Your thoughts on easy revving engine..

I have finally managed to get flat water, no rain and a bit of time to pop down to the boat and do the WOT test..

Load was 2 people, full water and 1/2-3/4 tank of fuel..

So starting at about 1500rpm I rolled on the throttle to full.. The revs climbed to 3600-3800 pretty quickly.. As it came onto a plane it was already probably 4000-4200rpm.. Then the speed started climbing and with the trim all the way it the rpm climbed to about 4700-4800.. I then trimmed it a little and the speed increased and the revs climbed to 5000 and seem to settle there but I didn't stay there long to to see if it would go any more, it seemed to have stopped.. Speed was about 28-30kn according to my phone gps and I think I was going into the tide so water speed was probably 30-31ish (boat speedo isn't working)..

So the current prop is 15x17.. My thinking is either a 4 blade 14.25x19 (http://www.steeldevelopments.net/volvo-single-prop-drives-1/4-blade-aluminum-sx-e-series.html) or 3 blade 14.5x19 (http://www.steeldevelopments.net/volvo-single-prop-drives-1/3-blade-aluminum-sx-e-series.html)..

What do you think?
 
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I think everything points in the direction of a higher pitch prop. You should experience a good acceleration (also known as hole shot) with the current prop as a too low pitch equals a low gear in a car.

Provided this is was the load you aim to carry at most times you should get a higher pitch (drop 200 rpm per inch). Provided your current prop works ok apart from the rpm (did it keep a good and constant grip in the water during pull and did it keep during turns at speed?), the prop design apparently is ok.

A new prop or alteration of the one you have would be my recommendation.
 
I wouldn't say the acceleration was brilliant but then again its not a "speed" boat.. It's a cruiser.. :)

Cornering seemed to hold ok..

I am thinking the 4 blade 14.25x19 sounds like the way to go.. The 2" change in pitch will drop the revs to ~400rpm.. The 3/4" drop in diameter will probably raise it about 100rpm and from what I have read the extra blade will drop it about that much again.. So I should end up at about 4600rpm which seems perfect..

As far as load we were probably a little light being only two people on board, usually at least three people.. Even so everything seems to point to the 19" pitch prop..

What do you think?

Makes of props available seem to be the "Vortex" and the "Solas" props.. Solas seem to have two options, one with a rubber hub and one with a pressed hub.. Are these brands ok? Is the rubber hub one better than the fixed hub or is fixed better?

Thanks..
 
It would also be a good idea to get the accuracy of the rev counter checked, as if it is not reading true all these experiments will produce misleading results.
 
I think your considerations make sense. Penfold is right about the tacho although I normally find them to be pretty accurate if not obviously defective. Easiest (inexpensive - sub £10) check is a universal laser reader (stick a bit of reflective tape to any rotating item, point the laser and get a reading on it's display) or engine tester.

BTW used a laser tacho to determine gear ratio by reading engine and prop at the same engine rpm ;)


If your 3-blade prop works flawlessly there isn't much to gain from switching to 4 blades but it may leave more choice when ordering. Customizing of the current prop normally cost less but that option should be discussed with your preferred prop shop. Both cupping, rake, diameter and pitch can be worked on.

Of course getting a new prop would make the old one a spare to carry onboard.
 
penfold said:
It would also be a good idea to get the accuracy of the rev counter checked, as if it is not reading true all these experiments will produce misleading results.
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This is true but when it was reading 5000rpm it sounded like a NASCAR.. :cool: So my guess is its pretty accurate.. :)
 
A V8 doing 5000 sounds busy but so does 4600. Stating rpm by ear isn't reliable.

You need to be accurate also re. idle speed. The recommended speed is crucial for engaging gears right.
 
Spi D said:
I think your considerations make sense. Penfold is right about the tacho although I normally find them to be pretty accurate if not obviously defective. Easiest (inexpensive - sub £10) check is a universal laser reader (stick a bit of reflective tape to any rotating item, point the laser and get a reading on it's display) or engine tester.

BTW used a laser tacho to determine gear ratio by reading engine and prop at the same engine rpm ;)
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Yes, I have one of those testers.. I had thought about using it to determine the ratio as well.. Issue is that in the water fully trimmed up the prop is still a little over half in the water.. Could get very wet attempting it and the laser probably wouldn't be able to detect anything before it drowned.. :)

If your 3-blade prop works flawlessly there isn't much to gain from switching to 4 blades but it may leave more choice when ordering. Customizing of the current prop normally cost less but that option should be discussed with your preferred prop shop. Both cupping, rake, diameter and pitch can be worked on.
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I don't think the existing prop is a very good one.. For example the leading edges of the blades are very square where the new ones seem quite "shaped" probably making them much better at cutting through the water..

