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Discussion Starter · #1 ·
I've just got my rebuilt coastworker out on the water for the first time since buying the boat and it was a real pleasure to use, but I'm not getting the engine revving out.

This is the boat - 18' console - similar to whaler I think:


Engine is a 1996 BF75A Honda, and as i understand it the WOT revs should be able to reach 6000rpm.
At full speed on the water I was hitting 34mph on the gps, 2 up, and very light load, at 5200rpm. Couldn't get it to rev higher.
This is with a 19" pitch 3 blade alloy prop, which I thought might be a bit much but only one I have currently.

This was on pretty much flat sea, pretty much perfect conditions for a speed run.


Now of course the obvious thing to do is try a 17" prop if I want the engine to rev higher, but what I'm not sure is if there's really any point or if 5200 is an acceptable WOT rpm?
If I change to a 17" prop, then I would guess the outcome will be I'll get the extra revs and be able to hit 6000RPM, but would then need to use the extra revs to maintain the same speed as before, so maybe still get 34mph, but at WOT and using more fuel I guess as a result?

I understand that if the boat has more people/load then the 17" prop would enable it to get on plane easier and reach WOT at any point, but it's not often I'll have more than 2 up, and I can't help but think that keeping the 19" would give better economy.

Appreciate any input and opinions from prop boffins as I'm not sure what's the best course of action.

Thanks!

There is another factor actually - which is that I'm trying to upgrade the engine to 90 from 75. I think with that extra oomph the 19" prop would then be ok and the engine would be able to rev out, and hit perhaps 37/38mph at 6000rpm. I need to get a 90 carb though as I only have 3 of them currently on my bench.
 

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I've just got my rebuilt coastworker out on the water for the first time since buying the boat and it was a real pleasure to use, but I'm not getting the engine revving out.

This is the boat - 18' console - similar to whaler I think:


Engine is a 1996 BF75A Honda, and as i understand it the WOT revs should be able to reach 6000rpm.
At full speed on the water I was hitting 34mph on the gps, 2 up, and very light load, at 5200rpm. Couldn't get it to rev higher.
This is with a 19" pitch 3 blade alloy prop, which I thought might be a bit much but only one I have currently.

This was on pretty much flat sea, pretty much perfect conditions for a speed run.


Now of course the obvious thing to do is try a 17" prop if I want the engine to rev higher, but what I'm not sure is if there's really any point or if 5200 is an acceptable WOT rpm?
If I change to a 17" prop, then I would guess the outcome will be I'll get the extra revs and be able to hit 6000RPM, but would then need to use the extra revs to maintain the same speed as before, so maybe still get 34mph, but at WOT and using more fuel I guess as a result?

I understand that if the boat has more people/load then the 17" prop would enable it to get on plane easier and reach WOT at any point, but it's not often I'll have more than 2 up, and I can't help but think that keeping the 19" would give better economy.

Appreciate any input and opinions from prop boffins as I'm not sure what's the best course of action.

Thanks!

There is another factor actually - which is that I'm trying to upgrade the engine to 90 from 75. I think with that extra oomph the 19" prop would then be ok and the engine would be able to rev out, and hit perhaps 37/38mph at 6000rpm. I need to get a 90 carb though as I only have 3 of them currently on my bench.
You don"t have to be able to "reach" 6,000rpm with your motor.
The WOT band on the motor is 5,000-6,000 rpm.
The motor produces 75 hp in this band so at 5,200 rpm it is releasing it"s 75hp.
I personally would say that 5,200 is a little low especially if you were loaded lighter than you would be normally & I would prop to be around the middle of your WOT band with a normall load.
Dropping the prop pitch 1" should give another 200rpm so a drop to a 17" pitch should give about 5,600 rpm with the same load that produced 5,200 & maybe around 5,400 with a heavier load.
It is up to you really as you are in your WOT band already as to whether you re-prop.
The answer really would be to try the boat with what will be it"s normal load & see what rpm it pulls & take it from there.
For your information I have a Honda 115 which also has a WOT band of 5,000-6,000 rpm.
I have this propped so that with 2 up & all our gear the motor revs to about 5,600-5,700 rpm & with 3 up plus gear it revs to about 5,400 rpm.

headlight
 

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Discussion Starter · #3 ·
Thanks for the reply headlight.

That's interesting when you mention the 200rpm thing - it vaguely rings a bell that years and years ago that my engineer mentioned this 'rule of thumb' that 2" change in pitch would vary rpm by about that. Might not have been the case though.

If that's the case then a 17 might be better. In terms of light load i suppose that might not have been the best description since most planned fishing trips will be 2 people and what I had with me yesterday but with rods and tackle boxes, so not much extra weight at all.

I'm of the impression that Honda engines basically have Mercury gearboxes, so does this mean I can look out for either a honda or mercury prop? Or any others?

Be handy to have a compatibility chart or something about what brand props fit different engines or something.
 

