Guest 000 Posted August 30, 2014 Share Posted August 30, 2014 My dear old H28 can make 5 knots if I'm lucky and the breeze is only a zephyr - which isn't very flash considering that the engine is a 27 HP Perkins in the prime of its life. I'm afraid that I can't tell you the size of the current fixed 3 blade prop, only that it is way too small. Irrespective of the hull aperature which I can enlarge if necessary,can anyone advise me of an optimum prop size? The engine drives through a 2:1 reduction box and seems happiest at about 1900 - 2000 rpm. Thanks. Link to post Share on other sites
Island Time 1,235 Posted August 31, 2014 Share Posted August 31, 2014 Chrisc, what sort of speed are you looking for? The hull speed of an h28 is about 6.5 knots, you won't get it over that. I'll have a look and see if I can find some prop prediction data and formula I had around somewhere..... Link to post Share on other sites
wheels 543 Posted August 31, 2014 Share Posted August 31, 2014 You may be able to simply re pitch it. Don't look at going bigger in Diameter unless you have plenty of room. 10% of daimeter needs to be the gap between Blade tip and Bottom of Hull. No less. Easiest way to work out if you have enough pitch.....In Neutral, open throttle all the way and note RPM. Then engage Fwd and open throttle all the way and note RPM. If RPM is the same or under 10%, you can increase pitch. If the engine slows 10%, you are bang on. If the engine slows more than 10%, you have too much pitch. If you have the same Prop from the days of the original motor, chances are it is well under pitched. The engines used to be around 10 to 12Hp and some of the newer boats or repowerd had up to 20Hp engines. 27hp is getting up there for an H28. Link to post Share on other sites
Guest 000 Posted August 31, 2014 Share Posted August 31, 2014 Thanks guys, great info. I know that I have at least 10% of diameter prop clearance, probably more. I can get 5 knots in flat calm weather, most of the time. What I would like is to maintain 5 knots with a couple of knots of breeze on the nose which I cannot do with my present prop. I'm going to do the engine run-up test under load and out of gear and see what I get. I guess I can do this on my pile mooring - the boat doesn't have to be under way? Link to post Share on other sites
Island Time 1,235 Posted August 31, 2014 Share Posted August 31, 2014 Chris, when the prop and boat are completely clean, does it reach max revs easily when underway - like at 3/4 throttle? If so then it's under propped. If the gearbox is 2:1, then engine revs are very low at 1900-2000. Most props turn at about 1400rpm flat out. That would mean 2800 engine rpm, and between 70 - 90% of that for cruising revs. Do you know the max rev spec for your engine? Link to post Share on other sites
idlerboat 116 Posted August 31, 2014 Share Posted August 31, 2014 Do a search for propcalc.ods. This is a broad based spread sheet. I have found it very accurate. (In the real world).... It will give you a couple of prop size / pitch solutions. You can then compare what you have. 27 Hp is reasonable for a H28. (Me invited member of H28 society). Dont forget that there is a lot of bullshit with SOG. HUll speed is fine, as long as you take into account all other factors like tide and current. Even a small amount of chop (head sea) can seriously reduce boat speed. I wont go into it, but prop theory, efficiency, and sizing is well explained in a couple of articles on the web. I do apologise for not being able to direct you straight to it, but there is a commercial based educational site from (Alaska?) that is brilliant in the no bullshit, easy to understand explanation of "best prop practice". If I find it I will pm it to you. An important point to remember though is that on smaller engines, the head room is also smaller. What I mean by that is you can size and pitch your prop to what may seem perfection, but then find that when pushing the boat into a sea, and running electronics which loads up the alternator, that you suddenly end up with the dreaded "black smoke".... That is an overload of the engine. Black "stuff" coming from the exhaust and settling on the water, and the engine throttling back. Better to have power at all times (like when you really need it) and sacrifice some top speed. cheers Link to post Share on other sites
idlerboat 116 Posted August 31, 2014 Share Posted August 31, 2014 Chris, when the prop and boat are completely clean, does it reach max revs easily when underway - like at 3/4 throttle? If so then it's under propped. What ? Link to post Share on other sites
Island Time 1,235 Posted August 31, 2014 Share Posted August 31, 2014 Totally agree IB, but 1st let's establish that there is an issue with the existing one first. The engine revs must be accurate, and for that gear ratio, they sound low! If Chris is cruising at 2000 rpm, and the engine cruise is actually meant to be 2700 rpm, that's likely the problem right there.... Start with the engine specs, check the actuals against theoretical, go from there. In my own boat, the (factory) tacho was almost 30% fast..... Link to post Share on other sites
