props

[Shaun]

Hi to all on Ribnet

Firstly may I say what a great site this is. I was wondering if anyone could give me some advise as i've not been ribbing for that long. I have a Delta 6.5m high bow with twin suzuki 65s the props are standard and have a 15 degree pitch I want to go for stainless ones but keep reading about different pitch etc. Could someone please advise me on what to go for as I have no idea.
Thanks Shaun.

[Richard B]

Hi Shaun,

Welcome to RIB net!

You need to know what rpm the engines are achieving at WOT (wide open throttle) and what the manufacturer's recommended (and maximum) rpm ranges are for your engine(s). Then you'll know if your engines are currently operating efficiently.

Then you can start to work out if you need a different size prop.

I did this recently for mine and gained another 4kts.... until the new prop fell apart!

Are you using aluminium props at the moment?

I guess that most of the prop calculator programs don't cater for twin engine installations?

BTW did you buy your RIB from Nationwide Boat Sales?

Richard.

[Shaun]

Hi Richard
Thanks for replying, yes the props are alimimium,and yes I did buy it from Nationwide boat sales. Do you know anything about it?

[Richard B]

No - no probs... just that I nearly drove up the M1 to look at that RIB as it looked like an excellent bargain. But by the time I'd got my cash together, it was sold to someone!

[Mike C]

Originally posted by Shaun
Hi to all on Ribnet

.... the props are standard and have a 15 degree pitch I want to go for stainless ones but keep reading about different pitch etc.

I think you mean 15 INCH pitch.... That's the theoretical distance your prop would travel through a solid during one revolution.

I had 15" props on my Ribcraft 5.85 with twin 65hp mariners. They were fine and the engines got up to the manufacturers recomended WOT RPM..... just!

Since yours is a bigger and presumably heavier boat.... and since you're talking about s/s props which, I am told, grip the water better, you may find it worth trying out a pair of 13" props just to see how it goes...... but at about £250 each, I'd borrow them first. You should get aluminium ones for around £100 each!

As Richard B said, it really is suck it and see.... what your WOT brings! I certainly wouldn't think any more than 15" though!

Mike C

[simmons0]

Keep trying different props untill you get the right one for your boat. You could try 4 bladed props too...theoretically less efficent on a high reving engine but always worth a try.

Also dont chose your prop WOT on flat water trimmed right out because you will never reach that RPM out at sea. I had me boat on windermere last week and i have never ever seen the rev counter go that high in 3 years of ribing on the sea.

[Richard B]

Not quite the same in a deep-V larger RIB. 'specially in a Delta which should punch it's way thru' the chop very nicely indeed.

Be as scientific as possible - saves on purchasing lots of useless props. Let us know how you get on Shaun, if only to tell us what rpm and speed you're getting at WOT at the moment.

[Dirk Diggler]

Richard B,

I'm suprised to see you handing out advice on subject that you clearly have no knowledge of !!!

[Louise]

oops, wrong login!

[Richard B]

Be a little more introspective, Dirk. You're the one who advised his customer to buy a "get you home" prop which clearly won't do what it says on the tin. Have you recalled that faulty component from your customer? Or would that mean that you'd have to admit that you've got it wrong? Heaven forbid!

[Dirk Diggler]

See, your at it again, more advice on a subject that you know Jack Sh*t about. I didn't advise my customer, I merely did what he asked, and that was to supply a "budget" priced prop for his 115hp outboard as a get you home prop. Something that the Solas is ideal for. I don't remember it arriving in a tin, and it's certainly not a "faulty component". As for never getting it wrong, sorry I can't admit to that, had the odd FU on a few occasions, that's how I know wrong from right.

But anyway, thats not the point, I just find it hard to understand that you've only owned this rib for a few months, spent one of those trying to fix a leak in the hydraulic steering, haven't learnt to throttle in any sort of sea and now your handing out advice like an expert. Your starting to suffer from the " Manos syndrome".

All IMHO, and lighthearted of course!!!!

[simmons0]

Sounds a bit harsh Dirk!

Everything Richard said was perfectly right and was definatly not in any way misleading.

Whats your problem with that?

[Steel Dev]

Hi Shaun

I have seen your question here and I will try and help you. Next time you use your rib make a note of the maximum rpm you can get from your engines. You will also need to find out what the max rpm the engine manufacturer recommends. You need to decide what you want to use your rib for most of the time, ie. general purpose, water sking, transporting divers or heavy loads etc. You may think that the use of the boat will make no difference however to pick the best prop for your boat you need to think about use. For instance if you want accleration for sking you would use a low pitch propeller to give accleration but this also increases rpm so would not be a good choice if you intend going around flat out racing your buddies. A higher pitch will usually give better top end or you can go for a compromise in the middle. When you find out the details either post a reply here or email me whichever you prefer and we can take it from there.

Julie

[Aging Youth]

Well it seems from the advice from both Richard B & Julie at Steel Delv, come to the same conclusion.

I think that Mr Diggler again is suffering from the large mouthed over opinionated bull shit syndrome.

