Boat prop pub conundrum

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Kingruar

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Hi all,

The other day in the pub we were chatting about props, and their various differences. I myself am potentially in the market for a new set. This in turn opened up a big debate (still ongoing) as to what route I should go down.

Currently I cruise around 20knots with each engine running around 3000 rpm burning around 21ltr/hr per engine, running at approx 85deg c. This is a very comftable cruising speed for us, but upon a little research my engine rev window should be a bit higher if I go by manufacturers spec. My propeller range should be between 3800 and 4100 rpm.
These are Volvo D3 160’s and have been very reliable to me.

So, I get the feeling that my current prop is not correct for my boat as when I put the throttles wide open, things start to top out at 3500rpm along with an increase in engine temps.

Upon scouting through web articles forums etc, I have come across quite frequently the need for the correct props to stop any vibration related damages and their knock on effects.

Should I be looking for a prop that offers a faster engine speed? Or leave things where they are?
Will fueling change, engine temps change? Etc
The pub debate continues to the forum.
I am happy with my setup but obviously if there is room for improvement, I’m all ears.
 
The first thing to ask is whether your hull and sterngear were absolutely clean when you found the engines would only rev to 3500rpm. Assuming they were than the boat is overpropped and that may account for the rise in engine temps too. You will probably be using more fuel than you need to as well. When the props are correctly sized and the hull is clean you should achieve slightly more than the max rated rpm by, say, 100-200rpm. The extra rpm will ensure that when the hull is moderately fouled or heavily loaded the engines are running at their rated max rpm. Well worth talking to a prop specialist
 
Hi
Just a couple of things maybe.
In all the VP DP units ive had and been familiar with, I can honestly say that it would be very unusual for a DP set up to be anything other than almost perfectly propped in terms of pitch and match. God knows how they do it, but they get it right all the time. The exception would be if someone has messed with it previously or it’s been re engined
Presuming the latter is not the case, I’d ask if you are trimming it up and if the hull is clean. This would be the first reason for missing rpm. Even a few hundred, easily.
I’d also maybe say that although the rpm range you say is correct, I’d also say that the 190 version has the same rpm range too. Anyway. Sorry for asking but are you trimming it right and with tabs up at full speed ?
I would think it very unlikely that it would be wrongly set up when new. They usually just aren’t at all. As mentioned.
 
HI,

The hull is fairly clean at all times, i do see the fluctuation in RPM's between clean and dirty. The units are an SX drive, not sure why this was chosen when the boat was re-engined in 2006. I do notice that i do not have a great deal of trim movement before cavitaion / ventilation (cant remember) starts to happen and i loose drive / can hear it about to go.
I'm going to ask some boat yards etc if they happen to have some props in different pitches and see if i can test to see what happens.
 
If you are overpropped & it would appear that you are then ventilation is a symptom of being overpropped. Reducing pitch to get your rpm up to max rated rpm with a clean hull should help with trim range and reduce ventilation
 
If the drives were replaced, I’m wondering if they were set at the right height now.
What kind of boat is it ?
 
It’s a fairline targa 27, I have the drives able to trim right through the range so no limits are functioning through the evc, only the bypass when the travel has reached fully down.
Will keep this posted as we go, hopefully I’ll get my hands on some props this weekend we shall see. Fingers crossed.
 
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