Changing from 5-blade to 3-blade prop on a 3.0L Mercruiser

[annageek]

So we've been in the market for a new prop for a while now, and have done nothing about it, other than ask for people's opinions. Nevertheless, it's bit the bullet time and sort it out.

At present, we have a 19" pitch Mercury High 5 (5 blade, stainless steel) prop. At WOT, on our Mercruiser 3.0L, we hit about 4750rpm (spot on for that engine). We cruise at about 3300-3400rpm which gives us about 22-23kts which just 'feel' like the boat's sweet spot.

We're planning to get a 19" Mercury Vengeance (3 blade, stainless steel) prop. I am hoping that this will maintain about 4700-4800rpm at WOT. I am also hoping this will enable us to cruise at about 3200rpm (more efficient thn 3300-3400rpm).
I believe that with the High 5, the rule of thumb of knocking off an inch of pitch for every extra blade does not apply. I read somewhere that the High 5 blade design is quite specific to watersports (slippier than it should be for better hole shot) and doesn't therefore follow this rule. Is this correct?

Two things have changed since we last ran on the High 5:

1. This winter, We removed the years of old, crusty antifoul paint off the bottom (since we're dry stacked - and loving it) and now have a smooth, shiny, more hydrodynamic bottom - gotta love a hydronamic bottom .
2. We have an exra 80L of fuel capacity on board (180L vs 100L previously).

My feeling is that the increase in maximum fuel load combined with the slight improvement in hydrodynamicy will, roughly speaking cancel out. Thoughs on this theory are welcomed!

The reason we want to change is to improve efficiency at cruising speeds. This is NOT to provide a financial benefit (with the cost of the prop, this'll never come near to making financial sense - we know that!), but to improve crusing range (a bit). If we could improve range at cruise by 10-20%, that'd be worthwhile for us.

We never do watersports (but would keep the High 5 incase we did / incase the next owner wants to / as a spare prop). We're based in the Solent, and therefore regularly have to tackle the 'Solent chop'. Will we likely notice much difference with a 3-blade prop, as I understand 5-bladers are supposed to have better grip and can therefore handle a bit better in confused waters like those found in t'Solent?

FWIW, the reason I'm looking at the 19" Vengeance is I remember reading on here that oGaryo, when he had his Fletcher 19gts with 3.0L Merc fitted the same. Naturally, I will be PMing him shortly to kindly seek his opinion
Sense check please? Am I expecting too much? Am I missing anything?

Thanks!!
Anna.

 

[BruceK]

I can only offer you my experience here, I have 6 props in the garage to bear testament to the experimentation, but my cuddy on a 140 3l GL works best on a 14.5 x 19 3 blade prop for general use. Strangely enough on a SS quicksilver prop of 14 x 21 she cruised quicker and more efficiently without labouring the engine albeit this is regarded as over propping her. (I stopped using it on two counts there is no ability to prop slip as the prop hub is splined and mates directly to the drive shaft and there are a lot of sandbanks where I am - cue one leg later, and the p-factor. Coming out the hole was slower and the engine torque would make the boat list quite alarmingly to port when given some welly until you got on the plane) However as it is one of my spares now you are most welcome to give it a try as in the end there is no real substitute but to try each of the given ranges to see what works best for you and your boat as even in two of my 14x17's by different manufacturers, gave different results, yet to the casual glance are near identical.

Edit. iBoats.com would probably be your best stream of information on this topic (am I allowed to say that?) It's a hot repetitive topic there.

 

[MapisM]

gotta love a hydronamic bottom

LOL, that's a statement everybody around here will agree with, if I've ever seen one...!

Back to the point, when considering prop alternatives on small speedboats, the slip is a very important consideration.
But you don't give us enough numbers to understand how the boat is performing at the moment, from this standpoint.
In fact, we should know the very top rpm and speed that you can currently achieve with the boat as unloaded as possible, and the newly polished bottom. And also the outdrive ratio, of course.
With these numbers, it's possible to make an accurate estimation of the prop slip, and any evaluation of alternatives should start from there.