For christ sake shut the Fu&*% up will you. If you have nothing helpfull to say then say Nothing

[Mosser]

I was talking recently to the guy who sold me my boat ( a RIB dealer in Cork) about props. My own 90hp struggles under load and I asked him about props, prop sizes, alum vs steel and so on. He told me some stuff that I didn't know and might be worth repeating here (I'm sure many realise it already). He said that alum props flex under the huge pressures at high revs. This leads to the idea that alum props are not as efficient as steel props which flex a lot less. He also said that
when switching from alum to stell, you should drop pitch size (say a 19"alum might be replaced with a 17" steel). His final comment was that steel props make more sense for offshore boats where the likelihood of whacking them is less (obvious enough statement but worth repeating).

[Steel Dev]

Alum Vs Stainless Steel Props.

Aluminium props do not flex enough to have a marked effect on performance. The reason why stainless steel propellers give better performance is because the blade section is usually thinner. In the propeller world the thinner the blade section the faster the boat will go. As stainless steel is much stronger than aluminium the blade section can be made thinner this is the reason why you usually get better acceleration & top end with a s/s prop. Racing propellers are in fact so thin that you can laterally shave with them, but it is accepted by the racer that thin blades are more prone to damage. As to size selection for a s/s prop to aluminium, usually you keep the same size pitch if you are getting the correct rpm. If you change the pitch size you will change what revs you can achieve from the engine and thus performance. The only reason you may need to have a different size pitch for s/s vs. aluminium matching is if the s/s prop has more or less cupping or a different blade area than the original aluminium prop. With a stainless steel prop if you do touch the bottom or are prone to prop damage, it is an unavoidable problem in some areas with a lot of floating debris, you do stand a higher chance of damage to the lower unit if you hit an obstruction. However most outboard and sterndrive props are fitted with a rubber bush which should sacrifice itself rather than your engine in most cases it depends what you hit a large rock for instance it can be quite unforgiving. This is the draw back with s/s props and needs to be considered by the skipper if it is an acceptable risk.

Mosser,
You state that your boat struggles to get on the plane when loaded? Does the prop have any damage, if it does however minimal it may seem it will affect performance. If the prop is not damaged did you know that you can actually have your propeller altered to improve acceleration. It is possible to alter the pitch on your existing prop if its alum price £33.50+vat or s/s £55.50+vat this includes any repairs needed and alteration of pitch, maybe something to consider? For your range of engine props available would normally be in uneven size pitch's i.e. 15", 17" 19" etc but if your boat does not get the desired performance with a standard pitch and the next size is too high or low for your set up the best course of action is to have it modified to suit your boat exactly. Boating is supposed to be fun, well most of the time anyway and if you can get the best from the boat the fun is increased, well hopefully anyway!

Julie

[Dirk Diggler]

Originally posted by Aging Youth
Well it seems from the advice from both Richard B & Julie at Steel Delv, come to the same conclusion.

I think that Mr Diggler again is suffering from the large mouthed over opinionated bull shit syndrome.

For christ sake shut the Fu&*% up will you. If you have nothing helpfull to say then say Nothing

Thanks for that, but isn't it rather "do as I say, not as I do"!

Hope I can be of more help in the future

[Richard B]

Originally posted by Dirk Diggler
:Hope I can be of more help in the future

Hope you can

[Mosser]

Thanks Julie. It's clear that there's a lot to learn about props.

As for my own prop, it's not damaged but it's probably the wrong size. I've even gathered that from reading some of the comments from other forum users here. She has a 19" prop currently and with a light load (4 people) will hit 5500 rpm. This sounds ok to me as I believe the rev limiter kicks in soon after.

It doesn't take much extra load to prevent her going on the plane which probably isn't a great situation. The cheaper option is to look at the prop and see what can be done. The obvious step after that is engine size.

I didn't know that props could be altered in that way. Worth considering. A good winter project I think...trying to get the bucket setup properly!

[jwalker]

Originally posted by Steel Dev
Alum Vs Stainless Steel Props.

...... The reason why stainless steel propellers give better performance is because the blade section is usually thinner. In the propeller world the thinner the blade section the faster the boat will go. ......

Some may consider me rather arrogant to contradict you but I'm not sure you are correct about this. The thinner the aerofoil section, the less lift it will generate.

It is true that in high speed applications, where prop speed is also high and the blade loading is relatively light, a thick section would give too high a drag or even produce a partial stall, then a thinner section is desirable. However, at lower prop speeds and/or heavy loads, the thin section will generate insufficient lift and the section needs to be thicker.

As I hope you are aware, propellers do not, as is often thought, screw through the water, they fly in the water using their aerofoil section to generate lift. ** Because the prop is vertically mounted the lift is horizontal and, therefore, often called thrust.

** I accept that at very low speeds such as maneouvring, it is probable that the blade will be virtually stalled and initial movement is generated by the deflection of the water and the prop working as an Archimedes screw.

I'll be interested to read your justification for the comment.