TOPIC: Ventilation issue

I have a '58 Thunderbird Geronimo with a '66 Merc 650 short shaft. The transom height on this boat is 16". I have a problem with ventilation on turns. The original owner that changed the original Mark 58e(?) motor to this one wished he had never done it. He said the orignal motor never had these problems. I have two questions. Did the shaft length on the Mark 58e run a little longer than 15" and/or is there any help for the motor that is on here now?

if your RPM's are correct get a lower pitch prop with a CUP. if your prop is on the High RPM range just get yours cupped. otherwise trim some of the transom down 1/4in at a time.

I tested five or six props last year on my boat, and I couldn't believe how much difference it makes. The least expensive way would be to have some cupping added to your prop as Dave stated. If you don't have a prop shop near you, there are often good deals on eBay for decent used props (Aluminum) in the $50 range.

Are you currently running a two blade or three, many of the older props had no cupping whatsoever!?

I have a 2 blade AL roughly 13 1/4" x 17 and a 3 blade AL same diam. and pitch. They both have the same venting issue. I ordered a 15p 3 blade that should be here tomorrow. Not sure about the blade geometry, though. It's a Michigan Wheel Vortex. Would rather not cut into the transom, if it can be helped.

Also, not sure about the RPM's. No gauges whatsoever on this thing. Thought about adding tach and speedo, but haven't got that far yet.

Good idea.

Still looking for an answer on the Mark 58 shaft length. I'm still looking for one to try and put the boat back to original. I'm thinking the 16" transom was designed for that motor.

No brainer on this one. The Mark 58 and all others of that ilk are to be used on a 16 1/2 inch transom, not a 15 1/2 like your 650 calls for. A good cupped prop should help however. My 60's runabout has a 15 inch transom and the 1960 Merc 400 I am running plows like the devil, I have to raise the transom a full inch and a half to get the thing to run right.

Thanks to everyone for their input. I've been struggling with this for several years. Just haven't taken the time to do anything about it. Now I have a new problem. Ordered a 13.5 x 15 Vortex prop and XHS II 202 Hub Kit. There is a huge difference in the shape and length of the original thrust washer and the one that came in the kit. The new prop is longer in the body than my originals, so I figured that was why the new thrust washer is shorter. The problem is that the new prop sits below where my threads stop on my drive shaft. With the nut tightened all the way down, there is still about 1/4"-3/8" play where the prop will slide on the shaft. Anyone ever run into anything like this? This is supposed to be the right hub kit for this motor, but now I'm wondering if this is the original lower unit that came with it. The picture I'm attaching shows both of the thrust washers.

Sorry about the pic. Having problems getting it to upload. Lol

Here's the pic, hopefully.

Played around with it this afternoon. If I use the old thrust washer, it looks like it might work. The front edge of the prop body sets just inside the cavity on the gear case. Not quite as far as the old prop, but I think far enough to channel the exhaust thru the prop like it should.

there are a few versions of thrust washers must get the correct on or damage can happen.

Thanks, Dave. I have an email sent to a rep from Michigan Wheel to see if he can shed some light on which kit/thrust washer that I need. Won't actually try the new prop until I hear from him.

The boat was not designed for a particular motor. You want to make sure the height of the motor is correct too

Michael R Cole
Son of the original designer
Richard C Cole

Your engine has the older square shoulder propshaft, you may have to get a thrust hub machined for the newer props, or replace the propshaft with the later model tapered shaft, they are much stronger to boot. The old ones could break at the shoulder in high mounted surfacing applications, Merc actually offered a tapered shaft as a heavy duty upgrade back then.

The drive shaft on this one is tapered. Don't know if the lower unit is original to this motor. The two thrust washers in the picture (old one on the left) just had me puzzled. The new one from the hub kit (that is supposed to be for this motor) sits too far down in the gear housing, since it doesn't have the longer sleeve that the old washer has. The new prop won't even touch it. It bottoms out in the housing. I tried the old washer with the new prop (see photo). Have sent pics to rep at Michigan Wheel to see what their take on this issue is.

eagle47803 wrote:

The drive shaft on this one is tapered.

Do you mean the prop shaft is tapered? I.E., it has a beveled shoulder where the propeller thrust hub sits?

If that's the case, the old-style hub with the long "snout" isn't meant to be used with the bevel. You'll note that the newer-style flat hub has a bevel whereas the old-style has a square shoulder. The old-style propshaft had a square shoulder.

Around '65-'66 (as I recall), Merc changed to a beveled propshaft and the thrust hub changed as well. It looked like your old-style long-snout thrust hub, except the "snout" was cut-off and the end beveled.

If your new prop has a completely flat back, it should be using the new-style flat hub and if that's not stacking-up properly to your lower unit, there's something wrong.

If the prop has a shoulder where the old-style thrust hub would fit, and your propshaft has a beveled thrust-hub-edge, you need to be using the shorter old-style thrust
hub, which is a bit thicker than the newer-style flat hub.

Part number for this thrust hub is 65091 and it's been superseded a few times. Here's a very inexpensive NOS part on eBay:



Here's another option: if you do have a square-shouldered prop shaft, there is a special spacer which is used with a "modern" prop along with the later-style flat thrust hub: P/N 86918. If you need one of them, you should be able to find on on eBay or at www.marineengine.com

I've attached some pics which may help.

One last thought, if the prop is too far away from the trailing edge of the gearcase, exhaust gasses will spill out, flow over the propeller and it'll ventilate like mad. So, what you want is for the prop to fit as closely to the gearcase as possible without rubbing. Be sure to push the prop forward against the pressure of the shift clutch spring, when checking prop-to-gearcase clearance.

HTH.......ed

Thanks Ed. Looks like I was wrong (Happens a lot. Lol), or wasn't following what you guys were asking. I see what you mean about the square (90 degree) shoulder. My prop shaft is the old style, so my old thrust washer is correct. The one that came in the new hub kit is tapered. Guess I will look for one of those spacers. My guess is tha guys at MW are going to write back and tell me the same thing.

eagle47803 wrote:

Thanks Ed. Looks like I was wrong (Happens a lot. Lol), or wasn't following what you guys were asking. I see what you mean about the square (90 degree) shoulder. My prop shaft is the old style, so my old thrust washer is correct. The one that came in the new hub kit is tapered. Guess I will look for one of those spacers. My guess is tha guys at MW are going to write back and tell me the same thing.

That being the case, the spacer should fix the problem. There's a really good deal on one at eBay, check it out:



Cheapest one at marineengine.com is $21.80 + shipping, so the eBay item is a $$teal!!!

Cheers.......ed

Thanks again, Ed. You're right. Spacers on Ebay were really good deal. Picked up both of them. Should be here on Monday.

There you go. If you do ever have to replace the propshaft for any reason, get the later beveled one, lots around.

Just got the thrust washer spacer and it worked like a charm. New prop sits right where it should. Now to get it on the lake and see if the prop will stay in the water.

eagle47803

WELCOME ABOARD.

yours, satx