under powered or under pitched

[rbh]

having done are tune up this year and making sure the timing was in at full advance we headed out on the lake, as I have never fully opened it up to WOT for any distance,
I thought I would give it a try!
the boat is a sundancer 27 foot 10 on the beam, twin 260 mercs alpha legs
14x20 pitch props and they are aluminum, estimated weight 7500 pounds, max RPM obtained 4000.
If the magic number is 4700 rpm max per motor, what way should I be looking?
thanks
rob

 

[Bondo]

Re: under powered or under pitched

Ayuh,... Probably 17" of prop pitch.... Maybe 18"... Depends on blade design...

 

[Don S]

Re: under powered or under pitched

The first thing you have to do is get the WOT rpm. Without that number, anything else is a WAG.

PS With twin 260 Mercs, you are not underpowered.

 

[rbh]

Re: under powered or under pitched

thank you gentleman-
with the 4 blade 20 pitch props, is it like being stuck in 2 gear?
would going to a 17,18,19, pitch raise rpms as well as speed but I would loose my bottom end torque/hole shot.
as well going to a stainless prop, would that cut down on the blade deflection as well as lower my prop loss?
what prop is my best bang for the buck?
thanks
rob

 

[TilliamWe]

Re: under powered or under pitched

Rob, if you are trying to relate this to gearing, just keep in mind that your outdrive is like your car's transmission and rear axle ratio, all in one.

You need to drop your pitch size, which would be a little like putting a lower rear end ratio (higher number) in it. It will get out of the hole faster, actually. It will probbably still run faster on the top end (although it might not), since you will turn the motors 4700 not just the 4000 you get now.

Yiou realize that pitch represent the theoretical distance that the prop will travel per 1 revolution, right? So a 20 pitch prop will go 20" in one rotation, where an 18p will go 18". So at your current 4000rpm WOT, the 20p prop would be faster than an 18p. But at 4700 the 18p should be faster than the 20p was at 4000. See what I mean?

Stainless steel can be an improvement over aluminum for a few reasons. But since you don't know a lot about props, stick with the much cheaper aluminum for now, okay?

 

[rbh]

Re: under powered or under pitched

thank you for the input-
I believe that my thoughts are more on the weight of the craft/hp over
pitch, meaning if I go to a lesser pitch prop it will allow the motor to rev higher and achieve a faster top end but I would sacrifice torque through the total range?
the boats on plane in 10 seconds now, and maybe 40 seconds with a smaller pitch.
thanks
rob

 

[junior1113]

Re: under powered or under pitched

just the opposite down in pitch= increase hole shot performance and overall torque. example: a 15" pitch would jump on a plane where a 23" pitch would take a week. easier to turn a lower pitch. id shoot for 4400-4600 wot rpm. do you know your wot rpm is it 4000? if it is i'd be looking for a pair off 18's

 

[TilliamWe]

Re: under powered or under pitched

thank you for the input-
I believe that my thoughts are more on the weight of the craft/hp over
pitch, meaning if I go to a lesser pitch prop it will allow the motor to rev higher and achieve a faster top end but I would sacrifice torque through the total range?
the boats on plane in 10 seconds now, and maybe 40 seconds with a smaller pitch.
thanks
rob

I'll type this again...

"You need to drop your pitch size, which would be a little like putting a lower rear end ratio (higher number) in it. It will get out of the hole faster, actually. It will probably still run faster on the top end (although it might not), since you will turn the motors 4700 not just the 4000 you get now."

 

[rbh]

Re: under powered or under pitched

ok then if I am to achieve the 700 rpms I should get drop 1 or 2 pitches
thanks rob

 

[junior1113]

Re: under powered or under pitched

going to a 4x18p would increase rpms about 300-350. id try 17's if it was mine to get about 500 more rpms.

 

[rbh]

Re: under powered or under pitched

you know the one thing I didnt try is trimiming up a couple of degrees!
by all the chatter that I have been reading that may be a slight fix?
any takers,
just remember its a big, heavy boat
thanks rob

 

[TilliamWe]

Re: under powered or under pitched

Geez, yes, of course you have to trim the drives up a little! With no trim, the front of the hull "plows" in the water. Just a little bit of trim raises the bow up and will increase rpms and speed. You aren't done with your current props, yet.
For example on my 21' Chaparral, WOT with no trim was only 4200rpm. Trim it up properly, and "viola", it went to 4900rpm, right at the top of it's range.

 

[hwsiii]

Re: under powered or under pitched

Rbh, what is the gear ratio for your outdrives.

H

 

[hwsiii]

Re: under powered or under pitched

If your gear ratio is 1.32 don't buy any props. It appears you have a motor problem and not props.

H

 

[TilliamWe]

Re: under powered or under pitched

He has no motor problems, probably doesn't have any prop problems either. He has "operator issues".

 

[rbh]

Re: under powered or under pitched

I WOULD GO WITH OPERATER PROBLEMS,
I deal with problems as they come, and finesse if I have time or the knowledge. BUT when your throwing around a chunk of fiberglass this big, I want to have a knowlegeble person to direct me to my faults!
You are never to old to learn!
Rob