Newboy Posted April 14, 2004 Report Share Posted April 14, 2004 What is pitch? Isn't it the height of one complete turn?! More importantly, what does it do? What's the difference between big and small pitch? Quote Link to comment Share on other sites More sharing options...
duncan Posted April 14, 2004 Report Share Posted April 14, 2004 distance the boat would travel in one complete turn of the screw for normal OB gearing this equates to approx 100 engine rpm per inch of pitch and they come in 2" pitch sizes (normally) looked at as a gearing the lower the pitch the lower the gearing - and correct gearing will enable the engine to attain between it's max revs and 200rpm lower for a normally loaded boat. Quote Link to comment Share on other sites More sharing options...
Newboy Posted April 14, 2004 Author Report Share Posted April 14, 2004 Call me thicky, I'm still a bit muddled, For example, say an emgine uses 10" pitch, how would a 12" and 8" differ? Does it make it faster or more economical? Quote Link to comment Share on other sites More sharing options...
Afishionado Posted April 14, 2004 Report Share Posted April 14, 2004 Call me thicky, I'm still a bit muddled, For example, say an emgine uses 10" pitch, how would a 12" and 8" differ? Does it make it faster or more economical? The answer is simple Yes it makes it faster/No it doesn't ........ Same answerers for economy. Right now the silly stuff is over (but both answers totaly correct) I'll try and explain. First off an O/B does not have a 1/2/3/4 gear box it just has one fixed ratio. Second the efficient running (fuel economy/speed of PROPELLER in rev's) is in a very narrow rev band. Usually this is between 4800 and 5200 rpm on a two stroke. These are fixed perameters, in other words one can't change up or down a gear like a car. Now imagine a very heavy boat with a poor hull shape that only just planes and has to have a big powerful engine to shift it. A coarse pitch (big number of inches) tries to grab big chunks of water and push them back BUT it cant 'cos the boat wont move through the water fast enough to START with so the engine cant rev' high enough to develop it's full potential and is very uneconomical at say 2500rpm struggling like mad. OK says the owner what we need is a much finer pitch (small number in inches) and goes and puts on a super fine pitched propeller. The engine now rev's it's little heart out at 5800 rpm whilst the prop slices super fine shreds of water and sends them out in a shower behind the boat. Because there is so little water being moved in relation to the engine rev's the boat is constantly at full throttle to produce far less than it's best speed. In essence the two scenarios have the same result .....The boat does not go as fast as it should and is uneconomical. The answer is that the boat, the engine and the prop ALL have to match each other. The hull shape (displacement/shallow V, deep V, Cathederal etc) The engine (HP, 2 or 4 stroke, most efficient rev range) The use (Racing one up, skiing, three blokes and fishing tackle. long distance cruising etc) Will all mean different props are needed. AND we haven't even started to talk about fluid dynamics and cavitation (don't even go there ) yet My advice is to talk to a supplier of your sort of engine, tell him what sort of use you'll be putting the boat to and get his recommendation. There is NO 'ONE' ideal prop for you or anyone really, we all use the best compromise. Unless of course there's someone out there who is very rich and daft who has a selection of props and cahnges to suit the day Mike PS if you still want to know more just come back and I'll try my best to explain Quote Link to comment Share on other sites More sharing options...
Coddy Posted April 14, 2004 Report Share Posted April 14, 2004 Ok Why is a stainless prop more efficent than a aliminium prop? The cost difference is huge so is it worth it? Codfather Quote Link to comment Share on other sites More sharing options...
duncan Posted April 14, 2004 Report Share Posted April 14, 2004 Mike - come on lets have cavitation and resistance humps too.... more seriously whilst I can agree with almost everything I can't help feeling the statement regarding efficient rev range as given is somewhat misleading. maximum revs is going to be in the range 5000 - 5600 for most 2 stroke OBs and the last place you want to be running ins in the last 10% of these revs for your engine where the manufacturers will be using everytrick in the book to maximise power - normally through a second choke on the carbs. on a planning hull, with an appropriate engine, the most effiecient throttle setting will be the one that has the hull fully planning as designed - around 12-15knots for most. this should give you the most miles per gallon and have the engine running at a load it is happy with. a well balanced rig should be happy running between this and around 85% revs without too much change in hull trim or fuel efficiency but above that things tail off and hit the wallet! whilst it doesn't have gears the analogy with cars helps if you think of a car going up a hill in too higher gear (pitch) it is labouring and ultimately inefficient on fuel too whilst too lower gear (pitch) leaves the engine comfortable but using more fuel to cover a given distance than necessary. Quote Link to comment Share on other sites More sharing options...