Considering 23 knots @ 3400rpm, and assuming/guessing that your Merc Alpha outdrive has a 1.98 ratio, the resulting slip would be 14%.
A tad on the high side, particularly for a high 5 prop, but maybe these numbers were still taken with the crusty a/f? If so, it's bound to improve significantly with the clean bottom.
But for a proper comparison, the slip should be calculated at WOT, anyway. Therefore, I would start checking carefully the current max rpm/speed.
Btw, an accurate slip estimation requires STW speed, not SOG. Otoh, STW speed given by the sounder wheel (if you've got one) is rarely accurate, so I would choose a spot without current, using the GPS to check SOG speed, eventually assuming it's =STW.

All that said, my gut feeling is that the Vengeance is better suited for your 4 cylinders engine.
The high 5 is a great prop, but mostly aimed at very torquey V8s.
In fact, if you reached 4750 with the high 5 and the old a/f, I wouldn't be surprised if you could pull a 21" vengeance.
But as always with props, there's only one way to know for sure, and it takes three things: test, test, test.
Haven't you got some local dealer kind enough to borrow some props to try before buying one? Just a thought...

 

[piratos]

So we've been in the market for a new prop for a while now, and have done nothing about it, other than ask for people's opinions. Nevertheless, it's bit the bullet time and sort it out.

At present, we have a 19" pitch Mercury High 5 (5 blade, stainless steel) prop. At WOT, on our Mercruiser 3.0L, we hit about 4750rpm (spot on for that engine). We cruise at about 3300-3400rpm which gives us about 22-23kts which just 'feel' like the boat's sweet spot.

We're planning to get a 19" Mercury Vengeance (3 blade, stainless steel) prop. I am hoping that this will maintain about 4700-4800rpm at WOT. I am also hoping this will enable us to cruise at about 3200rpm (more efficient thn 3300-3400rpm).
I believe that with the High 5, the rule of thumb of knocking off an inch of pitch for every extra blade does not apply. I read somewhere that the High 5 blade design is quite specific to watersports (slippier than it should be for better hole shot) and doesn't therefore follow this rule. Is this correct?

Thanks!!
Anna.

I know the High 5, but its many years ago I had sterndrives, so I dont know the Vengeance.
What I would say however is that with the speeds you will obtain on a Fletcher 21 with a 3,0 Mercruiser a SS prop and specially cruising with 23-25 knots is waste of money.
A standard Alu prop will do the job, and a if you use a local company they should be happy to lend you one or two propellers to test the right size.

 

[TheOrs]

I made a similar change but on a smaller Fletcher & did some measurements when I did so. Although not directly comparable, they may be of some interest:

Fletcher Arrowflyte with Mercury 75HP outboard (all up weight about 500Kg; so similar power/weight to your GTS)

I was running a 20" Hi-5 (small hub, so even pitch numbers) & I tried a 20" Lazer II & 18" Vengeance, before settling on the Lazer II and getting it re-pitched to 19".

20" Hi-5: Holeshot - 4s, 0-30 - 7s, deepwater start towing 90ish Kg skier - 15s, top speed (@ 5250 rpm) - 42mph
20" Lazer II: Holeshot - 3s, 0-30 - 6s, deepwater start towing 90ish Kg skier - 14s, top speed (@ 5250 rpm) - 45mph
18" Vengeance: Holeshot - 3s, 0-30 - 6s, deepwater start towing 90ish Kg skier - 7s, top speed (@ 5500 rpm) - 42mph

All speeds taken from the boat speedo (pitot tube).

As mentioned by Mapis, the Hi-5 is really meant for bigger, torquey engines and not really meant for something like my 75 Merc (which I believe is the smallest engine you can fit one to). One thing that really struck me was how much more grip the Hi-5 had during cornering compared to the aluminium 21" that preceded it. The Lazer II is slightly less grippy but will still tax your neck muscles if you stick the boat into a hard turn. The 21" aluminium would just ventilate and force you to slow down in similar circumstances.

As others have said; you really need to try some different props. I bought secondhand from eBay & then sold them on afterwards.

Edit: Also, be aware that some of these Quicksilver props have a 'PVS' system (not Vengeance though) that allows you to vary the amount of exhaust gas flowing over the prop at tick-over. More gas allows the prop to spin more and the engine to hit it's 'power-band' at a lower (boat) speed. That's the theory anyway. It works by swapping out plugs on the prop hub. Different plugs have different diameter gas orifices (!). I only had the plugs that came with the Lazer II but removing them (max gas flow) meant that the boat was unable to plane! I did buy some more various sizes to try but never got round to it as you have to remove the prop to get them out without damaging them, so I don't know whether they actually work or not.