idlerboat 116 Posted August 31, 2014 Share Posted August 31, 2014 OK, almost no chance of governor full stop being changed... so ...check that throttle stop hard up at full lever position. In neautral check engine spec against tacho. As I said with prop calc, you obviously need all the "Bits specs" to be able to input the data...I just took that as a given. Second go for a swim...hey its a bit above 10c here and I just went in. Take a plastic ruler and go check. All of the above gives every piece of data except pitch. As you well know, on a small engine prop system even a few small barnys can change things dramatically. I constantly live with people who say that their SOG is about 0.5 above hull speed when launched and five months later come back and say they are close to a knot under...... I say "so ?" and they are surprised. The bench mark for setup should be conservative. As in a half dirty bum, a third discharge of batteries, and full lights and instruments running. ...and thats in smooth seas with no current !! Add a fridge compressor at that HP (and yes there are some) and well...... (for me I would rather lose a little top end speed and go with lower shaft output rpm, more torque and greater pitch) Dont forget that an 8 HP single with a folding two blade prop will bring an H28 up to hull speed in an ideal situation : ) Link to post Share on other sites
Guest 000 Posted August 31, 2014 Share Posted August 31, 2014 Thanks again for call the good info. To clarify a few points: I am calling the 3 cyl Perkins a 27hp engine. For some reason an internet search of my engine model number will produce stated variances from 24 to 27 HP with the balance of opinion being toward the 27hp end. I don't know if the rev counter is accurate but I suspect that it is OK, given that I have run the machinery up to 3300rpm on the gauge - and with a little to spare. Am I right in thinking that modern high speed diesels should max out at about 3500 to 3600rpm? The engine does not produce any smoke at high rpms, doesn't produce much extra knots, either. The main difficulty I have is that any wind or sea just stops her dead. I think that I should have sufficient HP on board to largely overcome this if my HP was fed into the right prop. And finally, I spent much of my life at sea as the principal driver of large multipurpose cargo vessels where our normal cruise was 90rpm. Occasionally owners or charterers would give us the hurry-up so we would run flat out at 110rpm - wohoo, hold onto her hats, fellers...... Still trying to come to terms with running my boat engine at 2400rpm.. Link to post Share on other sites
John B 106 Posted August 31, 2014 Share Posted August 31, 2014 Sounds like a perama? 24 or 27 hp is tons for an H28, sounds under propped to me. The H28's have a club and all the prop people will have a knowledge base on em. Nearly every yacht I've been on has had a sweet spot around that 24 or 2500 mark. Maybe human tolerance comes into it too, I know I'm cruising in a fairly relaxed way at those revs and 27 to 28 is fanging it, crap starts to rattle and the noise is getting up. Link to post Share on other sites
Island Time 1,235 Posted August 31, 2014 Share Posted August 31, 2014 OK Chris, if the last say 1/4 throttle produces no real extra performance gain, then the thrust of the prop cannot overcome the additional loading that going faster makes. For whatever reason. From a quick check of likely perkins models, without an exact model number, it's likely your cruise RPM is about 2700. BUT you really need to find out the exact model specs to be sure, as either too high or to low can cause engine damage long term, and makes it impossible to fit the right prop. Causes of the lack of additional speed can be, obviously, the wrong prop. But also can be a fouled prop or hull. Is the boat clean? Especially the prop - even a few barnacles on the prop makes a surprising difference. IB is right - the governor will limit max revs, and this is as it should be. On most of the older Perkins, there is a mechanical governor. There is provision for adjustment, but it should not be adjusted, and the manuals state "factory set, do not adjust". It can be moved, but only if you are absolutely certain that everything else is working perfectly (cooling, fuel system, engine condition), and then only to put it within factory spec. All other things being in good order, often, lack of additional speed in the top end 1/4 throttle is either engine overloading (max HP cannot turn the prop faster - over propped ), lack of blade area, or an under size or pitched prop. Alternately the engine is not producing the HP it should... The Kiwi Prop is a great idea. I use one myself. The guy at Kiwiprop is very knowledgeable, and will be able to specify the correct prop for you, as well as be pretty accurate as to pitch settings required in your circumstances. Slight pitch adjustments may be required to get perfection, but these are easy enough to do in the water, by the owner. This prop will increase both your motoring and sailing performance. Prop selection/setup Again, IB is right. Your prop should be selected with the boat fully loaded, and with "average" cleanliness in mind, unless of course we were talking of a race boat First, find out what you have! Just a note for John B's post above, which is fine, but dependent on the engine and particular install. I know this motor is not turbo'd but for those of you who do have turbo'd motors, you need to run the engine fast enough to make the turbo work and heat up, or the turbo blades will carbon up, which can lead to turbo failure. Working the engine at the manufactures recommended cruising rev range is what you should do to all engines to get the best out of them. The manufactures know more about them than the guy down the dock, the local mechanic, the local dealer, or anyone else. If you are unsure, you can always contact the manufacturer, and ask. Often you even get an answer! Link to post Share on other sites
