Duo prop conundrum - experience so far

[BruceK]

During the last few years I have been experimenting with props to try and find an ideal fit for my boat. All of these are fully laden boat. Full fuel, water, pax and luggage.

I bought her with C3 which was wildly under propped and would hit 4k rpm quickly but only make 20 knts. At 3200 cruise revs she's make 17 knts
I replaced C3 with C4 and lost 400 revs in the bargain but struggled to make 4k and at 3200 rpm would make 22 knts.
Thinking that B5 splits the volvo chart between C3 and C4 I went B5 which gave a whole different metric. Quick to plane but struggled in the higher revs and actually felt coarser than the C4. At 3200 rpm they were making 24 knts. After just 200nm they started to show blade root cavitation indicating too much torque been driven through them so reverted to C4

So we have
C3 at 3200 making 17 knts
C4 at 3200 making 22 knts
B5 at 3200 making 24 knts (when the charts predicted a figure more like 21knts)

This year I tried B4 where I should loose 400 revs and put me where I thought B5 should be

B4 at 3200 rpm making 28knts

(and lo and behold my turbo on starboard is shot thanks forum for the jinx so I cant see if she'll make 4krpm. But I believe she will as port will spin to 3650 carrying the sbd engine if pushed)

But can someone make sense how a smaller and finer pitched prop is more efficient at the same revs?




Some pics of this weekend for entertainment

 

[BruceK]

Props. C series are volvo genuine
B series are BSP Pro patterns

 

[Portofino]

Drag is less at speed , both real and induced with the smaller dia finer pitched .

Also given on each test the variables were more / less equal for your meaningful comparisons .....we assume so those 28 knot props might develop more lift with your outdrives ,a greater vertical component in % wise compared with the other rival .

Seeing as the boat weight was identical then the extra lift means less hull drag = more speed for every cm it rise out of the sea .

There also might be a speed say 24.7 knots whereby the max efficiency of the lifting strips takes precedence and as I say the hull lifts up a few cm more and the final few knots snowball so to speak .
A sort of 2nd coming in the planing , it goes up again above X knots and a tiny rpm increase leads to a disproportionate speed gain .I have it with mine above 25 knots
If you had load / EGT guages you would see this .
This is because there are two surfaces at play here .
The deadrise V ie the hull which is angled and variable , flattens aft , forwards it sharpens .
And the hard chines which tend to be constant angle wise + the spray rails which morph into lifting strips as they go aft.
If you have a greater % SA of the none variable bits the chines + lifting strips you are more likely to get a second lift effect thats more profound and noticeable.

Your hull was designed around much more Hp yankie petrols remember in or around thunderbolt ally .

Or the labelling is false on one or more I say this because you say they are different brands .

There will be an explanation.

 

[Greg2]

Not sure if this is of any help but we used to have a Fairline Targa 35 with KAD42s and if memory serves the props were B6. At WOT she made 34 knots, not that we did that very often. She cruised best in the 23-26 knots range.

 

[BruceK]

@Portofino The labelling is unlikely to be false and I know the B4's were smaller diameter from the simple fact my wedge block when tightening / loosening the props didnt fit anymore with the B4's. BSP-Pro are a highly regarded prop manufacturer and I can accept they may be different to original Volvo B series in some small way that would skew the table but for the B series I stuck to them and C series to Volvo.

 

[BruceK]

Not sure if this is of any help but we used to have a Fairline Targa 35 with KAD42s and if memory serves the props were B6. At WOT she made 34 knots, not that we did that very often. She cruised best in the 23-26 knots range.

My boat came from the manufacturer with B5 which I found to be a tad coarse and the blade root cavitation erosion seemed to uphold that. My boat should make between 34 and 36 knt with these engines and so far the B4's are the only props that look likely to reach that figure. Like you I dont like pushing to 3900 / 4k rpm but cruise at 2900 to 3200 which is most comfortable in the Irish sea

 

[BruceK]

There also might be a speed say 24.7 knots whereby the max efficiency of the lifting strips takes precedence and as I say the hull lifts up a few cm more and the final few knots snowball so to speak .
A sort of 2nd coming in the planing , it goes up again above X knots and a tiny rpm increase leads to a disproportionate speed gain .I have it with mine above 25 knots
If you had load / EGT guages you would see this .
This is because there are two surfaces at play here .
The deadrise V ie the hull which is angled and variable , flattens aft , forwards it sharpens .
And the hard chines which tend to be constant angle wise + the spray rails which morph into lifting strips as they go aft.
If you have a greater % SA of the none variable bits the chines + lifting strips you are more likely to get a second lift effect thats more profound and noticeable.

