H28 prop

[waikiore 399]

Yes 29hp at 3600 -- which is ample for an H, but to put it in perspective - that engine would be no heavier than a Bukh 10. So according to my calcs and with the 2:1 ratio you should be swinging something like a 15 x 10 three blade for best performance. Of course this has to be proven in practice, clean fuel, clean bottom etc. It is crucial that she makes the full revs though - (lightship). And therefore standard cruising revs around 3000 - 3200 RPM. Another thing worth checking - and tricky in an H is the alignment to the prop shaft and half coupling. I bet it is out, this will release a few more horses to the prop where they belong.

[grantmc 59]

I've been quite amazed at the discussion that has gone on into fitting a prop. It's obviously quite a science.

When I had a new motor fitted to replace a very tired Yanmar I had hoped to use the old prop. Trouble was that it was an anti clockwise prop and a clockwise prop was needed. The technician doing the job made it really simple. He just asked how much do you want to spend?

Oh and I have been really pleased with the results. Boat zooms through the water at 6.5 knots (it maxed at 5 before) and there's heaps of spare revs.

But I do have a question. I have noted a few observations earlier about looking for black smoke at full noise. What causes this/why is it caused?

[Island Time 1,235]

Grant, black smoke is overfueling - it means that despite the aditional fuel provided, the engine cannot produce the power to go faster - it is bad for the engine! Back off a bit. If it happens at WOT and full revs, and stops even if you back off just a fraction, that's ok - you don't run your engine like that right?

[Chewing Gum 17]

Translation

 

"If it happens at Wide Open Throttle and full revs, and stops smoking if you back off just a fraction, that's OK"

 

Yep motor should get close to full revs at WOT. If it doesn't prop probably has too much pitch. Black smoke is definitely not good for the engine, make sure you back off the revs until no black smoke. Also not good for a diesel engine is running it too slow. Most of these small diesels are happy at around 2400 to 2500rpm. Lower revs wlll just glaze the bores and as mentioned before coke up the turbo if you have one. Each time we go out we run at about 3100 (under load ie under way) for ten minutes or so to give the engine a good clean out. For the same reason in the marina we run the motor every couple of weeks at 1500rpm in both forward and reverse gear for about an hour to keep the salt out of it and oil circulating. Diesels don't like idling.

[grantmc 59]

Translation

"If it happens at Wide Open Throttle and full revs, and stops smoking if you back off just a fraction, that's OK"

That's awesome info thanks.

Also good to know my motor is doing what it is supposed to.

Cheers

[Guest 000]

Well, I finally decided to deal to the prop issue so the boat is now in drydock awaiting a new propeller.

I knew that the old one would be difficult to remove so I fabricated a very cunning prop puller out of 8mm plate. But, I didn't expect it to suddenly break free of the taper with the force that it did, in the process of which it tore off my thumbnail and split my thumb open like a sausage. Unfortunately I had my young grandson with me who waited until a lull in the dinner time conversation to pipe up 'mum, what's a bleep bleep bleep bleeping propeller?' Now both my wife and my daughter in law aren't talking to me.

But I digress. The old prop was a 13 x 7 three bladed sailor type which explains why I didn't go anywhere. The 13" gave me two inches of tip clearance beneath the hull which was more than enough. My prop maker advises going to a 15" prop which reduces the clearance to one inch. He says that with the the aft sections of the H28 being so fine, hull drumming won't be a probLem.

Any advise here?

[wheels 543]

Ouch!!

If you have had advise from a Prop Expert, go with the Expert advise every time. If you have issues with the new one, you have someone you can go back to.

[Guest 000]

I don't know much about tip clearance, but from what I have read, one inch clearance on a 15 inch prop is pretty minimal. However, I vwill take the prop builders advise and go with the 15 inch. Its easy to take a bit off the blades if I have to, a lot harder to put it on should I need it.

Having regard for the future wellbeing of my fingers, is it normal to use a bit of anti seize between prop and shaft?

Thanks

[Guest]

Having regard for the future wellbeing of my fingers, is it normal to use a bit of anti seize between prop and shaft?

Thanks

Definitely not. Anti sieze will form a film between the two tapers that will wash out (melt out on an internal shaft coupling) and create a clearance, ie loose prop. Some aircraft use a prescribed amount of antiseize on their prop tapers but that is an entirely different set of circumstances.

Not sure exactly how the finger crush happened but the standard procedure for a prop removal is to back off the nut but still leave it on the threads with about 1/8th inch clearance between nut & prop. Tension puller and give the end a smart smack with a hammer. If this doesn't work after a couple of tries you might need to use a bit of heat.

We will have a further tutorial regarding the fitting a new prop including lapping in and checking for key binding.

[Guest 000]

I had the nut on the end of the shaft.

With the prop puller tensioned up I was rotating the prop by turning the aft end of the puller as I tapped the prop with a hammer.

The prop came off the taper with a sudden incredible force and my thumb was caught between the prop puller and the forward end of the rudder.

It bought tears to my eyes; I uttered words I hadn't used in decades, and you may be sure that I'll never do that again.

Thanks for your heads-up on anti seize. You have probably saved me from yet another disaster.

[Guest]

Ahh, I see. I still bear the scars of a similar incident with a truck shackle pin. The idea is to hit the puller tensioning bolt, not the prop. But I'm guessing your rudder was in the way. Don't feel bad, I've been doing this sort of stuff for 35 years and still bleed occasionally.

[wheels 543]

I've been doing this sort of stuff for 35 years and still bleed occasionally.

Darn near daily for me.

 

Pour a Jug of boiling water over the body of the Prop when the puller is under tension.

Usually a ratio of 10% the diameter of the prop should be the minimum distance between the tips and the Hull. What happens is that the Water gets compressed between the blade and hull and this is heard as a thump on the bottom of the Hull. It doesn't affect the performance, but for a large flat surface, it can get really annoying.

[Chewing Gum 17]

I would go with the 15in as advised, the bigger the better with the displacement type hull of a yacht especially in rough conditions to get maximum bite. As mentioned previously you just need to make sure the motor gets close to maximum revs at full throttle.

[Guest 000]

Woohoo!!

New 15 x 9 three bladed prop.

That shook a bit of soot down the funnel.

That made the old girl sit up and take notice.

6.5 knots at 1800 engine rpm.

Happy.happy.

[wheels 543]