Outboard won't back up

[pvanv]

I'm going through a 1981 Merc 7.5 OB, s/n 5848475, 8-tooth prop spline. Motor is pristine, and runs great.

The motor is used as the aux on a 23', heavy sailboat. The current prop is marked 7P A1 (I believe that means a 7" pitch) and visually, is a relatively shallow-pitched 3-blade aluminum unit.

The boat will not back up.

Seems the blades get washed in exhaust, and won't bite the water. I have found this to be typical of the through-the-hub-exhaust arrangement on sailboats. Goes fine in forward, and the prop seems perfect in every way. The LU is fine, and the prop is indeed turning in reverse.

On an old OMC 9.9/15 used as a sail aux, I have seen the exhaust housing relieved (just above the ventilation plate) by drilling a 1" hole all the way through the leg. Worked pretty well. I may need to do this on the Merc.

For the newer 4-stroke Tohatsu/Nissan/Merc 8, Solas makes a rather radical-looking 4-blade High Thrust prop, (both 5 pitch and 7 pitch) which is very effective for these low-speed, high load applications. It has an oversized, relieved hub, allowing the reverse exhaust to slip inside the hub, rather than washing over the blades. But I don't see one for the 81 Merc 7.5 (8-spline) application.

Does anyone know of a similar HT or "Bigfoot" prop for the Merc 7.5? Will my only option be to bore a hole through the side of the exhaust housing? Maybe there is a conversion sleeve to use a modern (Tohatsu/Nissan/Merc) HT Solas prop on this motor?

 

[Ross]

Are you quite sure that the prop in deep enough in the water? If it isn't below the transom the prop wash will be hitting the transom and pushing while it is trying to pull.

 

[Benny]

What's the point, sailboats do not back up well at all, irregardless.

 

[Scott On Eagles Wings]

I have a 23' O'Day with a Johnson 9.9. The prop that came with the boat and motor was not for a sailboat. It did not back up very well at all. When you order a new prop tell them specifically what the outboard will be used for and they will set you up. I will look at mine if you like but you have to match the boat to the prop to the motor for the best results.
As Ross stated the prop has to be pretty far down in the water for reverse, how long is your shaft? You may have more than one issue to resolve.

 

[tommays]

I looked at a few prop sights and so far its is SOL ,there seems to be just 7" 8" 9"

 

[pvanv]

Good thoughts all. The motor is indeed a long shaft, and is in the water quite deep. It's on a Garelick articulating mount, and even when fully raised (but not tilted), the skeg is in the water. So when deployed, it's plenty deep.

When I say it doesn't back up, I mean you can push the boat by hand and get more results than the motor at WOT reverse. It just churns the exhaust.

I had this same issue with my Hinterhoeller HR28 and an old Evinrude 15. That was solved by boring a 1" exhaust relief hole completely through the leg, a little above the ventilation plate. But on the current OB's, the Solas HT prop works wonders. It has the exhaust relief at the oversized hub, allowing the exhaust to slip ahead of the skeg when in reverse.

I'd rather change props than bore a hole through the side of this very-nice-looking motor, if possible. But finding a prop (or an adapter sleeve for the splines) seems to be a problem.

 

[Ross]

I often see 4 blade low pitch props on outboard engines on sailboats. A prop that will push a light dinghy at fifteen may not generate enough low speed thrust for a fairly heavy sailboat. Some outboards look like a large mixmaster running on the back of a boat. More blade area and less pitch is the route you need to take .

 

[Scott On Eagles Wings]

Paul the prop I have on mine is a Michigan Wheel 3-blade Aluminum Prop. 9.5" Dia X 10 pitch and cost me $70.00 from Overton's It made all the difference in the world. But again you have to match the prop to your motor and boat. So don't order the same prop without talking to them first.

 

[sailaway78]

I remember seeing

I remember seeing somewhere not too long ago a thread about drilling some 1/2 in holes in the base of the prop blade. This is supposed to help or cure this problem. For the life of me I can't remember where I saw this.

 

[Stu Jackson]

Merc makes a

"Sailboat" prop to overcome exactly what you describe. Have you called Mercury and asked them?

 

[tommays]

When you get to BIG motors and Sterndrives it is just a 4" of 4-1/2" hub and if they use and XHS hub you can put almost anything on anything

BUT with small outboards you end up with a very limited selection because there are so many spline sizes ,when i search the prop finder for a 1981 7.5 merc it comes up blank and i had to dig a bit deeper and still did not find the kind of selection that is out there for a new motor


Even a BRAND new small mercury/ Tohatsu has ONE high thrust prop

 

[Mac Daddy]

Maybe a prop shop can modify what you have.

 

[pvanv]

It would appear that the problem is that Merc has abandoned the older small OB's. The prop selection for the 8-spline units is very limited.

Yes, there are two HT Solas props (both 4-blade, either 5 pitch or 7 pitch) for the current 8/9.8 Tohatsu/Nissan/Merc. They have huge hubs, which allow reverse exhaust relief. I have that setup on my personal boat. Works brilliantly.

But I don't want to tell the owner of the O'Day 23 that she needs to buy a new $2000 OB, when it's clear that correct exhaust relief is the answer.

I know from firsthand experience that relieving the exhaust on the older OMC 9.9/15 works well. But again, convincing the owner that the only solution will be to bore a one-inch hole all the way through her motor may be quite a task.

Surely there is an aftermarket fix for this -- either a high-thrust prop, or an adapter sleeve that will allow me to run the modern Merc 8 HT prop on the old 7.5. Or maybe a trick prop shaft that fits the old gearcase, and allows use of the modern props.

Finding the fix seems to be quite the quest.

 

[pvanv]

Problem solved !

Problem solved !

I traded emails and a phone call with Marc Feinman, the US marketing rep for Solas Science and Engineering. He came up with the correct Solas part number (1113-100-07) which is a 4-blade, high-thrust, 7-pitch prop to fit the old 8-spline prop shaft on the Merc 7.5 OB. No special hub sleeve needed. Retail is about $92.00 US. Our store can source it through Coast Distributors.

Marc is a great guy, very knowledgeable and quite helpful. I can't say enough about how he handled my customer service request. He's in their Tampa, Florida office. Bottom line is that he knows the applications very well. The prop selector charts didn't help me much. Most OB and prop manufacturers concentrate on the faster, bigger and newer motors, and go-fast boats. They tend to ignore sail applications. The correct part is in fact available for most small OB's -- IF you get Marc to help find it.

I know from experience that these props are excellent for sailboats -- I have one on my boat. I was able to downsize from 15 hp to 9.8 hp, and still get better thrust, especially in reverse. Anyone who wants to improve the low-speed thrust on their outboard-aux sailboat may want to investigate these high-thrust Solas props.