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As well as previous advise if you can look at your engine when motoring and see if the cavitation plate is visible just at water level if not lift another hole reduce drag & give you couple hundred revs.
First thing the yanks do when setting an engine and prop up is lift it up as far as possible.
Your know if its to high it will cavitate in turns

Nice boat youv'e worked hard on her

now enjoy
 

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Discussion Starter · #5 ·
DOH!! Thanks for that, but you've just reminded me of one of the things I'd intended to check while out there yesterday and I completely forgot.

Will have to get back out as soon as I can then.... ;)

Cheers for the feedback too :)
 

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Forgot to mention that my figures were obviously for swapping like for like props ie 3 blade ally for another 3 blade of the same design/diameter just different pitch.
If fitting a stainless prop then in general a stainless 3 blade will pull 100rpm less than an identical ally prop, but give better performance.
With the figures you have posted I doubt that engine height is an issue although on my set up I had to lift the engine up 1.5" as the 115 Honda is a very heavy lump & dragged to much leg in the water when sat on the transom.
As to compatible props then Merc/Mariner/Honda & Yamaha are generally interchangeable (you might have to add a washer).
I don"t think that the Honda uses a Merc gearbox just it is the same spline count.

headlight
 

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Discussion Starter · #7 ·
Thanks for the reply again - hope you don't mind the questions - I want to understand properly (as far as you can without working with this stuff for a living) about stainless props, so if you get fewer revs, but better performance, where does that performance come from, and in what form? For instance, is that performance in terms of outright speed, or something else, and what is it that makes the stainless prop able to give this (less flex on the blades for instance?).

Will go and look for some articles to read about this to try and save so many questions.
I ask a lot because after getting the boat to this stage my budget is pretty much gone, so anything I buy has to be carefully researched to make sure (as far as possible) it's the right choice. I know props 'can' be a tricky one too.

Re' the mercury thing, it's something my engineer once explained to me. He'd said that when Honda began doing big outboards they just got gearboxes from Mercury and bolted them to the Honda powerhead. No idea if that's still the case, but I've yet to find any info from this guy that wasn't absolute gospel!
 

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Thanks for the reply again - hope you don't mind the questions - I want to understand properly (as far as you can without working with this stuff for a living) about stainless props, so if you get fewer revs, but better performance, where does that performance come from, and in what form? For instance, is that performance in terms of outright speed, or something else, and what is it that makes the stainless prop able to give this (less flex on the blades for instance?).

Will go and look for some articles to read about this to try and save so many questions.
I ask a lot because after getting the boat to this stage my budget is pretty much gone, so anything I buy has to be carefully researched to make sure (as far as possible) it's the right choice. I know props 'can' be a tricky one too.

Re' the mercury thing, it's something my engineer once explained to me. He'd said that when Honda began doing big outboards they just got gearboxes from Mercury and bolted them to the Honda powerhead. No idea if that's still the case, but I've yet to find any info from this guy that wasn't absolute gospel!
The reason for the performance increase of a stainless prop over an ally one is partly the fact that the stainless does not waste power through flexing & also the fact that the blades, especially the leading edge are thinner so therefore "cleave" through the water easier.
On small hp motors there is not a justification in the extra money a stainless prop costs but with bigger hp"s (75 hp is certainly big enough) there is IMO.
This increase in perormance will show itself in 3 ways, that is better holeshot, better max speed & the most important to us the ability to cruise at a given speed for less revs than with an ally prop so therefore using less fuel.
As to the Honda/Merc gearbox question then your engineer may well be correct & Honda may well originally have sourced gearboxes from Merc, the same as for years Merc sourced power heads from Yamaha.

headlight
 

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Discussion Starter · #9 ·
Brilliant thanks - that all makes perfect sense.

Well, my sister in law mentioned a stainless prop she's got for her Yam 90, so I'm still a bit doubtfull it'll fit the honda shaft but it'll be worth at the very least seeing what pitch it is, and if it's in the region of 17 it might be worth giving a try.

Is it right that stainless ones also come in odd sizes as well as the normal 2" variations, or have I made that up out of thin air?!
 

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Two threads have diverged into the same subject, so in an attempt to bring them both into this one, from the other thread:


What I was getting at is that 5-6000 rpm is quite a wide band for your WOT rpm - Headlight says that the max power is developed at 5200 rpm (might not be the same for the 90 once converted though), so that seems a sensible WOT rpm to aim for (or whatever the max power rpm for the 90 is if you go ahead with that conversion).

It's the stainless being stiffer and therefore thinner blades that gives the performance advantage over aluminium - then all sorts of options as to blade shape etc. When I changed mine, revs seemed to pick up quicker (giving better acceleration or 'hole shot') and as I said, added just over 3 knots to my top speed for the same rpm. The 2" increment thing maybe true for smaller engines (maybe even the case for manufacturers' OE props - I've been lucky in that dealers got it right first time with the two brand new engines I've bought so never had to investigate size options of OE props) but with performance props there are lots of sizes available. This is where mine came from along with what appeared to be top advice too: http://www.keypart.co.uk/. Props can be a complicated subject, so probably better to ask the experts.
 