Maverick Martin Posted April 15, 2004 Report Share Posted April 15, 2004 Re: Stainless versus ally props: Ally props are fine on lower horse power engines and will work ok on larger o/b's . The benefit of a stainless prop on larger motors is they dont flex or give so are more efficeint. Stainless is heavier than ally thats why on some engines when you select a fwd/rev you hear a clunk as the engine starts to throw the prop. Stainless is also harder then ally so does not damages as easily but is more difficult to repair. The downside is the cost, 2 or 3 times dearer than the equivilent ally. I have had stainless props for 5 years and havent had to have one repired yet but Im sure if they were ally they would have been in for repair 2 or 3 times. Martin Quote Link to comment Share on other sites More sharing options...
great white Posted April 15, 2004 Report Share Posted April 15, 2004 Hi all Some very in depth discussiions on this one. Let me add a little. Most outboards come with a range of about 11 props listed to them. But are supplied with a Standard Prop [ The most common size for that engine] The Hull size shape and total displacement weight then starts to take effect. and will determine wether the prop is correct. The worst part of running with the wrong size prop is that the engine is overloaded at all speeds if the pitch is to large. If the boat is standard with a common size /type of engine The boat builders would know from trial and error what size prop should be used. but they may recomend more than one size depending on the use of the boat. A ski boat may drop a size to allow for towing the skier and a smoother pick up. If it is older, non standard or you have changed the engine is can be checked. The engine needs to be running well and the hull clean. Fill the boat as normal with crew, equipment and liquids then give it a full speed run into and out of the tide flow noting the maximum engine revs, If you do not have a fitted RPM gauge a hand held one is required. Check the maximun engine speed that is listed for the engine and see if that is being reached. if the RPM is low the prop is to big on Pitch or Diameter [usually Pitch] But is only worth changing is the differance is over 300 RPM as new props tend to change pitch by 2" which is about 300-400RPM. Existing prop pitches can be adjusted by someone like Prop Revelutions at Cobbs Quay by about an inch +200RPM. Putting the correct prop on does reflect on effiecancy and running cost as well as stopping the engine being overloaded [ which can cause long term damage] The cost of the change [ally prop] would be recovered from fuel cost savings. If the boat has been oK with the prop before and the prop is not damaged the lack of revs may be caused by another fault that a new prop would not fix. I hope that helps Charlie Quote Link to comment Share on other sites More sharing options...
Afishionado Posted April 15, 2004 Report Share Posted April 15, 2004 What is pitch? Isn't it the height of one complete turn?! More importantly, what does it do? What's the difference between big and small pitch? SEE NOW LOOK AT WOT YOU DUN!!!!! Mike Quote Link to comment Share on other sites More sharing options...
Newboy Posted April 15, 2004 Author Report Share Posted April 15, 2004 What is pitch? Isn't it the height of one complete turn?! More importantly, what does it do? What's the difference between big and small pitch? SEE NOW LOOK AT WOT YOU DUN!!!!! Mike I've only just starting ......... Quote Link to comment Share on other sites More sharing options...
Afishionado Posted April 15, 2004 Report Share Posted April 15, 2004 Describe a spiral stair case whilst sitting on both hands....... That's pitch. The more turns round you go to get up, the finer (low number) is the pitch. But it's all academic really 'cos unless you want to go racing or skiing then the manufacturers recomended prop will serve. If after a few trips you feel your engine is reving a bit high (technical term Screaming it's guts out) you could get your prop re pitched to be a bit coarser (higher number) useing the scale 1" of pitch = 100 rpm. The opposite is also aplicable if you think your engine is plodding and not getting full revs then a reduction in pitch would help to raise the revs. However aly' props should only be re pitched 2" either side of what they were manufactured at. So if you take your prop in and say "Take this down in pitch 2" my good man" , but some begger has done that already before you bought it youm stuffed as they say here abouts Mike Quote Link to comment Share on other sites More sharing options...
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