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Outboard Propeller Selection

In this video I go through some of the basics of boat propellers including diameter, pitch, cavitation and ventilation. Choosing the right prop for your outboard is key.

Outboard Propeller Selection – Video Transcript

Hey there, Dangar Stu here. Today's video is all about the humble outboard propeller and is proudly sponsored by MarineEngine.com.

Most people when they own a boat have a propeller that came with the outboard and it's kind of what they use, and it's been okay. If you're buying a new outboard for an existing boat quite often they'll actually take the boat for a bit of a run and doing what they call "propping it" which is finding the right propeller for the boat or hull. The real purpose of that process is trying to find a prop that has your outboard run in its ideal rev range when you're at full throttle. There's a lot of other stuff going on though so I thought I'd take this chance just to start going through some of that and give you at least some of the basics.

There are a few characteristics to a prop that I think are really important to understand. You'll often see two numbers on the side not always but most often. This prop says nine and a quarter by 11. The first number, the nine and a quarter is referring to the diameter of the prop. and the eleven is referring to the pitch of the prop. When you hold the prop this way. When they talk about the pitch, you need to imagine that the props sitting in a solid and that for the prop do one rotation, as it turned one rotation, it would have moved 11 inches along. In this case this has got a nine and a quarter inch diameter and 11 inch pitch when it does one rotation in the solid it moves 11 inches forward. when you have a prop with a smaller pitch you can see that as it rotates it cuts this sort of a sharper arc. What that means is that by the time it's done this one rotation it actually hasn't gone very far when you change to a higher pitch. ie one that's more sort of forward slanting for each rotation.

You can see that shallow or actually scribes a longer line through the water and that's why this measurement between one rotation is the way they describe pitch that movement of eleven inches for it doesn't really happen in the real world because you have slippage it's a bit like driving a car in a wet day where the wheels are spinning in the water. So when a boat moves through water you have about a 10 or 30 percent slippage of the prop in the water. What that means is as your prop spins in the water for every complete rotation it's actually only going to move about eight to say 10 inches forward if you're really into your math you can actually figure out what slippage you're getting on your boat by figuring out what RPM your motor is doing, what gear ratio you've got, how many rotations you props actually doing per minute. With this rpm figure out how many knots you should get, figure out what you're actually getting and that'll tell you the slippage you're having. So there's a bit of a fun thing if you can't sleep one night.

Probably now what I consider the most important characteristic of a propeller that allows you to use it at all or not is the number of splines it has. If you look in here, you can see the number of splines and the diameter of the prop shaft if that's not right the props not going to go on the boat at all if a propeller goes on a boat chances are you can start to move forward if it doesn't go on you're going nowhere but just because it fits, on doesn't mean it's going to give you the performance you need to actually use the boat in the sort of practical way. Another characteristic of each prop is what it's made out of. You get some plastic sort of composite props, and you get aluminum props and you get stainless steel props. they're the most common ones in outboards. On large boats you get sort of brass prop to this kind of thing with older boats. but generally when outboard it's plastic aluminum or stainless each of these have advantages and disadvantages.

Plastics very cheap, aluminum is in the middle, stainless is the most expensive a lot of very high performance props tend to come in stainless because stainless is quite strong and won't deform over time as much as aluminum will. but to be honest with you it's a little bit of diminishing returns if you ask me I think aluminum has the best bang for buck. they will get damaged a bit more if you hit something I don't think that's necessarily a bad thing. The bushing inside, that we'll talk about it later on protects the gear box a little bit but a dinged-up aluminum prop isn't going to be that expensive to replace down the track where is if you start hitting your nice stainless prop into some rocks it's probably gonna fare better but there's probably still gonna be tears when you look at the cost of replacing it. another characteristic that's really apparent is how many blades it's got. a lot of Honda outboards I think you'll find have four-blade propellers most of the others are three blade.

Three blades by far the most common and there's not a huge difference between them ultimately one of the things that's very important about a prop is the surface area of the blades so as long as that total surface .area is in this sort of required parameters then you're kind of good to go. Obviously, if you've got four blades that can be smaller, three blades they need to be larger. the diameter or a prop tends to increase with horsepower that's pretty intuitive you see a big outboard you see a big prop so here I've got two propellers. This is the original propeller I looked at which has an 11 inch pitch. And this one has a 9 inch pitch it's a slightly smaller diameter but not actually significantly smaller so what it means is this particular propeller is like having a lower gear ratio which means your acceleration is going to be faster. On the other hand this propeller has your higher pitch so it's gonna have a higher top speed and often it's a bit of a compromise depending on how you like to use your boat.

