prop on my 76 Rude 70hp

[jim_in_day]

I have a glastron 15 1/2 foot trihull with a cmc tilt and trim and a 1976 70 hp Evenrude that I have put together. The problem is getting RPMs out of the engine. the 17" prop i started with would only do about 4700-4800 RPM. I switched to a 15" prop (OEM) and get about 5100 R's It doesn't want to rev freely unless the wieght is just perfect in the boat. With a 13" prop i can get 5400 R's, but that seems like an awfully small prop with this boat. I am not so worried about how fast I am as i mainly fish with it. Should I be concerned about that small a prop, or just leave it be?

 

[walleyehed]

Re: prop on my 76 Rude 70hp

I'd start by verifying the engine is healthy..compression check, clean carbs, fresh 87 octane fuel, etc...Might take a good look at the hull to make sure there isn't any hard-core blemishes that would add drag.<br />I might add I know that combination well, and at sea-level (or close to it) it should turn a 15 pitch 5500+<br />Is there any hydrofoil or trolling plate on the lower unit??<br />I should also ask: Where is the AV plate (above prop) in relation to the flat part of the bottom of hull (center)Flush, up, down, etc...

 

[jim_in_day]

Re: prop on my 76 Rude 70hp

Engine is healthy (I work at a Johnson shop) no trolling plate, I took it off. I will get exact measurement tomorrow at shop. Yes the bottom of the boat has more that a couple of good gouges in it. This winter i intend to get the bottom fiberglass re-gelcoated (if the budget allows that)

 

[jim_in_day]

Re: prop on my 76 Rude 70hp

I think the AV plate is parallel with the bottom, will make sure tomorrow. I can adjust it really simply on the CMC unit without disturbing the transom.

 

[jim_in_day]

Re: prop on my 76 Rude 70hp

checked it out, AV plate is 1/2 inch below bottom of hull, am moving it up one inch on CMC by raising it 2 holes. will post when it gets tested

 

[jim_in_day]

Re: prop on my 76 Rude 70hp

Reread your post Walleyehed, and came up with another question, I am running at 3200 ft right now, but use it frequently above 6000 ft. I really feal the effects of altitude in this boat with a load in it, am i better off using the 13 inch prop at 6000 ft, or with the AV plate set properly and the bottom repaired will the 15 inch prop still work at 6000 ft. (I have both props in my boat, the 17 inch shop prop just didn't want to work with a load in the boat.)

 

[walleyehed]

Re: prop on my 76 Rude 70hp

I deal with quite a few altitude issues as well, and for 1 thing, you need to jet for 3000ft first of all, Verify the sea-level jets are installed and drop .002 (2 sizes) from there...main jets only.<br />You can raise the engine up and start with the AV plate even with boat bottom.<br />I run from sea-level to 9100ft, and for the most part, plan on 2" less pitch for every 2500-3000ft up.<br />If you are testing by yourself, throw a bit of gear or another person in and shoot for about 5700-5800.<br />The CMC puts the engine back a bit...about how far back from the transom is it???

 

[jim_in_day]

Re: prop on my 76 Rude 70hp

It has the stock jets in it, i believe they are 54's should i go to 52's? The CMC unit puts it back 5-6 inches from the transom

 

[jim_in_day]

Re: prop on my 76 Rude 70hp

A couple of more questions, but first an explanation, my partner is a johnson master certified mechanic with 20+ years experience and I have been working with him for over 5 years now, but we do 90% of our work on small outboards (15hp or less) so we don't do too much with bigger engines. When we rig a boat with a larger motor (like mine) we try to prop it to stay in the middle of the factory recommended rpm range. From everything i read on this site (especially you and DHadley) you seem to recommend running these motors at rpm's above what the factory literature seems to indicate is the max rpm's. Obviously this is due to your experience, (which i do not question at all) which is part of the reason I ask so many questions. I also wonder about changing the jets on my boat, not because I don't believe that that is the proper course, but because i fish at many different altitudes and wonder if I should be constantly changing jets in the carbs. I am not trying to race this boat (not a bass racer) just get the best out of it i can with what I have. I really look forward to reading what you have to say so I can test it and gain experience as this is as much for fun as it is for learning how to better set up customer boats.

 

[jim_in_day]

Re: prop on my 76 Rude 70hp

WHOOO EEEEE I raised the motor all the way up, ended up about 1 3/8 inches above the transom. 30 mph @ 6000 rpm with 13 in pitch prop (speed with GPS) Tach checked with shop inductive tach. Wow it flies now, I will be able to use the 15 in prop and stay at 5800. Walleyehed you really hit the nail on the head with raising the motor. Now all I have to do is solve my other problem which really belongs on the other forum.

 

[walleyehed]

Re: prop on my 76 Rude 70hp

Glad to hear it!!<br />I would still go down 2 sizes in main jets though.....it'll run ok down to 2000ft with those jets, so unless you go below that, you won't need to change jets anymore..it'll be good for roughly 2000-6000ft...a compromise in between.<br />If you go below 2500ft, just don't run WOT for extended periods.