prop pitch

[Benny01]

can someone please explain to me what changing prop pitch. im about to get a new prop and was wondering what affect changin it will be. it is a 4.2 blusfin with a 40hp suzuki

[saurian]

Benny , pitch affects the distance the prop will push the boat per engine revolution.
It gets complicated as each boat is different.
But basically wide open throttle (wot), max rev's recommended for your motor by the maker can be acheived by using a larger pitch to use up available revs or it could be a case of not getting enough revs and a smaller pitch could help.
You can also change the diameter of the prop and the choices go on and on.
Hope I havn't confused the crap out of you.
Ta

[aussiebasser]

Prop "Pitch" is the theoretical distance the prop will travel through the water in one revolution. ie. 13" pitch will travel 13 inches through water per revolution if there is no slippage. A smaller pitch prop will give you better hole-shot, however your engine may over rev. A larger pitch prop may give you more top speed, but may restrict you motor from getting maximum rpm. With older outboards, full throttle opening, can give you full mechanical spark advance. If the engine is not at maximum revs the ignition could be too far advanced and cause a hole in a piston. The best bet is to go to Solas at Maroochydore and they normally let you try different pitch props. Ideally, you should have a tacho to ensure the correct revs are being reached. A Stainless prop will give better performance because of thinner blades, and a larger exhaust opening through the prop. (A bit like extractors on a car.) Some props have a different "rake" which is the angle the blade is welded to the hub. This can change the amount of bow lift you get, however on a 4.2 tinnie with a 40 it's probably not an issue.
Working out prop requirements is quite difficult, look here: http://www.boatpropellers.com./propcalc.html
It's much easier to try a few different pitches, with the boat loaded as it would be when fishing.

[Stroadman]

Good explanation on prop pitch, was wondering about that same issue myself, & thanks for prop calculator site Cheers

[Jono_SS]

nice one aussiebasser. i'm going to talk to Solas about props for sure.

[BM]

Ummmm.....

Mercury Marine have for many, many, many, many, and one more time MANY years been regarded as having invested the most money on prop design and hence havng the very best props on the market.

Some I speak with have suggested that the Solas props are antiquated designs which are based in part on old, old Merc designs.

Mercury now manufacture their props to suit ANY engine and sell hubs to suit the different manufacturers requirements.

I would buy a Merc designed prop 100 times over before I bought ANY OTHER COMPANIES PROP.........

Now I don't posess a Merc (or connected) Franchise at all and I repair all brands of engines so there is no bias whatsoever, but when the leader in an area produces props to suit all brands, I find it an easy decision for me...

Cheers

[aussiebasser]

The benefit of Solas over Merc props is that you can go to Solas, and they will allow you try different props to get the correct one. If a Merc dealer has the stocks and is willing to allow that that'd be great. We are talking about a 4.2 tinnie with a 40 on it. Not a race boat, or a ski boat. I don't think the latest prop shapes or rakes and cups are going to make a huge amount of difference, it will just be a choice between a couple of pitches.

[Wahoo]

The benefit of Solas over Merc props is that you can go to Solas, and they will allow you try different props to get the correct one. .

Hi Aussie
merc has been doing this for quite a while, i am on my 3rd prop now and ordered the 4th on monday, the prop im useing step down in 1-inch pitch increments, next prop will be the one we think
Ken Reeves
KR@Propgods.com




Daz

[FNQCairns]

I am with BM above if a person is going to shell out the big dollars needed for a new prop in Australia they should consider mercury as their first choice if at all possible.

40hp is borderline but good prop design makes a big difference, a bigger difference that changing from alum to SS etc.

cheers fnq

[Noelm]

hhmm sometimes it must be remembered when dealing with the smaller HP motors that a very small increase can mean a hell of a lot, lets look at it in a simple ,but not entirely true explanation just a way to point out something, if you have (say) a 200HP motor and find an increase of equal to a few HP you will notice next to nothing, but get the same increase in 40 HP and the performace boost could be as much as 10% total power output, remember a small outboard is pushed to the max almost every time it is used, but a big motor can sort of "coast" when full power is not needed.

