21 pitch or 19 pitch?

[Benny and his jets]

I have been boating for a few years now, but I am in no way an expert. I recently lost my 21 pitch 5 blade highfive SS prop off my 86 SeaRay. I did not have the tabs down on the washer. GRRR. I'll get it in the winter when I can see the bottom. I know where it is.

Anyway, I have a spare 19 pitch SS prop that works fine. Here is the issue. I have a 5.7L V8 Mercruiser Alpha 1 engine. When I had the 21 pitch on, the RPM's would hit the proper WOT RMP's. Now, with the 19 inch prop at WOT, the RPM's don't get to the proper RPM's. I thought if the RPM's were too low, you go down a pitch. My senario seems backwards being the RPM's dropperd when I went down 2 inches. Hmmm. The boat runs great though and it does not labor. Any ideas? Oh, yeah. Is it true that fuel consumption is worse with a 5 blade? Thanks!!

 

[Triton II]

Re: 21 pitch or 19 pitch?

I have been boating for a few years now, but I am in no way an expert. I recently lost my 21 pitch 5 blade highfive SS prop off my 86 SeaRay. I did not have the tabs down on the washer. GRRR. I'll get it in the winter when I can see the bottom. I know where it is.

Anyway, I have a spare 19 pitch SS prop that works fine. Here is the issue. I have a 5.7L V8 Mercruiser Alpha 1 engine. When I had the 21 pitch on, the RPM's would hit the proper WOT RMP's. Now, with the 19 inch prop at WOT, the RPM's don't get to the proper RPM's. I thought if the RPM's were too low, you go down a pitch. My senario seems backwards being the RPM's dropperd when I went down 2 inches. Hmmm. The boat runs great though and it does not labor. Any ideas? Oh, yeah. Is it true that fuel consumption is worse with a 5 blade? Thanks!!

Yes, normally a 2-inch drop in pitch would raise the rpm for any given speed. However it's not only pitch that affects rpm, it's the number of blades, the cup and the diameter. If your 19 has been "cupped" it will require more torque to turn it. One way to find out what's going on is to compare the speed of the boat at any given rpm with both props - assuming you have that data from when you were using the 21 before you had a bad experience with the locking tabs. If the boat is going faster with the 19 at a set rpm than it was with the 21, you know there's another variable in the mix. Are you still getting WOT rpm WITHIN the range recommended by the motor manufacturer? If you are, then enjoy the boat using the 19 even if its WOT rpm is not as high, until you retrieve the 21 as ideally boats should run towards the upper end of the WOT recommended rpm range at your normal loaded weight.

Not sure about increased fuel consumption with a five blade... I would assume from my limited knowledge of hydrodynamics that the consumption of a 5-blade prop which achieves the same speed at the same rpm as the equivalent thrust 4-blade or 3-blade would be the same... but I have been wrong before, so I'll leave that one to someone smarter than me! BTW, welcome to iboats!

TII

 

[Benny and his jets]

Re: 21 pitch or 19 pitch?

TII, Thanks for the info. When I had the 21 pitch prop on, 3000RPM's showed 30mph on the GPS, and the spedmoter was actually dead on with the GPS. WOW!! Anyway. the 19 inch prop shows around 34mph at 3k. The recommended RPM range for my engine WOT is 42-4600. When I go WOT with the 19 pitch, it shows just under 4k, yet the boat runs great and I never go WOT anway. I plain out and bring her back to 3000rpm's for cruising. I guess I'll keep this prop and hope not to loose it. I guess what is frustraiting is the fact the engine manuals don't talk about diameter of the props only the pitch. My gas milage is actually pretty good too. I guess it's because I drive like a grandpa. I don't understand the whole WOT thing. I mean, why would anyone want to put the hammer down and keep it down? All that does is suck way too much gas.

TII, can you explain what "cupped" is?

 

[Triton II]

Re: 21 pitch or 19 pitch?

TII, can you explain what "cupped" is?

I could, but this site does a good job of it...

http://www.thepropellerspecialist.com.au/services/propellers101/

The ideal WOT is something that the manufacturers have every powerboater strive for - and in your case they give you a 400 rpm range. Basically if you run the boat at maximum capacity the engine should run at the lower of the two rpm figures and at light weight (one up and minimum fuel) the higher. This is in an ideal world. From the information in your post above, it would appear that you're about 10% over-propped, so it would be best to change back to the 21 when you retrieve it. Good luck...

TII

 

[Benny and his jets]

Re: 21 pitch or 19 pitch?

Thanks, TII!! Well, I never go WOT so, I should be fine. I will find the prop. I know where it is within 1000 feet or so. I'll get it in the winter when I can see in the water. Thanks for the link. I will check it out!!

 

[Robbabob]

Re: 21 pitch or 19 pitch?

Benny,

the fact of the motor specs at wot allow the motor to run most efficiently at lower rpm as well. Nobody in their right (or left) mind would go wot for more than a minute, else, the gas tank would empty very quickly.

Keeping within the range of max rpm at wot means you are taking the best care of your motor and ANY rpm. Too much prop and you'll lug the engine, causing premature wear and higher temps.

my $.02

(I spent 5 years in Medford... very far from you?)

 

[Triton II]

Re: 21 pitch or 19 pitch?

Good explanation Robb. Benny, you should be fine with that 19 for now, but please consider getting that 21 back on when you can as Robb's $.02 above is spot on. Cheers,

TII

 

[Benny and his jets]

Re: 21 pitch or 19 pitch?

Benny,

the fact of the motor specs at wot allow the motor to run most efficiently at lower rpm as well. Nobody in their right (or left) mind would go wot for more than a minute, else, the gas tank would empty very quickly.

Keeping within the range of max rpm at wot means you are taking the best care of your motor and ANY rpm. Too much prop and you'll lug the engine, causing premature wear and higher temps.

my $.02

(I spent 5 years in Medford... very far from you?)

Rob, thanks for the info. Yeah, I am still confused as to why the drop in my prop caused the RPM's to drop. I thought if you go lower in pitch, RPM's increase. The 19 prop seems to have a larger diamitar but not cupped as sharp. Oh, well. Like I said, the engince runs smooth, quiet and no vibrations at all. Medford is quite a bit north of me. I am west of Oshkosh about 10 miles in the same county.

My only beef is the boat leans to the port side when I plain. Fuel tank is in the middle. Not sure if a trim tab on the lower unit would help this. When I'm cruising, I'll take my hand off the wheel and the boat stays straight and does not veer off left of right. So that's good. The darn thing just tilts to the left. GRRRR!!