Chewing Gum 17 Posted September 1, 2014 Share Posted September 1, 2014 Variable pitch props like the kiwi prop is the best way to go. Then you can get both best motoring speed and sailing speed when feathered. Feathering props are not cheap but worth every cent. Ours is a stainless Australian built 3 blade Autostream prop. Our Yanmar maxs at 3400rpm. We adjusted the pitch so it just reaches those revs or slightly less say 3350rpm at max throttle. We cruise at 2400 to 2500rpm. Out of 3000 odd miles around NZ around a third of the distance was under motor at those sort of revs. Needed an oil change half round ! In my opinion the bigger diameter prop you can fit the better the push into big seas but as wheels said you do need a bit of tip clearance. Also 3 blade is ten times better than 2 blade, fixed or feathering. Link to post Share on other sites
Guest 000 Posted September 1, 2014 Share Posted September 1, 2014 I realize that I am really being unfair in asking help but not providing all the details. If the weather warms a little over the next day or so then I will commit my ancient hide to the deep and take some propeller measurements. That, together with an engine run-up to maximum rpm both under load and out of gear should provide some decent info to work with. Link to post Share on other sites
Island Time 1,235 Posted September 1, 2014 Share Posted September 1, 2014 Chris, the prop should have the size and pitch stamped on it somewhere.... probably.... Link to post Share on other sites
wheels 543 Posted September 1, 2014 Share Posted September 1, 2014 I have a mate with an H28 with the old Buhk motor still running sweet. But one day he made a comment that he may have to replace the engine, because he had little power, heaps of black smoke and no speed. So I told him to check the Air Filter. Several months had past and once again he mumbled about engine needing replacing. So I went through a wee check list with him. That was when I found he was slightly mechanically challenged. I volunteered to come out to his boat on it's mooring and take a look. So first step was, OK start her up. It started instantly from cold and idled away really sweetly and no smoke. OK, open the throttle all the way up with it in Neutral please. It rev'd full RPM, no smoke. OK take her back to idle, now engage fwd. I instantly heard the engine load right down working really hard. Hmmm. I then asked for full throttle and as he opened up the throttle I took a look over the stern at the water flow and was viewing bits of Mussel and Weed and rubbish going out the back, along with no engine RPM. I then got in the Inflatable and looked under the Boat. The water was really clear and all I could see was this huge bundle of Weed and debris underneath. I had no show of a visual on the Prop. Hmmm, when last did you have the Hull cleaned. Oh yeah she's been needing a clean for a few years now. Ohhhkayyyy. In the end, he had to get the Boat towed to Westhaven for a clean and Antifoul, because he had no drive whatsoever due to the Hull and Prop being completely covered. Link to post Share on other sites
erice 732 Posted September 1, 2014 Share Posted September 1, 2014 kind of linked this table seems to show ideal prop sizes when a line is drawn from engine power to max prop rpm http://file:///C:/Users/leno/Downloads/Propellers%20Nomogram.pdf and http://www.vicprop.com/displacement_size.php Link to post Share on other sites
waikiore 399 Posted September 2, 2014 Share Posted September 2, 2014 Assuming that we are talking about a standard NZ Compass yachts H28? If you give us the exact engine model and gearbox reduction ratio then and only then I can give you a proper prop recommendation. Without these facts to start with it is all feelings, and quite frankly waffle, as evidenced by a number of posts above. PS have done 7.7 knots in an H28 fitted with the standard Yanmar YSE12 and a fixed two blade prop many years ago, and yes we were digging a hole. Link to post Share on other sites
Guest 000 Posted September 2, 2014 Share Posted September 2, 2014 The vessel is a standard CompasYachts H28. The engine is a Perkins Perama M-30, 27hp. The gearbox is 2:1 reduction. Link to post Share on other sites
Island Time 1,235 Posted September 2, 2014 Share Posted September 2, 2014 OK, spec says WOT 3600 rpm for this engine. Your cruise revs should be 2700 - 2800 rpm. If you have not been running the engine at these revs, it may object! When was the heat exchanger last cleaned, air filter cleaned/replaced etc. Normal maintenance. Check the accuracy of the rev counter as said above. Will it achieve these revs unloaded ( out of gear) ? In gear? With clean hull and prop, how's your speed now? Link to post Share on other sites
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