Duo prop conundrum - experience so far

LBRodders said:
reckon the smaller prop is 'grabbing the most water whilst slipping less' and so is more efficient and hence the higher rpm/speed.

As an 'unknown art' and more variables than you can shake a stick at,
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I'm not so sure. It's a conundrum for sure. One thing about the B4 is they seem just the opposite. No bite. Of all the props they are the slowest to plane and the revs climb up regardless of the boat sitting at 13knts and then it lets go and whoosh we are away accelerating faster than any other combo. To make any displacement speed I'm sitting at 1200 rpm just to make 6knts, yet 2.5x those revs and the speed is 4x at 24knts

It's mad magic and then to out perform the next size up given the same rpm ????
 
BruceK said:
I'm not so sure. It's a conundrum for sure. One thing about the B4 is they seem just the opposite. No bite. Of all the props they are the slowest to plane and the revs climb up regardless of the boat sitting at 13knts and then it lets go and whoosh we are away accelerating faster than any other combo. To make any displacement speed I'm sitting at 1200 rpm just to make 6knts, yet 2.5x those revs and the speed is 4x at 24knts

It's mad magic and then to out perform the next size up given the same rpm ????
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It is mad. There's no free lunch, acceleration or top end - choose your poison!

There is also flow over the props given the attitude / trim of the boat at different speeds. That may explain why you take off, as you reach a specific 'speed' and the boat attitude suits the prop for water flow underneath the hull. That's another variable. There's just too many!

Sounds like you have more propellers than the Royal Navy. :p
 
Rebecca & Steve said:
Looking at photograph No3 your boat seems to be significantly unbalanced - I would fit one of those stainless steel afterburner things to the starboard side as well - that should sort it.
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More like the dead cow thats just about to go on it - stowed aft it prevents Bruce getting out the hole!
 
blimey Bruce, so you own 4X4=16 props? That must have cost a bit!
I'm seriously underpropped as well and was thinking of altering props a bit, but don't know of any reputable firm that will do it down here, nor how much to alter the cupping (is that how you call that or is it the barrista term?) so planning not to bother
 
vas said:
blimey Bruce, so you own 4X4=16 props? That must have cost a bit!
I'm seriously underpropped as well and was thinking of altering props a bit, but don't know of any reputable firm that will do it down here, nor how much to alter the cupping (is that how you call that or is it the barrista term?) so planning not to bother
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In 5 years 21 props Vas. Trying to find the sweet spot for cruising, engine temp, engine load and then top end performance. No one is best but so far B4 for standard cruising. C3 for Holeshot. I mean that was blistering. Something like 6 seconds to 18 knts. Best sub 2k rpm was B5. C4 gave best average performance but seemed to cause the engine to load more and record higher temps. Even over a (coarser?) B5. B4's are the engine's coolest running to date and surprising give the highest cruising speed. (but no top end recorded for them as yet)
 
interesting, curious about the odd number of props though :rolleyes:
I thought the idea is only to offer drinks to Poseidon, not props as well!
 
Some were to Poseidon, some were due to ahem err discovering the channel groin had collapsed, at least, changed, maybe. Admiral faffing with the fenders was a distraction.
 
LBRodders said:
It is mad. There's no free lunch, acceleration or top end - choose your poison!

There is also flow over the props given the attitude / trim of the boat at different speeds. That may explain why you take off, as you reach a specific 'speed' and the boat attitude suits the prop for water flow underneath the hull. That's another variable. There's just too many!

Sounds like you have more propellers than the Royal Navy. :p
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He carries more props than beer.
 
Interesting thread Bruce.
What though do you mean by coarser, and also what are the signs of root cavitation?

I'm interested about the engine load and high temps, as I think this might be a symptom I am experiencing.
 
BruceK said:
In 5 years 21 props Vas. Trying to find the sweet spot for cruising, engine temp, engine load and then top end performance. No one is best but so far B4 for standard cruising. C3 for Holeshot. I mean that was blistering. Something like 6 seconds to 18 knts. Best sub 2k rpm was B5. C4 gave best average performance but seemed to cause the engine to load more and record higher temps. Even over a (coarser?) B5. B4's are the engine's coolest running to date and surprising give the highest cruising speed. (but no top end recorded for them as yet)
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Contradicts your mantra on fully serviced they do not loose Hp with age and simultaneously supports my alternative and I believe correct reasoning due to overloading and excess EGTs they do indeed insidiously loose Hp with age .The near the “edge “when new the sooner and greater the difference you feel .
Hence new props every 5 mins and chasing shadows .

