Prop? Low WOT RPM? New engine.

[LAC_STS]

Hi all!

I have a 25ft Donzi with a brand new OMC 5.7 350 with Cobra outdrive.

The boat currently has a 15.5 dia 15pitch prop on it.


I just got a new tach because my old one didnt work so now I can try to get the right prop.


At WOT I can only reach 4200 RPMs. I dont think I would want to go down in pitch from a 15.

When I went to a Marine Surplus near me they showed me the props they had. they had some actual OMC props. All of the ones they had that were bigger in pitch were smaller in dia.

they had a 21 pitch one that was 14 or 14 1/2 (forget) dia.


Question is if I go up in pitch but down in dia can I stay at the same or get more RPM?

Any idea on what I should do? I was more top speed and would like to get to the correct WOT RPM.


Any and all suggestions are greatly appreciated.

 

[Durimar]

Re: Prop? Low WOT RPM? New engine.

I'm having the similar issue with the same engine and prop. My boat is a little bit heavier and I only get 3800 with 15.5x15 prop. I've just tried 13' pitch and boat performs like crap. Maybe a 14 1/4 x 15 would do better? Could anyone help?

 

[NEWYORIKAN007]

Re: Prop? Low WOT RPM? New engine.

hello, people. im new to this forum n dont no how to post, but if anyone can give me a few corners, id apreciated. im trying to post that im searching for a new or used propeller model # 48-86903A1 or #48-70453A17. i need it for my 4hp 2 stroke Mercury outboard serial #9188755. or any suggestions or alternatives available, like similar propellers which i can use for that particular outboard. please call 508-441-9611, jay. or you can email me with some tips or hints at mrjlruiz@gmail.com THANX, Everyone, any help will be greatly appreciated, Have a safe n great 4th july.

 

[Durimar]

Re: Prop? Low WOT RPM? New engine.

hello, people. im new to this forum n dont no how to post, but if anyone can give me a few corners, id apreciated. im trying to post that im searching for a new or used propeller model # 48-86903A1 or #48-70453A17. i need it for my 4hp 2 stroke Mercury outboard serial #9188755. or any suggestions or alternatives available, like similar propellers which i can use for that particular outboard. please call 508-441-9611, jay. or you can email me with some tips or hints at mrjlruiz@gmail.com THANX, Everyone, any help will be greatly appreciated, Have a safe n great 4th july.

Dude, create your own post. Don't hijack other people topics with unrelated questions.

 

[craze1cars]

Re: Prop? Low WOT RPM? New engine.

Diameter doesn't hardly effect RPM at all. Only pitch does. Going from a 15 to a 21 in your boat would be a HUGE mistake...your RPMS will drag down big time.

If a boat cannot get low enough in pitch to push the smallest prop available, then it is geared too steep. Conventional wisdom says you just need to regear the outdrive to a numerically higher ratio in order to run bigger props.

What is your current drive ratio?

 

[1980Coronado]

Re: Prop? Low WOT RPM? New engine.

Diameter doesn't hardly effect RPM at all. Only pitch does. Going from a 15 to a 21 in your boat would be a HUGE mistake...your RPMS will drag down big time.

If a boat cannot get low enough in pitch to push the smallest prop available, then it is geared too steep. Conventional wisdom says you just need to regear the outdrive to a numerically higher ratio in order to run bigger props.

What is your current drive ratio?

Hmmmm.....I would strongly disagree with the statement

Diameter doesn't hardly effect RPM at all. Only pitch does.

.....I tested a 14x16 4-blade and a 13x16 4-blade from the same manufacturer on my inboard. There is a 300 rpm difference between the two props on my boat with diameter being the only difference.

 

[steelespike]

Re: Prop? Low WOT RPM? New engine.

Hmmmm.....I would strongly disagree with the statement .....I tested a 14x16 4-blade and a 13x16 4-blade from the same manufacturer on my inboard. There is a 300 rpm difference between the two props on my boat with diameter being the only difference.

Extremes in diameter will affect rpm but I would suspect that while the props were same brand were very likely different models. Straight drive inboards are a different animal.
Most are direct drive with no gear reduction. Those with gear reduction would likely require different diameter props in the same pitch.

 

[craze1cars]

Re: Prop? Low WOT RPM? New engine.

Fair enough. And I too have experienced small 300-ish RPM differences in 2 props, even of the SAME pitch and same diameter...some prop designs simply bite better than others.

So I will correct my statement with reference to the original post:

Dropping 1" in diameter will have a minimal effect on RPM, especially compared to the drastic and undesired reduction in RPM that will occur if the proposed 6" of pitch is added....despite 1" to 1.5" reduction in diameter

 

[1980Coronado]

Re: Prop? Low WOT RPM? New engine.

Extremes in diameter will affect rpm but I would suspect that while the props were same brand were very likely different models. Straight drive inboards are a different animal.
Most are direct drive with no gear reduction. Those with gear reduction would likely require different diameter props in the same pitch.

Both props were same mfg, and model. Both were OJ "4Force" props with the only difference being diameter. Both props were cupped equally as well..

Yes, straight inboards are different, but the principles involved are the same regardless of gear ratio. A reduction in diameter will affect rpm in the same manner as a reduction in pitch, although not necessarily equally with respect to the amount. The resultant rpm increase from a 1" diameter reduction does not necessarily equal a 1" pitch reduction. On this we agree. However, to say that diameter doesn't hardly affect rpm at all, is inaccurate. I achieved a 300 rpm increase with a 1" diameter reduction. This is a significant increase. The fact that it's on a 4 blade has a lot to do with it. A 1" reduction in diameter on a 3 blade would not produce as much rpm increase because you are removing 25% less area.....none of this is helping the solve the problem....I just wanted to provide a real world example of what a diameter reduction can achieve.

 

[1980Coronado]

Re: Prop? Low WOT RPM? New engine.

Fair enough. And I too have experienced small 300-ish RPM differences in 2 props, even of the SAME pitch and same diameter...some prop designs simply bite better than others.

So I will correct my statement with reference to the original post:

Dropping 1" in diameter will have a minimal effect on RPM, especially compared to the drastic and undesired reduction in RPM that will occur if the proposed 6" of pitch is added....despite 1" to 1.5" reduction in diameter

Agree 100%.