[Coontakinta]

You can also change the diameter of the prop and the choices go on and on.


What effect does changing the diameter have on rpm and WOT speed? Also would a drop of 1" pitch change the WOT rpm by as much 900 rpm?

[Wahoo]

Also would a drop of 1" pitch change the WOT rpm by as much 900 rpm?

no it wont, more like 200rpm

Daz

[FNQCairns]

What effect does changing the diameter have on rpm and WOT speed? Also would a drop of 1" pitch change the WOT rpm by as much 900 rpm?

No probably not, but it can change it by 400-600rpm, it depends on what speed the hull is doing with the old prop, the faster a hull travels the more lift it creats, if that 1 inch drop in pitch gets it traveleing fast enough to find the hulls sweet spot then it off like a robbers dog as slip and drag (hull) both decrease together.
The effect is most noticable on light craft, craft in fishing trim (or already lugging) may never see that spot.

My pressed tinnys spot is 58 ish km/h in fairly light trim if I can reach it, it's off to 67km/h like a rocket with trim.

Expect less than 200rpm more from your 1 inch on average, diameter needs to be looked at for the type of hull - very important in achieving RPM goals with the ability to still achieve a good economical cruise speed.

cheers fnq

[fairpractice]

hi benny01
you have received some good advice
2 things to consider
1. how do you know the original prop needed a pitch change without knowing your motors rpm at wot?
2. Think of the pitch of a prop like your car gears. If you stayed in 2nd gear and floored the pedal you would over-rev the motor (and go slow)so we change gears.this is why you need to relate rpm of motor 5500-6000 to the pitch of your prop. to get top speed within the operating rpm of your motor.
that's my 2c. worth.

[cormorant]

Don't know how worn your existing prop is but it is often more ecconomical to have your existing prop repitched and re-cupped. by a prop shop. Either way you must make sure your motor is set up right and get it serviced so you know where your starting point is.

These days with props that have different cupping and varying pitch along the blades it is very hard to compare one prop with another by using the numbers on them unless they are all of the same era, condition and design.

If you can find a mate who has a similar motor on a similar boat try his prop as a test.

Solas will only lend you props if you commit to buying one so not exactly a free trial either.

Some wreckers with a large range will allow you to buy one and then exchange it a couple of times as you try and find one that is suitable.

Last thing about new props- put a lock on them as they are a high theft item especially when shiny and new!!!! and don't forget to always use new split pins ar tab washers as it isn't worth losing a prop for a 20 cent item.

We also keep our old prop on the boat with a fresh nut , pin and spanner as a back up if we hit something and need to replace it while on the water rather than get towed.

c

[Coontakinta]

FNQCairns & Wahoo, have a question for u then, When I took delivery of my 5m Razerline and 90TLDI, the 15" alloy blade on the motor wouldnt spin much past 5000rpm. This with three aboard a full fuel load of 135 litres, a full kill & live bait tank (full of water). Spoke with the tohatsu dealer who said here try this 14" yamaha S/S blade and see what u get. Well I managed to get around 5800-5900 rpm. Max rating is 5150 - 5850.

With the 15" I would get 56 -58Km/h switching to the 14" I now get 54 - 56km/h @ WOT & 5850-5900rpm. I didnt think that dropping in pitch would make that much difference to the rpm either but the dealer seemed to think it was about right. The cav plate sits just under the waters surface @WOT so I'm going to try lifting the motor one hole and see what effect this has. When I rang Solas about the problem they recommended lifting also which should give an additinal 250rpm approx. for each hole raised and fitting a 15" S/S cupped blade.

So if this is right, why the large variation between the 15" and the 14"?

Before lifting the motor, i need to find a reliable source for blades to try. Sending blades back and forth to Qld really isnt an option.

[FNQCairns]

Hi Mate, the brands are all over the place, if brand no 1 say's it is a 16p and brand no2 says 16p and a person gets them scanned no1 could actually be 15.5p and no2 - 16.5p.

