lot of water up there, so as the resulting walls of water either side fall in to fill the hole, alots of air gets into the mix & you know what happens next.


Theory of the plate is that it helps to keep the water above the prop air free by not allowing anything in form the top, so water is pulled in from the sides to fill the gap (where it is still 100% water) rather than a water / air mix air form above. Not a spectacularly teccie description, but hopefully it clears things up a bit.

Thanks for that. Phew I'm glad we got that cleared up, I'll be ordering the ss props tomorrow after tonight's little run, I've been told that the Rebel props give a reasonable saving in fuel, apparently a lot of the commercial ribs run with these. What do think Vipers or Rebels????

[Chris]

Cyclone TBX Size range from 15" to 23" pitch. This is a stainless steel 4 blade propeller that brings excitement to the offshore cruiser and runabout market. Boat testing has proven speed gains of 2-4 mph in midrange cruising. The new Cyclones are also more efficient with reduced slip and a boost in fuel economy with a bottom line savings in fuel cost. When you need a prop that can hold course straight and true in all water condition, trust the Cyclone TBX line up of props. (available for v-6 15-spline in standard and counter rotation. now available in 1" pitch increments. when replacing a previous model Cyclone propeller, a 1 to 2" drop in pitch is suggested as a result of the new propellers improved efficiency)

Rebel TBX Size range from 15" to 25" pitch. This is a stainless steel 3 blade propeller that is designed and engineered to deliver improved midrange cruising speeds and top end speeds. Larger diameter and efficient blade contour provides faster speeds at lower RPM's to deliver significant improvements in fuel economy. Use the REBEL TBX propeller on offshore boats, large runabouts, and even larger pontoon boats to make them perform at their maximum potential. (available for v-6 15-spline in standard and counter rotation. now available in 1" pitch increments)

Renegade Bass TBX Size's 23, 25, and 27" pitch.This is a stainless steel 4 blade propeller that are designed to provide a ride that is smooth, powerful, and fast. When speed counts, count on the Renegade Bass Prop. Sharp leading edges and contoured cupping bite and hold the water. Ideal for boats with a heavy load of fisherman and gear. Renegade Bass TBX models feature the TBX hub system for improved durability in high load, high temperature situations. (available for v-6 15-spline models only)

Raker II and Raker Sizes from 18" to 28" pitch. This is a stainless steel 3 blade design for building up engines RPM's and getting you on plane faster. A great prop for surfacing conditions especially on high speed boats that require bow lift. Raker II TBX models feature the TBX hub system for improved durability in high load, high temperature situations and are now available in a wider pitch range. (raker II TBX available for v-6 15-spline modles only. raker available for 13-spline models only)

Viper TBX and Viper Sizes from 14" to 22" pitch. This is a stainless steel 3 blade that offers great overall performance. Its longer swept blades and larger diameter give it great grip on the water, faster acceleration, and good top end speed. Excellent on any receational boat, from runabouts to cruisers to deck and pontoon boats. (viper TBX available for v-6 15-spline models in standard and counter rotation. V-6 15-spline models now available in 1" pitch increments. viper available for 13-spline only)

Rogue Prop Sizes 13,15,17,19 and 21" pitch.This is a stainless steel 4 blade that provides faster acceleration and top speeds in stern heave boats. Flats boat operators will appreciate this propellers ability to lift the transon and accelerate cleaner in shallow water. (available for 13-spline models only)

Hydrus Sizes 9,11 and 13" pitch. This is a aluminum 3 blade propeller that is designed and manufactured to meet the specific performance needs of pontoon boats and work boats. Provides and over-sized blade area with a specially designed hub for maximum forward and reverse thrust, while aggressive cupping allows for tight balanced turning. (available for 13-spline only)

Aluminum This is a aluminum 3 blade propeller that offers you more performance for the money. Precision casting provides greater strength and longer life. Baked on corrosion resistant paint gives added protection. Engineered to match the power curve of your outboard for superior performance.

[Chris]

Originally Posted by GordyP

Thanks for that. Phew I'm glad we got that cleared up, I'll be ordering the ss props tomorrow after tonight's little run, I've been told that the Rebel props give a reasonable saving in fuel, apparently a lot of the commercial ribs run with these. What do think Vipers or Rebels????

Gordy, did you check your Anti Ventilation Plates? were thy riding on top of the water?

I think the Rebels may be a good choice, what size are you considering to order?

[willk]

Originally Posted by Cookee

I'm sorry but I still don't understand your advice -

That's OK, obviously I don't claim to either - just repeating parrot-like the few things I've seen and read. A little information can be a dangerous thing.

Originally Posted by Cookee

just because a manufacturer and an RNLI Coxwain set it up like that certainly doesn't make it right.

Good point, I'm learning to be more sceptical about what rib manufacturers tell me

Originally Posted by Cookee

Any engine manufacturer will tell you not to over prop any motor whether it is single twin, or triple inboard or outboard, it will shorten the life of the motor - no doubt at all! Running below the recommended rev range means the motor will be labouring and I have never come across any good reason for this - ever!

And again, I defer to your experience in matters mechanical

[Bigmuz7]

Originally Posted by willk

That's OK, obviously I don't claim to either - just repeating parrot-like the few things I've seen and read.

Hmm yess .. we have codprawn for that ,..... are you wanting his prize ? ( I wouldnt reccomend it)

Originally Posted by willk

A little information can be a dangerous thing.

no comment

[GordyP]

Originally Posted by Chris

Gordy, did you check your Anti Ventilation Plates? were thy riding on top of the water?

I think the Rebels may be a good choice, what size are you considering to order?

Thanks for the info Chris, yep the Anti-cavitation/Anti-ventilation plates are running just at water level at full throttle. I'm getting 49 knots at 5250 WOT with two people on board and 46 knots at just over 5000 with a load on the boat with the ali 22". The dealer is going to speak to the guys at BRP for their advice but looks like a Rebel 20" maybe a 19". The hole shot with these engines is hilarious!!

[GordyP]

Here's a couple of pics of the new set up, haven't got any of it on the move yet, been concentrating on holding on!

[Grouse]

Very Nice.