1975 Evinrude 70HP Outboard

[Ozi Darklighter]

Greetings fellow boaters!

First of all, let me thank you for this awesome forum. I've already done a bunch of reading, and I have learned a lot more than I knew before (which was nothing) and it's amazing how knowledgeable people are. That being said, I'm hoping some of you will help me with my current problem...

The good news:
The outboard is running pretty well, I can move the boat around the lake, and it starts reliably.

The bad news:
I seem to be losing horsepower as I go through a typical day. I had the boat out on the lake today, and we were doing a bit of tubing, and after about 2 hours or so, I couldn't get the boat to ride across the surface (not sure if theres a term for this?), and thus go 'zoom zoom'.

Here's what I know...
- It takes about 5 minutes of idling in order for the outboard to stay running (without stalling) going at the slowest speed available.
- Once we get going, I usually have to stand towards the front of the boat, to alter the weight to the front in order to level off.
- When the boat fails to 'zoom zoom' even after moving to the front of the boat, we slow way down, and I hear the outboard rev way up (as if it's out of gear) and then it kicks throwing me backwards in the boat a bit.
- I took a look at the prop, and it was pretty banged up when I got it, and I've had a few 'mishaps' that have added a few more. There are several places where there is a bur, so the metal isn't strait

I'm considering what I should do next, since the boat is in the water, and I don't have a whole lot of knowledge / time to make repairs myself. If it's rather basic (like changing a prop) I can handle this, however carb rebuilds, etc, i'd take it in for.

And now for the fun part. What do ya'll think? Let me know if you'd like me to post pictures of the outboard, prop, etc, if it will help. =) Here's a picture of the boat to get an idea of what it looks like.

 

[SS140]

Re: 1975 Evinrude 70HP Outboard

You need to be using your trim. And the term your looking for is "To get on plain" When the boat is out of the water (out of the hole) it is on "plain" which is riding on top of the water, you want to ride that way with the bow slightly out of the water, but allowing yourself to still turn.

The RPM change Im willing to bet is the weight up front pulling the motor out of the water.

You do this too many times you'll burn up your engine or worse.

 

[SS140]

Re: 1975 Evinrude 70HP Outboard

That is a sweet little bow rider though I hope you enjoy it for a while.

 

[Ozi Darklighter]

Re: 1975 Evinrude 70HP Outboard

So I just replaced the prop. The one that was on there was the original, and it was a bit dinged up. I've now got an aluminum prop (not sure if it will perform different though...). It's the same size as was original... 13.25x17. I'm debating between getting a 13.25x15 prop the next time to expirement a little bit, but it will be just that - an experiment. Mainly because there are no gauges (no tach) to see where my RPM's are.

Here's some pictures of the old prop...
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[ezeke]

Re: 1975 Evinrude 70HP Outboard

The way your boat sits on the water makes it look as though you may be getting some saturated foam in the hull - it might be worth investigating.

 

[Ozi Darklighter]

Re: 1975 Evinrude 70HP Outboard

"saturated foam in the hull" - sorry, i'm a bit new... could you describe a bit more what this means? From my quick google search, it looks like this means that boat itself is soaking up water?

What would I do to address this? (I'm guessing I'll probably want to do this at the end of the season?)

And thanks a bunch for the comment + keen eyes =)

 

[SS140]

Re: 1975 Evinrude 70HP Outboard

I wouldnt worry about that much Ozi..... That boat is fine, its lasting this long. LOL

I've got a rotten transom, but my motor aint goin no where. And its a 240lb 70 horse.

Its going to be hard to find the correct pitch without a tach but you need to be running a 13-15 pitch prop. Anything bigger will bog your motor down big time and smaller will be really slow. I run a 14 on my Tri Hull with a set of med. size dol-fins. (Dol-Fins help you plane faster, they are little wings that mount to the lower unit on the engine)

Good luck!

 

[SS140]

Re: 1975 Evinrude 70HP Outboard

I tell you what, with a prop that bad Im suprised you didnt notice the vibration.. I bet that thing was AWFUL! lol Glad you got a new one Sailor

 

[Ozi Darklighter]

Re: 1975 Evinrude 70HP Outboard

Ah, so it's the mounting area for the outboard that was in question. Yeah, I do know that is very solid

Is it a 'hydro-foil' that you're talking about (dol-fin)? I've looked at those, but from what the guys at the store recommended, I needed to replace the prop before I decide anything.

Is the tach / gauge something that can be done when I take it into the mechanic at the end of the season? I wonder how much it would cost to get one of those added (or if it would be worth it).

Once again, thanks for the tips guys. I enjoy reading all the comments and suggestions, and since my boat has been around since the bi-centenial (and has the sticker to prove it) - I'm sure I'll have more fun and interesting questions to come =)

-Ozi

 

[jay_merrill]

Re: 1975 Evinrude 70HP Outboard

By your photo, your boat appears to be about 15 - 16 feet in length. If that is correct, I doubt that you need a 15" pitch prop. The 17" wheel is probably about right and I wouldn't change it until you get a tach hooked up. Until then, you will be just shooting in the dark in regard to proper WOT rpm.