The best way to think of it is to imagine that a car that's only got one forward gear which is what our boards have. So if you jumped in the car and you said what gear what I like to have? What I like to only have first, only have second, only have third, etc. So if i jump in a car that only has first, I can take off pretty quickly from the lights but pretty quickly I'm going to hit the redline of the motor. This one I'm gonna be a bit more sluggish off the Lights, I'm taking off and say second or third but I'm not going to hit the redline of the motor until a much higher speed and that's pretty much what the decision of choosing your pitch is if, for example you're towing a water skier you might want a lower gear ratio or a lower pitch on the prop that way you can sort of get the boat out of the water onto the plane you can pull the skier out of the water but you don't necessarily need to go and a million miles an hour is your top speed. You might also consider a lower gear ratio if it's like a work barge or something you're gonna be carrying a lot of weight and you just need that low gear ratio to push it much as you would that sort of truck analogy again.

If however, you're not really pulling that weight and your boat is quite light or whatever, you might find that that gear ratio is so low that very very quickly you're hitting the redline of the motor. so say for example the redline of your motor is 5500 rpm but at full throttle it's wound up towards six. Maybe you're hitting a rev limiter, maybe you're not but you know you're well under geared. Generally you don't change the diameter of your prop, the manufacturer all sort of have a recommendation saying for this horsepower motor run with this diameter. what you normally do though is change to a prop that has a different pitch and the general rule of thumb there is that for every one inch of change in pitch of a propeller you'll either gain or lose 200 rpm at the top end. So in this case we might say we're doing 6,000 rpm way too high for the motor past the redline of the motor so we need to increase the pitch of the prop by two and a half inches by doing that we're giving ourselves those 500 rpm. You don't really know until you've got the boat in the water what the exact effect is but it's good to have a general rule of thumb to predict what's likely to happen.

If you've got access to loads of props even still it's great, gives you a good idea of picking the one you want and then you can tweak from there. You can get a little bit expensive if you're buying all these props you're buying by going actually it's not enough maybe I need to go a 3 inch greater pitch but I think as long as you use that formula you're gonna get somewhere pretty close. There's a huge amount that goes into designing propellers you know it's quite a complex science but hopefully this gives you an idea of what at least those two numbers mean the idea of the diameter at times pitch that you see written on a lot of props but also just some of the things to bear in mind the construction and more importantly the splines I have the past seen outboard manufacturers where you can say it's a Evinrude 9 horsepower and they actually come in 9 11 or 10 splines whatever they are so you really do need to have that spot on if the prop is going to go on at all. last thing I mentioned about props is the hubs on them.

Previously props used to have shear pins in them so it was a softer metal pin that went through if the prop hit something the shear pin would sheer, not surprisingly, and the prop would then spin on its hub and that was designed to avoid damage to the gearbox and the prop shaft. all that kind of stuff. These days though you have rubber bushings. This rubber bushing is this bit in here, so you've got this inner hub with the splines on it. the propeller itself and the rubber bushings in between the two. the advantage to that is it can take a few knocks, absorb that shock load and then keep working normally. But when they fail they're pretty much impossible to replace. I have heard people trying to press new ones in but it's easier said than done. I've got another video on the symptoms of that bushing failing and ways you can fix it temporarily but just something to really be aware of with modern props. The next thing I want to talk about with props is cavitation and ventilation. You hear these terms used a lot and they're not necessarily correctly understood a lot of time. Ventilation, I think is a reasonably easy concept to understand it's where air from an external source is being drawn into the propeller.

A little bit maybe like when you pull the plug out of a bath and you get that little sort of vortex sucking air down. that is either going to come from the surface of the water or potentially from the exhaust gases that come out through the center of the prop. Quite commonly. The exhaust gases coming through the prop aren't generally a big issue when they're going forward a little bit when you're going astern props by the way, before I forget to mention that. are very much tuned towards going forward. they favor that because that's the direction you go at speed and it's the direction you're going most of the time. so when props are designed they're not symmetrical they're designed predominantly to go forward to have sufficient effectiveness in Reverse but to compromise that to give better forward performance. Where you have issues with ventilation generally is if the prop is sucking air from above. This can be because the motor is mounted too high on the transom, or the motors trimmed up too high.

Another thing that happens when you turn a boat is if you have it trimmed, as you turn the boat the prop sits there and say this is the water level and as the boat turns the prop actually comes higher in the water so you might find your experience a bit of ventilation during a hard turn an obvious time your boat's suffering from ventilation is that you'll hear that air sucking sometimes but you'll also hear the revs increase. it's the sort of viscosity or the non-compressible nature of water that gives that motor something to work against so when that propellers got air around it there's not that resistance on the motor and so the revs will just sort of go through the roof. So if ever you hear your outboard suddenly rev, you can either suspect the bushing in the prop, or ventilation. To help with that you can simply trim the motor down before you do any heart turns. I also recommend that you trim the boat down when you come into a dock. you come off of plane, you're coming in to do some maneuvering in a marina or something.

