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Is My Prop Letting Me Down?


nic_ally_king

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Hi all,

I run an 8yr old honda 4stroke 75hp on a 5.35m ally. At WOT i'm only getting about 5100rpm (i think i'll have to double check but it's close to that either way). Does that sound too low?? I'd definitelty like some more top end speed. I'm unsure of the pitch on the prop will check soon.

If that's the case i'm planning on bringing the boat to you Huey - but I can't really go s/s as my old boy also uses the boat up the hawksbury on occasion and has a strong tendency to test out the sea bed!

Thanks Nic

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Hi Nic, yes mate you are way overpropped and the performance will suffer greatly. You are lugging your engine and ideally you want to be seeing more like 5800RPM at WOT with your normal load. Your engine life, fuel economy and as you mention performance will improve with the right prop to match your rig.

Get the pitch of the prop on it now and we can point you in the right direction,

Cheers,

Huey.

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Did you ever get proper WOT after motor was run in? Have you added heaps of weight to the boat?

Nicked props worn or bent blades can chew up a lot of revs along with incorrect engine height. Along with the prop numbers off the hub it might be worth your while to put up a photo of transom from the side with motor down and prop in case motor height is also an issue.

Pel

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Did you ever get proper WOT after motor was run in? Have you added heaps of weight to the boat?

Nicked props worn or bent blades can chew up a lot of revs along with incorrect engine height. Along with the prop numbers off the hub it might be worth your while to put up a photo of transom from the side with motor down and prop in case motor height is also an issue.

Pel

Thanks Huey. I'll get the pitch size either today or tomorrow and get back to you. Appreciate it.

Pelican - no added weight to the boat at all (small livey tank and two blokes). Yrs ago I didn't know much about this sort of stuff so I can't comment on the WOT after run-in. Can't say i've ever noticed a change though. The prop is in good condition though (after a repair!). I'll try get a pic of transom height and will be sure to ask today/tomorrow as she's getting a service. Thanks guys.

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If your motor is set up right, running right etc etc

I would ask the guy doing the mechanical work if he has a test prop you can borrow to see if you can get the WOT revs correct. Lot of mechanics have stuff sitting around and he would also look the repaired prop as often many repairs unless done properly can really screw up the performance ( not that many revs I wouldn't say unless obviously bent but). Ask the guy servicing it if he thinks the motor is the right height?

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  • 2 weeks later...

If your motor is set up right, running right etc etc

I would ask the guy doing the mechanical work if he has a test prop you can borrow to see if you can get the WOT revs correct. Lot of mechanics have stuff sitting around and he would also look the repaired prop as often many repairs unless done properly can really screw up the performance ( not that many revs I wouldn't say unless obviously bent but). Ask the guy servicing it if he thinks the motor is the right height?

Hi Huey,

Only got round to checking prop specks today - it's a solas 13.5 x 15 - which I would have thought would be pretty good. But anyway i'm still waiting on a throttle cable so still havn't managed to properly test it... but as I said It's def not going over 5200 and the engine is very healthy and boat is 'stock; with no options/extras adding weight. What kind of prop would you try on it knowing what it's already got (I was under the impression the prop on it now is already on the smaller end of the scale)?? Is it a matter of just trying a few out and seeing how it goes? I'm finishing off a yr of completely unpaid full-time work as a physio so i'm broke as buggery but it's a priority so would like to bring the boat round in next few months. Out of interest how much would an ally prop set me back?

Thanks Nic.

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Hi Nic, yes a 15 inch prop is on the low side, but if you are only seeing 5200 at WOT you are overpropped and with experience dealers shoudl know 4-Strokes need to run a lower pitch to get the revs up so that they can perform. An alloy prop would not be the go because a 13 inch prop would not rev much more than your 15 inch SST. Have you made sure the engine is not too low in the water, by raising an engine on the transom there is less gearcase in the water and therefore you can often pick up RPM (I still do not think you will be in the ideal RPM range, but closer at least) and this does not cost anything to do. Also SST props can run higher on the transom.

If I was you I would raise it a hole, take it for a run, trim the engine out to its ideal trim and see what the WOT RPM with your normal load. Than we cna determine what needs to be done and prop places can work on SST prop to lower the pitch and get your revs up.

Cheers,

Huey.

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Hi Nic, yes a 15 inch prop is on the low side, but if you are only seeing 5200 at WOT you are overpropped and with experience dealers shoudl know 4-Strokes need to run a lower pitch to get the revs up so that they can perform. An alloy prop would not be the go because a 13 inch prop would not rev much more than your 15 inch SST. Have you made sure the engine is not too low in the water, by raising an engine on the transom there is less gearcase in the water and therefore you can often pick up RPM (I still do not think you will be in the ideal RPM range, but closer at least) and this does not cost anything to do. Also SST props can run higher on the transom.

If I was you I would raise it a hole, take it for a run, trim the engine out to its ideal trim and see what the WOT RPM with your normal load. Than we cna determine what needs to be done and prop places can work on SST prop to lower the pitch and get your revs up.

Cheers,

Huey.

Thanks huey will do. ps the solas on it now is alloy.

nic

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Thanks huey will do. ps the solas on it now is alloy.

nic

Hi Nic, I just assumed it had a SST on it already. Still make sure height is right and than if the buget can afford a 15 inch SST prop will perform better than that alloy.

You will not be able to do much tweaking with that alloy prop.

Cheers,

Huey.

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silly question - is that a 3 or 4 bladed prop?

Yeh cheers huey finally got the engine serviced today and the mech said the same thing about alloy and advising on a SST. He did however straighten it out a little - I didn't even notice it! Nothing too major but def needed some work. He thinks he has a test SST 15 so i'll try test it next w/e and see how it goes.

Pel, it's a 3 blader. The heights pretty much fine too, could be a smidgen higher but more or less where it should be

Thnaks guys

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Yeh cheers huey finally got the engine serviced today and the mech said the same thing about alloy and advising on a SST. He did however straighten it out a little - I didn't even notice it! Nothing too major but def needed some work. He thinks he has a test SST 15 so i'll try test it next w/e and see how it goes.

Pel, it's a 3 blader. The heights pretty much fine too, could be a smidgen higher but more or less where it should be

Thnaks guys

Hi fella's...Just curious to know if a S/S prop that is the same spec's as an alloy on the same boat will improve the performance much???

Cheers...Sparra

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Depends on prop brands but the theory is the you can have stronger thinner blades , less flex, with edges that last longer. The stainless will hold a cup better. Thinner blades and slightly better shape, less corrosion in salt water, thinner hub ( means better exhause gas flow) means better performance increase.

Some of the better ally props are now very thin with cupping and they have incorporated thinner castings on hubs etc so ........it is very much dependant on the exact props you are talking about. A lot of people I reckon get upsold on poor quality or design stainless and never get the best out of the motor.

You really have to try em out as there are good and bad ones but overall like for like stainless should give better performance at higher speed and a bit better ecconomy.

Ally has one benefit that it may break if you strike something and protect your gearbox from failure but if you aren't up rivers and banging into rocks and sand a lot maybe not as big a beneft.

Like all products you pay for it is about if you are doing it for a reason and if it suits that reason and how much the cost difference is. Stainless props have come down in price a lot.

Edited by pelican
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Finally got round to fitting a new SS prop on my Haines 445F and the difference across the board was suprising with an extra 600RPM @WOT and an extra 6MPH along with a whole boot load of grunt, bar crossings might be a little easier now, should have done it years ago.

Cheers Dave

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