Of course getting a new prop would make the old one a spare to carry onboard.
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This was a plan too.. Especially if I do any longer trips.. Although I don't think changing it in the water would be easy..
 
wipe_out said:
This was a plan too.. Especially if I do any longer trips.. Although I don't think changing it in the water would be easy..
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You may not have a choice if you hit something.. A spare can be seen as a safety item as it is your only means of converting engine power to movement :cool:
 
Spi D said:
A V8 doing 5000 sounds busy but so does 4600. Stating rpm by ear isn't reliable.

You need to be accurate also re. idle speed. The recommended speed is crucial for engaging gears right.
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Yes, I know an ear isn't accurate.. :) Wouldn't like to be revving any higher though, it was screaming a bit.. Might attempt a laser measurement at ~2000 rpm in the marina.. Wouldn't want to try and test it on the water in front is a spinning v8 with all the pulleys.. Certainly not at full speed out on the water..

Good advice.. I might do that and get the idle speed set using the laser.. Engaging gears does seem a little rough which could be down to idle speed.. What is the recommended idle speed either in gear or out of gear?
 
Well it looks like 19" was too much.. Put the 4 blade 19" on today and took it for a run.. Couldn't get on a plane.. At full throttle it got to about 2800rpm and pretty much stayed there.. Granted the water wasn't that smooth today and we were going into the wind but still.. The engine just had no power at all..

Looks like a 4 blade 17" prop is next on the cards.. Will see if Steel Developments can do anything with the 19" to offset the price of the 17" otherwise will have to put it onto ebay.. It's one of the Solas props with the Rubex hub so will work with many other engines.. Obviously I will keep the hub for the next prop..
 
Spi D said:
Or get the 19" altered to a lower pitch
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I'll chat to Steel Developments next week.. I think in a previous conversation they said they could change it by 1" up or down if it was wrong.. Will see what they say but I don't think that a 1" change will be enough..

Spi D said:
Might be that both adding a blade and upping pitch was too much.
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Yes, seems that way.. The engine can't get into the torque band it needs to get over the "hump"..

I suspect that the old prop wan't a very good one either.. Looking at the blade finish on the new prop its easy to see it far more "shaped" with rounded leading edges and some cupping on the trailing edge where the original one had square leading edges and the blades are quite "flat"..

The better design and the extra blade plus the 2" pitch change all added together to be quite a big leap because I wasn't comparing the performance of the same make and style of prop.. Now that I am able to get the same brand I can make a better assessment..

Of course there is no telling if the original prop had been re-pitched either so really it was a "best guess" based on whet information was available..

If nothing else I have a point of reference now..
 
Does the 19" have holes in the hub? Are they blocked up? Many props have holes in the hub to enable exhaust gases to escape when there is some back pressure (in displacement mode) to enable the prop to spin up and get into the power band of the engine.
 
Cookee said:
Does the 19" have holes in the hub? Are they blocked up? Many props have holes in the hub to enable exhaust gases to escape when there is some back pressure (in displacement mode) to enable the prop to spin up and get into the power band of the engine.
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There are holes through the prop, similar to the original prop so should work the same as the original.. The new one with the extra blade, 2" of additional pitch and some cupping on the blades (more than the original) I think has been too much of a jump for a boat of this weight.. So the engine gets to about 2800rpm and you can hear the carb drawing air quite a lot more than before so its certainly trying but not easily managing to get on a plane.. In completely smooth water it may have got there but it was a little rough and it battled..
 
The holes referred to are on the hub, in front of the blades. As exhaust runs inside the hub it will get out in the prop blade water stream and make a mix of exhaust/water, preventing the prop from getting a grip. Hence allowing the prop to increase rpm and once the water passing + drag behind the prop gets up, the exhaust will stay inside the hub and get out behind the prop. I've seen it work only on high hp/weight ratio setups, where fast oats are slightly overpropped to get top speed while on plane.

Quicksilver registerede their design as PVS


Pretty sure that your Solas prop does not feature this.
 
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Worth noting that in general if you're playing with props, it's a good policy to only change one thing at a time-eg ally to one particular brand of stainless, changing pitch,changing diameter OR changing number of blades. Changing multiples just makes it hard work and you just erase your benchmarks.
 
Spi D said:
The holes referred to are on the hub, in front of the blades. As exhaust runs inside the hub it will get out in the prop blade water stream and make a mix of exhaust/water, preventing the prop from getting a grip. Hence allowing the prop to increase rpm and once the water passing + drag behind the prop gets up, the exhaust will stay inside the hub and get out behind the prop. I've seen it work only on high hp/weight ratio setups, where fast oats are slightly overpropped to get top speed while on plane.

Quicksilver registerede their design as PVS


Pretty sure that your Solas prop does not feature this.
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No, neither prop has those holes..
 
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