Only props within a brand lineup will allign as apples to apples and sometimes they can be borderline also.

The 15p was a Tohatsu? Yamaha in my experience is a little under it's P, possibly the tohatsu was a little over??

There is also the wildcard of diameter also, don't ignore the difference that makes to rpm.

When I get a chance I will run your good numbers to try and estimate whats up with the yam prop (cannot help it I find this stuff interesting, bit like a riddle:)

cheers fnq

[Coontakinta]

FNQCairns that would be Gr8. Since the Razerline is rated to take a max of 90hp I was really expecting alot better in the way of top end from her.

What i didnt mention earlier, that will have some effect to your calcs is the fact that the dealer took the original 15" alloy blade and had it modified to 14". So perhaps my 14 isnt a 14 @ all. I cant recall now whether the yam blade was a smaller diameter or not now.

[FNQCairns]

Ok that makes it a little harder, how well do you trust your yamaha 14p SS numbers?

I am at a little bit of an advantage here as my Yam90 uses exactly the same leg ratio, close CC and is 3cyl so they are almost identical, I have been all over mine like a rash.
We are both at a disadvantage though as our engines are both low torque engines so proping becomes a tighter issue to get just right, if you or I chose a 4cyl 90hp 2st for our rigs we would traveling an entire pitch faster as a trade for the extra fuel.

cheers fnq

[Wahoo]

hi mate, sounds good, never heard of picking up 900rpm by changeing i"pitch, when you swaped the props over did you have the same weight in the boat? was the motor trimmed out the same?
thats good figures,
Scott thats a bit of info i did not look at, as you know i just swapped from a 19p down to a 18p, i noticed an improvement in holeshot, but only picked up 150rpm WOT, 17p is on its way to try out

Daz

[Kleyny]

i'm a little confused on merc prop. Is that a genuine mercury prop as in mercury the motor.

Were can i find a merc prop dealer that will lend me props until i find the one to suit my needs.
I have a 15ft open quinny with a 60hp mecury on the back. at the moment it has a 13p prop (genuine) and it revs to the limiter of 5500 far to easy and comes out of the hole like a rocket. It doesnt matter if i have one person or a whole heap of fishing gear. looking to get a better cruise speed more than a faster overall speed.

neil

[Wahoo]

i'm a little confused on merc prop. Is that a genuine mercury prop as in mercury the motor.

Were can i find a merc prop dealer that will lend me props until i find the one to suit my needs.
I have a 15ft open quinny with a 60hp mecury on the back. at the moment it has a 13p prop (genuine) and it revs to the limiter of 5500 far to easy and comes out of the hole like a rocket. It doesnt matter if i have one person or a whole heap of fishing gear. looking to get a better cruise speed more than a faster overall speed.

neil



Hi Neil
yes they are props made by mercury, they wont lend you one, you more or less have to buy the first one, if its not right then you can swap it as long its not marked or scratched, shame your not any closer as i have spare 15p S/S prop to suit your motor sitting in the shed that you could of tried
go in to any merc dealer and have a chat to one of the guys

Daz

[FNQCairns]

coona,

Had a look, with your 14p at 5875 you should be traveling at 63km/h in a perfect world. So either the Yam prop is less than 14p (which they are known for) and/or your hull is traveling too slow to lift and become efficient ie engine HP a little small.

I would raise the engine put a 15p on and just suck it to see, already it seems that at this speed and slower you need no increase in prop slip, the 15 may increase top end speed but I would be very mindfull of dropping max WOT in your instance as even with the 14, your on the outside of slip at that wot speed.

The faster your prop spins the less efficient it becomes, the faster your hull travels the more efficient it becomes, your prop is spinning fast and your hull is traveleing relativly slow for the prop speed.
The wildcard is how much you gain from a leg lift and is it enough to counter the extra pitch at ALL speeds, don't forget if it is not enough there are brands of 15p that have less diameter, you could consider these right now ithout the lift or if the lift doesn't work as planned, consider Mercury props (vengeance 15p?? or even the 14p-less diameter than what you have and possibly over pitch)with flow torque hub to suit your engine, soon there will be lot's of aftermarket propeller choices for the flowtorque hub.

