Kiwi prop - another update

[Jim Seamans]

Got out on the water yesterday early before the winds kicked up.All speeds are GPS speed over ground. Typically ran a straight track for a half mile.Have noticed a definite change in speed vs rpm. First of all the max rated rpm is 3600 for the 3GM30, I was only able to achieve 3200. At that 3200 rpm the boat speed was 6.8, where I noticed the biggest difference was at lower rpm's. At 2000rpm spd was 5 - 5.4 on different courses, at 2500 rpm it was 6 - 6.4. At 2800rpm spd was around 6.8, so the difference at higher rpms didn't seem to help push the boat any faster. With this it appears I can run at lower rpms and still achieve a decent speed.Under sail, I was broad reaching with a wind of 5 - 5.5kts on two different tacks. Boat spead was running about 4.5 - 4.8, this is an improvement over the fixed three blade prop. The prop feathered as designed and at this speed had very little turbulence at the transom. Will need to see how this works when the speed starts hitting 6 and above. Wasn't able to get more data - the wind QUIT/DIED/WENT SOME WHERE ELSE. *cryQUESTION FOR THE COLLECTIVE: Will it hurt the engine to be run at 2000 - 2500 rpm range for long periods? I usually power along at 2800. I'll give another report in June after some more use.

 

[Rick D]

Similar Issue, Jim...

I am slightly over propped too. Almost identical to you. My Yanmar tops at 3800. I find that 2800 RPM is the practical limit and going to 3200 only gets a few tenths of a knot. I contacted the Yanmar distributor who said it was OK, but on the lower side of it. So, in your case, 70% would be 2500 RPM and 80% 2800. I wouldn't go below 70% and if it's easy to re-pitch, do it next haul (mine is not so easy). However, if you do push it, watch your temp. If you are over propped, you may find it creeping up. Rick D.

 

[ED]

Pitch

You wont do damage running at that rpm, but you should re pitch to get most efficient performance. YOU should be able to let the engine run its max rpm.The way your set up now you are putting a bit extra load on the engine at lower rpm. I think your overall performace would be better with less pitch and it won't work the engine quite so hard.

 

[Paul356]

Kiwi

Jim,I'm doing this from memory, but I think your results are consistent with mine. I meant to check the logbook this weekend, but forgot. Will try tonight.I had the same top-end result as you, although perhaps dropping the max only to about 3450. I had been at 3600 with the fixed three blade. I was at first a bit disappointed in this and the corresponding loss in top-end motoring performance. I then remembered my wife's sage advice -- "It IS a sailboat, you know" -- and the improved performance under sail always brings a smile to my face. That is a sweet hull, and it's nice not to have the big fixed prop mucking it up. I'd probably go back to the fixie for a long motor trip, for example if we ever went down the Mississippi/Tombigbee. But for now, the Kiwi stays.I think the Yanmar likes to run in the 2000 - 2500 range. We've done that for long periods of motorsailing, including 8 hours coming back across teh lake last summer. It gives nice performance, allows me to keep the sails up, is relatively quiet and uses almost no fuel. I'd add that with teh Kiwi, we can sail longer, in lighter air, before turing on the engine, so that's a related benefit.I still have some problems with shaft alignment, and it seems to me the Kiwi might be more sensitive to those than the fixie. What appears to happen is that the hull deflects slightly during the winter hallout (despite constant efforts to keep the cradle just so, and despite Hunter's general reluctance to admit this). That in turn puts the shaft out of alignment, and causes some thumping at certain rpms. So, I avoid those and try to get the boatyard to make good on its promise to come out and realign. Not the best situation.Other than that (which isn't really a Kiwi issue), I do like the Kiwi. Probably another month until we launch here. Drat.--paul

 

[Shippy]

props

Hey Jim, I completed the yanmar course a month or so ago and the one thing that I got out of it more than anything else is run the engine hard. If the prop doesn't allow you to do so, repitch the prop. The 80% rule of max prm is the ideal cruising RPMs. Less than that for long times causes problems. more and you overheat. also, I was told that the tach on the boat is pretty useless. It could be off by 100-300 rpms. Only a handheld tack and reflective tape is accurate. I know on my boat, we are off by about 200 rpms.T

 

[Jim Seamans]

Thanks Tim

I might leave the pitch alone for this season and change it next fall when we get pulled. Then again....

 

[Paul356]

data

Jim,Here's what I found in my log on some speed tests after installing the Kiwi. Speeds are GPS over ground, a bit of a chop and crosswind, no current that I perceived.rpmmphknts20005.14.4230065.225006.35.530007.36.334007.96.9350087.0I had a note in my log from earlier that with the fixed 3-blade I'd made 8.1 (7.1K) at 3000 rpm, so there was a drop. Also, I'd been able to rev 3600 with the fixed, but only 3500 or really a bit less with the Kiwi.Your results appear slightly better, for example you said you're getting 6.3K at 2800, where I had to go to 3000. That could be a product of what I continue to think is chronic shaft misalignment in my boat.To clarify my earlier comment, when I was talking about cruising all day at 2000 or 2500, I meant at less than full load, as indicated here, and didn't mean that 2000 or 2500 would be the top end under full load. Not sure what all this means, except it makes me hungry to get out on the water.Always enjoy your posts and site. Hope this helps.--paul

 

[Jim Seamans]

Question for you Paul

When you received the prop was it already assembled or did you have to do it?Mine came put together with the pitch set for engine/transmission. Based on the differences in rpm's I would say my pitch is a little greater than yours.One of the concerns on cruise rpm is running the engine at 2500 for hours on end, but with a load. I wouldn't expect any problems.

 

[Paul356]

Together

It was assembled with the pitch set by Kiwi for the 356 with the 3-cyl Yanmar. I checked the pitch on a later haulout, and found nothing to quarrel with.