RPM issue under load

[Sirius 26]

Ok i was on the boat this past weekend and measured the prop and it's 14" as it specs for volvo original 14x8" for my 10hp engine. Couldn't find any marks for size and pitch but i measured the diameter to 14".
So prop is correct size, even prop calc for my boat specs came up with 14x14" (which i find it very odd for pitch to be 14").
I don't know now where the problem in low wot rpms lays. In reverse i can go 3600rpm easy and maybe even higher but didn't want to break the tiller in case it tilted it sideways from my hands. In neutral fwd and reverse it also hits 3600 easy maybe even more. In gear fwd won't go over 2800rpm and i get max speed 6knots. I guess the speed is standard for a 26ft sailboat with 10hp engine. I just don't get the low wot rpms in fwd gear.

[alaskaflyfish]

I just went thru this same scenario with my 27ft 8hp Yanmar. See my previous posts. I finally went with a 13x9 two blade Martec folder prop instead of a 3 blade 12x12 Campbell Sailor. I was assured by the prop mfg that I would get 3400 WOT. Now instead of getting 2500rpm wot with the 3 blade, I get 2600 rpm wot with a 2 blade. Not much improvement. It does get 3400rpm in reverse. I went thru the fuel system extensively with new filters, supply pump, injection pump and injector. New air filter, exhaust elbow, exhaust hose, muffler, compression check,,etc.. You can see I spent a lot of cash. At the end of it all I determined that everybody was wrong about the prop calculator. It only gets you ballpark and you need to customize the pitch to your application. 13x7 would probably work best for me now. The good part in all this is I no longer have black smoke and the transom stays clean, so a marked improvement in that regard for a 100rpm increase. Also, Im thinking that a lower WOT RPM than the engine MFG recommends is not all that abnormal, although I would still like to hit the proper numbers.

[Sirius 26]

This is so stupid, I'm not gonna go waste all the money and time to replace everything to gain 100 or 200 rpm. Makes no sense and it doesn't even go close to mfg specs for engine wot. I read a lot about changing this and that and cleaning this and swaping that and in the end we end up like u with no improved result at all (as if those 100-200 gained rpms make any difference after all).
I guess I'm just gonna leave it the way it is then and cruise at 2500rpm knowing 2800 is my wot.
Tnx mate
Sasha

[Q Xopa]

OK good, you have some more information than before. The puzzle is being progressed. But you still need more clues, dont give up.

Any chance you can take it off and take it to a prop shop? If you are talking to a shop maybe they can tell you what prop would work for your installation.

If the original is meant? to be 14x8. But you still arent sure what pitch is.

Unfortunately there is not much getting around this is kind of a relevant number.

The only other possible alternatives I can think of is if you can borrow a known pitch one from someone/ prop shop and try it or compare it.

You could also try to measure the pitch angle and compare it to the what it should be.

This is not as easy as it easy as it would seem. Its usually done properly on a surface plate and a Dial test indicator/ Height gauge or accurate protractor. Different readings are taken at different 'blade stations'. The Pitch generally varies along it's span on most efficient designs. Then you have to know or find out what 'nominal pitch designation' those measurements correspond to.

But you might be able to get close with a smart phone protractor. Say holding it horizontal. Perhaps taking reference readings from the propshaft or leading edges it they're straight. Especially if you had one to compare or some good information from a prop shop. Or perhaps the prop manufacturer or online etc

If your prop checks out ok then at least you can eliminate this from the list of possibilities. But that would be my most likely suspect and where I would start.

Keep up the good work.

[alaskaflyfish]

It has always been said that a wise man learns from the mistakes of others. I did a lot of research on the subject and did learn a lot about theoretical normalities, just not enough about practical field realities. One helpful bit of information I learned is that a fixed pitch prop can be re-pitched 2 degrees from standard. Local prop shops can do this easily at a cost of $40 US. Knowing this could have saved me beaucoup $$$.

[Q Xopa]

You can leave it. In fact a lot of people do over prop deliberately, thinking they will save fuel. That's probably how yours got that way.

It's not the best for two reasons. You aren't getting max power from your engine. You might want that one day if you are caught in bad conditions.

