Prop for Islander MK2

[seaboat]

I recently bought an Islander MK2 32'.
I noticed that at times of adverse weather I barely make headway and
one of the reasons for this could be an improperly matched prop.

The boat has a Volvo 2002, a 2 cylinder diesel with about 14 hp for a boat weighing around 11,000lb.
The max rpm of the little diesel is 3,500, but I don't know what the optimal rpm is.
The prop I believe is a 3 blade maxiprop, each blade is 6.5" long (from the center).

How can I best figure out if this is the right prop for my boat.
From what I read the diesel should be capable of easily pushing the boat at hull speed, which, for a boat of 25' water line, is around 7 knots.

I do not achieve that.

Thanks for insights Uwe

[fstbttms]

Originally Posted by seaboat

The prop I believe is a 3 blade maxiprop, each blade is 6.5" long (from the center).

How can I best figure out if this is the right prop for my boat.

Talk to PYI (U.S. Max Prop distributor.) They can tell you to which pitch settings you should have your Max Prop adjusted.

PYI Inc. | Contact PYI Inc.

[DougR]

The engine that is installed in your boat is rated 18 h.p. @ 3200 RPM and although you don't tell us, the most common reduction ration that was used with that engine was 2.39:1. So what follows is based on that ratio.....

Given the particulars of your boat, the thetorical hull speed is just shy of 7kts., but it will take about 29-30 h.p. to achieve that speed. With only 18 h.p. you should expect something closer to 6 Kts. With the engine/ gear ratio that you have the OPTIMUM three blade propeller for your boat will be something around a 16 X 9 x 3. This assumes, of course, that a 16 inch prop can be installed on the boat and still maintain proper tip clearance.

The 13 inch prop that you currently have is really too small ( lack of blade area) to absorb the power that you have without experiencing cavitation, but it can probably have the pitch increased to absorb more power and produce somewhat more thrust. Currently the engine is "up against the governor" and not producing its full output. It should only be able to turn the prop up to 3200rpm.

Try increasing the pitch and see if that helps.

DougR

[seaboat]

Thank you very much DougR for the very informative answer.
You are right about the lacking gear info.
I am very tall and for me to get in there and check which gear I have is quite an ordeal.
But modern technology in the form of a digital camera, which can be held at arms length and still make usable photos, was the solution.
I have a MS2B 8527 and I believe that is the 2.37:1 gear ratio.
Another poster had recommended to call Max prop, which I did,
and they also thought the propeller was a bit small, but asked to check the pitch.
I found a pitch of 20 or 22 degrees (one of their numbers pointed to 20, the other to 22).
They recommended a 26 degree pitch.
Right now there is not enough space to mount a 14" or even a 16" propeller, but the shaft could be changed for a longer version and then there would be plenty of space. The question is how much could the shaft and propeller stick out past the external strut before repositioning of that strut would be necessary.
I think for now I will adjust the pitch and see what happens.

A problem I also have to address is the at times erratic display of my rpm.
I don't know if I have to look at the input side of this, eg. the alternator, or if I need to troubleshoot the display end of it, I have never done this.
I just feel that without a reliable rpm info many of the previous advise
can't be implemented.

Thanks again for the help.

[DougR]

The recommended overhang between the rear of the cutlass bearing and the front of the propeller hub is generally considered to be not more than one shaft diameter. Of course you will see overhangs which are greater than that, but that's the generally recommended distance. Likewise the generally recommended diameter of the prop should not exceed 15 shaft diameters.

The tachometer on the 2002 engine was driven from two different sources depending on the vintage of the engine. Earlier versions were driven by an inductive sender which was mounted on the top/ front of the timing case immediately behind the oil fill opening. The hex shaped sender had two grey wires connected on the top and these wires ran thru the harness to the tach.

Later engine versions were driven by the alternator. One grey wire (or maybe grey/black) was connected to the "W" terminal on the alternator and ran to the tach.

In either case its probably a case of looking for loose/corroded wires in the connections at the harness ends or at the sender or tach.

DougR

[seaboat]

My tachometer gets it's signal from the alternator.
I did dig in there at one point, checking all the wires for a bad contact
and even putting my fluke on the alternator to see if the signal there was as erratic as the display. The test was inconclusive if I remember right and I will have to do this again.
The problem is that the Volvo people do not divulge what is in their tachometer, so if that seems to be the culprit it will be difficult to troubleshoot, not knowing what is in there
and how it works.
At least now I own an external tachometer to see the rpm of the diesel engine.

Uwe

[DougR]

Nothing magic about the Volvo tach....its built by VDO. Like almost all solid state tachs it counts pulses and converts to an RPM reading. You could probably send it to someone like Lauderdale Speedometer and have it checked out or repaired, or just get a similar replacement unit.

I think there is a thread here on CF dealing with the hour meter repair on VP tachs, and these same guys probably can do other types of repair as well.

DougR