TOPIC: 57' Mark75 finally running good

Well after over a year of fighting all of the small bugs out of the Mark75, i am happy to report that she finally seems to be running great.

I launched the boat last night and it ran like a watch. Next order of business is to re-prop her. I dont have a tach, but it seems to be turning an awful lot of rpms at top end, and top speed is only about 35 on a 14' boat.

The motor currently has a 13" pitch Michigan wheel prop (boat is only 14'long). I'm thinking a 15" or 16" (if i can find one) would suit the boat much better. Any other suggestions?

Here is a video of it planing the boat last night



Scott

put a tach and find out the RPM's the you can get the right one right away.

Thanks Dave.

What should WOT RPM be?

Also, are there any tach kits that will work and arent worth their weight in gold?

Scott

5500rpm ain't gonna hurt the beast! It's probably doing close to that already, perhaps a bit more. A 15"-pitch prop would lessen the hole shot a bit but unless you're water skiing with it, it'd be a better choice.

My old 700 Dockbuster on a 13' Sea King runabout would do 35mph with a prop that would rip a slalom skier right off the beach.

BTW an automotive tach, set on 6-cylinders, and connected to one ign coil, will give you an accurate reading. You might find a cheap tach on Craigslist or even have an old one kicking around. Just temporarily connect it for a quick rpm reading and then do a permanent installation later, if you so desire.

Nice video, she sounds purty good!........ed

even 5600 is good ## to shoot for. for each inch pitch change look for 200-300 rpm change. to bring the # down go up in pitch to bring it up go down in pitch. rule of thumb.

I'll try an see what LR DR props I have tomorrow.
I went out and looked at the pile today, but forgot what I was looking.
I have this tagged for the AM.
Life on Morphine sucks.
You young fellows need to take care of your spine and related issues.
Taking an enraged 2010 Senior Line-Backer into custody when his emotions are blinding rationality can really ruin the 'Golden Years'.
SIGH

doc

Thanks to all for the great info!

I think i have an old auto tach laying around i will wire up for the next time i bring the ol girl out. Ill keep everyone posted on what i find out for the RPM.

I actually live about 15 minutes from Michigan Wheel company, maybe i will stop in and see what they have if Doc doesnt have one he will sell.

Scott

I'll try and get some help moving things out of the way so I can see what pitch props are in the pile.
Sorry about the delay.
doc

Okay,...Here ya go:
I wrote the diameter and pitch on all the Mk 75-78 props I have.
I couldn't find the cross reference for the (2) Michigan props, but I'm pretty sure I have the stamped numbers correct.
One prop in the pile suggests that it started life as a 13P, but 'might have been' modified to a 17P, which would probably make it very desirable.
Based on this suspicion,...That one will cost you $100 + postage with no guarantee it is a 17P.
The others,...Or any other LH DR prop I have - whether through prop exhaust or earlier style - as you are hunting - are $50 plus actual postage.
doc F

Thanks doc. I really appreciate it!

Let me hook up my Tach to see what RPM i am running now, so i know about what pitch i will need. Looks like you have (1) 15" pitch prop in the pile.... Im thinking that will be right, but ill let you know after i water test with a tach.

Thanks again!
Scott

Listening to it sounds a little high on R's too. Does come out the hole nice though! I think maybe the 15 might be a good choice but you can figure if that, or the 17. I can't tell from Docs pic if they is any cup on either, but add some cup to the 15 maybe?
Ya really gotta hook a tach up to see where ya at. GOOD LUCK and motor does sound "sweet"!