You also need to get your engine running right, before trying to figure out appropriate prop pitch. Until you are certain that the motor is developing full power, trying to determine if it is operating within its proper WOT rpm range, is impossible.

 

[SS140]

Re: 1975 Evinrude 70HP Outboard

yeah. he is right.

Im right there with you buddy, I have an ancient boat that I just love and this is all new to me, it's fun to learn. I cant wait until I know a lot.

Some of these guys have been doing it a LONG time and some of them just know their stuff. This is a great place to spend hours online, LOL

Much healthier then any games, aye?

 

[Ozi Darklighter]

Re: 1975 Evinrude 70HP Outboard

Once again, great advice guys.

I actually took the new prop out on the water and I get to plain a lot easier. But what I'm finding is I can't get it to plain when I'm sitting in the driver's seat =(. If I move to the front (about two feet further forward), we get to plain just fine, and I haven't had any problems getting going with the new prop. I'm wondering now if the hydro-foil is worth getting, since that's supposed to help get to plain faster. Also, I can tell I have plenty of power, because it lifts the boat right up out of the water (at a pretty steep angle, much better than before).

 

[jay_merrill]

Re: 1975 Evinrude 70HP Outboard

If you need a 15" wheel to plane a 15 foot boat with 70hp, something is still wrong. I have a Johnson 65hp on my 17.5 foot trihull, which is most likely wider than your boat as well, and it planes easily. When I am doing a SAR with it, and am carrying aux tanks and a total of 48 gallons (about 298 pounds) of fuel, it takes a bit longer to get on plane, but still does so quickly. With this comparison in mind, you can see that your boat should do better than it is.

The first thing that I would do is a compression check. While the motor should produce decent power with anything over 100 psi per cylinder, I would want to see 120 psi or better.

You might also want to take your boat/trailer to a commercial scale to see what they weigh. If you can find a scale that can read a light enough weight to get a reading on the trailer alone, you can do the math to figure out the weight of the boat. At about 15 feet, it should weigh about 800 pounds, empty. Do the math, once again to account for fuel, gear, etc. If the computed weight seems to be exceptionally high, you may have a bilge full of water saturated foam.

 

[leysathcj]

Re: 1975 Evinrude 70HP Outboard

A quick note on saturated foam: I recently planned on repairing a soft spot in the floor of my tri-hull only to end up opening up the entire thing. Much of the foam was holding water and needed to replace some rotted wood. The moisture was doing some real damage! I replaced the foam and just completed 'glassing in the new floor. It's not a hard job, just a bit messy. I'll have her in the water this weekend. I think your soaking up water and adding alot of weight.

 

[SS140]

Re: 1975 Evinrude 70HP Outboard

I was an idiot, dont listen to me.

Ezeke and these other guys are right, I bet you have some water stuck inside. My boat does the same thing, I have a tri hull. I just run the bildge pump and do some turns and most of the water goes out.


Take the thing out of the water, pull your plug and jar the boat back and forth, it aint heavy. Im handicapped and I did it with mine.

I gotta redo the floor one day, my stringers are totally rotted in some spots, making the hull a little soft. But it's ok for now.

 

[Ozi Darklighter]

Re: 1975 Evinrude 70HP Outboard

Thanks for the posts again guys. Sounds like the experts have spoken - need to check out the back part of the boat for water saturation.

Wow, it definitely sounds like redoing the floor is a pain in the ***. Is there foam or something directly under the wood floor? Just wondering what to expect should I try and pull it up.

Obviously I'm waiting until the off season to work on this though. As the boat will be in the water for a couple more months before I have a place to store it out of water. Add this to the other improvements I'm planning for next season, and I'll have an even better boat.

 

[leysathcj]

Re: 1975 Evinrude 70HP Outboard

It's not a difficult job as I did it and I'm no mechanic:
Pulled seats out, I cut floor out in sections w/ radial arm saw, Pulled out all foam and kept only dry pieces, reinforced "soft" stringers w/ synthetic decking (I had some left over from pool deck), Cut new foam to fit & installed, Re-installed floor using pressure treated exterior plywood (you'll find lots of opinions regarding flooring material but I went w/ treated plywood), 1 layer of fiberglass over plywood to waterproof, sanded rough spots and layed synthetic outdoor carpet. The fiberglass sucked but everything else went pretty smooth. Had her out today for test and will be fishing all weekend for the 4th.

 

[ezeke]

Re: 1975 Evinrude 70HP Outboard

There is not much point in tearing up the deck without knowing if there is a problem.

Most soaked foam problems start near the transom because that is the lowest point on the boat's deck.

You can use inspection ports to close the holes needed to check the foam. Just get the ports that are as large as you want, carefully cut out the correct diameter for the inspection port and take a core sample of the foam. http://www.iboats.com/Seadog_Watert...72838190--**********.694175345--view_id.39787

 

[leysathcj]

Re: 1975 Evinrude 70HP Outboard

Ohh, these are very nice. I'll be installing one for sure.