1994 omc 5.0 prop question

[mpilot]

I hope someone can help me shed some light on my prop selection. I have a new to me boat with the above setup. It is the Ford block and the joint venture Volvo setup. The boat runs well but there are a few things I'd like to get out of it by changing the prop around. Here is what I know:

14x23 aluminum three blade prop that came brand new on the boat. Unsure of the manufacturer.

I assume it has the 1.6 gear ratio from the little bit of research I've done.

Currently I'm running 52 by gps with two passengers and quarter to half tank of fuel.

The tach is reading around 5300 which appears to be over revving if this is accurate.

To run on comfortable plane I am having to run around 3400-3500 by the tachometer and the speed varies depending on weather, boat traffic, trim, etc.

This setup is on a 1994 Sunbird 200 Corsair (listed as 19.5 on the title and 20 ft in the literature I have).

I would like to know first does it sound like I am indeed over revving and if so what prop should I start my search with?

Second, I would like to achieve a plane at a lower rpm even if I have to sacrifice top speed as I rarely run wide open. Would a different pitch, four blade prop, whale tail, etc help me achieve this?

Thanks in advance for any help.

 

[jestor68]

Re: 1994 omc 5.0 prop question

The first thing I'm going to suggest is to verify the accuracy of the tachometer; because that ratio and pitch at 3500 rpm(10% slip) works out to 43 mph.

Looking at it from your stated top speed, again using a 10% slip; it works out to about 4300 rpm. And your tach is showing 5300.

Any dealer/shop can hook up a shop tach and compare readings with your tach.

It sometimes helps to clean the connectors on the back of the tach. When they get dirty, it can cause the tach to read high.

 

[mpilot]

Re: 1994 omc 5.0 prop question

I'm working on that but haven't been able to yet. I imagine I'm running a good bit more slip than that though. The boat is a heavy boat for the size (3300 lbs dry) and the fuel consumption running 3500 on plane is close to what I would think would be on par for that rpm.

 

[mpilot]

Re: 1994 omc 5.0 prop question

I'm going to try and clean the connectors and try turning the selector on back tomorrow and see if that makes a difference in the rpms.

 

[mpilot]

Re: 1994 omc 5.0 prop question

Okay. Didn't get a chance to actually go out and run it in the water but I cleaned the connections and all on the tach. It might be this weekend before I get to actually run it though cause it looks like we have bad weather coming but if the weather breaks tomorrow evening I might go jump on it and take it for a spin. I did do a little more searching at it appears that the recommended props are all in the 14 1/2 - 15 inch diameter range and that gives you more thrust at lower speed I would imagine. Shouldn't this in theory allow me to plane out a little earlier and stay on plane easier at a lower RPM (say below 3k rpm...again assuming the tach is right)? Also it looks like they recommend props in 17-21 pitch range in those diameters. Looking forward to any feedback. I would like to make her not as much of a gas guzzler

FYI right now assuming I run her for around an hour (three 20-25 minute rides) at what my tach is saying is around 4,000 rpm I'm burning 10ish gallons of fuel so I think the tach can't be that far off.

 

[riecken]

Re: 1994 omc 5.0 prop question

CADET.. am having the EXACT same issue right now, down to the boat weight and everything. Only I am on a 22' Chris. I have found that a 23 pitch originally came on my boat, when I bought it it had a 19. I tried a 21, then the 23. The 23 has poor hole shot because the boat is so heavy. The top end is 5250 +/-, and I think it is the weight/horse ratio. Not enough power for too much boat. I plan doing some engine work this winter to increase HP, then put on a 25. I think that will do perfect. So it's not us I believe, it was the design from the plant that was poor to begin with.