Is my new Kiwi Propeller the right size??

[swanny]

New Beta 43hp with Hydraulic gearbox with 2:1 reduction in both directions. Ordered new Kiwi prop which arrived but was right hand not left hand so they replaced with left hand prop sized 17inch pitch 22 ( I believe that with flat bladed props, pitch is calculated differently). Too many assumptions on boths sides led them to believe I had the standard mechanical gearbox. Exchanged with no quibble, very good service. Perfect. Strangley though the new left hand was smaller than the first one they sent to me. They had not come across my combination of hydraulic gearbox and engine on my size of boat but I was assured that after much consideration this was the perfect size and pitch. Now after first sea trials I feel it is not quite right but am unsure as to why. My experience is non-existant with these matters. Maximum revs of engine is 2800rpm, boat weighs approx 9 tons fully loaded with a fairly large wetted area but was say only 8.5 tons on day of test - Contest 36k. 1800rpm gives me 6knots, 2000 gives me 6.4, 2500rpm gives me 6.7 but no increase in speed at maximum revs. With this engine and power I would have expected to have reached 6.5knots at lower revs. Hull speed is 7.23. I thought perhaps with this engine which is eight hp more than old volvo would give me lots of grunt to perhaps reach hull speed. Engineer who fitted the engine told me I should reach hull speed with less than 2000rpm! He has same engine on a southerly 35 with much the same weight but perhaps less wetted area and he can with his set up. His prop is fixed 3 blader. The other interesting issue is the prop noise. My old two blader made some whooshing but very little, you could only hear it in the aft cabin. The Kiwi Prop makes enough whooshing that can be heard from the cockpit. Irony is the new engine is so smooth and quiet, I often think it has stalled but now I have intrusive noise from the prop! Question is this: How do I test if the prop is the right size and pitch? Stoping by the way is awsome! astern the prop is set at maximum pitch. Marinas hold much less fear now than before! Second question: I thought with a complete feathering prop I would not need a shaft brake but the prop still turns albeit slowly - I can stop it easily with my hand. Hydraulic gearbox has no effect if put in gear. Will this damage my gearbox?

 

[wotayottie]

the prop is adjustable for pitch as below

"The Kiwi Feather Prop™ will have been set at the recommended pitch for your installation based on the engine model number, the reduction gear fitted and the particular characteristics you supplied of your vessel. You may however wish to take advantage of the simple pitch adjustment feature to accommodate the many variations between individual vessels and operating preferences and obtain the optimal motoring performance for your particular requirements. One turn of the 8 mm pitch screw in a clockwise direction to each blade in turn will equate to 3 degrees of pitch [not inches of pitch] and substantially increase the power required from the engine and drive train. This will translate to lower engine revs. We would recommend adjustments be made in no more than exact half turn increments to each blade, which has the effect of varying engine revs by some 300 ~ 400 rpm. Each installation is unique and only experience can deliver the appropriate settings and optimal cruising revs for your vessel. A pitch setting of 20 degrees equates to a normal pitch of ~ 11 to 12 inches."

If the boat stops well in reverse, it suggests that you need a bit more pitch going forward. The engineer might be right knowing the boat or he might be talking nonsense - what in effect he is saying is that you should reach hull speed on way less than the engine's rated output suggesting the engine is big for the boat. Mine is a 34 footer of 6.5 tonnes (allegedly - according to the crane ) and reaches hull speed at max revs on an old 2003 ie 29 bhp absolute max. So my guess would be that you should reach hull speed at something less than full revs, but I doubt the 2000. Whats the power output at those revs?

 

[swanny]

Thanks for your informed reply wotayottie!. To answer your question, the power at 2000rpm is 35hp. Maximum torque however is produced at 1600rpm. I went for the larger engine size to reduce the revs required to cruise and therefore the fuel usage and comfort levels. I figured that this engine would give me good cruising at around 6.5knots at about where the maximum torque would be. but as I said before, I am a novice at this stuff. You sound almost conneced to to Kiwi Props or are involved in the prop industry. Do you think it is a matter of simple pitch adjustment or is the actual size of the prop perhaps wrong too considering the first prop I got was larger? It is a pain to have it wrong as unless I want to go swimming with a allen key in my mouth in the not sooo warm Baltic I have to remove both masts in order for my local crane to lift me out - a helluva lot of work At the moment my new set up feels like it is performing like my old volvo MD17c (35hp) without the smoke. I think and I will have to go to the boat to get the details, that the pitch right now is set at 22 which I believe only gives me one more degree to increase to as I remember the maximum pitch available was 23. All will be clearer tomorrow when I can speak with Kiwi Prop Uk who I have to say up to now have been most helpful. I am interested in hearing more from you though!...