Secondly the engine is not designed to run so labored so much of the time. So it will effect the long term health of the engine.

So another possibility is that your engine is simply not making the power it should anymore. Perhaps from being over propped? In this case you will need to find out why to fix it. Injectors, filters etc.

It could still do more RPMs in reverse because props arent as efficient in reverse and the ratio is lower.

I try to look after my engine as best I can in the hope that it will give me good service and get me outta the 'you know what' if and when I need it to.

Your boat, your cash, your call.

[Sirius 26]

my boat is not overproped, prop calc calls for a 2 blade 14 x 14" and volvo original calls for a 14 x 8". i got a 14" and the pitch is unknown but looks pretty mellow to me, not steep like outboard engines at all.


borrowing someones propeller to try it on mine and swapping it under the water is nearly impossible without scuba tank and i don't have that.


i wonder if reverse really goes slower on high rpms than forward speed on lower rpms. i'd like to measure with speedometer but afraid to floor it with tiller in hand.


i will take a look at exhaust elbow and heater exchange while changing antifreeze, but filters have all been replaced and tank cleaned and all...

[Compass790]

Originally Posted by alaskaflyfish

I just went thru this same scenario with my 27ft 8hp Yanmar. See my previous posts. I finally went with a 13x9 two blade Martec folder prop instead of a 3 blade 12x12 Campbell Sailor. I was assured by the prop mfg that I would get 3400 WOT. Now instead of getting 2500rpm wot with the 3 blade, I get 2600 rpm wot with a 2 blade. Not much improvement. It does get 3400rpm in reverse. I went thru the fuel system extensively with new filters, supply pump, injection pump and injector. New air filter, exhaust elbow, exhaust hose, muffler, compression check,,etc.. You can see I spent a lot of cash. At the end of it all I determined that everybody was wrong about the prop calculator. It only gets you ballpark and you need to customize the pitch to your application. 13x7 would probably work best for me now. The good part in all this is I no longer have black smoke and the transom stays clean, so a marked improvement in that regard for a 100rpm increase. Also, Im thinking that a lower WOT RPM than the engine MFG recommends is not all that abnormal, although I would still like to hit the proper numbers.

i tried to say our prop was 12x7 2 blade with same water line length & Yanmar ysm8 . I think in an earlier post. That would be a close fit for you assuming you have same gbox ratio

[alaskaflyfish]

Originally Posted by Compass790

i tried to say our prop was 12x7 2 blade with same water line length & Yanmar ysm8 . I think in an earlier post. That would be a close fit for you assuming you have same gbox ratio

Yes I remember you saying that. You were correct and the prop suppliers were wrong. Not just a little wrong, but big time wrong. I was assured by multiple prop manufactures that the 13x9 was correct. This is what ultimately led me to redo the fuel system (which was not the problem) due to the low RPM. The funny thing is that many people with a boat/engine combo like mine also stated that 13x9 was a good fit. I think they just believe that 2600RPM is normal, even though the engine MFG says otherwise. Good call on your part, just wished I would have listened.

[Q Xopa]

Originally Posted by Sirius 26

my boat is not overproped, prop calc calls for a 2 blade 14 x 14" and volvo original calls for a 14 x 8". i got a 14" and the pitch is unknown but looks pretty mellow to me, not steep like outboard engines at all.


borrowing someones propeller to try it on mine and swapping it under the water is nearly impossible without scuba tank and i don't have that.


i wonder if reverse really goes slower on high rpms than forward speed on lower rpms. i'd like to measure with speedometer but afraid to floor it with tiller in hand.


i will take a look at exhaust elbow and heater exchange while changing antifreeze, but filters have all been replaced and tank cleaned and all...

Good news if you are sure you are not over propped. Doesnt sound like a particularly conclusive assessment of your prop pitch. You can see better than us only able to go off your descriptions.

As you say you can concentrate on zeroing in on something on the engine as being the issue.

Always best to try the easiest things first.

Someone here is sharing their experiences of wild goose chases from incorrect but well meaning assumptions, prop calculators etc and advice. Take from that what you will.