Your hull was designed around much more Hp yankie petrols remember in or around thunderbolt ally .

You are spot on here. At 23 knts she comes alive and seems to skip. From this speed upwards I can feel when just 1 pax moves position in the roll axis. However all but the C3's could get her to this speed. I use the 3200 rpm baseline simply because it's the most comfortable cruising speed (~22 knts average) and what others boats in company are cruising at..... except Roy. He's dead slow!

 

[madabouttheboat]

Messing around with Volvo C series on a twin engined boat sounds like an expensive pastime

 

[[2068]]

Start of the season ... no fouling?

 

[Mr Googler]

Just buy J series....you know you want to

 

[BruceK]

Start of the season ... no fouling?

Boat has been in the river since July last year. With ablative AF on the constant 3-4 knt water flow ensures the hull is always spotless. The transom however is another story but that has no negative effect on the plane.

 

[BruceK]

Just buy J series....you know you want to

I believe so far that B4 is the optimum prop for me. I just need to replace a turbo to prove it. Leaving the boat in the river this winter I think the turbo exhaust has grown enough rust crystals to stop the fan completely because the turbo isn't creating boost. Squat diddly. I can hear Scottie in the engine room screaming "she aint gonna blow Captain"

 

[BruceK]

Messing around with Volvo C series on a twin engined boat sounds like an expensive pastime

yes and it dint help I would periodically loose the aft prop. Ya! did you know you were supposed to change the cone bolt every time you took a prop off to do the ring anode? Apparently they stretch or something and are use once. These days it's not just new bolts but wodges of bearing locktite. Takes a man full of porridge to get them off now. Bast$%^£%'s!!

 

[LBRodders]

My boat came from the manufacturer with B5 which I found to be a tad coarse and the blade root cavitation erosion seemed to uphold that. My boat should make between 34 and 36 knt with these engines and so far the B4's are the only props that look likely to reach that figure. Like you I don't like pushing to 3900 / 4k rpm but cruise at 2900 to 3200 which is most comfortable in the Irish sea

Bruce, what HP are you? 230 per side?

 

[Mr Googler]

Bruce, what HP are you? 230 per side?

20 years ago ???

 

[BruceK]

Bruce, what HP are you? 230 per side?

yes

 

[LBRodders]

yes

I 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, I think its difficult to pinpoint [and understand] a prop / boat / engine combo.

With our tub, they did some straight line variable speed runs at slack water with test props. They sent the laptop data to Mercruiser who came up with a prop spec. These were ordered, fitted and then tested. It worked - as final efficiency at the prop was pretty impressive. Something along the lines of 1 degree [slippage] per rev. But the devil was in the detail, i.e. knowing how hard the engine was working through the diagnostics in all scenarios. Porto's infamous EGT's but you need good accurate instrumentation - albeit there's time lag.

That gave us on day 1 - 37knts WOT. But that was bareboat, light fuel / water, no canopy, 2 crew and flat water.

Best we have got since is 34knts. So the variables have changed again for us and it will continue to do so [in one direction!]

Yours is a big boat for 460hp and with all your cruising gear its even bigger. If you are making 28knts with a combo [and 1 turbo down?] I think that is pretty good and probably the best you will get. It will end up being a case of diminishing returns.

Anyway, looks like you had another good weekend. Top effort, great pics

 

[scottie]

I believe so far that B4 is the optimum prop for me. I just need to replace a turbo to prove it. Leaving the boat in the river this winter I think the turbo exhaust has grown enough rust crystals to stop the fan completely because the turbo isn't creating boost. Squat diddly. I can hear Scottie in the engine room screaming "she aint gonna blow Captain"

Don’t blame me

 

[BruceK]

if I can push 28 knts at 3200 rpm but the engines can rev another 800 rpm you think I wont improve on 28knts?

I know a turbo is down, both were ropey and on last legs 3 years ago, then with lockdown and sitting idle, they are gone. The port engine's is slightly better and it could lead the starboard to 3650 rpm so I am confident the boat is good for it's 35ish knts. The petrol versions with the big V8's are pushing them just shy of 60 knts.


on a plus note the engines are running very cool at the moment. 83C on IR gun as opposed to 90-92 on C4 and B5

 

[Tanqueray]

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.