Outdrive upgrade requires new prop

[talon8]

I upgraded my Cobra outdrive to a Cobra SX. The original ratio was 1.41:1 and the replacement is 1.51:1. The engine is a 235 hp 5.8 liter OMC. The boat is a 2155 Ciera Sunbridge. I knew the ratio change would require a new prop pitch and first sea trial yesterday confirmed it. Advertised WOT 4000-4400 RPM. I was able to get 5100 RPM so I am 700-1100 over spec with my 14.25X17 SST Rapture prop. I have found online that 1" of pitch change equals about 225 RPM. I am thinking I need to go to a 14-15 X 20-21 to. I'm not sure how much diameter affects RPM. Any Ideas?

 

[Maclin]

Re: Outdrive upgrade requires new prop

This would get a lot of help over in the Props section, I would repost it over there or maybe a moderator can move it.

Diameter does not affect RPM's in theory when the engine has enough power to keep moving the larger prop at the higher RPM's. With that engine I do not think it would lower top RPM's much. Obviously a V8 cannot spin a battleship prop, but within the parameters of the props that fit the drive a V8 is not affected much. The pitch is the biggest factor on the RPM's.

 

[talon8]

Re: Outdrive upgrade requires new prop

Thanks Maclin, maybe the moderator will move. I didn't even see a Prop section. Moderator?

 

[Reinell-BRXL-191]

Re: Outdrive upgrade requires new prop

When you took it out, how much people and stuff where on the boat? Honestly I would try a 14.5X19 at first, I think you will see more RPM drop than you are noting... I have seen as much as 400 rpm drop per inch - I think if you get up to the 21 inch area and have a full load of people, its going to dog and have a hard time getting up - I know chasing an RPM with props can get expensive... so I would look for used, banged up ones on craigslist before settling in on a brand new shiny one...

 

[talon8]

Re: Outdrive upgrade requires new prop

It was half fuel and just me. I picked up a 14.25X21 Aluminum and will try it tomorrow with 2-3 adults, 2-3 kids and ice chest with food and drinks. I can return the prop if it is in pristine condition.

 

[talon8]

Re: Outdrive upgrade requires new prop

Reinell,
It seems you were right, the 21 was too much with 3/4 fuel, 3 adults and 2 small kids and heavy ice chest. I eventually moved the ice chest as far forward as possible to help get it on plane. I'll try a 19 next weekend.

The weird thing is, it would lug and plow at about 3500 WOT. Then when I moved some weight forward and got it to break over on plane, the RPM would go to about 4600. My engine is suppose to WOT at 4000-4400.

 

[Bondo]

Re: Outdrive upgrade requires new prop

The weird thing is, it would lug and plow at about 3500 WOT. Then when I moved some weight forward and got it to break over on plane, the RPM would go to about 4600. My engine is suppose to WOT at 4000-4400.

Ayuh,... Nothin' Weird 'bout it,... that's the way it works...

 

[talon8]

Re: Outdrive upgrade requires new prop

Bond-o
I guess I should have explained it better. I understand that the noted behavior was correct (RPM increase once on plane) What I thought was strange was I went from a 17 pitch to a 21 pitch and only corrected WOT by 500 RPM. I expected 700-900 RPM correction. I know it only shows me as Cadet in my profile but I have owned over 10 boats, skippered my own commercial fishing vessel for 18 years, lived on lake front property for over 20 years and have worked on the ocean in some form for almost 40 years.

So what do you do if a prop lugs and won't get you on plane, but if you adjust weight distribution or tabs and get it on plane and then the prop over revs? The prop should be sized for recommended WOT while on plane. The boat is very stern heavy with a 351 V8, 3 batteries and people all in the stern.

 

[jimmbo]

Re: Outdrive upgrade requires new prop

Redistribute the load, and if still sluggish drilling the prop is an option

 

[Bondo]

Re: Outdrive upgrade requires new prop

Redistribute the load, and if still sluggish drilling the prop is an option

Ayuh,... Or another prop, from another manufacturer, of a different style, 'n rake maybe...

It sounds like yer hull/ load, Don't like that particular prop...

 

[talon8]

Re: Outdrive upgrade requires new prop

I tried a 14 1/8 X 19 4 blade this weekend, boat was heavy with 3/4 fuel, 4 adults, 2 kids and a heavy ice chest. Holeshot was still troublesome but once on plane all was perfect. 4400 RPM, 35 MPH. Then I trimmed the bow up a little and it went to 42 MPH and 4600 RPM. This is about the same at with my old outdrive and the 17 pitch propeller. I guess that is as good as it gets. My old prop is at a shop being re-pitched to about a 19 pitch.

 

[Maclin]

Re: Outdrive upgrade requires new prop

Sounds like you need to pitch up sooner, a 7mph increase indicates the bow was plowing. You will get a lot of bow steer when not pitched up properly, pretty strange feeling in rollers.

 

[Lelo]

Re: Outdrive upgrade requires new prop

Talon what is the outcome on your prop? I'm repowering same boat from v6 to 5.0. I'm in the same process and looking at 3bld 15 to 16 with a pitch of 19. Presently I have a 16x17. It got up to almost 5600 RPM. But it almost media me sit getting out of the hole it plane at 2800. Gear ratio now is 1:65. I my have to change that.......still looking for the right combination

 

[talon8]

Re: Outdrive upgrade requires new prop

Lelo,
What is the HP of your 5.0? My 5.8L is rated at 235 HP. Gear ratio for OMC/Volvo SX outdrives seems to be tied to HP rating. As I said when I changed my OD I knew it would require a prop change and I tried to figure it out mathamatically but was unable to so I tried the boat with the 14.25X17 SS Michigan Rapture prop that was on my old OD just to see where I was at. Holeshot was best it ever has been but speed was horrible and RPM was too high. Next I tried a 3 blade 21P aluminum and it was too much pitch, no hole shot and couldn't attain proper RPM's. I traded it in on a 14X19 Solas SS 4 blade. I was heavy with people and had to put one person in the cabin to get it out of the hole and on plane, then it was fine once on plane. As I said, I think it is as good as gets. My boat has always been stern heavy when loaded with people. Everyone wants to be outside which on this boat puts all the weight aft of center. A prop shop in Phoenix is re-pitching my old 17P prop to about 19 but i will not be able to get back to the lake until the last week in Aug. For you, 5600 RPM is about 1000 RPM too high so a 21P might work for you. Before you buy, ask if you can return/trade the prop if it doesn't work. Some will let you and some will not. In any event, the prop must be in pristine condition to be able to return it. Also, I think 1.66 is the correct ratio for a 5.0 L 220 HP engine. If this is what you have, 1.65 is close enough.