Good luck

Darren

 

[annageek]

Thanks for all the replies! Wasn't sure how well I'd get on here, but this info has all been really useful!

I think I have made a decision... I've decided to defer my decision until I've gone for a blat up So'ton water with a light load on board, with the new, smooth bottom.

I presume that an alternative to timing it so I can run the test at slack water and rely on our pitot tube based speedo, I could do a run in either directions and just average the two GPS derived SOG readings to get an equivalent STW?

I've tried to track down some prop suppliers around here who would be willing to lend me a prop or two to try, and even after offering to accept sale or return with a restocking charge, no joy. If anybody has any suggestions, it'd be much appreciated!

RE the PVS system. One thing I do know is that on our High Five would have been shipped with solid PVS hole plugs. However, there is one (I think) missing, meaning that one blade is getting overly ventillated. It was like that when we bought the boat, and I've just never given it much attention. However, perhaps this could partially account for some of the more than expected prop slip (assuming the figures with which MapisM calculated it are accurate enough).

I have been keeping my ear to the ground on eBay to see if anything suitable crops up, knowing that if I can buy a second hander now, I should be able to resell through the season without too much loss.

I take piratos' point RE the alu vs SS prop. However, I have seen a couple of people at the dry stack who have gone through Alu props, and just like the fit and forget nature of an SS prop - even if it is very expensive for minimal convenience! Also, we are dry stacked, and you just can't put a price on the 'bling factor' of a SS prop when its sitting up on the rack!

BruceK - Thanks for your kind offer of the loan! Once I've had chance to test our current set-up once more, I may just take you up on that (assuming you'd be able to post - if I cover the cost, of course!). I'll be in touch by PM!

 

[MapisM]

I could do a run in either directions and just average the two GPS derived SOG readings to get an equivalent STW?

Yup, absolutely.

RE the PVS system. One thing I do know is that on our High Five would have been shipped with solid PVS hole plugs. However, there is one (I think) missing, meaning that one blade is getting overly ventillated. It was like that when we bought the boat, and I've just never given it much attention. However, perhaps this could partially account for some of the more than expected prop slip (assuming the figures with which MapisM calculated it are accurate enough).

I really don't think that the missing plug can make any difference.
The idea of ventilating the prop is strictly meant to get more revs upon startup, hence having more power available to get the boat over the hump, but in a boat like yours that's a solution to a problem that doesn't exist.
Once at P speed, that PVS thing should be irrelevant.
Re. my slip number, I can assure you that it's mathematically correct, 'cause I just threw your numbers (plus my assumption on your outdrive ratio) in my Excel prop calculator - just pm me your mailbox and I can send it to you if you're interested, though there are also several prop calculators available online.
Trouble is, the slip is normally calculated/compared at WOT, so in this respect when I said that 14% is a bit high I was actually assuming that in your boat it's the same also at cruise speed, and that could well be a wrong assumption.

I take piratos' point RE the alu vs SS prop. However...

TBH, his point is a good one indeed.
Personally, on a small speedboat, I would also stick to alu, with boats capable of up to 40 knots or so.
The s/s higher efficiency is really worth the higher cost only when you're talking of going north of 50 kts, possibly with a surface or semi/surface setup (=very aggressive X dimension, i.e. outdrive/outboard installed higher than normal).
Can't argue with your "however", though...

 

[annageek]

I can assure you that it's mathematically correct, 'cause I just threw your numbers (plus my assumption on your outdrive ratio) in my Excel prop calculator

That's what JP Morgan was thinking before they realised they were out by around $6bn! Hehe!

Wasn't suggesting the maths was wrong though... just that the data I supplied was! It's all from memory based on testing last summer!

I think I may well be rethinking the alu vs SS prop then. Given that you can pick up a mercury black maxx brand new for £120, I can buy nearly three for the cost of one SS prop! The saving can go to the next exhaust manifold we'll need next winter - gotta love raw water cooling!!