Good luck, not much help i know you are a hard case just like mine.

cheers fnq

[Coontakinta]

coona,



not much help i know you are a hard case just like mine.

cheers fnq

No, youve been very helpful particularly with this last post. helps to explain a few things to me. I have sometimes felt that the hull is underpowered, even though the 90 is max hp for the hull. Heading into a sea, the boat sometimes doesnt feel as though its got enough oomph to get over a wave, feels like it is just riding it. Does this make sence? Perhaps its just me.......limited knowledge of these things! Gone from a 3.8m tinnie to the Razerline. Different characteristics etc.

Obviously in this conditions, trim is everything so am I doing it right? nose down (trim in into the sea) nose up (trim out in a following see).

Thanks all for your continued support

[FNQCairns]

Coona I would bet you are doing the right thing on the water, your engine is doing it easy as your wot rpm shows.

We own fishing rigs they are heavy for planing length, turn the igniton off at full speed and a person had better hang on or get propelled forwards.

Because our engines are small displacement 3cyl 90hp a little extra weight costs us a lot, it's a trade.

If you are ever going to add more weight to your boat stay with the prop you have, if the solas prop is their equivalent to the 15p yam black SS then it will be tight because of no prop diameter change, again relies on how much you get from the lift.

The hard part is not wasting money on useless props, with easy access like a dealer has it's all very simple but with us a mistake is costly and time consuming, I suspect the vengeance 15p Vengeance is the go if the leg lift works.


13 1/4
15
SS
3 - NEW -
N/A
854354A46 (http://www.mercurypartsexpress.com/PartsExpress/ProductSelection/Accessories/getProduct.asp?CID=174163189068228062271&sku=854354A46&store=Accessories)
Vengeance


The diameter above is 1/4 inch less than your current prop I think, you can afford a little less RPM wot, and your leg lift may see an extra couple of hundred rpm or bugger all depending on where it actualy is now - here is the kicker if your current yam prop is behaving like less than a 14p and the vengeance behaves a bit bigger than 15p then it might all turn pear shaped a little. There is no good way to tell what will happen till you test, personally I would use a merc prop before a solas but that's me, the prices in the US are reasonable if you can find somewhere that ships to you.

I just last night bought a brand spanking new Vengeance 14p for $180.00 AUD on ebay.

One last option is buy the 15p yam prop (because it's an apple to apple, do they make a 15p blackSS) and if it doesn't all work when fitted and after the lift, give it to a prop doc to tip it or take some cup out, cup emulates rake, rake benefits boats that go fast enough to use a lot of trim out, this is not our boats. lastly on the yam 14 prop, get an extra 1 inch pitch pounded into it:), not a whole lot to loose than, can give you the number of a guy that does mine, he is good, has a prop scan and doesn't over charge which is unusual:)

cheers fnq

[Coontakinta]

FNQCairns. I can get access to a 16p (think its 16 might be 17?) vengence from the old mans 75 Optimax. If I was to lift motor and record the details obtained with this prop. Would I then have enough data to suggest whether a 15p vengence will be a worthwhile investment?

If i use a dealers prop, I'm committed to purchasing from them and @ $600+ thats an expensive trial if there is neglible gain.

Your comments on the merc props are appreciated and I have heard similar from others. This plus the performace from the old mans 75 opti suggests the same. His boat has a hull weight of 580kg, heavier than mine and the thing fly's.

[FNQCairns]

yes, try that and get the engine as high as possible (you will need the appropriate hub kit to suit your engine though:( approx $100 ), too high and it will be a pain in the lumpy stuff so don't go silly if you test on flat water, I would give it a miss if it is a 17p but you might pull it off with the 16p as it is a 13 1/8 diameter, at least enough to get some numbers, try and record speed & rpm at your cruise as well as WOT.

