1991 20 HP 2 stroke just mounted on sailboat

[Mohawkriverman]

I just recently mounted a 1991 20 HP Evinrude 2 stroke to my MacGregor 25 Sailboat which weighs about 2700 lbs now with the upgrades. I did not notice the prop pitch when I put the motor on, but I will be taking it out of the water soon, so will know for sure then. Given the past owners use, I am guessing a 13 pitch. I have been told this pitch is way too high for my displacement hull, and wish to upgrade the blade this winter. I cruise now at 2,900 RPM, 6.5 MPH., at like 65% throttle. I am fouling the top plug pretty regularly, but the motor seems pretty smooth. What should I do about the propeller?

here is the motor running at 2900 RPM: https://vimeo.com/184059973

 

[tpenfield]

That boat probably only needs 8 hp to reach hull speed, so maybe the 20 hp is just not a good match for it. Any particular reason you put a 20 HP motor on it?

 

[Mohawkriverman]

Hi Ted,

Yes, we are on the Mowhawk river, the Hudson river & the Erie canal, so we do a lot of motor cruising. Many of the bridges are too low for the mast at all! It handles so well with the extra power headroom. Plus the 8 HP that we still have really has to work hard in adverse weather. The new motor has remote steering, throttle, transmission & ignition controls & electric start - love that. Much more agile when the traffic gets heavy also. The other factor is that we do not want to have only one motor when we are going for weekend cruises. This way we have the 8 HP as a back up.

 

[boobie]

Best thing to do is make sure the motor is running on both cylinders and then get a prop to put in the right WOT rpm range. JMO.

 

[Mohawkriverman]

Yes, she is running on both cylinders - I am thinking a 7 pitch. Right now WOT RPM is 3900, should be 5000 - 5500.

 

[Vic.S]

Hi Ted,

Yes, we are on the Mowhawk river, the Hudson river & the Erie canal, so we do a lot of motor cruising. Many of the bridges are too low for the mast at all! It handles so well with the extra power headroom. Plus the 8 HP that we still have really has to work hard in adverse weather. The new motor has remote steering, throttle, transmission & ignition controls & electric start - love that. Much more agile when the traffic gets heavy also. The other factor is that we do not want to have only one motor when we are going for weekend cruises. This way we have the 8 HP as a back up.

I believe however that the maximum recommended hp for a MacGregor 25 is 10hp.

http://glenmoresailboats.com/wp/wp-content/uploads/2013/09/Mac25_Manual_xxxx.pdf

 

[Mohawkriverman]

Hi Vic,

Yes, that is because the motor mount is only rated for up to 10 HP. The 'new' Mac 26 is very similar in hull design to the 25, and they are rated at much higher HP. I removed the swimming ladder on the starboard side of the transom after consulting my father & brother, and 2 local professional boat repair guys who have all renovated boats for years. I then used construction adhesive to attach a 3 1/2' 2" x 8" inside the transom. Next I glued 4 industrial vibration mats 4" w x 1/4" h x 4" d with awesome weather & pressure ratings to the exterior of the transom where the motor mount would be placed. Then I installed a motor mount (20 HP rating) with 8 stainless steel screws through the transom and into the 2" x 8". So far no problems with the upgrade, in fact she has never handled any better.

 

[Mohawkriverman]

The final decision -

I have the boat out of the water & the prop is a 3 blade 10 1/2 pitch - so I have decided on the Evinrude 'special 7' as I call it. It is obviously designed with use at higher RPM pushing a heavier displacement hull working the motor less, fouling fewer plugs & wasting less energy on cavitation in mind!

http://boatpropellers.iboats.com/Evi...t_id=703243474


Thanks everyone!
Bill

 

[racerone]

So---" the top plug is fouling regularly "----That means there is a problem with the top cylinder.----The cylinder is not generating enough heat to keep the plug clean.----Best to trouble shoot that issue.----Does spark jump a gap of 7/16" on it ?----What is the compression on the top cylinder ?---Thermostat in place and working properly ?------Other opinions will vary of course.

 

[oldboat1]

If a cavitation issue, the 4-bladed prop might help. And with the 7 pitch, you should be able to run at a lower hull speed with higher, more comfortable rpms -- maybe a little more control as well. I use a 4-bladed Solas 5 pitch on a 9.9 trolling motor, swim platform center-mounted on a 26 foot cruiser. Aesthetics aside (not great), performance is pretty good for slow trolling. I wanted to run at higher rpms than with the standard prop, and I get that while trolling at one or two mph. Control is OK, but I don't think I could easily use it for docking (tight slip).

On a prior boat, I did something similar to what you describe to beef up the mounting point. The only thing I would suggest is through-bolting rather than screwing into the backing plate.

As was mentioned, the plug fouling needs to be addressed before relaunching. If it's not tucked away yet for the winter, you could do compression and spark tests and go from there.

 

[Mohawkriverman]

She is all tucked in - I will take her to my local Johnson outboard dealer in the spring before launch to have the compression test done and have it checked out - thanks!

 

[Sea Rider]

She is all tucked in - I will take her to my local Johnson outboard dealer in the spring before launch to have the compression test done and have it checked out - thanks!

On 2 strokes OB's it's normal, been wondering why upper plug fouls slight more than lower one, see this all the time when removing spark plugs to clean them regularly say each 50 metered worked hours and that's while running at constant 3/4 to full throttle. Don't ask me why though...LOL!!

With new 7 prop pitch, what's the wot rpm OB is achieving now at full throttle as currently loaded ? My combo is propped to run full wot revs as currently loaded with a under powered OB. Works fantastic well.

Happy Boating

 

[Mohawkriverman]

Compression test is perfect, better prop solved everything, running 4,800 - 5,200 rpm, great power, smooth sailing!

 

[Mohawkriverman]

Thanks to all you experienced boaters!