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Brilliant thanks - that all makes perfect sense.

Well, my sister in law mentioned a stainless prop she's got for her Yam 90, so I'm still a bit doubtfull it'll fit the honda shaft but it'll be worth at the very least seeing what pitch it is, and if it's in the region of 17 it might be worth giving a try.

Is it right that stainless ones also come in odd sizes as well as the normal 2" variations, or have I made that up out of thin air?!
I"ve run Yam props on my Honda :thumbs:
A prop off a 90 Yam will go straight on your Honda.
Yes you can get stainless props in 1" pitch increments :thumbs:

headlight
 

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Discussion Starter · #12 ·
That makes it sound rather promising then - will enquire further asap.
Thanks

And sorry Clinker - that's my bad for pursuing the subject on the other thread instead of keeping it here!

Would I be right in thinking that when comparing like for like, an alloy prop when flexing, effectively ends up with more than the measured/stated pitch due to the water pressure pushing the blades back (flexing them) further, whereas the stainless one retains the actual stated pitch under load, and thus "gains" rpm compared with an ally equivalent? That would be my rationale to it but I could be talking rubbish. It's been known on occasion!
 

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Never asked the question myself, but your rationale sounds entirely feasible, but as said before, prop dynamics seem to baffle even the experts, so you get to a point where you need advice from people who really know the subject inside out.
 

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A 19" pitch prop will theoretically move 19 inches per revolution if there is no prop slip.
When a blade flexes it distorts & moves a lot less than the 19" therefore the boat dosen"t move as far for the same revs.

Clinker we have been posting almost identical replies on different threads :g:

headlight
 

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Discussion Starter · #15 ·
Exactly - I get the impression that really knowing your stuff with prop expertise is a certain element of sourcery or black magic, but for a lot of people they can usually get more or less the best scenario by following 'rule of thumb' or what suits others with similar setup etc.

I'm not after every last possible knot that I get from the boat as such, but a setup where it's reasonably quick, healthy for the engine and can cope with an extra person on the odd occasion and so on.
 

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I'd be very interested to hear how much of this prop flexing actually goes in with aluminium props? I wonder if anybody has measured it or is it something stated by the people who want to sell us stainless ones? Try clamping one in a vice and trying to bend the blades, they are pretty tough.

TBH on a boat of the size and type in question I'd stick a 17" Aluminium prop on and enjoy the boat, it isn't reckoned to be good practice to "lug" the motor too much.

Another downside of stainless which I was discussing with another boat owner recently is that should it inadvertently hit something in the water (other than water) it can potentially transmit a harsher shock load to the gearbox. Just a thought.
 

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I'd be very interested to hear how much of this prop flexing actually goes in with aluminium props? I wonder if anybody has measured it or is it something stated by the people who want to sell us stainless ones?
No idea what the answer is to the amount of flexing, but having experienced the difference of swapping from aluminium to stainless, I wouldn't run an aluminium prop again on anything but a small engine. It gave me 75% of the performance advantage of swapping from a 70hp to a 90hp at a cost of £250 v a cost of £3400!
 

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I would use a 17" prop as it will give you more accelaration and now you got the boat up and running there will be times when more than 2 of you are going out for a [email protected] fishing ! The17" would be more efficent with a heavier load and not put extra srain on the gearbox etc.
chris
 

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I'd be very interested to hear how much of this prop flexing actually goes in with aluminium props? I wonder if anybody has measured it or is it something stated by the people who want to sell us stainless ones? Try clamping one in a vice and trying to bend the blades, they are pretty tough.

TBH on a boat of the size and type in question I'd stick a 17" Aluminium prop on and enjoy the boat, it isn't reckoned to be good practice to "lug" the motor too much.

Another downside of stainless which I was discussing with another boat owner recently is that should it inadvertently hit something in the water (other than water) it can potentially transmit a harsher shock load to the gearbox. Just a thought.
Typically the performance increase between a stainless & ally prop is said to be around 10%"ish .
Some of this is down to less flexing (how much I don"t know) & some is down to the thinner blade section & thinner leading edge.
Clinkers increase was about 3kts iirc which would be about 10% & my own & other peoples results have been similar so I think the general consensus is about right.
If you think that the leading edge of a stainless prop is half or less than half the thickness of an ally & the prop is spinning at several thousand rpm the reduced effort for it to cut through the water must be quite significant
As to the increased risk of damage obviously there is a possiblity but the rubber bush in the stainles prop is designed to try & stop this.

headlight
 

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On this second page the fact that the OP is considering uprating the engine from 75 to 90hp seems to be getting lost. I would do this first, then see how the current prop performs, then think about changing size and/or swapping for a stainless prop.
 
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