I have a 16p veng I have been running on my boat while it is still light, just dropped the engine 2 holes from way high (higher than standard holes) after finding a major fault with the pod that I rectified with the grinder and ran at it's heavyest so far, It impacted the wot GREATLY as expected. The more info you have before paying for a new prop the better.

Think about getting your 14 hammered, cost to me was $80 to get a SS yam K 17p down to 15.5p incedently this prop scanned at 16.5p originally and was in PC.

Was your alum prop a 14dia or 13.75 dia?, just to highlight how much diameter matters.

Those carby merc 3cy 75/90hp engines are beasts! not sure if the opti stayed the same CC - see if you can do swap:)

If you do purchase a torq flo hub kit you then can run any merc prop that suits your case size, they are better than yamaha and the S brand - we both need whatever help we can get with our engines IMO.

cheers fnq

[Coontakinta]

yes, try that and get the engine as high as possible (you will need the appropriate hub kit to suit your engine though:( approx $100 ),

I've been lead to believe that merc props will go straight on. Is this incorrect?




Was your alum prop a 14dia or 13.75 dia?, just to highlight how much diameter matters.


I've been told that mods made to the prop make it a 14p. Cant say I'm happy with the dealer I got the motor from and his lack of assistance. Was never really interested in setting the thing up properly>:(

[Kleyny]

Hi Neil
yes they are props made by mercury, they wont lend you one, you more or less have to buy the first one, if its not right then you can swap it as long its not marked or scratched, shame your not any closer as i have spare 15p S/S prop to suit your motor sitting in the shed that you could of tried
go in to any merc dealer and have a chat to one of the guys

Daz

Very disapointing as i have been told that a 15p would be a bout what i need.:(

[FNQCairns]

Coonta, what prop is on it now? Is it the SS 14p Yam? and the alum is a yam 15p banged to 14p (so you were told) These are the only props you have talked about and have any numbers for so far? I re-read your post and not sure now if I am reading it right. can you tell me what WOT speed and RPM the 14ss yam prop got?

re fitting: I don't think so but I am not totally sure, never played with a Tohatsu, the standard mercury torq-flo hub will not fit the Yamaha and needs a kit, if you say your prop is a yamaha and fits your tohatsu yet you are sourcing it from a merc, someone here who reads this might know for sure - my head is spinning:)

Fairly sure the merc and yam/tohatsu all have the same number of splines but the merc prop shaft diameter is a bit too large, will check later to see if Merc sells a kit.

Every Merc prop I have bought for my Yam secondhand and from a mercury engine, the torqu flo insert splines were the wrong dia also the forward thrust washer was wrong the yam one should be splined.

cheers fnq

[Coontakinta]

FNQCairns, The prop that is on there now is the rehashed alloy Tohatsu prop. The dealer offered me the yam prop but it was a bit too banged up for my liking, which is why he offered to have the standard prop modified.

As far as WOT speed / rpm from the yam blade, I’m drawing on memory now, but believe it to be around 54Km/h. Not certain but I don’t believe that blade was achieving as much rpm as the nodified alloy one. 5600+ perhaps? (this sounds odd huh? As I’d expect that it would wind out to the same??) Is it possible that the motor was still a little tight and that this had some effect on the actual rpms when tested?

Fitment……I don’t think there was any adapting the hub when swapping the yam and tohatsu blades. Think they just swapped over. Is this usually the case? The prop was not sourced from a merc dealer. The dealer is a Tohatsu / Suzuki dealer. My information on the ease of swaping from a tohie blade to a merc blade maybe misinformation or a misunderstanding. Not sure.

[FNQCairns]

Coontakinta, It might be best to wait till you have some test numbers on a known prop in good condition, just offhand I suspect you may end up somewhere like me, this week I am getting a Yam alum 13.5x15p reworked to a true 13x15p to test, less diameter than I would like but as a trade for a higher pitch.

You may in the end need to do the same thing but targeted toward your setup but you need some numbers you trust to gain a baseline first with the engine height set where it will stay. sometimes the standard range of props to select from leaves a lot to be desired.

cheers fnq