If you trim the motor all the way down you'll get better bite in Reverse and you'll also get sort of stronger maneuvering. Ventilation can be helped a little bit by a hydrofoil which is something I've been putting on the green machine in a week or two, and I'll definitely do some before and after tests, how high can I trim it without the hydrofoil, how sharp can I turn, all that kind of stuff. but that'll be coming in a video soon. Now the other one is cavitation. Now cavitation is actually air or gas that's created under the water it's not air that sucked down. And the way it's created is by having a low pressure on the front of the propeller. and that low pressure actually causes the water to boil it's not boiling in the sense of getting to 100 degrees although propellers actually can get surprisingly hot underwater. I've had a propeller fracture a whole blade after running upriver for a bit of an emergency we had and eventually just the heat fracture went through it because the friction the water so it can happen but that's not what cavitation is.

Cavitation is the water boiling or becoming a gas because the pressures reduced. So the more you reduce the pressure the lower the boiling point is. But it's a little bit like when you open a bottle of champagne all of a sudden all these gases, carbon dioxide that was dissolved in the liquid suddenly becomes a gas again. So propellers work by having this low pressure on the front bit of a high pressure on the back and that sort of gives us thrust. Now when you have cavitation there's a few obvious signs of it. One is that you'll get a bit of a vibration the other is you might start to see a bit of pitting on the propeller. That cavitation actually acts like a bit of a water hammer bit on that knocking you kind of get in your plumbing system sometimes. That little microscopic hammering action will actually start to put some pitting on your propeller. so definitely if you see that you're feeling a vibration you've got to consider cavitation the cause. Now where our ventilation is pretty easy to fix you are that mount the motor lower, keep it trimmed down lower, avoid really hard turns whatever, install the hydrofoil.

Cavitation is a bit tricky. Cavitation can be caused by a bit of damage to the prop causing sort of unusual areas of low pressure around the prop. but it can also be caused by the surface area of the prop or the total surface area the blades and propeller being low relative to the total diameter. So by having large blades more of them, maybe going to a four blade prop instead of a three you can solve some of your cavitation problems but it tends to be a little bit trickier to sort out. Because of that you might want to get some advice from a propeller specialist. Speaking of propeller specialist, I once spoke to the guy who owned SOLAS which is a propeller manufacturer. some of these I think are SOLAS props, one of them is I think Yamaha. whatever and he said to me yes you know people often talking about choosing props and they said yeah after selling 50,000 props or whatever you know you start to know a thing or two and I could respect that you know I think experience is great. unfortunately never got around to organizing to sort of meet up with him and try and do this video in conjunction with him because I think you would add lots of great information share but one thing SOLAS do have is a prop selector.

So I'll show you that on a computer it's actually something you can get to from the MarineEngine.com website and it's really good to help you just sort of generally leave the prop that's most likely to be right for your motor. I don't think there's any substitute for trial and error really with props because hull shape all sorts of things come into it. The weight of the hull, but I'll show you this prop selector because I think it's actually a really useful tool to have. On the front page to marineengine.com you'll see in the middle down here at the bottom there's this SOLAS outboard propeller selector. and if you click on that you'll see down this side it's got a sort of four step prop finder. Here for example I can select my outboard which is a Honda it's a 40 horsepower, and then here you will see this is the thing I was saying about 13 spline being the critical thing. My outboard is about a 2010 so it's definitely 95 and newer. which means it's one of the 13 spline. and then here for the whole time this is kind of a critical thing that an outboard manufacturer can't take into account when they're selling you an outboard off the Shelf is what sort of bows going onto.

In this case I'm just going to call it an aluminum Bay boat I'll say I want an aluminum prop I want a standard right hand turning propeller and a three blade propeller. here now if I click search I'll get a set of results that show me all the propellers that I can use and ideally I guess I'm going to start somewhere around the middle range here go for about a eleven and a half inch diameter start with a twelve inch pitch and then see how I fare. Alright, well I probably bored you senseless with propellers now I think the next related video will definitely be doing that Hydrofoil I think it's definitely a part of the whole sort of interaction of a surface of the water and the propeller, but for now I think we'll call it a day

I've got apologize for this video being a bit late I actually filmed a video on the weekend spent all of Sunday doing a video on Danger Island, because I often get asked about the island so I thought I'd do a whole sort of lap around the island talk a bit about it show you the whole you know four sides whatever but unfortunately the audio was really bad on that videos by the time I sat down that night try to edit it I thought this is unusable you know I can't do this I decided not to give up on that folks I'd like to do a lot more filming on water over the summer so I've ordered a new mike that I'm hoping is going to solve that problem so I'll keep you posted and hopefully get that one out redone sometime soon as well. Alright, well take care and I'll catch you soon. Bye.