It might be achieving rpm buts what’s it’s pushing out Hp wise at said rpm shove the lardy tub theses days ?

Ok you say a turbos knackered this time , but I say it’s been canned once too many times .....inadvertently agreed .

The thing about mushroomed exhaust valves you only know with a drop off of Performance.Its insidious.
 
Oh god. Could you be any more boring? My turbos are knackered because the exhaust housing rusts out. Am I bothered? No. Unlike yours my turbo's cost less than a fuel fill up. Can you please put your little EGT genie back in it's box we've heard it so many times it's become a right PITA. KAD Turbo's rust out. Regularly and predictably. It's almost a service item. Thankfully they are cheap.

And I am chasing props because it came with the incorrect props on. I've not been able to find the perfect pair and Volvo bless them dont make this very clear either, nor mercruiser from what I can tell. Duo prop is a dark art. Lets leave it there.
 
Helps is if know the starting point H p wise with what ever empirical prop chart you use .
Re work the calcs ( within the “ dark art “ cop out ) for the correct HP which is not as it left the factory .

Why you have lost the Hp or the mechanism is just for curiosity.It does not change the outcome knowing why .I’d you are not interested fine .But it’s nice to know why it can’t swing big enough wheels to achieve brochure / mag boat test performance numbers .
8/10 knots are missing now .It was a gradual process hence the prop shadow chasing you describe and are in .
 
Bruce is wondering why his boat is slow...it’s easy....new davits...tender....bbq.....anti Seacock devices......family.....kit.....junk....oh....and 1100 litres of diesel. You can bet your bottom dollar formula (and Itama ?) ran a stripped out hull with a dribble of fuel onboard for brochure figures.

I still think you should go J series. Better hole shot without top end loss. J6 should do it. You get an extra blade too!

I got 34 knots in the sqeeker on j5 with a cruise of 25 knots at 3200 revs but it was slightly under prop and could easy hit 4K revs. The hole shot was brilliant
 
You guys have completely missed the point. I am trying to find what prop best works for me and how I use the boat. None of the props have have shown decreased performance as if the engine is tiring. I ride with a full boat always because I am a river boat and a family boat. Going into a marina to bunker up is a PITA I avoid because of gate times queue etc so I usually fill up on a return journey and never let the tanks get to half. My question was the unusual performance I was getting out of B4 comparative to coarser pitched props given the same rpm before this became the usual witch hunt from the usual suspect.
For those that recommend the B6 my boat never came from factory with B6. It came with B5. As B5 is no longer available I used BSP PRO B5 pattern and was pushing too much torque through them as evidenced by blade root cavitation erosion. I dare say at the rate of erosion those props would have had a life of one season.
Mr Googler your recommendation of J5 has been considered but given the expense of going through 2 series already and having a good idea that based on performance B4 would be most suitable I was not inclined to start a 3rd series. I was expecting a drop of 400 rpm from B5 and a corresponding drop in performance speed. What surprised me was to find the opposite. A less stressed engine pushing the boat faster.
As for the usual suspect's accusation of cop out when I say dark art, then why does Volvo not openly publish figures for props. If it is not as simple as A B C D 1 2 3 4 then why the fancy programs followed by sea trials at OEM. You tell me why a B4 out performs a B5 with less load and better performance for the same cruise rpm. Given the odd results I have had I may well try a B6 at some point if the B4's turn out not to have the legs they currently seem to have
 
Two new Turbo's winging their way to me. I thought about doing the starboard only but I figure doing both will keep the engines paired. Cant give sweeties to one and not the other eh Porto?
 
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vas said:
exchange units or brand new? can't you recon the ones you have?
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Brand new. You can recon them and even have the exhaust housing changed rather than sleeved. Problem is a potential lead time on just the exhaust housing which is the bit that corrodes and is the main culprit (NOT EGT PORTO!!) in their demise. They have gone up in price a bit since 2019 but £2040 for a new pair isn't going to hurt the bank too much and if the kids get an orange for Christmas well that's just a fine and dandy tradition we grew up with, they have enough electronic toys and gubbins to build a space shuttle already.
With luck I'll whip the old ones off and replace both before heading out on Friday. Weather looks good ?
 
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