 

[davidbains]

Swanny if you're doing 6.7kts at 2500rpm then that sounds pretty good to me.
22 degrees is near max pitch. So if you want a few more revs then decrease the pitch slightly.
What revs does your handbook recommend for max horsepower?
You can do it swimming, just remember to unwind the screws (slightly) the same on each blade.
The max pitch you experience in reverse is much greater than the pitch available in fwds.
As you say reversing is a revelation. Are you sure your reverse ratio is the same as fwds?

My experience of the Kwi prop is that the motoring speed fwds only increased slightly, but astern is fantastic for power and manouvreability, and of course sailing speeds are up especially in light airs. I would not go back to a fixed prop now.

The gentleman at the importers is very helpful. Talk to him on the phone.

 

[swanny]

Hi David. I suppose 6.7knots is not sooo bad but the revs are higher than I was led to expect from my engineer so I thought I would question it. I have just learn't also that a hydraulic gearbox can also suck more power from the engine so with the two alternators and the gearbox, I am probably not getting the added advantage of the extra 8hp over the old engine. I dunno. I have no spoken with Alan at Kiwi Prop UK and they are getting back to me. They are very nice. Do you have a kiwi prop David? If so, do you notice much more noise than say on a traditional prop?

 

[bedouin]

I've just been looking at replacing an old Volvo. If you look at the details you will probably see that the old Volvo has a lot more torque low down than the beta, so while the replacement will have more power at the top end, at cruising revs there may be little noticeable difference.

You say nothing about the conditions you experienced in your "sea trials" - it makes a big difference whether it was flat calm or a rough sea on the nose.

However roughly speaking, if the prop is too large then you will not be able to get the engine to full revs. If the prop is too small then you will reach maximum revs at less than full throttle.

 

[swanny]

Hi Bedouin. The old volvo MD17c had maximum torque of 81lbs at 2200 rpm. The Beta has maximum torque of 95lbs at 1600rpm so it is the reverse of what. Having now spoken with Kiwi Props it appears I have a pitch setting of 23 degrees not 22. The tag on the prop when it arrived read wrong. So, increasing the pitch is not really an option as I am at maximum pitch now. The only option to enable maximum revs would be to increase the size of the prop. Kiwi props will do some more calculations and I will conduct another test perhaps if I can in flat water to see how I get on. Thanks for all your comments.

 

[bedouin]

I wonder if your prop is actually over-pitched?

If that is the case then you would expect to achieve hull speed in a flat calm at very low revs, but would struggle in any sea conditions as you would get too much slippage. That might also account for the greater noise you hear from the prop.

 

[davidbains]

My Kiwi prop is not noisy and I have slightly increased the pitch.
I don't think there's much wrong with your setup but you could try slightly less pitch.
Expecting max speed at 1600 to 2000 revs doesn't sound right to me.
How big are those twin alternators?

 

[swanny]

The alternators are 70 and 175amp but the batteries were completely charged so the alternators would have been working very little. The larger one was only putting out 1.7amps during the trial. Its flat calm today, beautiful. Later I will take her for another run to see if the small chop the other day really made so much difference. I hope not! It could be my expectations have been too great. I had thought that with more torque and similiar revs compared to my old engine but with more power would give me relaxed motoring with great fuel economy and the power to punch into waves. The old volvo would cruise at 5knots and although I never pushed it due to the age and smoke, she would do about 6.2knots. Into the waves though and she would lose way from say 5 to 3knots and these were only say a metre and a half of wave heights but the wind was on the nose at 24knots and she is a ketch with fair freeboard..etc...etc. We shall see. Not sure about reducing the pitch but after today I will decide. thanks guys. Interesting stuff