1977 Lund tyee offshore prop size?

[Danc1]

Hey all
I am having issues with prop size to use with this boat, right now I have a 17p on it and max rpm is 3900, the other issue this boat has had in the last few years is blown couplers, I have changed it twice in 4 yrs, mechanic that I talked to thinks it needs to be re propped because he thinks it is getting loaded up to much with the 17 and wants me to go to a 15p
The outdrive is lined up right and everything else seems to also be ok also

I should also mention that this is a heavy 21 foot boat and it is usually loaded right up
thank you

 

[Rick Stephens]

What motor and what is the gear ratio on the drive?

A 17 inch moving down to a 15 inch is not enough difference to account for the RPMs you need to gain.

 

[Danc1]

Shoot sorry forgot to say it’s a 165 mercruiser on it with stock bottom end so not sure what the gears that are in it

 

[Rick Stephens]

Some things to think about.

Boat has some years on it. If the motor was running right, and everything else was in good shape, it should have no issue with a 17 inch prop. Being a 40 plus year old boat, it can be water logged, too heavy. You might weight it and see what it comes out to. The motor may be weak and not putting out enough power. Might do a compression check and see what the status of the motor is as a baseline. Your transom may be soft, causing the drive to sag and blowing out couplers. You might jump up and down on the drive and see if anything moves at all in the transom to gimbal housing junction. That should not move in the least with two people jumping on the drive.

I think it foolish to think a 2 inch drop in prop size will change things enough to get the motor into operating range. You might post drive serial number, we should be able to figure out gearing from that.

Rick

 

[achris]

6 cylinder or 4 cylinder?

If it's a 6, then the revs should be 3900 to 4300 and a change from 17" to 15" will get you to the top end of the range. :thumb:

Chris...

 

[Rick Stephens]

6 cylinder or 4 cylinder?

If it's a 6, then the revs should be 3900 to 4300 and a change from 17" to 15" will get you to the top end of the range. :thumb:

Chris...

Yes indeedy. Thanks Chris.

 

[Danc1]

Ok thx guys
I changed the motor in this a couple years ago with a super low hr one that still has the cross hatch in it but was out of an omc now wondering if it was a 160 maybe? Instead of a 165 it didn’t have any info on the motor itself but just assumed it was the same as it was somewhere around the same vintage
i did order a 15p prop for it but didn’t really want to pay for one and not be the right one for my boat and couldn’t find one just to try as almost all around here have newer boats now and nobody stocks the props in these pitches anymore
thx again

 

[kenny nunez]

All OMC in line 6 cylinder engInes were known as 165s.

 

[Rick Stephens]

6 cylinder or 4 cylinder?

If it's a 6, then the revs should be 3900 to 4300 and a change from 17" to 15" will get you to the top end of the range. :thumb:

Chris...

So I been on the hunt for an actual maximum WOT throttle goal for the 165 HP 250 cubic inch chev I6. What an adventure. I even owned a Steury with one of those in it back in the early 80's. Unfortunately, I don't recall a thing about that motor from back then other than the carb rebuilding jobs I did on it and the props we dinged up.

From what I have found, early 70's Chevy I6 was rated low in HP and low in RPMs. By late 70's the RPM were up to 4600 on some Merc spec sheets.

Michigan Motorz says the 250 C.I. remanufactured engine produces 165 H.P. @ 4800 RPM.

 

[achris]

So I been on the hunt for an actual maximum WOT throttle goal for the 165 HP 250 cubic inch chev I6. What an adventure. I even owned a Steury with one of those in it back in the early 80's. Unfortunately, I don't recall a thing about that motor from back then other than the carb rebuilding jobs I did on it and the props we dinged up.

From what I have found, early 70's Chevy I6 was rated low in HP and low in RPMs. By late 70's the RPM were up to 4600 on some Merc spec sheets.

Michigan Motorz says the 250 C.I. remanufactured engine produces 165 H.P. @ 4800 RPM.

Using Merc factory manuals, and my own ownership knowledge and experience....

Mercury started using the inline 6 in 1964. They ran is as a 230cubic inch, 150hp, unit for 3 years, and in 1967 it became 250 cubic inch and 160hp. 3 years later the 160 became 165, and remained as a 165hp engine until it was discontinued, being replaced by the notorious '470', in 1981 (although the 470 had already been in production for 5 years by then). In 1969 Merc also used the longer stroke version of the inline 6, the 292, as a 200hp engine. Not sure why it ran only 1 year. I've heard that it used too much fuel, but can't verify that (and it seems unlikely). The other possibility is that it overlapped and was 'poaching' sales of the 5 litre V8 (rated at 198hp)... (I think this is the more likely reason).

Used by Mercury, all inline 6s had a recommended WOT of 3900-4300rpm (screenshot of manual #1, #2 and #3, which fully covers the inline 6s, at the bottom)...

I have no idea why or how Michigan are getting the engine to spin at 4800rpm. A different cam, or higher compression ratio perhaps... :noidea: But I'd be skeptical, as it's what I'd call a 'lazy' engine. It's happy to make its maximum power lower down the range rather than scream its box off. While not a great power producer, it was a reliable engine despite being VERY old tech (not that anything coming out of Mercruiser since has been much better. Still OHV engines. :facepalm. The 165 did suffer one Achilles heal, gudeon knock. It never really stopped an engine, but it was an annoying knock at idle. Once the engine was doing 1,000rpm, it went away...

HTH....

Chris......

 

[Rick Stephens]

Using Merc factory manuals, and my own ownership knowledge and experience....

Thanks Chris. I owned one of these, think it was also a '77. Good motor. I was always having to do carb work and replace the points. Good to have the book numbers. Researching I found all sorts of numbers. All higher than those.

 

[achris]

Thanks Chris. I owned one of these, think it was also a '77. Good motor. I was always having to do carb work and replace the points. Good to have the book numbers. Researching I found all sorts of numbers. All higher than those.

I had mine (a 1972 engine) from purchase of the boat (1986) to replacing the entire power package in 1994 with a 4.3LX. I had solved the points problem (which is because conventionally they are used as a high current switch on an inductive load, so that promotes arcing), by building a CDI unit that just used the points as a low current switch. Stopped all the arcing and pitting and provided a better quality spark. Had I kept the engine for any longer I would have replaced the points themselves with an optical switch. I never had carb problems.

Those manual screengrabs are from Merc genuine factory manuals. Not sure where you're seeing higher numbers, but those are the numbers right though all of Merc's documentation, including the sales brochures of the day (see below), and the owner's handbook.

Cheers,

Chris.........

 

[Rick Stephens]

Wow. Look how much weight that drive added to the 302!

 

[achris]

Wow. Look how much weight that drive added to the 302!

Yeah, the TR drive weighed a ton!